JP2004139001A - Optical element, optical modulating element and image display apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element having a periodical structure with a smaller period than the wavelength of the light used and having higher performance than a conventional one. <P>SOLUTION: The optical element is obtained by layering a structural body having a smaller periodical structure than the wavelength of the light used, and the element is controlled to satisfy conditions to obtain desired phase difference at least two wavelengths in the wavelength region to be used. <P>COPYRIGHT: (C)2004,JPO

Description

ただし、
Δｎ_ｉ：ｉ番目の第１方向に周期を有する周期構造体のλ_１に対するＴＥ偏光とＴＭ偏光の屈折率の差
ｄ_ｉ：ｉ番目の前記第１方向に周期を有する周期構造体の層の厚さ
Δｎ_ｊ：ｊ番目の前記第２方向に周期を有する周期構造体のλ_１に対するＴＥ偏光とＴＭ偏光の屈折率の差
ｄ_ｉ：ｊ番目の前記第２方向に周期を有する周期構造体の層の厚さ
Δｎ_ｉ’：ｉ番目の前記第１方向に周期を有する周期構造体のλ_２に対するＴＥ偏光とＴＭ偏光の屈折率の差
Δｎ_ｊ’：ｊ番目の前記第２方向に周期を有する周期構造体のλ_２に対するＴＥ偏光とＴＭ偏光の屈折率の差
である。
【００２９】
（５）式は、以下のようにして求められる。
【００３０】
まず、厚さｄの格子の、λ_１に対する屈折率差Δｎ、λ_２に対する屈折率差Δｎ’とすると、
位相差φは、
【外３】

これより、
【外４】

ここで、λ_１に対するΔｎｄ、λ_２に対するΔｎ’ｄを求める。
まず、λ_１に関して、第１方向に周期を有する周期構造が複数層（ｋ層）、あるとし、ｉ番目の層のＴＥ偏光とＴＭ偏光の屈折率差をｎｉ，厚さをｄｉとすると、これらの周期構造の各層のΔｎｄの和Δｎｄ（１）は、
【外５】

同様に、上記の周期構造と直交する第２方向に周期を有する周期構造が複数層（ｌ層）あるとし、ｊ番目の層のＴＥ偏光とＴＭ偏光の屈折率差をｎｊ，厚さをｄｊとすると、これらの周期構造の各層のΔｎｄの和Δｎｄ（２）は、
【外６】

となる。ただし、前記第２方向に周期を有する周期構造が存在しない場合（ｌ＝０）、ｊ＝０となる。
【００３１】
仮に第１方向と第２方向が同一だとすると、λ_１に対するΔｎｄは（８）式＋（９）式、つまりΔｎｄ＝Δｎｄ（１）＋Δｎｄ（２）となる。しかし、第１方向と第２方向は直交している。これはどういうことかというと、ある偏向方向の光が第１方向の周期構造でＴＥ偏光として入射して透過し、第２方向の周期構造に入射したとすると、第２方向の周期構造ではＴＭ偏光になるということである。つまり、ＴＥ偏光とＴＭ偏光が入れ替わることになる。
【００３２】
従って、ある特定の偏向方向の光に対して、第１方向の周期構造では
Δｎ＝ｎ（ＴＥ）−ｎ（ＴＭ）　　　（１０）
であったとすると、この光が第２方向の周期構造に入射すると
−ｎ（ＴＥ）＋ｎ（ＴＭ）＝−Δｎ　　　（１１）
となる。従って、実際には、λ_１に対するΔｎｄは（８）式−（９）式となる。従って、
【外７】

となる。
同様にして、λ_２に対するΔｎ’ｄは、
【外８】

となる。
【００３３】
（１２）式、（１３）式を（７）式に代入して、
【外９】

を得る。
【００３４】
次に、本発明の様々な実施形態を、詳細な実施例に基づき説明する。
【００３５】
【実施例】
［実施例１］
本実施例１の概略図を図１に示す。本実施例の光学素子は、微細周期構造を用いた位相板（λ／４板）である。図２は、図１の位相板の横方向からの断面図である。図２において、５は基板、１５は微細周期構造である一次元格子を表わしている。一次元格子１５は使用光の波長よりも周期の短い２つの周期構造からなる。入射側媒質１６は空気である。一次元格子１５には、屈折率及び分散の異なる材料からなる２つの周期構造を用いている。ここでは一例として、基板をＴａ_２Ｏ_５、基板側から第１の周期構造６（以下、第１格子６と称する）をＴａ_２Ｏ_５（屈折率ｎ１＝２．１３９）、第２の周期構造７（以下、第２格子７と称する）をＳｉＯ_２（屈折率ｎ２＝１．８）で構成した。第１及び第２格子の周期Λは０．１６μｍ、第１格子６の厚みｄ１を０．３９μｍ、第２格子７の厚みｄ２を０．１０μｍとし、フィリングファクターＦＦ（＝ｗ／Λ）を０．８５とした。このような構造は、例えばエッチングなどの手法によって作製が可能である。ここでは、可視光領域において０次以外の高次の回折光が発生しない、すなわち格子構造が０次格子として振舞う格子周期にした。
【００３６】
格子が０次格子として振舞う格子周期条件は、文献（Ｅ．Ｂ．Ｇｒａｎｎｅｔａｌ，　Ｊ．Ｏｐｔ．Ｓｏｃ．Ａｍ．　ＡＶｏｌ．　１３，　Ｎｏ．５，　ｐ９８８−９９２，　１９９６）に記載されており、以下の式（１５）で与えられる。
【００３７】
（Λｍａｘ）＝（λｍｉｎ）／（ｎｓ＋ｎｉ｜ｓｉｎθｉ｜）　　（１５）
ここで、Λｍａｘは０次格子として振舞う格子周期の最大値、λｍｉｎは入射波長の最小値、ｎｓは一方の格子材料の屈折率、ｎｉは他方の格子材料の屈折率、θｉは入射角を示す。入射波長λｍｉｎ＝０．４０μｍ、一方の格子材料をＴａ_２Ｏ_５（ｎｓ＝２．１３９）、他方の格子材料を空気（ｎｉ＝１．０００）、入射角θｉ＝０とした場合、Λｍａｘは、約０．１８７μｍとなり、格子周期Λ１及びΛ２が０．１６μｍの時には０次格子として振舞うことがわかる。また、ベクトル解析である厳密波結合理論を用いた厳密な透過・反射率計算からも高次の回折光が発生しないことを確認している。
【００３８】
０次以外の高次回折光が発生するような格子周期を採用した場合には、０次光の回折効率が低下し光学系の光利用効率が低下するばかりでなく、高次の回折光がゴーストやフレアとして現れ、光学性能を著しく劣化させることが予想される。このため、０次格子として振舞う格子周期が望ましい。また、位相板の設計には有効屈折率法（ＥＭＴ）にて算出したｎ（ＴＥ）及びｎ（ＴＭ）を用いて屈折率差Δｎを算出し、設計初期値を見積もった。ここでは２つの格子を使用しているため、第１格子６及び第２格子７に対する屈折率差をそれぞれΔｎ１、Δｎ２とし、格子厚みをそれぞれｄ１、ｄ２とすると、つぎの式（１６）
Δｎ１×ｄ１＋Δｎ２×ｄ２＝λ／４　　（１６）
を、可視光領域においてほぼ満たすような格子形状を採用した。本実施例では、Δｎ１＝０．２８４、Δｎ２＝０．１４７である。最終的には、ベクトル解析（厳密結合波解析）によって反射・透過率および位相差を厳密に算出した。
【００３９】
本実施例のλ／４板の位相差特性を図３に示す。図３において横軸に波長、縦軸に位相差をとっている。従来の水晶を用いた波長板と比較すると位相差を表わす直線の傾きが小さく、０．５０μｍ以下の短波長領域にて特性が平坦になっていることが特徴であり、位相差の波長依存性が改善されている。また、本実施例の位相板の透過率特性を図４に示す。横軸に波長、縦軸に透過率をとっている。水晶板を用いた位相板の可視光領域における透過率がおよそ９５％であるのに対して、本実施例の位相板においては可視光領域のほぼ全域において透過率９７％以上となっており優れた透過率特性を実現している。更に、格子材料の屈折率、格子周期、格子厚み、フィリングファクターを変数として位相差を制御することができるため、様々な特性の位相板を設計することが可能である。
【００４０】
本実施例においては、格子の材料としてＴａ_２Ｏ_５とＳｉＯ_２を用いたが、格子材料はこれに限定するものではなく、屈折率および分散の異なる２種類以上の材料を用いれば何ら問題はない。また、入射側媒質である空気に最も近い格子（実施例１においては第２格子７）の材料がそれ以外の格子（実施例１においては第１格子６）の材料に比べて低い屈折率である。また、基本的な機能及び性能を有するような条件を満たしさえすれば、格子の材料としてどのような屈折率の材料の組み合わせ（高屈折率材料と低屈折率材料の組み合わせ）を用いても構わない。ここで、大きな構造性複屈折を実現するためには屈折率差を大きくとることが必要であり、低屈折率材料として、空気を用いた場合に最も大きな構造性複屈折が得られる。
【００４１】
また、実施例１における第１格子６と第２格子７のように、格子全体が複数の材料で構成されている。実施例１では２種類の材料を用いて第１格子６、第２格子７を設けたが、３種類以上の材料を用いて第１格子、第２格子、第３格子・・・・と３つ以上の格子を有する微細周期構造を形成しても構わない。
【００４２】
［実施例２］
次に、実施例２について述べる。
【００４３】
実施例２の概略図を図５に示し、図５のｘｚ平面断面図を図６に、ｙｚ平面断面図を図７に示す。
【００４４】
図５、図６、図７においては格子形状を横方向に拡大した模式図を表わしているため実際の形状とは異なる場合があるため注意が必要である。図５に示した本実施例２は３つの格子からなる構造の位相板（λ／４板）である。この実施例２では、第１格子６と、第２格子７及び第３格子８との格子の周期方向が直交していることが特徴である。これ以降、このような構成の位相板を積層型位相板と呼ぶ。この場合は積層型λ／４板である。本実施例では、基板５をＴｉＯ_２、入射側媒質１６を空気とした。最も基板側の第１格子６は材料をＴｉＯ_２、格子周期Λ１を０．１５μｍ、格子深さｄ１を１．８５μｍ、フィリングファクターＦＦ１（＝ｗ１／Λ１）を０．９０とし、２段目の第２格子７は材料をＴｉＯ_２、３段目の第３格子８は材料をＳｉＯ_２とし、第２格子７及び第３格子８は、格子周期Λ２を０．１５μｍ、フィリングファクターＦＦ２（＝ｗ２／Λ２）を０．８２とし、第２格子７の格子深さｄ２を１．７５μｍ、第３格子８の格子深さｄ３を０．１０μｍとした。格子周期Λ１およびΛ２は、可視光領域において０次格子として振舞うような格子周期とした。
【００４５】
ここで、設計手法としてＥＭＴを用いて各格子における有効屈折率を算出した。
まず１段目の第１格子６における常光と異常光の屈折率差Δｎ１は、つぎの式（１７）
Δｎ１＝ｎ１（ＴＥ）−ｎ１（ＴＭ）　　（１７）
によって与えられる。
【００４６】
一方、第２格子７においてＴｉＯ２が与える屈折率差Δｎ２及び第３格子８においてＳｉＯ２が与える屈折率差Δｎ３は同様に、つぎの式（１８）及び式（１９）
Δｎ２＝ｎ２（ＴＥ）−ｎ２（ＴＭ）　　（１８）
Δｎ３＝ｎ３（ＴＥ）−ｎ３（ＴＭ）　　（１９）
によって与えられる。ただし、ここで注意するべきなのは、第１格子と、第２及び第３格子の格子構造の周期方向が直交するので、第１格子６の格子構造におけるＴＥ偏光は第２及び第３格子の格子構造７，８ではＴＭ偏光に、第１格子６の格子構造におけるＴＭ偏光は第２及び第３格子の格子構造７，８ではＴＥ偏光となる。このような屈折率の与え方の違いは、異方性結晶における正結晶と負結晶との違いに相当するものである。
よって、つぎの式（２０）
Δｎ１×ｄ１−Δｎ２×ｄ２−Δｎ３×ｄ３＝１／４λ　　（２０）
を可視光領域の２つの波長で満たすような解を式（５）より算出して採用し、最終的にはベクトル解析である厳密結合波解析によって反射・透過率及び位相差を厳密に算出した。
【００４７】
図８に実施例２における積層型λ／４板の位相差特性を示す。横軸は波長を、縦軸は位相差を表わしている。従来の水晶結晶板を用いたλ／４板と比較すると、積層型λ／４板においては位相差が略９０°の波長領域が広帯域化しており、波長依存性が改善されている。また、透過率特性を図９に示す。横軸は波長を、縦軸は透過率を示している。可視光領域のほぼ全域において９６％以上の高い透過率で、優れた透過率特性を実現している。更に、格子材料の屈折率、格子周期、格子厚み、フィリングファクターを変数として位相差を制御することができるため、様々な特性の位相板を設計することが可能である。
【００４８】
実施例２においては、格子材料として、ＴｉＯ_２とＳｉＯ_２の２種類を用いたが、これに限定するものではなく、屈折率および分散の異なる２種類以上の格子材料を用いれば何ら問題はない。また、本実施例においては、第１格子６及び第２格子７の材料として共にＴｉＯ_２を用いたが、これに限定するものではなく、第１の格子６と第２の格子７の格子材料としてＴｉＯ_２以外の格子材料を用いても良いし、第１格子６と第２格子７の格子材料として屈折率の異なる材料を用いても何ら差し支えない。更に付け加えるならば、入射側媒質である空気１６に最も近い格子（実施例２においては第３格子８）の格子材料は、他の格子（実施例２においては第１格子、第２格子）に比べ屈折率が低い材料を用いた。基本的な機能及び性能を有するような条件を満たしさえすれば、格子の材料としてどのような屈折率の材料の組み合わせ（高屈折率材料と低屈折率材料の組み合わせ）を用いても構わない。
【００４９】
ここで、実施例２の変形例を図１０に示す。図１０は、基板５の上に、基板側から第１格子６、第２格子７、第３格子８、第４格子９を設けた位相板の実施例である。このように、基板の上に格子を４つ積層して位相板を構成しても良い。ここでは、基板及び第１〜３格子をＴｉＯ_２、第４格子をＳｉＯ_２で作成した。しかしながら、第１〜３格子は異なる材料で構成しても構わない。また、第１〜３格子は、フィリングファクターが同じであっても異なるようにしても構わない。
【００５０】
［実施例３］
実施例３として、偏光変換素子において使用されているλ／２板を例に挙げる。液晶プロジェクタなどに搭載されている偏光変換素子において、光源からの無偏光の白色光は、偏光ビームスプリッタを用いて偏光方向が互いに９０°異なる２つの直線変更に変換される。このとき、光利用効率を向上させるために、一方の偏光方向を９０°回転させ、他方の偏光と偏光方向をそろえ、両者を同一方向に出射させている。そして、偏光方向を９０°回転させる為にλ／２板が使用されている。従来のλ／２板においては屈折率差の波長依存性があった。
【００５１】
本実施例におけるλ／２板を偏光変換素子に用いた例を図１１に示す。偏光ビームスプリッタ１２に入射した、光源からの無偏光の白色光は、偏光ビームスプリッタを透過したｐ偏光と、偏光ビームスプリッタで反射したｓ偏光とに分離する。分離した２つの偏光のうち、ｐ偏光は、λ／２板を透過し、ｓ偏光となる。そして、偏光ビームスプリッタを反射し、ミラー１１を反射したｓ偏光と同一方向に射出する。従来と比較して、位相差の波長依存性が少なく、設計波長以外の波長においても光利用効率の高い光学系を実現できる。このため、明るさの向上及び画質の向上により従来以上に高性能な液晶プロジェクタの実現が可能である。また、材料としてＴｉＯ_２などの誘電体材料を用いることによって、熱による体積膨張や屈折率の変化を小さく抑えることができ安定した性能を発揮できるという利点がある。また、構造性複屈折がもつ大きな複屈折量による素子の薄型化が可能である。更に、格子材料の屈折率、格子周期、格子厚み、フィリングファクターを変数として位相差を制御することができるため、様々な特性の位相板を設計することが可能で、使用用途に最適な性能を実現することができる。
【００５２】
また、本実施形態における位相板の応用例は偏光変換素子に限定するわけではなく、例えば、実施例３の偏光変換素子を有する液晶プロジェクタ（画像表示装置）や、液晶プロジェクタと、それに画像情報を送信する画像送信手段（テレビやパーソナルコンピューターやデジタルカメラ等）を有する画像表示システムなどに適用しても構わない。勿論、それに限らず、λ／２板、λ／４板等を有する様々な光学機器・デバイスへの応用が可能である。
【００５３】
また、本実施例においては、格子構造（微細周期構造）として、格子材料と空気が周期方向に交互に配列された構造を用いたが、本発明はこれに限らず、第１の格子材料と空気以外の第２の格子材料を周期方向に交互に配列して、格子を構成してもかまわない。
【００５４】
［実施例４］
次に、実施例４について述べる。
【００５５】
図１２は実施例４の積層型位相板の構成を示す斜視図、図１３はそのｘｚ平面の断面図、図１４はｙｚ平面の断面図である。図中１は基板、２は基板１上に一次元に一定周期で配列された第１格子である。また、３は第１格子２の周期方向に対し直交する方向に一定周期で配列された第２格子３である。第２格子３は格子３ａと格子３ｂの屈折率の異なる２種類の材料を積層して構成されている（本実施例では、３ａ、３ｂの二つの格子をまとめて１つの１次元格子として扱い、第２格子と呼ぶことにする）。第１及び第２格子２，３の２つの１次元格子は、各々高屈折率材料（格子部分）と低屈折率材料（空気）とが交互に配列された構造となっている。また、第１及び第２格子２，３の周期は入射波長（ここでは可視光）よりも小さくなっている。なお、入射側媒質として、ここでは空気としているが、ガラス等のカバーを用いて入射側媒質としてもよい。
【００５６】
また、第１格子２の幅はＷ１、深さ（厚み）はｄ１、格子の周期はΛ１である。第２格子３の幅はＷ２、格子周期はΛ２である。第２格子３のうち格子３ａの深さ（厚み）はｄ３、格子３ｂの深さ（厚み）はｄ２である。
【００５７】
ここで、本実施例では、一次元格子を２段積層した構造のλ／４板を示しており、第１格子２と第２格子３の格子の周期方向が直交していることが特徴である。以下、本実施例では、この構成を二段積層型λ／４板という。この二段積層型λ／４板の一例として、第２格子３を構成する二種類の材料について、格子３ａとしてＳｉＯ_２、格子３ｂとしてＴａ_２Ｏ_５を用い、第１格子２を構成する格子材料としてＴａ_２Ｏ_５を用いている。格子周期Λ１及びΛ２は０．１６μｍとし、Λ１及びΛ２は、可視光領域において０次以外の高次の回折光が発生しない条件、即ち、０次格子として振舞うような格子周期としている。
【００５８】
本実施例では、第１格子と、第２格子２，３の周期方向が直交するので、第１格子２における屈折率差をΔｎ１、第２格子３における屈折率差をΔｎ２とする。
【００５９】
よってλ／４板の場合、次式、
Δｎ１×ｄ１−Δｎ２×ｄ２−Δｎ３×ｄ３＝λ／４　（２１）
を可視光領域の２つの波長において満たすような解を採用している。ここで、式（２１）を２つの波長において満たすような解は無数に存在するが、素子製造の観点から見ると、フィリングファクターＦＦと格子深さが適当な値をとるようにすることが望ましい。一般的に、フィリングファクターＦＦが極端に小さく、且つ、格子深さが深い場合、格子構造が倒れやすくなり製造が困難である。よって、ＦＦは０．２０から０．９５程度の範囲内の値をとることが望ましい。
【００６０】
更に、付け加えるならば、本実施形態において、第１格子２のフィリングファクターＦＦ１がとりうる範囲は概ね０．７５以上０．９０以下の範囲、第２格子３のフィリングファクターＦＦ２がとりうる範囲は概ね０．３０以上０．７０以下であることが望ましい。一例として具体的な値を挙げると、位相差１／４λを満たすためには、ＦＦ１＝０．８１、ＦＦ２＝０．６０の場合、格子材料の深さとしてそれぞれｄ１＝１．０７μｍ、ｄ２＝０．３１μｍ、ｄ３＝０．１０μｍ程度の厚みが必要である。但し、必ずしもこの値と一致しなければ位相差１／４λを実現できないわけではなく、基本的な性能を満たす範囲であるならば、ここに示した値よりも大きな値、あるいは小さな値をとっても良い。
【００６１】
次に、式（２１）に示した位相差条件を満たし、且つ、素子の反射率を低減するための例について説明する。ここで目的とすることは、位相差１／４λを満たし、且つ、使用光の各偏光方向に対して反射率が低減できる位相板を実現することである。格子構造は二段積層型λ／４板とし、格子材料はＳｉＯ_２、Ｔａ_２Ｏ_５を用い、格子周期Λ１＝Λ２＝０．１６μｍとし、変数としてフィリングファクターＦＦ１，ＦＦ２、格子深さｄ１，ｄ２，ｄ３とする。
【００６２】
本実施例の２段積層型λ／４板は、一方の偏光方向に対しては、入射側媒質から基板１に向かって有効屈折率が徐々に高くなっていくような屈折率分布であり、もう一方の偏光方向に対しては、２層積層された第１格子と第２格子間の屈折率差が小さくなるような屈折率を実現していることを特徴としている。
【００６３】
この場合、例えば、ＦＦ１＝０．８１、ＦＦ２＝０．６０とする。フィリングファクターの組み合わせは無数に存在するが、二つの格子深さが極端に大きくならない値を選択したほうが作製しやすいので留意したほうが望ましい。格子深さは、ＦＦ１及びＦＦ２を用いて算出される有効屈折率と、（４）から（７）式を用いて算出することができる。本実施例においては、格子深さが極端に深くならない構成として、ＦＦ１がとりうる範囲は概ね０．７５以上０．９０以下の範囲、ＦＦ２がとりうる範囲は０．３０以上０．７０以下であることが望ましい。
【００６４】
この時の各偏光に対する有効屈折率を図１５に示す。但し、直交する格子でＴＥ偏光とＴＭ偏光が入れ替わることを考慮し、混乱を避けるために、ここでは第１格子２の周期と直交する方向をＡ方向、第１格子２の周期と平行な方向をＢ方向とする。そして、各格子のＡ方向の偏光の有効屈折率をｎ（Ａ）、Ｂ方向の偏光の有効屈折率をｎ（Ｂ）とする。つまり、第１格子２において、ｎ（Ａ）＝ｎ（ＴＥ）となるが、第２格子３においてはｎ（Ａ）＝ｎ（ＴＭ）となる（図１２参照）また、図１５において、ｎｉは入射側媒質（ここでは空気）、ｎ１は第１の格子２、ｎ２は第２の格子３における格子３ｂ、ｎ３は第２の格子３における格子３ａ、ｎｓは基板、の各屈折率を示す。
【００６５】
はじめに、ｎ（Ａ）について考えると、ＥＭＴから算出した有効屈折率（λ０＝ｄ線）を詳しくみると、
ｎｉ（Ａ）＝１（ａｉｒ）、
ｎ３（Ａ）＝１．２３０、
ｎ２（Ａ）＝１．４３６、
ｎ１（Ａ）＝１．９９３、
ｎｓ＝２．１３９、
となっており、入射側から基板方向に向かって屈折率が徐々に高くなる屈折率分布になっている。このような屈折率分布をとることにより、基板表面でのフレネル反射を低減し広帯域で反射防止効果が得られる。これは、例えば、傾斜膜を用いて屈折率が徐々に変化する屈折率分布を実現したときに得られる反射防止効果と同様である。
【００６６】
次に、ｎ（Ｂ）に対して考えると、ＥＭＴから算出した有効屈折率は、
ｎｉ（Ｂ）＝１（ａｉｒ）、
ｎ３（Ｂ）＝１．３１３、
ｎ２（Ｂ）＝１．８２３、
ｎ１（Ｂ）＝１．７３３、
ｎｓ＝２．１３９、
となっており、ｎ１（Ｂ）とｎ２（Ｂ）の屈折率差（即ち、格子間における屈折率差）が小さい構成となっている。ここでのｎ１（Ｂ）とｎ２（Ｂ）との屈折率差は０．０９という値となっており、ほぼ一致している。なお、屈折率差は完全なゼロである必要はなく、おおよその目安として望ましくは屈折率差が０．１程度、あるいは、０．２程度前後の値を満たしていることが望ましい。このように、ＴＭ偏光に関しては、複数積層された格子間の屈折率の差がＴＥ偏光に比べて小さくなっている。また、ここで、差の小さいｎ１（Ｂ）とｎ２（Ｂ）をほぼ同じ屈折率と考えると、ｎ１とｎ２との間で生じるフレネル反射を低減することができる。また、このようにｎｉ（Ｂ）＜ｎ３（Ｂ）＜ｎ２（Ｂ）≒ｎ１（Ｂ）＜ｎｓ（Ｂ）という屈折率分布をとることで反射防止効果が得られる。
【００６７】
ここで、ｎ３の光学膜厚を最適に設計するとさらに反射防止効果が得られる。具体的には、第２格子３における格子３ａの厚みｄ３を設計波長の１／４程度の光学膜厚にすることで、ＴＭ偏光に対する反射防止効果が得られる。但し、ｎ３の膜厚ｄ３はＴＥ偏光に対する反射率にも影響を与えるので、実際には、ＴＥ偏光とＴＭ偏光に対する反射率を両立するような膜厚とすることが重要であり、設計値の目安として設計波長の１／４に近い値とすることが望ましい。
【００６８】
なお、ＴＥ偏光とＴＭ偏光の有効屈折率の関係は逆であっても成立する。つまり、ＴＥ偏光に関して格子間の有効屈折率の差が小さく、且つ、ＴＭ偏光に関しては入射側媒質から基板に向かって有効屈折率が徐々に高くなるような構成をとっても同様の効果が得られる。
【００６９】
更に、有効屈折率は、格子周期Λ、格子のフィリングファクターＦＦによって制御することが可能であるため、格子材料の屈折率によらず、有効屈折率をある程度の範囲で制御可能である。よって、以上のような有効屈折率を実現するための格子材料の積層順番を必ずしも限定する必要はない。
【００７０】
また、基板材料の屈折率ｎｓと基板上の格子の有効屈折率ｎ１（ＴＥ）（もしくはｎ１（ＴＭ））の屈折率差が大きいと境界面で生じるフレネル反射によって反射率が増加するため、ｎｓとｎ１（ＴＥ）（もしくはｎ１（ＴＭ））の屈折率差が小さいほうが望ましい。但し、ここで注意しなければならないことは、一次元格子形状においては屈折率に異方性を持っているためｎ１（ＴＥ）とｎ１（ＴＭ）とは同じ屈折率をとることは原理的にあり得ない。よって、ＴＥ方向に対する屈折率差ｎｓ−ｎ１（ＴＥ）とＴＭ方向に対する屈折率差ｎｓ−ｎ１（ＴＭ）を同時にゼロとすることはできない。そのため、ＴＥ・ＴＭ両方の偏光方向に対して反射防止効果を得るためには、ＴＥ偏光に対する屈折率差ｎｓ−ｎ１（ＴＥ）とＴＭ方向に対する屈折率差ｎｓ−ｎ１（ＴＭ）の値が両方小さくなるように設定することが望ましい。
【００７１】
次に、格子深さについて述べる。位相差については位相差Δｎｄが１／４λを満たすような深さが必要であるが、それと同時に反射防止効果が得られるような格子深さとする必要がある。反射防止効果を得るためには、一般的に、膜厚ｎｄが波長λ０に対して１／４λ０の整数倍であることで実現することが可能であるから設計の目安にすると良い。
【００７２】
位相差条件から求めた設計初期値を元に上記手法を用いて求めた偏光方向に依存しない反射防止効果を得る格子形状を求めたところ、以下に示すような結果が得られた。即ち、二段積層型λ／４板の一例として、第２格子３における格子３ａの材料としてＳｉＯ_２、格子３ｂの材料としてＴａ_２Ｏ_５を用い、格子周期Λ２を０．１６μｍ、格子深さｄ３を０．１０μｍ、格子深さｄ２を０．２４μｍ、フィリングファクターＦＦ２（＝ｗ２／Λ２）を０．６０とする。また、第１格子２の格子材料としてＴａ_２Ｏ_５を用い、格子周期Λ１を０．１６μｍ、格子深さｄ１を０．９６μｍ、フィリングファクターＦＦ１（＝ｗ１／Λ１）を０．８１とする。格子周期Λ１及びΛ２は、可視光領域において０次以外の高次の回折光が発生しない条件、即ち、０次格子として振舞うような格子周期とする。
【００７３】
図１６に実施例４における二段積層型λ／４板の位相差特性を示す。図１６では水晶の位相差特性も併せて示している。横軸は波長、縦軸は位相差を表わしている。従来の水晶結晶板を用いたλ／４板と比較すると、二段積層型λ／４板においては位相差が略９０°の波長領域が広帯域化している。また、誤差範囲は、９０°に対して−５°から＋１０°程度に抑えられ、波長依存性が改善されていることが注目すべき点である。
【００７４】
また、透過率特性を図１７及び図１８に示す。ここでは、ベクトル解析である厳密結合波解析によって反射・透過率及び位相差を厳密に算出している。図１７はＴＥ偏光及びＴＭ偏光に対する透過率特性を示している。横軸は波長を、縦軸は透過率を示しており、ＴＥ・ＴＭ偏光の両方に対して優れた透過率特性が得られていることがわかる。また、図１８は各偏光方向に対する透過率の平均値を示す。可視光領域のほぼ全域において約９７％以上の高い透過率が得られ、優れた性能を実現している。
【００７５】
なお、ここではλ／４板を例に挙げたが、本発明はこれに限定するものではなく、格子材料の屈折率、格子周期、格子深さ、フィリングファクターを変数として位相差を制御することが可能で、且つ、反射防止効果も得られるため、例えば、λ／２板等様々な特性の位相板に適用することが可能である。
【００７６】
また、本実施例においては、第２格子３の２種類の格子材料としてＴａ_２Ｏ_５とＳｉＯ_２を用いたが、これに限定するものではなく、屈折率及び分散の異なる２種類以上の格子材料を用いれば何ら問題はない。更に、本実施形態においては、第１格子２と第２格子３の格子３ｂの材料として共にＴａ_２Ｏ_５を用いているが、これに限定するものではなく、Ｔａ_２Ｏ_５以外の材料を用いても良いし、第１格子と第２格子の格子３ｂの材料として屈折率の異なる材料を用いても何ら差し支えない。また、基本的な機能及び性能を有するような条件を満たしさえすれば、第１格子及び第２格子の格子３ａ、格子３ｂの材料としてどのような屈折率の材料の組み合わせを用いても構わない。また、第２格子全体が複数の材料で構成されている方が好ましく、本実施例では２種類の材料を用いて第２格子３を形成したが、３種類以上の材料を用いても構わない。
【００７７】
［実施例５］
次に、実施例５について説明する。図１９は実施例５の構成を示す斜視図、図２０はそのｘｚ平面の断面図、図２１はそのｙｚ平面の断面図である。図中１は基板、２は基板１上に一次元に一定周期で配列された第１格子、３は第１格子２の周期方向に対して直交する方向に一定周期で配列された第２格子である。また、第２格子３上に第２格子３の周期方向に直交する方向に第３格子４が一定周期で配列されている。第３格子４は格子４ａと格子４ｂの、屈折率の異なる２種類の材料を積層して構成されている（本実施例では、４ａ、４ｂの二つの格子をまとめて１つの１次元格子として扱い、第３格子と呼ぶことにする）。第１〜第３格子による１次元格子構造は、それぞれの周期構造の周期方向に各々高屈折率材料（格子部分）と低屈折率材料（空気）が交互に配列された構成となっている。また、第１〜第３格子２〜４の格子周期は、実施例４と同様に入射波長（可視光）よりも小さい。入射側媒質としては、例えば、空気としているが、ガラス等のカバーを用いて入射側媒質としてもよい。
【００７８】
また、第１格子２の格子の幅はＷ１、深さ（厚み）はｄ１、格子周期Λ１である。第２格子３の格子の幅はＷ２、深さ（厚み）はｄ２、格子周期はΛ２である。第３格子４の格子の幅はＷ３、格子周期はΛ３であり、そのうち格子４ａの深さ（厚み）はｄ４、格子４ｂの深さ（厚み）はｄ３である。
【００７９】
実施例５は、一次元格子を３段積層した構造のλ／４板であり、第１格子２と第２格子３が、そして第２格子３と第３格子４が、それぞれ周期方向が直交していることが特徴である。第１格子２の材料としてＴａ_２Ｏ_５を用い、格子周期Λ１を０．１６μｍ、厚みｄ１を０．５６５μｍ、フィリングファクターＦＦ１（＝Ｗ１／Λ１）を０．８１としている。第２格子３の材料としてＴａ_２Ｏ_５を用い、格子周期Λ２を０．１６μｍ、厚みｄ２を０．１９０μｍ、フィリングファクターＦＦ２（＝Ｗ２／Λ２）０．６としている。第３格子４における格子４ａの材料としてＳｉＯ_２を、格子４ｂの材料としてＴａ_２Ｏ_５を用い、格子周期Λ３を０．１６μｍ、格子４ａの厚さｄ４を０．１００μｍ、格子４ｂの厚さｄ３を０．２５０μｍ、フィリングファクターＦＦ３（＝Ｗ３／Λ３）を０．８１としている。格子周期Λ１、Λ２及びΛ３は、可視光領域において０次以外の高次の回折光が発生しない条件、即ち、０次格子として振舞うような格子周期としている。
【００８０】
第１〜第３格子の有効屈折率は実施例４と同様である。即ち、ｘ軸方向の偏光に関して、入射側媒質側から、第３格子４の格子４ａ、格子４ｂ、第２格子３、第１格子２と基板側へ向かうにつれ、有効屈折率が高くなっている。また、ｙ軸方向の偏光に関しては複数積層された格子間、即ち、第３格子４の格子４ｂと第２格子３、第２格子３と第１格子２の有効屈折率の差が小さくなっている。この屈折率の差は前述のように０である必要はなく、０．１あるいは０．２程度前後であればよい。もちろん、ＴＥ偏光とＴＭ偏光の有効屈折率の関係は第１の実施形態の場合と同様に反対の関係であってもよい。
【００８１】
実施例５の三段積層型λ／４板の位相差特性を図２２、透過率特性（偏光平均）を図２３に示す。図２２では水晶の場合の特性を併せて示している。図２２から位相差の波長依存性が小さく、９０°からの誤差範囲は−１０°から＋１０°程度に抑えられていることは特筆すべきことである。また、図２３から分かるように優れた透過率特性が実現でき、可視領域において概ね９５％以上の透過率を実現している。三段積層型の場合は二段積層型に比べ、格子材料の厚みを更に薄くすることが可能であり、一つ一つの格子構造を作製することが容易になると期待できる。
【００８２】
［実施例６］
次に、実施例６について説明する。実施例６は、実施例４の位相板を用いた光学系の実施形態である。図２４は実施例６を示す図であり、実施例４の位相板をダブルパスで用いた場合の光学系を示している。図２４において、１９は実施例４の位相板（λ／４板）、２０はミラーである。また、１７は使用光、１８は出射光（観測する光）である。ここでは、偏光方向の揃った使用光（直線偏光）１７が積層型λ／４板１９に入射し、円偏光に変換された後、ミラー２０で反射され、再び積層型λ／４板１９に入射し、出射した光１８の偏光方向が使用光１７に対して９０°回転する。
【００８３】
ここで、偏光方向が回転した時の光の透過率については、以下に示す式（２２）で算出することができる。
【００８４】
Ｔ＝ｃｏｓ^２（Γ／２−π／２）　　　（２２）
Ｔは偏光の回転を考慮した場合の透過率、Γは位相差である。この式について横軸を位相差Γ、縦軸を透過率Ｔとして表わしたグラフを図２５に示す。透過率Τについて補足すると、ダブルパス光学系に入射する直線偏光１７の光強度を１とした場合、ダブルパス光学系を通って出射した偏光方向が回転した光１８に対して、入射する直線偏光に９０°直交する偏光子を通して観測した光強度の割合を表わしている。具体的にはダブルパス光学系の使用光１７と出射光１８との位相差が１８０°である時は、観測される出射光１８の偏光方向は入射する直線偏光１７の偏光方向に対して丁度９０°回転した偏光であるので、使用光に対して観測される光強度の割合は、１／１＝１となる。
【００８５】
また、ここで注意すべき点は、式（２２）で算出される透過率においては位相板素子の表面反射や多重反射による反射光は考慮されていない。よって、実質的な透過率を検討するためには、位相板素子の表面反射・多重反射を考慮した透過率と式（２２）から得られる、偏光の回転を考慮した透過率を同時に考える必要がある。以上のことからダブルパスを用いた光学系における実質的な透過率は、位相板がもつ透過率と式（２２）から算出される偏光の回転による透過光強度損失を考慮した透過率との掛け算によって評価できる。
【００８６】
実施例６のダブルパス光学系における位相差特性を図２６に示す。図２６では水晶の位相差特性も併せて示す。従来の水晶板と比較して、波長依存性が小さく抑えられていることがわかる。また、上記手法によって算出した実質的な透過率を図２７に示す。図２７には比較のために従来の水晶を用いたλ／４板における実質的な透過光強度も同時に示している。従来の水晶λ／４板におけるフレネル反射は、水晶の屈折率（ｎ＝１．５５）を用いて算出される値を計算に用い、透過率９５％として算出している。
【００８７】
また、水晶厚みは設計波長λ０＝０．５５μｍで位相差９０°を実現できる値とし、厚みｄ＝１５．３μｍとしている。従来の水晶λ／４板を用いた場合には、位相差の波長依存性の影響を大きく受け、設計波長付近での実質的な透過率は優れた特性を有しているが、短波長側及び長波長側において実質的な透過率が減少していることがわかる。これは、図２６に示すように水晶においては位相差の波長依存性が大きな問題となっているためである。
【００８８】
一方、実施例６の二段積層型位相板を用いたダブルパス光学系の場合には、位相差の波長依存性を大幅に改善でき、各偏光方向に対する反射率を低減した構成をとっているので、可視領域全域（概ね０．４μｍから０．７μｍの範囲）において、概ね９６％以上の優れた透過率を実現することが可能である。
【００８９】
このようなダブルパスを用いた光学系は、例えば、反射型カラー液晶表示装置等に使用が可能である。従来のλ／４板を用いた場合と比較して、本発明における積層型位相板を用いた場合は、位相板そのものの反射率を低く抑えることが可能であり、光損失が小さいばかりでなく、従来の位相板と比較して波長依存性が小さく抑えられているため色再現性が非常に優れ、画質の向上が期待できる。更に、格子材料として誘電体材料を用いることで熱による体積膨張や屈折率変化を小さく抑えることが可能となり、安定した性能を発揮することが可能である。また、構造性複屈折がもつ大きな複屈折量により素子の薄型化が期待できる。更に付け加えるならば、位相板の基板１としてＡｌ等の金属を用いるか、あるいは誘電体多層膜を用いて作製した広帯域反射膜の上に本実施例の積層型位相板を作製することによって位相板１９とミラー２０を一体化して素子の一体化、薄型化も可能である。
【００９０】
［実施例７］
次に、実施例７の積層型位相板について説明する。本実施例の積層型位相板は、基板と格子からなる格子型素子部を複数個、互いの格子面が向かい合うように積層したことを特徴としている。
【００９１】
図２８から図３０は、実施例７の位相板の概略構成を示している。なお、図２８（Ａ）は上記位相板の斜視図であり、図２８（Ｂ）は上記位相板の製作工程を示している。図２９は上記位相板のｘｚ平面の断面図であり、図３０はｙｚ平面の断面図である。
【００９２】
この位相板は、第１の基板２１と、入射する光の波長よりも微小な格子周期を有する第１格子２２と、入射する光の波長よりも微小な格子周期を有し、周期方向が第１格子２２の周期方向に対して略直交する第２格子２４と、第２の基板２３とをこの順で積層した構成を有する。
【００９３】
具体的な製造方法としては、例えば、第１および第２の基板２１，２３上にそれぞれ高さ（深さ）ｄ１，ｄ２を有する格子２２，２４を、エッチング、電子ビーム描画、ＬＩＧＡプロセス、フォトリソグラフィー、多光束レーザー干渉法、多積層薄膜などの手法によって作製する。
【００９４】
そして、できあがった第１の基板２１および第１格子２２からなる第１の格子型素子部Ａと、第２の基板２３および第２格子２４からなる第２の格子型素子部Ｂとを、図２８（Ｂ）に示すように、格子２２，２４が向かい合い、かつ格子２２，２４の周期の方向が互いに略直交するように積層する。このとき、光学密着法（ｏｐｔｉｃａｌ　ｃｏｎｔａｃｔ）などによって両格子の格子面を密着させ、積層することができる。
【００９５】
このような製造方法を採用することで、格子のみを積層させる場合に比べて積層型位相板を容易に製造することができる。
【００９６】
第１の基板２１の材料としてはガラス（屈折率ｎｓ１＝１．８）を、第１格子２２の材料としてはＴａ_２Ｏ_５を、第２の基板２３の材料としてはガラス（屈折率ｎｓ２＝１．６）を、第２格子２４の材料としてはＴａ_２Ｏ_５（屈折率ｎ＝２．１３９）を用いている。また、格子の周囲を満たす媒質は空気とした。
【００９７】
第１格子２２は格子周期Λ１＝０．１６μｍ、格子深さｄ１＝０．９０μｍ、フィリングファクターＦＦ１＝０．８とし、第２格子２４については格子周期Λ２＝０．１６μｍ、格子深さｄ２＝０．２５μｍ、フィリングファクターＦＦ２＝０．６とした。格子周期Λ１およびΛ２については、可視光領域において０次回折光以外の高次の回折光が生じないような格子周期を採用した。
【００９８】
ここで、図２８（Ａ）における、ｙ軸方向をＡ方向、ｘ軸方向をＢ方向とし、各方向の偏光に対する有効屈折率をｎ（Ａ），ｎ（Ｂ）とする。
【００９９】
第１格子に対する有効屈折率ｎ１（Ａ），ｎ１（Ｂ）、第２格子に対する有効屈折率ｎ２（Ａ），ｎ２（Ｂ）をＥＭＴにより見積もると次のような値になる。波長λ＝０．５５μｍにおいて、
第１格子２のＴＥ偏光に対する屈折率ｎ１（Ａ）＝１．９６５
第１格子２のＴＭ偏光に対する屈折率ｎ１（Ｂ）＝１．６３３
第２格子４のＴＥ偏光に対する屈折率ｎ２（Ａ）＝１．３７２
第２格子４のＴＭ偏光に対する屈折率ｎ２（Ｂ）＝１．７７４
となる。
【０１００】
上記設計値を採用した理由について述べる。まず、ｘ方向の偏光に注目した場合、第１格子２２におけるｘ方向の偏光に対する屈折率ｎ１（Ｂ）と、第２格子２４におけるｘ方向の偏光に対する屈折率ｎ２（Ｂ）はそれぞれ、
ｎ１（Ｂ）＝１．６３３、ｎ２（Ｂ）＝１．７７４
となっており、有効屈折率が略等しい（差が０．２以下である）。これにより、ｘ方向の偏光に対して、第１格子２２と第２格子２４との境界においてフレネル反射をほとんど生じさせないようにすることができる。
【０１０１】
次に、ｙ方向の偏光に注目した場合、第１格子２２におけるｙ方向の偏光に対する屈折率ｎ１（Ａ）と、第２格子２４におけるｙ方向の偏光に対する屈折率ｎ２（Ａ）はそれぞれ、
ｎ１（Ａ）＝１．９６５、ｎ２（Ａ）＝１．３７２
となっており、有効屈折率に若干の違いがみられる。
【０１０２】
このままでは、ｙ方向の偏光に対して第１格子２と第２格子４との間でフレネル反射が生じる。さらに、第１の基板２１と第１格子２２との屈折率差および第２の基板２３と第２格子２４との屈折率差によって生じるフレネル反射を考慮すると、透過率の減少が懸念される。
【０１０３】
以上のことを考慮すると、互いに隣接して積層されている第１の基板２１と第１格子２２において、第１の基板２１の屈折率ｎｓ１は、第１格子２２におけるｙ方向の偏光に対する有効屈折率ｎ１（Ａ）および第１格子２２におけるｘ方向の偏光に対する有効屈折率ｎ１（Ｂ）の両方に対して、屈折率差がバランスよく小さくなるように設定するとよい。
【０１０４】
具体的には、第１の基板２１の屈折率ｎｓ１が、第１格子２２におけるｙ方向の偏光に対する有効屈折率ｎ１（Ａ）および第１格子２２におけるｘ方向の偏光に対する有効屈折率ｎ１（Ｂ）との間の値であることが好ましい。
【０１０５】
なお、このことは、第２の基板２３の屈折率ｎｓ２と、第２格子２４におけるｙ方向の偏光に対する有効屈折率ｎ２（Ａ）および第２格子２４におけるｘ方向の偏光に対する有効屈折率ｎ２（Ｂ）との関係においても同様である。
【０１０６】
ここでは、第１の基板２１の屈折率をｎｓ１＝１．８０とし、第２の基板２３の屈折率をｎｓ２＝１．６０として、第１の基板２１と第１格子２２との境界におけるフレネル反射、ならびに第２の基板２３と第２格子２４との境界におけるフレネル反射の両方を同時に低減し、透過率の減少を最小限にしている。
【０１０７】
但し、本発明は、上記各値を示す構成に限定されるものではない。例えば、ｙ方向の偏光およびｘ方向の偏光のうち一方のみを通過させるような用い方をする場合には、互いに隣接する基板と格子において、基板の屈折率を格子におけるｙ方向の偏光に対する有効屈折率およびｘ方向の偏光に対する有効屈折率のうち一方に略等しくしてもよい。これにより、通過させる偏光のフレネル反射を減少させることができる。
【０１０８】
また、上記２組の互いに隣接した基板および格子において、一方の組の基板および格子の屈折率の関係のみを上記のようにしてもよい。これにより、一方の組の基板と格子との境界におけるフレネル反射のみを低減した構成を採ることができる。これらは応用事例によって適宜選択すればよい。
【０１０９】
なお、本実施例のように設定することで、両方の基板と格子との境界におけるフレネル反射を同時に低減することによって、位相板全体として最も透過率の減少が抑えられる。
【０１１０】
図３１は、本実施例の位相板における入射角度０°での透過率特性を示している。図３１に示すように、可視光領域０．４０μｍから０．７０μｍの全域においてほぼ９７％以上の透過率を実現している。
【０１１１】
また、位相差特性を図３２に示す。従来の水晶薄板による位相板と比較すると、格段に波長依存性が改善されており、可視光領域０．４０μｍから０．７０μｍの全域においてほぼ８０°から９５°の範囲の位相差を実現しており、可視光波長領域の全域においてほぼλ／４板として機能することができる。
【０１１２】
そして、本実施形態では、積層型位相板の格子の格子面が基板２１，２３によって覆われるため、手油や粉塵、ほこりによる格子の乱れ、引っかきや摩擦による格子破壊などによる性能劣化が防止できるという利点がある。
【０１１３】
また、積層型位相板の入射面でのフレネル反射を防止するために、入射面に一般的な単層膜や多層膜などの反射防止コートを施すか、あるいは基板表面に入射波長よりも微細な凹凸構造（例えば、微細なピラミッド構造や円錐構造が規則的あるいはランダムに配列した構造）を作製する必要がある。その場合でも、反射防止コート等を微細な各格子の端面に設けるのではなく、ある程度の面積を有する基板の表面に設ければよいので、前述したように格子のみのものに比べて積層型位相板の製造がし易く、かつ反射防止効果も得易い。
【０１１４】
さらに、従来のフィルム型位相板はポリマーなどの材料を使用しているため、熱による性能劣化が著しかった。このため、高温環境下においては期待される十分な性能が得られなかったり、使用できないかったりし、また、使用する場合には十分な冷却システムの工夫が必要であった。本実施形態の積層型位相板は材料としてＴａ２Ｏ５などの誘電体材料を用いているので、熱変動による性能劣化がほとんど生じず、高温環境下においても高性能な位相板として機能することができる。例えば、常に高温環境にさらされている液晶プロジェクタなどの光学系において非常に有効である。
【０１１５】
［実施例８］
次に、本発明の実施例８について説明する。実施例８は、実施例７の積層型位相板を備えたダブルパス光学系である。その構成は、実施例６のダブルパス光学系において、位相板１９を実施例７の積層型位相板に置き換えたものと等しい。従って、図２４を流用して本実施例の光学系を説明する。つまり、図２４において、１９が実施例７の積層型位相板である。その他の構成は実施例６と等しい。
【０１１６】
当然であるが、本実施形態の積層型位相板１９は、可視光領域内の広範囲でλ／４板として機能するものである。
【０１１７】
このダブルパス光学系の積層型位相板１９の、入射角度０°における光学特性について述べる。
【０１１８】
まず、使用光１７に対する出射光１８の透過率特性を図３３に示す。ここで示す透過率特性は、基板表面におけるフレネル反射の影響は無視している。図３３からわかるように、可視光波長領域０．４０μｍから０．７０μｍの全域においてほぼ９５％から９９％程度の透過率を実現している。
【０１１９】
基板表面におけるフレネル反射を減少させるためには、先にも説明したが、一般的な単層膜や多層膜などの反射防止コートを基板表面に施すか、あるいは基板表面に入射波長よりも微細な凹凸構造（例えば、微細なピラミッド構造や円錐構造が規則的あるいはランダムに配列した構造）を作製することによって実現可能である。
【０１２０】
また、使用光１７に対する出射光１８の位相差特性について図３４に示す。図３４からわかるように、可視光波長領域０．４０μｍから０．７０μｍの全域においてほぼ１６０°から１９０°の範囲の位相差を実現しており、広い波長帯域でほぼ１８０°の位相差を実現している。このことは、積層型位相板１４を光が２回通過したとき、可視光波長領域の全域において積層型位相板１４がほぼ１／２波長板として機能するダブルパス光学系を実現できることを示している。また、従来の水晶を用いた場合の位相板と比較しても、位相差の波長依存性の少ないことがわかる。
【０１２１】
次に、このダブルパス光学系の積層型位相板１９の、入射角度２０°における光学特性について述べる。使用光１７に対する出射光１８の透過率特性を図３５に、位相差特性を図３６に示す。
【０１２２】
透過率特性は、可視光波長領域においてほぼ９５％から９９％程度の透過率を有し、入射角度に敏感でない透過率を実現している。同様に、位相差特性について見ると、やはり、可視光波長領域においてほぼ１５５°から１８５°の位相差を実現しており、幅広い波長領域においてほぼ１８０°の位相差を得ることが示されている。このように、本実施例では、ダブルパス光学系において、入射角度依存性が小さく、かつ可視光波長領域の全域において光が２回通過することによりほぼ１／２波長板として機能する素子を実現している。
【０１２３】
但し、ここで示した入射角度は、第２の基板から第２格子に入射する角度を意味している。本実施例においては、第２格子の屈折率をｎ２＝１．６０としているため、空気から第２の基板に入射する光の入射角度については約３３°となる。
【０１２４】
すなわち、実際に積層型位相板を使用する場合には、第２の基板に入射する光線の入射角度が３３°程度の光線に対しても同等な性能が得られることを示している。
【０１２５】
以上のように、本実施形態においては、２０°程度の入射角度変動に対して透過率特性および位相差特性がほとんど変化せず、かつ可視光波長領域の全域において使用光１７に対して出射光１８の位相差がほぼ１８０°となるような位相板を実現することができる。
【０１２６】
従来の水晶による波長板やポリマーフィルムによる波長板では、位相差の波長依存性および入射角度依存性が顕著に現れ、これらを改善するためには、複数枚の位相板の光学軸をずらして貼り合せる等、煩雑な工程が必要であった。また、複数枚のフィルムを貼り合せるという点からも位相板素子自体の薄型化が図りにくいという欠点があった。
【０１２７】
これに対し、実施例８および前述した実施例７の積層型位相板においては、基板上に作製した格子を互いに周期方向が直交するように格子面を向かい合わせて貼り合せることで容易に作製が可能である。そして、位相差の波長依存性および入射角度依存性を同時に改善することができる。
【０１２８】
［実施例９］
次に、本発明の実施例９について説明する。実施例９は、実施例７の積層型位相板を用いた反射型ディスプレイを示している。図３７に実施例９の模式断面図を示す。
【０１２９】
図３７において、２５はリフレクタ、２６は液晶、２７はカラーフィルター、２８は実施例７の積層型位相板（λ／４板）、２９は偏光板である。
【０１３０】
実施例７，８で説明したように、上記積層型位相板２８は、位相差の波長依存性が小さく、かつ入射角依存性が小さい。このため、ディスプレイなど様々な入射角度で入射する光に対して位相板として機能しなければならない光学系では特に有用である。
【０１３１】
例えば、従来のフィルム位相板を用いた場合には、波長依存性によって表示される色純度が低下し、色再現性が悪くなったり、斜めからディスプレイを覗いた場合に波長シフトが生じて色が変化して見えたりするなどの問題があった。
【０１３２】
これに対して、本実施例の積層型位相板２８を採用すれば、上記問題を大幅に改善することが期待できる。つまり、色再現性が優れ、かつ入射角度変化による色シフトの少ない、美しく表現力の高い画像を表示できる、見やすいディスプレイを実現することができる。
【０１３３】
例えば、携帯情報端末や携帯電話、プロジェクタ用液晶ディスプレイなどの小型の反射型液晶ディスプレイから、ＬＣＤモニタなどの大型ディスプレイに至るまで、幅広い応用が考えられる。
【０１３４】
本実施例では反射型ディスプレイについて述べたが、実施例７の積層型位相板は透過型ディスプレイにも適用できる。この場合、積層型位相板はλ／２板として機能するものを用いる。
【０１３５】
［実施例１０］
次に、本発明の実施例１０について説明する。実施例１０は、実施例９の反射型ディスプレイを用いた液晶プロジェクタ（光学機器）を示している。図３８に、その液晶プロジェクタの構成を示した。
【０１３６】
図３８において、光源５０は、ハロゲンランプやキセノンランプ等からなる点に近い白色光源である。光源５０から放射された光はリフレクタ５１で反射され、略平行光の光束として出射され、白色の偏光シート５２ａに照射される。この偏光シート５２ａは、リフレクタ５１からの光の内、第１の偏光成分（Ｓ偏光又はＰ偏光）を透過し、第２の偏光成分（Ｐ偏光又はＳ偏光）を吸収する特性を有しており、偏光シート５２ａを透過した光束は、偏光ビームスプリッタ５３の偏光面で反射され、色分解光学系としての合成クロスプリズム５４に照射する。
【０１３７】
合成クロスプリズム５４は、４個の直角プリズムの直角を挟む面をそれぞれ接着剤により貼り合わせ、貼り合わせ面に第１ダイクロイックミラー面５４ａと第２ダイクロイックミラー面５４ｂが略十字状になるように設けられ、第１ダイクロイックミラー面５４ａ及び第２ダイクロイックミラー面５４ｂの法線と使用光の主光線とのなす角が略４５度程度となるように構成されている。また、第１ダイクロイックミラー面５４ａ及び第２ダイクロイックミラー面５４ｂは、誘電体多層膜で構成され、例えばＴｉＯ２及びＳｉＯ２の薄膜を数十層交互に積層することで波長選択反射特性を得ている。
【０１３８】
第１ダイクロイックミラー面５４ａは、第１の色光である赤色光を反射しかつ第２の色光である緑色光及び第３の色光である青色光を透過する。また、第２ダイクロイックミラー面５４ｂは、青色光を反射しかつ赤色光及び緑色光を透過するように構成されている。第１ダイクロイックミラー面５４ａで反射された赤色光は、実施例９の反射型ディスプレイ（液晶パネル）５６ａに入射する。第２ダイクロイックミラー面５４ｂで反射された青色光は、同様に積層型位相板を備えた反射型ディスプレイ（液晶パネル）５６ｂに入射する。また、第１ダイクロイックミラー面５４ａ及び第２ダイクロイックミラー面５４ｂを透過した緑色光は、同様に積層型位相板を備えた緑色光用の反射型ディスプレイ（液晶パネル）５６ｃに入射する。
【０１３９】
赤色光用の反射型ディスプレイ５６ａにて変調された画像光は、合成クロスプリズム５４に入射し、第１ダイクロイックミラー面５４ａで反射し、第２ダイクロイックミラー面５４ｂを透過する。また、青色光用の反射型ディスプレイ５６ｂにて変調された画像光は、合成クロスプリズム５４に入射し、第１ダイクロイックミラー面５４ａを透過し、第２ダイクロイックミラー面５４ｂで反射する。同様に、緑色光用の反射型ディスプレイ５６ｃにて変調された画像光は、合成クロスプリズム５４に入射し、緑色の画像光は第１ダイクロイックミラー面５４ａ及び第２ダイクロイックミラー面５４ｂを透過する。
【０１４０】
以上のように合成クロスプリズム５４でカラー合成された画像光は、偏光ビームスプリッタ５３及び偏光シート５２ｂを透過し、投影レンズ５７によってスクリーン５８に拡大投影される。
【０１４１】
本実施例の反射型液晶ディスプレイは、積層型位相板を用いており、従来の反射型液晶ディスプレイに比べ、位相差の波長依存性や入射角度特性が改善している。なお、本実施形態では、反射型液晶ディスプレイへの応用について説明したが、本実施例の位相制御素子は、透過型液晶ディスプレイにも適用でき、この場合も位相差の波長依存性や入射角度依存性が大幅に改善される。また、本発明の積層型位相板は、これら以外も様々な光学装置の光学系に対して応用することができる。
【０１４２】
【発明の効果】
以上説明したように、本発明によれば、使用光の波長よりも微小な周期の周期構造を有する光学素子であって、従来知られたものに比べより高性能な光学素子が実現できる。
【図面の簡単な説明】
【図１】本発明の実施例１における位相板の概略図。
【図２】本発明の実施例１における位相板の断面図。
【図３】本発明の実施例１における位相板の位相差特性。
【図４】本発明の実施例１における位相板の透過率特性。
【図５】本発明の実施例２における位相板の概略図。
【図６】本発明の実施例２における位相板のｘｚ平面断面図。
【図７】本発明の実施例２における位相板のｙｚ平面断面図。
【図８】本発明の実施例２における位相板の位相差特性を示す図。
【図９】本発明の実施例２における位相板の透過率特性を示す図。
【図１０】本発明の実施例２の変形例における位相板の概略図。
【図１１】本発明の実施例３における偏光変換素子の概略図。
【図１２】本発明の実施例４における位相板の斜視図である。
【図１３】図１２のｘｚ平面断面図である。
【図１４】図１２のｙｚ平面断面図である。
【図１５】実施例４の各格子材料のＴＥ偏光、ＴＭ偏光に対する有効屈折率を示す図である。
【図１６】実施例４の位相差特性を水晶の場合と比較して示す図である。
【図１７】実施例４のＴＥ偏光、ＴＭ偏光に対する透過率特性を示す図である。
【図１８】実施例４のＴＥ偏光、ＴＭ偏光に対する透過率の平均値を示す図である。
【図１９】実施例５の斜視図である。
【図２０】実施例５のｘｚ平面断面図である。
【図２１】実施例５のｙｚ平面断面図である。
【図２２】実施例５の位相差特性を水晶の場合と比較して示す図である。
【図２３】実施例５の透過率特性を示す図である。
【図２４】実施例６の光学系を示す図である。
【図２５】偏光方向が回転したときの位相変化に対する透過率特性を示す図である。
【図２６】実施例６の位相差特性を水晶の場合と比較して示す図である。
【図２７】実施例６の実質的な透過率特性を水晶の場合と比較して示す図である。
【図２８】実施例７の積層型位相板の概略構成図（斜視図）および製造工程図。
【図２９】実施例７の積層型位相板のｘｚ平面断面図。
【図３０】実施例７の積層型位相板のｙｚ平面断面図。
【図３１】実施例７の透過率特性を示すグラフ図。
【図３２】実施例７の位相差特性を示すグラフ図。
【図３３】実施例８の入射角度０°における積層型位相板の透過率特性を示すグラフ図。
【図３４】実施例８の入射角度０°における積層型位相板の位相差特性を示すグラフ図。
【図３５】実施例８の入射角度０°における積層型位相板の透過率特性を示すグラフ図。
【図３６】実施例８の入射角度２０°における積層型位相板の位相差特性を示すグラフ図。
【図３７】実施例９の積層型位相板を用いた反射型液晶ディスプレイの断面図。
【図３８】実施例１０の反射型液晶ディスプレイを用いた液晶プロジェクタの構成図。
【図３９】従来の１次元微細周期構造を示す図。
【図４０】従来の１次元微細周期構造を示す図。
【図４１】従来の水晶板における位相差特性を示すグラフ図。
【符号の説明】
１　基板
２　第１の格子
３　第２の格子
４　第３の格子
５　基板
６　第１格子
７　第２格子
８　第３格子
２１　第１の基板
２２　第１格子
２３　第２の基板
２４　第２の格子[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical element utilizing a structural birefringence effect of a grating structure having a periodic structure with a period smaller than the wavelength of light used, and an optical modulator, an optical device, and an image display device having the same. It is.
[0002]
[Prior art]
Conventionally known as birefringent materials include crystalline materials such as calcite and quartz, liquid crystal materials, plastic materials having birefringence, and polymer materials. These birefringent materials are applied to phase plates (1/4 wavelength plates, 1/2 wavelength plates), low-pass filters, and the like. Recently, the importance of birefringent materials has increased in products such as liquid crystal projectors, liquid crystal displays, and digital still cameras.
[0003]
On the other hand, it is possible to impart birefringence to the material by producing a periodic structure having a period smaller than the wavelength of the light used in the substrate material. Here, the used light refers to light in a wavelength region in which a desired effect is to be obtained among light incident on the element. For example, in the case of a phase plate used in the visible light region (wavelength 0.40 μm to 0.70 μm), assuming that light to be incident on the phase plate is white light, light having a wavelength of 0.40 μm or less or a wavelength of 0.70 μm The above light is also incident. However, in this case, the phase plate only needs to be able to give a desired phase difference to light having a wavelength of 0.40 μm to 0.70 μm. 70 μm light is used light. Birefringence caused by such a structure is well known as structural birefringence. (Non-Patent Document 1)
Features of structural birefringence include:
{Circle around (1)} The amount of birefringence can be arbitrarily controlled by designing a fine periodic structure.
{Circle around (2)} has a larger birefringence than conventional birefringence such as quartz.
[0004]
Here, an example of a one-dimensional lattice shape having a structural birefringence action is shown in FIG. In order to realize the structural birefringence, two materials 31 and 32 having different refractive indexes with respect to the periodic direction A of the grating are used. In FIG. 39, air is used as the low-refractive-index material 32. However, as shown in FIG.
[0005]
In the example of FIG. 40, a structure having a structural birefringence effect using two lattice materials (lattice material 41 and lattice material 42) having different refractive indexes is shown.
[0006]
The above structure can be manufactured by, for example, a technique such as etching, electron beam drawing, a LIGA process, photolithography, a multi-beam laser interferometry, or a multi-layer thin film.
[0007]
Further, the birefringence amount in the one-dimensional lattice shape can be controlled using the refractive index of the material, the lattice period Λ, and the filling factor FF as variables. The filling factor FF is a ratio of the width w of one of the two materials constituting the lattice shape (31, 32 in FIG. 39, the width w of one 31) to the lattice period Λ, and FF = It is expressed as w / Λ. An effective refractive index method (Effective {Medium} Theory: EMT) can be used for estimating an apparent refractive index (hereinafter referred to as an effective refractive index) for ordinary light and extraordinary light.
[0008]
However, Patent Literature 1, which is a conventional technique, describes a birefringent structure having a periodic structure of 以下 or less of the wavelength of light, but only describes the provision of the birefringent structure. However, there is no description about a phase plate having small wavelength dependence.
[0009]
Further, Patent Document 2 describes a phase plate such that a phase difference is matched at many wavelengths. However, a material constituting the phase plate uses a crystal plate of a large number of anisotropic crystals. No mention is made of a phase plate utilizing birefringence.
[0010]
Patent Document 3 proposes a transmission phase grating, but describes only a one-dimensional grating as a grating structure.
[0011]
Further, in Patent Document 4, a first substance and a second substance having different refractive indexes are applied to a phase plate as structural refractors, but two materials are used in a plane orthogonal to the direction of a used light beam. Structural birefringence is realized by alternately arranging materials, and nothing is mentioned about the improvement of the wavelength dependence of the phase difference.
[0012]
In recent academic papers (Non-Patent Document 2), there is a description of a phase plate using a one-dimensional lattice shape. However, the anti-reflection film has a structure in which an antireflection film is added to the surface of the one-dimensional lattice shape. The function is added, and the phase difference is given only to the grating portion, and the antireflection film itself does not give the phase difference. Therefore, this does not realize a phase difference between ordinary light and extraordinary light by using two or more kinds of materials for the grating portion.
[0013]
[Patent Document 1]
JP-A-5-107412
[Patent Document 2]
JP-A-5-331321
[Patent Document 3]
JP-A-8-254607
[Patent Document 4]
JP-A-9-145921
[Non-patent document 1]
Born & Wolf, Principles of optics 6th edition, P.E. 705
[Non-patent document 2]
H. Kikuta @ et @ al, @ Apply. {Opt. {Vol. No. 36, No. 7, @p. 1566-1572, 1997
[0014]
[Problems to be solved by the invention]
In conventional birefringent materials, it was difficult to control the wavelength dependence of the amount of birefringence, so it was difficult to control the wavelength dependence of the phase difference between ordinary light and extraordinary light. When a monochromatic light source such as a laser is used only at a single wavelength, the phase difference at the design wavelength can be optimized.However, in an optical system that handles light in various wavelength regions such as white light, the wavelength dependence of the phase plate If there is, there is a big problem. For example, in the case of a liquid crystal projector that uses light in the visible light region, optical loss occurs in the liquid crystal panel and the color separation element, which causes a reduction in light use efficiency of the entire system and deterioration of an image.
[0015]
From the above, the use of a phase plate having a small wavelength dependence of the phase difference in the visible light region makes it possible to improve the light use efficiency and the image quality in the above-described optical system, and to make the element thinner. It is important in doing.
[0016]
In view of the above, the present invention provides an optical element having a periodic structure with a period smaller than the wavelength of light used, taking into account the conventional example, and an element having a novel configuration for obtaining higher performance. With the goal.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, the present invention discloses an optical element in which structures having a periodic structure smaller than the wavelength of light to be used are stacked in various forms. For example, a form in which a desired phase difference can be obtained at least two wavelengths in the used wavelength band, a form in which the periodic directions of a plurality of structures are orthogonal, or a difference in the refractive index of each structure of a predetermined polarization component is determined. A mode in which the value is equal to or less than the value is disclosed.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Prior to the description of embodiments of the present invention, a phase plate using quartz, which is a conventional technique, will be described in order to facilitate understanding of the present invention. In addition, the effective refractive index of the fine periodic structure, which is a value corresponding to the refractive indexes of ordinary light and extraordinary light of quartz, will be described.
[0019]
A λ / 4 plate using a quartz crystal utilizing a birefringence effect will be described as an example. The refractive indexes (no, ne) of the ordinary light and the extraordinary light of the quartz crystal at the wavelength of 0.5893 μm are no = 1.5443, ne = 1.5534, respectively, and the refractive index difference Δn is 0.0091.
[0020]
Here, the design method of the λ / 4 plate includes a design wavelength λ₀The thickness d of the crystal that gives a phase difference of λ / 4 at 0.55 μm is calculated from the following equation (1).
[0021]
Δnd = λ₀/ 4 (1)
As a result, it is understood that the λ / 4 plate using quartz needs to have a thickness of 15.3 μm. FIG. 41 shows the characteristics of the phase plate at this time. The horizontal axis indicates the wavelength, and the vertical axis indicates the phase difference between the ordinary light and the extraordinary light. As can be seen from FIG. 41, at a design wavelength of 0.55 μm, the phase difference satisfies 90 ° (λ / 4), which satisfies the performance as a λ / 4 plate. On the short wavelength side, the phase difference deviates from 90 °. Further, as the distance from the design wavelength λ increases, the deviation of the phase difference from 90 ° increases. That is, a λ / 4 plate using quartz as a birefringent material has wavelength dependency in the visible light region.
[0022]
In the case of quartz, the amount of birefringence depends on the refractive index difference Δn = no−ne using the refractive indexes (no, ne) of ordinary light and extraordinary light. In the case of a fine periodic structure, an effective refractive index is used as a value corresponding to this. Specifically, the polarized light having a polarization direction perpendicular to the periodic direction of the grating of the fine periodic structure is referred to as TE polarized light, the polarized light having a polarization direction parallel to the periodic direction of the grating as TM polarized light, and TE polarized light, TM polarized light. The effective refractive index is represented by n (TE) and n (TM). In this case, the refractive index difference Δn = n (TE) −n (TM). The effective refractive index can be calculated using an effective refractive index method (Effective {Medium} Theory: EMT).
[0023]
The calculation method of the effective refractive index of the one-dimensional periodic structure is described in Born & Wolf, Principles of optics 6th edition, P.K. 706-707. Specifically, it is represented by the following equation
n (TE) = ｛ns²FF + ni²(1-FF)｝^1/2(2)
n (TM) = ｛(1 / ns²) FF + (1 / ni)²) (1-FF)｝^-1/2
(3)
Here, ns is the refractive index of one of the two materials constituting the fine periodic structure, and ni is the refractive index of the other material. FF is a filling factor, and is represented by a ratio of the length w of the material having the refractive index ns in the period direction to the grating period Λ (FF = w / Λ). Also, the refractive index difference Δn is
Δn = n (TE) −n (TM) (4)
Becomes
[0024]
Based on the above, embodiments of the present invention will be described below.
[0025]
One embodiment of the present invention discloses an optical element in which structures having a periodic structure smaller than the wavelength of light to be used are stacked in various forms. For example, a form in which a desired phase difference can be obtained at least two wavelengths in the used wavelength band, a form in which the periodic directions of a plurality of structures are orthogonal, or a difference in the refractive index of each structure of a predetermined polarization component is determined. A mode in which the value is equal to or less than the value is disclosed.
[0026]
First, an embodiment in which a desired phase difference is obtained at at least two wavelengths in the used wavelength band will be described.
[0027]
In the quartz phase plate shown in FIG. 41, the wavelength at which the phase difference is exactly 90 ° is one design wavelength. As the distance from the design wavelength λ increases, the deviation of the phase difference from 90 ° increases. In contrast, in the present embodiment, two design wavelengths are used. By setting the phase difference to 90 ° (in the case of a λ / 4 plate) at the two design wavelengths, the deviation of the phase difference in a wide visible light region is reduced, and the wavelength dependence of the phase difference is improved. In order to use two design wavelengths, the following conditions may be satisfied.
[0028]
In a periodic structure having a plurality of layers laminated on a substrate and having a period shorter than the wavelength of the used light, the plurality of periodic structures are arranged in a first direction parallel to the substrate of k layers (k: an integer of 1 or more). In the case of a periodic structure having a period and a periodic structure having a period in a second direction orthogonal to the first direction of l layers (1 is an integer of 0 or more), two design wavelengths λ1 in a visible light region; With respect to λ2, the following expression (5) is satisfied.
[Outside 2]

However,
Δn_i: Λ of a periodic structure having a period in the i-th first direction₁Difference between TE-polarized light and TM-polarized light
d_i: Thickness of a layer of a periodic structure having a period in the i-th first direction
Δn_j: Λ of the j-th periodic structure having a period in the second direction₁Difference between TE polarized light and TM polarized light
d_i: Thickness of the layer of the j-th periodic structure having a period in the second direction
Δn_i': Λ of a periodic structure having a period in the i-th first direction₂Difference between TE-polarized light and TM-polarized light
Δn_j': Λ of the j-th periodic structure having a period in the second direction₂Difference between TE-polarized light and TM-polarized light
It is.
[0029]
The expression (5) is obtained as follows.
[0030]
First, λ of the lattice of thickness d₁Index difference Δn, λ₂And the refractive index difference Δn ′ with respect to
The phase difference φ is
[Outside 3]

Than this,
[Outside 4]

Where λ₁Δnd, λ₂Δn′d with respect to.
First, λ₁Assuming that there are a plurality of layers (k layers) having a period in the first direction, the refractive index difference between TE-polarized light and TM-polarized light of the i-th layer is ni, and the thickness is di, these periodic structures The sum Δnd (1) of Δnd of each layer of
[Outside 5]

Similarly, it is assumed that there are a plurality of layers (1 layer) having a periodic structure having a period in the second direction orthogonal to the above-mentioned periodic structure, the refractive index difference between the TE-polarized light and the TM-polarized light of the j-th layer is nj, and the thickness is dj. Then, the sum Δnd (2) of Δnd of each layer of these periodic structures is
[Outside 6]

Becomes However, when there is no periodic structure having a period in the second direction (l = 0), j = 0.
[0031]
If the first direction and the second direction are the same, λ₁Is given by (8) + (9), that is, Δnd = Δnd (1) + Δnd (2). However, the first direction and the second direction are orthogonal. This means that if light in a certain deflection direction is incident and transmitted as TE-polarized light in the periodic structure in the first direction, and is incident on the periodic structure in the second direction, it is TM in the periodic structure in the second direction. That is, it becomes polarized light. That is, the TE polarized light and the TM polarized light are exchanged.
[0032]
Therefore, for light in a specific deflection direction, the periodic structure in the first direction
Δn = n (TE) −n (TM) (10)
If this light is incident on the periodic structure in the second direction,
−n (TE) + n (TM) = − Δn (11)
Becomes Thus, in practice, λ₁Δnd with respect to is expressed by Expression (8)-(9). Therefore,
[Outside 7]

Becomes
Similarly, λ₂Δn′d is
[Outside 8]

Becomes
[0033]
Substituting equations (12) and (13) into equation (7),
[Outside 9]

Get.
[0034]
Next, various embodiments of the present invention will be described based on detailed examples.
[0035]
【Example】
[Example 1]
FIG. 1 shows a schematic diagram of the first embodiment. The optical element of this embodiment is a phase plate (λ / 4 plate) using a fine periodic structure. FIG. 2 is a cross-sectional view of the phase plate of FIG. 1 as viewed from the lateral direction. In FIG. 2, 5 indicates a substrate, and 15 indicates a one-dimensional lattice which is a fine periodic structure. The one-dimensional grating 15 has two periodic structures having a shorter period than the wavelength of the used light. The incident side medium 16 is air. As the one-dimensional grating 15, two periodic structures made of materials having different refractive indices and dispersions are used. Here, as an example, the substrate is Ta.₂O₅The first periodic structure 6 (hereinafter, referred to as a first grating 6) from the substrate side is Ta.₂O₅(Refractive index n1 = 2.139), the second periodic structure 7 (hereinafter referred to as the second grating 7)₂(Refractive index n2 = 1.8). The period Λ of the first and second gratings is 0.16 μm, the thickness d1 of the first grating 6 is 0.39 μm, the thickness d2 of the second grating 7 is 0.10 μm, and the filling factor FF (= w / Λ) is 0. .85. Such a structure can be manufactured by, for example, a technique such as etching. Here, the diffraction period is set such that no higher-order diffracted light other than the 0th order is generated in the visible light region, that is, the grating structure behaves as a 0th order grating.
[0036]
The lattice period condition under which the lattice behaves as a zero-order lattice is described in the literature (EB Grannetal, J. Opt. Soc. Am. AVol. 13, No. 5, p988-992, 1996), It is given by equation (15).
[0037]
(Λmax) = (λmin) / (ns + ni | sin θi |) (15)
Here, Λmax is the maximum value of the grating period acting as a zero-order grating, λmin is the minimum value of the incident wavelength, ns is the refractive index of one grating material, ni is the refractive index of the other grating material, and θi is the incident angle. . Incident wavelength λmin = 0.40 μm, one of the lattice materials is Ta₂O₅(Ns = 2.139), when the other grating material is air (ni = 1.000) and the incident angle θi = 0, Λmax is about 0.187 μm, and the grating periods Λ1 and Λ2 are 0.16 μm. It can be seen that sometimes it behaves as a zero-order lattice. In addition, strict transmission / reflectance calculations using strict wave coupling theory, which is a vector analysis, have confirmed that high-order diffracted light is not generated.
[0038]
When a grating period that generates a higher-order diffracted light other than the 0th order is employed, not only the diffraction efficiency of the 0th-order light is reduced and the light use efficiency of the optical system is reduced, but also the higher-order diffracted light is ghosted. It is expected that this will appear as a flare or a flare, and will significantly degrade the optical performance. For this reason, a grating period that behaves as a zero-order grating is desirable. In designing the phase plate, a refractive index difference Δn was calculated using n (TE) and n (TM) calculated by the effective refractive index method (EMT), and a design initial value was estimated. Here, since two gratings are used, assuming that the refractive index differences with respect to the first grating 6 and the second grating 7 are Δn1 and Δn2, and the grating thicknesses are d1 and d2, respectively, the following equation (16) is used.
Δn1 × d1 + Δn2 × d2 = λ / 4 (16)
Was adopted in the visible light region. In this embodiment, Δn1 = 0.284 and Δn2 = 0.147. Finally, the reflection / transmittance and the phase difference were strictly calculated by vector analysis (strict coupled wave analysis).
[0039]
FIG. 3 shows the phase difference characteristics of the λ / 4 plate of this embodiment. In FIG. 3, the horizontal axis represents wavelength and the vertical axis represents phase difference. Compared to a conventional wavelength plate using quartz, the slope of the straight line representing the phase difference is smaller, and the characteristic is flat in the short wavelength region of 0.50 μm or less. Has been improved. FIG. 4 shows the transmittance characteristics of the phase plate of this embodiment. The horizontal axis represents wavelength, and the vertical axis represents transmittance. The phase plate using a quartz plate has a transmittance of about 95% in the visible light region, whereas the phase plate of the present embodiment has a transmittance of 97% or more in almost the entire visible light region, and is excellent. Transmission characteristics. Furthermore, since the phase difference can be controlled by using the refractive index of the grating material, the grating period, the grating thickness, and the filling factor as variables, it is possible to design phase plates having various characteristics.
[0040]
In this embodiment, the material of the lattice is Ta.₂O₅And SiO₂Was used, but the lattice material is not limited to this, and there is no problem if two or more materials having different refractive indices and dispersions are used. Further, the material of the grating (the second grating 7 in the first embodiment) closest to the air as the incident side medium has a lower refractive index than the material of the other gratings (the first grating 6 in the first embodiment). is there. Also, any combination of materials having a refractive index (combination of a high-refractive-index material and a low-refractive-index material) may be used as the material of the grating, as long as conditions satisfying basic functions and performance are satisfied. Absent. Here, in order to realize a large structural birefringence, it is necessary to increase the refractive index difference, and the largest structural birefringence is obtained when air is used as the low refractive index material.
[0041]
Further, like the first grating 6 and the second grating 7 in the first embodiment, the entire grating is made of a plurality of materials. In the first embodiment, the first grating 6 and the second grating 7 are provided by using two kinds of materials. However, the first grating, the second grating, the third grating,. A fine periodic structure having two or more lattices may be formed.
[0042]
[Example 2]
Next, a second embodiment will be described.
[0043]
FIG. 5 shows a schematic diagram of the second embodiment, and FIG. 6 shows a cross-sectional view of the yz plane in FIG. 5 and FIG.
[0044]
It should be noted that FIGS. 5, 6, and 7 are schematic diagrams in which the lattice shape is enlarged in the horizontal direction, and may be different from the actual shape. Example 2 shown in FIG. 5 is a phase plate (λ / 4 plate) having a structure including three gratings. The second embodiment is characterized in that the first grating 6 is orthogonal to the second grating 7 and the third grating 8 in the period direction of the grating. Hereinafter, the phase plate having such a configuration is referred to as a laminated phase plate. In this case, a laminated λ / 4 plate is used. In this embodiment, the substrate 5 is made of TiO.₂The incident side medium 16 was air. The first grating 6 closest to the substrate is made of TiO.₂The lattice period Λ1 is 0.15 μm, the lattice depth d1 is 1.85 μm, the filling factor FF1 (= w1 / Λ1) is 0.90, and the second lattice 7 in the second stage is made of TiO 2.₂The third grid 8 in the third stage is made of SiO 2₂The second grating 7 and the third grating 8 have a grating period Λ2 of 0.15 μm, a filling factor FF2 (= w2 / Λ2) of 0.82, a grating depth d2 of the second grating 7 of 1.75 μm, The grating depth d3 of the third grating 8 was set to 0.10 μm. The grating periods Λ1 and と し た 2 are such that they behave as zero-order gratings in the visible light region.
[0045]
Here, the effective refractive index in each grating was calculated using EMT as a design technique.
First, the refractive index difference Δn1 between ordinary light and extraordinary light in the first grating 6 of the first stage is expressed by the following equation (17).
Δn1 = n1 (TE) −n1 (TM) (17)
Given by
[0046]
On the other hand, the refractive index difference Δn2 given by TiO2 in the second grating 7 and the refractive index difference Δn3 given by SiO2 in the third grating 8 are similarly expressed by the following equations (18) and (19).
Δn2 = n2 (TE) −n2 (TM) (18)
Δn3 = n3 (TE) −n3 (TM) (19)
Given by However, it should be noted that since the periodic direction of the first grating and the grating structure of the second and third gratings are orthogonal to each other, the TE polarization in the grating structure of the first grating 6 is the same as that of the second and third gratings. The structures 7 and 8 become TM polarized light, and the TM polarized light in the lattice structure of the first lattice 6 becomes TE polarized light in the lattice structures 7 and 8 of the second and third lattices. Such a difference in how to give a refractive index corresponds to a difference between a positive crystal and a negative crystal in an anisotropic crystal.
Therefore, the following equation (20)
Δn1 × d1-Δn2 × d2-Δn3 × d3 = 1 / 4λ (20)
Is calculated using equation (5) and adopted as a solution that satisfies the two wavelengths in the visible light region. Finally, the reflection / transmittance and the phase difference were strictly calculated by strictly coupled wave analysis which is a vector analysis. .
[0047]
FIG. 8 shows a phase difference characteristic of the laminated λ / 4 plate in the second embodiment. The horizontal axis represents wavelength, and the vertical axis represents phase difference. Compared with a conventional λ / 4 plate using a quartz crystal plate, the laminated λ / 4 plate has a wider wavelength region where the phase difference is approximately 90 °, and the wavelength dependency is improved. FIG. 9 shows the transmittance characteristics. The horizontal axis indicates the wavelength, and the vertical axis indicates the transmittance. Excellent transmittance characteristics are realized with a high transmittance of 96% or more in almost the entire visible light region. Furthermore, since the phase difference can be controlled by using the refractive index of the grating material, the grating period, the grating thickness, and the filling factor as variables, it is possible to design phase plates having various characteristics.
[0048]
In Example 2, TiO 2 was used as the lattice material.₂And SiO₂However, there is no problem if two or more types of lattice materials having different refractive indices and dispersions are used. In this embodiment, both the first lattice 6 and the second lattice 7 are made of TiO 2.₂However, the present invention is not limited to this, and the first grid 6 and the second grid 7 are made of TiO 2 as a grid material.₂Other grating materials may be used, and materials having different refractive indices may be used as the grating materials of the first grating 6 and the second grating 7. In addition, the lattice material of the lattice closest to the air 16 as the incident side medium (the third lattice 8 in the second embodiment) is different from other lattices (the first lattice and the second lattice in the second embodiment). A material having a lower refractive index was used. Any combination of materials having a different refractive index (combination of a material having a high refractive index and a material having a low refractive index) may be used as a material of the grating as long as conditions satisfying basic functions and performances are satisfied.
[0049]
Here, a modified example of the second embodiment is shown in FIG. FIG. 10 shows an embodiment of a phase plate in which a first grating 6, a second grating 7, a third grating 8, and a fourth grating 9 are provided on a substrate 5 from the substrate side. Thus, a phase plate may be formed by laminating four gratings on a substrate. Here, the substrate and the first to third lattices are made of TiO.₂, The fourth lattice is SiO₂Created in. However, the first to third gratings may be made of different materials. The first to third gratings may have the same filling factor or different filling factors.
[0050]
[Example 3]
As a third embodiment, a λ / 2 plate used in a polarization conversion element will be described as an example. In a polarization conversion element mounted on a liquid crystal projector or the like, unpolarized white light from a light source is converted into two linear changes in which the polarization directions are different from each other by 90 ° using a polarization beam splitter. At this time, in order to improve the light use efficiency, one polarization direction is rotated by 90 °, the other polarization is aligned with the polarization direction, and both are emitted in the same direction. A λ / 2 plate is used to rotate the polarization direction by 90 °. In a conventional λ / 2 plate, the refractive index difference has wavelength dependence.
[0051]
FIG. 11 shows an example in which the λ / 2 plate in this embodiment is used for a polarization conversion element. The unpolarized white light from the light source that has entered the polarization beam splitter 12 is separated into p-polarized light transmitted through the polarization beam splitter and s-polarized light reflected by the polarization beam splitter. Of the two separated polarized lights, the p-polarized light passes through the λ / 2 plate and becomes the s-polarized light. Then, the light is reflected by the polarization beam splitter and emitted in the same direction as the s-polarized light reflected by the mirror 11. Compared with the related art, an optical system having less wavelength dependence of the phase difference and having high light use efficiency even at wavelengths other than the design wavelength can be realized. For this reason, it is possible to realize a liquid crystal projector with higher performance than before by improving the brightness and the image quality. Moreover, TiO is used as a material.₂The use of such a dielectric material has an advantage that the volume expansion and the change in the refractive index due to heat can be suppressed to a small level, and stable performance can be exhibited. In addition, the element can be made thinner due to the large amount of birefringence of the structural birefringence. Furthermore, since the phase difference can be controlled by using the refractive index of the grating material, the grating period, the grating thickness, and the filling factor as variables, it is possible to design phase plates with various characteristics, and to optimize the performance for the intended use. Can be realized.
[0052]
Further, the application example of the phase plate in the present embodiment is not limited to the polarization conversion element. For example, a liquid crystal projector (image display device) having the polarization conversion element of Example 3, a liquid crystal projector, The present invention may be applied to an image display system having an image transmitting means (television, personal computer, digital camera, or the like) for transmission. Of course, the present invention is not limited to this, and can be applied to various optical apparatuses and devices having a λ / 2 plate, a λ / 4 plate, and the like.
[0053]
Further, in the present embodiment, a structure in which a lattice material and air are alternately arranged in the periodic direction is used as the lattice structure (fine periodic structure). The lattice may be configured by alternately arranging second lattice materials other than air in the periodic direction.
[0054]
[Example 4]
Next, a fourth embodiment will be described.
[0055]
FIG. 12 is a perspective view showing the configuration of the laminated phase plate of the fourth embodiment, FIG. 13 is a cross-sectional view of the xz plane, and FIG. 14 is a cross-sectional view of the yz plane. In the figure, reference numeral 1 denotes a substrate, and 2 denotes a first lattice arranged on the substrate 1 in a one-dimensional manner at a constant period. Reference numeral 3 denotes a second grating 3 arranged at a constant period in a direction orthogonal to the period direction of the first grating 2. The second grating 3 is formed by laminating two kinds of materials having different refractive indices of the grating 3a and the grating 3b (in the present embodiment, the two gratings 3a and 3b are collectively treated as one one-dimensional grating). , The second grating). The two one-dimensional gratings of the first and second gratings 2 and 3 each have a structure in which high-refractive-index materials (lattice portions) and low-refractive-index materials (air) are alternately arranged. The period of the first and second gratings 2 and 3 is smaller than the incident wavelength (here, visible light). Although the air on the incident side is air here, a medium such as glass may be used as the incident side medium.
[0056]
The width of the first grating 2 is W1, the depth (thickness) is d1, and the period of the grating is Λ1. The width of the second grating 3 is W2, and the grating period is Λ2. Of the second grating 3, the grating 3a has a depth (thickness) d3, and the grating 3b has a depth (thickness) d2.
[0057]
Here, the present embodiment shows a λ / 4 plate having a structure in which one-dimensional gratings are stacked in two stages, and is characterized in that the periodic directions of the first grating 2 and the second grating 3 are orthogonal to each other. is there. Hereinafter, in this embodiment, this configuration is referred to as a two-stage laminated λ / 4 plate. As an example of the two-stage laminated λ / 4 plate, two kinds of materials constituting the second grating 3 are made of SiO 2 as the grating 3 a.₂, Ta as the lattice 3b₂O₅And using Ta as a lattice material constituting the first lattice 2₂O₅Is used. The grating periods Λ1 and Λ2 are 0.16 μm, and Λ1 and Λ2 are conditions under which no diffracted light of a higher order other than the 0th order is generated in the visible light region, that is, a grating period that acts as a 0th order grating.
[0058]
In the present embodiment, since the first grating and the second gratings 2 and 3 are perpendicular to each other in the periodic direction, the difference in refractive index between the first grating 2 and the second grating 3 is defined as Δn1 and Δn2.
[0059]
Therefore, for a λ / 4 plate,
Δn1 × d1-Δn2 × d2-Δn3 × d3 = λ / 4 (21)
Is satisfied at two wavelengths in the visible light region. Here, there are countless solutions satisfying the expression (21) at two wavelengths, but from the viewpoint of device fabrication, it is desirable that the filling factor FF and the grating depth have appropriate values. . In general, when the filling factor FF is extremely small and the grating depth is deep, the grating structure is likely to collapse and manufacturing is difficult. Therefore, it is desirable that the FF take a value in the range of about 0.20 to 0.95.
[0060]
In addition, in this embodiment, the range that the filling factor FF1 of the first lattice 2 can take is approximately 0.75 to 0.90, and the range that the filling factor FF2 of the second lattice 3 can take is approximately. It is desirable that it is 0.30 or more and 0.70 or less. Taking a specific value as an example, in order to satisfy the phase difference １ ／ λ, when FF1 = 0.81 and FF2 = 0.60, the depth of the lattice material is d1 = 1.07 μm and d2 = A thickness of about 0.31 μm and d3 = 0.10 μm is required. However, a phase difference of ４λ cannot necessarily be realized unless the value matches this value, and a value larger or smaller than the value shown here may be used as long as the range satisfies basic performance. .
[0061]
Next, an example for satisfying the phase difference condition shown in Expression (21) and reducing the reflectance of the element will be described. The purpose here is to realize a phase plate that satisfies the phase difference of λλ and that can reduce the reflectance in each polarization direction of the used light. The lattice structure is a two-stage laminated λ / 4 plate, and the lattice material is SiO₂, Ta₂O₅Is used, the lattice period is set to Λ1 = Λ2 = 0.16 μm, and the filling factors are FF1 and FF2 and the lattice depths are d1, d2 and d3.
[0062]
The two-stage laminated λ / 4 plate of this embodiment has a refractive index distribution such that the effective refractive index gradually increases from one side of the incident medium toward the substrate 1 for one polarization direction. With respect to the other polarization direction, it is characterized by realizing a refractive index such that the refractive index difference between the first grating and the second grating in which two layers are stacked is small.
[0063]
In this case, for example, FF1 = 0.81 and FF2 = 0.60. There are countless combinations of filling factors, but it is desirable to keep in mind that it is easier to fabricate if two lattice depths are selected so as not to become extremely large. The grating depth can be calculated using the effective refractive index calculated using FF1 and FF2 and the equations (4) to (7). In the present embodiment, as a configuration in which the lattice depth does not become extremely deep, the range that FF1 can take is approximately 0.75 to 0.90, and the range that FF2 can take is 0.30 to 0.70. Desirably.
[0064]
FIG. 15 shows the effective refractive index for each polarized light at this time. However, considering that the TE polarized light and the TM polarized light are switched in the orthogonal grating, in order to avoid confusion, here, the direction orthogonal to the period of the first grating 2 is the direction A, and the direction parallel to the period of the first grating 2. Is the B direction. The effective refractive index of polarized light in the A direction of each grating is defined as n (A), and the effective refractive index of polarized light in the B direction is defined as n (B). In other words, n (A) = n (TE) in the first grating 2, but n (A) = n (TM) in the second grating 3 (see FIG. 12). Denotes an incident side medium (here, air), n1 denotes a refractive index of the first grating 2, n2 denotes a grating 3b of the second grating 3, n3 denotes a grating 3a of the second grating 3, and ns denotes a refractive index of the substrate. .
[0065]
First, considering n (A), the effective refractive index (λ0 = d-line) calculated from EMT will be described in detail.
ni (A) = 1 (air),
n3 (A) = 1.230,
n2 (A) = 1.436,
n1 (A) = 1.993,
ns = 2.139,
The refractive index distribution is such that the refractive index gradually increases from the incident side toward the substrate. By taking such a refractive index distribution, Fresnel reflection on the substrate surface is reduced, and an antireflection effect can be obtained in a wide band. This is the same as the anti-reflection effect obtained when, for example, a gradient film is used to realize a refractive index distribution in which the refractive index changes gradually.
[0066]
Next, considering n (B), the effective refractive index calculated from EMT is
ni (B) = 1 (air),
n3 (B) = 1.313,
n2 (B) = 1.823,
n1 (B) = 1.733,
ns = 2.139,
And the configuration is such that the refractive index difference between n1 (B) and n2 (B) (that is, the refractive index difference between lattices) is small. Here, the refractive index difference between n1 (B) and n2 (B) is 0.09, which is almost the same. Note that the refractive index difference does not need to be completely zero, and it is preferable that the refractive index difference satisfies a value of about 0.1 or about 0.2 as an approximate standard. As described above, regarding the TM polarized light, the difference in the refractive index between the plurality of stacked gratings is smaller than that of the TE polarized light. In addition, if n1 (B) and n2 (B) having a small difference are considered to have substantially the same refractive index, Fresnel reflection occurring between n1 and n2 can be reduced. In addition, antireflection effect can be obtained by taking the refractive index distribution such that ni (B) <n3 (B) <n2 (B) ≒ n1 (B) <ns (B).
[0067]
Here, if the optical film thickness of n3 is optimally designed, a further antireflection effect can be obtained. Specifically, by setting the thickness d3 of the grating 3a in the second grating 3 to an optical film thickness of about の of the design wavelength, an antireflection effect on TM polarized light can be obtained. However, since the film thickness d3 of n3 also affects the reflectance for TE polarized light, it is actually important that the film thickness be such that the reflectance for both TE polarized light and TM polarized light is compatible. As a guide, a value close to 値 of the design wavelength is desirable.
[0068]
It should be noted that the relationship between the effective refractive indices of TE polarized light and TM polarized light is established even if they are reversed. In other words, the same effect can be obtained even if the effective refractive index difference between the gratings for the TE polarized light is small and the effective refractive index for the TM polarized light is gradually increased from the medium on the incident side toward the substrate.
[0069]
Furthermore, since the effective refractive index can be controlled by the grating period Λ and the filling factor FF of the grating, the effective refractive index can be controlled within a certain range regardless of the refractive index of the grating material. Therefore, it is not always necessary to limit the stacking order of the lattice materials for realizing the above effective refractive index.
[0070]
Also, if the refractive index difference between the refractive index ns of the substrate material and the effective refractive index n1 (TE) (or n1 (TM)) of the grating on the substrate is large, the reflectance increases due to Fresnel reflection occurring at the interface, and ns It is desirable that the difference in refractive index between n1 and n1 (TE) (or n1 (TM)) is small. However, it should be noted here that n1 (TE) and n1 (TM) have the same refractive index in principle because the one-dimensional lattice shape has anisotropic refractive index. impossible. Therefore, the refractive index difference ns-n1 (TM) in the TE direction and the refractive index difference ns-n1 (TM) in the TM direction cannot be made zero simultaneously. Therefore, in order to obtain an anti-reflection effect for both the TE and TM polarization directions, the value of the refractive index difference ns-n1 (TE) for the TE polarized light and the value of the refractive index difference ns-n1 (TM) for the TM direction are both required. It is desirable to set it to be small.
[0071]
Next, the lattice depth will be described. The phase difference needs to have a depth such that the phase difference Δnd satisfies １ ／ λ, but at the same time, it is necessary to set the grating depth so that an antireflection effect can be obtained. In general, the antireflection effect can be achieved by setting the film thickness nd to an integral multiple of 1 / 4λ0 with respect to the wavelength λ0.
[0072]
Based on the design initial value obtained from the phase difference condition, a lattice shape that obtains an antireflection effect independent of the polarization direction obtained by using the above method was obtained, and the following results were obtained. That is, as an example of a two-stage lamination type λ / 4 plate, SiO 2 is used as the material of the lattice 3 a in the second lattice 3.₂And Ta as a material of the lattice 3b.₂O₅, The grating period ０．１2 is set to 0.16 μm, the grating depth d3 is set to 0.10 μm, the grating depth d2 is set to 0.24 μm, and the filling factor FF2 (= w2 / Λ2) is set to 0.60. Further, Ta is used as a lattice material of the first lattice 2.₂O₅And the grating period Λ1 is set to 0.16 μm, the grating depth d1 is set to 0.96 μm, and the filling factor FF1 (= w1 / Λ1) is set to 0.81. The grating periods Λ1 and Λ2 are conditions under which no higher-order diffracted light other than the 0th order is generated in the visible light region, that is, a grating period that acts as a 0th-order grating.
[0073]
FIG. 16 shows the phase difference characteristic of the two-stage laminated λ / 4 plate in the fourth embodiment. FIG. 16 also shows the phase difference characteristics of quartz. The horizontal axis represents the wavelength, and the vertical axis represents the phase difference. Compared with a λ / 4 plate using a conventional quartz crystal plate, the wavelength range where the phase difference is approximately 90 ° is broadened in the two-stage laminated λ / 4 plate. Also, it should be noted that the error range is suppressed to about −5 ° to + 10 ° with respect to 90 °, and the wavelength dependency is improved.
[0074]
17 and 18 show the transmittance characteristics. Here, the reflection / transmittance and the phase difference are strictly calculated by a strictly coupled wave analysis which is a vector analysis. FIG. 17 shows transmittance characteristics for TE polarized light and TM polarized light. The horizontal axis indicates the wavelength and the vertical axis indicates the transmittance, which indicates that excellent transmittance characteristics are obtained for both TE and TM polarized light. FIG. 18 shows the average value of the transmittance for each polarization direction. A high transmittance of about 97% or more is obtained in almost the entire visible light region, and excellent performance is realized.
[0075]
Here, a λ / 4 plate is taken as an example, but the present invention is not limited to this, and the phase difference is controlled by using the refractive index of the lattice material, the lattice period, the lattice depth, and the filling factor as variables. And a reflection preventing effect can be obtained, so that the present invention can be applied to, for example, a phase plate having various characteristics such as a λ / 2 plate.
[0076]
In the present embodiment, two kinds of lattice materials of the second lattice 3 are Ta.₂O₅And SiO₂However, the present invention is not limited to this, and there is no problem if two or more types of lattice materials having different refractive indices and dispersions are used. Furthermore, in the present embodiment, both the first grating 2 and the second grating 3 are made of Ta as a material of the grating 3b.₂O₅, But is not limited to this.₂O₅Other materials may be used, and materials having different refractive indices may be used as materials for the first grating and the second grating 3b. Also, any combination of materials having any refractive index may be used as the materials of the first and second gratings 3a and 3b as long as the conditions for having the basic function and performance are satisfied. . In addition, it is preferable that the entire second grating is formed of a plurality of materials. In the present embodiment, the second grating 3 is formed using two kinds of materials, but three or more kinds of materials may be used. .
[0077]
[Example 5]
Next, a fifth embodiment will be described. 19 is a perspective view showing the configuration of the fifth embodiment, FIG. 20 is a cross-sectional view of the same in the xz plane, and FIG. 21 is a cross-sectional view of the same in the yz plane. In the drawing, reference numeral 1 denotes a substrate, 2 denotes a first grating arranged on the substrate 1 in a one-dimensional manner at a constant period, and 3 denotes a second grating arranged at a constant period in a direction orthogonal to the period direction of the first grating 2. It is. Further, the third gratings 4 are arranged on the second grating 3 at regular intervals in a direction orthogonal to the periodic direction of the second grating 3. The third grating 4 is formed by laminating two kinds of materials having different refractive indexes, that is, a grating 4a and a grating 4b (in the present embodiment, the two gratings 4a and 4b are combined into one one-dimensional grating). Treat it as the third lattice). The one-dimensional lattice structure of the first to third lattices has a configuration in which high-refractive-index materials (lattice portions) and low-refractive-index materials (air) are alternately arranged in the periodic direction of each periodic structure. Further, the grating periods of the first to third gratings 2 to 4 are smaller than the incident wavelength (visible light) as in the fourth embodiment. As the incident side medium, for example, air is used, but a cover made of glass or the like may be used as the incident side medium.
[0078]
The width of the first grating 2 is W1, the depth (thickness) is d1, and the grating period is Λ1. The width of the second grating 3 is W2, the depth (thickness) is d2, and the grating period is Λ2. The width of the grating of the third grating 4 is W3, and the grating period is Λ3. The depth (thickness) of the grating 4a is d4, and the depth (thickness) of the grating 4b is d3.
[0079]
Example 5 is a λ / 4 plate having a structure in which three-dimensional one-dimensional gratings are stacked. The first grating 2 and the second grating 3, and the second grating 3 and the third grating 4 each have a periodic direction orthogonal to each other. The feature is that Ta as a material of the first lattice 2₂O₅, The grating period Λ1 is 0.16 μm, the thickness d1 is 0.565 μm, and the filling factor FF1 (= W1 / Λ1) is 0.81. Ta as a material of the second grating 3₂O₅And the grating period Λ2 is set to 0.16 μm, the thickness d2 is set to 0.190 μm, and the filling factor FF2 (= W2 / Λ2) is set to 0.6. SiO 4 is used as the material of the lattice 4 a in the third lattice 4.₂Is used as the material of the lattice 4b.₂O₅The grating period Λ3 is 0.16 μm, the thickness d4 of the grating 4a is 0.100 μm, the thickness d3 of the grating 4b is 0.250 μm, and the filling factor FF3 (= W3 / Λ3) is 0.81. The grating periods Λ1, Λ2, and Λ3 are conditions under which no higher-order diffracted light other than the 0th order is generated in the visible light region, that is, a grating period that acts as a 0th-order grating.
[0080]
The effective refractive indices of the first to third gratings are the same as in the fourth embodiment. That is, with respect to the polarization in the x-axis direction, the effective refractive index becomes higher as it goes from the incident side medium side to the gratings 4a, 4b, the second grating 3, and the first grating 2 of the third grating 4 and the substrate. . Further, with respect to the polarization in the y-axis direction, the difference between the effective refractive indices between a plurality of stacked gratings, that is, the effective refractive index between the grating 4b and the second grating 3 of the third grating 4 and between the gratings 2 and 3 becomes smaller. I have. The difference in the refractive index does not need to be 0 as described above, but may be about 0.1 or 0.2. Of course, the relationship between the effective refractive indices of the TE polarized light and the TM polarized light may be the opposite relationship as in the case of the first embodiment.
[0081]
FIG. 22 shows the phase difference characteristic of the three-stage laminated λ / 4 plate of Example 5, and FIG. 23 shows the transmittance characteristic (polarization average). FIG. 22 also shows the characteristics in the case of quartz. It should be noted from FIG. 22 that the wavelength dependence of the phase difference is small and the error range from 90 ° is suppressed to about −10 ° to + 10 °. Further, as can be seen from FIG. 23, excellent transmittance characteristics can be realized, and a transmittance of approximately 95% or more in the visible region is realized. In the case of the three-layer structure, it is possible to further reduce the thickness of the lattice material as compared with the two-layer structure, and it can be expected that it is easy to fabricate each lattice structure.
[0082]
[Example 6]
Next, a sixth embodiment will be described. Example 6 is an embodiment of an optical system using the phase plate of Example 4. FIG. 24 is a diagram showing the sixth embodiment, and shows an optical system when the phase plate of the fourth embodiment is used in a double pass. In FIG. 24, reference numeral 19 denotes a phase plate (λ / 4 plate) of the fourth embodiment, and reference numeral 20 denotes a mirror. Reference numeral 17 denotes used light, and 18 denotes emitted light (light to be observed). Here, the used light (linearly polarized light) 17 having a uniform polarization direction is incident on the laminated λ / 4 plate 19, is converted into circularly polarized light, is reflected by the mirror 20, and is again transmitted to the laminated λ / 4 plate 19. The polarization direction of the incident and emitted light 18 is rotated by 90 ° with respect to the working light 17.
[0083]
Here, the transmittance of light when the polarization direction is rotated can be calculated by the following equation (22).
[0084]
T = cos²(Γ / 2-π / 2) (22)
T is the transmittance in consideration of the polarization rotation, and Γ is the phase difference. FIG. 25 shows a graph in which the horizontal axis represents the phase difference Γ and the vertical axis represents the transmittance T for this equation. Supplementing the transmittance Τ, if the light intensity of the linearly polarized light 17 incident on the double-pass optical system is set to 1, the linearly-polarized light incident on the double-pass optical system has a polarization direction of 90 degrees. ° Indicates the ratio of light intensity observed through orthogonal polarizers. Specifically, when the phase difference between the used light 17 and the output light 18 of the double-pass optical system is 180 °, the observed polarization direction of the output light 18 is exactly 90 degrees with respect to the polarization direction of the incident linearly polarized light 17. Since the polarization is rotated by °, the ratio of the observed light intensity to the used light is 1/1 = 1.
[0085]
It should be noted that the transmittance calculated by the equation (22) does not take into account the surface reflection of the phase plate element or the reflected light due to multiple reflection. Therefore, in order to study the substantial transmittance, it is necessary to consider simultaneously the transmittance taking into account the surface reflection and multiple reflection of the phase plate element and the transmittance taking into account the polarization rotation obtained from the equation (22). is there. From the above, the substantial transmittance in the optical system using the double pass is obtained by multiplying the transmittance of the phase plate by the transmittance calculated from Expression (22) and taking into account the transmitted light intensity loss due to the rotation of the polarized light. Can be evaluated.
[0086]
FIG. 26 shows a phase difference characteristic in the double-pass optical system of the sixth embodiment. FIG. 26 also shows the phase difference characteristics of quartz. It can be seen that the wavelength dependency is suppressed to be smaller than that of the conventional quartz plate. FIG. 27 shows the substantial transmittance calculated by the above method. FIG. 27 also shows the substantial transmitted light intensity in a λ / 4 plate using a conventional crystal for comparison. Fresnel reflection in a conventional quartz λ / 4 plate is calculated using a value calculated using the refractive index (n = 1.55) of quartz as a transmittance of 95%.
[0087]
The thickness of the quartz crystal is set to a value that can realize a phase difference of 90 ° at the design wavelength λ0 = 0.55 μm, and the thickness d is set to 15.3 μm. When a conventional quartz λ / 4 plate is used, the wavelength dependence of the phase difference is greatly affected, and the substantial transmittance near the design wavelength has excellent characteristics. Also, it can be seen that the substantial transmittance decreases on the long wavelength side. This is because, as shown in FIG. 26, the wavelength dependence of the phase difference is a major problem in quartz.
[0088]
On the other hand, in the case of the double-pass optical system using the two-stage laminated phase plate of the sixth embodiment, the wavelength dependence of the phase difference can be significantly improved, and the reflectance in each polarization direction is reduced. In the entire visible region (approximately in the range of 0.4 μm to 0.7 μm), it is possible to realize an excellent transmittance of approximately 96% or more.
[0089]
An optical system using such a double pass can be used, for example, in a reflection type color liquid crystal display device. Compared with the case where the conventional λ / 4 plate is used, when the laminated phase plate of the present invention is used, the reflectance of the phase plate itself can be suppressed low, and not only the light loss is small but also In addition, since the wavelength dependency is reduced as compared with the conventional phase plate, the color reproducibility is very excellent, and an improvement in image quality can be expected. Further, by using a dielectric material as the lattice material, it is possible to suppress a volume expansion and a change in refractive index due to heat to be small, and it is possible to exhibit stable performance. In addition, the thinness of the element can be expected due to the large birefringence of the structural birefringence. In addition, the phase plate can be formed by using a metal such as Al as the substrate 1 of the phase plate, or by forming the laminated phase plate of the present embodiment on a broadband reflection film formed by using a dielectric multilayer film. By integrating the mirror 19 and the mirror 20, the elements can be integrated and the thickness can be reduced.
[0090]
[Example 7]
Next, a laminated phase plate of Example 7 will be described. The laminated phase plate of the present embodiment is characterized in that a plurality of lattice-type element portions each composed of a substrate and a lattice are laminated so that their lattice surfaces face each other.
[0091]
28 to 30 show a schematic configuration of the phase plate of the seventh embodiment. FIG. 28A is a perspective view of the phase plate, and FIG. 28B shows a manufacturing process of the phase plate. FIG. 29 is a cross-sectional view of the phase plate on the xz plane, and FIG. 30 is a cross-sectional view of the yz plane.
[0092]
This phase plate has a first substrate 21, a first grating 22 having a smaller grating period than the wavelength of the incident light, and a grating period smaller than the wavelength of the incident light, and the periodic direction is the second direction. It has a configuration in which a second grating 24 substantially orthogonal to the periodic direction of one grating 22 and a second substrate 23 are stacked in this order.
[0093]
As a specific manufacturing method, for example, gratings 22 and 24 having heights (depths) d1 and d2 are respectively formed on the first and second substrates 21 and 23 by etching, electron beam drawing, LIGA process, and photolithography. It is manufactured by a technique such as lithography, multi-beam laser interferometry, or multi-layer thin film.
[0094]
Then, the completed first lattice type element portion A composed of the first substrate 21 and the first lattice 22 and the second lattice type element portion B composed of the second substrate 23 and the second lattice 24 are shown in FIG. As shown in FIG. 28 (B), the layers are stacked so that the gratings 22 and 24 face each other and the directions of the periods of the gratings 22 and 24 are substantially orthogonal to each other. At this time, the lattice surfaces of both lattices can be brought into close contact with each other by an optical contact method (optical contact) or the like and can be laminated.
[0095]
By employing such a manufacturing method, a laminated phase plate can be easily manufactured as compared with a case where only the grating is stacked.
[0096]
The material of the first substrate 21 is glass (refractive index ns1 = 1.8), and the material of the first lattice 22 is Ta.₂O₅As a material of the second substrate 23, glass (refractive index ns2 = 1.6), and as a material of the second grating 24, Ta.₂O₅(Refractive index n = 2.139). The medium filling the periphery of the grid was air.
[0097]
The first grating 22 has a grating period Λ1 = 0.16 μm, a grating depth d1 = 0.90 μm, and a filling factor FF1 = 0.8. The second grating 24 has a grating period Λ2 = 0.16 μm and a grating depth d2 = 0.25 μm and filling factor FF2 = 0.6. Regarding the grating periods # 1 and # 2, a grating period that does not generate high-order diffracted light other than the 0th-order diffracted light in the visible light region is employed.
[0098]
Here, in FIG. 28A, the y-axis direction is the A direction, the x-axis direction is the B direction, and the effective refractive indices for polarized light in each direction are n (A) and n (B).
[0099]
When the effective refractive indices n1 (A) and n1 (B) for the first grating and the effective refractive indices n2 (A) and n2 (B) for the second grating are estimated by EMT, the following values are obtained. At a wavelength λ = 0.55 μm,
Refractive index n1 (A) of first grating 2 for TE polarized light = 1.965
Refractive index n1 (B) for TM polarized light of first grating 2 = 1.633
Refractive index n2 (A) for TE polarized light of second grating 4 = 1.372
Refractive index n2 (B) for TM polarized light of second grating 4 = 1.774.
Becomes
[0100]
The reason why the above design values are adopted will be described. First, when focusing on the polarization in the x direction, the refractive index n1 (B) of the first grating 22 for the polarization in the x direction and the refractive index n2 (B) of the second grating 24 for the polarization in the x direction are respectively:
n1 (B) = 1.633, n2 (B) = 1.774
And the effective refractive indices are substantially equal (the difference is 0.2 or less). This makes it possible to cause almost no Fresnel reflection at the boundary between the first grating 22 and the second grating 24 with respect to the polarized light in the x direction.
[0101]
Next, when focusing on the polarization in the y direction, the refractive index n1 (A) of the first grating 22 for the polarization in the y direction and the refractive index n2 (A) of the second grating 24 for the polarization in the y direction are respectively:
n1 (A) = 1.965, n2 (A) = 1.372
And there is a slight difference in the effective refractive index.
[0102]
In this state, Fresnel reflection occurs between the first grating 2 and the second grating 4 for the polarized light in the y direction. Furthermore, in consideration of the refractive index difference between the first substrate 21 and the first grating 22 and the Fresnel reflection caused by the refractive index difference between the second substrate 23 and the second grating 24, there is a concern that the transmittance may decrease.
[0103]
In consideration of the above, in the first substrate 21 and the first grating 22 that are stacked adjacent to each other, the refractive index ns1 of the first substrate 21 is the effective refraction of the first grating 22 with respect to the polarization in the y direction. The refractive index difference may be set to be small in a well-balanced manner with respect to both the index n1 (A) and the effective refractive index n1 (B) for the polarized light in the x direction in the first grating 22.
[0104]
Specifically, the refractive index ns1 of the first substrate 21 is equal to the effective refractive index n1 (A) of the first grating 22 for polarized light in the y direction and the effective refractive index n1 (B) of the first grating 22 for polarized light in the x direction. ) Is preferable.
[0105]
This means that the refractive index ns2 of the second substrate 23, the effective refractive index n2 (A) of the second grating 24 for polarized light in the y direction, and the effective refractive index n2 ( The same applies to the relationship with B).
[0106]
Here, the refractive index of the first substrate 21 is set to ns1 = 1.80, the refractive index of the second substrate 23 is set to ns2 = 1.60, and the Fresnel at the boundary between the first substrate 21 and the first grating 22 is set. Both the reflection and the Fresnel reflection at the boundary between the second substrate 23 and the second grating 24 are simultaneously reduced to minimize the decrease in transmittance.
[0107]
However, the present invention is not limited to the configuration showing each of the above values. For example, in a case where only one of the polarized light in the y direction and the polarized light in the x direction is used, the refractive index of the substrate in the adjacent substrate and the grating is set to the effective refractive index for the polarized light in the y direction in the grating. It may be substantially equal to one of the index and the effective refractive index for the polarized light in the x direction. Thereby, Fresnel reflection of the polarized light to be transmitted can be reduced.
[0108]
In the two sets of adjacent substrates and gratings, only the relationship between the refractive indices of the one set of substrates and gratings may be as described above. This makes it possible to adopt a configuration in which only Fresnel reflection at the boundary between one set of substrates and the grating is reduced. These may be appropriately selected depending on the application example.
[0109]
By setting as in the present embodiment, Fresnel reflection at the boundary between both substrates and the grating is reduced at the same time, whereby the decrease in transmittance of the entire phase plate is suppressed most.
[0110]
FIG. 31 shows the transmittance characteristics of the phase plate of this example at an incident angle of 0 °. As shown in FIG. 31, a transmittance of about 97% or more is realized in the entire visible light region from 0.40 μm to 0.70 μm.
[0111]
FIG. 32 shows the phase difference characteristics. Compared with the conventional phase plate made of a thin quartz plate, the wavelength dependency is remarkably improved, and a phase difference in the range of approximately 80 ° to 95 ° is realized in the entire visible light region from 0.40 μm to 0.70 μm. Thus, it can function almost as a λ / 4 plate over the entire visible light wavelength region.
[0112]
In the present embodiment, since the lattice surface of the lattice of the laminated phase plate is covered by the substrates 21 and 23, it is possible to prevent the performance from being deteriorated due to the disorder of the lattice caused by hand oil, dust, or dust, and the destruction of the lattice caused by scratching or friction. There is an advantage.
[0113]
In order to prevent Fresnel reflection on the incident surface of the laminated phase plate, a general anti-reflection coating such as a single-layer film or multilayer film is applied to the incident surface, or the substrate surface is finer than the incident wavelength. It is necessary to manufacture an uneven structure (for example, a structure in which fine pyramid structures and conical structures are regularly or randomly arranged). Even in such a case, the antireflection coat or the like may be provided on the surface of the substrate having a certain area instead of being provided on the end face of each fine grating. It is easy to manufacture a plate, and it is easy to obtain an antireflection effect.
[0114]
Further, since the conventional film-type phase plate uses a material such as a polymer, the performance is significantly deteriorated by heat. For this reason, in a high-temperature environment, the expected performance cannot be obtained or cannot be used, and when used, a sufficient cooling system must be devised. Since the laminated phase plate of this embodiment uses a dielectric material such as Ta2O5 as a material, performance degradation due to heat fluctuation hardly occurs, and can function as a high-performance phase plate even in a high-temperature environment. For example, it is very effective in an optical system such as a liquid crystal projector which is always exposed to a high temperature environment.
[0115]
Example 8
Next, an eighth embodiment of the present invention will be described. Example 8 is a double-pass optical system including the multilayer phase plate of Example 7. The configuration is the same as that of the double-pass optical system of the sixth embodiment except that the phase plate 19 is replaced with the laminated phase plate of the seventh embodiment. Therefore, the optical system of this embodiment will be described with reference to FIG. That is, in FIG. 24, reference numeral 19 denotes the laminated phase plate of the seventh embodiment. Other configurations are the same as those of the sixth embodiment.
[0116]
Needless to say, the laminated phase plate 19 of the present embodiment functions as a λ / 4 plate over a wide range in the visible light region.
[0117]
The optical characteristics of the double-pass optical system laminated phase plate 19 at an incident angle of 0 ° will be described.
[0118]
First, the transmittance characteristics of the outgoing light 18 with respect to the use light 17 are shown in FIG. The transmittance characteristics shown here ignore the influence of Fresnel reflection on the substrate surface. As can be seen from FIG. 33, a transmittance of approximately 95% to 99% is realized in the entire visible light wavelength region from 0.40 μm to 0.70 μm.
[0119]
As described above, to reduce Fresnel reflection on the substrate surface, an antireflection coat such as a general single-layer film or a multilayer film is applied to the substrate surface, or the substrate surface is finer than the incident wavelength. This can be realized by manufacturing an uneven structure (for example, a structure in which fine pyramid structures or conical structures are regularly or randomly arranged).
[0120]
FIG. 34 shows the phase difference characteristics of the outgoing light 18 with respect to the working light 17. As can be seen from FIG. 34, a phase difference of approximately 160 ° to 190 ° is realized in the entire visible light wavelength range of 0.40 μm to 0.70 μm, and a phase difference of approximately 180 ° is realized in a wide wavelength band. are doing. This indicates that when light passes through the laminated phase plate 14 twice, a double-pass optical system can be realized in which the laminated phase plate 14 functions almost as a half-wave plate in the entire visible light wavelength region. . In addition, it can be seen that the wavelength dependence of the phase difference is small even when compared with a conventional phase plate using quartz.
[0121]
Next, optical characteristics of the laminated phase plate 19 of the double-pass optical system at an incident angle of 20 ° will be described. FIG. 35 shows the transmittance characteristic of the emitted light 18 with respect to the working light 17, and FIG. 36 shows the phase difference characteristic.
[0122]
The transmittance characteristic has a transmittance of about 95% to 99% in a visible light wavelength region, and realizes a transmittance that is not sensitive to an incident angle. Similarly, looking at the phase difference characteristics, it is also shown that a phase difference of approximately 155 ° to 185 ° is realized in the visible light wavelength region, and a phase difference of approximately 180 ° is obtained in a wide wavelength region. . As described above, in the present embodiment, in the double-pass optical system, an element that has a small incident angle dependency and functions as an almost half-wave plate by passing light twice in the entire visible light wavelength region is realized. ing.
[0123]
However, the angle of incidence shown here means the angle of incidence from the second substrate to the second grating. In the present embodiment, since the refractive index of the second grating is n2 = 1.60, the incident angle of light incident on the second substrate from air is about 33 °.
[0124]
That is, it is shown that in the case where the laminated phase plate is actually used, the same performance can be obtained even for a light beam whose incident angle on the second substrate is about 33 °.
[0125]
As described above, in the present embodiment, the transmittance characteristic and the phase difference characteristic hardly change with respect to the incident angle variation of about 20 °, and the emitted light is compared with the used light 17 in the entire visible light wavelength region. It is possible to realize a phase plate in which the phase difference of 18 is approximately 180 °.
[0126]
In the case of conventional wave plates made of quartz or polymer films, the wavelength dependency and the incident angle dependency of the phase difference appear remarkably, and in order to improve these, the optical axes of multiple phase plates are staggered. Complicated steps such as combining were required. Further, there is a disadvantage that it is difficult to reduce the thickness of the phase plate element itself from the viewpoint of bonding a plurality of films.
[0127]
On the other hand, in the laminated phase plates of Example 8 and Example 7 described above, the fabrication was easily performed by bonding the gratings fabricated on the substrate with the grating surfaces facing each other so that the periodic directions were orthogonal to each other. It is possible. The wavelength dependence and the incident angle dependence of the phase difference can be simultaneously improved.
[0128]
[Example 9]
Next, a ninth embodiment of the present invention will be described. Example 9 shows a reflective display using the multilayer phase plate of Example 7. FIG. 37 shows a schematic sectional view of the ninth embodiment.
[0129]
In FIG. 37, 25 is a reflector, 26 is a liquid crystal, 27 is a color filter, 28 is a laminated phase plate (λ / 4 plate) of the seventh embodiment, and 29 is a polarizing plate.
[0130]
As described in the seventh and eighth embodiments, the laminated phase plate 28 has a small wavelength dependence of the phase difference and a small incident angle dependence. Therefore, it is particularly useful in an optical system such as a display which must function as a phase plate for light incident at various incident angles.
[0131]
For example, when a conventional film phase plate is used, the color purity displayed by the wavelength dependency is reduced, and the color reproducibility is deteriorated. There were problems such as changing appearance.
[0132]
On the other hand, if the laminated phase plate 28 of the present embodiment is employed, the above problem can be expected to be greatly improved. That is, it is possible to realize a display that is excellent in color reproducibility, has little color shift due to a change in incident angle, and can display a beautiful and highly expressive image.
[0133]
For example, a wide range of applications are conceivable, from small reflective liquid crystal displays such as portable information terminals, cellular phones, and liquid crystal displays for projectors to large displays such as LCD monitors.
[0134]
In this embodiment, the reflection type display is described. However, the multilayer type phase plate of the seventh embodiment can be applied to a transmission type display. In this case, a laminated phase plate that functions as a λ / 2 plate is used.
[0135]
[Example 10]
Next, a tenth embodiment of the present invention will be described. Embodiment 10 shows a liquid crystal projector (optical apparatus) using the reflection type display of Embodiment 9. FIG. 38 shows the configuration of the liquid crystal projector.
[0136]
In FIG. 38, a light source 50 is a white light source close to a point including a halogen lamp, a xenon lamp, or the like. The light emitted from the light source 50 is reflected by the reflector 51, emitted as a substantially parallel light beam, and applied to the white polarizing sheet 52a. The polarizing sheet 52a has a property of transmitting the first polarized light component (S-polarized light or P-polarized light) and absorbing the second polarized light component (P-polarized light or S-polarized light) of the light from the reflector 51. The luminous flux transmitted through the polarizing sheet 52a is reflected by the polarization plane of the polarizing beam splitter 53, and irradiates a synthetic cross prism 54 as a color separation optical system.
[0137]
The synthetic cross prism 54 is provided such that the surfaces sandwiching the right angles of the four right-angle prisms are bonded to each other with an adhesive, and the bonded surfaces are such that the first dichroic mirror surface 54a and the second dichroic mirror surface 54b are substantially cross-shaped. The angle formed between the normal line of the first dichroic mirror surface 54a and the second dichroic mirror surface 54b and the principal ray of the used light is approximately 45 degrees. Further, the first dichroic mirror surface 54a and the second dichroic mirror surface 54b are formed of a dielectric multilayer film, and obtain a wavelength selective reflection characteristic by alternately stacking several tens of TiO2 and SiO2 thin films.
[0138]
The first dichroic mirror surface 54a reflects red light, which is the first color light, and transmits green light, which is the second color light, and blue light, which is the third color light. The second dichroic mirror surface 54b is configured to reflect blue light and transmit red light and green light. The red light reflected by the first dichroic mirror surface 54a enters the reflective display (liquid crystal panel) 56a of the ninth embodiment. The blue light reflected by the second dichroic mirror surface 54b similarly enters a reflective display (liquid crystal panel) 56b having a laminated phase plate. The green light transmitted through the first dichroic mirror surface 54a and the second dichroic mirror surface 54b similarly enters a green light reflective display (liquid crystal panel) 56c having a laminated phase plate.
[0139]
The image light modulated by the reflective display 56a for red light enters the synthetic cross prism 54, is reflected by the first dichroic mirror surface 54a, and transmits through the second dichroic mirror surface 54b. The image light modulated by the reflective display 56b for blue light enters the synthetic cross prism 54, passes through the first dichroic mirror surface 54a, and is reflected by the second dichroic mirror surface 54b. Similarly, image light modulated by the reflective display 56c for green light enters the synthetic cross prism 54, and green image light passes through the first dichroic mirror surface 54a and the second dichroic mirror surface 54b.
[0140]
The image light color-combined by the combining cross prism 54 as described above passes through the polarizing beam splitter 53 and the polarizing sheet 52b, and is enlarged and projected on the screen 58 by the projection lens 57.
[0141]
The reflective liquid crystal display of this embodiment uses a laminated phase plate, and has improved wavelength dependence and incident angle characteristics of the phase difference as compared with the conventional reflective liquid crystal display. In this embodiment, the application to the reflection type liquid crystal display has been described. However, the phase control element of this example can also be applied to the transmission type liquid crystal display. In this case as well, the phase difference depends on the wavelength and the incident angle. The performance is greatly improved. In addition, the laminated phase plate of the present invention can be applied to optical systems of various optical devices other than the above.
[0142]
【The invention's effect】
As described above, according to the present invention, it is possible to realize an optical element having a periodic structure with a period smaller than the wavelength of the used light and having higher performance than conventionally known ones.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a phase plate in Embodiment 1 of the present invention.
FIG. 2 is a sectional view of a phase plate according to the first embodiment of the present invention.
FIG. 3 shows a phase difference characteristic of the phase plate in the first embodiment of the present invention.
FIG. 4 shows transmittance characteristics of a phase plate according to the first embodiment of the present invention.
FIG. 5 is a schematic diagram of a phase plate in Embodiment 2 of the present invention.
FIG. 6 is an xz plane cross-sectional view of a phase plate according to the second embodiment of the present invention.
FIG. 7 is a yz-plane cross-sectional view of a phase plate in Embodiment 2 of the present invention.
FIG. 8 is a diagram illustrating phase difference characteristics of a phase plate according to the second embodiment of the present invention.
FIG. 9 is a diagram illustrating transmittance characteristics of a phase plate according to the second embodiment of the present invention.
FIG. 10 is a schematic diagram of a phase plate according to a modification of the second embodiment of the present invention.
FIG. 11 is a schematic diagram of a polarization conversion element according to a third embodiment of the present invention.
FIG. 12 is a perspective view of a phase plate in Embodiment 4 of the present invention.
FIG. 13 is a cross-sectional view in the xz plane of FIG. 12;
FIG. 14 is a yz plan cross-sectional view of FIG.
FIG. 15 is a view showing the effective refractive index of each grating material of Example 4 for TE polarized light and TM polarized light.
FIG. 16 is a diagram showing phase difference characteristics of Example 4 in comparison with the case of quartz.
FIG. 17 is a diagram showing transmittance characteristics of Example 4 for TE polarized light and TM polarized light.
FIG. 18 is a diagram showing average values of transmittance for TE polarized light and TM polarized light in Example 4.
FIG. 19 is a perspective view of a fifth embodiment.
FIG. 20 is an xz plane sectional view of a fifth embodiment.
FIG. 21 is a yz-plane sectional view of a fifth embodiment.
FIG. 22 is a diagram showing the phase difference characteristics of Example 5 in comparison with the case of quartz.
FIG. 23 is a diagram showing transmittance characteristics of Example 5.
FIG. 24 is a diagram illustrating an optical system according to a sixth embodiment.
FIG. 25 is a diagram illustrating transmittance characteristics with respect to a phase change when the polarization direction is rotated.
FIG. 26 is a diagram showing phase difference characteristics of Example 6 in comparison with the case of quartz.
FIG. 27 is a diagram showing substantial transmittance characteristics of Example 6 in comparison with the case of quartz.
FIG. 28 is a schematic configuration diagram (perspective view) and a manufacturing process diagram of the laminated phase plate of Example 7.
FIG. 29 is a cross-sectional view in the xz plane of the laminated phase plate of the seventh embodiment.
FIG. 30 is a yz-plane cross-sectional view of the laminated phase plate of the seventh embodiment.
FIG. 31 is a graph showing transmittance characteristics of Example 7.
FIG. 32 is a graph showing a phase difference characteristic of the seventh embodiment.
FIG. 33 is a graph showing transmittance characteristics of the laminated phase plate at an incident angle of 0 ° in Example 8.
FIG. 34 is a graph showing the phase difference characteristics of the laminated phase plate at an incident angle of 0 ° in Example 8.
FIG. 35 is a graph showing the transmittance characteristics of the laminated phase plate at an incident angle of 0 ° in Example 8.
FIG. 36 is a graph showing the phase difference characteristics of the laminated phase plate at an incident angle of 20 ° in Example 8.
FIG. 37 is a cross-sectional view of a reflective liquid crystal display using the laminated phase plate of Example 9.
FIG. 38 is a configuration diagram of a liquid crystal projector using the reflective liquid crystal display of the tenth embodiment.
FIG. 39 is a view showing a conventional one-dimensional fine periodic structure.
FIG. 40 is a view showing a conventional one-dimensional fine periodic structure.
FIG. 41 is a graph showing phase difference characteristics in a conventional quartz plate.
[Explanation of symbols]
1 substrate
2 First grid
3 2nd grid
4 Third grid
5mm board
6 1st lattice
7 Second grid
8 3rd lattice
21 First substrate
22 First grid
23 Second substrate
24 ° second grid

Claims (12)

An optical element having a substrate and a periodic structure having a plurality of layers laminated on the substrate and having a period shorter than a wavelength of a predetermined use light, wherein the plurality of layers of the periodic structure have a period in a first direction. a periodic structure of k layers (k: an integer of 1 or more) and a periodic structure of 1 layers (l: an integer of 0 or more) having a period in a second direction orthogonal to the first direction; When the polarized light having a polarization direction perpendicular to the periodic direction is TE polarized light and the polarized light having a polarized direction parallel to the periodic direction of the grating is TM polarized light, the two design wavelengths λ ₁ and λ ₂ in the visible light region are On the other hand,
[Outside 1]

However,
Δn _i : the difference between the refractive index of the TE-polarized light and the TM-polarized light with respect to λ ₁ of the i-th periodic structure having a period in the first direction _di : the layer of the i-th periodic structure having a period in the first direction thickness [Delta] n _j: difference _j-th of the TE polarized light to the lambda ₁ of the periodic structure having a period in the second direction and the TM polarized light refractive index of the d _i: period has a period in the j-th second direction structure Body layer thickness Δn _i ′: difference in refractive index between TE-polarized light and TM-polarized light with respect to λ ₂ of the periodic structure having a period in the i-th first direction Δn _j ′: in the j-th second direction wherein the optical element satisfies the difference in the refractive index of the TE and TM polarizations become conditions for lambda ₂ of the periodic structure having a periodicity. The optical element according to claim 1, wherein the visible light region is a wavelength region from 400 nm to 700 nm. A substrate, a first structure disposed on the substrate and having a periodic structure having a period shorter than the wavelength of the light used in the first direction, and a second structure disposed on the substrate and perpendicular to the first direction. A second structure having a periodic structure with a period shorter than the wavelength of the light used. A third structure provided on the substrate and having a periodic structure having a period shorter than the wavelength of the light used in a third direction, the first structure, the second structure, and the The third structure is stacked, wherein the first direction and the third direction are orthogonal to each other, and the second direction and the third direction are parallel to each other. Optical element. An effective refractive index of each periodic structure for a first polarized light having a first polarization component having a first polarization component of the used light, the substrate having a substrate and a periodic structure having a period shorter than the wavelength of the used light laminated on the plurality of substrates; Wherein the difference between the optical elements is 0.2 or less. 6. The optical element according to claim 5, wherein the plurality of periodic structures have an effective refractive index for a second polarized light having a second polarized light component orthogonal to the first polarized light component, the closer to the substrate, the higher the effective refractive index. . In order from the light incident direction to the light emitting direction, there are a first substrate, a first structure having a periodic structure having a period shorter than the wavelength of the used light, and a periodic structure having a period shorter than the wavelength of the used light. An optical element comprising: a second periodic structure having a periodic direction substantially orthogonal to a periodic direction of the first structure; and a second substrate. The refractive index of the first substrate is substantially equal to the effective refractive index of the first structure with respect to polarized light having a polarization direction substantially parallel to the periodic direction of the first structure, and substantially equal to the periodic direction of the first structure. 8. The optical element according to claim 7, wherein the value is a value between the effective refractive index of the first structure and the polarized light having the orthogonal polarization direction. The refractive index of the first substrate is substantially equal to the effective refractive index of the first structure with respect to polarized light having a polarization direction substantially parallel to the periodic direction of the first structure, or substantially equal to the periodic direction of the first structure. 8. The optical element according to claim 7, wherein the effective refractive index of the first structure is equal to the effective refractive index of the polarized light having the orthogonal polarization direction. Forming, on the first substrate, a first structure having a periodic structure having a shorter period than light having a predetermined wavelength of the used light; and forming, on the second substrate, light having a predetermined wavelength of the used light. Forming a second structure having a periodic structure having a short period, and forming the first substrate on which the first structure is formed and the second substrate on which the second structure is formed into two structures. Facing each other, and laminating the two structures such that the directions of the periods of the two structures are substantially orthogonal to each other. A polarizing beam splitter for converting non-polarized light into linearly polarized light, the optical element according to any one of claims 1 to 9, and a reflecting member, and one of the linearly polarized lights transmitted or reflected by the polarizing beam splitter is An optical modulation element, wherein one of the linearly polarized lights transmitted or reflected by the polarizing beam splitter is emitted in the same direction as the other by a reflecting member, after being incident on the optical element and the deflection direction is rotated by 90 °. An image display device comprising: a polarizing plate; the optical element according to claim 1; and a liquid crystal.

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