JP2005010377A - Optical retardation element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical retardation element capable of producing a stable retardation for light of wide wavelength region. <P>SOLUTION: The optical retardation element has a rugged structure in which the projected parts 11 having substantially rectangular cross-section are arrayed with a period T shorter than a designed wavelength, on the upper surface thereof. In the rugged structure of a substrate part 10, the occupy ratio t/T of the projected parts which is a ratio between the width (t) in the periodical direction of the projected part 11 and the period T of the rugged structure becomes one value of 0.6 to 0.9. Therein, a cover film 20 having a refractive index n<SB>2</SB>higher than that of the projected part 11 is disposed so that a first cover film is formed at least on the upper surface of the projected part 11 in the substrate part 10. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４０】
【数１５】

【００４１】
第２の実効層（ｊ＝２）
【００４２】
【数１６】

【００４３】
【数１７】

【００４４】
第３の実効層（ｊ＝３）
【００４５】
【数１８】

【００４６】
【数１９】

【００４７】
数式１４〜数式１９を「Ｒｙｔｏｖの２次近似式」と呼ばれる式に適用することにより、数式２０及び２１に示す有効屈折ｎ_ＴＥ（ｊ）及び ｎ_ＴＭ（ｊ）を得ることができる（参考文献：Ｓ． Ｍ． Ｒｙｔｏｖ， ”Ｅｌｅｃｔｒｏｍａｇｎｅｔｉｃ Ｐｒｏｐｅｒｔｉｅｓ ｏｆ Ｆｉｎｅｌｙ Ｓｔｒａｔｉｆｉｅｄ Ｍｅｄｉｕｍ， Ｊｏｕｒｎａｌ ｏｆ Ｅｘｐｅｒｉｍｅｎｔａｌ ａｎｄ Ｔｈｅｏｒｅｔｉｃａｌ Ｐｈｙｓｉｃｓ ＵＳＳＲ， ｖｏｌ．２９ （１９５５） ６０５−６１６）。
【００４８】
【数２０】

【００４９】
【数２１】

【００５０】
ここで、ｎ_３及びｎ_４は、ｊ＝１の場合、ｎ_３＝１、ｎ_４＝ｎ_２、ｊ＝２の場合、ｎ_３＝１．０、ｎ_４＝ｎ_１、ｊ＝３の場合、ｎ_３＝ｎ_２、ｎ_４＝ｎ_１となる。
【００５１】
図２（ａ）及び（ｂ）は、図１に示した光学位相差素子において、凸部占有率ｔ／Ｔを変化させたときの位相遅延差δφの入射光の波長λに対する依存性を示したグラフである。光学位相差素子としては、凸部占有率ｔ／Ｔの値が上述した０．６〜０．９の範囲を越える０．４及び０．５のものも比較のために示している。
【００５２】
これらのグラフにおいて、凸部１１の屈折率ｎ_１は、上述のように石英ガラスの屈折率の１．４６である。また、図２（ａ）においては、周期Ｔを３００ｎｍとし、被覆膜２０の屈折率ｎ_２が２．１となっており、図２（ｂ）においては、周期Ｔを１８０ｎｍとし、被覆膜２０の屈折率ｎ_２が３．５となっている。
【００５３】
図示のように、ここに示した光学位相差素子は、Ｈｅ−Ｎｅレーザー光の波長である６３３ｎｍの入射光に対して９０°（０．５πラジアン）の位相遅延差δφを発生させる１／４波長板となっているが、凸部占有率ｔ／Ｔの値が小さいものでは、入射光の波長λが長くなるにつれて位相遅延差δφが減少している。
【００５４】
凸部１１の周期方向の幅ｔは凹部１２の周期方向の幅と同じ幅に形成され、凸部占有率ｔ／Ｔは０．５とされるのが一般的であるが、凸部占有率ｔ／Ｔが０．５を越えるものではこの位相遅延差低下の傾向が改善されており、位相遅延差δφが入射光の波長に依存しない、又は、ほぼ依存しないようになっている。例えば、ｔ／Ｔが０．５の場合、位相変調量は入射光の波長に強く依存しているが、ｔ／Ｔ＞０．７になると、位相遅延差δφは入射光の波長λの変化に対して余り変化しなくなっている。
【００５５】
第１の実施形態に係る光学位相差素子１００においては、凸部占有率ｔ／Ｔが、０．６から０．９までのいずれかの値となっており、上記効果を奏する。したがって、位相遅延差δφの入射光の波長λに対する変動を抑制して、広い波長域の光に対して安定した位相遅延差を発生させることができ、１つの光学位相差素子を広い波長域の用途に適用することができる。
【００５６】
尚、図２に示されているように、凸部占有率ｔ／Ｔは、０．７から０．９までのいずれかの値となっていることがより望ましく、０．８から０．９までのいずれかの値となっていることがさらに望ましい。これにより、位相遅延差δφを入射光の波長λの変換に対してより変動しないものとすることができ、広い波長域の光に対してより安定した位相遅延差を発生させることができる。
【００５７】
図３は、本発明の第２の実施形態に係る光学位相差素子を模式的に示した断面図である。
【００５８】
図示のように、本発明の第２の実施形態に係る光学位相差素子１００Ａは、基板部１０と、被覆膜２０Ａとを備えているが、基板部１０については、第１の実施形態に示したものと同様であるので、その詳細な説明を省略する。
【００５９】
本実施形態においても、被覆膜２０Ａは、基板部１０のうち凸部１１の上面に形成された第１被覆膜２１Ａと、凹部１２の上面に形成された第２被覆膜２２Ａとを有し、第１の実施形態に示したものと同様に形成される。しかし、本実施形態では、凹部の底面からの高さである第２被覆膜２２Ａの高さＤ_２は、凸部１１の高さＤ_１より高くなっている。
【００６０】
本実施形態では、第２被覆膜２２Ａの凹部１２の底面からの高さＤ_２と凸部１１の高さＤ_１との差である被覆高差Ｄ_２−Ｄ_１が、設計波長をλ、第２被覆膜２２Ａの屈折率をｎ_２、０以上の任意の整数をＮとしたとき、数式１で示される範囲内のいずれかの値となっている。
【００６１】
２Ｎλ／４ｎ_２ ＜Ｄ_２−Ｄ_１＜（２Ｎ＋１）λ／４ｎ_２ … 数式１
本実施形態においても、第１の実施形態の場合と同様に、図３に示すような第１〜３の実効層からなる三層構造が形成される。第１の実効層（ｊ＝１）が、第１被覆膜２１Ａ（屈折率ｎ_２）及び空気（屈折率１．０）、第３の実効層（ｊ＝３）が、凸部１１（屈折率ｎ_１）及び第２被覆膜２２Ａ（屈折率ｎ_２）によって構成されているのは同様である。しかし、第２の実効層（ｊ＝２）は、第１被覆膜２１Ａ（屈折率ｎ_２）及び第２被覆膜２２Ａ（屈折率ｎ_２）、即ち被覆膜２０Ａだけで構成された均一な層となる。
【００６２】
この第２の実効層には光学的異方性がなく、層の厚さが偏光成分間の位相遅延差に影響を与えないので、被覆高差Ｄ_２−Ｄ_１の値を調整しても１／４波長の位相遅延差を保持することができる。被覆高差Ｄ_２−Ｄ_１が数式１で示される範囲のいずれかの値となっているので、ＴＥ波及びＴＭ波に対して界面での反射を打ち消すことができる。したがって、本実施形態に係る光学位相差素子１００Ａによれば、設計波長λに対して凹凸構造での反射を抑制することができる。以下、これについて説明する。
【００６３】
光学位相差素子１００Ａに光が入射すると、第１〜３の実効層の上面となる界面１〜３、及び第３の実効層の下面となる界面４の４つの界面において反射が起こる。被覆膜２０Ａの屈折率ｎ_２の値が大きい程、この反射が強くなり、透過率が低下する。以下では、界面１に入射する光（電場）の振幅をＡ_０、界面１からの反射光の振幅をＢ_０とし、第１〜３の各実効層内を伝播する入射光の振幅をＡ_１〜_３、反射光の振幅をＢ _１〜_３とし、界面４に入射する光の振幅をＡ_４とする。
【００６４】
界面１からの反射光の振幅Ｂ_０は、各界面での反射の総和として生じるものであり、その値は各界面での反射率と界面間の光学距離とで決まる。特に、界面間の光学距離は光波の干渉効果に関係するので、各実効層の膜厚を調節することにより、干渉効果を制御して反射光を抑制することができる。反射光の強さを示す振幅Ｂ_０は、多層膜干渉理論を用いて概算される。
【００６５】
まず、界面１より上方では、入射と反射との光波が存在するので、界面１より上方における電場Ｅ_０は数式２２で表すことができる。各実効層における電場Ｅ_ｊは、同様に数式２３で表すことができる。ここで、ｎ_ｊの値はその実効層における屈折率であり、数式２０及び数式２１に示した有効屈折率ｎ_ＴＥ（ｊ）及びｎ_ＴＭ（ｊ）となる。ただし、第２の実効層においてはｎ_２となる。また、式中のｚは、図３において基板部１０の上面に対して垂直上向きに示されたｚ軸上の変位を表す。
【００６６】
【数２２】

【００６７】
【数２３】

【００６８】
一方、各界面における電場Ｅ_ｊ（ｚ_ｊ）、及びその伝播方向の微分は連続でなければならないので、ｚ＝ｚ_ｊにおいて数式２４及び２５の境界条件が満足される。
【００６９】
Ｅ_ｊ（ｚ_ｊ） ＝ Ｅ_ｊ−１（ｚ_ｊ） … 数式２４
ｄＥ_ｊ−１（ｚ）／ｄｚ ＝ ｄＥ_ｊ（ｚ）／ｄｚ （ｚ＝ｚ_ｊ）… 数式２５
上記の数式２２〜数式２５を用いて、界面１からの反射光の振幅Ｂ_０を、ＴＥ波に対しては数式２６〜３０、ＴＭ波に対しては数式３１〜３５のように計算することができる。通常、両者の振幅Ｂ_０は異なる値となっている。
【００７０】
【数２６】

【００７１】
【数２７】

【００７２】
【数２８】

【００７３】
【数２９】

【００７４】
【数３０】

【００７５】
【数３１】

【００７６】
【数３２】

【００７７】
【数３３】

【００７８】
【数３４】

【００７９】
【数３５】

【００８０】
上記数式２６〜３０、及び数式３１〜３５を用い、Ｈｅ−Ｎｅレーザー光（波長６３３ｎｍ）に適した１／４波長板である本実施形態に係る光学位相差素子１００Ａの場合を計算する。基板部１０の屈折率ｎ_１は１．４６、凹凸構造の周期Ｔは３２０ ｎｍ、凸部占有率ｔ／Ｔは０．６、凸部１１の高さＤ_１は３２５ ｎｍである。
【００８１】
被覆膜２０Ａの屈折率ｎ_２が２．２の場合、１／４波長板とするためには、第１被覆膜２１Ａ及び第２被覆膜２２Ａの高さＤ_２は３２５ ｎｍであればよい。しかしながら、この場合、上述した界面での反射のために、ＴＥ波の透過率は８７％、ＴＭ波の透過率は９０％になってしまう。これに対し、本実施形態では、Ｄ_１が３２５ ｎｍであるのに対し、上記数式１を満足するようにＤ_２は３４５ ｎｍとしている。
【００８２】
図４は、第２被覆膜２２Ａの高さＤ_２を変化させたときの界面１からの反射光のＴＥ波及びＴＭ波に対する振幅Ｂ_０の依存性を示したグラフである。
【００８３】
図示のように、第２被覆膜２２Ａの高さＤ_２が本実施形態である３４５ ｎｍの場合、ＴＥ波とＴＭ波の透過率が共に９５％以上となり、反射を５％以下に抑制することができるのがわかる。
【００８４】
図５は、本発明の第３の実施形態に係る光学位相差素子を模式的に示した断面図である。
【００８５】
図示のように、本発明の第３の実施形態に係る光学位相差素子１００Ｂは、基板部１０Ｂと、被覆膜２０Ｂとを備えている。
【００８６】
基板部１０Ｂについては、凸部１１Ｂの周期方向の幅ｔと凹凸構造の周期Ｔとの比である凸部占有率ｔ／Ｔが０．４から０．６までのいずれかの値となっている。その他の点では第１の実施形態と同様であるので、その詳細な説明を省略する。
【００８７】
被覆膜２０Ｂは、第１の実施形態の場合と同様に、凸部１１Ｂより高い屈折率を有して凹凸構造上に形成されており、凸部１１Ｂの上面に形成された第１被覆膜２１Ｂと、凹凸構造の凹部１２Ｂの底面に形成された第２被覆膜２２Ｂとを有する。本実施形態では、さらに凸部１１Ｂの側面に形成された第３被覆膜２３Ｂを有する。
【００８８】
図５に示したような構造は、スパッタリング法などの指向性の弱い成膜方法により、基板部１０Ｂの凹凸構造上に被覆膜２０Ｂを成長させることによって得られる。このような成膜方法では、凹部１２Ｂの底面及び凸部１１Ｂの側面に比べて、凸部の上面の方が膜が成長し易いので、第２被覆膜２２Ｂ及び第３被覆膜２３Ｂの膜厚は、第１被覆膜２１Ｂの膜厚より薄くなっている。また、凸部の上面に形成される膜は成長と共に、その周期方向の幅が広くなり、最大幅ｔ１を越えると狭くなって縮小している。したがって第１被覆膜２１Ｂは、凸部の周期方向の幅ｔより広い最大幅ｔ１を周期方向に有する拡張部を含み、前記凸部の上面から前記拡張部を経て第１被覆膜２１Ｂの上端に至るまで周期方向の幅が滑らかに変化した形状となっている。最大幅ｔ１と凹凸構造の周期Ｔとの比である第１被覆膜占有率ｔ１／Ｔは、０．６から０．９までのいずれかの値となっている。
【００８９】
本実施形態では、被覆膜２０Ｂが曲面状に形成されているため厳密ではないが、第１の実施形態の場合と同様に、図５に示すような第１〜３の実効層からなる三層構造が形成されると近似することができる。即ち、第１の実効層（ｊ＝１）は、第１被覆膜２１Ｂ（屈折率ｎ_２）及び空気（屈折率１．０）、第２の実効層（ｊ＝２）は、凸部１１Ｂ（屈折率ｎ_１）及び空気（屈折率１．０）、第３の実効層（ｊ＝３）は、凸部１１Ｂ（屈折率ｎ_１）及び第２被覆膜２２Ｂ（屈折率ｎ_２）によって概ね構成される。
【００９０】
したがって、位相遅延差δφについても、第１〜３の実効層に基づき、数式１２の場合と同様に数式３６で求めることができる。ここで、Ｄ_２は、第１被覆膜２１の高さ、Ｄ_４は、第２被覆膜２２Ｂの高さ、Ｄ_３は、凸部１１の高さＤ_１と第２被覆膜２２Ｂの高さＤ_４との差をそれぞれ示している。
【００９１】
【数３６】

【００９２】
本実施形態に係る光学位相差素子１０Ｂにおいては、第１及び第３の実効層における位相遅延差δｎ（１）及びδｎ（３）に比べて、第２の実効層における位相遅延差δｎ（２）の値は小さい。また、第１被覆膜２１の高さＤ_２及び凸部１１の高さＤ_１と第２被覆膜２２Ｂの高さＤ_４との差Ｄ_３と比べて、第２被覆膜２２Ｂの高さＤ_４の値は小さい。したがって、本実施形態に係る光学位相差素子１０Ｂにおける位相遅延差においては、第１の実効層による複屈折効果が支配的となる。
【００９３】
本実施形態に係る光学位相差素子１００Ｂによれば、基板部１０が屈折率の低い誘電体基板であっても、また、その上面に形成された凹凸構造における凸部占有率ｔ／Ｔが製造し易い０．４から０．６の値であっても、第１被覆膜占有率ｔ１／Ｔが０．６から０．９までのいずれかの値となるように、屈折率の高い第１被覆膜２１Ｂを凹凸構造上に形成することができる。したがって、第１の実施形態の場合と同様に、位相遅延差δφの入射光の波長λに対する変動を抑制して、広い波長域の光に対して安定した位相遅延差を発生させることができる。さらに、基板部１０の凸部占有率ｔ／Ｔを０．５前後とすることができるので、製造が容易となり、成型などに有利である。
【００９４】
また、被覆膜２０Ｂ各部の幅が高さ方向で滑らかに変化する形状となるので、有効屈折率が徐々に変化する構造となり、反射光を抑制することができる。
【００９５】
図６は、図５に示した光学位相差素子を近似的に示した光学位相差素子の断面図であり、図７は、図６に示した光学位相差素子において、被覆膜の屈折率を変化させたときの位相遅延差δφの入射光の波長λに対する依存性を示したグラフである。
【００９６】
図６に示した光学位相差素子１００Ｃは、位相遅延差δφの計算を容易にするために、図５に示した光学位相差素子１００Ｂにおける被覆膜２０Ｂを被覆膜２０Ｃに近似したものとなっている。詳細には、第１被覆膜２１Ｂを５つの第１被覆膜部２２Ｃ１〜２２Ｃ５に分割し、第２被覆膜２２Ｂを第２被覆膜２２Ｃに、第３被覆膜２３Ｂを第３被覆膜２３Ｃにそれぞれ近似している。
【００９７】
尚、凸部占有率ｔ／Ｔは０．５とし、第１被覆膜部２２Ｃ１及び２２Ｃ５の幅は０．７Ｔ、第１被覆膜部２２Ｃ２及び２２Ｃ４の幅は０．７５Ｔ、第１被覆膜部２２Ｃ３の幅は０．８Ｔとして、第１被覆膜占有率ｔ１／Ｔを０．７から０．８まで変化させた。また、第３被覆膜２３Ｃの幅は０．１Ｔとした。さらに、第２被覆膜２２Ｃの高さＤ_４を０．２Ｄ_２とし、被覆膜２０Ｃの屈折率には２．０、２．５及び３．０を適用し、凹凸構造の周期Ｔは３２０ｎｍ、２５０ｎｍ及び２１０ｎｍとしている。
【００９８】
図示のように、図２に示した第１の実施形態の場合と同様に、入射光の波長λが長くなるにつれて位相遅延差δφが減少する傾向が見られる。図中黒丸で示した水晶の位相板は従来の技術を示しており、本実施形態に係る光学位相差素子１００Ｃによれば、これと比べて位相遅延差低下の傾向が改善されており、位相遅延差δφが入射光の波長に依存し難くなっている。
【００９９】
以上本発明の実施形態について詳細に説明したが、本発明は上記実施形態に制限されるものではなく種々の変更が可能である。例えば、本発明は、入射光λに対してλ、λ／２など、λ／４以外の位相遅延差を発生させる波長板にも適用可能である。また、設計波長、周期Ｔ等についても種々の変更が可能である。
【０１００】
【実施例】
光学位相差素子の一例として、Ｈｅ−Ｎｅレーザーに使用する１／４波長板を作成した。基板部１０を１．４６の低い屈折率を持つ石英ガラス基板で構成し、上面には周期Ｔが３２０ｎｍ、線幅ｔが１４０ｎｍ、深さＤ_１が５００ｎｍの凹凸構造を作成した。次いで、スパッタ装置を用いて、被覆膜として屈折率ｎ_２が２．０３のＺｎ_２ＳｎＯ_４（ＺＴＯ）を凹凸構造上に５００ｎｍの厚さで堆積した。
【０１０１】
図８（ａ）は、石英ガラス基板上に形成した凹凸構造の断面電子顕微鏡写真であり、図８（ｂ）は、図８（ａ）に示した凹凸構造上にＺＴＯ膜を被覆させて形成した光学位相差素子表面の畝状構造の断面電子顕微鏡写真である。
【０１０２】
上記のようにして製造した光学位相差素子にＨｅ−Ｎｅレーザー光（波長６３３ｎｍ）と
Ｔｉ：サファイアレーザー光（波長７８０ｎｍ）とを透過させて光学特性を測定した。ＴＥ波とＴＭ 波との間の位相遅延差は、Ｈｅ−Ｎｅレーザー光では約８６度であり、Ｔｉ：サファイアレーザー光では約７８度であった。また、Ｈｅ−Ｎｅレーザー光でのＴＥ波、ＴＭ波の透過率は、それぞれ９０．６％及び８８．２％であった。
【０１０３】
【発明の効果】
本発明に係る光学位相差素子は、上面に凹凸構造を有する基板部と、前記凹凸構造の凸部の上面に形成された屈折率の高い第１被覆膜を有し、凸部占有率ｔ／Ｔが０．６から０．９までのいずれかの値となっていることを特徴とする。
【０１０４】
また、本発明に係る他の光学位相差素子は、上面に凹凸構造を有する基板部と、前記凹凸構造の凸部の上面に形成された第１被覆膜と、第１被覆膜より薄い凹部底面に形成された第２被覆膜と、前記凸部側面に形成された第３被覆膜とを有し、前記第１被覆膜が、前記凸部の周期方向の幅ｔより広い最大幅ｔ１を周期方向に有する拡張部を含み、前記凸部の上面から前記拡張部を経て前記第１被覆膜の上端に至るまで周期方向の幅が滑らかに縮小した形状となっており、凸部占有率ｔ／Ｔが０．４から０．６までのいずれかの値、第１被覆膜占有率ｔ１／Ｔが０．６から０．９までのいずれかの値となっていることを特徴とする。
【０１０５】
これらの構造に基づき、本発明に係る光学位相差素子によれば、位相遅延差の入射光の波長に対する変動を抑制して、広い波長域の光に対して安定した位相遅延差を発生させることができる。したがって、従来の光学位相差素子では実現できなかった広帯域の光学位相差素子を実現することができる。
【０１０６】
また、凹凸構造に被覆膜を形成した構造となっているので、製造が容易であり、特に凹凸構造に成形により製造することにより低価格を実現し量産性に優れている。また、成形製造により曲面状に湾曲したレンズや反射部材等の光学素子の表面に適用することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態に係る光学位相差素子を模式的に示した断面図である。
【図２】（ａ）及び（ｂ）は、図１に示した光学位相差素子において、凸部占有率ｔ／Ｔを変化させたときの位相遅延差δφの入射光の波長λに対する依存性を示したグラフである。
【図３】本発明の第２の実施形態に係る光学位相差素子を模式的に示した断面図である。
【図４】第２被覆膜の高さＤ_２を変化させたときの界面１からの反射光のＴＥ波及びＴＭ波に対する振幅Ｂ_０の依存性を示したグラフである。
【図５】本発明の第３の実施形態に係る光学位相差素子を模式的に示した断面図である。
【図６】図５に示した光学位相差素子を近似的に示した断面図である。
【図７】図６に示した光学位相差素子において、被覆膜の屈折率を変化させたときの位相遅延差δφの入射光の波長λに対する依存性を示したグラフである。
【図８】（ａ）は、石英ガラス基板上に形成した凹凸構造の断面電子顕微鏡写真であり、（ｂ）は、（ａ）に示した凹凸構造上にＺＴＯ膜を被覆させて形成した光学位相差素子表面の畝状構造の断面電子顕微鏡写真である。
【符号の説明】
１０、１０Ｂ 基板部
１１、１１Ｂ 凸部
１２、１２Ｂ 凹部
２０、２０Ａ、２０Ｂ、２０Ｃ 被覆膜
２１、２１Ａ、２１Ｂ、２１Ｃ 第１被覆膜
２２、２２Ａ、２２Ｂ、２２Ｃ 第２被覆膜
２３Ｂ、２３Ｃ 第３被覆膜
１００、１００Ａ、１００Ｂ、１００Ｃ 光学位相差素子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical phase difference element called a birefringent wave plate or an optical phase plate.
[0002]
[Prior art]
An optical phase difference element is an optical element that gives a different phase delay for each polarization component of incident light. It is an optical disk pickup, an optical isolator in optical communication, a surface analyzer by polarization analysis, and a liquid crystal display that combines and separates polarized light. It has been applied to various fields such as devices, and has become a necessary and indispensable device in these fields.
[0003]
In particular, an optical phase difference element provided in an optical disk pickup or the like can be used together with a polarizing beam splitter, so that the utilization efficiency of light energy can be increased, and at the same time, it functions as an optical isolator. In CDs and DVDs, laser beams having different wavelengths (780 nm and 650 nm) are used. In recent years, an apparatus that realizes these with one optical system has been proposed. Also in an optical isolator in optical communication, light in a wide wavelength range can be transmitted for multiplexing optical signals. Accordingly, there has been a demand for an optical phase difference element that generates a stable phase delay difference for light in a wide wavelength range.
[0004]
The optical phase difference element is provided with a natural birefringence crystal such as calcite, mica, or quartz, a birefringent polymer, or an artificial periodic structure shorter than the wavelength used. There is something that was formed.
[0005]
As the birefringent crystal, there is a flat plate cut parallel to the crystal axis. The light incident perpendicular to the cut surface is divided into a light component called an ordinary ray and a light component called an extraordinary ray. Since the propagation speed is different between the ordinary ray and the extraordinary ray, a difference occurs in the amount of phase delay between the two rays as they propagate through the optical phase difference element. Since the difference in the phase delay is proportional to the propagation distance in the optical phase difference element, the difference in the phase delay amount as the target value can be obtained by controlling the thickness of the optical phase difference element. However, the optical phase difference element of the birefringence crystal has a drawback that it is very expensive because it uses a crystal.
[0006]
Even the birefringent polymer is the same as the birefringent crystal described above in principle. In this case, the birefringence expressed between the average molecular direction of the polymer constituting the optical retardation element and the direction perpendicular thereto is used. Such an uneven arrangement of polymers can be produced simply by applying a force in a specific direction to the plastic material and making it a thin plate, so that even a large size can be obtained at a low cost. However, the birefringent polymer is inferior to the birefringent crystal in optical properties and durability, and is insufficient for application to a high-performance optical element.
[0007]
As an artificially formed periodic structure, there is a structure in which a periodic uneven structure is provided on a transparent dielectric substrate. The period of the uneven structure is shorter than the wavelength used. Since the propagation speed of light perpendicularly incident on the concavo-convex structure is different between the polarization component parallel to the groove direction of the concavo-convex structure and the polarization component perpendicular to the polarization component, a phase delay difference occurs between the two components. In this case, this phase delay difference is proportional to the depth of the concavo-convex structure. Therefore, a desired phase delay difference can be obtained by controlling this depth. An optical phase difference element provided with such a fine periodic concavo-convex structure was developed in 1983 by D.C. C. This is realized by Flanders (see Non-Patent Document 1).
[0008]
The concavo-convex structure (surface relief grid) is manufactured by manufacturing a mold by electroforming using a grid pattern formed of a photoresist and injecting a thermoplastic resin into the mold. That is, mass production by molding is possible. Furthermore, in the case of an optical element manufactured by molding, even if it has a curved surface such as a lens, the periodic concavo-convex structure as described above is incorporated into the surface constituting the element, and It can be manufactured as one optical phase difference element.
[0009]
However, in the optical retardation element having the concavo-convex structure, the phase delay difference strongly depends on the refractive index of the material in addition to the depth of the concavo-convex structure, and in particular, the higher the refractive index of the material, the stronger the birefringence. . Since the refractive index of optical materials such as glass and plastic is about 1.5, a very deep concavo-convex structure is required to realize a practical optical phase difference element using them. For example, when an optical phase difference element having a phase delay difference of ¼λ corresponding to He—Ne laser light (wavelength λ = 632.8 nm) is realized using a glass substrate, the period of the concavo-convex structure is 400 nm, When the line width is 200 nm, the depth is about 1800 nm. When such a structure is manufactured by injection molding or press molding, there is a problem that the thermoplastic resin does not peel well and remains in the mold.
[0010]
Further, as a method for manufacturing such an optical phase difference element, there is a method in which a lattice pattern of a photoresist is directly formed on a dielectric substrate and etching is performed using this as a mask. However, this method requires an expensive apparatus such as an ultraviolet exposure apparatus, an electron beam drawing apparatus, and a plasma etching apparatus, which is difficult to form a resist pattern having a thickness that can withstand etching, and requires a long time for manufacturing. There was a problem such as need.
[0011]
Also, an optical phase difference element has been devised that can realize a large phase delay even in a shallow uneven structure by forming an uneven structure in an optical material having a high refractive index. However, an optical material having a high refractive index is not suitable for molding, and thus has a drawback of poor productivity and high cost.
[0012]
In order to cope with this problem, a surface relief grating having a shallow groove is provided in the substrate dielectric, and a dielectric material having a higher refractive index than the substrate dielectric is coated or filled on the surface relief grating. An optical phase difference element has been proposed (see Patent Document 1).
[0013]
[Non-Patent Document 1]
Dee. Sea. F. Flanders, “Submicrometer periodicities as anotropic dielectrics”, Vol. 42, No. 6, Applied. Physics Letter (Applied Physics Letter), March 15, 1983, p. 492-494
[0014]
[Patent Document 1]
Japanese Patent Publication No. 7-99402
[0015]
[Problems to be solved by the invention]
However, the optical phase difference element described in Patent Document 1 has a problem that the strength of birefringence hardly depends on the incident wavelength. Since the phase delay difference is proportional to the ratio of the birefringence strength to the wavelength, there is a sufficient phase delay difference in the vicinity of the light wavelength (hereinafter referred to as the design wavelength) that is assumed to be used for the optical phase difference element at the time of design. Even if it is configured to occur, it is usual that a sufficient phase delay difference cannot be obtained in other wavelength regions far from the vicinity of the design wavelength.
[0016]
The present invention has been made in order to solve the above-described problem, and suppresses fluctuation of the phase delay difference with respect to the wavelength of the incident light, thereby generating a stable phase delay difference for light in a wide wavelength range. An object of the present invention is to provide an optical phase difference element that can be used.
[0017]
[Means for Solving the Problems]
In order to solve the above-described problems, an optical phase difference element according to the present invention includes a substrate portion having a concavo-convex structure on a top surface, in which convex portions having a substantially rectangular shape in cross section are arranged with a period T shorter than a design wavelength, and the convex portions An optical retardation element having a higher refractive index than that of a portion and having a coating film formed on the concavo-convex structure, wherein the coating film is at least on an upper surface of the convex portion of the substrate portion A convex covering ratio t / T, which is a ratio of a width t in the periodic direction of the convex portion and a period T of the concave-convex structure; It is characterized by any value.
[0018]
According to the optical phase difference element, since the convex portion occupation ratio t / T is any value from 0.6 to 0.9, the fluctuation of the phase delay difference with respect to the wavelength of the incident light is suppressed. Thus, a so-called achromatic optical phase difference element capable of generating a stable phase delay difference for light in a wide wavelength range can be realized.
[0019]
Further, the coating film has a second coating film formed on the bottom surface of the concave portion of the concavo-convex structure in the substrate portion, and the height D of the second coating film from the bottom surface of the concave portion ₂ And the height D of the projection ₁ Difference in coating height D ₂ -D ₁ Is the design wavelength λ, and the refractive index of the second coating film is n ₂ , Where N is an arbitrary integer greater than or equal to 0, it is preferably any value within the range represented by Equation 1.
[0020]
2Nλ / 4n ₂ <D ₂ -D ₁ <(2N + 1) λ / 4n ₂ ... Formula 1
According to the optical phase difference element, the coating height difference D ₂ -D ₁ Is within the range of Equation 1 above, it is possible to suppress reflection at the concavo-convex structure with respect to the design wavelength λ.
[0021]
In addition, another optical phase difference element according to the present invention includes a substrate portion having a concavo-convex structure in which convex portions having a substantially rectangular shape in a cross-sectional view are arranged at a period T shorter than a design wavelength, and a higher refractive than the convex portion. An optical phase difference element comprising a coating film formed on the concavo-convex structure, wherein the coating film is formed on the upper surface of the convex portion, A second coating film formed on a bottom surface of the concave portion of the concavo-convex structure; and a third coating film formed on a side surface of the convex portion, the second coating film and the third coating film. The extension portion has a maximum width t1 in the periodic direction that is wider than the width t in the periodic direction of the convex portion. Including a shape in which the width in the period direction smoothly changes from the upper surface of the convex portion to the upper end of the first coating film through the extended portion, The convex portion occupation ratio t / T, which is the ratio of the width t in the periodic direction of the portion and the period T of the concave-convex structure, is any value from 0.4 to 0.6, and the maximum width t1 The first coating film occupation ratio t1 / T, which is a ratio to the period T of the uneven structure, is any value from 0.6 to 0.9.
[0022]
According to the optical phase difference element, even when the convex portion occupancy t / T in the concavo-convex structure is a value of 0.4 to 0.6 which is easy to manufacture, the first coating film occupancy t1 / T is set to 0. By setting the value to any value from 6 to 0.9, it is possible to suppress the fluctuation of the phase delay difference with respect to the wavelength of the incident light and generate a stable phase delay difference with respect to light in a wide wavelength range. .
[0023]
In addition, since the width of each part of the coating film changes smoothly in the height direction, the effective refractive index gradually changes, and reflected light can be suppressed.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, taking as an example a quarter wave plate that generates a phase delay difference of λ / 4 with respect to incident light λ as an optical phase difference element. To do.
[0025]
FIG. 1 is a cross-sectional view schematically showing an optical phase difference element according to the first embodiment of the present invention. The illustrated cross-sectional structure is continuous in the direction perpendicular to the paper surface, and has a bowl-like surface shape.
[0026]
As shown in the drawing, the optical retardation element 100 according to the first embodiment of the present invention has a concavo-convex structure on the upper surface of which convex portions 11 having a substantially rectangular shape in cross section are arranged with a period T shorter than the design wavelength. Refractive index n of part 10 and convex part 11 ₁ Higher refractive index n ₂ And a coating film 20 formed on the concavo-convex structure.
[0027]
In the present embodiment, the substrate portion 10 is made of quartz glass (refractive index: 1.46). However, the substrate portion 10 may be made of another dielectric substrate having a high transmittance such as a plastic or a photocurable resin.
[0028]
The period T of the concavo-convex structure formed on the upper surface of the substrate unit 10 is shorter than the wavelength λ of the light to be used. In this embodiment, the period t of the convex part 11 and the period T of the concavo-convex structure The ratio of convex portion occupancy t / T, which is a ratio, is any value from 0.6 to 0.9. In addition, the period T is set to 300 nm or 180 nm, the He—Ne laser light (wavelength 633 nm) and the GaAlAs semiconductor laser light (wavelength 780 nm) are assumed as the design wavelengths, and the wavelength region in the vicinity thereof is assumed as the use light. However, the design wavelength and the value of the period T are not limited to this.
[0029]
Such a concavo-convex structure is preferably formed by mass production techniques such as injection molding and press molding in consideration of cost and productivity, but is also formed by etching using a photoresist having a lattice pattern as a mask. Is possible. In FIG. 1, the convex portion 11 is formed of the same material as the main body of the substrate portion 10. However, the convex portion 11 is formed on the substrate by forming a dielectric layer on the substrate and patterning it by etching. It is good also as a material different from the main body of the part 10. FIG.
[0030]
The coating film 20 has a first coating film 21 formed on the upper surface of the convex portion 11 in the substrate portion 10 and a second coating film 22 formed on the upper surface of the concave portion 12. In the present embodiment, the first coating film 21 and the second coating film 22 are simultaneously formed under the same conditions, and the first coating film 21 and the second coating film 22 have the same height D. ₂ However, there may be a difference in film formation conditions due to the shape effect of the unevenness, and the height may be different. However, in the present embodiment, the height D of the second coating film 22 is the height from the bottom surface of the recess. ₂ Is the height D of the convex part 11 ₁ It is lower.
[0031]
In the present embodiment, the coating film 20 is made of Si. ₃ N ₄ (Refractive index 2.1) or silicon (Si) (refractive index 3.5). ₂ It is also possible to configure with another dielectric substrate having a high refractive index. The coating film 20 is formed by an electron beam evaporation method, a sputtering method, an ion beam evaporation method, or the like.
[0032]
Thus, by providing the film | membrane with a refractive index different from the board | substrate part 10 on the uneven structure of the board | substrate part 10, the three-layer structure which consists of a 1st-3rd effective layer as shown in FIG. 1 is formed. The first effective layer (j = 1) is the first coating film 21 (refractive index n ₂ ) And air (refractive index 1.0), the second effective layer (j = 2) is a convex portion 11 (refractive index n). ₁ ) And air (refractive index 1.0), the third effective layer (j = 3) ₁ ) And the second coating film 22 (refractive index n) ₂ ) Respectively.
[0033]
Since the period T of the concavo-convex structure is short enough not to generate a diffracted wave with respect to incident light as described above, each of the first to third effective layers is a thin film having optical anisotropy due to artificial birefringence. Can be considered. When light including a light wave (TE wave) oscillating in a direction parallel to the groove of the concavo-convex structure and a light wave (TM wave) oscillating in a direction perpendicular to the groove is incident on each effective layer, the TE wave and the TM wave are Each has a different effective refractive index (effective refractive index). In the following, j (= 1 to 3) is made to correspond to the first to third effective layers, and the effective refractive index of the TE wave and TM wave in the first to third effective layers is expressed as n. _TE (J), n _TM This will be described in detail with reference to (j), in which different phase delays occur between the polarization direction and the direction perpendicular to the transmitted light due to the optical anisotropy of each effective layer.
[0034]
First, the period T is shorter than the wavelength λ of light in each effective layer, which is perpendicularly incident on each effective layer, and no diffracted light other than the 0th order light is generated at the time of this normal incidence. The condition of such a period T is that the refractive index n of the convex portion 11 ₁ The effective refractive index n of the coating film 20 _TE (J), n _TM Using (j), the following Expression 11 is given.
[0035]
T <λ / max [n ₁ , N _TE (J), n _TM (J)] ... Formula 11
Refractive index n of coating film 20 ₂ Is large, the depth of the concavo-convex structure for generating a necessary phase delay difference is made shallow, but the effective refractive index n _TE (J), n _TM Since the value of (j) also increases, it can be seen that the necessary period T is shortened as shown in Equation 11.
[0036]
In the present embodiment, since the period T satisfies Equation 11, the light incident perpendicularly to the concavo-convex structure does not generate high-order diffracted light, and the transmitted light is directed in the same direction as the incident light. Within the concavo-convex structure, the effective refractive index n of the TE wave _TE The value of (j) is the effective refractive index n of the TM wave _TM Since it is larger than the value of (j), the TE wave passes through the concavo-convex structure at a slower speed than the TM wave. As a result, when the light wave incident in the same phase passes through the concavo-convex structure, a phase delay different between the polarization direction and the direction perpendicular thereto is generated. At this time, the phase delay difference δφ generated between the TE wave and the TM wave has two effective refractive indexes n for the TE wave and the TM wave as shown in Expression 13. _TE (J) and n _TM The difference of (j) is given by Equation 12 with δn (j). For a quarter-wave plate, the value is 0.5π radians and for a half-wave plate is π radians.
[0037]
δφ = 2π / λ (δn (1) D ₂ + Δn (2) (D ₁ -D ₂ ) + Δn (3) D ₂ ) ... Formula 12
δn (j) = n _TE (J) -n _TM (J) Expression 13
As in this embodiment, the coating film 20 having a high refractive index is formed on the concavo-convex structure to form a three-layer structure in which the values of δn (1) and δn (3) are large. In addition, the depth D of the uneven structure ₁ A large phase delay difference can be obtained without increasing the depth.
[0038]
Effective refractive index n for TE and TM waves in the entire effective layer _TE (J) and n _TM (J) is the effective refractive index n in each effective layer _TE0 (J) and n _TM0 Using (j), it is expressed by the following equation. In the formula, f is an abbreviation of the convex portion occupancy t / T, which is the ratio of the width t in the periodic direction of the convex portion 11 to the period T of the concave-convex structure.
First effective layer (j = 1)
[0039]
[Expression 14]

[0040]
[Expression 15]

[0041]
Second effective layer (j = 2)
[0042]
[Expression 16]

[0043]
[Expression 17]

[0044]
Third effective layer (j = 3)
[0045]
[Expression 18]

[0046]
[Equation 19]

[0047]
By applying Expressions 14 to 19 to an expression called “Rytov's quadratic approximate expression”, effective refraction n shown in Expressions 20 and 21 is obtained. _TE (J) and n _TM (J) (Reference: S. M. Rytov, “Electromagnetic Properties of Finely Stratified Medium, Journal of Experimental and Theoretics.
[0048]
[Expression 20]

[0049]
[Expression 21]

[0050]
Where n ₃ And n ₄ Is n if j = 1 ₃ = 1, n ₄ = N ₂ , J = 2, n ₃ = 1.0, n ₄ = N ₁ , J = 3, n ₃ = N ₂ , N ₄ = N ₁ It becomes.
[0051]
2A and 2B show the dependency of the phase delay difference δφ on the wavelength λ of the incident light when the convex portion occupancy t / T is changed in the optical phase difference element shown in FIG. It is a graph. As optical phase difference elements, those having a convex portion occupation ratio t / T of 0.4 and 0.5 exceeding the above-described range of 0.6 to 0.9 are also shown for comparison.
[0052]
In these graphs, the refractive index n of the convex portion 11 ₁ Is 1.46 of the refractive index of quartz glass as described above. In FIG. 2A, the period T is set to 300 nm, and the refractive index n of the coating film 20 is set. ₂ In FIG. 2B, the period T is 180 nm, and the refractive index n of the coating film 20 is ₂ Is 3.5.
[0053]
As shown in the figure, the optical phase difference element shown here generates a phase delay difference δφ of 90 ° (0.5π radians) with respect to incident light of 633 nm which is the wavelength of the He—Ne laser light. Although a wave plate is used, when the convex portion occupation ratio t / T is small, the phase delay difference δφ decreases as the wavelength λ of the incident light increases.
[0054]
The width t of the convex portion 11 in the periodic direction is formed to be the same width as the width of the concave portion 12 in the periodic direction, and the convex portion occupation ratio t / T is generally set to 0.5. When t / T exceeds 0.5, the tendency of the phase delay difference to decrease is improved, and the phase delay difference δφ does not depend on the wavelength of incident light or almost does not depend on it. For example, when t / T is 0.5, the phase modulation amount strongly depends on the wavelength of the incident light. However, when t / T> 0.7, the phase delay difference δφ is a change in the wavelength λ of the incident light. Has not changed much.
[0055]
In the optical phase difference element 100 according to the first embodiment, the convex portion occupancy t / T is any value from 0.6 to 0.9, and the above-described effect is exhibited. Therefore, the fluctuation of the phase delay difference δφ with respect to the wavelength λ of the incident light can be suppressed, and a stable phase delay difference can be generated for light in a wide wavelength range. It can be applied for use.
[0056]
As shown in FIG. 2, the convex portion occupation ratio t / T is more preferably any value from 0.7 to 0.9, and 0.8 to 0.9. It is further desirable that the value is any of the above. Thereby, the phase delay difference δφ can be made less variable with respect to the conversion of the wavelength λ of the incident light, and a more stable phase delay difference can be generated for light in a wide wavelength range.
[0057]
FIG. 3 is a cross-sectional view schematically showing an optical phase difference element according to the second embodiment of the present invention.
[0058]
As shown in the figure, the optical retardation element 100A according to the second embodiment of the present invention includes a substrate unit 10 and a coating film 20A. The substrate unit 10 is the same as that of the first embodiment. Since it is the same as that shown, its detailed description is omitted.
[0059]
Also in the present embodiment, the coating film 20 </ b> A includes a first coating film 21 </ b> A formed on the upper surface of the convex portion 11 in the substrate portion 10 and a second coating film 22 </ b> A formed on the upper surface of the concave portion 12. And formed in the same manner as shown in the first embodiment. However, in the present embodiment, the height D of the second coating film 22A, which is the height from the bottom surface of the recess. ₂ Is the height D of the convex part 11 ₁ Higher.
[0060]
In the present embodiment, the height D from the bottom surface of the concave portion 12 of the second coating film 22A. ₂ And the height D of the projection 11 ₁ Difference in coating height D ₂ -D ₁ Is the design wavelength λ and the refractive index of the second coating film 22A is n ₂ , Where N is any integer greater than or equal to 0, the value is any value within the range represented by Equation 1.
[0061]
2Nλ / 4n ₂ <D ₂ -D ₁ <(2N + 1) λ / 4n ₂ ... Formula 1
Also in the present embodiment, as in the case of the first embodiment, a three-layer structure including the first to third effective layers as shown in FIG. 3 is formed. The first effective layer (j = 1) is the first coating film 21A (refractive index n ₂ ) And air (refractive index 1.0), the third effective layer (j = 3) ₁ ) And the second coating film 22A (refractive index n) ₂ ) Is the same. However, the second effective layer (j = 2) includes the first coating film 21A (refractive index n ₂ ) And the second coating film 22A (refractive index n) ₂ ), That is, a uniform layer constituted only by the coating film 20A.
[0062]
Since this second effective layer has no optical anisotropy and the thickness of the layer does not affect the phase delay difference between the polarization components, the coating height difference D ₂ -D ₁ Even if this value is adjusted, the phase delay difference of ¼ wavelength can be maintained. Coating height difference D ₂ -D ₁ Is any value within the range represented by Equation 1, reflection at the interface can be canceled with respect to the TE wave and the TM wave. Therefore, according to the optical phase difference element 100A according to the present embodiment, it is possible to suppress reflection at the concavo-convex structure with respect to the design wavelength λ. This will be described below.
[0063]
When light enters the optical phase difference element 100A, reflection occurs at four interfaces, that is, the interfaces 1 to 3 that are the upper surfaces of the first to third effective layers and the interface 4 that is the lower surface of the third effective layer. Refractive index n of coating film 20A ₂ The larger the value of, the stronger this reflection and the lower the transmittance. In the following, the amplitude of the light (electric field) incident on the interface 1 is A ₀ , The amplitude of the reflected light from the interface 1 is B ₀ And the amplitude of incident light propagating in each of the first to third effective layers is A ₁ ~ ₃ , The amplitude of the reflected light is B ₁ ~ ₃ And the amplitude of the light incident on the interface 4 is A ₄ And
[0064]
Amplitude B of reflected light from interface 1 ₀ Occurs as the sum of reflections at each interface, and its value is determined by the reflectance at each interface and the optical distance between the interfaces. In particular, since the optical distance between the interfaces is related to the interference effect of light waves, the interference effect can be controlled to suppress the reflected light by adjusting the film thickness of each effective layer. Amplitude B indicating intensity of reflected light ₀ Is approximated using multilayer interference theory.
[0065]
First, since there are incident and reflected light waves above the interface 1, the electric field E above the interface 1. ₀ Can be expressed by Equation 22. Electric field E in each effective layer _j Can be similarly expressed by Equation 23. Where n _j Is the refractive index in the effective layer, and the effective refractive index n shown in Equation 20 and Equation 21. _TE (J) and n _TM (J). However, in the second effective layer, n ₂ It becomes. Further, z in the equation represents a displacement on the z-axis that is shown vertically upward with respect to the upper surface of the substrate portion 10 in FIG.
[0066]
[Expression 22]

[0067]
[Expression 23]

[0068]
On the other hand, the electric field E at each interface _j (Z _j ), And its propagation direction derivative must be continuous, z = z _j , The boundary conditions of Equations 24 and 25 are satisfied.
[0069]
E _j (Z _j ) = E _j-1 (Z _j ) ... Formula 24
dE _j-1 (Z) / dz = dE _j (Z) / dz (z = z _j ) ... Formula 25
Using the above Equations 22 to 25, the amplitude B of the reflected light from the interface 1 ₀ Can be calculated as Equations 26-30 for the TE wave and Equations 31-35 for the TM wave. Usually, both amplitudes B ₀ Are different values.
[0070]
[Equation 26]

[0071]
[Expression 27]

[0072]
[Expression 28]

[0073]
[Expression 29]

[0074]
[30]

[0075]
[31]

[0076]
[Expression 32]

[0077]
[Expression 33]

[0078]
[Expression 34]

[0079]
[Expression 35]

[0080]
The case of the optical retardation element 100A according to the present embodiment, which is a ¼ wavelength plate suitable for He—Ne laser light (wavelength 633 nm), is calculated using the above Expressions 26-30 and 31-35. Refractive index n of the substrate unit 10 ₁ Is 1.46, the period T of the concavo-convex structure is 320 nm, the convex portion occupation ratio t / T is 0.6, and the height D of the convex portion 11 ₁ Is 325 nm.
[0081]
Refractive index n of coating film 20A ₂ Is 2.2, the height D of the first coating film 21A and the second coating film 22A is required to obtain a quarter-wave plate. ₂ May be 325 nm. However, in this case, due to the reflection at the interface described above, the TE wave transmittance is 87% and the TM wave transmittance is 90%. On the other hand, in this embodiment, D ₁ Is 325 nm, and D satisfies the above formula 1 ₂ Is 345 nm.
[0082]
FIG. 4 shows the height D of the second coating film 22A. ₂ Of the reflected light from the interface 1 with respect to the TE and TM waves ₀ It is the graph which showed dependence of.
[0083]
As shown, the height D of the second coating film 22A ₂ However, in the case of 345 nm, which is the present embodiment, both the TE wave and TM wave transmittances are 95% or more, and reflection can be suppressed to 5% or less.
[0084]
FIG. 5 is a cross-sectional view schematically showing an optical phase difference element according to the third embodiment of the present invention.
[0085]
As illustrated, the optical phase difference element 100B according to the third embodiment of the present invention includes a substrate unit 10B and a coating film 20B.
[0086]
For the substrate portion 10B, the convex portion occupation ratio t / T, which is the ratio of the width t in the periodic direction of the convex portion 11B and the period T of the concave-convex structure, is any value from 0.4 to 0.6. Yes. Since the other points are the same as those of the first embodiment, detailed description thereof is omitted.
[0087]
As in the case of the first embodiment, the coating film 20B has a higher refractive index than the convex portion 11B and is formed on the concave-convex structure, and the first coating formed on the upper surface of the convex portion 11B. A film 21B and a second coating film 22B formed on the bottom surface of the concave portion 12B having the concave-convex structure are included. In the present embodiment, it further includes a third coating film 23B formed on the side surface of the convex portion 11B.
[0088]
The structure as shown in FIG. 5 is obtained by growing the coating film 20B on the concavo-convex structure of the substrate portion 10B by a film forming method having a low directivity such as a sputtering method. In such a film forming method, since the film is more likely to grow on the top surface of the convex portion than the bottom surface of the concave portion 12B and the side surface of the convex portion 11B, the second coating film 22B and the third coating film 23B The film thickness is smaller than the film thickness of the first coating film 21B. Further, the film formed on the upper surface of the convex portion becomes wider in the periodic direction as it grows, and narrows and shrinks when it exceeds the maximum width t1. Accordingly, the first coating film 21B includes an extended portion having a maximum width t1 wider than the width t in the periodic direction of the convex portion in the periodic direction, and the first coating film 21B passes through the extended portion from the upper surface of the convex portion. It has a shape in which the width in the period direction smoothly changes until reaching the upper end. The first coating film occupation ratio t1 / T, which is the ratio between the maximum width t1 and the period T of the concavo-convex structure, is any value from 0.6 to 0.9.
[0089]
In the present embodiment, since the coating film 20B is formed in a curved surface shape, it is not strict, but as in the case of the first embodiment, three coating layers including first to third effective layers as shown in FIG. It can be approximated when a layered structure is formed. That is, the first effective layer (j = 1) includes the first coating film 21B (refractive index n ₂ ) And air (refractive index 1.0), the second effective layer (j = 2) is a convex portion 11B (refractive index n). ₁ ) And air (refractive index 1.0), the third effective layer (j = 3) is a convex portion 11B (refractive index n). ₁ ) And the second coating film 22B (refractive index n) ₂ ).
[0090]
Therefore, the phase delay difference δφ can also be obtained by Expression 36 as in Expression 12, based on the first to third effective layers. Where D ₂ Is the height of the first coating film 21, D ₄ Is the height of the second coating film 22B, D ₃ Is the height D of the convex part 11 ₁ And the height D of the second coating film 22B ₄ The difference is shown respectively.
[0091]
[Expression 36]

[0092]
In the optical phase difference element 10B according to the present embodiment, the phase delay difference δn (2 in the second effective layer compared to the phase delay differences δn (1) and δn (3) in the first and third effective layers. ) Is small. Further, the height D of the first coating film 21 ₂ And the height D of the projection 11 ₁ And the height D of the second coating film 22B ₄ Difference D ₃ Compared to the height D of the second coating film 22B ₄ The value of is small. Therefore, the birefringence effect by the first effective layer is dominant in the phase delay difference in the optical retardation element 10B according to the present embodiment.
[0093]
According to the optical phase difference element 100B according to the present embodiment, even if the substrate unit 10 is a dielectric substrate having a low refractive index, the convex portion occupation ratio t / T in the concavo-convex structure formed on the upper surface thereof is manufactured. Even if the value is 0.4 to 0.6, the first coating film occupancy t1 / T has a high refractive index so that the first coating film occupancy t1 / T is any value from 0.6 to 0.9. One coating film 21B can be formed on the concavo-convex structure. Therefore, as in the case of the first embodiment, it is possible to suppress the fluctuation of the phase delay difference δφ with respect to the wavelength λ of the incident light and generate a stable phase delay difference for light in a wide wavelength range. Further, since the convex portion occupancy t / T of the substrate portion 10 can be set to around 0.5, manufacturing is facilitated, which is advantageous for molding and the like.
[0094]
Further, since the width of each part of the coating film 20B changes smoothly in the height direction, the effective refractive index gradually changes, and reflected light can be suppressed.
[0095]
6 is a cross-sectional view of the optical phase difference element that schematically shows the optical phase difference element shown in FIG. 5, and FIG. 7 shows the refractive index of the coating film in the optical phase difference element shown in FIG. 6 is a graph showing the dependence of the phase delay difference δφ on the wavelength λ of incident light when the angle is changed.
[0096]
The optical phase difference element 100C shown in FIG. 6 is obtained by approximating the coating film 20B in the optical phase difference element 100B shown in FIG. 5 to the coating film 20C in order to facilitate the calculation of the phase delay difference δφ. It has become. Specifically, the first coating film 21B is divided into five first coating film portions 22C1 to 22C5, the second coating film 22B is the second coating film 22C, and the third coating film 23B is the third. Each of them is approximate to the coating film 23C.
[0097]
The convex portion occupation ratio t / T is 0.5, the width of the first coating film portions 22C1 and 22C5 is 0.7T, the width of the first coating film portions 22C2 and 22C4 is 0.75T, The width of the covering film portion 22C3 was 0.8T, and the first covering film occupation ratio t1 / T was changed from 0.7 to 0.8. The width of the third coating film 23C was set to 0.1T. Further, the height D of the second coating film 22C ₄ 0.2D ₂ The refractive index of the coating film 20C is 2.0, 2.5, and 3.0, and the period T of the concavo-convex structure is 320 nm, 250 nm, and 210 nm.
[0098]
As shown in the figure, the phase delay difference δφ tends to decrease as the wavelength λ of the incident light increases as in the case of the first embodiment shown in FIG. The quartz phase plate shown by the black circles in the figure shows the prior art, and according to the optical phase difference element 100C according to the present embodiment, the tendency of the phase delay difference to be reduced is improved as compared with this, The delay difference δφ is unlikely to depend on the wavelength of incident light.
[0099]
Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications can be made. For example, the present invention is also applicable to a wave plate that generates a phase delay difference other than λ / 4, such as λ, λ / 2, etc. with respect to incident light λ. Various changes can be made to the design wavelength, the period T, and the like.
[0100]
【Example】
As an example of the optical phase difference element, a quarter wavelength plate used for a He—Ne laser was prepared. The substrate part 10 is composed of a quartz glass substrate having a low refractive index of 1.46, and the upper surface has a period T of 320 nm, a line width t of 140 nm, and a depth D. ₁ Produced a concavo-convex structure of 500 nm. Next, using a sputtering apparatus, a refractive index n as a coating film ₂ Zn with 2.03 ₂ SnO ₄ (ZTO) was deposited on the concavo-convex structure to a thickness of 500 nm.
[0101]
FIG. 8A is a cross-sectional electron micrograph of the concavo-convex structure formed on the quartz glass substrate, and FIG. 8B is formed by covering the concavo-convex structure shown in FIG. 8A with a ZTO film. 3 is a cross-sectional electron micrograph of a saddle-like structure on the surface of an optical retardation element.
[0102]
The optical phase difference element manufactured as described above is subjected to He—Ne laser light (wavelength 633 nm) and
Ti: sapphire laser light (wavelength 780 nm) was transmitted to measure optical characteristics. The phase delay difference between the TE wave and the TM wave was about 86 degrees for the He—Ne laser light and about 78 degrees for the Ti: sapphire laser light. Moreover, the transmittance | permeability of the TE wave and TM wave with He-Ne laser light was 90.6% and 88.2%, respectively.
[0103]
【The invention's effect】
The optical phase difference element according to the present invention includes a substrate portion having a concavo-convex structure on the upper surface and a first coating film having a high refractive index formed on the upper surface of the convex portion of the concavo-convex structure, and has a convex portion occupation ratio t. / T is any value from 0.6 to 0.9.
[0104]
Further, another optical phase difference element according to the present invention includes a substrate portion having a concavo-convex structure on an upper surface, a first coating film formed on an upper surface of the convex portion of the concavo-convex structure, and being thinner than the first coating film. A second coating film formed on the bottom surface of the concave portion and a third coating film formed on the side surface of the convex portion, wherein the first coating film is wider than a width t in a periodic direction of the convex portion; It includes an extended portion having a maximum width t1 in the periodic direction, and has a shape in which the width in the periodic direction is smoothly reduced from the upper surface of the convex portion to the upper end of the first coating film through the extended portion, The convex portion occupation ratio t / T is any value from 0.4 to 0.6, and the first coating film occupation ratio t1 / T is any value from 0.6 to 0.9. It is characterized by that.
[0105]
Based on these structures, the optical phase difference element according to the present invention suppresses the fluctuation of the phase delay difference with respect to the wavelength of the incident light, and generates a stable phase delay difference for light in a wide wavelength range. Can do. Accordingly, it is possible to realize a broadband optical phase difference element that could not be realized with a conventional optical phase difference element.
[0106]
Moreover, since the coating film is formed on the concavo-convex structure, the manufacturing is easy. In particular, by manufacturing the concavo-convex structure by molding, a low price is realized and the mass productivity is excellent. Moreover, it can be applied to the surface of an optical element such as a lens or a reflecting member that is curved into a curved shape by molding.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an optical phase difference element according to a first embodiment of the present invention.
2A and 2B show the dependence of the phase delay difference δφ on the wavelength λ of incident light when the convex portion occupancy t / T is changed in the optical phase difference element shown in FIG. It is the graph which showed.
FIG. 3 is a cross-sectional view schematically showing an optical phase difference element according to a second embodiment of the present invention.
FIG. 4 shows the height D of the second coating film. ₂ Of the reflected light from the interface 1 with respect to the TE and TM waves ₀ It is the graph which showed dependence of.
FIG. 5 is a cross-sectional view schematically showing an optical phase difference element according to a third embodiment of the present invention.
6 is a cross-sectional view schematically showing the optical phase difference element shown in FIG. 5;
7 is a graph showing the dependence of the phase delay difference δφ on the wavelength λ of incident light when the refractive index of the coating film is changed in the optical phase difference element shown in FIG. 6;
8A is a cross-sectional electron micrograph of a concavo-convex structure formed on a quartz glass substrate, and FIG. 8B is an optical image formed by coating the concavo-convex structure shown in FIG. It is a cross-sectional electron micrograph of a bowl-like structure on the surface of a retardation element.
[Explanation of symbols]
10, 10B Board part
11, 11B Convex part
12, 12B recess
20, 20A, 20B, 20C Coating film
21, 21A, 21B, 21C First coating film
22, 22A, 22B, 22C Second coating film
23B, 23C Third coating film
100, 100A, 100B, 100C Optical phase difference element

Claims (3)

A convex portion having a substantially rectangular shape in cross section in a top surface has a concave-convex structure arranged with a period T shorter than the design wavelength, and a substrate having a higher refractive index than the convex portion and formed on the concave-convex structure. An optical retardation element comprising a covering film,
The coating film has a first coating film formed on the upper surface of the convex portion of at least the substrate portion,
The convex portion occupancy t / T, which is the ratio of the width t in the periodic direction of the convex portion and the period T of the concave-convex structure, is any value from 0.6 to 0.9. Optical phase difference element. The coating film has a second coating film formed on the bottom surface of the concave portion of the concavo-convex structure in the substrate portion,
The coating height difference D ₂ -D ₁ , which is the difference between the height D ₂ from the bottom surface of the concave portion of the second coating film and the height D ₁ of the convex portion, has a design wavelength of λ and the second coating film. _2. The optical phase difference according to claim 1, wherein when the refractive index of the covering film is n ₂ and an arbitrary integer equal to or greater than 0 is N, the optical retardation is any value within the range represented by Formula 1. element.
2Nλ / 4n ₂ <D ₂ −D ₁ <(2N + 1) λ / 4n ₂ ... A convex portion having a substantially rectangular shape in cross section in a top surface has a concave-convex structure arranged with a period T shorter than the design wavelength, and a substrate having a higher refractive index than the convex portion and formed on the concave-convex structure. An optical retardation element comprising a covering film,
A first coating film formed on an upper surface of the convex portion; a second coating film formed on a bottom surface of the concave portion of the concave-convex structure; and a first coating film formed on a side surface of the convex portion. 3 coating films,
The film thickness of the second coating film and the third coating film is thinner than the film thickness of the first coating film,
The first coating film includes an extension portion having a maximum width t1 wider than the width t in the periodic direction of the convex portion in the periodic direction, and the first coating film passes through the extension portion from the upper surface of the convex portion. It has a shape in which the width in the period direction smoothly changes to the top,
The convex portion occupancy t / T, which is the ratio of the width t in the periodic direction of the convex portion and the period T of the concave-convex structure, is any value from 0.4 to 0.6,
The first coating film occupation ratio t1 / T, which is a ratio of the maximum width t1 and the period T of the concavo-convex structure, is any value from 0.6 to 0.9. Phase difference element.

Images