JP2007122017A - Phase plate, optical element, and image projection apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new phase plate that facilitates manufacturing thereof and has satisfactory optical characteristics as the phase plate over a wide wavelength band, to provide a new optical element, and also to provide a new image projection apparatus that uses them. <P>SOLUTION: At least on one surface of a glass flat plate 11 of the phase plate, a thin layer 12a is formed by using materials having refractive indexes that are ≥1.6, and on the surface of the thin layer 12a, a fine irregular structure of cross-sectional rectangular wave-shapes due to lands and spaces is formed as a sub-wavelength structure. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a phase plate / optical element and an image projection apparatus.

  A phase plate known as one type of optical element has an optical function of providing a phase difference between mutually orthogonal polarization components, and is used in various optical devices. Conventionally, phase plates using artificial or natural rutile, calcite, quartz and other “uniaxial anisotropic crystals exhibiting birefringence” are known, but artificial ones “grow crystals uniformly. Natural crystals are “optically uniform and large in shape” and are difficult to obtain and expensive. Such a “phase plate using a crystal material” has a narrow operating wavelength range.

Recently, a phase plate having a sub-wavelength structure (SWS) has been proposed as a “phase plate capable of functioning over a wide wavelength range” (Patent Documents 1 and 2).
The optical action of the subwavelength structure is described in Non-Patent Document 1 and the like. A non-patent document 2 is known as an example of a sub-wavelength structure forming method.

  Patent Document 1 discloses a phase plate in which a sub-wavelength structure is formed as a “surface structure of a glass substrate”. Patent Document 2 discloses a “phase plate in which a lattice structure is transferred in a plane using a parallel plate as a substrate”, but does not describe at all the material of the portion to which the parallel plate or the lattice structure is transferred.

  Non-Patent Document 2 discloses a thermal nanoimprint method using a resin bulk material as a method of forming a subwavelength structure. This thermal nanoimprint method can form a sub-wavelength structure relatively easily, but considering the case of using PMMA as an example of a resin bulk material, there are the following problems.

  That is, the “phase difference given to polarization components orthogonal to each other” by the sub-wavelength structure is proportional to the aspect ratio of the sub-wavelength structure. The ratio is close to 10, and it is difficult for the resin to enter the mold sufficiently, making it difficult to form an accurate subwavelength structure.

  Also, when the sub-wavelength structure is formed by etching in the resin bulk of PMMA, since the aspect ratio is large, the etching species do not sufficiently enter the depth of the recess, and the formed cross-sectional shape tends to be tapered. That is, the fine pitch of the sub-wavelength structure and the high aspect ratio are in a trade-off relationship.

JP-A-2005-44429 JP 2005-106901 A Hisao Kikuta Structural birefringence and its application to optical elements Introduction to diffractive optical elements (May 20, 1997, first edition, first issue) p158-167 Optronics Publishing KONICA MINOLTA TECHNOLOGY REPORT VOL. 2 (2005) p97-100 “Fabrication of subwavelength structured broadband waveplates using nanoimprint technology”

  In view of the above, this invention is a novel phase plate that is easy to manufacture and has good optical properties (transmittance, retardation is substantially constant) as a phase plate in a wide wavelength band, a novel optical element, It is an object of the present invention to provide a novel image projection apparatus using these.

The phase plate of the present invention is as follows (claim 1).
That is, a thin layer made of a material having a refractive index of 1.6 or more is formed on at least one surface of a glass flat plate, and “a fine concavo-convex structure having a rectangular wave section with lands and spaces” is formed on the surface of the thin layer. It is formed as. That is, the sub-wavelength structure is one-dimensional, has a concavo-convex structure in one direction, and has a rectangular wave shape in cross section. The rectangular wave shape naturally means “approximate rectangular wave shape”. Although “land and space” will be described later, simply speaking, “land” is a portion corresponding to a convex portion in the fine uneven structure, and “space” is a portion corresponding to a concave portion in the fine uneven structure.

  Further, the fact that the fine concavo-convex structure having a rectangular wave cross section is “formed as a sub-wavelength structure” means that the pitch of the rectangular wave shape forming the fine concavo-convex structure is smaller than the wavelength used (the wavelength of light when used as a phase plate). However, since the phase plate of the present invention has a constant optical characteristic in a wide wavelength region, the wavelength used in this case means “the lower limit value of the wavelength region”.

  Since the thin layer made of a material having a refractive index of 1.6 or more is formed on “at least one surface of the glass flat plate”, it may be formed only on one surface of the glass flat plate or on both surfaces. The glass flat plate may be a “parallel flat plate glass” whose both surfaces are parallel, but may be a wedge-shaped flat plate. When forming a thin layer on both surfaces of a glass flat plate, the material of the thin layer formed on each surface may be the same, but may be different from each other on the condition that the refractive index is 1.6 or more.

  The phase plate according to claim 2 is characterized in that “the width of the land of the fine uneven structure is wider than the width of the space”.

The parameters characterizing the fine concavo-convex structure include pitch: P and fling factor: f. The “fine concavo-convex structure in which the land width is wider than the space width” in the phase plate according to claim 2 is the pitch of the fine concavo-convex structure: P, filling factor: f, conditions:
(1) 0.1 μm <P <0.4 μm
(2) 0.5 <f <0.8
Is preferably satisfied (Claim 3).

  The phase plate according to any one of claims 1 to 3 can have a configuration in which an antireflection film is formed on a fine relief structure (claim 4). The “configuration in which the antireflection film is formed on the fine concavo-convex structure” is a configuration in which the antireflection film is formed on the top of the convex portion and the bottom of the concave portion of the fine concavo-convex structure.

  The phase plate according to any one of claims 1 to 3 may also be “a configuration in which an antireflection film is formed only on a land having a fine relief structure” (claim 5). The “configuration in which the antireflection film is formed only on the land having the fine concavo-convex structure” is a configuration in which the antireflection film is formed on “only the top of the convex portion of the fine concavo-convex structure”.

The optical element according to claim 6 is formed by integrating the phase plate according to any one of claims 1 to 5 on “a flat surface of an optical substrate having an optical function such as a prism function and a lens function”.
The optical element according to claim 7 is provided such that a thin layer made of a material having a refractive index of 1.6 or more is formed on the surface of an optical substrate having an optical function such as a prism function and a lens function. And a fine concavo-convex structure having a rectangular wave cross section by a space ”, and the fine concavo-convex structure functions as a phase plate for the optical substrate. Since the “thin layer” is formed on the “surface of the optical substrate having an optical function”, the “surface on which the thin layer is formed” of the optical substrate is not limited to a flat surface but may be a curved surface such as a lens surface.

Also in the optical element according to claim 7, “the width of the land in the fine uneven structure is preferably wider than the width of the space (claim 8). In this case also, the pitch P of the fine uneven structure and the filling factor f are It is preferable that the above conditions (1) and (2) are satisfied.
Also in the optical element according to claim 7 or 8, it can be set as “a structure in which an antireflection film is formed on a fine uneven structure in a thin layer” (claim 9). “A configuration in which an antireflection film is formed only on a land, that is, a convex portion” may be adopted.

  The optical element according to any one of claims 6 to 10, wherein the optical substrate includes a polarization separation surface that divides incident light into reflected light and transmitted light whose polarization surfaces are orthogonal to each other, and the reflected light and transmitted light. A reflective surface that reflects one of them and faces in the same direction as the other traveling direction, and has a refractive index of 1.6 or more on the surface side where the transmitted light and the reflected light that are aligned in the same direction are emitted. A thin layer made of a material is arranged, and a fine concavo-convex structure having a rectangular wave cross-section with a land space formed as a sub-wavelength structure on the thin layer changes the polarization plane of at least one of reflected light and transmitted light. It can be set as the structure which has a function as a phase plate which makes the polarization planes correspond.

  That is, a thin layer made of a material having a refractive index of 1.6 or more may be directly formed and disposed on the “surface on which transmitted light and reflected light that are aligned in substantially the same direction are emitted” of the optical substrate. The phase plate according to any one of Items 1 to 5, wherein the phase plate is formed on one or both sides of a flat glass plate, and the phase plate is integrated with the above-described “surface on which transmitted light and reflected light are aligned in substantially the same direction”. It may be arranged in an integrated manner by the converting means. When a thin layer is formed directly on the “surface on which transmitted and reflected light aligned in substantially the same direction” of the optical substrate, “transmitted light and reflected light aligned in approximately the same direction are emitted. The “surface” is not a flat surface, but may be a curved surface such as a lens surface.

  An image projection apparatus according to claim 12 is an image projection apparatus that guides a light beam from a light source to a liquid crystal display element and projects a display image of the liquid crystal display element onto a display surface by a projection lens. 5. The phase plate according to any one of 5 and / or the optical element according to any one of claims 6 to 11 is used. The “display surface” is usually a screen.

  The image projection apparatus according to claim 12 includes three liquid crystal display elements that individually form images of the respective colors corresponding to the three primary colors, and a cross prism that combines the image lights of the respective colors emitted from the respective liquid crystal display elements. The phase plate according to any one of claims 1 to 5, wherein one or more of the three optical paths between each liquid crystal display element and the cross prism is a "phase plate corresponding to a half-wave plate". The optical element according to any one of 6 to 10 may be arranged (claim 13).

The image projection apparatus according to claim 13 also synthesizes three reflective liquid crystal display elements that individually form images of each color corresponding to the three primary colors, and video light of each color emitted from each of the reflective liquid crystal display elements. 6. The apparatus according to claim 1, comprising a cross prism, wherein one or more of the three optical paths between each reflective liquid crystal display element and the cross prism is a “phase plate corresponding to a quarter-wave plate”. A phase plate or an optical element according to any one of claims 6 to 10 may be arranged (claim 14).
The image projection apparatus according to the thirteenth or fourteenth aspect can include the optical element according to the eleventh aspect as "an optical element that aligns light from a light source with linearly polarized light" (claim 15).

“Inorganic materials” such as TiO ₂ , Nb ₂ O ₅ , Ta ₂ O ₅ , ZrO ₂ , ITO (SnO ₂ + In ₂ O ₅ ), etc. , TiO ₂ , ZrO ₂ , Sb ₂ O ₅ , ITO, Al ₂ O _3, etc. are combined in the “sol-gel material”, and further, in the sol-gel material having SiO ₂ as a skeleton A “mixed material” in which fine particles (5 nm to 100 nm) of the inorganic material are dispersed, or a photo-curing resin or a thermosetting resin having a refractive index of 1.6 or more can be used. The mixed material can be mixed and blended according to the characteristics. As the photocurable resin or thermosetting resin, for example, “excellent optical characteristics” including a light transmitting adhesive is used.

  The inorganic material forms a thin layer by a film forming technique such as sputtering or vapor deposition. A sol / gel material, a mixed material, a photocurable resin, or a thermosetting resin forms a thin layer on a glass plate by spin coating or the like. The “thin layer made of an inorganic material” has a heat resistance of 200 ° C. or higher and is suitable as a thin layer for a phase plate or an optical element used in a high temperature environment.

  The fine concavo-convex structure formed by lands and spaces is formed, for example, by forming a “resist layer on which a latent image is formed by scanning an electron beam” on a thin layer, and forming a “pattern corresponding to the fine concavo-convex structure” on this resist. A latent image is formed by drawing with an electron beam and developed to obtain a “resist pattern corresponding to a fine concavo-convex structure”. By using this resist pattern as a mask, etching such as RIE (reactive ion etching) is used to form a “thin layer Is etched to a desired groove depth ".

  Further, when the thin layer is composed of a thermosetting resin, it can be formed by a thermal nanoimprint method described in Non-Patent Document 2.

  As described above, in the phase plate and optical element of the present invention, since the thin layer configured as the sub-wavelength structure is configured of “a material having a refractive index of 1.6 or more”, the phase plate or the optical element is provided with a level that can be given to transmitted light. Even if the phase difference is increased, the aspect ratio of the fine concavo-convex structure does not become excessive, and manufacturing is easy. In particular, when forming a “thin layer having a fine concavo-convex structure with a material having a refractive index of 1.6 or more” on both surfaces of a parallel flat glass plate, the aspect ratio of the fine concavo-convex structure formed on the thin layer on each surface is Further, it can be made smaller than when it is formed only on one side.

  As in the examples described later, the transmittance and retardation of the thin layer in the phase plate and optical element of the present invention are constant over a wide wavelength region.

  In the optical element of the present invention, the optical substrate itself has a lens function, a prism function, or an optical function as set forth in claim 11, and these optical functions and a function as a phase plate are combined to provide a composite optical function. realizable.

  By using the phase plate and optical element of the present invention, an image projection apparatus having a good image projection function can be realized.

Embodiments of the invention will be described below.
FIG. 1 is a diagram for explaining one embodiment of a phase plate.
The phase plate of this embodiment has a configuration in which thin layers 12a and 12b are formed on both surfaces of a parallel flat glass plate 11 as shown in FIG.
The thin layers 12a and 12b are formed of a material having a refractive index of 1.6 or more, and “a fine concavo-convex structure having a rectangular wave section with lands and spaces” is formed as a sub-wavelength structure.

Here, for the following explanation, terms will be explained with reference to FIG.
FIG. 1B schematically illustrates a “fine concavo-convex structure having a rectangular wave shape in cross section” formed in a thin layer. The unevenness of the fine unevenness structure has a “rectangular wave shape” in cross-sectional shape, and such rectangular wave-like unevenness is formed in a uniform cross-sectional shape in a direction orthogonal to the drawing. Accordingly, the convex portion in the fine concavo-convex structure forms a long “projection” in a direction orthogonal to the drawing, and the concave portion forms a long “concave” in a direction orthogonal to the drawing. The convex portion forming the ridge is called “land”, and the concave portion forming the ridge is called “space”.

  As shown in the figure, the pitch P of the fine concavo-convex structure having a rectangular wave cross section is a pair of land and space land width (hereinafter referred to as “land width” as shown): a and space width ( Hereinafter, it is referred to as “space width” as shown in the drawing): sum of b: (a + b). The height of the land with respect to the bottom of the space is defined as “groove depth: H”.

  At this time, the filling factor is “a / P” and the aspect ratio is “H / a”. That is, “the filling factor is large” means that the land width: a occupying the pitch: P is large (the space width: b is small). As the “aspect ratio is large”, the groove depth with respect to the land width: a. : H is large. The aspect ratio is preferably “less than 10”, more preferably about 5 or less, from the viewpoint of easy formation of a fine relief structure.

  As known from Non-Patent Documents 1 and 2 and the like, if the fine concavo-convex structure is a sub-wavelength structure, light having a wavelength larger than the pitch is not diffracted and is transmitted as it is as “0th-order light” (at this time The transmittance is referred to as “0th-order transmittance”) and exhibits birefringence with respect to incident light.

  That is, as shown in FIG. 1C, in the incident light “incident from the air region” to the fine concavo-convex structure, polarization components that vibrate in parallel to the periodic direction (left-right direction in the figure) of the fine concavo-convex structure: TM, land The fine concavo-convex structure acts like a “medium having a different refractive index” with respect to a polarized light component TE that vibrates parallel to the longitudinal direction (direction orthogonal to the drawing).

Assuming that the effective refractive index in the fine concavo-convex structure portion is n (TM) for the polarization component: TM and n (TE) for the polarization component: TE, these effective refractive indexes are the values of the thin layer on which the fine concavo-convex structure is formed. It is expressed as follows using the refractive index of the material: n and the filling factor of the fine relief structure: f.
n (TE) = √ {fn ² + (1−f)} (1)
n (TM) = √ [n ² / {f + (1−f) n ² }] (2)
For this reason, the phase of the polarization component TE in the transmitted light is delayed by “δ” as shown in FIG.

That is, when the groove depth: H is used, the “optical thickness” of the fine concavo-convex structure is “H · n (TM)” for the polarization component: TM and “H · n” for the polarization component: TE. (TE) ", a“ phase delay: δ ”is generated according to the difference in optical thickness: H {n (TE) −n (TM)}. This “phase delay: δ” is “retardation”.
Difference in optical thickness: where H {n (TE) −n (TM)} is D and wavelength is λ, δ = 2πD / λ, but in the fine concavo-convex structure, “wavelength: wide region of λ” A substantially constant retardation "is obtained.

  n (TE) and n (TM) are determined by the refractive index of the thin layer material: n and the filling factor: f. Retardation: δ is the refractive index: n, the filling factor: f, and the groove depth: H. As a result, retardation can be obtained by adjusting the material of the thin layer (n is determined) and the form of the fine concavo-convex structure (filling factor: f and groove depth: H are determined). Can do.

  When fine concavo-convex structures are formed on both surfaces of the glass flat plate 11 as in the embodiment shown in FIG. 1A, the retardation by the fine concavo-convex structure 12a is “δ1”, and the retardation by the fine concavo-convex structure 12b is “δ2”. Then, the retardation as the phase plate is “δ1 + δ2”.

  Therefore, in the case of the phase plate shown in FIG. 1A, by adjusting the retardation: δ1 + δ2, the phase difference with respect to the polarization components TM, TE can be set to, for example, “π or π / 2”, and various wavelengths can be set. A board can be realized.

  FIG. 2 shows a “relation between the refractive index of the thin layer material and the aspect ratio” for realizing “desired retardation (eg,“ π / 2 ”)” by the fine uneven structure formed in the thin layer. . It can be seen that, in order to realize the desired retardation, “the smaller the refractive index of the thin layer material: n, the larger the aspect ratio is required”.

  If the aspect ratio is 10 or less as described above, the refractive index of the thin layer material needs to be 1.6 or more because of the ease of forming the fine uneven structure. That is, in claim 1, when the refractive index of the thin layer material is 1.6 or more, the aspect ratio is 10 or less and the formation of a fine uneven structure is facilitated (desired pitch: P, filling factor: f, Groove depth: a condition for reliably forming a fine relief structure with H).

  In the case shown in FIG. 2, a thin layer material having a refractive index of 2.0 or more is required to set the aspect ratio to “more preferably about 5 or less”.

FIG. 3 shows the relationship between the groove depth H (horizontal axis: μm unit) and retardation: δ (vertical axis: wavelength unit) with the refractive index: n and the filling factor: f as parameters. Yes. It can be seen that the retardation: δ increases linearly as the groove depth: H increases.
This is because the difference between the “optical thickness” of the fine concavo-convex structure with respect to the polarization components: TM and TE: H {n (TE) −n (TM)} is a linear function of the groove depth: H. .

  FIG. 4 shows the relationship between the “polarization direction (unit: degree) of incident light on the fine concavo-convex structure” and the “0th-order transmittance” when the pitch P of the fine concavo-convex structure is used as a parameter. The refractive index of the thin layer material at this time is n = 2.25.

  When a phase plate as an optical element having a fine concavo-convex structure is viewed from the viewpoint of optical characteristics, it is preferable that the 0th-order transmittance is large and “the dependency on the polarization direction of incident light is small”. When such a preferable characteristic as an optical function is assumed to be “the range of the polarization direction of incident light: in the range of −180 degrees to +180 degrees, 0th-order transmittance: 0.8 or more”, from FIG. It can be seen that it should be 0.4 or less.

  FIG. 5 shows the refractive index of the thin layer material: n = 2.29 (wavelength: 420 nm), 2.23 (wavelength: 470 nm), 2.20 (wavelength: 520 nm), 2.17 (wavelength: 570 nm), 2 .16 (wavelength: 620 nm), 2.14 (wavelength: 700 nm), when the groove depth in the fine relief structure is H = 1 μm, the filling factor is f = 0.7, and the pitch is changed with P as a parameter Retardation: how δ varies with wavelength. In order to provide a phase plate with stable retardation in a wide wavelength region, it is desired that “a change in retardation is small and substantially constant” in the wavelength range of the horizontal axis.

  This condition is satisfied when the parameter pitch: P is in the range of 0.2 to 0.4.

  Therefore, it is understood that “a phase plate having a substantially constant retardation in a wide wavelength range” can be obtained by satisfying the condition (1) for the pitch: P and the filling factor: f described above.

  6 shows refractive indexes: n = 2.29 (wavelength: 420 nm), 2.23 (wavelength: 470 nm), 2.20 (wavelength: 520 nm), 2.17 (wavelength: 570 nm), 2.16 (wavelength). : 620 nm), 2.14 (wavelength: 700 nm) on a thin layer with a pitch: P = 0.3 μm and a groove depth: H = 1 μm. , 420 nm, 470 nm, and 520 nm (FIG. 6A), when changed to 520 nm, 570 nm, and 620 nm, and when changed to 620 nm and 700 nm (FIG. 6B). (C)) shows the relationship between the filling factor (horizontal axis) and retardation (vertical unit of vertical axis).

Assuming that the wavelength region used is 420 nm to 700 nm, in order to obtain a retardation (retardation variation: Δδ <± 10%) that can be regarded as substantially the same with respect to incident light in this wavelength region, the filling factor of A range of 0.5 to 0.8 (a range surrounded by a broken line in FIGS. 6A to 6C) is preferable. That is,
0.5 <f <0.8
This is the significance of the condition (2).

  By satisfying the conditions (1) and (2), it is possible to realize a phase plate with retardation variation: Δδ <± 10% in the range of polarization direction of incident light: ± 180 degrees, zero-order transmittance: 80%.

  The material of the thin layer in the phase plate of the present invention has a high refractive index of 1.6 or more. For this reason, since the refractive index difference with the air in contact with the fine concavo-convex structure is large, reflection on the surface of the thin layer tends to be large.

As in the phase plate according to claim 4, it is preferable to suppress reflection on the surface of the thin layer by forming an antireflection film on the fine concavo-convex structure.
FIG. 7 is a view for explaining an embodiment in which an antireflection film is formed in this way.
The thin layer 70 shown in FIG. 7A is formed of a material having a refractive index: n = 2.25, and the fine uneven structure has a pitch: P = 0.22 μm, a groove depth: H = 1.86 μm, and a filling factor. : F = 0.7. An antireflection film 71 is formed on the fine concavo-convex structure 70, that is, on the land of the fine concavo-convex structure 70 and the bottom of the space.

The antireflection film 71 has a four-layer structure, and as shown in FIG. 7B, the first layer counted from the thin layer side is a “high refractive index layer having a refractive index of 2.35 and a thickness of 76 nm. The second layer has a refractive index of 1.46 and a thickness of 7 nm, a “low refractive index layer”, the third layer has a refractive index of 2.35 and a thickness of 18 nm, a “high refractive index layer”, the fourth The layer is a “low refractive index layer” having a refractive index of 1.46 and a thickness of 78 nm.
FIG. 16A shows the zero-order transmittance of the polarization component: TE (“TE direction” in FIG. 16) when no antireflection film is formed. “R” is a red wavelength region, “G” is a green wavelength region, and “B” is a blue wavelength region. As shown in the legend, the incident angles are 0 degrees, 5 degrees, 10 degrees, and 15 degrees. When the incident angle is 0 degree, the wavelength is 480 nm or less, and the 0th-order transmittance is 80% or less. FIG. 16B shows the zero-order transmittance of the polarization component: TM (“TM direction” in FIG. 16).

  In each case of FIGS. 16A and 16B, the polarization component when the antireflection film (“AR coating” in the drawing) having the configuration shown in FIG. 7B is formed on the fine concavo-convex structure: FIG. 16C shows the zero-order transmittance of TE, and FIG. 16D shows the zero-order transmittance of the polarization component: TM.

As shown in FIGS. 16 (c) and 16 (d), the zero-order transmittance is 88% or more in the range of wavelength: 420 nm to wavelength: 700 nm (B to R wavelength region) for both polarization components: TE and TM. .
In FIGS. 16A and 16B, there are cases where the 0th-order transmittance decreases in a “whisker shape” on each short wavelength side of each color wavelength region indicated by reference characters R, G, and B. This portion is where the 0th order transmittance is reduced by the “first order diffracted light”. The “whisker-shaped portion” in which the 0th-order transmittance decreases exists in each short wavelength side of the B and G wavelength regions in FIG. 16C, and also in FIG. Although slightly present on the wavelength side, “a high zero-order transmittance of about 99% or more” can be secured in FIGS. 16C and 16D for such “region where no first-order diffracted light is generated”. ing.

  FIGS. 16C and 16D show the case where the antireflection film is provided on the land and the space of the fine convex structure as shown in FIG. 7, but the antireflection film is “only on the land”. The effect similar to the above can be acquired also by providing.

  That is, FIGS. 17C and 17D show polarization components: TE and TM (TE direction and TM direction in FIG. 17) when the same antireflection film as described above is formed only on the land. FIGS. 17A and 17B are the same as FIGS. 16C and 16D.

  As is clear from comparison between FIGS. 17A and 17C and FIGS. 17B and 17D, the zero-order transmittance of the polarization components: TE and TM is that the antireflection film is placed only on the land. Even when it is formed (although slightly inferior to the case where the antireflection film is formed on the land and the space), the zero-order transmittance of 88% or more can be secured.

  FIG. 18 is a diagram for explaining “the influence on the phase difference caused by the formation of the antireflection film”. FIG. 18A is a diagram illustratively showing the “fine concavo-convex structure”. Pitch: P = 0.2 μm, Filling factor: f = 0.8, Groove depth: H = 0.3 μm, Refractive index: 2.15. Light having wavelength: λ = 660 nm is incident. Retardation generated when this is done is 0.11λ.

  FIG. 18B is a diagram for explaining the antireflection film. As shown in the figure, the first antireflective film having a refractive index of 1.4 and a thickness of 100 nm is formed on the surface of a parallel plate having a refractive index of 2.15 from the air side. Layer, refractive index: 2.3, thickness: 30 nm, second layer, refractive index: 1.4, thickness: 10 nm, third layer, refractive index: 2.3, thickness: 80 nm In this four-layer structure, an antireflection film having a pitch: P = 0.2 μm and a filling factor: f = 0.8 is formed. The retardation generated when light having a wavelength of λ = 660 nm is incident on the antireflection film is 0.02λ.

  FIG. 18C shows a case where the antireflection film having the four-layer structure described in conformity with FIG. 18B is formed only on the land having the fine concavo-convex structure shown in FIG. In the same manner as described above, the retardation when light having a wavelength of λ = 660 nm is incident is 0.19λ.

  FIG. 18D shows a case where the antireflection film having the four-layer structure described in accordance with 18B is formed on the land and space having the fine concavo-convex structure shown in FIG. Retardation when λ = 660 nm light is incident is 0.12λ. In the case of FIG. 18C in which the antireflection film is formed only on the land, the retardation is increased as compared with the case of FIG. 18D in which the antireflection film is formed on the land and the space. Is possible.

  As described above, even when the antireflection film is formed only on the land, an antireflection function is exhibited, and it is possible to increase the retardation while ensuring a high zero-order transmittance. In addition, such a “configuration for forming an antireflection function only on a land” refers to other sub-wavelength structure elements (polarization-selective diffractive elements or certain specific ones) described in Non-Patent Documents 1 and 2. The anti-reflection function can be secured in the same manner with respect to a filter that transmits light only in the wavelength region.

  In addition, as a method of forming an antireflection function only on the land, after forming a fine concavo-convex structure with the above-mentioned “high refractive index thin layer”, the space is once filled with a “temporary member” for reflection. A method of depositing an anti-reflection film and then removing the “anti-reflection film deposited on the temporary member in the space together with the filled member” may be used.

Hereinafter, embodiments of the image projection apparatus will be described.
FIG. 8 shows an embodiment of the image projection apparatus according to claims 13 and 14, and three liquid crystal display elements 110, 111, and 112 that individually form images of respective colors corresponding to the three primary colors, and the respective liquid crystals. A cross prism 113 is provided for synthesizing video light of each color emitted from the display element. Phase plates 116, 117, and 118 corresponding to half-wave plates are provided in three optical paths between each liquid crystal display element and the cross prism 113. Have.
White light emitted from the white light source 101 is reflected by the reflector 102 and enters the dichroic mirror 103. The dichroic mirror 103 transmits light having a blue wavelength or less and reflects light having a wavelength longer than the blue wavelength. Accordingly, of the white light incident on the dichroic mirror 103, the blue component is transmitted through the dichroic mirror 103, and the green component and the red component are reflected by the dichroic mirror 103 and incident on the dichroic mirror 104.

  The dichroic mirror 104 transmits light having a wavelength longer than the red wavelength and reflects light having a wavelength shorter than the red wavelength. Accordingly, among the light incident on the dichroic mirror 104, the green component is reflected by the dichroic mirror 104 and the red component is transmitted through the dichroic mirror 104. In this manner, the white light from the white light source 101 is color-separated by the dichroic mirrors 103 and 104 into three primary color component lights of red, green, and blue.

  The “blue component light” transmitted through the dichroic mirror 103 is reflected by the mirror 105 and enters the liquid crystal display element 110, and the “green component light” reflected by the dichroic mirror 104 enters the liquid crystal display element 111. Further, the “red component light” transmitted through the dichroic mirror 104 enters the liquid crystal display element 112 along an optical path constituted by the relay lens 108, the mirror 106, the relay lens 109, and the mirror 107. The relay lenses 108 and 109 perform optical path length correction of red component light.

The liquid crystal display elements 110, 111, and 112 have “a liquid crystal layer sandwiched between a pair of polarizers”, and the pair of polarizers sandwiching the liquid crystal layer have their polarization directions orthogonal to each other.
Each color component light becomes linearly polarized light and enters the liquid crystal layer when passing through the incident side polarizer of the corresponding liquid crystal display element. Image signals are applied to the liquid crystal display elements 110, 111, and 112 so as to display a blue image, a green image, and a red image, respectively, and light transmitted through the liquid crystal layer at the position of “image pixel to be projected” is polarized. The surface rotates 90 degrees, and has the same polarization direction as that of the exit-side polarizer and passes through the exit-side polarizer.

  In this way, the liquid crystal display element 110 emits “blue component light that is two-dimensionally intensity-modulated in accordance with a blue image (hereinafter referred to as“ blue image light ”). Similarly, the liquid crystal display element 111 emits “green component light that is two-dimensionally intensity-modulated in accordance with a green image (hereinafter referred to as“ green image light ”), and the liquid crystal display element 112 emits a“ red image. Accordingly, red component light (hereinafter referred to as “red image light”) that is two-dimensionally intensity-modulated is emitted. That is, the liquid crystal display elements 110, 111, and 112 individually form images corresponding to the three primary colors (blue, green, and blue).

Each color image light emitted from each liquid crystal display element has a polarization direction parallel to the plane of the drawing.
Blue video light emitted from the liquid crystal display element 110 is incident on the phase plate 116, and green video light and red video light emitted from the liquid crystal display elements 111 and 112 are incident on the phase plates 117 and 118, respectively.

  The phase plates 116, 117, and 118 are “corresponding to a half-wave plate” and give a phase difference of ½ wavelength to two orthogonal components of transmitted light. Since each color image light incident on these phase plates is polarized in a plane parallel to the drawing as described above, the transmitted light is rotated 90 degrees from the direction of polarization of the transmitted light in a direction orthogonal to the drawing. It becomes a polarized light beam and enters the cross prism 113 from the corresponding surface.

  The cross prism 113 is a rectangular parallelepiped made of a light-transmitting material having a square cross-sectional shape when viewed from a direction orthogonal to the drawing, and has reflection surfaces 113a and 113b orthogonal to each other. The reflective surface 113a is “a dichroic mirror that reflects light having a wavelength less than or equal to the blue wavelength and transmits light having a wavelength longer than the blue wavelength”, and the reflective surface 113b is “reflects light having a wavelength greater than or equal to the red wavelength, It is a “dichroic mirror that transmits light having a wavelength shorter than the red wavelength”.

  The reflecting surfaces 113a and 113b have directionality with respect to the “polarization axis of the reflected light”. The reflecting surface 113a reflects light of a polarization component orthogonal to the drawing (blue image light) out of light having a blue wavelength or less, but transmits green image light and red image light of the polarization component orthogonal to the drawing well. Let The reflecting surface 113b reflects red image light having a polarization component orthogonal to the drawing well, and transmits blue image light and green image light having a polarization component orthogonal to the drawing well.

  Of the color image lights incident on the cross prism 113, the blue image light is reflected on the reflection surface 113a, the red image light is reflected on the reflection surface 113b, and the green image light is transmitted through the reflection surfaces 113a and 113b. The colors are synthesized and emitted from the cross prism 113. The emitted light beam is incident on the projection lens 114 and is enlarged and projected on the screen 115 which is a “display surface” by the projection lens 114 to display a projected image.

  FIG. 9 is a modification of the embodiment shown in FIG. In order to avoid confusion, the same reference numerals as those in FIG. 8 are given to those which are not likely to be confused, and the description of FIG.

  The image projection apparatus in FIG. 9 is obtained by adding a uniform illumination means 201 (optical integrator) and a polarization conversion element 202 between the white light source 101 and the dichroic mirror 103 to the image projection apparatus in FIG.

  The uniform illumination means 201, which is an “optical integrator”, is a means for making the amount of light irradiated to the liquid crystal display element substantially uniform. For example, a well-known device composed of, for example, a fly-eye lens, a rod lens, a rectangular lens array, or the like is appropriately used. it can.

  The polarization conversion element 202 is used to effectively use the light from the white light source 101 and align the polarization direction of the light incident on the liquid crystal display element in order to brighten the projected image on the screen.

  The liquid crystal display elements 110, 111, and 112 use the polarization characteristics of the liquid crystal and can achieve high contrast. However, since the liquid crystal is sandwiched between a pair of polarizers, the embodiment shown in FIG. As described above, when the irradiation light incident on each liquid crystal display element is in a naturally polarized state, half of the amount of illumination light is blocked when passing through the “incident side polarizer” of each liquid crystal display element. As a result, the light utilization efficiency is poor.

  The polarization conversion element 202 converts the polarization state of the illumination light from the light source side from “natural polarization state to linear polarization state while substantially preserving the light intensity of the illumination light”. If the polarization direction of the linearly polarized illumination light is aligned with the polarization direction of the incident-side polarizer in the liquid crystal display element, approximately 100% of the illumination light from the light source side can be used for displaying a projected image. Can do.

  FIG. 10 is a diagram for explaining a state in which the white light beam from the light source is made uniform by the uniform illumination unit 201 and the polarization state is converted by the polarization conversion unit 202.

  The white light beam from the light source side is transmitted through a known uniform illumination unit 201 “configured by arranging a pair of condensing lens arrays (fly-eye lens arrays) to face each other” and enters the polarization conversion unit 202.

The polarization conversion means 202 constitutes an embodiment of the optical element according to claim 11 and has an optical base 202A and a phase plate portion 202B.
As shown in FIG. 10A, the portion of the optical base 202A alternately has polarization separation surfaces 2021 and reflection surfaces 2022 inclined by 45 degrees with respect to the optical axis of the illumination light.

  The polarization separation surface 2021 divides incident light into reflected light S (hereinafter referred to as “S component”) and transmitted light P (hereinafter referred to as “P component”) whose polarization surfaces are orthogonal to each other. The reflecting surface 2022 reflects the S component and directs it in the direction substantially the same as the traveling direction of the P component.

  The combination of the polarization separation surface 2021 and the reflection surface 2022 is one unit, and a plurality of such units are provided over the transmission region of the illumination light to form a so-called “polarization prism lens array”, and the individual elements constituting the array For each polarizing prism, the transmitted illumination light is divided into an S component and a P component.

  The phase plate portion 202B rotates the polarization plane of the S component emitted from the optical base 202A by 90 degrees and aligns it with the polarization direction of the P component. In this way, linearly polarized illumination light having a uniform polarization direction is obtained. The polarization direction of the illumination light is of course the same as “the polarization direction of the incident-side polarizer of each liquid crystal display element”.

  FIG. 10B illustrates the phase plate portion 202B. The phase plate portion 202B has a sub-wavelength of fine concavo-convex structures 2021B, 2022B, and 2023B having a rectangular cross section on the surface of a thin layer 202B2 made of “material having a refractive index of 1.6 or more” formed on one surface of a glass flat plate 202B1. The fine concavo-convex structure 2021B is provided with a function as a “phase plate corresponding to a 2/1 wavelength plate”. The glass flat plate 202B1 is integrated with “a surface on the side where illumination light with a uniform direction is emitted” in the optical base 202A, and the fine concavo-convex structure 2021B and the like are formed on “a portion where the S component is emitted”. The phase plate portion 202B and the optical base body 202A may be joined with the thin layer 202B2 on the side of the optical base body 202A. In this way, the fine uneven structure can be better protected by the glass flat plate 202B1.

  Further, the thin layer 202B2 may be directly formed on the optical base 202A without using the glass flat plate 202B1.

  FIG. 11 is a view for explaining one embodiment of the image projection apparatus according to claim 14. In order to avoid complications, the same symbols as in FIG.

  When the white light emitted from the white light source 101 is reflected by the reflector 102 and enters the dichroic mirror 103, the blue component light is separated and transmitted through the dichroic mirror 103, and the green component light and the red component light are reflected to reflect the dichroic mirror. 104 is incident.

  The green component light is reflected by the dichroic mirror 104, and the red component light is transmitted through the dichroic mirror 104. In this way, white light is separated into three primary colors of blue, green, and red. The blue component light is reflected by the mirror 105 and enters the polarization beam splitter 301, and the S polarization component is reflected and enters the reflective liquid crystal display element 304 via a phase plate 307 corresponding to a quarter wavelength plate.

  The green component light enters the polarization beam splitter 302, the S-polarized component is reflected, and enters the reflective liquid crystal display element 305 through a phase plate 308 corresponding to a quarter wavelength plate. The red component light enters the polarization beam splitter 303, the S-polarized component is reflected, and enters the reflective liquid crystal display element 306 via a phase plate 309 corresponding to a quarter wavelength plate.

  Each color component light passes through the corresponding phase plate to become circularly polarized light and enters the corresponding reflective liquid crystal display element. Each of the reflective liquid crystal display elements 304, 305, and 306 modulates reflected light according to an image signal, and converts the modulated image light into circularly polarized light that is reverse to the incident light. By passing through the phase plates 307, 308, and 309, the polarization plane rotates 90 degrees from the forward state, and becomes a P-polarized component with respect to the corresponding polarizing beam splitters 301, 302, and 303, and the polarizing beam splitter 301 , 302, 303, and enters the cross prism 113, and is color-combined and emitted from the cross prism 313 in the same manner as in the embodiment of FIG. 8, and is enlarged and projected as a color image on the screen 115 by the projection lens 114. . In this way, an image is displayed on the screen 115.

  Of course, in the embodiment of FIG. 11 as well, the uniform illumination means 201 (optical integrator) and the polarization conversion element 202 are added between the white light source 101 and the dichroic mirror 103 as in the embodiment of FIG. Thus, the light use efficiency can be increased.

  Specific examples of the phase plates 116, 117, and 118 in the embodiment of the image projection apparatus shown in FIGS.

Each of the phase plates 116, 117, and 118 is made of parallel plate-like quartz glass having a thickness of 1.0 mm and a refractive index of 1.45 as “glass flat plate”, and “refractive index of 1.6 or more is provided on both surfaces thereof”. “TiO ₂ (refractive index: 2.25)” is used as the “material”, thin layers having a thickness of 1 μm are formed by sputtering, and a fine concavo-convex structure having a rectangular wave cross section is formed on the surface of these thin layers as a subwavelength structure. Formed.

The fine concavo-convex structure forms a “resist layer on which a latent image is formed by scanning an electron beam” on a thin layer, and a pattern corresponding to the fine concavo-convex structure is drawn on the resist by an electron beam to form a latent image. This was developed to obtain a “resist pattern corresponding to the fine concavo-convex structure”, and the thin layer was etched to a desired groove depth by RIE (reactive ion etching) using this resist pattern as a mask.
The specifications and characteristics of each phase plate are as follows.

Phase plate 116 (phase plate equivalent to a half-wave plate for blue image light)
Fine uneven structure on the surface of the thin layer Pitch: P = 0.20 μm
Filling factor: f = 0.6
Land width: a = 0.120 μm
Space width: b = 0.080 μm
Groove depth: H = 0.380 μm
optical properties
Wavelength used: 420-520 nm
Retardation: 50 ± 3% (50% is phase difference: π)
Zero-order transmittance: 0.88 ± 7% (polarization component: TE)
0.93 ± 7% (polarization component: TM).

Phase plate 117 (phase plate equivalent to a half-wave plate for green image light)
Fine uneven structure on the surface of the thin layer Pitch: P = 0.22 μm
Filling factor: f = 0.7
Land width: a = 0.154 μm
Space width: b = 0.066 μm
Groove depth: H = 0.543 μm
Optical properties Wavelength used: 520-620 nm
Retardation: 50 ± 3% (50% is phase difference: π)
Zero-order transmittance: 0.88 ± 9% (polarization component: TE)
0.93 ± 5% (polarization component: TM).

Phase plate 118 (a phase plate equivalent to a half-wave plate for red image light)
Fine uneven structure on the surface of the thin layer Pitch: P = 0.30 μm
Filling factor: f = 0.5
Land width: a = 0.150 μm
Space width: b = 0.150 μm
Groove depth: H = 0.475 μm
Optical properties Wavelength used: 620-700 nm
Retardation: 50 ± 4% (50% is phase difference: π)
Zero-order transmittance: 0.88 ± 5% (polarization component: TE)
0.93 ± 4% (polarization component: TM).

  FIG. 12 shows “retardation characteristics” of the phase plates 116, 117, and 118 in the first embodiment. Stable retardation is secured in each wavelength region of the three primary colors (blue (B): 420 to 520 nm, green (G): 520 to 620 nm, red (R): 620 to 700 nm).

“0th-order transmittance” of the phase plates 116, 117, and 118 is shown in FIG. A zero-order transmittance of 80% or more can be secured in the range of the three primary colors. FIG. 14 shows the retardation in each of the phase plates when the incident angle is 0 and 10 degrees. FIG. 15 shows how the zero-order transmittance varies with the incident angle for each of the phase plates.
When the liquid crystal display element is disposed immediately after the liquid crystal display element, such as the phase plates 116, 117, and 118, it is only necessary to estimate the change in the incident angle within a range of ± 10 degrees. .

  In Example 1, although the thin layer in the phase plate is formed on both surfaces of the glass flat plate, the thin layer is formed only on one surface of the glass flat plate, and the groove depth of the fine unevenness formed in this thin layer: H may be twice that of the first embodiment (the sum of the groove depths of the thin layers on both sides).

  In addition, if the thin layer on one side of the thin layers on both sides of the flat glass formed in Example 1 is omitted, the phase plate is “equivalent to a quarter-wave plate”, and the phase plate in the embodiment of FIG. 307, 308, and 309 can be used.

  8 and 9 show a case in which the polarization axis needs to be rotated by 90 degrees for all three colors, but only 1/2 for any one of the three primary colors. A configuration requiring a phase plate equivalent to a wave plate is also possible.

It is a figure for demonstrating one Embodiment of a phase plate. It is a figure for demonstrating the relationship between the refractive index of a thin layer material, and an aspect ratio. It is a figure for demonstrating the relationship between a groove depth and retardation. It is a figure for demonstrating the relationship between the polarization direction of the incident light to a fine grooving | roughness structure, and 0th-order transmittance | permeability. It is a figure which shows how retardation changes with a wavelength by using the pitch of a fine uneven structure as a parameter. It is a figure which shows the relationship between a filling factor and retardation. It is a figure for demonstrating embodiment which formed the anti-reflective film. It is a figure for demonstrating one Embodiment of an image projection apparatus. It is a figure for demonstrating another form of implementation of an image projection apparatus. It is a figure for demonstrating a polarization conversion element. It is a figure for demonstrating the other form of implementation of an image forming apparatus. It is a figure which shows the wavelength dependence of the retardation of each phase plate of Example 1. FIG. It is a figure which shows the wavelength dependence of the 0th order transmittance | permeability of each phase plate of Example 1. FIG. It is a figure which shows the incident angle dependence of the retardation characteristic of each phase plate of Example 1. FIG. It is a figure which shows the incident angle dependence of the 0th-order transmittance | permeability of each phase plate of Example 1. FIG. It is a figure for demonstrating the effect by an antireflection film. It is a figure for demonstrating the effect when forming an antireflection film only in a land. It is a figure for demonstrating the effect | action with respect to the retardation of an antireflection film.

Explanation of symbols

11 Glass flat plate 12a thin layer 12b thin layer

Claims (15)

  A thin layer made of a material having a refractive index of 1.6 or more is formed on at least one surface of the glass flat plate, and a fine concavo-convex structure having a rectangular wave section with lands and spaces is formed as a subwavelength structure on the surface of the thin layer. A phase plate characterized by that. The phase plate according to claim 1,
A phase plate characterized in that the land width in the fine relief structure is wider than the space width. The phase plate according to claim 2,
Pitch of fine concavo-convex structure: P, filling factor: f, conditions (1) 0.1 μm <P <0.4 μm
(2) 0.5 <f <0.8
A phase plate satisfying the requirements. In the phase plate according to any one of claims 1 to 3,
A phase plate, wherein an antireflection film is formed on a fine concavo-convex structure. In the phase plate according to any one of claims 1 to 3,
A phase plate, wherein an antireflection film is formed only on a land having a fine concavo-convex structure.   An optical element formed by integrating the phase plate according to any one of claims 1 to 5 on a flat surface of an optical substrate having optical functions such as a prism function and a lens function.   A thin layer made of a material having a refractive index of 1.6 or more is formed on the surface of an optical substrate having an optical function such as a prism function and a lens function. An optical element, wherein a structure is formed, and the fine concavo-convex structure functions as a phase plate for the optical substrate. The optical element according to claim 7.
An optical element characterized in that a land width in a fine relief structure is wider than a space width. The optical element according to claim 7 or 8,
An optical element, wherein an antireflection film is formed on a fine uneven structure in a thin layer. The optical element according to claim 7 or 8,
An optical element, wherein an antireflection film is formed only on a land having a fine uneven structure in a thin layer. The optical element according to any one of claims 6 to 10,
The optical substrate has a polarization separation surface that divides incident light into reflected light and transmitted light whose polarization planes are orthogonal to each other, reflects one of the reflected light and transmitted light, and is in substantially the same direction as the other traveling direction. A reflective surface to face,
A thin layer made of a material having a refractive index of 1.6 or more is disposed on the side of the surface from which the transmitted light and reflected light exit in substantially the same direction, and a land and a space formed as a subwavelength structure in the thin layer. An optical element characterized in that the fine concavo-convex structure having a rectangular wave shape in section has a function as a phase plate that changes the polarization plane of at least one of the reflected light and the transmitted light to make the polarization planes coincide with each other. In an image projection apparatus that guides a light beam from a light source to a liquid crystal display element and projects a display image of the liquid crystal display element on a display surface with a projection lens.
An image projection apparatus using the phase plate according to any one of claims 1 to 5 and / or the optical element according to any one of claims 6 to 11. The image projection apparatus according to claim 12, wherein
Three liquid crystal display elements corresponding to the three primary colors and individually forming images of the respective colors;
It has a cross prism that synthesizes video light of each color emitted from each liquid crystal display element,
The phase plate according to any one of claims 1 to 5 or a phase plate according to any one of claims 1 to 5, as a phase plate corresponding to a half-wave plate in one or more of the three optical paths between the liquid crystal display elements and the cross prism. An image projection apparatus comprising: the optical element according to any one of 10; The image projection apparatus according to claim 13.
Three reflective liquid crystal display elements that individually form images of each color corresponding to the three primary colors;
Having a cross prism for synthesizing video light of each color emitted from each reflective liquid crystal display element;
The phase plate according to any one of claims 1 to 5 as a phase plate corresponding to a quarter-wave plate in one or more of the three optical paths between each of the reflective liquid crystal display elements and the cross prism. 6. An image projection apparatus comprising the optical element according to any one of 6 to 10. The image projection apparatus according to claim 13 or 14,
An image projection apparatus comprising the optical element according to claim 11 as an optical element for aligning light from a light source with linearly polarized light.

Images