JP2005148591A - Reflective optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element capable of suppressing the reflection of unnecessary light in an optical system and suppressing the occurrence of ghost and flare etc. <P>SOLUTION: The optical element has at least one light reflection surface and has a micro rugged periodical structure of a pitch P smaller than wavelength of visible light on a surface part A other than an optical effective range part C of the light reflection surface. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a reflective optical element having a fine uneven periodic structure having a pitch smaller than the wavelength of incident light.

When light passes through the boundary between two substances having a difference in refractive index, light is reflected according to the difference in refractive index, and this reflected light may have an optically undesirable effect. In the reflective optical system, it is set so that light is reflected at the boundary, but in the optical system of the observation optical system and the imaging optical system, if unnecessary light is reflected from a part other than the optical effective range part, May have an optically undesirable effect. For example, optical devices that have observation optical systems and imaging optical systems such as cameras and endoscopes use many reflective optical elements such as prisms, but they are not necessary because ghosts and flares occur due to unnecessary reflected light. Reducing light reflections has been performed. In general, in order to prevent reflection from a portion other than the optical effective range portion of the reflecting surface, a black paint is applied to a portion other than the optical effective range portion of the optical surface (the surface on which the reflected light ray is incident or the reflecting surface). Or a light shielding part is formed by installing a light shielding mask. Alternatively, when a metal thin film such as aluminum or silver is formed on the reflecting surface, it is optically effective so that the metal thin film is not formed as much as possible outside the optical effective range part by mechanically covering the part other than the optical effective range part. The reflected light from outside the range is prevented. In addition, a diaphragm member is arranged on the optical path, or a groove is formed in the optical element. However, in order to cope with these problems, the shape of the optical element is complicated. For example, Patent Document 1 discloses a color separation prism device in which a light shielding frame is provided on a prism dividing surface in order to suppress variation in the light shielding effect and prevent flare from occurring. Patent Document 2 discloses an image display device in which a light shielding plate having an opening is provided between an eyepiece optical system and an eyeball of an observer to block flare light and the like.
JP-A-5-346501 JP-A-9-65245

  When forming a light-shielding part with a black paint etc. on parts other than the optical effective range part of the optical surface, it is necessary to manually take measures to prevent the paint from adhering to other optical surfaces, for example, masking with a tape. This is very time consuming and low in productivity. In addition, when the optical surface that is a target for forming the light shielding portion is reduced, the adhesion prevention treatment becomes a delicate operation, which becomes very difficult and further decreases the productivity.

  In the case of forming a metal thin film on the reflecting surface, vapor deposition or sputtering is generally used. In these methods, when the surface other than the optical effective range portion of the reflecting surface is covered with a jig or the like, if there is a slight gap, a vapor deposition material such as aluminum or silver wraps around from there, and other than the optical effective range portion. A metal film also adheres to the surface, and reflection occurs at the attached portion. Even if the metal film can be successfully prevented from adhering to a portion other than the optical effective range portion, reflection occurs due to a difference in refractive index (a difference in refractive index between the optical element material and air) on the surface other than the optical effective range portion. There is a case.

  An object of the present invention is to provide a reflective optical element that suppresses unnecessary light reflection in an optical system and suppresses generation of ghosts, flares, and the like in view of the above-described problems of the prior art.

  The reflective optical element of the present invention that achieves the above object is an optical element having at least one light reflecting surface, and has a fine uneven period having a pitch of visible light wavelength or less on the surface portion other than the optical effective range portion of the light reflecting surface. It has a structure.

  In addition, the reflective optical element of the present invention is an optical element having at least one light reflecting surface, and has a fine concavo-convex periodic structure having a pitch of visible light wavelength or less on the surface portion other than the optical effective range portion of the light reflecting surface. An aluminum thin film is formed on the light reflecting surface, and aluminum is anodized on the surface portion other than the optically effective range portion.

  Further, the reflective optical element of the present invention has a fine concavo-convex periodic structure having a pitch of visible light wavelength or less on a surface other than the optical effective range portion of the light reflective surface in an optical element having at least one light reflective surface. The light reflecting surface has a shape other than a flat surface.

  According to the present invention, a reflective optical element that suppresses generation of ghost and flare due to stray light in an optical system by forming a fine uneven periodic structure having an antireflection effect on a surface portion other than the optical effective range portion of the reflective optical element. Can be provided. In addition, since a fine uneven periodic structure can be formed by using a molding method or anodizing method suitable for mass production, it is possible to provide a reflective optical element with extremely excellent productivity.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Prior to the description of the embodiment of the present invention, basic matters regarding the reflection prevention of light by the fine uneven periodic structure will be described.
FIG. 1 is a cross-sectional view illustrating one embodiment of a fine concavo-convex structure formed outside the optical effective range portion of the reflecting surface of the reflecting optical element of the present invention. FIG. 2 shows a specific structure of the fine uneven periodic structure. (A) shows an example in which the fine concavo-convex periodic structure is composed of substantially conical protrusions. (b) shows an example in which the fine concavo-convex periodic structure is formed of substantially triangular pyramidal protrusions. (c) is a modification of (a). (d) and (e) are other variations of (a). FIG. 3 shows another example of a fine concavo-convex periodic structure in which a large number of fine holes h are regularly and repeatedly (periodically) arranged at pitch intervals.
In the figure, H is the difference in height between the bottom B and the top T of the concavo-convex portion of the fine concavo-convex structure, that is, indicates the height. P is the distance between the vertex T of the concavo-convex portion of the fine concavo-convex structure and the vertex T of the adjacent concavo-convex portion, or the midpoint of the convex portion or concave portion and the adjacent convex portion or concave portion as viewed in cross section. Is the distance between the midpoints. For example, when the concavo-convex portion, which is a constituent unit of the fine concavo-convex periodic structure, has a cone shape, it corresponds to the pitch between the apex T of the projection of this cone and the apex T of the adjacent projection.
In the fine concavo-convex periodic structure in the present invention, the concavo-convex portion which is a structural unit is formed of a cone, a pyramid or the like, and a large number of projections of these cones are arranged on the surface. The vertices T have a structure formed by being regularly (periodically) arranged at a pitch (interval) P less than or equal to the wavelength of visible light. Further, the fine uneven periodic structure in the present invention includes the structure shown in FIG.

  When a large number of fine irregularities arranged at a pitch shorter than the wavelength of visible light are present on the surface, the material forming the fine irregularities occupies most of the bottom B of the fine irregularities. The refractive index of the material has the characteristics of the material, but as it approaches the surface of the fine irregularities, that is, the top portion T, the volume occupancy of the material forming the fine irregularities decreases, and instead the substance that is in contact with the fine irregularities (usually Since the ratio of air) increases, it appears to have the same effect as a layer in which the refractive index continuously changes between the bottom and top of the fine irregularities. The reflection of light occurs due to a difference in refractive index when light passes through the interface of transparent materials having different refractive indexes. In the fine uneven periodic structure, since the refractive index is continuously changed, light reflection is prevented.

  In order to obtain a good antireflection effect, it is preferable that the fine irregularities as illustrated in FIG. 2 have a substantially pyramid shape such as a substantially quadrangular pyramid, a substantially triangular pyramid, and a substantially conical shape. In addition, the pitch P of the fine uneven periodic structure is shorter than the wavelength λ of light of interest, and the ratio of the height H of the fine unevenness to the pitch P of the fine uneven periodic structure is preferably 0.2-4. In this embodiment, P and H are 100 to 400 nm, respectively. (See Figure 1)

  The antireflection effect can be obtained even with a fine hole periodic structure that forms a fine concavo-convex structure in which the fine holes h as shown in FIG. 3 are regularly and repeatedly (periodically) arranged. In this case, the pitch P is also smaller than the wavelength λ of the target light, and the ratio of the fine hole pitch P to the fine hole depth H is preferably 1 or more.

Here, the relationship between the critical angle, the refractive index, and the antireflection effect, which are conditions under which light is totally reflected, will be described.
The critical angle θc at which light is totally reflected at the interface between two transparent materials is expressed by the following equation.
θc = sin ⁻¹ (n1 / n2) where n1 and n2 are the refractive indexes of the respective transparent materials.
As described above, the fine concavo-convex periodic structure can provide the same effect as a layer whose refractive index is continuously changed. Therefore, the fine concavo-convex layer has n1≈n2 in a microscopic range, and has a very large criticality. It becomes a corner. For this reason, in the fine uneven periodic structure, a good antireflection effect can be obtained even when the incident angle of light increases.

  For this reason, even when stray light that cannot be expected in optical design is irradiated at a large incident angle on the reflecting surface, an antireflection effect can be obtained for light having a wide range of incident angles.

  The fine uneven periodic structure used here may be produced by any method. For example, by using microfabrication technology used in the formation of integrated circuits using semiconductors, or after applying a lithography technique to form a resist pattern by electron beam drawing or laser interferometry, using atomic beams, ion beams, or chemicals The fine uneven periodic structure can be directly formed on the surface of the optical element other than the optically effective range portion by a method such as etching.

  In addition, by applying the above-described lithography technology, a method of forming a reverse fine irregular periodic structure on a mold base, or a predetermined fine irregular periodic structure on a substrate such as glass, and then a metal such as nickel There is a method of forming a mold for molding by electroforming by plating and removing the plating layer to form a mold, and molding an optical element using the mold to form a fine uneven periodic structure at a predetermined position. is there. Similarly, a fine pattern etching method using a lithography technique can be applied to a metal thin film on a surface other than an optical effective range portion in an optical element having a metal thin film formed on a reflection surface.

When the metal thin film is aluminum, a fine pore periodic structure can be formed by an anodic oxidation method. Therefore, the optical effective range can be easily removed by removing the protective material such as the resist material after applying the anodizing method after protecting the optical effective range portion of the reflecting surface with the aluminum thin film formed on the entire surface with the resist material. A fine uneven periodic structure composed of a fine pore periodic structure can be formed in addition to the portion. Furthermore, the anodized portion changes from aluminum (Al) to alumina (Al ₂ O ₃ ) and becomes transparent and transmits light, so that light reflection can be further suppressed.

  When the metal thin film is aluminum as described above, the fine hole period can be obtained without using very expensive equipment such as an electron beam drawing apparatus, a laser interference exposure apparatus, or a fine pattern mask, which are necessary for the application of the above-mentioned lithography technology. A fine uneven periodic structure having a structure can be formed.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. However, the present invention is not limited to these embodiments.
Embodiment 1
FIG. 4 is an optical path diagram of the decentration optical system 9 used in the head-mounted image display device.
In the figure, 1 indicates a pupil position. On the other hand, a free-form curved prism 6 which is a decentered optical system 9 is placed. 2 is an axial principal ray, 3 is the first surface of the free-form surface prism 6, 4 is the second surface, and 5 is the third surface 5. Reference numeral 7 denotes an image plane. A display element such as a liquid crystal display element is disposed on the image plane 7. The image displayed on the image plane 7 is transmitted through the third surface 5 of the free-form surface prism 6, totally reflected by the first surface 3, reflected again by the second surface 4, transmitted through the first surface 3, and transmitted through the pupil. 1 is reached. An aluminum thin film is formed on the entire second surface 4 by vapor deposition.

  In the portion other than the optical effective range portion of the first surface 3 of the free-form curved prism 6 shown in the hatched portion A of FIG. 5, that is, in a portion within a range of 1 mm from the outer periphery, a fine uneven periodic structure is formed on the surface. . In the present embodiment, the pitch P of the formed fine uneven periodic structure is 150 nm, and the uneven height H is 155 nm. (See Figure 1)

  The free-form surface prism 6 of the present embodiment was produced by injection molding an amorphous polyolefin-based plastic (ZEONEX (registered trademark) 480R manufactured by Nippon Zeon). The mold used for injection molding is shown in FIG. 1 in a portion other than the optical effective range portion C shown in FIG. 5 by applying lithography technology to the mold surface corresponding to the first surface 3 of the free-form curved prism 6. The one having a fine uneven periodic structure having a shape opposite to the fine uneven periodic structure was used.

FIG. 8 shows the reflectance of the portion where the fine irregular periodic structure is formed on the first surface 3 of the free-form surface prism 6. Amorphous polyolefin-based plastic has a refractive index n ₀ = 1.52. On the untreated surface, the reflectance is about 4%, but as shown in FIG. 8, a sufficient antireflection effect is obtained. Yes. Further, when the free-form curved prism 6 was incorporated into a head-mounted image display device and evaluated for visual image display, no ghost light was seen and it was good.
Embodiment 2
FIG. 6 shows an example of an imaging optical system using a free-form surface prism.
In the figure, the first free-form curved prism 10 has a first surface 11, a second surface 12 and a third surface 13, and the second free-form surface prism 20 comprises a first surface 21, a second surface 22 and a third surface 13. It has a surface 23. Light rays from the first surface 11 side of the first free-form surface prism 10 pass through the incident surface 11 of the free-form surface prism 10, are reflected by the first reflection surface 12, and are emitted from the exit surface 13. The light beam emitted from the first free-form surface prism 10 and passed through the stop 14 is transmitted through the incident surface 21 of the second free-form surface prism 20 and reflected by the first reflection surface 22, and this time on the first surface 21. The light is totally reflected, passes through the exit surface 23, and forms an image on the image surface 7.

Here, an aluminum thin film having a thickness of 0.1 μm is formed on the entire surface of the second surface 12 of the first free-form surface prism 10 and the second surface 22 of the second free-form surface prism 20. Further, as shown in FIG. 7, a fine hole periodic structure formed by anodic oxidation of aluminum is formed in the thin film of the portion B (hatched portion) other than the optical effective range portion C of the second surface 22.
In the fine hole periodic structure formed in this embodiment, the hole diameter h is 50 nm and the pitch P is 100 nm.

  The first free-form surface prism 10 and the second free-form surface prism 20 of Embodiment 2 were produced by injection-molding amorphous polyolefin-based plastic (ZEONEX (registered trademark) 480R manufactured by Nippon Zeon). After molding, an aluminum thin film (film thickness: 0.1 μm) was formed on the entire surface of the second surface 12 of the first free-form curved prism 10 and the second surface 22 of the second free-form curved prism 20 by a normal vacuum deposition method.

Further, a photo-curing resin soluble in an organic solvent is applied to the entire second surface 22 of the second free-form curved prism, and the resin is cured only in the optically effective range, and then washed to obtain an uncured resin. Was removed, and the exposed aluminum thin film was anodized to form a periodic micropore structure in the aluminum thin film portion other than the optically effective range portion. Thereafter, the resin in the optically effective range portion was removed with an organic solvent. Although not shown, the same processing was performed on the second surface 12 on which the aluminum thin film of the first free-form curved prism 10 was formed, and a fine hole periodic structure was formed in a portion other than the optical effective range portion. .
As for the conditions for anodization, the first and second free-form surface prisms, which were anodized in a 0.3 mol / l oxalic acid solution at a voltage of 40 V, were incorporated in an imaging evaluation housing, and imaging evaluation was performed. However, no ghost was generated and good results were obtained. Similarly, a slight ghost was observed in the free-form curved prism in which the aluminum thin film was not treated. In this embodiment, the film thickness of the aluminum thin film is 0.1 μm, but the film thickness can be selected and used in the range of 0.06 μm to 0.4 μm.

It is sectional drawing of one form of the fine uneven | corrugated periodic structure used for this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of various embodiments of a fine uneven periodic structure used in the present invention. For example, (c) is a modification of (a), and (d) and (e) are modifications of (c). It is an external appearance perspective view of an example of the fine uneven | corrugated periodic structure used for this invention. It is explanatory drawing of the optical system of Embodiment 1 of this invention. It is an external appearance perspective view of the free prism surface in Embodiment 1 of this invention. It is explanatory drawing of the optical system of Embodiment 2 of this invention. It is an external appearance perspective view of the free prism surface in Embodiment 2 of this invention. It is a graph which shows the reflectance characteristic of embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pupil position 2 Axial principal ray 3 First surface of prism 4 Second surface of prism 5 Third surface of prism 6 Free-form surface prism 7 Image surface 10 First prism 11 First surface 12 of first prism Second surface 13 Third surface 14 of first prism Aperture 21 First surface 22 of second prism Second surface 23 of second prism Third surface A, B of second prism Surface portion H other than optical effective range portion H High Length P pitch B bottom position T top position h pore

Claims (3)

  An optical element having at least one light reflecting surface, the reflecting optical element having a fine concavo-convex periodic structure having a pitch smaller than the wavelength of visible light on a surface portion other than the optical effective range portion of the light reflecting surface .   2. The reflective optical element according to claim 1, wherein a thin film of aluminum is formed on the light reflecting surface, and the surface portion other than the optically effective range portion of the light reflecting surface is anodized aluminum.   The reflective optical element according to claim 1, wherein the light reflecting surface has a shape other than a flat surface.