JP2009042472A - Optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element having optical performance such as superior reflection suppression performance and high design flexibility. <P>SOLUTION: The optical element has a first layer 14, a second layer 15 and a base member 11 laminated in order from the light incidence side. The first layer has an irregular structure in which a convex part 14a and a concave part 14b are alternately formed at a pitch smaller than the wavelength λ of the incident light. The second layer includes a plurality of layers 13 and 12 of which refractive indexes are different from each other. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical element having an optical function such as a reflection suppressing function and an optical apparatus using the optical element.

  Many optical devices use various transmissive optical elements. For example, in an imaging optical system of a digital camera, an object image is formed on the imaging element by a lens as a transmissive optical element.

  However, optical glass or optical plastic generally used as a material for the transmission optical element has a high refractive index because of its high refractive index, and the use of a plurality of such transmission optical elements reduces the amount of transmitted light.

  In order to suppress reflection at the transmissive optical element, the optical element is often provided with a reflection suppressing function. Several methods are known as methods for imparting a reflection suppressing function to an optical element.

  The most general method is a method of forming a reflection suppressing film on the surface of the transmission optical element. Specifically, a thin film is formed on the surface of the transmission optical element by a thin film formation technique represented by vapor deposition, sputtering, etc., and the reflectance is lowered by using optical interference.

  There is also a method of using a structure finer than the wavelength of incident light (also referred to as a working wavelength). The most famous structure is the “moth-eye”. The eyelids can obtain very low reflectivity due to its unique microstructure.

  In a structure finer than the wavelength of incident light, the light cannot recognize the structure and behaves like a uniform medium. The structure exhibits a refractive index according to the volume ratio of the materials constituting the structure. As a result, a structure having a low refractive index that cannot be obtained with a normal material can be realized, and reflection can be satisfactorily suppressed.

As a reflection suppressing method using a fine structure, a method of applying a film in which fine particles having a particle diameter smaller than the wavelength of incident light are dispersed (see Patent Document 1), or a method of forming a fine periodic structure by a fine pattern processing technique There is a method (see Patent Document 2).
Japanese Patent No. 3135944 Japanese Patent Laid-Open No. 50-70040

  However, in order to form a structure finer than the wavelength of incident light, a complicated process is required, and the materials constituting the structure are limited, so the degree of freedom in design is low. For this reason, only a transmission optical element under limited conditions can obtain a high reflection suppression performance by such a fine structure.

  The present invention provides an optical element having optical performance such as good antireflection performance and high design freedom.

  An optical element according to one aspect of the present invention includes a first layer, a second layer, and a base member in order from the light incident side. The first layer has a concavo-convex structure in which convex portions and concave portions are alternately formed at a pitch smaller than the wavelength λ of incident light, and the second layer includes a plurality of layers having different refractive indexes. Features.

  According to the present invention, it is possible to realize an optical element having good optical performance such as reflection suppression performance excellent in broadband characteristics and incident angle characteristics and having a high degree of design freedom.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  In FIG. 1, the structure of the optical element which is an Example of this invention is shown. L represents light incident on the optical element. The optical element includes a first layer 14, a second layer 15 (13, 12), and a base member 11 in order from the light incident side.

  The base member 11 corresponds to an optical transparent member (transmission optical member) such as a lens or a prism to which the reflection suppressing film 201 by the first and second layers 14 and 15 is added.

  The first layer 14 has a concavo-convex structure in which convex portions 14a and concave portions 14b are alternately formed at a pitch p smaller than the wavelength λ of the incident light L to the optical element. The convex portion 14a and the concave portion 14b have widths wa and wb, respectively. The pitch p here is a distance from the convex portion 14a to the next convex portion 14a. The width means the dimension in the direction in which the convex portions 14a and the concave portions 14b are alternately arranged.

  The first layer 012 has a concavo-convex structure constituted by convex portions 012a and concave portions 012b having a uniform width in the thickness direction.

  The second layer 15 is composed of two layers 13 and 12 having different refractive indexes. Hereinafter, these two layers are referred to as the second a layer 13 and the second b layer 12 in order from the light incident side. The second a layer 13 and the second b layer 12 are homogeneous layers that do not have an uneven structure like the first layer 14.

  Here, when the pitch p in the concavo-convex structure forming the first layer 14 is λ / 20 or more, the first layer 14 includes a medium constituting the convex portion 14a and a medium (for example, air) filling the concave portion 14b. Is preferable because it functions as a layer having a mixed equivalent refractive index.

  The uneven structure may be a periodic structure in which all the convex portions 14a and all the concave portions 14b are regularly arranged with the same width (that is, the convex portions and the concave portions are alternately formed at the same pitch). . However, as long as the average pitch is smaller than λ, a non-periodic structure in which the convex portions 14a and the concave portions 14b are irregularly arranged may be used.

  Furthermore, at least one of the second a layer 13 and the second b layer 12 may not be the homogeneous layer described above, and may have a concavo-convex structure in which convex portions and concave portions are alternately formed at a pitch smaller than λ. Good. In the present embodiment, the case where the second layer 15 includes two layers having different refractive indexes will be described. However, the second layer may include three or more layers.

  The refractive index structure of the optical element shown in FIG. 1 is shown in FIG. In FIG. 2, 21 indicates the refractive index of the base member 11, and 22 indicates the refractive index of the second b layer 12. Reference numeral 23 denotes a refractive index of the second a layer 13, and 24 denotes a refractive index (equivalent refractive index) of the first layer 14. Further, the vertical axis in FIG. 2 indicates the refractive index, and the horizontal axis indicates the position in the thickness direction.

  As can be seen from FIG. 2, the refractive index of the plurality of layers (in this embodiment, the second a layer 13 and the second b layer 12) included in the second layer 15 is equal to the equivalent refractive index of the first layer 14 and the base. The refractive index is between the refractive index of the member 11 and the layer closer to the first layer 14 is smaller. However, the arrangement of the refractive index layer of the second layer 15 is merely an example, and does not necessarily have to be satisfied.

  Since the first layer 14 has an uneven structure sufficiently smaller than the wavelength of the incident light L, it apparently has optical characteristics like a homogeneous layer. Specifically, it has an equivalent refractive index obtained from the filling amount ff of the material constituting the convex portion 14a.

Equivalent refractive index ns is, when the material constituting the convex portion 14a and the refractive index n _0, are simply represented by the following formula (a).

  As can be seen from this equation (a), the higher the filling rate of the material constituting the convex portion 14a, the higher the equivalent refractive index of the first layer 14. By utilizing this fact, it is possible to create a low refractive index layer that cannot be obtained with a conventional homogeneous layer.

  The optical element of the present embodiment has a configuration in which a layer (second layer 15) that generates another optical interference is inserted between the first layer 14 and the base member 11. In such a configuration, the refractive index of the first layer 14 that is the outermost layer can be lowered to suppress the change in the Fresnel coefficient with respect to the incident angle. Furthermore, the incidence angle characteristic can be improved by causing optical interference in the second layer 15.

  In addition, since the refractive index difference between the first layer 14 as the outermost layer and the air in contact with the first layer 14 can be reduced, the amplitude of the wave of optical interference can be reduced, and the reflection suppressing function with excellent broadband characteristics. Can be obtained.

  In order to obtain the above performance or function more effectively, it is better to satisfy the following condition (1).

  Here, n0 is the equivalent refractive index of the first layer 14, and n1 is the layer closest to the first layer among the plurality of layers included in the second layer 15 (the second a layer 13 in the example of FIG. 1). Is the refractive index. Further, n2 is the refractive index of the layer closest to the base member (the second b layer 12 in the example of FIG. 1) among the plurality of layers, and n3 is the refractive index of the base member 11.

  By satisfying this condition (1), the amplitude of the reflected wave becomes the same for each boundary surface of each layer, so that the above performance and mechanism can be exhibited over a wider wavelength range.

  Furthermore, it is more effective if the following conditions are satisfied.

  As described above, since the refractive index of each layer is set, the light whose incident angle characteristics and broadband characteristics are corrected by the first layer 14 which is the outermost layer can be further reduced by optical interference, so that A high performance antireflection structure can be obtained.

  FIG. 3 shows a comparative example of an optical element in which a first layer 32 having a concavo-convex structure having a uniform width in the thickness direction is directly formed on the base member 31 (that is, having no second layer). As shown. Even in this case, the refractive index can be adjusted by changing the structure of the first layer 32. In this case, by adopting a structure that satisfies the following condition (3), a reflectance characteristic as indicated by 203 in FIG. 4 can be obtained. Note that reference numeral 202 in FIG. 4 denotes the reflectance of the reflection suppressing structure of this embodiment shown in FIG.

  However, the structure shown in FIG. 3 is equivalent to a single-layer film, and when a base member 31 having a high refractive index is used, the refractive index difference from the first layer 31 is large. For this reason, the wavelength band is narrow and the incident angle characteristic is also poor.

  FIG. 5 shows, as a comparative example, an optical element in which only one homogeneous layer 52 is provided between the first layer 53 having a concavo-convex structure with a uniform width in the thickness direction and the base member 51.

  As shown by 204 in FIG. 4, a high-performance reflection suppressing performance can be obtained as compared with the optical element shown in FIG. However, when the base member 51 has a high refractive index, it is not possible to obtain sufficient reflection suppression performance in the wavelength band by providing only one homogeneous layer 52.

  On the other hand, in the reflection suppression structure of this embodiment shown in FIG. 1, sufficient reflection suppression performance can be obtained in a wide wavelength band as indicated by 202 in FIG.

  Further, in the optical element having the same configuration as the optical element shown in FIG. 1, the uneven structure provided in the first layer may be a structure as shown in FIG. 61 is a base member. Reference numeral 65 denotes a second layer, which includes a second a layer 63 and a second b layer 62, which are homogeneous layers.

  Reference numeral 64 denotes a first layer having a concavo-convex structure in which convex portions 64a and concave portions 64b are alternately formed at a pitch smaller than the wavelength λ of the incident light L. However, the widths of the convex portions 64 a and the concave portions 64 b change in the thickness direction of the first layer 64. Specifically, the width of the convex portion 64a is larger as it is closer to the second layer 65 and the base member 61, and the width of the concave portion 64b is reversed.

  The refractive index structure of this optical element is shown in FIG. The equivalent refractive index 74 of the first layer 64 changes in the thickness direction. Reference numeral 71 denotes a refractive index of the base member 61, 72 denotes a refractive index of the second b layer 62, and 73 denotes a refractive index of the second a layer 63. In addition, the vertical axis in FIG. 7 indicates the refractive index, and the horizontal axis indicates the position in the thickness direction.

  Since the convex portion 64a of the first layer 64 has a tapered shape from the second layer side, the equivalent refractive index gradually decreases from the second layer side toward the air side (light incident side).

  In such a refractive index structure, incident light interferes innumerably in the first layer 64 and enters the second layer 65. In this case, unlike the conventional optical interference film, light is incident on the second layer 65 while being attenuated corresponding to the gradient of the refractive index and the thickness of the first layer 64. Therefore, in order to suppress reflection of the remaining light on the base member 61, the refractive index and the thickness of the second layer 65 (the second a layer 63 and the second b layer 62) are adjusted, and the optical characteristics are excellent. A reflection suppressing structure is obtained.

  Note that n0 in the conditions (1) and (2) when this structure is adopted is an equivalent refractive index on the most base member side of the concavo-convex structure.

  According to the reflection suppressing structure of this embodiment, light can be attenuated without using a conventional optical interference film. Since the conventional optical interference film is sensitive to the thickness and refractive index of the layer, it is difficult to obtain a reflection suppressing structure having excellent broadband characteristics and incident angle characteristics. On the other hand, in this embodiment, since it becomes insensitive to the incident angle and wavelength of incident light, it is easy to obtain a high-performance reflection suppressing structure.

  Further, in this embodiment, by introducing a second layer suitable for the structure of the first layer, there is a reflection suppressing structure that can be applied to various transmission optical members regardless of the material and shape of the first layer. can get. Furthermore, since the light attenuated in the first layer is incident on the second layer of this embodiment, the second embodiment has a refractive index of the second layer as compared with the case where a conventional optical interference film is used. Insensitive to thickness. For this reason, the optical element of the present embodiment has a larger margin of manufacturing accuracy than the conventional one.

  FIG. 8 shows a comparative example having a first layer 82 whose structure changes in the thickness direction. FIG. 8 shows a case where the base member 81 and the first layer 82 formed of a different material are adjacent to each other (when the second layer is not provided).

  FIG. 9 shows a refractive index structure in the structure of FIG. 91 is the refractive index of the base member 81, and 92 is the refractive index (equivalent refractive index) of the first layer 92. In the structure of FIG. 8, since different materials are used for the first layer 82 and the base member 81, a large difference in refractive index occurs at the interface between them. For this reason, even if the light incident from the air side is attenuated by the first layer 82, the reflection at the interface between the first layer 82 and the base member 81 is large, and the optical performance is inferior. Reference numeral 207 in FIG. 11 shows the reflectance characteristics of the structure in FIG. Further, reference numeral 206 in FIG. 11 shows reflectance characteristics according to the structure of this embodiment shown in FIG.

  FIG. 10 shows a comparative example in which the first layer 103 whose structure changes in the thickness direction is provided, and only one homogeneous layer 102 is provided between the first layer 103 and the base member 101. Show.

  With this structure, a relatively high reflection suppression performance as indicated by 208 in FIG. 11 is obtained. However, when the refractive index of the base member 101 is as high as 2.0 or more, it is not possible to obtain a sufficient reflection suppression performance in a wide band by providing only one homogeneous layer 102.

  On the other hand, according to the structure of the present embodiment shown in FIG. 6, sufficient reflection suppression performance can be obtained in a wide band as indicated by 206 in FIG.

12 and 13 show application examples of the reflection suppressing structure described in this embodiment to an optical element. FIG. 12 is a cross-sectional view of a lens that is one of optical elements, and FIG. 13 is a cross-sectional view of a prism that is another optical element.

  In these drawings, reference numerals 112 and 122 denote a lens body and a prism body as a base member (optical transparent member) for adding a reflection suppressing function, respectively. Reference numerals 111 and 121 denote first layers each having a concavo-convex structure with a pitch smaller than the wavelength λ of incident light. Reference numerals 113 and 123 denote second layers.

  The lens body and prism bodies 112 and 122 have a thickness that can be mounted on a general optical device, and the first layers 111 and 121 and the second layers 113 and 123 have a wavelength λ or less of incident light. Have uneven pitch and thickness.

  Such optical elements such as lenses and prisms can be used in many optical devices. For example, FIG. 17 shows a digital camera as an optical apparatus using the optical element of this embodiment.

  Reference numeral 20 denotes a camera body, and reference numeral 21 denotes a photographing optical system configured using a lens which is an optical element of the present embodiment. The photographing optical system 21 is composed of a plurality of lenses, and at least one of these lenses is composed of the optical element of this embodiment. Reference numeral 22 denotes a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor that receives a subject image formed by the photographing optical system 21 and is built in the camera body 20.

  Reference numeral 23 denotes a memory for recording information corresponding to the subject image photoelectrically converted by the image sensor 22, and reference numeral 24 denotes an electronic viewfinder which is constituted by a liquid crystal display panel or the like and for observing the subject image formed on the solid-state image sensor 22. It is.

  As described above, by configuring the imaging optical system using the optical element according to the embodiment, it is possible to realize a camera having high optical performance in which unnecessary reflection in the imaging optical system is suppressed.

  The optical element of the present embodiment can also be used for a camera finder optical system, a liquid crystal projector illumination optical system, and a projection optical system. Since the optical element has the above-described reflection suppressing structure, the amount of transmitted light is sufficiently large, and generation of ghosts and flares due to unnecessary reflection can be sufficiently suppressed.

  As described above, according to the above-described embodiment, it is possible to realize an optical element having good optical performance such as reflection suppression performance excellent in broadband characteristics and incident angle characteristics and having a high degree of design freedom.

  Hereinafter, numerical examples (simulation examples) corresponding to the respective embodiments will be described. In all numerical examples, the wavelength of incident light is 500 nm. However, this does not mean that the wavelength of incident light in the embodiment (numerical example) of the present invention is limited to 500 nm. The layer thickness was the physical layer thickness (film thickness).

(Numerical example 1)
In the optical element of this numerical example, two layers (second a and 2b layers) are provided in the second layer, and the refractive indexes of these two layers are expressed by the refractive index of the first layer and the refractive index of the base member. Different from each other within the range. The 2a layer and the 2b layer were both homogeneous layers. As the base member, optical glass having a refractive index of 2.0 was used. The first layer was made of a material having a refractive index of 1.46, and the material filling rate was 30%. The equivalent refractive index at this time is 1.13. The first layer has a uniform structure in the thickness direction.

  In this case, the ratio (n2 / n1) of the refractive index of the second layer a on the first layer side to the refractive index of the second layer b on the base member side shown in the conditional expression (1) is 1.13 or more. It is desirable to set the refractive index of each layer to be in the range of 43 or less.

In this numerical example, a 2b layer having a refractive index of 1.86 is formed on the base member with a thickness of 55 nm, and a second a layer having a refractive index of 1.55 is formed thereon with a thickness of 70 nm. Further, a first layer having a thickness of 100 nm was formed thereon. In this case, n2 / n1 was 1.2.

  FIG. 14 shows the reflectance characteristics of the optical element of this numerical example. As can be seen from FIG. 14, according to this numerical example, a sufficiently low reflectance (sufficiently high reflection suppression performance) of 0.5% or less was obtained in a wide wavelength range from 400 nm to 700 nm.

  In FIG. 14, as a comparative example, the reflection of an optical element in which a first layer having an equivalent refractive index of 1.13 is provided on a base member having a refractive index of 2.0 and no second layer is provided. The rate characteristic is shown.

(Numerical example 2)
In the optical element of this numerical example, three layers (second a, 2b, and 2c layers) are provided in the second layer, and the refractive indexes of these three layers are the refractive index of the first layer and the refractive index of the base member. And different from each other. The three layers were homogeneous layers. As the base member, optical glass having a refractive index of 2.0 was used. The first layer was made of a material having a refractive index of 1.46, and the material filling rate was 30%. The equivalent refractive index at this time is 1.13. The first layer has a uniform structure in the thickness direction.

  In this case, it is desirable to set the refractive index of each layer so that n2 / n1 is in the range of 1.13 to 1.43.

In this numerical example, the 2a layer having a refractive index of 1.71 was formed on the base member with a thickness of 74 nm, and the second layer b having a refractive index of 1.51 was formed thereon with a thickness of 84 nm. Further, a second c layer having a refractive index of 1.33 was formed thereon with a thickness of 96 nm, and a first layer was formed thereon with a thickness of 109 nm. In this case, n2 / n1 was 1.13.

  FIG. 15 shows the reflectance characteristics of the optical element of this numerical example. As can be seen from FIG. 15, according to this numerical example, a sufficiently low reflectance (sufficiently high reflection suppression performance) of 0.5% or less was obtained in a wide wavelength range from 400 nm to 700 nm.

(Numerical example 3)
In the optical element of this numerical example, two layers (second a and 2b layers) are provided in the second layer, and the refractive indexes of these two layers are expressed by the refractive index of the first layer and the refractive index of the base member. Different from each other within the range. In addition, the 2a layer was a concavo-convex structure, and the 2b layer was a homogeneous layer. As the base member, optical glass having a refractive index of 1.8 was used.

  The first layer was made of a material having a refractive index of 1.46, and the material filling rate was 30%. The equivalent refractive index at this time is 1.13. The first layer has a uniform structure in the thickness direction. In this case, it is desirable to set the refractive index of each layer so that n2 / n1 is in the range of 1.06 to 1.36.

  The 2a layer was made of a material having a refractive index of 1.46, and the material filling rate was 65%. The equivalent refractive index at this time is 1.30. The 2a layer had a uniform structure in the thickness direction.

In this numerical example, a second b layer (homogeneous layer) having a refractive index of 1.60 is formed on the base member with a thickness of 70 nm, and a second a layer (uneven structure) having an equivalent refractive index of 1.30 is formed thereon. Layer) was formed with a thickness of 80 nm, and a first layer was formed thereon with a thickness of 93 nm. In this case, n2 / n1 was 1.23.

  FIG. 16 shows the reflectance characteristics of the optical element of this numerical example. As can be seen from FIG. 16, according to this numerical example, a sufficiently low reflectance (sufficiently high reflection suppression performance) of 0.3% or less was obtained in a wide wavelength range from 400 nm to 700 nm.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention. For example, in the above-described embodiment, the case where the reflection suppressing function is obtained by providing the first and second layers on the base member has been described. However, in order to obtain other optical functions, the first and second layers are formed. You may provide on a base member.

Schematic which shows the structure of the optical element which is an Example of this invention. The figure which shows the refractive index structure of the optical element of FIG. Schematic which shows the structure of the optical element as a comparative example. The figure which shows the reflection suppression performance in the optical element of FIG.1, FIG3 and FIG.5. Schematic which shows the structure of the optical element as another comparative example. Schematic which shows the structure of the optical element which is another Example of this invention. The figure which shows the refractive index structure of the optical element of FIG. Schematic which shows the structure of the optical element as another comparative example. The figure which shows the refractive index structure of the optical element of FIG. Furthermore, the schematic which shows the structure of the optical element as another comparative example. The figure which shows the reflection suppression performance in the optical element of FIG.6, FIG8 and FIG.10. The figure which shows the example of application to the lens of the reflection suppression structure of an Example. The figure which shows the example of application to the prism of the reflection suppression structure of an Example. The figure which shows the reflectance characteristic of the numerical example 1 corresponding to an Example. FIG. 6 is a diagram showing the reflectance characteristics of Numerical Example 2. FIG. 6 is a diagram showing the reflectance characteristics of Numerical Example 3. 1 is a schematic diagram of a digital camera using an optical element of an embodiment.

Explanation of symbols

11, 61 Base member 12, 62 2b layer 13, 63 2a layer 14, 64 1st layer 15, 65 2nd layer L Incident light

Claims (5)

In order from the light incident side, it has a first layer, a second layer, and a base member,
The first layer has a concavo-convex structure in which convex portions and concave portions are alternately formed at a pitch smaller than the wavelength λ of incident light,
The optical element, wherein the second layer includes a plurality of layers having different refractive indexes.   The refractive index of the plurality of layers included in the second layer is a refractive index between the equivalent refractive index of the first layer and the refractive index of the base member, and is close to the first layer. The optical element according to claim 1, wherein the layer is smaller. The optical element according to claim 1, wherein the following condition is satisfied.

Where n0 is the equivalent refractive index of the first layer, n1 is the refractive index of the layer closest to the first layer among the plurality of layers included in the second layer, and n2 is the refractive index of the plurality of layers. Of these, the refractive index of the layer closest to the base member, n3 is the refractive index of the base member.   The width of the convex portion and the concave portion is changed in the thickness direction of the first layer, and n0 is an equivalent refractive index closest to the base member in the concavo-convex structure. The optical element according to claim 3.
An optical apparatus comprising the optical element according to claim 1.