Optical element

Abstract

Compared to conventional metalens, this invention provides an optical element equipped with a light-gathering structure that allows the direction of propagation of focused light to be significantly tilted from the normal direction of the light incident surface. [Solution] The optical element 1 comprises an APD 10 and a light-gathering structure. The APD 10 has a light incident surface 101 and a light-receiving region, and converts light incident on the light incident surface 101 into electrons in the light-receiving region and performs avalanche multiplication of electrons. The light-gathering structure has a nanostructure 20 provided on the light incident surface 101 of the APD 10. The nanostructure 20 has a plurality of unit structures 201 arranged along the light incident surface 101. The period of the plurality of unit structures 201 is greater than or equal to the wavelength of light. The light-gathering structure focuses the diffracted light L3 into the light-receiving region by individually modulating the phase of the ±n-th order (n>0) diffracted light L3 in each unit structure 201.

Description

[Technical Field]

[0001] This disclosure relates to optical elements. [Background technology]

[0002] Patent Document 1 discloses a configuration in which a metalens is positioned above a photodiode. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2020-98814 [Patent Document 2] Japanese Patent Publication No. 2006-100515 [Non-patent literature]

[0004] [Non-Patent Document 1] Philippe Lalanne and Pierre Chavel, "ON THE PREHISTORY OFOPTICAL METASURFACES," Photoniques, Vol. 119, pp. 41-45 [Overview of the project] [Problems that the invention aims to solve]

[0005] The technique of focusing light using a metalens is used in optical elements such as photodiodes. Conventional metalens have multiple unit structures arranged along the light incident surface of the optical element, and the period of these unit structures is less than the wavelength of the incident light. The direction of light emitted from a conventional metalens with such characteristics coincides with the direction of the incident light, for example, being perpendicular to the light incident surface. On the other hand, depending on the optical element, it may be desirable to significantly tilt the direction of focused light from the normal direction of the light incident surface in the region close to the light incident surface. For example, in a photodiode, the longer the optical path in the light-receiving region that converts light into electrons, the higher the quantum efficiency of photoelectric conversion can be. With conventional metalens, since the direction of emitted light coincides with the direction of incident light, it is difficult to lengthen the optical path of focused light in the region close to the light incident surface.

[0006] The present disclosure aims to provide an optical element equipped with a light-gathering structure that can significantly tilt the direction of propagation of focused light from the normal direction of the light incident surface. [Means for solving the problem]

[0007] [1] An optical element relating to one aspect of the present disclosure comprises a photodiode and a light-gathering structure. The photodiode has a light-incident surface and a light-receiving region, and converts light incident on the light-incident surface into electrons in the light-receiving region. The light-gathering structure has a nanostructure provided on the light-incident surface of the photodiode. The nanostructure has a plurality of first unit structures arranged along the light-incident surface. The period of the plurality of first unit structures is greater than or equal to the wavelength of the light. The light-gathering structure focuses the diffracted light into the light-receiving region by individually modulating the phase of the ±n-th order (n>0) diffracted light in each of the plurality of first unit structures.

[0008] [2] An optical element relating to one aspect of the present disclosure comprises an optical member and a light-gathering structure. The optical member has a light-incident surface to which light is incident. The light-gathering structure has a nanostructure provided on the light-incident surface of the optical member. The nanostructure has a plurality of first unit structures arranged along the light-incident surface. The period of the plurality of first unit structures is greater than or equal to the wavelength of the light. The light-gathering structure focuses the diffracted light by individually modulating the phase of the ±n-th order (n>0) diffracted light in each of the plurality of first unit structures.

[0009] In the optical elements described in [1] and [2] above, a nanostructure with a light-gathering structure is provided on the light incident surface of the photodiode or optical member. The period of the multiple first unit structures of the nanostructure is greater than or equal to the wavelength of the incident light, and the light-gathering structure focuses the diffracted light by individually modulating the phase of the ±n-th order (n>0) diffracted light of the incident light in each of the multiple first unit structures. Such a light-gathering structure focuses the ±n-th order (n>0) diffracted light, such as ±1st order and ±2nd order, rather than focusing the 0th-order light like a conventional metalens. The direction of propagation of the ±n-th order diffracted light is tilted significantly with respect to the direction of propagation of the incident light. In other words, with the optical elements described in [1] and [2] above, the direction of propagation of the focused light can be tilted significantly from the normal direction of the light incident surface compared to a conventional metalens. Furthermore, when a photodiode is provided, the optical path in the light-receiving region can be lengthened, thereby increasing the quantum efficiency of photoelectric conversion. Furthermore, if the photodiode is an avalanche photodiode, the avalanche probability can be increased by forming a focal point within the light-receiving region. As a result, the photoreceiving sensitivity of the photodiode can be increased.

[0010] [3] The optical elements described in [1] or [2] above may further comprise a metalens. The nanostructure is located between the metalens and the light incident surface. The metalens has a plurality of second unit structures aligned along the light incident surface. The period of the plurality of second unit structures is less than the wavelength of light. The metalens focuses the zeroth order light toward the nanostructure. By injecting light into the nanostructure through such a metalens, the aperture ratio can be increased, thereby increasing the amount of light incident on the photodiode or optical element.

[0011] [4] In any of the optical elements described in [1] to [3] above, the light-gathering structure may further comprise a first reflecting portion. The first reflecting portion is provided on the side opposite to the nanostructure with respect to the light incident surface and is arranged to surround the nanostructure when viewed from a direction perpendicular to the light incident surface. The first reflecting portion has a first light-reflecting surface facing inward within the enclosure. In this case, at least a portion of the diffracted light from the nanostructure is reflected by the first light-reflecting surface and then converges at the focal point. This further lengthens the optical path in the light-receiving region and further increases the quantum efficiency of photoelectric conversion.

[0012] [5] In any of the optical elements described in [1] to [4] above, the light-gathering structure may further comprise a second reflecting portion. The second reflecting portion is provided on the side opposite to the nanostructure with respect to the light incident surface. The second reflecting portion has a second light-reflecting surface facing the direction of the light incident surface. In this case, at least a portion of the diffracted light from the nanostructure is reflected by the second light-reflecting surface and then converges at the focal point. This further lengthens the optical path in the light-receiving region, thereby further increasing the quantum efficiency of photoelectric conversion.

[0013] [6] In any of the optical elements described in [1] to [5] above, the focusing structure may focus the diffracted light by individually modulating the phase of the diffracted light of order ±2 or higher in each of the multiple first unit structures. The higher the order of the diffracted light emitted from the nanostructure, the greater the inclination of the direction of propagation of the diffracted light with respect to the normal direction of the incident light surface. According to the optical element described in [6] above, the inclination of the direction of propagation of the diffracted light can be further increased, the optical path in the light-receiving region can be further lengthened, and the quantum efficiency of photoelectric conversion can be further increased.

[0014] [7] In any of the optical elements [1] to [6] above, the phase distribution of light emitted from the nanostructure is given by the following formula (where φ Lens (x,y) is the phase at coordinate position (x,y) on the plane of incident light, λ is the wavelength of light, f is the focal length due to the nanostructure, k xHere, \(\omega\) is an arbitrary coefficient including 0, \(\alpha=\frac{\pi}{2}-\theta\) (rad), \(\theta\) is the diffraction angle, and it may satisfy \(\alpha\geq0\). For example, with such a configuration, the nanostructure can condense diffracted light.

Number

[0015] [8] In any of the optical elements of [1] to [7] above, each of the plurality of first unit structures may have a gap extending in a predetermined direction. And the relative position of the center of the gap of each of the plurality of first unit structures with respect to the center of each of the plurality of first unit structures in a direction intersecting the predetermined direction may be determined according to the phase. For example, with such a configuration, the phase of the diffracted light can be individually modulated in each of the plurality of first unit structures.

[0016] [9] In any of the optical elements of [1] to [8] above, the plurality of first unit structures may mainly contain metal. In this case, by plasmonic diffraction, the inclination of the traveling direction of the diffracted light can be further increased, the optical path in the light-receiving region can be further lengthened, and the quantum efficiency of photoelectric conversion can be further enhanced.

[0017]

[10] In any of the optical elements of [1] to [8] above, the plurality of first unit structures may mainly contain a dielectric. Even in this case, diffracted light of ±nth order can be emitted from the nanostructure, and the effects of the optical elements of [1] and [2] can be obtained.

[0018]

[11] In any of the optical elements of [1], [3] to

[10] above, the photodiode may be an avalanche photodiode. In that case, a condensing point can be formed in the light-receiving region to increase the avalanche probability. As a result, the light-receiving sensitivity of the photodiode can be further enhanced.

Advantages of the Invention

[0019] According to this disclosure, it is possible to provide an optical element equipped with a light-gathering structure that can significantly tilt the direction of propagation of focused light from the normal direction of the light incident surface. [Brief explanation of the drawing]

[0020] [Figure 1] Figure 1 is a cross-sectional view showing the configuration of an optical element according to one embodiment of the present disclosure. [Figure 2] Figure 2 schematically shows a side cross-section of a single unit structure contained in a nanostructure when the film mainly contains metal. [Figure 3] Figure 3 is a graph showing the relationship between the reflectance (%) of the nanostructure and the wavelength (nm) of the incident light when the film mainly contains metal. [Figure 4] Figure 4 schematically shows a side cross-section of a single unit structure contained in a nanostructure when the film mainly consists of a dielectric material. [Figure 5] Figure 5 is a graph showing the relationship between the reflectance (%) of the nanostructure and the wavelength (nm) of the incident light when the film mainly contains a dielectric material. [Figure 6] Figure 6 is a graph showing the relationship between wavelength and optical absorption amplification factor. [Figure 7] Figures 7(a) to (e) are plan views of the unit structure. [Figure 8] Figures 8(a) through (e) are side views of the unit structure, corresponding to Figures 7(a) through (e). [Figure 9] Figure 9 is a conceptual diagram illustrating the diffraction of light in multiple unit structures. [Figure 10] Figure 10 is a conceptual diagram illustrating the diffraction of light in multiple unit structures. [Figure 11] Figure 11 is a conceptual diagram illustrating how light is collected by nanostructures in an optical element. [Figure 12] Figure 12 is a conceptual diagram illustrating how light is collected by nanostructures in an optical element. [Figure 13] Figure 13 is a conceptual diagram illustrating how light is collected by nanostructures in an optical element. [Figure 14] Figure 14 is a conceptual diagram illustrating how light is collected by nanostructures in an optical element. [Figure 15] Figure 15 is a conceptual diagram illustrating how light is collected by nanostructures in an optical element. [Figure 16] Figure 16 is a conceptual diagram illustrating, as a reference example, the light focusing behavior when a metalens is provided on the light incident surface instead of a nanostructure. [Figure 17] Figure 17 is a cross-sectional view showing the configuration of an optical element according to a reference example. [Figure 18] Figure 18 is a cross-sectional view showing the configuration of an optical element according to another reference example. [Figure 19] Figure 19 is a plan view showing a modified unit structure. [Figure 20] Figure 20 is a plan view showing a unit structure related to a modified example. [Figure 21] Figure 21 is a plan view showing a modified unit structure. [Figure 22] Figure 22 is a plan view showing a unit structure related to a modified example. [Figure 23] Figure 23 is a plan view showing the unit structure related to the modified example. [Figure 24] Figure 24 is a side view showing a unit structure related to a modified example. [Figure 25] Figure 25 is a schematic diagram showing the configuration of a modified example. [Figure 26] Figure 26 shows the process of fabricating the nanostructure. [Figure 27] Figure 27 is a schematic diagram showing the cross-sectional configuration of an optical element according to a modified example. [Figure 28] Figure 28 is a graph showing the relationship between wavelength and optical absorption amplification factor in the simulation. [Figure 29] Figure 29 is a graph showing the relationship between wavelength and optical absorption amplification factor in the simulation. [Figure 30] Figure 30 is a graph showing the relationship between wavelength and optical absorption amplification factor in the simulation. [Figure 31]Figure 31 is a graph showing the relationship between wavelength and the optical absorption and optical absorption amplification factor of the APD in the simulation. [Figure 32] Figure 32 is a graph showing the relationship between wavelength and the reflectance and transmittance of a nanostructure in a certain structure in the simulation. [Figure 33] Figure 33 is a graph showing the relationship between wavelength and the optical absorption of the APD in a certain structure of the simulation. [Figure 34] Figure 34 is a graph showing the relationship between wavelength and reflectance in the simulation. [Figure 35] Figure 35 is a graph showing the simulation results when the nanostructured film is made of silicon (Si) and an anti-reflective coating made of Al2O3 is provided. [Figure 36] Figure 36 is a graph showing the simulation results when the nanostructured film is made of Si and an anti-reflective coating made of Al2O3 is provided. [Figure 37] Figure 37 is a graph showing the simulation results when the nanostructured film is made of Si and an anti-reflective coating made of Al2O3 is provided. [Figure 38] Figure 38 is a graph showing the simulation results when the nanostructured film is made of Si and an anti-reflective coating made of SiO2 is provided. [Figure 39] Figure 39 is a graph showing the simulation results when the nanostructured film is made of Si and an anti-reflective coating made of SiO2 is provided. [Figure 40] Figure 40 is a graph showing the simulation results when the nanostructured film is made of Si and an anti-reflective coating made of SiO2 is provided. [Figure 41] Figure 41 is a graph showing the simulation results. [Figure 42] Figure 42 is a graph showing the simulation results. [Figure 43] Figure 43 is a graph showing the simulation results. [Figure 44]Figure 44 is an optical microscope image showing the nanostructure. [Figure 45] Figure 45 is a scanning electron microscope image showing the nanostructure. [Figure 46] Figure 46 is a graph showing the relationship between the wavelength and reflectivity of the fabricated nanostructure, and the relationship between the wavelength and the optical absorption amplification factor of the optical element. [Modes for carrying out the invention]

[0021] Specific examples of the present disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as defined by the claims. In the following description, identical elements in the drawings are denoted by the same reference numerals, and redundant descriptions are omitted.

[0022] Figure 1 is a cross-sectional view showing the configuration of an optical element 1 according to one embodiment of the present disclosure. As shown in Figure 1, the optical element 1 of this embodiment comprises an avalanche photodiode (APD) 10, a reflective portion 15, a nanostructure 20, a light-transmitting layer 31, and a metalens 32. The nanostructure 20 is provided on the light incident surface 101 of the APD 10. The light-transmitting layer 31 is provided on the light incident surface 101 of the APD 10 and covers the nanostructure 20. The metalens 32 is provided on the surface of the light-transmitting layer 31. Light L1 is incident on the surface of the light-transmitting layer 31 via the metalens 32. Light L1 is, for example, infrared light. The metalens 32 focuses the light L1 toward the nanostructure 20 to generate focused light L2, and causes the focused light L2 to be incident on the light-transmitting layer 31. The focused light L2 travels through the light-transmitting layer 31 and reaches the nanostructure 20. The nanostructure 20 diffracts the focused light L2 to generate diffracted light L3, and works in cooperation with the reflector 15 to focus the diffracted light L3 within the APD 10. The APD 10 generates a current signal corresponding to the light intensity of the diffracted light L3. The light-transmitting layer 31 and the metalens 32 may be omitted if they are not needed. In that case, the nanostructure 20 diffracts light L1 to generate diffracted light L3. The components of the optical element 1 will be described in detail below.

[0023] The metalens 32 has a plurality of unit structures 321 (second unit structures) arranged along the light incident surface 101 of the APD 10. Only two unit structures 321 are shown in the figure, and the other unit structures 321 are not shown. The arrangement period of the plurality of unit structures 321 is less than the wavelength of light L1. The metalens 32 individually modulates the phase of the 0th order light contained in the light L1 in each of the plurality of unit structures 321. As a result, the metalens 32 converts the light L1 into focused light L2 that converges toward the nanostructure 20. Specifically, the metalens 32 consists of a plurality of films. The plurality of films are, for example, metal films or dielectric films. Each of the plurality of unit structures 321 has a gap extending in a predetermined direction. The relative position of the center of the gap of each of the plurality of unit structures 321 with respect to the center of each of the plurality of unit structures 321 in a direction intersecting the predetermined direction is determined according to the phase.

[0024] The focused light L2 passes through the light-transmitting layer 31 to the nanostructure 20. The light-transmitting layer 31 is made of a material such as SiO2 or Si.

[0025] The nanostructure 20 constitutes part of the light-gathering structure in this embodiment. The nanostructure 20 is located between the metalens 32 and the light incident surface 101 of the APD 10, and in the illustrated example, it is provided on the light incident surface 101. The nanostructure 20 includes a plurality of films 21 arranged along the light incident surface 101. The films 21 mainly consist of metals or dielectrics. Examples of metals that constitute the films 21 include gold (Au), silver (Ag), and aluminum (Al). Examples of dielectric materials that constitute the films 21 include TiO2, SiO2, HfO2, SiN, and a-Si.

[0026] Figure 2 schematically shows a side cross-section of a single unit structure 201 contained in a nanostructure 20 when the film 21 mainly contains metal. The nanostructure 20 has a plurality of unit structures 201 arranged along the light incident surface 101. The plurality of unit structures 201 are arranged, for example, in a lattice (matrix) pattern. The period P of the plurality of unit structures 201 is greater than or equal to the wavelength of light L1. That is, the period P of the plurality of unit structures 201 is the same as the wavelength of light L1 or longer than the wavelength of light L1. The period P is, for example, 520 nm. Each unit structure 201 is composed of a pair of adjacent films 21 and a gap 22 provided between the pair of films 21. The gap 22 extends in a predetermined direction along the light incident surface 101. As shown in Figure 7, which will be described later, the pair of films 21 do not reach both ends of the unit structure 201 in the predetermined direction and are spaced apart from the pair of films 21 of adjacent unit structures 201 in the predetermined direction. When the film 21 of each unit structure 201 mainly contains metal, the nanostructure 20 diffracts the focused light L2 (light L1 if a metalens 32 is not provided) by surface plasmon resonance, causing the ±n-th order (n>0, in one example n≧2) diffracted light L3 to propagate in a direction intersecting the normal direction of the light incident surface 101. Figure 2 shows the 0th-order light L30, the ±1st-order diffracted light L31, and the ±2nd-order diffracted light L32. The higher the order of the diffracted light L3, the greater the inclination of the direction of propagation of the diffracted light L3 with respect to the normal direction of the light incident surface 101. Also, the negative-order diffracted light L3 diffracts in the opposite direction to the positive-order diffracted light L3. In this embodiment, the ±1st-order diffracted light L31 or the ±2nd-order diffracted light L32 is used as the diffracted light L3, and the 0th-order light L30 is not used. The angle between the direction of propagation of the diffracted light L3 and the normal direction of the light incident surface 101 is, for example, 80°.

[0027] When the film 21 mainly contains metal, the gap 22 is formed, for example, by wet etching, and the width of the gap 22 increases with distance from the light incident surface 101. Therefore, the inner surface 211 of the gap 22 is inclined with respect to the direction normal to the light incident surface 101. The angle β between the light incident surface 101 and the inner surface 211 is, for example, 50° or more and less than 90°. The angle β is, for example, 65°. The thickness of the film 21, i.e., the height H of the surface of the film 21 with respect to the light incident surface 101, is, for example, 200 nm. The diffraction efficiency of the nanostructure 20 depends on the height H. The minimum width G of the gap 22 is, for example, 60 nm or more and 100 nm or less. Figure 3 is a graph showing the relationship between the reflectance (%) of the nanostructure 20 and the wavelength (nm) of light L1 when the film 21 mainly contains metal. In Figure 3, curve C11 shows the case where the width G is 60 nm, curve C12 shows the case where the width G is 80 nm, and curve C13 shows the case where the width G is 100 nm. Curve C14 shows the case where the nanostructure 20 is not provided. As shown in Figure 3, the reflectance of the nanostructure 20 varies depending on the wavelength of the incident light, but there is a wavelength band in which the reflectance is lower than when the nanostructure 20 is not provided. Therefore, by appropriately setting the width G of the gap 22, the nanostructure 20 can also function as a wavelength filter.

[0028] Figure 4 schematically shows a side cross-section of one unit structure 201 included in the nanostructure 20 when the film 21 mainly contains a dielectric. In this example as well, the period P of the multiple unit structures 201 is greater than or equal to the wavelength of light L1. The period P is, for example, 330 nm. Even when the film 21 of each unit structure 201 mainly contains a dielectric, the nanostructure 20 diffracts the focused light L2 (light L1 if the metalens 32 is not provided) and causes the ±n-th order (n>0) diffracted light L3 to propagate in a direction intersecting the normal direction of the light incident surface 101. Figure 4 shows the 0th-order light L30 and the ±1st-order diffracted light L31. In this embodiment, the ±1st-order diffracted light L31 is used as the diffracted light L3, and the 0th-order light L30 is not used. When the film 21 mainly contains a dielectric, the gap 22 is formed, for example, by dry etching, and the width of the gap 22 is constant regardless of the distance from the light incident surface 101. Therefore, the inner surface 211 of the gap 22 is perpendicular to the light incident surface 101.

[0029] If the film 21 mainly contains a dielectric, an anti-reflective film 16 is provided on the APD 10 as needed. The anti-reflective film 16 may be made of a material that has etching selectivity with respect to the film 21. In that case, the anti-reflective film 16 also functions as an etching stop layer for the film 21. If the film 21 mainly contains silicon, the anti-reflective film 16 mainly contains, for example, SiO2 or Al2O3. The anti-reflective film 16 constitutes the light incident surface 101.

[0030] Figure 5 is a graph showing the relationship between the reflectance (%) of the nanostructure 20 and the wavelength (nm) of light L1 when the film 21 mainly contains a dielectric material. In Figure 5, the dashed line shows the case where the width G of the gap 22 is 120 nm. The solid line shows the case where the nanostructure 20 is not provided. As shown in Figure 5, the reflectance of the nanostructure 20 varies depending on the wavelength of the incident light, but in all wavelength bands, the reflectance is lower than when the nanostructure 20 is not provided. Therefore, the nanostructure 20 can also function as an anti-reflective coating.

[0031] FIG. 6 is a graph showing the relationship between the wavelength of the light L1 and the absorption enhancement of the APD 10 when the width G is 100 nm. The value of the absorption enhancement is a normalized value with the case where the nanostructure 20 is not provided being 1. Specifically, the absorption enhancement PE is calculated by the following formula. However, I sig is the photocurrent generated in the APD 10 when the nanostructure 20 is provided, and I ref is the photocurrent generated in the APD 10 when the nanostructure 20 is not provided, and I dark is the dark current generated in the APD 10. PE = (I sig - I dark ) / (I ref - I dark ) (a) of FIG. 6 shows the case where the film 21 mainly contains a metal, and (b) of FIG. 6 shows the case where the film 21 mainly contains a dielectric. As shown in FIG. 6, when the film 21 mainly contains a metal, the sensitivity of the optical element 1 can be improved at a specific wavelength (for example, 870 nm to 900 nm), and when the film 21 mainly contains a dielectric, the sensitivity of the optical element 1 can be improved in a broad wavelength range.

[0032] (a) to (e) of FIG. 7 are plan views of the unit structure 201. (a) to (e) of FIG. 8 are side views of the unit structure 201 corresponding to (a) to (e) of FIG. 7 respectively. As shown in FIGS. 7 and 8, in each unit structure 201, the relative position of the center line A2 of the gap 22 with respect to the center line A1 of the unit structure 201 is individually set for each unit structure 201. Then, as shown in FIG. 8, the phase of the diffracted light L3 is controlled according to the relative position of the center line A2 of the gap 22 with respect to the center line A1 of the unit structure 201. The point Q in FIG. 8 indicates the magnitude of the phase of the diffracted light L3 (mπ (rad) to -mπ (rad), m is an integer of 1 or more) corresponding to the position of the center line A2. In the example shown in FIG. 8, the phase of the diffracted light L3 is the largest in (a) and the smallest in (e). Note that the diffraction angle θ is also shown in (e).

[0033] Thus, in each unit structure 201, the relative position of the center line A2 of the gap 22 with respect to the center line A1 is determined according to the phase to be set for the diffracted light L3 emitted from each unit structure 201. As a result, the nanostructure 20 individually modulates the phase of the diffracted light L3 in each of the multiple unit structures 201. The nanostructure 20 then functions as a focusing lens, focusing the diffracted light L3 into the interior of the APD 10 (specifically, the semiconductor region 11 described later). In one embodiment, the phase distribution of the diffracted light L3 emitted from the nanostructure 20 satisfies the following equation (1), where φ Lens (x,y) is the phase at coordinate position (x,y) on the light incident surface 101, λ is the wavelength of light, f is the focal length due to the nanostructure 20, k x Here, k in equation (1) is an arbitrary coefficient including 0, α is α = π / 2 - θ (rad), and α ≥ 0. x sinα is the phase correction term.

number

[0034] Figures 9 and 10 conceptually illustrate the diffraction of light in multiple unit structures 201. Figure 9 shows the case where there is no phase correction term in equation (1), and Figure 10 shows the case where there is a phase correction term in equation (1). In the figures, arrow B1 represents the direction of propagation of the diffracted light L3, and point Q represents the phase of the diffracted light L3. The semicircle B2 represents the case where there is no phase correction term in equation (1) (i.e., k xThe curve B3 and point U1 represent the combined wavefront and focal point, respectively, when there is no phase correction term in equation (1). The semicircle B4 represents the wavefront of the diffracted light L3 emitted from each unit structure 201 when there is a phase correction term in equation (1). The curve B5 and point U2 represent the combined wavefront and focal point, respectively, when there is a phase correction term in equation (1). As shown in Figure 9, the nanostructure 20 can focus the diffracted light L3 to point U1 by individually modulating the phase of the diffracted light L3 in each unit structure 201. Furthermore, as shown in Figure 10, the nanostructure 20 can focus the diffracted light L3 to point U2, which is closer to the incident light surface 101 than point U1, by individually modulating the phase of the diffracted light L3 in each unit structure 201 and also correcting the phase.

[0035] Furthermore, the nanostructure 20 exhibits diffraction and focusing effects only within a plane intersecting the extension direction (predetermined direction) of the gap 22. Therefore, since it diffracts and focuses only the light components having a polarization plane along that plane, the nanostructure 20 can also function as a polarizing filter.

[0036] Refer again to Figure 1. The APD10 is an optical element in this embodiment. The APD10 has a light incident surface 101 and a back surface 102. The light incident surface 101 and the back surface 102 are, for example, flat surfaces that are parallel to each other. The APD10 is a photodiode that utilizes avalanche multiplication, converting light incident on the light incident surface 101 into electrons in the light-receiving region and performing avalanche multiplication of the electrons. In the APD10, electrons are multiplied by applying a reverse bias voltage. The APD10 has one or more pixel sections 18. Figure 1 shows one pixel section 18. Multiple pixel sections 18 may be arranged, for example, in a grid (matrix) or aligned in one direction along the light incident surface 101.

[0037] The pixel region 18, when viewed from a direction perpendicular to the light incident surface 101, exhibits a rectangular shape, for example. The pixel region 18 is fabricated within a semiconductor substrate, such as a silicon (Si) substrate. The pixel region 18 has an n-type semiconductor region 11, a high-density n-type semiconductor region 12, a p-type semiconductor region 13, and a high-density p-type semiconductor region 14. The semiconductor regions 12 and 14 perform electron avalanche multiplication. The semiconductor region 11 is, for example, a substrate region. The semiconductor region 11 constitutes the back surface 102 of the APD 10. The semiconductor regions 12, 13, and 14 are formed, for example, by ion implantation into the semiconductor region 11. The semiconductor regions 12 and 14 form a pn junction with each other. The semiconductor region 11, together with the semiconductor regions 12 and 14, functions as a sensitive region (light-receiving region) that is sensitive to incident light. The sensitive region is sensitive to light in the near-infrared region (e.g., 750 nm to 2.5 μm). The semiconductor region 14 constitutes a part of the light incident surface 101. The semiconductor region 11 constitutes the remainder of the light incident surface 101.

[0038] The APD10 has electrodes on, for example, the light incident surface 101 and the back surface 102. A reverse bias voltage is applied to the APD10 through these electrodes. The APD10 operates, for example, in Geiger mode, in which case a reverse bias voltage greater than or equal to the breakdown voltage is applied. The APD10 may also operate in linear mode, in which case a reverse bias voltage less than the breakdown voltage is applied.

[0039] The reflective portion 15 (first reflective portion) is provided around the entire outer edge of the pixel portion 18. The reflective portion 15 is provided on the side opposite to the nanostructure 20 with respect to the light incident surface 101 and is arranged to surround the nanostructure 20 when viewed from a direction perpendicular to the light incident surface 101. The reflective portion 15 extends from the light incident surface 101 in the thickness direction of the APD 10. The reflective portion 15 functions as a low-sensitivity region that has no sensitivity (or low sensitivity) to incident light. The reflective portion 15 is formed, for example, by embedding a metallic material in a groove (trench) formed in the light incident surface 101.

[0040] The reflective portion 15 constitutes part of the light-gathering structure in this embodiment and has a light-reflecting surface 151 (first light-reflecting surface). The light-reflecting surface 151 faces inward, within the enclosure that surrounds the nanostructure 20, where the reflective portion 15 encloses the nanostructure 20. The light-reflecting surface 151 reflects the diffracted light L3, which is in the process of focusing and emitted from the nanostructure 20, toward the focal point within the APD 10.

[0041] Figures 11 to 15 conceptually illustrate the focusing behavior of the nanostructure 20 in the optical element 1. Figure 16, as a reference example, conceptually illustrates the focusing behavior when a metalens 32 is provided on the light incident surface 101 instead of the nanostructure 20. Each figure shows x and z coordinate axes. The x-axis is parallel to the light incident surface 101. The z-axis is parallel to the normal direction of the light incident surface 101. As shown in Figure 16, when the metalens 32 is provided on the light incident surface 101, the metalens 32 modulates the phase of the 0th-order light, so the center line of the converged light L2 becomes perpendicular to the light incident surface 101, and a focusing point R is formed at a focal length fb from the center of the metalens 32 along the z-axis direction (i.e., the normal direction of the light incident surface 101).

[0042] Figure 11 shows the present embodiment, which includes a nanostructure 20, in a state where the reflective portion 15 is not provided. The center line (z' axis) of the +n-th order (n>0) diffracted light L3a emitted from the nanostructure 20 forms a diffraction angle θ with respect to the normal direction (z axis) of the light incident surface 101. When the reflective portion 15 is absent, a focal point R is formed at a position with a focal length fa from the center of the metalens 32 in a direction inclined with respect to the normal direction of the light incident surface 101. The distance between the light incident surface 101 and the focal point R is significantly shorter compared to the case where the metalens 32 is provided instead of the nanostructure 20.

[0043] Figure 12 shows the state in which the reflective section 15 is provided. By providing the reflective section 15 during the convergence of the diffracted light L3, the +nth order diffracted light L3a during convergence is reflected by the light-reflecting surface 151. As a result, the position of the focal point R moves to directly below the center of the nanostructure 20, that is, near the center line of the APD 10 perpendicular to the light incident surface 101. Even in this case, the distance between the light incident surface 101 and the focal point R is significantly shorter compared to the case in which a metalens 32 is provided instead of the nanostructure 20.

[0044] Figure 13 further shows the -n-th order (n>0) diffracted light L3b in addition to the +n-th order (n>0) diffracted light L3a. The direction of propagation of the -n-th order diffracted light L3b is symmetric with respect to the direction of propagation of the +n-th order diffracted light L3a with respect to the center line of the APD10 perpendicular to the light incident surface 101. Therefore, after the -n-th order diffracted light L3b is reflected at the light reflection surface 151 while converging, the position of the focal point R of the -n-th order diffracted light L3b coincides with the position of the focal point R of the +n-th order diffracted light L3a.

[0045] However, as shown in Figure 14, the ±n-th order diffracted light L3a and L3b are focused at the focal point R and then diffuse downward from the focal point R. Therefore, as shown in the equation (1) above, a phase correction term k is added to the phase distribution of the diffracted light L3a and L3b emitted from the nanostructure 20. x The sinα can also be introduced. As a result, as shown in Figure 15, the propagation direction of the ±n-th order diffracted light L3a and L3b approaches the direction along the light incident surface 101, and the ±n-th order diffracted light L3a and L3b can be focused in the direction along the light incident surface 101, i.e., in the transverse direction. Therefore, the diffusion of the ±n-th order diffracted light L3a and L3b downward after being focused at the focal point R can be suppressed.

[0046] The effects obtained by the optical element 1 of this embodiment, as described above, will now be explained. In the optical element 1 of this embodiment, a nanostructure 20 with a light-gathering structure is provided on the light incident surface 101 of the APD 10. Unlike conventional metalens, the period P of the multiple unit structures 201 of the nanostructure 20 is greater than or equal to the wavelength of light L1. The light-gathering structure focuses the diffracted light L3 into the APD 10 by individually modulating the phase of the ±n-th order (n>0) diffracted light L3 in each unit structure 201. Such a light-gathering structure does not focus 0th-order light like conventional metalens, but focuses ±n-th order (n>0) diffracted light L3 such as ±1st order and ±2nd order. The direction of propagation of the ±n-th order diffracted light L3 is significantly tilted with respect to the direction of propagation of the converged light L2 (light L1 if a metalens 32 is not provided). In other words, with the optical element 1 of this embodiment, compared to conventional metalens, the direction of propagation of the focused diffracted light L3 can be significantly tilted from the normal direction of the light incident surface 101. This lengthens the optical path in the light-receiving region (semiconductor region 11), thereby increasing the quantum efficiency of photoelectric conversion. Furthermore, by forming the focal point R within the light-receiving region, the avalanche probability can also be increased. As a result, the light-receiving sensitivity of the APD 10 can be increased.

[0047] Figure 17 is a cross-sectional view showing the configuration of an optical element 1A according to a reference example. Optical element 1A has the same configuration as optical element 1 of this embodiment, except that it does not have a nanostructure 20. In optical element 1A, focused light L2 emitted from the metalens 32 is incident on the APD 10. In this case, the aperture ratio can be improved and the avalanche probability can be increased by focusing the light within the light-receiving region. However, if the optical path within the light-receiving region of APD 10 is short, and especially if the wavelength of light L1 is a long wavelength such as in the infrared region, the quantum efficiency will be kept low. Figure 18 is a cross-sectional view showing the configuration of an optical element 1B according to another reference example. Optical element 1B has a structure in which a nanostructure 20A is further provided on the optical element 1A described above. The nanostructure 20A is provided on the light incident surface 101 of APD 10. Unlike the nanostructure 20 of this embodiment, the nanostructure 20A does not individually modulate the phase for each unit structure 201, and therefore does not have the function of focusing diffracted light L3. In that case, although the optical path within the light-receiving region of the APD10 can be lengthened by the diffracted light L3, the avalanche probability is kept low because the diffracted light L3 is not focused. The optical element 1 of this embodiment overcomes the shortcomings of optical elements 1A and 1B and can improve the aperture ratio, avalanche probability, and quantum efficiency of photoelectric conversion.

[0048] As in this embodiment, the optical element 1 may include a metalensor 32. By injecting focused light L2 into the nanostructure 20 via the metalensor 32, the aperture ratio can be increased, thereby increasing the amount of light incident on the APD 10. Thus, the light-receiving sensitivity of the optical element 1 can be increased.

[0049] As in this embodiment, the light-gathering structure may further include a reflecting portion 15. In this case, at least a portion of the diffracted light L3 from the nanostructure 20 is reflected by the light-reflecting surface 151 and then converges at the focal point R. This further lengthens the optical path in the light-receiving region, thereby further increasing the quantum efficiency of photoelectric conversion.

[0050] The focusing structure may focus the diffracted light L3 by individually modulating the phase of the diffracted light L3 of order ±2 or higher (i.e., n≧2) in each unit structure 201. The higher the order of the diffracted light L3 emitted from the nanostructure 20, the greater the inclination of the direction of propagation of the diffracted light L3 with respect to the normal direction of the light incident surface 101. With this optical element 1, the inclination of the direction of propagation of the diffracted light L3 can be further increased, the optical path in the light-receiving region can be further lengthened, and the quantum efficiency of photoelectric conversion can be further increased.

[0051] As in this embodiment, the phase distribution of the diffracted light L3 emitted from the nanostructure 20 may satisfy the aforementioned equation (1). For example, with such a configuration, the nanostructure 20 can focus the diffracted light L3.

[0052] As in this embodiment, each unit structure 201 may have a gap 22 extending in a predetermined direction. The relative position of the center line A2 of the gap 22 of each unit structure 201 with respect to the center line A1 of each unit structure 201 in a direction intersecting the predetermined direction may be determined according to the phase. For example, with such a configuration, the phase of the diffracted light L3 can be individually modulated in each unit structure 201.

[0053] As mentioned above, the multiple unit structures 201 may mainly contain metals. In this case, plasmonic diffraction can further increase the gradient of the propagation direction of the diffracted light L3, thereby lengthening the optical path in the light-receiving region and further increasing the quantum efficiency of photoelectric conversion.

[0054] As mentioned above, the multiple unit structures 201 may mainly contain dielectrics. Even in this case, the effects of this embodiment described above can be obtained by emitting ±n-th order diffracted light L3 from the nanostructure 20.

[0055] (First variation) Figures 19 to 23 (a) to (e) are plan views showing modified unit structures 202 to 206 according to the above embodiment. In unit structure 202 shown in Figure 19, the pair of films 21 reach both ends of unit structure 201 in the direction of extension of the gap 22 (a predetermined direction), and are connected to the pair of films 21 of adjacent unit structures 201 in the same predetermined direction. In unit structure 203 shown in Figure 20, the pair of films 21 are connected to each other at both ends of the gap 22 in the direction of extension of the gap 22, and the gap 22 is surrounded by films 21. In unit structures 202 and 203 shown in Figures 19 and 20, the relative position of the center line A2 of the gap 22 with respect to the center line A1 of the unit structure is set individually for each unit structure. The phase of the diffracted light L3 changes according to the relative position of the center line A2 of the gap 22 with respect to the center line A1 of the unit structure.

[0056] Furthermore, the unit structure 204 shown in Figure 21 has a gap 23 instead of the gap 22 described above. The gap 23 extends in a direction intersecting (e.g., orthogonal) to the extending direction of the gap 22 in the above embodiment. Thus, the extending direction of the gap in the nanostructure 20 can be arbitrarily determined. Therefore, the polarization plane of the diffracted light L3 diffracted and focused by the nanostructure 20 can also be arbitrarily determined. In the unit structure 204 shown in Figure 21, the relative position of the center line A4 of the gap 23 with respect to the center line A3 of the unit structure 204 is set individually for each unit structure 204. Then, the phase of the diffracted light L3 changes according to the relative position of the center line A4 of the gap 23 with respect to the center line A3 of the unit structure 204. As shown in Figure 22, the unit structure 205 may have both the two gaps 22 and 23. In this case, the phase of each polarization component can be controlled individually, and circularly polarized, elliptically polarized, or unpolarized light can be diffracted and focused.

[0057] Figures 24(a) to (e) are side views of the unit structure 206, corresponding to Figures 23(a) to (e). Unlike the above embodiment, in the unit structures 206 shown in Figures 23 and 24, the center line of the gap 22 coincides with the center line A1 of the unit structure 206 in all unit structures 206. The width G of the gap 22 is set individually for each unit structure 206. In this case, the phase of the diffracted light L3 changes according to the width G of the gap 22. Point Q in Figure 24 indicates the magnitude of the phase of the diffracted light L3 (mπ(rad) to -mπ(rad), where m is an integer greater than or equal to 1) corresponding to the width G of the gap 22. In the example shown in Figure 24, the smaller the width G of the gap 22, the larger the phase of the diffracted light L3, and the larger the width G of the gap 22, the smaller the phase of the diffracted light L3. Note that the diffraction angle θ is also shown in (e).

[0058] (Second variation) Figure 25 is a schematic diagram showing a modified configuration of the above embodiment. As shown in Figure 25, the light-gathering structure may further include a reflecting portion 17 (second reflecting portion). The reflecting portion 17 is provided on the side opposite to the nanostructure 20 with respect to the light incident surface 101. The APD 10 is located between the light incident surface 101 and the reflecting portion 17. The reflecting portion 17 has a light-reflecting surface 171 (second light-reflecting surface) facing the direction of the light incident surface 101. In one example, the light-reflecting surface 171 is parallel to the light incident surface 101. The reflecting portion 17 may be, for example, a metal mirror, or it may be a metal wiring provided on a circuit board.

[0059] In this modified example, the diffracted light L3 emitted from the nanostructure 20 is reflected by the light-reflecting surface 151 of the reflecting section 15, then further reflected by the light-reflecting surface 171 of the reflecting section 17, and then further reflected by the light-reflecting surface 151 opposite to the light-reflecting surface 151, before reaching the focal point R. In this way, at least a portion of the diffracted light L3 from the nanostructure 20 is reflected by the light-reflecting surface 171 and then converges at the focal point R. This further lengthens the optical path in the light-receiving region of the APD 10, thereby further increasing the quantum efficiency of photoelectric conversion.

[0060] (Third variation) Figures 26(a) to (d) show the fabrication process of the nanostructure 20. As shown in Figure 26(d), in this example, semiconductor regions 12 to 14 are positioned closer to the back surface 102 side of the APD 10. Semiconductor region 14 constitutes a part of the back surface 102. Semiconductor region 11 constitutes the entire light incident surface 101. The other structures are the same as in the above embodiment. The nanostructure 20 of the above embodiment can also be fabricated by the same process as in this modified example.

[0061] As shown in Figure 26(a), first, an anti-reflective film 16 (for example, an SiO2 film) is deposited on the light incident surface 101, for example, by sputtering. At this time, the thickness of the deposited film is for example 2 nm, and when combined with the thickness of the native oxide film, the thickness of the anti-reflective film 16 is for example 5 nm. Next, a film 210 made of the material for film 21 (for example, amorphous silicon) is deposited on the anti-reflective film 16, for example, by sputtering. The thickness of film 210 is for example 200 nm. Next, as shown in Figure 26(b), an electron beam resist 212 is applied to film 210. The thickness of the electron beam resist 212 is for example 300 nm. As the electron beam resist 212, for example, ZEP520A (manufactured by Nippon Zeon) is used. Next, as shown in Figure 26(c), electron beam drawing and development are performed on the electron beam resist 212. Subsequently, as shown in Figure 26(d), dry etching is performed on the film 210 using the electron beam resist 212 as an etching mask. During this process, etching is carried out until the anti-reflective film 16 is exposed. The anti-reflective film 16 functions as an etching stop layer. In this way, a nanostructure 20 having multiple films 21 aligned along the light incident surface 101 is formed.

[0062] (Fourth variation) Figure 27 is a schematic diagram showing the cross-sectional configuration of an optical element 2 according to a modified example of the above embodiment. The optical element 2 comprises a nanostructure 20 and an optical integrated circuit 40 as an optical component. The nanostructure 20 is provided on the light incident surface 401, which is the surface of the optical integrated circuit 40. The optical integrated circuit 40 is an SOI (Silicon On Insulator) substrate and comprises a first Si layer 41, an SiO2 layer 42, and a second Si layer 43. The SiO2 layer 42 is provided on the first Si layer 41, and the second Si layer 43 is provided on the SiO2 layer 42. The refractive index of the SiO2 layer 42 is smaller than the refractive index of the first Si layer 41 and the second Si layer 43. The second Si layer 43 constitutes the light incident surface 401.

[0063] In this modified example, diffracted light L3 emitted from the nanostructure 20 at a diffraction angle θ is incident on the second Si layer 43 and propagates within the second Si layer 43. In this case, since the optical integrated circuit 40 does not have a reflector 15, the form of light collection by the nanostructure 20 is as shown in Figure 11. In this modified example as well, compared to a conventional metalens, the direction of propagation of the light emitted from the nanostructure 20 (diffracted light L3) can be significantly tilted from the normal direction of the light incident surface 401. Therefore, the diffracted light L3 can be easily confined within the thin second Si layer 43. In the illustrated example, light L1 is incident on the nanostructure 20, but as in the above embodiment, a metalens 32 may be provided above the nanostructure 20 and focused light L2 may be incident on the nanostructure 20. In this modified example, the nanostructure 20 focuses the diffracted light L3 inside the optical integrated circuit 40, but the diffracted light L3 may be focused inside an optical element separate from the optical integrated circuit 40, which is provided alongside the optical integrated circuit 40.

[0064] (First embodiment) The structure of the unit structure 201 was investigated by simulation when the film 21 is made of silicon (Si) and the anti-reflective film 16 is not provided. In this simulation, the width G of the gap 22 was set to one of 60 nm, 80 nm, 100 nm, and 120 nm, the period P was set to one of 310 nm, 330 nm, and 350 nm, and the height H of the film 21 was set to one of 100 nm, 150 nm, and 200 nm. Figures 28, 29, and 30 are graphs showing the relationship between wavelength and optical absorption amplification factor in this simulation. The value of optical absorption amplification factor is a normalized value with the case where the nanostructure 20 is not provided set to 1. Figure 28 shows the case when the period P is 310 nm. Figure 29 shows the case when the period P is 330 nm. Figure 30 shows the case when the period P is 350 nm. In each figure, (a) shows the case where the height H of the film 21 is 100 nm, (b) shows the case where the height H of the film 21 is 150 nm, and (c) shows the case where the height H of the film 21 is 200 nm. From these graphs, it can be seen that the optical absorption amplification rate can be controlled by setting structural parameters such as the period P, height H, and width G. Furthermore, if the unit structure 201 has, for example, the structural parameters of this simulation, the effects of the above embodiment can be obtained. The height H of the film 21 may be in the range of 100 nm to 500 nm, and the width G of the gap 22 may be in the range of 60 nm to 200 nm.

[0065] Figure 31(a) is a graph showing the relationship between wavelength and the optical absorption of APD10 in this simulation. In this graph, the height H of the film 21 is set to 200 nm, the width G of the gap 22 is set to 100 nm, and the period P is set to 350 nm. In Figure 31(a), the dashed line shows the case where the nanostructure 20 is provided, and the solid line shows the case where the nanostructure 20 is not provided. As shown in Figure 31(a), the optical absorption of APD10 varies depending on the wavelength of the incident light, but in all wavelength bands, the optical absorption of APD10 is greater than when the nanostructure 20 is not provided. Also, Figure 31(b) is a graph showing the relationship between wavelength and the optical absorption amplification factor in this simulation. The value of the optical absorption amplification factor is a normalized value with the case where the nanostructure 20 is not provided set to 1. As shown in Figure 31(b), when the film 21 is made of silicon (Si), it is possible to improve the sensitivity of the optical element 1 over a broad wavelength range.

[0066] Figure 32(a) is a graph showing the relationship between wavelength and the reflectance of nanostructure 20 in a certain structure in this simulation. Figure 32(b) is a graph showing the relationship between wavelength and the transmittance of nanostructure 20 in a certain structure in this simulation. In the figures, the dashed line shows the case where nanostructure 20 is provided, and the solid line shows the case where nanostructure 20 is not provided. As shown in Figure 32, the reflectance and transmittance of nanostructure 20 vary depending on the wavelength of the incident light, but when nanostructure 20 is provided, the reflectance is lower and the transmittance is higher in all wavelength bands than when nanostructure 20 is not provided. Therefore, by selecting the structural parameters, nanostructure 20 can also function as an anti-reflective coating.

[0067] Figure 33 is a graph showing the relationship between wavelength and optical absorption of APD10 in a certain structure in this simulation. In Figure 33, the dashed line represents the case where nanostructure 20 is provided, and the solid line represents the case where nanostructure 20 is not provided. As shown in Figure 33, the optical absorption of APD10 varies depending on the wavelength of the incident light, but when nanostructure 20 is provided, the optical absorption of APD10 is greater in all wavelength bands than when nanostructure 20 is not provided.

[0068] (Second example) The structure of the unit structure 201 was investigated by simulation when the film 21 is made of silicon (Si) and an anti-reflective film 16 made of an SiO2 film or an Al2O3 film is provided. Figures 34(a) and (b) are graphs showing the relationship between wavelength and reflectance in this simulation. In this simulation, the width G of the gap 22 was set to 60 nm, 80 nm, 100 nm, and 120 nm, and the period P was set to 350 nm. Figure 34(a) is the graph when the height H of the film 21 is 150 nm. Figure 34(b) is the graph when the height H of the film 21 is 200 nm. For comparison, these figures also show curves when the nanostructure 20 is not provided (w / o nano-grating). As shown in Figure 34, even when the anti-reflective film 16 is provided, the reflectance can be reduced over a broad wavelength range, and the sensitivity of the optical element 1 can be improved.

[0069] Figures 35 to 40 are graphs showing the simulation results when the film 21 is made of silicon (Si) and an anti-reflective coating 16 is provided. In this simulation, the period P was set to 310 nm, the height H of the film 21 was set to 200 nm, the width G of the gap 22 was set to one of 60 nm, 80 nm, 100 nm, or 120 nm, and the thickness of the anti-reflective coating 16 was set to one of 0 nm, 5 nm, or 10 nm. Figures 35 to 37 show the case when the anti-reflective coating 16 is made of an Al2O3 film. Figures 38 to 40 show the case when the anti-reflective coating 16 is made of an SiO2 film. Figures 35 and 38 show the case when the thickness of the anti-reflective coating 16 is 0 nm. Figures 36 and 39 show the case when the thickness of the anti-reflective coating 16 is 5 nm. Figures 37 and 40 show the case when the thickness of the anti-reflective coating 16 is 10 nm. Furthermore, in each figure, (a) shows the relationship between wavelength and the light absorption of APD10, (b) shows the relationship between wavelength and the transmittance of nanostructure 20, (c) shows the relationship between wavelength and the reflectance of nanostructure 20, and (d) shows the relationship between wavelength and the light absorption amplification factor of APD10. For comparison, these figures also show curves for the case where nanostructure 20 is not provided (w / o nano-grating). The value of the light absorption amplification factor is a normalized value with the case where nanostructure 20 is not provided set to 1. In all graphs, it can be seen that when nanostructure 20 is provided, the sensitivity of the optical element 1 can be improved compared to the case where nanostructure 20 is not provided.

[0070] Figures 41 to 43 are graphs showing the simulation results when the film 21 is made of silicon (Si), and the anti-reflective film 16 consists of an SiO2 film (thickness 2 nm) provided on the APD 10 and an Al2O3 film (thickness 10 nm) provided on the SiO2 film. In this simulation, the period P was set to 310 nm, the height H of the film 21 was set to 200 nm, 300 nm, or 400 nm, and the width G of the gap 22 was set to 60 nm, 80 nm, 100 nm, 120 nm, or 140 nm. Figure 41 shows the case when the height H of the film 21 is 200 nm. Figure 42 shows the case when the height H of the film 21 is 300 nm. Figure 43 shows the case when the height H of the film 21 is 400 nm. Furthermore, in each figure, (a) shows the relationship between wavelength and the light absorption of APD10, (b) shows the relationship between wavelength and the transmittance of nanostructure 20, (c) shows the relationship between wavelength and the reflectance of nanostructure 20, and (d) shows the relationship between wavelength and the light absorption amplification factor of APD10. For comparison, these figures also show curves for the case where nanostructure 20 is not provided (w / o nano-grating). The value of the light absorption amplification factor is a normalized value with the case where nanostructure 20 is not provided set to 1. In all graphs, it can be seen that when nanostructure 20 is provided, the sensitivity of the optical element 1 can be improved compared to the case where nanostructure 20 is not provided. In addition, by including both SiO2 film and Al2O3 film in the anti-reflective film 16, the etching resistance of the anti-reflective film 16 can be improved, and the etching selectivity for film 21 can be further enhanced.

[0071] (Third embodiment) Figure 44 is an optical microscope image showing the nanostructure 20. Figure 45 is a scanning electron microscope (SEM) image showing the nanostructure 20 fabricated by the method of this modified example. In these nanostructures 20, the period P is 330 nm and the height H of the film 21 is 200 nm. In Figures 44 and 45, (a) to (d) show the cases where the width G of the gap 22 is 60 nm, 80 nm, 100 nm, and 120 nm, respectively. The nanostructures 20 shown in these figures can be suitably fabricated, for example, by the method described in the third modified example.

[0072] Figure 46(a) is a graph showing the relationship between wavelength and reflectance of the fabricated nanostructure 20. Figure 46(b) is a graph showing the relationship between wavelength and the optical absorption amplification factor of the APD10 of an optical element having the fabricated nanostructure 20. The value of the optical absorption amplification factor is a normalized value with 100% being the case without the nanostructure 20. The period P of the nanostructure 20 is 310 nm, the height H of the film 21 is 200 nm, and the width G of the gap 22 is one of 60 nm, 80 nm, 100 nm, or 120 nm. For comparison, Figure 46(a) also shows the curve for the case without the nanostructure 20 (w / o nano-grating). As shown in Figure 46(a), the fabricated nanostructure 20 can reduce reflectance over a broad wavelength range and improve the sensitivity of the optical element. Furthermore, as shown in Figure 46(b), the provision of the nanostructure 20 can improve the sensitivity of the optical element compared to the case where the nanostructure 20 is not provided. In particular, in the wavelength band of 900 nm or higher, the value of the optical absorption amplification factor can be increased by more than 2.8 times compared to the case where the nanostructure 20 is not provided.

[0073] The optical elements according to this disclosure are not limited to the embodiments and modifications described above, and various other modifications are possible. For example, although APDs and optical integrated circuits were exemplified as optical members in the above embodiments and modifications, the optical members are not limited to these. Furthermore, the methods for modulating the phase for each unit structure are not limited to the above embodiments and modifications. For example, the phase can be modulated for each unit structure by making the shape of the film 21 different for each unit structure. However, as in the above embodiments and modifications, it is easier to control the phase for each unit structure by setting the relative position of the center line of the gap with respect to the center line of the unit structure or the width of the gap for each unit structure. Furthermore, although APDs were exemplified as photodiodes in the above embodiments, the photodiodes are not limited to APDs. That is, in the above embodiments, other types of photodiodes having a light incident surface and a light receiving region may be provided instead of APD 10. [Explanation of symbols]

[0074] 1,1A,1B,2…Optical element, 10…Avalanche photodiode (APD), 11~14…Semiconductor region, 15…Reflective part (first reflecting part), 16…Anti-reflective film, 17…Reflective part (second reflecting part), 18…Pixel part, 20,20A…Nanostructure, 21,210…Film, 22,23…Gap, 31…Transparent layer, 32…Metalens, 40…Optical integrated circuit, 41…First Si layer, 42…SiO2 layer, 43…Second Si layer, 101…Light incident surface, 102…Back surface, 151…Light reflective surface (first light reflective surface) ), 171...light reflection surface (second light reflection surface), 201~206...unit structure (first unit structure), 211...inner surface, 212...electron beam resist, 321...unit structure (second unit structure), 401...light incident surface, A1~A4...center line, B1...arrow, B2,B4...semicircle, B3,B5...curve, C11~C14...curve, fa,fb...focal length, G...width, L1...light, L2...focused light, L3,L31,L32...diffracted light, L30...0th order light, P...period, Q,U1,U2...point, R...focus point, θ...diffraction angle.

Claims

1. A photodiode having a light incident surface and a light receiving region, which converts light incident on the light incident surface into electrons in the light receiving region, A photodiode has a nanostructure provided on the light incident surface, the nanostructure has a plurality of first unit structures arranged along the light incident surface, the period of the plurality of first unit structures is greater than or equal to the wavelength of the light, and a focusing structure that focuses the diffracted light onto the light-receiving region by individually modulating the phase of the ±n-th order (n>0) diffracted light in each of the plurality of first unit structures, An optical element equipped with the following features.

2. An optical component having a light incident surface into which light is incident, The optical member has a nanostructure provided on the light incident surface, the nanostructure has a plurality of first unit structures arranged along the light incident surface, the period of the plurality of first unit structures is greater than or equal to the wavelength of the light, and the focusing structure focuses the diffracted light by individually modulating the phase of the ±n-th order (n>0) diffracted light in each of the plurality of first unit structures, An optical element equipped with the following features.

3. Equipped with a metal lens, The nanostructure is located between the metalens and the light incident surface, The metalens has a plurality of second unit structures arranged along the light incident surface, The period of the plurality of second unit structures is less than the wavelength of light. The optical element according to claim 1 or 2, wherein the metalens focuses the zeroth-order light toward the nanostructure.

4. The aforementioned light-gathering structure further comprises a first reflecting section, The first reflective portion is provided on the side opposite to the nanostructure with respect to the light incident surface, and is arranged to surround the nanostructure when viewed from a direction perpendicular to the light incident surface. The optical element according to claim 1 or 2, wherein the first reflective portion has a first light-reflecting surface facing inward within the enclosure.

5. The aforementioned light-gathering structure further comprises a second reflecting section, The second reflective portion is provided on the side opposite to the nanostructure with respect to the light incident surface, The optical element according to claim 1 or 2, wherein the second reflective portion has a second light-reflecting surface facing the direction of the light incident surface.

6. The optical element according to claim 1 or 2, wherein the light-gathering structure focuses the diffracted light by individually modulating the phase of the ±2nd order or higher diffracted light in each of the plurality of first unit structures.

7. The phase distribution of light emitted from the aforementioned nanostructure is given by the following formula (where φ Lens (x, y) is the phase at the coordinate position (x, y) on the light incident surface, λ is the wavelength of the light, f is the focal length due to the nanostructure, k ｘ The optical element according to claim 1 or 2, wherein is any coefficient including 0, α is α = π / 2 - θ (rad), θ is the diffraction angle, and α ≥ 0. [Math 1]

8. Each of the plurality of first unit structures has a gap extending in a predetermined direction, The optical element according to claim 1 or 2, wherein the relative position of the center of the gap between each of the plurality of first unit structures with respect to the center of each of the plurality of first unit structures in a direction intersecting the predetermined direction is determined according to the phase.

9. The optical element according to claim 1 or 2, wherein the plurality of first unit structures mainly contain metal.

10. The optical element according to claim 1 or 2, wherein the plurality of first unit structures mainly contain a dielectric.

11. The optical element according to claim 1, wherein the photodiode is an avalanche photodiode.

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