Patent Publication Number: US-2021191019-A1

Title: Resonator structure, imaging element, and electronic apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a resonator structure including a resonator, and an imaging element and an electronic apparatus each including the resonator structure. 
     BACKGROUND ART 
     There have been proposed solid-state imaging devices each including a multilayer interference filter that includes a plurality of Fabry-Perot resonator structures having different thicknesses, thereby allowing light of a plurality of wavelength bands to be selectively transmitted therethrough (e.g., PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2006-351800 
     SUMMARY OF THE INVENTION 
     An imaging element able to perform such multispectral dispersion preferably has a structure having superior mass productivity while acquiring a highly accurate optical spectrum. 
     A resonator structure that obtains a highly accurate optical spectrum while having a simple structure, and an imaging element and an electronic apparatus each including the resonator structure are therefore desired. 
     A resonator structure as an embodiment of the present disclosure includes a stacked structure that includes a semiconductor layer, a first resonator, a first reflection layer, a second resonator, a second reflection layer stacked in this order, allows light of a specific wavelength band to be transmitted therethrough, the semiconductor layer having a first average refractive index, the first resonator having a second average refractive index lower than the first average refractive index, and the first reflection layer having a third average refractive index higher than the second average refractive index. In addition, an imaging element and an electronic apparatus as embodiments of the present disclosure each include the resonator structure described above. 
     According to the resonator structure, the imaging element, and the electronic apparatus as the embodiments of the present disclosure, it is possible to obtain a highly accurate optical spectrum while having a simple configuration. 
     It is to be noted that the effects of the present disclosure are not limited to the effects described above, and may be any effect described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an entire configuration example of an imaging element according to a first embodiment of the present disclosure. 
         FIG. 2  is a schematic enlarged cross-sectional view of a main part of the imaging element illustrated in  FIG. 1 . 
         FIG. 3  illustrates an example of a periodic spectral arrangement by a resonator filter of the imaging element illustrated in  FIG. 1 . 
         FIG. 4A  is a cross-sectional view of a process of a method of manufacturing the imaging element illustrated in  FIG. 1 . 
         FIG. 4B  is a cross-sectional view of a process subsequent to  FIG. 4A . 
         FIG. 5  is a characteristic diagram illustrating an example of thickness dependency of a first resonator in transmittance of light transmitted through a resonator structure illustrated in  FIG. 1 . 
         FIG. 6  is a characteristic diagram illustrating an example of spectral characteristics in the resonator structure illustrated in  FIG. 1 . 
         FIG. 7  is a schematic cross-sectional view of an entire configuration example of an imaging element according to a first modification example of the first embodiment of the present disclosure. 
         FIG. 8  is a schematic enlarged cross-sectional view of a main part of the imaging element illustrated in  FIG. 7 . 
         FIG. 9  is a characteristic diagram illustrating spectral characteristics in a resonator structure illustrated in  FIG. 7 . 
         FIG. 10  is another characteristic diagram illustrating spectral characteristics in the resonator structure illustrated in  FIG. 7 . 
         FIG. 11  is a schematic cross-sectional view of an entire configuration example of an imaging element according to a second modification example of the first embodiment of the present disclosure. 
         FIG. 12  is a schematic plan view of a main part of the imaging element illustrated in  FIG. 11 . 
         FIG. 13  is a schematic plan view of an entire configuration example of a camera module according to a second embodiment of the present disclosure. 
         FIG. 14  is a schematic enlarged cross-sectional view of a main part of an imaging element illustrated in  FIG. 13 . 
         FIG. 15  is a characteristic diagram illustrating spectral characteristics of reflectance depending on an growth state of plants and the like. 
         FIG. 16  illustrates optical spectrum characteristics of reflectance of human skin. 
         FIG. 17  is a block diagram illustrating a configuration example of an imaging device mounted on an electronic apparatus. 
         FIG. 18  is a view depicting an example of a schematic configuration of an endoscopic surgery system. 
         FIG. 19  is a block diagram depicting an example of a functional configuration of a camera head and a camera control unit (CCU). 
         FIG. 20  is a block diagram depicting an example of a schematic configuration of an in-vivo information acquisition system. 
         FIG. 21  is a block diagram depicting an example of schematic configuration of a vehicle control system. 
         FIG. 22  is a diagram of assistance in explaining an example of installation positions of an outside-vehicle information detecting section and an imaging section. 
         FIG. 23  is a schematic cross-sectional view of an entire configuration example of an imaging element according to a third modification example of the first embodiment of the present disclosure. 
         FIG. 24  is a schematic cross-sectional view of an entire configuration example of an imaging element according to a first modification example of the second embodiment of the present disclosure. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present disclosure are described in detail below with reference to drawings. It is to be noted that description is given in the following order. 
     1. First Embodiment: Example of Imaging Element Including Resonator Structure 
     2. Modification Examples of First Embodiment: Modification Examples of Imaging Element Including Resonator Structure 
     3. Second Embodiment: Example of Camera Module Including a Plurality of Imaging Elements 
     4. Application Examples of Imaging Device 
     5. Example of Practical Application to Endoscopic Surgery System 
     6. Example of Practical Application to In-Vivo Information Acquisition System 
     7. Example of Practical Application to Mobile Body 
     8. Other Modification Examples 
     1. First Embodiment: Example of Imaging Element 
     [Configuration of Imaging Element  1 ] 
       FIG. 1  is a schematic cross-sectional view of an entire configuration example of an imaging element  1  as a first embodiment of the present disclosure.  FIG. 2  is a schematic enlarged cross-sectional view of any one pixel  22  of a plurality of pixels  22  included in the imaging element  1 . The imaging element  1  is, for example, a visible light spectral type COMS image sensor. 
     As illustrated in  FIG. 1 , the imaging element  1  includes a photodiode PD as a photoelectric converter, the plurality of pixels  22  ( 22 - 1  to  22 - 4 ) each including a resonator structure  10  that transmits light of a specific wavelength band toward a photodiode PD.  FIG. 1  illustrates four pixels  22 - 1  to  22 - 4  as an example; however, the number of pixels  22  included in the imaging element  1  is not limited thereto. 
     The photodiode PD is embedded in a semiconductor substrate  12 , for example. A wiring layer  11  is provided on a front surface  12 A of the semiconductor substrate  12 . The wiring layer  11  includes a wiring line  11 A included in a drive circuit that includes an MOSFET or the like used for driving of each of the plurality of pixels  22 . The semiconductor substrate  12  is one component included in the resonator structure  10 , and has a first average refractive index N 1 . The semiconductor substrate  12  includes a semiconductor material such as Si (silicon), for example. The first average refractive index N 1  is about 4, for example. In the present application, the “average refractive index” refers to a refractive index averaged by a volume ratio in a case where a target object, for example, the semiconductor substrate  12 , includes two or more kinds of components each having a different refractive index. For example, in a case where the semiconductor substrate  12  includes a mixture including a first material having a first refractive index n 1  and a second material having a second refractive index n 2  at a volume ratio of V 1 :V 2 , the average refractive index N is a value determined by: 
         N =( V 1* n 1+ V 2* n 2)/( V 1+ V 2). 
     The same applies to a case where the target object includes three or more kinds of components each having a different refractive index. In addition, in a case where the target object includes only one type of component, the refractive index of the component is the average refractive index N. 
     (Resonator Structure  10 ) 
     The resonator structure  10  includes the semiconductor substrate  12  and a stacked structure provided on a back surface  12 B of the semiconductor substrate  12 . The stacked structure includes, in order from the back surface  12 B of the semiconductor substrate  12 , a first resonator  13 , a first reflection layer  14 , a second resonator  15 , a second reflection layer  16 , and a transparent layer  17  that are stacked in order. Here, the first resonator  13  has a second average refractive index N 2  lower than the first average refractive index N 1 . In addition, the first reflection layer  14  has a third average refractive index N 3  higher than the second average refractive index N 2 . 
     The first resonator  13  includes a multilayer film structure in which a plurality of films is stacked. The plurality of films includes a material containing at least one kind of silicon oxide, silicon nitride, tantalum oxide, or aluminum oxide. Specifically, the first resonator  13  includes, for example, a first film  131  including silicon dioxide (SiO 2 ), a second film  132  including silicon nitride (SiN), and a third film  133  including silicon dioxide (SiO 2 ) that are stacked in this order from the back surface  12 B side. A thickness  13 T of the first resonator  13  is desirably, for example, 400 nm or less. 
     The first reflection layer  14 , the second resonator  15 , and the second reflection layer  16  form a so-called Fabry-Perot resonator structure. Accordingly, wavelength selectivity appears in transmittance of light L transmitted through the first reflection layer  14 , the second resonator  15 , and the second reflection layer  16 . The first reflection layer  14  and the second reflection layer  16  each include, for example, polycrystal silicon (Si), and function as a reflection film. Each of the thickness of the first reflection layer  14  and the thickness of the second reflection layer  16  may be 50 nm or less. Polycrystal silicon (Si) has a large light absorption coefficient in a short wavelength band, which causes reduction in transmittance of transmitted light. For this reason, reducing both the thickness of the first reflection layer  14  and the thickness of the second reflection layer  16  to 50 nm or less, for example, to about 30 nm, makes it possible to minimize light absorption in the first reflection layer  14  and the second reflection layer  16 . The second resonator  15  includes, for example, silicon dioxide (SiO 2 ). In the imaging element  1 , others, except for the first reflection layer  14 , the second reflection layer  16 , and the semiconductor substrate  12 , may include a material other than elemental silicon. In the imaging element  1 , for example, as in a schematic view of a pixel arrangement illustrated in  FIG. 3 , the plurality of pixels  22  including the second resonators  15  having thicknesses different from each other are periodically arranged in a stacked plane (in an XY plane orthogonal to a Z-axis direction that is a stacking direction). It is to be noted that  FIG. 3  illustrates an example in which pixels  22 - 1  to  22 - 16  each including the second resonator  15  having a different thickness  15 T are periodically arranged.  FIG. 1  illustrates the pixels  22 - 1  to  22 - 4  of the pixels  22 - 1  to  22 - 16  as an example. The pixels  22 - 1  to  22 - 4  respectively include the second resonators  15  having thicknesses  15 T 1  to  15 T 4 . That is, the pixels  22 - 1  to  22 - 16  are regarded as a pixel group  22 G for one cycle, and a plurality of pixel groups  22 G is repeatedly arranged in the stacked plane. Here, wavelength selectivity for the light L in the Fabry-Perot resonator structure including the first reflection layer  14 , the second resonator  15 , and the second reflection layer  16  is dependent on the thickness  15 T of the second resonator  15 . Accordingly, the plurality of pixels  22  including the second resonators  15  having different thicknesses is periodically arranged as in  FIG. 3 , which allows for multispectral dispersion in the imaging element  1 . 
     The transparent layer  17  includes a transparent body including, for example, silicon dioxide (SiO 2 ) as a main constituent material. Front surfaces  17 S of the transparent layers  17  in the plurality of pixels  22  are located at height positions substantially equal to each other. 
     Each of light shielding layers  18  and  19  may be provided in proximity to a boundary between adjacent pixels  22 . The light shielding layer  18  is embedded in the third film  133  of the first resonator  13 , for example. In addition, the light shielding layer  18  is provided in proximity to the back surface  12 B of the semiconductor substrate  12 , for example. This is to prevent light leakage between adjacent pixels  22  and avoid color mixture. 
     For example, a moth eye (Moth Eye) structure  21  is provided on a front surface  17 S of the transparent layer  17 . This is to suppress front surface reflection in the front surface  17 S of the transparent layer  17  and reduce spectrum vibration. The moth eye structure  21  is a structure having a plurality of pointed projections arranged on the front surface  17 S at a pitch of a wavelength λ or less, in particular at a pitch of ⅓×λ or less. It is possible to form the moth eye structure  21  as follows. First, for example, as illustrated in  FIG. 4A , a transparent ultraviolet curable resin layer  21 Z having a uniform thickness is applied by, for example, a spin coating method to cover the front surface  17 S of the transparent layer  17 . Meanwhile, a mold  23  in which a predetermined recessed and projected pattern  23 P is formed is prepared. The mold  23  has the fine recessed and projected pattern  23  formed by dry etching a substrate such as a Si substrate, which allows ultraviolet rays to be transmitted therethrough, with use of a resist pattern formed by electron beam lithography, for example. Next, as illustrated in  FIG. 4B , the recessed and projected pattern  23 P is pressed against the ultraviolet curable resin layer  21 Z. Further, while maintaining that state, that is, a state in which the recessed and projected pattern  23 P is pressed against the ultraviolet curable resin layer  21 Z, the ultraviolet curable resin layer  21 Z is irradiated with ultraviolet rays UV having a predetermined intensity for a predetermined period of time to cure the ultraviolet curable resin layer  21 Z. After the ultraviolet curable resin layer  21 Z is cured, the mold  23  is removed, thereby obtaining the moss eye structure  21 . 
     [Workings of Imaging Element  1 ] 
     In the imaging element  1 , for example, a resonator length in the first resonator  13 , that is, the thickness  13 T (see  FIG. 2 ) as a dimension in the Z-axis direction of the first resonator  13  is changed, which makes it possible to change a shape of a peak wavelength of transmitted light transmitted through the resonator structure  10 . As previously described, the first resonator  13  is provided between the semiconductor substrate  12  in which the photodiode PD is embedded and the first reflection layer  14  that forms the Fabry-Perot resonator structure. The second average refractive index N 2  of the first resonator  13  is lower than the first average refractive index N 1  in the semiconductor substrate  12  and lower than the third average refractive index N 3  in the first reflection layer  14 . Accordingly, the first resonator  13  exhibits coherence, and wavelength selectivity for transmitted light in the resonator structure  10  is improved. Si (silicon) is suitable as constituent materials of the semiconductor substrate  12  and the first reflection layer  14 . An average refractive index of silicon is about 4. For this reason, the second average refractive index N 2  of the first resonator  13  is desirably 1 or more and 4 or less. This is because a sufficient interference effect in the first resonator  13  is obtained. The second average refractive index N 2  of the first resonator  13  may be specifically 1 or more and 2 or less. This is because the interference effect in the first resonator  13  is further improved. 
       FIG. 5  illustrates an example of dependency of the first resonator  13  on the thickness  13 T at transmittance T of the light transmitted through the resonator structure  10 . In  FIG. 5 , an upper left graph represents a case of the thickness  13 T=1100 nm, an upper right graph represents a case of the thickness  13 T=1100 nm, a lower left graph represents a case of the thickness  13 T=400 nm, and a lower right graph represents a case of the thickness  13 T=200 nm. It is to be noted that  FIG. 5  illustrates results of simulating an optical spectrum by the effective Fresnel coefficient method in a case where the light L having entered the resonator structure  10  from the front surface  17 S is transmitted through the resonator structure  10 . In addition, in each of the cases,  FIG. 5  corresponds to a case where the semiconductor substrate  12  includes Si (silicon), the first film  131  includes SiO 2 , the second film  132  includes SiO 2 , the third film  133  includes SiO 2 , the first reflection layer  14  includes polycrystal silicon (Si) having a thickness of 31 nm, the second resonator  15  includes SiO 2  having a thickness of 147 nm, the second reflection layer  16  includes polycrystal silicon (Si) having a thickness of 31 nm, and the transparent layer  17  includes SiO 2  having a thickness of 100 nm. As illustrated in  FIG. 5 , it can be seen that in a case where the thickness  13 T of the first resonator  13  is 400 nm or less, an unimodal peak appearing in a wavelength band of 400 nm to 500 nm is obtained in a transmittance distribution of the light L transmitted, and wavelength selectivity suitable in performing multispectral dispersion is obtained. 
     In the imaging element  1 , for example, changing a resonator length in the second resonator  15 , that is, the thickness  15 T (see  FIG. 2 ) as the dimension in the Z-axis direction of the second resonator  15  makes it possible to change the position of a peak wavelength of visible light transmitted through the resonator structure  10 . Specifically, in the imaging element  1 , for example, changing the thickness  15 T in a range from 147 nm to 215 nm allows for dispersion into blue light, green light, and red light. 
       FIG. 6  illustrates an example of dependency of the second resonator  15  on the thickness  15 T at the transmittance T of the light L transmitted through the resonator structure  10 . In  FIG. 6 , a curve R 61  represents a case of the thickness  15 T=147 nm, a curve R 62  represents a case of the thickness  15 T=180 nm, and a curve R 63  represents a case of the thickness  15 T=215 nm. It is to be noted that  FIG. 6  corresponds to a case where the semiconductor substrate  12  includes Si (silicon), the first film  131  includes SiO 2  having a thickness of 10 nm, the second film  132  includes SiO 2  having a thickness of 56 nm, the third film  133  includes SiO 2  having a thickness of 20 nm, the first reflection layer  14  includes polycrystal silicon (Si) having a thickness of 31 nm, the second resonator  15  includes SiO 2 , the second reflection layer  16  includes polycrystal silicon (Si) having a thickness of 31 nm, and the transparent layer  17  includes SiO 2  having a thickness of 100 nm. As illustrated in  FIG. 6 , changing the thickness  15 T of the second resonator  15  allows for dispersion into three colors of blue, green, and red, for example. In addition, it is also possible to reduce the half width of each peak to a narrow width of 50 nm or less. Accordingly, in the imaging element  1 , the pixels  22 - 1  to  22 - 15  including the second resonators  15  having different thicknesses  15 T are periodically arranged as illustrated in  FIG. 3 , which allows for multispectral dispersion. 
     [Effects of Imaging Element  1 ] 
     As described above, the resonator structure  10  of the imaging element  1  includes the semiconductor substrate  12 , the first resonator  13 , the first reflection layer  14 , the second resonator  15 , and the second reflection layer  16  that are stacked in this order, and allows the light L of a specific wavelength band to be transmitted therethrough. The first resonator  13  has the second average refractive index N 2  lower than both the first average refractive index N 1  in the semiconductor substrate  12  and the third average refractive index N 3  in the first reflection layer  14 . Accordingly, the resonator structure  10  and the imaging element  1  obtain a highly accurate optical spectrum while having a simple structure including a relatively small number of layers. 
     In the imaging element  1 , the peak wavelength of the intensity of the light L transmitted is changed, for example, depending on the thickness  15 T of the second resonator  15  provided on a side opposite to the semiconductor substrate  12  as viewed from the first resonator  13  in the resonator structure  10 . In the imaging element  1 , the pixels  22  including a plurality of resonator structures  10  having different thicknesses  15 T are periodically arranged with use of this property, which allows for multispectral dispersion. 
     In particular, in the resonator structure  10 , the thickness  13 T of the first resonator  13  is 400 nm or less, thereby obtaining a unimodal peak appearing in a wavelength band of 400 nm to 500 nm in the transmittance distribution of the light L transmitted, and obtaining wavelength selectivity suitable in performing multispectral dispersion. 
     In addition, in the resonator structure  10 , setting each of the thickness of the first reflection layer  14  and the thickness of the second reflection layer  16  to 50 nm or less makes it possible to minimize light absorption in the first reflection layer  14  and the second reflection layer  16 . 
     2. Modification Examples of First Embodiment: Modification Examples of Imaging Element 
     2.1 First Modification Example 
     [Configuration of Imaging Element  1 A] 
       FIG. 7  is a schematic cross-sectional view of an entire configuration example of an imaging element  1 A as a first modification example of the first embodiment of the present disclosure.  FIG. 8  is a schematic enlarged cross-sectional view of an enlarged view of any one pixel  22  of the plurality of pixels  22  included in the imaging element  1 A. The imaging element  1 A is, for example, an infrared spectral type COMS image sensor. 
     In the imaging element  1  according to the first embodiment described above, the first resonator  13  having a three-layer structure has been described as an example. In contrast, the imaging element  1 A as the present modification example includes a resonator structure  10 A that includes a first resonator  13 A having a four-layer structure. Specifically, the first resonator  13 A in the resonator structure  10 A includes the first film  131 , the second film  132 , the third film  133 , and a fourth film  134  that are stacked in this order from the back surface  12 B side. Each of the first film  131  and the fourth film  134  includes, for example, silicon dioxide (SiO 2 ). The second film  132  includes, for example, aluminum oxide (AlO). The third film  133  includes, for example, tantalum oxide (TaO). A thickness  13 AT of the first resonator  13 A is desirably 400 nm or less, for example. The first resonator  13 A has the second average refractive index N 2  lower than both the first average refractive index N 1  in the semiconductor substrate  12  and the third average refractive index N 3  in the first reflection layer  14 . 
     In addition, the imaging element  1 A includes a visible light cut filter  24  instead of the moss eye structure  21  on the front surface  17 S of the transparent layer  17 . 
     Except for these points, the imaging element  1 A as a modification example 1 has substantially the same configuration as the imaging element  1 . 
     [Workings of Imaging Element  1 A] 
     The resonator structure  10 A in the imaging element  1 A has, for example, an overall thickness of 1 μm or less, and sufficiently suppresses color mixture. Furthermore, changing the thickness  15 T of the second resonator  15 , for example, in a range from 250 nm to 350 nm makes it possible to spectrally disperse infrared light. 
       FIG. 9  is a characteristic diagram illustrating an example of dependency of the second resonator  15  on the thickness  15 T at the transmittance T of the light L transmitted through the resonator structure  10 A. In  FIG. 9 , a curve R 91  represents a case of the thickness  15 T=250 nm, a curve R 92  represents a case of the thickness  15 T=275 nm, a curve R 93  represents a case of the thickness  15 T=300 nm, a curve R 94  represents a case of the thickness  15 T=325 nm, and a curve R 95  represents a case of the thickness  15 T=350 nm. It is to be noted that  FIG. 9  corresponds to a case where the semiconductor substrate  12  includes Si (silicon), the first film  131  includes SiO 2  having a thickness of 1 nm, the second film  132  includes AlO having a thickness of 7 nm, the third film  133  includes TaO having a thickness of 54 nm, the fourth film  134  includes SiO 2  having a thickness of 20 nm, the first reflection layer  14  includes polycrystal silicon (Si) having a thickness of 31 nm, the second resonator  15  includes SiO 2 , the second reflection layer  16  includes polycrystal silicon (Si) having a thickness of 31 nm, and the transparent layer  17  includes SiO 2  having a thickness of 110 nm. 
     In addition, in the resonator structure  10 A in the imaging element  1 A, changing the thickness  15 T of the second resonator  15  as a higher-order mode Fabry-Perot structure, for example, in a range from 500 nm to 625 nm makes it possible to obtain an optical spectrum having peaks, each with a narrow half-width. 
       FIG. 10  is another characteristic diagram illustrating an example of dependency of the second resonator  15  on the thickness  15 T at the transmittance T of the light L transmitted through the resonator structure  10 A. In  FIG. 10 , a curve R 101  represents a case of the thickness  15 T=500 nm, a curve R 102  represents a case of the thickness  15 T=525 nm, a curve R 103  represents a case of the thickness  15 T=550 nm, a curve R 104  represents a case of the thickness  15 T=575 nm, a curve R 105  represents a case of the thickness  15 T=600 nm, a curve R 106  represents a case of the thickness  15 T=625 nm, a curve R 107  represents a case of the thickness  15 T=650 nm, and a curve R 108  represents a case of the thickness  15 T=675 nm. It is to be noted that  FIG. 10  corresponds to a case where the semiconductor substrate  12  includes Si (silicon), the first film  131  includes SiO 2  having a thickness of 1 nm, the second film  132  includes AlO having a thickness of 7 nm, the third film  133  includes TaO having a thickness of 54 nm, the fourth film  134  includes SiO 2  having a thickness of 20 nm, the first reflection layer  14  includes polycrystal silicon (Si) having a thickness of 31 nm, the second resonator  15  includes SiO 2 , the second reflection layer  16  includes polycrystal silicon (Si) having a thickness of 31 nm, and the transparent layer  17  includes SiO 2  having a thickness of 110 nm. 
     [Effects of Imaging Element  1 A] 
     As illustrated in  FIGS. 9 and 10 , changing the thickness  15 T of the second resonator  15  allows for spectral dispersion into, for example, light having five different peak wavelengths in an infrared region. It is to be noted that an optical spectrum in the resonator structure  10 A also includes a peak of visible light. Accordingly, a component in a visible light region R 24  of the light L entering the resonator structure  10 A is removed by the visible light cut filter  24 , which makes it possible to obtain an optical spectrum only in the infrared region in the imaging element  1 A. 
     In addition, the first resonator  13 A has the second average refractive index N 2  lower than both the first average refractive index N 1  in the semiconductor substrate  12  and the third average refractive index N 3  in the first reflection layer  14 . Accordingly, the resonator structure  10  and the imaging element  1 A obtain a highly accurate optical spectrum while having a simple structure. 
     2.2 Second Modification Example 
     [Configuration of Imaging Element  1 B] 
       FIG. 11  is a schematic cross-sectional view of an entire configuration example of an imaging element  1 B as a second modification example of the first embodiment of the present disclosure.  FIG. 12  is a schematic plan view of a configuration example of the second resonator  15  of a resonator structure  10 B included in the imaging element  1 B. 
     In the imaging element  1  according to the first embodiment described above, a plurality of pixels each including the second resonator  15  having a different thickness  15 T are arranged, thereby changing the peak wavelength of the intensity of the light L transmitted through the respective pixels  22  to perform multispectral dispersion. In contrast, in the imaging element  1 B as the present modification example, while resonator lengths in the plurality of pixels  22 , i.e., the thicknesses  15 T of the second resonators  15  are uniform, microstructures different for each of the pixels  22  are formed in the second resonators  15  of the respective pixels  22 . Specifically, the second resonator  15  includes a first portion  151  having a first refractive index, and a second portion  152  as a microstructure that is dispersedly located in the first portion  151  in the stacked plane (in the XY plane) and has a second refractive index. Thus, the second resonator  15  has a refractive index distribution in the stacked plane. It is to be noted that “having a refractive index distribution” here means that the second resonator  15  has a different effective refractive index for each of the pixels  22 . Here, the first portion  151  includes, for example, Si 3 N 4 , and the second portion  152  includes, for example, SiO 2 .  FIG. 12  illustrates, as an example, a state in which 16 types of pixels  22 - 1  to  22 - 16  each having a different existence ratio of the first portion  151  and the second portion  152  in the second resonator  15  are periodically arranged in a matrix. 
     [Workings and Effects of Imaging Element  1 B] 
     Even in the imaging element  1 B as the second modification example of the first embodiment, it is possible to obtain an optical spectrum having 16 kinds of different peak wavelengths. 
     3. Second Embodiment: Example of Camera Module 
     [Configuration of Camera Module  2 ] 
       FIG. 13  is a schematic plan view of an entire configuration example of a camera module  2  as a second embodiment of the present disclosure.  FIG. 14  is a schematic enlarged cross-sectional view of any one of a plurality of imaging elements  1 C ( 1 C 1  to  1 C 9 ) included in the camera module  2 . The imaging element  1 C is, for example, a visible light spectral type COMS image sensor. 
     The camera module  2  includes nine imaging elements  1 C 1  to  1 C 9  arranged in, for example, three rows and three columns in the XY plane. Each of the imaging elements  1 C 1  to  1 C 9  includes an imaging lens  25  provided on the front surface  17 S of transparent layer  17  instead of the moth eye structure  21 . Here, one imaging lens  25  is provided common to a plurality of pixels  22  in each of the imaging elements  1 C 1  to  1 C 9 . The imaging lens  25  provides a refractive power to the light L (L 1  to L 9 ) toward the resonator structure  10 C. It is to be noted that the shapes and refractive powers of the imaging lenses  25  in the imaging elements  1 C 1  to  1 C 9  may be the same as or different from each other. Except for this point, the imaging element  1 C ( 1 C 1  to  1 C 9 ) has substantially the same configuration as the imaging element  1  according to the first embodiment described above. 
     The resonator structures  10 C in the imaging elements  1 C 1  to  1 C 9  respectively allow the light L having different peak wavelengths, that is, lights L 1  to L 9  having wavelengths λ 1  to λ 9  to be selectively transmitted therethrough. Accordingly, the imaging elements  1 C 1  to  1 C 9  selectively obtain the lights L 1  to L 9  having the wavelengths λ 1  to λ 9 , respectively. 
     [Workings and Effects of Camera Module  2 ] 
     As described above, the camera module  2  includes a plurality of imaging elements  1 C 1  to  1 C 9  that obtain respective different wavelengths. As described in the first embodiment described above, each of the imaging elements  1 C 1  to  1 C 9  has a simple structure including a relatively small number of layers, which makes it possible to achieve simplification and thinning of the camera module  2  as a whole. This makes it possible to manufacture the camera module  2  by a simple manufacturing process, which contributes to mass production. In addition, a highly accurate optical spectrum is obtained in the resonator structure  10 C, thereby also improving imaging performance in the camera module  2 . 
     4. Application Examples of Imaging Device 
     Next, applications to which the imaging element  1  is applicable are described with reference to  FIGS. 15 and 16 . 
     [4.1 Application Example to Monitoring of Growth State of Agricultural Crops, Etc.] 
     It is possible to use the imaging element  1  in a spectroscopic device that performs multispectral dispersion or hyperspectral dispersion for measurement of a normalized differential vegetation index (NDVI: Normalized Difference Vegetation Index) for growth of agricultural crops and plants (hereinafter, referred to as plants and the like).  FIG. 15  illustrates spectral characteristics of reflectance depending on a growth state of the plants and the like. 
     As illustrated in  FIG. 15 , in a wavelength range from 600 nm to 800 nm, the plants and the like have a different reflectance distribution depending on the growth state thereof. That is, the reflectance distribution different depending on whether the plants and the like are healthy, weak, or dead is illustrated. This reflectance distribution is formed by reflected light mainly on leaves of the plants and the like. It is possible from the results of the reflectance distribution in  FIG. 15  that obtaining spectral characteristics of two or more lights from the plants and the like at least in a wavelength band including wavelengths from 600 nm to 800 nm makes it possible to sense the growth state (vegetation state) of the plants and the like. 
     It is possible to sense the vegetation state from a relationship between two signal values with use of, for example, two imaging elements, that is, the imaging element  1  able to detect a wavelength band in a wavelength range of 600 nm to 700 nm, and another imaging element  1  able to detect a wavelength band in a wavelength range of 700 nm to 800 nm. Alternatively, to improve detection accuracy, three or more imaging elements  1  may be used to detect three or more plural wavelength bands and sense the vegetation state from a relationship among signal values of these wavelength bands. 
     The imaging element  1  able to detect such a wavelength band is mounted on, for example, a small unmanned aerial vehicle (a so-called drone), which makes it possible to monitor the growth state of agricultural crops from the sky, and manage and control growth of the crops. 
     [4.2 Application Example to Biometric Authentication] 
     It is possible to use the imaging element  1 , for example, in a spectroscopic device that performs multispectral dispersion or hyperspectral dispersion to measure reflectance of human skin in biometric authentication. In the multispectral dispersoin or hyperspectral dispersion, a plurality of spectral dispersions is performed in multiple bands of three or more primary colors of light.  FIG. 16  illustrates optical spectrum characteristics of reflectance of human skin. 
     As illustrated in  FIG. 16 , it can be seen that the reflectance greatly changes particularly in a wavelength range of 450 nm to 650 nm. These changes make it possible to authenticate whether or not a subject is human skin. 
     For example, detecting three spectra of a wavelength of 450 nm, a wavelength of 550 nm, and a wavelength of 650 nm with use of three imaging elements  1  makes it possible to authenticate whether or not the subject is human skin. For example, in a case where the subject is a material other than human skin, the spectral characteristics of the reflectance are changed, which makes it possible to distinguish the material from human skin. 
     Accordingly, the imaging element  1  able to detect such a wavelength band is mounted on, for example, a biometric authentication device, which makes it possible to apply the imaging element  1  to prevention of forgery of faces, fingerprints, irises, and the like, thereby enabling more accurate biometric authentication. 
     [4.3 Application Example to Electronic Apparatus] 
     The imaging element  1  as described above is applicable to various kinds of electronic apparatuses. Examples of the electronic apparatuses include an imaging system such as a digital still camera and a digital video camera, a mobile phone having an imaging function, and other apparatuses having imaging functions. 
       FIG. 17  is a block diagram illustrating a configuration example of an imaging device  101  mounted on an electronic apparatus. As illustrated in  FIG. 17 , the imaging device  101  includes an optical system  102 , an imaging element  103 , a signal processing circuit  104 , a monitor  105 , and a memory  106 , and is able to capture a still image and a moving image. 
     The optical system  102  includes one or a plurality of lenses, and guides image light (incident light) from a subject to the imaging element  103 , and forms an image on a light reception surface (a sensor section) of the imaging element  103 . 
     The imaging element  1  described above is applied as the imaging element  103 . Electrons are accumulated in the imaging element  103  for a fixed period of time in accordance with the image formed on the light reception surface via the optical system  102 . Thereafter, a signal corresponding to the electrons accumulated in the imaging element  103  is supplied to the signal processing circuit  104 . 
     The signal processing circuit  104  performs various kinds of signal processing on a pixel signal outputted from the imaging element  103 . An image (image data) captured by performing signal processing by the signal processing circuit  104  is supplied to and displayed on the monitor  105 , or is supplied to and stored (recorded) in the memory  106 . 
     In the imaging device  101  configured in such a manner, applying the imaging element  1  described above makes it possible to obtain a highly accurate optical spectrum while having a simple structure. This makes it possible to cature an image with higher image quality. 
     5. Example of Practical Application to Endoscopic Surgery System 
     The technology (present technology) according to the present disclosure is applicable to various products. For example, the technology according to the present disclosure may be applied to an endoscopic surgery system. 
       FIG. 18  is a view depicting an example of a schematic configuration of an endoscopic surgery system to which the technology according to an embodiment of the present disclosure (present technology) can be applied. 
     In  FIG. 18 , a state is illustrated in which a surgeon (medical doctor)  11131  is using an endoscopic surgery system  11000  to perform surgery for a patient  11132  on a patient bed  11133 . As depicted, the endoscopic surgery system  11000  includes an endoscope  11100 , other surgical tools  11110  such as a pneumoperitoneum tube  11111  and an energy device  11112 , a supporting arm apparatus  11120  which supports the endoscope  11100  thereon, and a cart  11200  on which various apparatus for endoscopic surgery are mounted. 
     The endoscope  11100  includes a lens barrel  11101  having a region of a predetermined length from a distal end thereof to be inserted into a body cavity of the patient  11132 , and a camera head  11102  connected to a proximal end of the lens barrel  11101 . In the example depicted, the endoscope  11100  is depicted which includes as a rigid endoscope having the lens barrel  11101  of the hard type. However, the endoscope  11100  may otherwise be included as a flexible endoscope having the lens barrel  11101  of the flexible type. 
     The lens barrel  11101  has, at a distal end thereof, an opening in which an objective lens is fitted. A light source apparatus  11203  is connected to the endoscope  11100  such that light generated by the light source apparatus  11203  is introduced to a distal end of the lens barrel  11101  by a light guide extending in the inside of the lens barrel  11101  and is irradiated toward an observation target in a body cavity of the patient  11132  through the objective lens. It is to be noted that the endoscope  11100  may be a forward-viewing endoscope or may be an oblique-viewing endoscope or a side-viewing endoscope. 
     An optical system and an image pickup element are provided in the inside of the camera head  11102  such that reflected light (observation light) from the observation target is condensed on the image pickup element by the optical system. The observation light is photo-electrically converted by the image pickup element to generate an electric signal corresponding to the observation light, namely, an image signal corresponding to an observation image. The image signal is transmitted as RAW data to a CCU  11201 . 
     The CCU  11201  includes a central processing unit (CPU), a graphics processing unit (GPU) or the like and integrally controls operation of the endoscope  11100  and a display apparatus  11202 . Further, the CCU  11201  receives an image signal from the camera head  11102  and performs, for the image signal, various image processes for displaying an image based on the image signal such as, for example, a development process (demosaic process). 
     The display apparatus  11202  displays thereon an image based on an image signal, for which the image processes have been performed by the CCU  11201 , under the control of the CCU  11201 . 
     The light source apparatus  11203  includes a light source such as, for example, a light emitting diode (LED) and supplies irradiation light upon imaging of a surgical region to the endoscope  11100 . 
     An inputting apparatus  11204  is an input interface for the endoscopic surgery system  11000 . A user can perform inputting of various kinds of information or instruction inputting to the endoscopic surgery system  11000  through the inputting apparatus  11204 . For example, the user would input an instruction or a like to change an image pickup condition (type of irradiation light, magnification, focal distance or the like) by the endoscope  11100 . 
     A treatment tool controlling apparatus  11205  controls driving of the energy device  11112  for cautery or incision of a tissue, sealing of a blood vessel or the like. A pneumoperitoneum apparatus  11206  feeds gas into a body cavity of the patient  11132  through the pneumoperitoneum tube  11111  to inflate the body cavity in order to secure the field of view of the endoscope  11100  and secure the working space for the surgeon. A recorder  11207  is an apparatus capable of recording various kinds of information relating to surgery. A printer  11208  is an apparatus capable of printing various kinds of information relating to surgery in various forms such as a text, an image or a graph. 
     It is to be noted that the light source apparatus  11203  which supplies irradiation light when a surgical region is to be imaged to the endoscope  11100  may include a white light source which includes, for example, an LED, a laser light source or a combination of them. Where a white light source includes a combination of red, green, and blue (RGB) laser light sources, since the output intensity and the output timing can be controlled with a high degree of accuracy for each color (each wavelength), adjustment of the white balance of a picked up image can be performed by the light source apparatus  11203 . Further, in this case, if laser beams from the respective RGB laser light sources are irradiated time-divisionally on an observation target and driving of the image pickup elements of the camera head  11102  are controlled in synchronism with the irradiation timings. Then images individually corresponding to the R, G and B colors can be also picked up time-divisionally. According to this method, a color image can be obtained even if color filters are not provided for the image pickup element. 
     Further, the light source apparatus  11203  may be controlled such that the intensity of light to be outputted is changed for each predetermined time. By controlling driving of the image pickup element of the camera head  11102  in synchronism with the timing of the change of the intensity of light to acquire images time-divisionally and synthesizing the images, an image of a high dynamic range free from underexposed blocked up shadows and overexposed highlights can be created. 
     Further, the light source apparatus  11203  may be configured to supply light of a predetermined wavelength band ready for special light observation. In special light observation, for example, by utilizing the wavelength dependency of absorption of light in a body tissue to irradiate light of a narrow band in comparison with irradiation light upon ordinary observation (namely, white light), narrow band observation (narrow band imaging) of imaging a predetermined tissue such as a blood vessel of a superficial portion of the mucous membrane or the like in a high contrast is performed. Alternatively, in special light observation, fluorescent observation for obtaining an image from fluorescent light generated by irradiation of excitation light may be performed. In fluorescent observation, it is possible to perform observation of fluorescent light from a body tissue by irradiating excitation light on the body tissue (autofluorescence observation) or to obtain a fluorescent light image by locally injecting a reagent such as indocyanine green (ICG) into a body tissue and irradiating excitation light corresponding to a fluorescent light wavelength of the reagent upon the body tissue. The light source apparatus  11203  can be configured to supply such narrow-band light and/or excitation light suitable for special light observation as described above. 
       FIG. 19  is a block diagram depicting an example of a functional configuration of the camera head  11102  and the CCU  11201  depicted in  FIG. 18 . 
     The camera head  11102  includes a lens unit  11401 , an image pickup unit  11402 , a driving unit  11403 , a communication unit  11404  and a camera head controlling unit  11405 . The CCU  11201  includes a communication unit  11411 , an image processing unit  11412  and a control unit  11413 . The camera head  11102  and the CCU  11201  are connected for communication to each other by a transmission cable  11400 . 
     The lens unit  11401  is an optical system, provided at a connecting location to the lens barrel  11101 . Observation light taken in from a distal end of the lens barrel  11101  is guided to the camera head  11102  and introduced into the lens unit  11401 . The lens unit  11401  includes a combination of a plurality of lenses including a zoom lens and a focusing lens. 
     The number of image pickup elements which is included by the image pickup unit  11402  may be one (single-plate type) or a plural number (multi-plate type). Where the image pickup unit  11402  is configured as that of the multi-plate type, for example, image signals corresponding to respective R, G and B are generated by the image pickup elements, and the image signals may be synthesized to obtain a color image. The image pickup unit  11402  may also be configured so as to have a pair of image pickup elements for acquiring respective image signals for the right eye and the left eye ready for three dimensional (3D) display. If 3D display is performed, then the depth of a living body tissue in a surgical region can be comprehended more accurately by the surgeon  11131 . It is to be noted that, where the image pickup unit  11402  is configured as that of stereoscopic type, a plurality of systems of lens units  11401  are provided corresponding to the individual image pickup elements. 
     Further, the image pickup unit  11402  may not necessarily be provided on the camera head  11102 . For example, the image pickup unit  11402  may be provided immediately behind the objective lens in the inside of the lens barrel  11101 . 
     The driving unit  11403  includes an actuator and moves the zoom lens and the focusing lens of the lens unit  11401  by a predetermined distance along an optical axis under the control of the camera head controlling unit  11405 . Consequently, the magnification and the focal point of a picked up image by the image pickup unit  11402  can be adjusted suitably. 
     The communication unit  11404  includes a communication apparatus for transmitting and receiving various kinds of information to and from the CCU  11201 . The communication unit  11404  transmits an image signal acquired from the image pickup unit  11402  as RAW data to the CCU  11201  through the transmission cable  11400 . 
     In addition, the communication unit  11404  receives a control signal for controlling driving of the camera head  11102  from the CCU  11201  and supplies the control signal to the camera head controlling unit  11405 . The control signal includes information relating to image pickup conditions such as, for example, information that a frame rate of a picked up image is designated, information that an exposure value upon image picking up is designated and/or information that a magnification and a focal point of a picked up image are designated. 
     It is to be noted that the image pickup conditions such as the frame rate, exposure value, magnification or focal point may be designated by the user or may be set automatically by the control unit  11413  of the CCU  11201  on the basis of an acquired image signal. In the latter case, an auto exposure (AE) function, an auto focus (AF) function and an auto white balance (AWB) function are incorporated in the endoscope  11100 . 
     The camera head controlling unit  11405  controls driving of the camera head  11102  on the basis of a control signal from the CCU  11201  received through the communication unit  11404 . 
     The communication unit  11411  includes a communication apparatus for transmitting and receiving various kinds of information to and from the camera head  11102 . The communication unit  11411  receives an image signal transmitted thereto from the camera head  11102  through the transmission cable  11400 . 
     Further, the communication unit  11411  transmits a control signal for controlling driving of the camera head  11102  to the camera head  11102 . The image signal and the control signal can be transmitted by electrical communication, optical communication or the like. 
     The image processing unit  11412  performs various image processes for an image signal in the form of RAW data transmitted thereto from the camera head  11102 . 
     The control unit  11413  performs various kinds of control relating to image picking up of a surgical region or the like by the endoscope  11100  and display of a picked up image obtained by image picking up of the surgical region or the like. For example, the control unit  11413  creates a control signal for controlling driving of the camera head  11102 . 
     Further, the control unit  11413  controls, on the basis of an image signal for which image processes have been performed by the image processing unit  11412 , the display apparatus  11202  to display a picked up image in which the surgical region or the like is imaged. Thereupon, the control unit  11413  may recognize various objects in the picked up image using various image recognition technologies. For example, the control unit  11413  can recognize a surgical tool such as forceps, a particular living body region, bleeding, mist when the energy device  11112  is used and so forth by detecting the shape, color and so forth of edges of objects included in a picked up image. The control unit  11413  may cause, when it controls the display apparatus  11202  to display a picked up image, various kinds of surgery supporting information to be displayed in an overlapping manner with an image of the surgical region using a result of the recognition. Where surgery supporting information is displayed in an overlapping manner and presented to the surgeon  11131 , the burden on the surgeon  11131  can be reduced and the surgeon  11131  can proceed with the surgery with certainty. 
     The transmission cable  11400  which connects the camera head  11102  and the CCU  11201  to each other is an electric signal cable ready for communication of an electric signal, an optical fiber ready for optical communication or a composite cable ready for both of electrical and optical communications. 
     Here, while, in the example depicted, communication is performed by wired communication using the transmission cable  11400 , the communication between the camera head  11102  and the CCU  11201  may be performed by wireless communication. 
     One example of the endoscopic surgery system to which the technology according to the present disclosure may be applied has been described above. The technology according to the present disclosure may be applied to (the image pickup unit  11402  of) the camera head  11102 , (the image processing unit  11412  of) the CCU  11201 , and the like) among the components described above. Specifically, for example, the imaging element  1  is applicable to the image pickup unit  10402 . Applying the technology according to the present disclosure to the image pickup unit  10402  makes it possible to capture a more accurate image of a surgical region, which allows a surgeon to reliably confirm the surgical region. 
     It is to be noted that the endoscopic surgery system has been described here as one example, but the technology according to the present disclosure may be additionally applied to, for example, a microscopic surgery system or the like. 
     6. Example of Practical Application to In-Vivo Information Acquisition System 
       FIG. 20  is a block diagram depicting an example of a schematic configuration of an in-vivo information acquisition system of a patient using a capsule type endoscope, to which the technology according to an embodiment of the present disclosure (present technology) can be applied. 
     The in-vivo information acquisition system  10001  includes a capsule type endoscope  10100  and an external controlling apparatus  10200 . 
     The capsule type endoscope  10100  is swallowed by a patient at the time of inspection. The capsule type endoscope  10100  has an image pickup function and a wireless communication function and successively picks up an image of the inside of an organ such as the stomach or an intestine (hereinafter referred to as in-vivo image) at predetermined intervals while it moves inside of the organ by peristaltic motion for a period of time until it is naturally discharged from the patient. Then, the capsule type endoscope  10100  successively transmits information of the in-vivo image to the external controlling apparatus  10200  outside the body by wireless transmission. 
     The external controlling apparatus  10200  integrally controls operation of the in-vivo information acquisition system  10001 . Further, the external controlling apparatus  10200  receives information of an in-vivo image transmitted thereto from the capsule type endoscope  10100  and generates image data for displaying the in-vivo image on a display apparatus (not depicted) on the basis of the received information of the in-vivo image. 
     In the in-vivo information acquisition system  10001 , an in-vivo image imaged a state of the inside of the body of a patient can be acquired at any time in this manner for a period of time until the capsule type endoscope  10100  is discharged after it is swallowed. 
     A configuration and functions of the capsule type endoscope  10100  and the external controlling apparatus  10200  are described in more detail below. 
     The capsule type endoscope  10100  includes a housing  10101  of the capsule type, in which a light source unit  10111 , an image pickup unit  10112 , an image processing unit  10113 , a wireless communication unit  10114 , a power feeding unit  10115 , a power supply unit  10116  and a control unit  10117  are accommodated. 
     The light source unit  10111  includes a light source such as, for example, a light emitting diode (LED) and irradiates light on an image pickup field-of-view of the image pickup unit  10112 . 
     The image pickup unit  10112  includes an image pickup element and an optical system including a plurality of lenses provided at a preceding stage to the image pickup element. Reflected light (hereinafter referred to as observation light) of light irradiated on a body tissue which is an observation target is condensed by the optical system and introduced into the image pickup element. In the image pickup unit  10112 , the incident observation light is photoelectrically converted by the image pickup element, by which an image signal corresponding to the observation light is generated. The image signal generated by the image pickup unit  10112  is provided to the image processing unit  10113 . 
     The image processing unit  10113  includes a processor such as a central processing unit (CPU) or a graphics processing unit (GPU) and performs various signal processes for an image signal generated by the image pickup unit  10112 . The image processing unit  10113  provides the image signal for which the signal processes have been performed thereby as RAW data to the wireless communication unit  10114 . 
     The wireless communication unit  10114  performs a predetermined process such as a modulation process for the image signal for which the signal processes have been performed by the image processing unit  10113  and transmits the resulting image signal to the external controlling apparatus  10200  through an antenna  10114 A. Further, the wireless communication unit  10114  receives a control signal relating to driving control of the capsule type endoscope  10100  from the external controlling apparatus  10200  through the antenna  10114 A. The wireless communication unit  10114  provides the control signal received from the external controlling apparatus  10200  to the control unit  10117 . 
     The power feeding unit  10115  includes an antenna coil for power reception, a power regeneration circuit for regenerating electric power from current generated in the antenna coil, a voltage booster circuit and so forth. The power feeding unit  10115  generates electric power using the principle of non-contact charging. 
     The power supply unit  10116  includes a secondary battery and stores electric power generated by the power feeding unit  10115 . In  FIG. 20 , in order to avoid complicated illustration, an arrow mark indicative of a supply destination of electric power from the power supply unit  10116  and so forth are omitted. However, electric power stored in the power supply unit  10116  is supplied to and can be used to drive the light source unit  10111 , the image pickup unit  10112 , the image processing unit  10113 , the wireless communication unit  10114  and the control unit  10117 . 
     The control unit  10117  includes a processor such as a CPU and suitably controls driving of the light source unit  10111 , the image pickup unit  10112 , the image processing unit  10113 , the wireless communication unit  10114  and the power feeding unit  10115  in accordance with a control signal transmitted thereto from the external controlling apparatus  10200 . 
     The external controlling apparatus  10200  includes a processor such as a CPU or a GPU, a microcomputer, a control board or the like in which a processor and a storage element such as a memory are mixedly incorporated. The external controlling apparatus  10200  transmits a control signal to the control unit  10117  of the capsule type endoscope  10100  through an antenna  10200 A to control operation of the capsule type endoscope  10100 . In the capsule type endoscope  10100 , an irradiation condition of light upon an observation target of the light source unit  10111  can be changed, for example, in accordance with a control signal from the external controlling apparatus  10200 . Further, an image pickup condition (for example, a frame rate, an exposure value or the like of the image pickup unit  10112 ) can be changed in accordance with a control signal from the external controlling apparatus  10200 . Further, the substance of processing by the image processing unit  10113  or a condition for transmitting an image signal from the wireless communication unit  10114  (for example, a transmission interval, a transmission image number or the like) may be changed in accordance with a control signal from the external controlling apparatus  10200 . 
     Further, the external controlling apparatus  10200  performs various image processes for an image signal transmitted thereto from the capsule type endoscope  10100  to generate image data for displaying a picked up in-vivo image on the display apparatus. As the image processes, various signal processes can be performed such as, for example, a development process (demosaic process), an image quality improving process (bandwidth enhancement process, a super-resolution process, a noise reduction (NR) process and/or image stabilization process) and/or an enlargement process (electronic zooming process). The external controlling apparatus  10200  controls driving of the display apparatus to cause the display apparatus to display a picked up in-vivo image on the basis of generated image data. Alternatively, the external controlling apparatus  10200  may also control a recording apparatus (not depicted) to record generated image data or control a printing apparatus (not depicted) to output generated image data by printing. 
     One example of the in-vivo information acquisition system to which the technology according to the present disclosure may be applied has been described above. The technology according to the present disclosure may be applied, for example, to the image pickup unit  10112  among the components described above. Specifically, the imaging element  1  in  FIG. 1  is applicable to the image pickup unit  10112 . Applying the technology according to the present disclosure to the image pickup unit  10112  makes it possible to acquire a more accurate image of the surgical region, thereby improving inspection accuracy. 
     7. Example of Practical Application to Mobile Body 
     For example, the technology (the present technology) according to the present disclosure may be achieved as a device mounted on any type of mobile body such as a vehicle, an electric vehicle, a hybrid electric vehicle, a motorcycle, a bicycle, a personal mobility, an airplane, a drone, a vessel, or a robot. 
       FIG. 21  is a block diagram depicting an example of schematic configuration of a vehicle control system as an example of a mobile body control system to which the technology according to an embodiment of the present disclosure can be applied. 
     The vehicle control system  12000  includes a plurality of electronic control units connected to each other via a communication network  12001 . In the example depicted in  FIG. 21 , the vehicle control system  12000  includes a driving system control unit  12010 , a body system control unit  12020 , an outside-vehicle information detecting unit  12030 , an in-vehicle information detecting unit  12040 , and an integrated control unit  12050 . In addition, a microcomputer  12051 , a sound/image output section  12052 , and a vehicle-mounted network interface (I/F)  12053  are illustrated as a functional configuration of the integrated control unit  12050 . 
     The driving system control unit  12010  controls the operation of devices related to the driving system of the vehicle in accordance with various kinds of programs. For example, the driving system control unit  12010  functions as a control device for a driving force generating device for generating the driving force of the vehicle, such as an internal combustion engine, a driving motor, or the like, a driving force transmitting mechanism for transmitting the driving force to wheels, a steering mechanism for adjusting the steering angle of the vehicle, a braking device for generating the braking force of the vehicle, and the like. 
     The body system control unit  12020  controls the operation of various kinds of devices provided to a vehicle body in accordance with various kinds of programs. For example, the body system control unit  12020  functions as a control device for a keyless entry system, a smart key system, a power window device, or various kinds of lamps such as a headlamp, a backup lamp, a brake lamp, a turn signal, a fog lamp, or the like. In this case, radio waves transmitted from a mobile device as an alternative to a key or signals of various kinds of switches can be input to the body system control unit  12020 . The body system control unit  12020  receives these input radio waves or signals, and controls a door lock device, the power window device, the lamps, or the like of the vehicle. 
     The outside-vehicle information detecting unit  12030  detects information about the outside of the vehicle including the vehicle control system  12000 . For example, the outside-vehicle information detecting unit  12030  is connected with an imaging section  12031 . The outside-vehicle information detecting unit  12030  makes the imaging section  12031  image an image of the outside of the vehicle, and receives the imaged image. On the basis of the received image, the outside-vehicle information detecting unit  12030  may perform processing of detecting an object such as a human, a vehicle, an obstacle, a sign, a character on a road surface, or the like, or processing of detecting a distance thereto. 
     The imaging section  12031  is an optical sensor that receives light, and which outputs an electric signal corresponding to a received light amount of the light. The imaging section  12031  can output the electric signal as an image, or can output the electric signal as information about a measured distance. In addition, the light received by the imaging section  12031  may be visible light, or may be invisible light such as infrared rays or the like. 
     The in-vehicle information detecting unit  12040  detects information about the inside of the vehicle. The in-vehicle information detecting unit  12040  is, for example, connected with a driver state detecting section  12041  that detects the state of a driver. The driver state detecting section  12041 , for example, includes a camera that images the driver. On the basis of detection information input from the driver state detecting section  12041 , the in-vehicle information detecting unit  12040  may calculate a degree of fatigue of the driver or a degree of concentration of the driver, or may determine whether the driver is dozing. 
     The microcomputer  12051  can calculate a control target value for the driving force generating device, the steering mechanism, or the braking device on the basis of the information about the inside or outside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030  or the in-vehicle information detecting unit  12040 , and output a control command to the driving system control unit  12010 . For example, the microcomputer  12051  can perform cooperative control intended to implement functions of an advanced driver assistance system (ADAS) which functions include collision avoidance or shock mitigation for the vehicle, following driving based on a following distance, vehicle speed maintaining driving, a warning of collision of the vehicle, a warning of deviation of the vehicle from a lane, or the like. 
     In addition, the microcomputer  12051  can perform cooperative control intended for automatic driving, which makes the vehicle to travel autonomously without depending on the operation of the driver, or the like, by controlling the driving force generating device, the steering mechanism, the braking device, or the like on the basis of the information about the outside or inside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030  or the in-vehicle information detecting unit  12040 . 
     In addition, the microcomputer  12051  can output a control command to the body system control unit  12020  on the basis of the information about the outside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030 . For example, the microcomputer  12051  can perform cooperative control intended to prevent a glare by controlling the headlamp so as to change from a high beam to a low beam, for example, in accordance with the position of a preceding vehicle or an oncoming vehicle detected by the outside-vehicle information detecting unit  12030 . 
     The sound/image output section  12052  transmits an output signal of at least one of a sound and an image to an output device capable of visually or auditorily notifying information to an occupant of the vehicle or the outside of the vehicle. In the example of  FIG. 21 , an audio speaker  12061 , a display section  12062 , and an instrument panel  12063  are illustrated as the output device. The display section  12062  may, for example, include at least one of an on-board display and a head-up display. 
       FIG. 22  is a diagram depicting an example of the installation position of the imaging section  12031 . 
     In  FIG. 22 , the imaging section  12031  includes imaging sections  12101 ,  12102 ,  12103 ,  12104 , and  12105 . 
     The imaging sections  12101 ,  12102 ,  12103 ,  12104 , and  12105  are, for example, disposed at positions on a front nose, sideview mirrors, a rear bumper, and a back door of the vehicle  12100  as well as a position on an upper portion of a windshield within the interior of the vehicle. The imaging section  12101  provided to the front nose and the imaging section  12105  provided to the upper portion of the windshield within the interior of the vehicle obtain mainly an image of the front of the vehicle  12100 . The imaging sections  12102  and  12103  provided to the sideview mirrors obtain mainly an image of the sides of the vehicle  12100 . The imaging section  12104  provided to the rear bumper or the back door obtains mainly an image of the rear of the vehicle  12100 . The imaging section  12105  provided to the upper portion of the windshield within the interior of the vehicle is used mainly to detect a preceding vehicle, a pedestrian, an obstacle, a signal, a traffic sign, a lane, or the like. 
     Incidentally,  FIG. 22  depicts an example of photographing ranges of the imaging sections  12101  to  12104 . An imaging range  12111  represents the imaging range of the imaging section  12101  provided to the front nose. Imaging ranges  12112  and  12113  respectively represent the imaging ranges of the imaging sections  12102  and  12103  provided to the sideview mirrors. An imaging range  12114  represents the imaging range of the imaging section  12104  provided to the rear bumper or the back door. A bird&#39;s-eye image of the vehicle  12100  as viewed from above is obtained by superimposing image data imaged by the imaging sections  12101  to  12104 , for example. 
     At least one of the imaging sections  12101  to  12104  may have a function of obtaining distance information. For example, at least one of the imaging sections  12101  to  12104  may be a stereo camera constituted of a plurality of imaging elements, or may be an imaging element having pixels for phase difference detection. 
     For example, the microcomputer  12051  can determine a distance to each three-dimensional object within the imaging ranges  12111  to  12114  and a temporal change in the distance (relative speed with respect to the vehicle  12100 ) on the basis of the distance information obtained from the imaging sections  12101  to  12104 , and thereby extract, as a preceding vehicle, a nearest three-dimensional object in particular that is present on a traveling path of the vehicle  12100  and which travels in substantially the same direction as the vehicle  12100  at a predetermined speed (for example, equal to or more than 0 km/hour). Further, the microcomputer  12051  can set a following distance to be maintained in front of a preceding vehicle in advance, and perform automatic brake control (including following stop control), automatic acceleration control (including following start control), or the like. It is thus possible to perform cooperative control intended for automatic driving that makes the vehicle travel autonomously without depending on the operation of the driver or the like. 
     For example, the microcomputer  12051  can classify three-dimensional object data on three-dimensional objects into three-dimensional object data of a two-wheeled vehicle, a standard-sized vehicle, a large-sized vehicle, a pedestrian, a utility pole, and other three-dimensional objects on the basis of the distance information obtained from the imaging sections  12101  to  12104 , extract the classified three-dimensional object data, and use the extracted three-dimensional object data for automatic avoidance of an obstacle. For example, the microcomputer  12051  identifies obstacles around the vehicle  12100  as obstacles that the driver of the vehicle  12100  can recognize visually and obstacles that are difficult for the driver of the vehicle  12100  to recognize visually. Then, the microcomputer  12051  determines a collision risk indicating a risk of collision with each obstacle. In a situation in which the collision risk is equal to or higher than a set value and there is thus a possibility of collision, the microcomputer  12051  outputs a warning to the driver via the audio speaker  12061  or the display section  12062 , and performs forced deceleration or avoidance steering via the driving system control unit  12010 . The microcomputer  12051  can thereby assist in driving to avoid collision. 
     At least one of the imaging sections  12101  to  12104  may be an infrared camera that detects infrared rays. The microcomputer  12051  can, for example, recognize a pedestrian by determining whether or not there is a pedestrian in imaged images of the imaging sections  12101  to  12104 . Such recognition of a pedestrian is, for example, performed by a procedure of extracting characteristic points in the imaged images of the imaging sections  12101  to  12104  as infrared cameras and a procedure of determining whether or not it is the pedestrian by performing pattern matching processing on a series of characteristic points representing the contour of the object. When the microcomputer  12051  determines that there is a pedestrian in the imaged images of the imaging sections  12101  to  12104 , and thus recognizes the pedestrian, the sound/image output section  12052  controls the display section  12062  so that a square contour line for emphasis is displayed so as to be superimposed on the recognized pedestrian. The sound/image output section  12052  may also control the display section  12062  so that an icon or the like representing the pedestrian is displayed at a desired position. 
     One example of the vehicle control system to which the technology according to the present disclosure may be applied has been described above. The technology according to the present disclosure may be applied to the imaging section  12031  among the components described above. Specifically, the imaging element  1  in  FIG. 1  is applicable to the imaging section  12031 . Applying the technology according to the present disclosure to the imaging section  12031  makes it possible to obtain a captured image that is easier to see. Hence, it is possible to reduce fatigue of the driver. 
     8. Other Modification Examples 
     The present disclosure has been described above with reference to some embodiments and modification examples; however, the present disclosure is not limited to the embodiments and the like described above, and may be modified in a variety of ways. For example, in the imaging elements  1  and  1 A according to the first embodiment described above, a case where the first resonator  13  or  13 A has a three-layer structure or a four-layer structure is described as an example; however, the present disclosure is not limited thereto, and the number of stacked layers in the first resonator is optionally settable. In addition, the materials and thicknesses of the respective layers of the resonator structure are not limited to those described above, and are optionally settable. 
     In addition, the imaging elements  1  and  1 A according to the first embodiment described above, the moth eye structure  21  is provided on the front surface  17 S of the transparent layer  17 ; however, the present disclosure is not limited thereto. For example, as with an imaging element  1 D according to a third modification example of the first embodiment illustrated in  FIG. 23 , an on-chip lens  26  may be provided for each of the pixels  22  on the front surface  17 S of the transparent layer  17 . 
     In addition, in the imaging elements  1 C 1  to  1 C 9  according to the second embodiment described above, the imaging lens  25  is provided on the front surface  17 S of the transparent layer  17 ; however, the present disclosure is not limited thereto. For example, as with an imaging element  1 E according to a first modification example of the second embodiment illustrated in  FIG. 24 , a space may be provided between the front surface  17 S of the transparent layer  17  and the imaging lens  25 . The imaging element  1 E further includes the moth eye structure  21  on the front surface  17 S. Alternatively, instead of the moth eye structure  21 , an on-chip lens may be provided on the front surface  17 S. 
     It is to be noted that the effects described in the present specification are merely illustrative and non-limiting, and there may be other effects. In addition, the present technology may have the following configurations. 
     (1) 
     A resonator structure comprising: 
     a stacked structure that includes a semiconductor layer, a first resonator, a first reflection layer, a second resonator, a second reflection layer stacked in this order, allows light of a specific wavelength band to be transmitted therethrough, 
     the semiconductor layer having a first average refractive index, 
     the first resonator having a second average refractive index lower than the first average refractive index, and 
     the first reflection layer having a third average refractive index higher than the second average refractive index. 
     (2) 
     The resonator structure according to (1), in which the second average refractive index is 1 or more and four or less. 
     (3) 
     The resonator structure according to (1), in which the second average refraction index is 1 or more and two or less. 
     (4) 
     The resonator structure according to any one of (1) to (3), in which the second resonator has a refractive index distribution in a stacked plane. 
     (5) 
     The resonator structure according to (4), in which the second resonator includes a first portion having a first refractive index, and a second portion being dispersedly located in the first portion in the stacked plane and having a second refractive index. 
     (6) 
     The resonator structure according to any one of (1) to (5), in which a thickness of the first resonator is 400 nm or less. 
     (7) 
     The resonator structure according to any one of (1) to (6), in which the first reflection layer and the second reflection layer include a material including polycrystal silicon. 
     (8) 
     The resonator structure according to (7), in which each of a thickness of the first reflection layer and a thickness of the second reflection layer is 50 nm or less. 
     (9) 
     The resonator structure according to any one of (1) to (8), in which the first reflection layer and the second reflection layer includes a material including polycrystal silicon, and except for the first reflection layer, the second reflection layer, and the semiconductor layer, others include a material other than elemental silicon. 
     (10) 
     The resonator structure according to (9), in which each of thicknesses of the first reflection layer and the second reflection layer is 50 nm or less. 
     (11) 
     The resonator structure according to any one of (1) to (10), further including a visible light cut filter that suppresses transmission of visible light. 
     (12) 
     The resonator structure according to any one of (1) to (11), in which the first resonator includes a multilayer film structure in which a plurality of films is stacked. 
     The resonator structure according to claim  1 .
 
(13)
 
     The resonator structure according to (12), in which the plurality of films in the first resonator includes a material containing at least one kind of silicon oxide, silicon nitride, tantalum oxide, or aluminum oxide. 
     (14) 
     An imaging element comprising: 
     a plurality of pixels each including a photoelectric converter and a resonator structure that allows light of a specific wavelength band to be transmitted toward the photoelectric converter, the resonator structure including a semiconductor substrate, a first resonator, a first reflection layer, a second resonator, a second reflection layer stacked in this order, 
     the semiconductor substrate including the photoelectric layer and having a first average refractive index, 
     the first resonator having a second average refractive index lower than the first average refractive index, and 
     the first reflection layer having a third average refractive index higher than the second average refractive index. 
     (15) 
     The imaging element according to (14), further including a light shielding layer between adjacent ones of the plurality of pixels. 
     (16) 
     The imaging element according to (14) or (15), in which 
     the plurality of pixels is provided with a lens that provides refractive power to light L toward the resonator structure, and 
     the respective resonator structures in the plurality of pixels allow light of wavelength bands different from each other to be transmitted therethrough. 
     (17) 
     An electronic apparatus comprising: 
     an imaging element, 
     the imaging element including a plurality of pixels each including a photoelectric converter and a resonator structure that allows light of a specific wavelength band to be transmitted toward the photoelectric converter, the resonator structure including a semiconductor substrate, a first resonator, a first reflection layer, a second resonator, a second reflection layer stacked in this order, 
     the semiconductor substrate including the photoelectric layer and having a first average refractive index, 
     the first resonator having a second average refractive index lower than the first average refractive index, and 
     the first reflection layer having a third average refractive index higher than the second average refractive index. 
     This application claims the benefit of Japanese Priority Patent Application JP2018-96234 filed with the Japan Patent Office on May 18, 2018, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.