Patent Publication Number: US-2021191141-A1

Title: Polarization spectral filter, polarization spectral filter array, and polarization spectral sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2019-0171993, filed on Dec. 20, 2019, in the Korean Intellectual Property Office, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Example embodiments consistent with the present disclosure relate to a polarization spectral filter, a polarization spectral filter array, and a polarization spectral sensor, and more particularly, to a polarization spectral filter and a polarization spectral filter array which are capable of selectively transmitting a light having a specified wavelength band and having a specified linear polarization component, and a polarization spectral sensor capable of simultaneously obtaining polarization information and spectral information on an incident light by using the polarization spectral filter array. 
     2. Description of Related Art 
     A spectroscope is widely used, for example, for analyzing agricultural conditions, mineral distribution, vegetation on a ground surface, pollution, and the like by capturing a ground image via a drone, satellite, an aircraft, and the like. Such analysis is used in various fields such as food safety, skin/face analysis, authentication and recognition, and biological tissue analysis. Recently, applications using a spectroscope have been expanded to other fields such as mobile healthcare. 
     A polarization image may provide additional information such as pressure, surface defects, and scratches in addition to general red, green, and blue (RGB) information. This additional information may be used in applications for industrial equipment, automotive application components, etc. In addition, a polarization image may enable more accurate object identification even in a cloudy or foggy weather. 
     Accordingly, the application fields of sensors capable of obtaining a spectral image or polarization image have been expanded. In addition, as an image sensor is miniaturized and a resolution thereof is increased, research has been conducted to obtain a spectral image or a polarization image with a high resolution by integrating the image sensor with other devices. 
     SUMMARY 
     One or more example embodiments provide a polarization spectral filter and a polarization spectral filter array which are both capable of selectively transmitting light in a specified wavelength band and having a specified linear polarization component. 
     In addition, one or more example embodiments provide a polarization spectral sensor capable of simultaneously obtaining polarization information and spectral information on an incident light by using the polarization spectral filter array. 
     According to an aspect of an example embodiment, there is provided a polarization spectral filter including: a first reflector; a second reflector, the first reflector and the second deflector being disposed to face each other in a first direction; and a grating layer disposed between the first reflector and the second reflector, wherein the grating layer includes a plurality of first grating elements and a plurality of second grating elements, the first grating elements and the second grating elements being alternately arranged with each other in a second direction perpendicular to the first direction, wherein each of the plurality of first grating elements includes a first dielectric material having a first refractive index, and wherein each of the plurality of second grating elements includes a second dielectric material having a second refractive index different from the first refractive index. 
     Each of the first grating elements and the second grating elements may have a rod shape, and the plurality of first grating elements and the plurality of second grating elements may be arranged one-dimensionally. 
     A first surface of each of the plurality of first grating elements and a first surface of each of the plurality of second grating elements may be in contact with the first reflector, and a second surface of each of the plurality of first grating elements, opposite to the first surface of each of the plurality of first grating elements, and a second surface of each of the plurality of second grating elements, opposite to the first surface of each of the plurality of second grating elements, may be in contact with the second reflector. 
     Based on at least one of thicknesses of the plurality of first grating elements and the plurality of second grating elements, arrangement periods of the plurality of first grating elements and arrangement periods of the plurality of second grating elements, and a ratio of the plurality of first grating elements to the plurality of second grating elements, the polarization spectral filter may be configured to transmit therethrough light in a first wavelength band, from among light having a first linear polarization component, and transmit therethrough light in a second wavelength band different from the first wavelength band, from among light having a second linear polarization component perpendicular to the first linear polarization component. 
     The thicknesses of each of the plurality of first grating elements and each of the plurality of second grating elements may be approximately 90 nm to approximately 350 nm. 
     The arrangement periods of the plurality of first grating elements and the arrangement periods of the plurality of second grating elements may be in a range from approximately 150 nm to approximately 300 nm. 
     The ratio of the plurality of first grating elements to the plurality of second grating elements may be in a range from approximately 0.2 to approximately 0.7. 
     The first dielectric material and the second dielectric material may be transparent with respect to the light in the first wavelength band and the light in the second wavelength band. 
     The polarization spectral filter may further include a band pass filter disposed on a surface of the first reflector, the band pass filter being configured to block the light in the first wavelength band and to transmit therethrough the light in the second wavelength band. 
     The polarization spectral filter may further include a quarter wave plate disposed on a surface of the first reflector. 
     The first reflector may include a plurality of first dielectric layers and a plurality of second dielectric layers, the plurality of first dielectric layers and the plurality of second dielectric layers being alternately stacked with each other in a third direction, the second reflector may include a plurality of third dielectric layers and a plurality of fourth dielectric layers, the plurality of third dielectric layers and the plurality of fourth dielectric layers being alternately stacked with each other in the third direction, each of the plurality of first dielectric layers may include a dielectric material having a refractive index that is different from a dielectric material included in each of the plurality of second dielectric layers, and each of the third dielectric layers may include a dielectric material having a refractive index that is different from a dielectric material included in each of the plurality of fourth dielectric layers. 
     Each of the plurality of first dielectric layers and each of the plurality of third dielectric layers may include the first dielectric material, and each of the plurality of second dielectric layers and each of the plurality of fourth dielectric layers may include the second dielectric material. 
     The grating layer further may include a plurality of third grating elements, each of the plurality of third grating elements including a third dielectric material having a third refractive index different from the first refractive index and the second refractive index, and the plurality of first grating elements, the plurality of second grating elements, and the plurality of third grating elements may be alternately with each other arranged in the second direction. 
     According to an aspect of an example embodiment, there is provided a polarization spectral filter array, including: a plurality of unit filter arrays that are two-dimensionally arranged, wherein each of the unit filter arrays includes a first polarization spectral filter set configured to transmit therethrough light in a first wavelength band, and a second polarization spectral filter set configured to transmit therethrough light in a second wavelength band different from the first wavelength band, the first polarization spectral filter set includes a first polarization spectral filter configured to transmit therethrough light having a first linear polarization component, from among the light in the first wavelength band, and a second polarization spectral filter configured to transmit therethrough light having a second linear polarization component perpendicular to the first linear polarization component, from among the light in the first wavelength band, the second polarization spectral filter set includes a third polarization spectral filter configured to transmit therethrough the light having the first linear polarization component, from among the light in the second wavelength band, and a fourth polarization spectral filter configured to transmit therethrough the light having the second linear polarization component, from among the light in the second wavelength band, each of the first polarization spectral filter, the second polarization spectral filter, the third polarization spectral filter, and the fourth polarization spectral filter includes a first reflector and a second reflector disposed to face each other in a first direction, and a grating layer disposed between the first reflector and the second reflector, the grating layer includes a plurality of first grating elements and a plurality of second grating elements, the plurality of first grating elements and the plurality of second grating elements being alternately arranged with each other in a second direction perpendicular to the first direction, each of the plurality of first grating elements includes a first dielectric material having a first refractive index, and each of the plurality of second grating elements includes a second dielectric material having a second refractive index different from the first refractive index. 
     The plurality of first grating elements and the plurality of second grating elements of the grating layer of the second polarization spectral filter may be rotated by 90 degrees, on a plane perpendicular to the first direction, with respect to the plurality of first grating elements and the plurality of second grating elements of the grating layer of the first polarization spectral filter, and the plurality of first grating elements and the plurality of second grating elements of the grating layer of the fourth polarization spectral filter may be rotated by 90 degrees, on the plane perpendicular to the first direction, with respect to the plurality of first grating elements and the plurality of second grating elements of the grating layer of the third polarization spectral filter. 
     Each of the plurality of first grating elements and each of the plurality of second grating elements may have a rod shape, and the plurality of first grating elements and the plurality of second grating elements may be one-dimensionally arranged. 
     With respect to each of the first polarization spectral filter, the second polarization spectral filter, the third polarization spectral filter, and the fourth polarization spectral filter, based on at least one of thicknesses of the plurality of first grating elements and the plurality of second grating elements, arrangement periods of the plurality of first grating elements and arrangement periods of the plurality of second grating elements, and a ratio of the plurality of first grating elements to the plurality of second grating elements, the first polarization spectral filter may be further configured to transmit therethrough the light in the first wavelength band from among the light having the first linear polarization component, the second polarization spectral filter may be further configured to transmit therethrough the light in the first wavelength band from among the light having the second linear polarization component, the third polarization spectral filter may be further configured to transmit therethrough the light in the second wavelength band from among the light having the first linear polarization component, and the fourth polarization spectral filter may be further configured to transmit therethrough the light in the second wavelength band from among the light having the second linear polarization component. 
     A width and the thickness of each of the plurality first grating element of the first polarization spectral filter, a width and the thickness of each of the plurality of second grating elements of the first polarization spectral filter, and the ratio of the plurality of first grating elements to the plurality of second grating elements of the first polarization spectral filter may be respectively the same as a width and the thickness of each of the plurality of first grating elements of the second polarization spectral filter, a width and the thickness of each of the plurality of second grating elements of the second polarization spectral filter, and the ratio of the plurality of first grating elements to the plurality of second grating elements of the second polarization spectral filter, and a width and the thickness of each of the plurality of first grating elements of the third polarization spectral filter, a width and the thickness of each of the plurality of second grating elements of the third polarization spectral filter, and the ratio of the plurality of first grating elements to the plurality of second grating elements of the third polarization spectral filter may be respectively the same as a width and the thickness of each of the plurality of first grating elements of the fourth polarization spectral filter, a width and the thickness of each of the plurality of second grating elements of the fourth polarization spectral filter, and the ratio of the plurality of first grating elements to the plurality of second grating elements of the fourth polarization spectral filter. 
     Each of the first polarization spectral filter, the second polarization spectral filter, the third polarization spectral filter, and the fourth polarization spectral filter may further include a band pass filter disposed on a surface of the first reflector, the band pass filter being configured to transmit therethrough the light in the first wavelength band and the second wavelength band and to block light in other wavelength bands. 
     The first polarization spectral filter set may further include a fifth polarization spectral filter configured to transmit therethrough light having a third linear polarization component rotated by 45 degrees with respect to the first linear polarization component, from among the light in the first wavelength band, the second polarization spectral filter set may further include a sixth polarization spectral filter configured to transmit therethrough the light having the third linear polarization component rotated by 45 degrees with respect to the first linear polarization component, from among the light in the second wavelength band, and each of the fifth polarization spectral filter and the sixth polarization spectral filter may include the first reflector, the second reflector, and the grating layer. 
     The plurality of first grating elements and the plurality of second grating elements of the grating layer of the fifth polarization spectral filter may be rotated by 45 degrees, on a plane perpendicular to the first direction, with respect to the plurality of first grating elements and the plurality of second grating elements of the grating layer of the first polarization spectral filter, and 
     the plurality of first grating elements and the plurality of second grating elements of the grating layer of the sixth polarization spectral filter may be rotated by 45 degrees, on the plane perpendicular to the first direction, with respect to the plurality of first grating elements and the plurality of second grating elements of the grating layer of the third polarization spectral filter. 
     The first polarization spectral filter set may further include a fifth polarization spectral filter configured to transmit therethrough the light having the first linear polarization component from among the light having the first wavelength band, the second polarization spectral filter set may further include a sixth polarization spectral filter configured to transmit therethrough the light having the first linear polarization component from among the light in the second wavelength band, each of the fifth polarization spectral filter and the sixth polarization spectral filter may include the first reflector, the second reflector, the grating layer, and a quarter wave plate disposed on a surface of the first reflector. 
     According to an aspect of an example embodiment, there is provided polarization spectral sensor including: a polarization spectral filter array including a plurality of unit filter arrays arranged two-dimensionally; and an image sensor including a plurality of sensing pixels arranged two-dimensionally, the plurality of sensing pixels being configured to sense intensity of light transmitting through the polarization spectral filter array, wherein each of the plurality of unit filter arrays includes a first polarization spectral filter set through which light in a first wavelength band transmits; and a second polarization spectral filter set through which light in a second wavelength band different from the first wavelength band transmits, wherein the first polarization spectral filter set includes a first polarization spectral filter configured to transmit therethrough light having a first linear polarization component, from among the light in the first wavelength band; and a second polarization spectral filter configured to transmit therethrough light having a second linear polarization component perpendicular to the first linear polarization component, from among the light having the first wavelength band, wherein the second polarization spectral filter set includes a third polarization spectral filter configured to transmit therethrough the light having the first linear polarization component, from among the light in the second wavelength band; and a fourth polarization spectral filter configured to transmit therethrough the light having the second linear polarization component, from among the light in the second wavelength band, wherein each of the first polarization spectral filter, the second polarization spectral filter, the third polarization spectral filter, and the fourth polarization spectral filter includes a first reflector and a second reflector disposed to face each other in a first direction; and a grating layer disposed between the first reflector and the second reflector, wherein the grating layer includes a plurality of first grating elements and a plurality of second grating elements, the plurality of first grating elements and the plurality of second grating elements being alternately arranged with each other in a second direction perpendicular to the first direction, wherein each of the plurality of first grating elements includes a first dielectric material having a first refractive index, and wherein each of the plurality of second grating elements includes a second dielectric material having a second refractive index different from the first refractive index. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain example embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to an example embodiment; 
         FIG. 2  is a perspective view schematically illustrating a configuration of a grating layer of the polarization spectral filter illustrated in  FIG. 1 ; 
         FIG. 3  shows graphs illustrating an example of transmission properties of the polarization spectral filter illustrated in  FIG. 1 ; 
         FIG. 4  is a graph illustrating an example of transmission properties according to polarization angles of two different transmission wavelength bands of the polarization spectral filter illustrated in  FIG. 1 ; 
         FIG. 5  shows graphs illustrating an example of a change in transmission properties of the polarization spectral filter illustrated in  FIG. 1 ; 
         FIG. 6  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment; 
         FIG. 7  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment; 
         FIG. 8  shows graphs illustrating an example of transmission properties of the polarization spectral filter illustrated in  FIG. 7 ; 
         FIG. 9  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment; 
         FIG. 10  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment; 
         FIG. 11  is a cross-sectional view schematically illustrating a configuration of the polarization spectral filter according to example embodiment; 
         FIG. 12  is a perspective view schematically illustrating a configuration of a polarization spectral filter array and a polarization spectral sensor including the polarization spectral filter array according to an example embodiment; 
         FIG. 13  illustrates an example of a configuration of the polarization spectral filter array illustrated in  FIG. 12 ; 
         FIG. 14  illustrates an example of an arrangement of polarization spectral filters in one polarization spectral filter set of the polarization spectral filter array illustrated in  FIG. 12 ; 
         FIG. 15  is a cross-sectional view taken along line A-A′ of  FIG. 14 ; 
         FIG. 16  illustrates another example of the arrangement of the polarization spectral filters in the one polarization spectral filter set of the polarization spectral filter array illustrated in  FIG. 12 ; 
         FIG. 17  illustrates an example of a cross-sectional view taken along line B-B′ of  FIG. 16 ; 
         FIG. 18  illustrates another example of a cross-sectional view taken along line B-B′ of  FIG. 16 ; and 
         FIG. 19  illustrates yet another example of a cross-sectional view taken along line B-B′ of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a polarization spectral filter, a polarization spectral filter array, and a polarization spectral sensor according to example embodiments will be described in detail with reference to the accompanying drawings. In the drawings, like reference numerals refer to like elements, and a size of each element in the drawings may be exaggerated for clarity and convenience of description. In addition, the example embodiments to be described below are merely examples, and various modifications are possible from the example embodiments. 
     Hereinafter, what is described as “over” or “on” may include not only directly over and in contact but also over without being in contact. A singular expression includes the plurality of expressions unless the context clearly indicates otherwise. In addition, when a part is described to “include” a certain configuration element, which means that the part may further include other configuration elements, except to exclude other configuration elements unless otherwise stated. 
     A term “above-described” and similar terminology may be used for the singular and the plural. If a sequence of steps configuring a method is apparently described or there is no contradictive description, the sequence may be performed in a proper order and is not limited to the described order. 
     In addition, terms such as “. . . unit/portion”, “module”, and the like described in the specification mean a unit for processing at least one function or operation, which may be implemented as hardware or software or a combination of the hardware and the software. 
     Connections of lines between configuration elements or connection members illustrated in the drawings represent functional connections and/or physical or circuit connections by way of example and may be replaced or represented as additional various functional connections, physical connections, or circuit connections in the actual device. 
     All examples or certain terms are used simply for the purpose of describing technical concepts in detail, and the scope is not limited by the examples or terms unless defined by the claims. 
     Expressions, such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. For example, the expression “at least one of a, b or c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof. 
       FIG. 1  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to an example embodiment. Referring to  FIG. 1 , a polarization spectral filter  100  according to an example embodiment may include a first reflector  110 , a grating layer  120  disposed on the first reflector  110 , and a second reflector  130  disposed on the grating layer  120 . Accordingly, the first reflector  110  and the second reflector  130  may be disposed to face each other in a thickness direction (or a top to bottom direction of the polarization spectral filter  100 ), and the grating layer  120  may be disposed between the first reflector  110  and the second reflector  130 . 
     The first reflector  110  and the second reflector  130  may be, for example, a distributed Bragg reflector (DBR) formed by stacking repeatedly and alternately two dielectric layers having different refractive indices. For example, the first reflector  110  may include a plurality of first dielectric layers  110   a  and a plurality of second dielectric layers  110   b  that are alternately stacked in the thickness direction. The second reflector  130  may include a plurality of third dielectric layers  130   a  and a plurality of fourth dielectric layers  130   b  that are alternately stacked in the thickness direction. Each of the first dielectric layers  110   a  and each of the second dielectric layers  110   b  may include dielectric materials having different refractive indices. In addition, each of the third dielectric layers  130   a  and each of the fourth dielectric layers  130   b  may also include dielectric materials having different refractive indices. For example, each of the first dielectric layers  110   a  may include a first dielectric material having a first refractive index, and each of the second dielectric layers  110   b  may include a second dielectric material having a second refractive index different from the first refractive index, and each of the third dielectric layers  130   a  may include a third dielectric material having a third refractive index, and each of the fourth dielectric layers  130   b  may include a fourth dielectric material having a fourth refractive index different from the third refractive index. 
     For example, the first dielectric layers  110   a  and the second dielectric layers  110   b  may include two different dielectric materials selected from Si, TiO 2 , SiO 2 , and Si 2 N 3 , and the third dielectric layers  130   a  and the fourth dielectric layers  130   b  may also include two different dielectric materials selected from Si, TiO 2 , SiO 2 , and Si 2 N 3 . For example, the first dielectric layers  110   a  may have a dielectric material that is selected from Si, TiO 2 , SiO 2 , and Si 2 N 3 , and the second dielectric layers  110   b  may have another dielectric material that is selected from Si, TiO 2 , SiO 2 , and Si 2 N 3 . In addition, the first dielectric layers  110   a  of the first reflector  110  and the third dielectric layers  130   a  of the second reflector  130  may include the same dielectric material, and the second dielectric layers  110   b  of the first reflector  110  and the fourth dielectric layers  130   b  of the second reflector  130  may include the same dielectric material. Alternatively, the first dielectric layers  110   a  of the first reflector  110  and the fourth dielectric layers  130   b  of the second reflector  130  may include the same dielectric material, and the second dielectric layers  110   b  of the first reflector  110  and the third dielectric layers  130   a  of the second reflector  130  may include the same dielectric material. 
     According to structures of the first reflector  110  and the second reflector  130 , reflection occurs at an interface between the first dielectric layers  110   a  and the second dielectric layers  110   b  having different refractive indices, and occurs at an interface between the third dielectric layers  130   a  and the fourth dielectric layers  130   b  having different refractive indices, and high reflectance may be obtained by matching phases of all the reflected light. To this end, an optical thickness (that is, a value obtained by multiplying a physical thickness by a refractive index of a material of the layer) of each of the first to the fourth dielectric layers  110   a,    110   b,    130   a,  and  130   b  may be selected as approximately a quarter of a wavelength band of light to be transmitted through the polarization spectral filter  100 . 
     The first reflector  110  and the second reflector  130  disposed to face each other may form a resonator to resonate light. The grating layer  120  may be disposed inside the resonator formed by the first reflector  110  and the second reflector  130 . Light incident through an upper surface of the first reflector  110  may be emitted through a lower surface of the second reflector  130  while resonating between the first reflector  110  and the second reflector  130 . The light passes through the grating layer  120  repeatedly while resonating between the first reflector  110  and the second reflector  130 . Thus, properties of the light emitted through the lower surface of the second reflector  130  may be mainly determined by a structure of the grating layer  120 . 
     In an example embodiment, the grating layer  120  may be configured to have polarization dependent properties. To this end, the grating layer  120  may include a plurality of first grating elements  120   a  and a plurality of second grating elements  120   b  alternately arranged in a horizontal direction perpendicular to the thickness direction. For example, the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  may be arranged such that lower surfaces of the plurality of first grating elements  120   a  in the thickness direction and lower surfaces of the plurality of second grating elements  120   b  in the thickness direction are in contact with the first reflector  110  and are located on the same plane, and upper surfaces of the plurality of first grating elements  120   a  in the thickness direction and upper surfaces of the plurality of second grating elements  120   b  in the thickness direction are in contact with the second reflector  130  and are located on the same plane. 
     The first grating elements  120   a  and the second grating elements  120   b  may include dielectric materials having different refractive indices. In other words, each of the first grating elements  120   a  may include a first dielectric material having a first refractive index, and each of the second grating elements  120   b  may include a second dielectric material having a second refractive index different from the first refractive index. For example, the first grating elements  120   a  and the second grating elements  120   b  may include two different dielectric materials selected from Si, TiO 2 , SiO 2 , and Si 2 N 3 . In addition to the above examples of materials, the first dielectric material and the second dielectric material respectively forming the first grating elements  120   a  and the second grating elements  120   b  may include materials transparent with respect to light of a transmission wavelength band of the polarization spectral filter  100 . 
     In addition, the first grating elements  120   a  may include the same dielectric material as the first dielectric layers  110   a  of the first reflector  110  and the third dielectric layers  130   a  of the second reflector  130 , and the second grating elements  120   b  may include the same dielectric material as the second dielectric layers  110   b  of the first reflector  110  and the fourth dielectric layers  130   b  of the second reflector  130 . Alternatively, the first grating elements  120   a  may include the same dielectric material as the first dielectric layers  110   a  of the first reflector  110  and the fourth dielectric layers  130   b  of the second reflector  130 , and the second grating elements  120   b  may include the same dielectric material as the second dielectric layers  110   b  of the first reflector  110  and the third dielectric layers  130   a  of the second reflector  130 . 
       FIG. 2  is a perspective view schematically illustrating a configuration of the grating layer  120  of the polarization spectral filter  100  illustrated in  FIG. 1 . Referring to  FIG. 2 , each of the first grating elements  120   a  and each of the second grating elements  120   b  of the grating layer  120  may have a shape of a rod extending in a longitudinal direction. In addition, the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  are arranged alternately and repeatedly in a width direction thereof. The first grating elements  120   a  and the second grating elements  120   b  have the same thickness T. In addition, the plurality of first grating elements  120   a  have the same width W 1 , and the plurality of second grating elements  120   b  have the same width W 2 . Thus, the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  are arranged in a constant period P. 
     Since the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  are one-dimensionally arranged in the above-described manner, the grating layer  120  and the polarization spectral filter  100  may have polarization dependent properties. For example, among light resonating between the first reflector  110  and the second reflector  130 , transmittance of the polarization spectral filter  100  for light having a polarization component parallel to a longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  may be different from transmittance of the polarization spectral filter  100  for light having a polarization component perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b.  Particularly, in the polarization spectral filter  100  according to an example embodiment, a transmission wavelength band may change depending on a polarization direction of light beam. 
       FIG. 3  illustrates graphs illustrating an example of transmission properties of the polarization spectral filter  100  illustrated in  FIG. 1 . Referring to  FIG. 3 , a wavelength of light transmitted through the polarization spectral filter  100  may have peaks in two narrow wavelength bands separated from each other. For example, the light transmitted through the polarization spectral filter  100  may have a first spectrum SP 1  having a center wavelength of approximately 825 nm and a second spectrum SP 2  having a center wavelength of approximately 875 nm. The first spectrum SP 1  and the second spectrum SP 2  may have narrow wavelength widths. For example, a full width at half maximum (FWHM) of the first spectrum SP 1  and a FWHM of the second spectrum SP 2  may range from approximately 1 nm to approximately 10 nm. Accordingly, the first spectrum SP 1  and the second spectrum SP 2  may not overlap each other, and transmittance of the polarization spectral filter  100  is approximately zero in a wavelength band between the first spectrum SP 1  and the second spectrum SP 2 . The FWHM of the first spectrum SP 1  and the FWHM of the second spectrum SP 2  may be reduced as the number of pairs of the first dielectric layers  110   a  and the second dielectric layers  110   b  included in the first reflector  110  and the number of pairs of the third dielectric layers  130   a  and the fourth dielectric layers  130   b  included in the second reflector  130  increase. 
     Particularly, light of the first spectrum SP 1  may have a polarization component perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120 , and light of the second spectrum SP 2  may have a polarization component parallel to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120 . Accordingly, the polarization spectral filter  100  may have two different transmission wavelength bands having polarization directions perpendicular to each other. In other words, the polarization spectral filter  100  may have two different transmission wavelength bands, and the polarization spectral filter  100  may have polarization properties perpendicular to each other for the two transmission wavelength bands. 
       FIG. 4  is a graph illustrating an example of the transmission properties according to polarization angles of the two different transmission wavelength bands of the polarization spectral filter  100  illustrated in  FIG. 1 . Referring to  FIG. 4 , in the light of the first spectrum SP 1 , light parallel to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  may have transmittance of approximately zero for the polarization spectral filter  100 . In the light of the first spectrum SP 1 , light perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  may have transmittance of approximately 0.9 for the polarization spectral filter  100 . On the other hand, in the light of the second spectrum SP 2 , light perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  may have transmittance of approximately zero for the polarization spectral filter  100 . In addition, in the light of the second spectrum SP 2 , light parallel to the longitudinal direction (or length direction) of the first grating elements  120   a  and the second grating elements  120   b  may have transmittance of approximately 0.9 for the polarization spectral filter  100 . 
     Specific transmission wavelength bands and polarization properties of the polarization spectral filter  100  may be determined by the thickness T of each of the first grating elements  120   a  and each of the second grating elements  120   b,  the arrangement period P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b,  and a ratio of the first grating elements  120   a  and the second grating elements  120   b,  and the like. For example, thicknesses of each of the first grating elements  120   a  and each of the second grating elements  120   b  may range from approximately 90 nm to approximately 350 nm. In addition, the arrangement period P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  may range from 150 nm to 300 nm. 
     Accordingly, a size of each of the first grating elements  120   a  and a size of each of the second grating elements  120   b  may be smaller than the transmission wavelength of the polarization spectral filter  100 . For example, the thickness of each of the first grating elements  120   a  and the thickness of each of the second grating elements  120   b  may be smaller than ½ or ⅓ of the transmission wavelength of the polarization spectral filter  100 . In addition, the arrangement period P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  may be less than ½ or ⅓ of the transmission wavelength of the polarization spectral filter  100 . 
     Since the first grating elements  120   a  and the second grating elements  120   b  have the same thickness T, a ratio of the first grating elements  120   a  to the second grating elements  120   b  may be the same as a ratio of a width W 1  of the first grating elements  120   a  to a width W 2  of the second grating elements  120   b.  For example, when the first refractive index of the first dielectric material forming the first grating elements  120   a  is lower than the second refractive index of the second dielectric material forming the second grating elements  120   b,  the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  may range from approximately 0.2 to approximately 0.7. The thickness T of the first grating elements  120   a  and the second grating elements  120   b  and the arrangement period P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  is fixed, and by adjusting the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b,  the transmission properties of the polarization spectral filter  100  may be adjusted. 
       FIG. 5  illustrates graphs illustrating an example of a change in the transmission properties of the polarization spectral filter  100  illustrated in  FIG. 1  according to a change in the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120 . In  FIG. 5 , a graph marked as “str 1 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.7, a graph marked as “str 2 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.5, and a graph marked as “str 3 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.3. In addition, in  FIG. 5 , three peaks on the left represent polarization components perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120 , and three peaks on the right represent polarization components parallel to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120 . Referring to  FIG. 5 , it may be seen that, as the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  increases, two transmission wavelength bands of the polarization spectral filter  100  are increased and thus gradually move toward a longer wavelength. 
     Although it is described above that the grating layer  120  includes only the first grating elements  120   a  and the second grating elements  120   b,  the grating layer  120  according to example embodiments is not limited thereto. The grating layer  120  may be configured by alternately arranging three, four, or more grating elements having different refractive indices. The number of grating elements alternately arranged in the grating layer  120  is not limited. 
       FIG. 6  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment. A polarization spectral filter  200  illustrated in  FIG. 6  is similar to the configuration of the polarization spectral filter  100  illustrated in  FIG. 1 , except that the grating layer  121  includes three grating elements. Referring to  FIG. 6 , the grating layer  121  may include a plurality of first grating elements  121   a,  a plurality of second grating elements  121   b,  and a plurality of third grating elements  121   c,  which are arranged alternately. Each of the first grating elements  121   a  includes a first dielectric material having a first refractive index, each of the second grating elements  121   b  includes a second dielectric material having a second refractive index different from the first refractive index, and each of the third grating elements  121   c  may include a third dielectric material having a third refractive index different from the first refractive index and the second refractive index. The plurality of first grating elements  121   a,  the plurality of second grating elements  121   b,  and the plurality of third grating elements  121   c  may be one-dimensionally arranged in a horizontal direction perpendicular to a thickness direction. 
     Referring back to the graphs of  FIGS. 3 and 5 , the polarization spectral filter  100  may have two different transmission wavelength bands having polarization properties perpendicular to each other. In addition, the two transmission wavelength bands may be completely separated from each other and may not overlap each other. Accordingly, if only one transmission wavelength band is selected from the two different transmission wavelength bands, the polarization spectral filter  100  may be configured to transmit only light having a specified polarization component from among light beams in specified transmission wavelength bands. Alternatively, a method of selecting one transmission wavelength band in a polarization spectral filter according to another example embodiment is described below. 
       FIG. 7  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment. A polarization spectral filter  300  illustrated in  FIG. 7  is similar to the configuration of the polarization spectral filter  100  illustrated in  FIG. 1  except that the polarization spectral filter  300  further includes a band pass filter  140  disposed on an upper surface of the second reflector  130 . In other words, the band pass filter  140  is disposed on a light incident surface of the polarization spectral filter  300 . The band pass filter  140  may be configured to allow only one wavelength band to pass therethrough from among the wavelength band of the first spectrum SP 1  and the wavelength band of the second spectrum SP 2  illustrated in  FIG. 3  and configured to block the other wavelength band. For example, the band pass filter  140  may be configured to block light in a wavelength range of 800 nm to 850 nm and configured to transmit light in a wavelength range of 850 nm to 900 nm therethrough. Then, the polarization spectral filter  300  may be configured to transmit only light of the second spectrum SP 2  having polarization components parallel to a longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  therethrough, and block light of the first spectrum SP 1  having polarization components perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b.    
       FIG. 8  illustrates graphs illustrating an example of transmission properties of the polarization spectral filter  300  of  FIG. 7 . In  FIG. 8 , a graph marked as “str 1 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.7, a graph marked as “str 2 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.5, and a graph marked as “str 3 ” represents a case where the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  is 0.3. Referring to  FIG. 8 , when the band pass filter  140  is used, light having a polarization component parallel to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  of the grating layer  120  passes through the polarization spectral filter  300 , and a wavelength band of light passing through the polarization spectral filter  300  may be adjusted according to the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b.    
     The band pass filter  140  may be configured to transmit the light in a wavelength range of, for example, 800 nm to 850 nm therethrough, and configured to block the light in a wavelength range of 850 nm to 900 nm. In this case, the polarization spectral filter  300  may transmit only light having a polarization component perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  therethrough. In addition, by adjusting the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b,  a transmission wavelength band of the light having a polarization component perpendicular to the longitudinal direction of the first grating elements  120   a  and the second grating elements  120   b  may be adjusted. 
       FIG. 9  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment. While  FIG. 7  illustrates that the band pass filter  140  is disposed on an upper surface of the second reflector  130 , a position of the band pass filter  140  is not limited thereto. The band pass filter  140  may be disposed anywhere outside a resonator formed by the first reflector  110  and the second reflector  130 . For example, referring to  FIG. 9 , a polarization spectral filter  400  may include the band pass filter  140  disposed on a lower surface of the first reflector  110 . In other words, the band pass filter  140  may be disposed on a light emission surface of the polarization spectral filter  400 . 
       FIG. 10  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment. Referring to  FIG. 10 , a polarization spectral filter  500  is similar to the configuration of the polarization spectral filter  300  illustrated in  FIG. 7  and may further include a quarter wave plate  150  disposed between the band pass filter  140  and the second reflector  130 . The quarter wave plate  150  serves to delay a phase of incident light by a quarter wavelength of a wavelength of the incident light beam. The quarter wave plate  150  may be formed by patterning a dielectric material having a relatively high refractive index into a nanoscale structure smaller than the wavelength of light beam. For example, the quarter wave plate  150  may be formed by a meta surface including Si, TiO 2 , or Si 2 N 3 . 
     If the phase of the incident light is delayed by a quarter wavelength of the wavelength of the incident light by the quarter wave plate  150 , a linear polarization component of the incident light is changed into a circular polarization component and the circular polarization component is changed into the linear polarization component. In other words, the quarter wave plate  150  may serve to change the linearly polarized light into a circularly polarized light and change the circularly polarized light into the linearly polarized light beam. For example, a first linear polarization component is changed into a first circular polarization component by the quarter wave plate  150 , and a second linear polarization component perpendicular to the first linear polarization component is changed into a second circular polarization component rotated in a direction opposite to the first circular polarization component by the quarter wave plate  150 . Accordingly, by using the quarter wave plate  150 , light having the circular polarization component passes through the polarization spectral filter  500 . 
       FIG. 11  is a cross-sectional view schematically illustrating a configuration of a polarization spectral filter according to example embodiment. While  FIG. 10  illustrates that the quarter wave plate  150  is disposed on the upper surface of the second reflector  130 , a position of the quarter wave plate  150  is not limited thereto. The quarter wave plate  150  may be disposed anywhere outside a resonator formed by the first reflector  110  and the second reflector  130 . For example, referring to  FIG. 11 , a polarization spectral filter  600  may include the quarter wave plate  150  disposed on the lower surface of the first reflector  110 . In other words, the quarter wave plate  150  may be disposed on a light emission surface of the polarization spectral filter  600 . 
     In addition, in the example embodiments illustrated in  FIGS. 10 and 11 , a position between the quarter wave plate  150  and the band pass filter  140  may be interchanged. For example, the band pass filter  140  may be disposed on the upper surface of the second reflector  130 , and the quarter wave plate  150  may be disposed on an upper surface of the band pass filter  140 . In addition, the band pass filter  140  may be disposed on the lower surface of the first reflector  110 , and the quarter wave plate  150  may be disposed on a lower surface of the band pass filter  140 . 
     In addition, the quarter wave plate  150  and the band pass filter  140  may be disposed opposite to each other. For example, the quarter wave plate  150  may be disposed on the upper surface of the second reflector  130 , and the band pass filter  140  may be disposed on the lower surface of the first reflector  110 . Alternatively, the band pass filter  140  may be disposed on the upper surface of the second reflector  130 , and the quarter wave plate  150  may be disposed on the lower surface of the first reflector  110 . 
     The polarization spectral filter according to the example embodiments described above may transmit light in a specified wavelength band having a specified linear polarization component or a specified circular polarization component therethrough without using a separate polarization filter and/or a separate spectral filter. In addition, the polarization spectral filter according to the example embodiments described above may be made in a small size such as a pixel size of an image sensor. Accordingly, an array of the polarization spectral filters according to the example embodiments described above may be integrated with the image sensor to be used to obtain polarization information and spectral information simultaneously. In addition, by integrating the image sensor with the array of the polarization spectral filters according to the example embodiments described above, for example, a miniaturized polarization spectral image sensor that may be mounted in a small mobile device such as a smartphone may be provided. 
       FIG. 12  is a perspective view schematically illustrating a configuration of a polarization spectral filter array and a polarization spectral sensor including the polarization spectral filter array according to an example embodiment. Referring to  FIG. 12 , a polarization spectral sensor  1000  according to an example embodiment may include an image sensor  1100  and a polarization spectral filter array  1200  disposed on the image sensor  1100 . The polarization spectral filter array  1200  may include an array of a plurality of unit filters UPF UPF 12 , UPF 21 , . . . that are two-dimensionally arranged (e.g., arranged in columns and rows). In addition, the image sensor  1100  may include a plurality of sensing pixels that are two-dimensionally arranged and convert intensity of incident light into an electrical signal. Accordingly, the sensing pixels of the image sensor  1100  may sense intensity of the light transmitting through the polarization spectral filter array  1200 . 
       FIG. 13  illustrates an example of a configuration of the polarization spectral filter array  1200  illustrated in  FIG. 12 . Referring to  FIG. 13 , the polarization spectral filter array  1200  may include the plurality of unit filters UPF 11 , UPF 12 , UPF 21 , . . . that are two-dimensionally arranged. Each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . of the polarization spectral filter array  1200  may be configured to analyze a plurality of different polarization states for light having a plurality of different wavelengths. Each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . is a minimum unit of the polarization spectral filter array  1200  for simultaneously obtaining both polarization information and spectral information on incident light. 
     Each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . may include a set WF of the plurality of polarization spectral filters through which the light having different wavelengths transmits.  FIG. 13  illustrates an example in which each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . includes 16 polarization spectral filter sets WF through which light λ1 through λ16 in first through sixteenth wavelength bands respectively transmit. However, the number of polarization spectral filters sets WF arranged in each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . is not limited thereto, and more polarization spectral filter sets WF may be arranged, or less polarization spectral filter sets WF may be arranged. The plurality of polarization spectral filters WF may be arranged in the form of a two-dimensional array within each of the unit filters UPF 11 , UPF 12 , UPF 21 , . . . 
       FIG. 14  illustrates an example of an arrangement of the polarization spectral filters in one polarization spectral filter set WF of each of the unit filter arrays UPF 11 , UPF 12 , UPF 21 , . . . of the polarization spectral filter array  1200  illustrated in  FIG. 12 . Referring to  FIG. 14 , each polarization spectral filter set WF may include, for example, first to fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  arranged in the form of a 2×2 array. The first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be configured to transmit light having linear polarization components in different directions therethrough. For example, the first polarization spectral filter PF 1  may be configured to transmit the light having the first linear polarization component therethrough. The second polarization spectral filter PF 2  may be configured to transmit the light having the second linear polarization component perpendicular to the first linear polarization component therethrough. The third polarization spectral filter PF 3  may be configured to transmit light having a third linear polarization component that is inclined 45 degrees with respect to the first linear polarization component therethrough. In addition, the fourth polarization spectral filter PF 4  may be configured to transmit light having a fourth linear polarization component inclined 135 degrees with respect to the first linear polarization component therethrough. 
       FIG. 15  is a cross-sectional view taken along line A-A′ of  FIG. 14 .  FIG. 15  schematically illustrates an example of an arrangement of the polarization spectral filters in one polarization spectral filter set WF illustrated in  FIG. 14  and a partial configuration of the polarization spectral sensor  1000  including the image sensor  1100 . Referring to  FIG. 15 , the first and the third polarization spectral filters PF 1  and PF 3  may have the same structure as, for example, the structure of the polarization spectral filter  300  illustrated in  FIG. 7 . While  FIG. 15  illustrates only the first and the third polarization spectral filters PF 1  and PF 3 , it should be understood that the above descriptions may equally apply to the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4 . In other words, each of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may include the first reflector  110 , the grating layer  120 , the second reflector  130 , and the band pass filter  140 . In the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4 , the first reflector  110 , the second reflector  130 , and the band pass filter  140  may extend integrally with each other as a common configuration. 
     The plurality of the first and the second grating elements  120   a  and  120   b  of the grating layer  120  may be arranged along different directions in the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  such that the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  to transmit the light having different linear polarization components therethrough. For example, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the second polarization spectral filter PF 2  may be arranged perpendicularly to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . In other words, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the second polarization spectral filter PF 2  may be rotated 90 degrees on a horizontal plane with respect to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . 
     In addition, the first and the second grating elements  120   a  and  120   b  of the third polarization spectral filter PF 3  may be arranged to be inclined 45 degrees with respect to the first and the second grating elements  120   a  and  120   b  of the first polarization spectral filter PF 1 . In other words, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the third polarization spectral filter PF 3  may be rotated 45 degrees on the horizontal plane with respect to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . In addition, the first and the second grating elements  120   a  and  120   b  of the fourth polarization spectral filter PF 4  may be arranged to be inclined 135 degrees with respect to the first and the second grating elements  120   a  and  120   b  of the first polarization spectral filter PF 1 . In other words, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the fourth polarization spectral filter PF 4  may be rotated 135 degrees on the horizontal plane with respect to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . 
     The first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  arranged in one same polarization spectral filter set WF may be configured to transmit light having the same wavelength band therethrough. As described above, the transmission bands of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be determined by the thickness T of the first grating elements  120   a  and the second grating elements  120   b,  the arrangement period P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b,  the ratio (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b,  and the like. Accordingly, in one same polarization spectral filter set WF, the thicknesses T of the first grating elements  120   a  and the second grating elements  120   b  of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be the same, and the arrangement periods P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  may be the same, and the ratios (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b  may be the same. 
     Light transmitted through the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be incident on different pixels of the image sensor  1100 . To this end, the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may arranged to correspond one-to-one to the pixels of the image sensor  1100 , respectively. Based on this configuration, by analyzing electrical signals output from pixels corresponding to the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  of the image sensor  1100 , information on intensity of light having the first linear polarization component, intensity of light having the second linear polarization component, intensity of light having the third linear polarization component, and intensity of light having the fourth linear polarization component may be extracted from among the light beams having the same wavelength band. 
     Each of the polarization spectral filter sets WF may include the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4 . The first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  arranged in different polarization spectral filter sets WF may be configured to transmit the light in different wavelength bands therethrough. For example, the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  in the polarization spectral filter set WF for analyzing the linear polarization component of the light λ1 in a first wavelength band may be configured to transmit the light λ1 in the first wavelength band, and the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  in the polarization spectral filter set WF for analyzing the linear polarization component of the light λ2 in a second wavelength band may be configured to transmit the light λ2 in the second wavelength band. 
     For the sake of a convenient manufacturing process, in an example embodiment, the thicknesses T of all the first grating elements  120   a  and the second grating elements  120   b  may be the same in the polarization spectral filter array  1200 . In this case, the transmission bands of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be determined mainly by the arrangement periods P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b,  and/or the ratio (W 1 /W 2 ) between the first grating element  120   a  and the second grating element  120   b.  In an example embodiment, the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  respectively arranged in the different polarization spectral filter sets WF may have different arrangement periods P of the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b,  and/or may have different ratios (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b.    
     In an example embodiment, the thicknesses T and the arrangement periods P of all the plurality of first grating elements  120   a  and the plurality of second grating elements  120   b  may be the same in the polarization spectral filter array  1200 . In this case, the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  respectively arranged in different polarization spectral filter sets WF may have different ratios (W 1 /W 2 ) between the first grating elements  120   a  and the second grating elements  120   b.    
     In addition, the transmission bands of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may also be determined by a pass band of the band pass filter  140 . For example, the pass bands of the band pass filters  140  in the different polarization spectral filter sets WF may be different. In another example, one common band pass filter  140  may be used in the entire polarization spectral filter array  1200 . In this case, the band pass filter  140  may be configured to transmit the light beams λ1 to λ16 in first to sixteenth wavelength bands therethrough and configured to block light in the remaining wavelength bands. 
       FIG. 16  illustrates another example of the arrangement of the polarization spectral filters in one polarization spectral filter set WF of each of the unit filter arrays UPF 11 , UPF 12 , UPF 21 , . . . of the polarization spectral filter array  1200  illustrated in  FIG. 12 . In addition,  FIG. 17  illustrates an example of a cross-sectional view taken along line B-B′ of  FIG. 16 .  FIG. 17  schematically illustrates an example of a partial configuration of an arrangement of the polarization spectral filters in one polarization spectral filter set WF illustrated in  FIG. 16  and a partial configuration of the polarization spectral sensor  1000  including the image sensor  1100 . 
     Referring to  FIG. 16 , each polarization spectral filter set WF may include, for example, first to sixth polarization spectral filters PF 1 , PF 2 , PF 3 , PF 4 , PF 5 , and PF 6  arranged in the form of a 2×3 array. The first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may be configured to transmit light having linear polarization components in different directions therethrough. Since configurations and operations of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  are the same as the configuration and operation described above with reference to  FIG. 14 , description thereof will be omitted. 
     The fifth polarization spectral filter PF 5  may be configured to transmit the light having a first circular polarization component therethrough and the sixth polarization spectral filter PF 6  may be configured to transmit the light having a second circular polarization component rotated in a direction opposite to the first circular polarization component. To this end, the fifth polarization spectral filter PF 5  may further include the quarter wave plate  150  disposed between the second reflector  130  and the band pass filter  140 , as illustrated in  FIG. 17 . Although not illustrated in  FIG. 17 , the sixth polarization spectral filter PF 6  may further include the quarter wave plate  150  disposed between the second reflector  130  and the band pass filter  140 . Accordingly, configurations of the fifth polarization spectral filter PF 5  and the sixth polarization spectral filter PF 6  may be the same as the configuration of the polarization spectral filter  500  illustrated in  FIG. 10 . 
     The first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the fifth polarization spectral filter PF 5  may be arranged in parallel to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . On the other hand, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the sixth polarization spectral filter PF 6  may be arranged perpendicularly to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the first polarization spectral filter PF 1 . Accordingly, the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the sixth polarization spectral filter PF 6  may be arranged in parallel to the first and the second grating elements  120   a  and  120   b  of the grating layer  120  of the second polarization spectral filter PF 2 . Accordingly, the light transmitted through the fifth polarization spectral filter PF 5  and the light transmitted through the sixth polarization spectral filter PF 6  may have information on circular polarization components rotated in opposite directions. 
     According to the example embodiments illustrated in  FIGS. 16 and 17 , by analyzing electrical signals output from pixels corresponding to the first to sixth polarization spectral filters PF 1 , PF 2 , PF 3 , PF 4 , PF 5 , and PF 6  of the image sensor  1100 , information on intensity of light having various linear polarization components and intensity of light having a circular polarization component may be extracted from light in the same wavelength band. 
     As illustrated in  FIG. 17 , the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4  may further include a spacer  160  disposed on the second reflector  130  to maintain a constant position of the band pass filter  140  in a thickness direction. A thickness of the spacer  160  may be the same as the thickness of the quarter wave plate  150 . 
       FIG. 18  illustrates another example of a cross-sectional view taken along line B-B′ of  FIG. 16 .  FIG. 18  schematically illustrates another example of an arrangement of the polarization spectral filters in one polarization spectral filter set WF illustrated in  FIG. 16  and a partial configuration of the polarization spectral sensor  1000  including the image sensor  1100 . Referring to  FIG. 18 , the band pass filter  140  may be disposed under the first reflector  110 . In this case, the band pass filter  140  may be disposed to face the image sensor  1100 . In addition, in the fifth and sixth polarization spectral filters PF 5  and PF 6 , the quarter wave plate  150  may be disposed between the first reflector  110  and the band pass filter  140 . In the example embodiment illustrated in  FIG. 18 , the quarter wave plate  150  may be buried in the band pass filter  140 . Then, a separate spacer may not be used. 
       FIG. 19  illustrates yet another example of a cross-sectional view taken along line B-B′ of  FIG. 16 .  FIG. 19  schematically illustrates a yet another example of an arrangement of the polarization spectral filters in one polarization spectral filter set WF illustrated in  FIG. 16  and a partial configuration of the polarization spectral sensor  1000  including the image sensor  1100 . Referring to  FIG. 19 , the band pass filter  140  may be disposed on an upper surface of the second reflector  130 , and the quarter wave plate  150  may be disposed on a lower surface of the first reflector  110  in the fifth and sixth polarization spectral filters PF 5  and PF 6 . In addition, the spacer  160  may be further disposed on the lower surface of the first reflector  110  of the first to the fourth polarization spectral filters PF 1 , PF 2 , PF 3 , and PF 4 . A thickness of the spacer  160  may be the same as the thickness of the quarter wave plate  150 . 
     Although the polarization spectral filter, the polarization spectral filter array, and the polarization spectral sensor are described with reference to the example embodiments illustrated in the drawings, these embodiments are merely examples, and it will be understood that various modifications and equivalent other embodiments may be implemented by those skilled in the art. Therefore, the disclosed embodiments should be considered in descriptive sense only and not for the purpose of limitation. The scope of rights is set forth in the claims rather than the foregoing description, and all differences within the scope shall be construed as being included in the scope of rights. 
     While the disclosure has been particularly illustrated and described with reference to example embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.