Patent Publication Number: US-2021173239-A1

Title: Light modulator, optical device including light modulator, and electronic apparatus including optical device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2019-0164156, filed on Dec. 10, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments of the present disclosure relate to a light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device. 
     2. Description of Related Art 
     Light modulators that change transmittance/reflectivity, polarization, phase, intensity, path, etc. of incident light are used in various types of optical devices. In addition, light modulators of various structures have been introduced to control the above-described properties of incident light in an optical device in a desired manner. 
     For example, liquid crystals having optical anisotropy, a microelectromechanical system (MEMS) structure using micromechanical movement of a light blocking/reflecting element, and the like have been widely used as general light modulators. Such optical modulators have an operational response time of several μs or more according to characteristics of a driving method thereof. 
     Recently, attempts have been made to apply a metasurface to a light modulator. The metasurface refers to a structure having a thickness, a pattern, a cycle, or the like with a value smaller than a wavelength of incident light. For example, an optical device using a tunable metasurface, which has variable optical properties (e.g., a refractive index) and is based on a semiconductor material with a multi-quantum well structure, has been used in various technical fields ranging from optical communications to optical sensing. For example, the light modulator using a tunable metasurface includes a pair of distributed Bragg reflectors (DBRs) or a Fabry-Perot resonator structure having a sandwich structure in which a semiconductor material is provided between one DBR and a grating reflector or between one DBR and a metal mirror. 
     SUMMARY 
     One or more example embodiments provide a light modulator including a reflective multilayer structure having precisely adjusted reflectivity or transmittance, an optical device including the light modulator, and an electronic apparatus including the optical device. 
     Aspects are not limited thereto and there may be additional aspects. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the example embodiments of the disclosure. 
     According to an aspect of an example embodiment, there is provided a light modulator including a substrate, and a resonator configured to modulate a phase of incident light by modulating a refractive index based on an external stimulus, the resonator comprising a first reflective structure provided on the substrate, a cavity layer provided on the first reflective structure, and a second reflective structure provided on the cavity layer, wherein at least one of the first reflective structure or the second reflective structure comprises first material layers, second material layers that are alternately stacked with the first material layers, and a third material layer, and wherein each of the first material layers has a first refractive index, each of the second material layers has a second refractive index that is different from the first refractive index, and the third material layer has a third refractive index that is different from the first refractive index. 
     A first optical thickness of each of the first material layers may be equal, and a third optical thickness of the third material layer may be different from the first optical thickness of each of the first material layers. 
     A first physical thickness of each of the first material layers may be equal, and a third physical thickness of the third material layer may be equal to the first physical thickness of the first material layers. 
     The first physical thicknesses of the first material layers and the third physical thickness of the third material layer may be odd-number multiples of λ/(4*n 1 ), where λ is a wavelength of the incident light and n 1  is the first refractive index of the first material layers. 
     A second physical thickness of the second material layers may be an odd-number multiple of λ/(4*n 2 ), where λ is a wavelength of the incident light and n 2  is the second refractive index of the second material layers. 
     A first reflectivity of the first reflective structure may be greater than a second reflectivity of the second reflective structure. 
     At least one of the first reflective structure and the second reflective structure may include a distributed Bragg reflector or a grating reflector. 
     The light modulator may further include a heating element provided between the substrate and the second reflective structure, provided between the first reflective structure and the second reflective structure, or provided above the second reflective structure, and the first reflective structure or the second reflective structure may include a material, a refractive index of the material changing based on temperature. 
     The cavity layer, the first reflective structure, or the second reflective structure may include an electro-optic material, a permittivity of the electro-optic material changing based on an electrical signal supplied thereto, and a voltage is applied through a first electrode in contact with the first reflective structure and a second electrode in contact with the second reflective structure. 
     The external stimulus may include at least one of heat, voltage, current, or a magnetic field. 
     According to another aspect of an example embodiments, there is provided a light modulator including a substrate, and a resonator configured to modulate a phase of incident light by modulating a refractive index based on an external stimulus, the resonator including a first reflective structure provided on the substrate, a cavity layer provided on the first reflective structure, and a second reflective structure provided on the cavity layer, wherein at least one of the first reflective structure or the second reflective structure includes repeatedly and alternately stacked first material layers and second material layers, wherein each of the first material layers have a first refractive index and each of the second material layers have a second refractive index, and wherein a thickness t 1 ′ of at least one of the first material layers is different from a thickness t 1  of other of the first material layers. 
     The thickness t 1  of the other of the first material layers may be an odd-number multiple of λ/(4*n 1 ), and wherein the thickness t 1 ′ of the at least one first material layer satisfies t 1 −0.75*λ/(4*n 1 )≤t 1 ′&lt;t 1  or t 1 &lt;t 1 ′≤2*t 1 , where λ is a wavelength of incident light and n 1  is the first refractive index of the first material layers. 
     A thickness of the second material layers may be an odd-number multiple of λ/(4*n 2 ), where λ is a wavelength of incident light and n 2  is the second refractive index of the second material layers. 
     A first reflectivity of the first reflective structure may be greater than a second reflectivity of the second reflective structure. 
     The other of the first reflective structure and the second reflective structure may include a distributed Bragg reflector or a grating reflector. 
     The external stimulus may include at least one of heat, voltage, current, or a magnetic field. 
     According to yet another aspect of an example embodiment, there is provided an optical device including a spatial light modulator including a plurality of light modulators, each of the plurality of light modulators including a substrate, and a resonator configured to modulate a phase of incident light by modulating a refractive index based on an external stimulus, the resonator comprising a first reflective structure provided on the substrate, a cavity layer provided on the first reflective structure, and a second reflective structure provided on the cavity layer, wherein at least one of the first reflective structure or the second reflective structure comprises first material layers, second material layers that are alternately stacked with the first material layers, and a third material layer, and wherein each of the first material layers has a first refractive index, each of the second material layers has a second refractive index that is different from the first refractive index, and the third material layer has a third refractive index that is different from the first refractive index, and wherein each of the plurality of light modulators are provided corresponding to one-dimensional pixels or two-dimensional pixels, the spatial light modulator being configured to modulate a phase of light for each of the pixels. 
     The optical device may further include a phase mask including a transparent support plate and a plurality of nanostructures provided on the transparent support plate, wherein each of the plurality of nanostructures are provided differently for each of the corresponding pixels to control a phase of light. 
     According to another aspect of an example embodiment, there is provided an electronic apparatus including a light source, an optical device configured to adjust a traveling direction of light emitted from the light source and transmit the light to travel toward a subject, the optical device including a spatial light modulator including a plurality of light modulators, each of the plurality of light modulators including a substrate, and a resonator configured to modulate a phase of incident light by modulating a refractive index based on an external stimulus, the resonator comprising a first reflective structure provided on the substrate, a cavity layer provided on the first reflective structure, and a second reflective structure provided on the cavity layer, wherein at least one of the first reflective structure or the second reflective structure comprises first material layers, second material layers that are alternately stacked with the first material layers, and a third material layer, and wherein each of the first material layers has a first refractive index, each of the second material layers has a second refractive index that is different from the first refractive index, and the third material layer has a third refractive index that is different from the first refractive index, and a phase mask including a transparent support plate and a plurality of nanostructures provided on the transparent support plate, wherein each of the plurality of light modulators are provided corresponding to one-dimensional pixels or two-dimensional pixels, the spatial light modulator being configured to modulate a phase of light for each of the pixels, and wherein each of the plurality of nanostructures are provided differently for each of the corresponding pixels to control the phase of light, a receiver configured to receive the light reflected from the subject and convert the light into an electrical signal, and a processor configured to process the electrical signal obtained by the receiver. 
     The electronic apparatus may include at least one of a light detection and ranging (LiDAR) apparatus, a three-dimensional (3D) image obtaining apparatus, a 3D sensor, a depth sensor, or a holographic display apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects, features, and advantages of 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 side cross-sectional view illustrating a structure of a light modulator according to an example embodiment; 
         FIG. 2  is a graph showing the relationship between reflectivity of a reflective multilayer structure and the number of third material layers; 
         FIG. 3  is an enlarged graph showing a section of  FIG. 2  with reflectivity of 95% to 100%; 
         FIG. 4  illustrates a result of sorting, in descending order, various reflectivities obtained from various combinations of a first material layer, a second material layer, and a third material layer; 
         FIG. 5  illustrates a change in a refractive index of silicon (Si) with temperature; 
         FIG. 6  is a side cross-sectional view illustrating a structure of a light modulator according to another example embodiment; 
         FIG. 7  is a side cross-sectional view illustrating a structure of a light modulator according to another example embodiment; 
         FIG. 8  is a side cross-sectional view illustrating a structure of a light modulator according to another example embodiment; 
         FIG. 9  is a side cross-sectional view illustrating a structure of a light modulator according to another example embodiment; 
         FIG. 10  is a plan view illustrating a structure of an optical device according to an example embodiment; 
         FIG. 11  is a plan view illustrating a structure of an optical device according to another example embodiment; 
         FIG. 12  is a plan view illustrating a structure of an optical device according to another example embodiment; 
         FIG. 13  is a block diagram illustrating a structure of an electronic apparatus including an optical device according to an example embodiment; 
         FIG. 14  is a diagram illustrating an electronic apparatus including an optical device according to an example embodiment; 
         FIG. 15  is a side view illustrating an example in which an electronic apparatus according to an example embodiment is applied to light detection and ranging (LiDAR) for a vehicle; and 
         FIG. 16  is a side view illustrating an example in which a beam scanning device according to an example embodiment is applied to LiDAR for a vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to example embodiments which are illustrated in the accompanying drawings. In this regard, the example embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the example embodiments are merely described below, by referring to the figures, to explain aspects. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 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, and c,” should be understood as including only a, only b, only c, both a and b, both a and c, both b and c, or all of a, b, and c. 
     In the drawings, the same reference numerals may represent the same elements, and a size of each element may be exaggerated for clarity and convenience of description. Example embodiments described below are merely examples and various modifications may be made therein. 
     Terms such as “first” and “second” may be used to describe various components but the components should not be limited by the terms. These terms are only used to distinguish one component from another. A light modulator, an optical device including the optical device, and an electronic apparatus including the optical device may be embodied in many different forms and are not limited to example embodiments set forth herein. 
     As used herein, the singular expressions are intended to include plural forms as well, unless the context clearly dictates otherwise. It will be understood that when an element is referred to as “including” another element, the element may further include other elements unless mentioned otherwise. It will be further understood that when one element is referred to as being “on” or “above” another element, the element may be on, below, or on a left or right side of the other element, in direct contact with or without contacting the other element. 
     Terms such as “unit”, “module,” and the like, when used herein, represent units for processing at least one function or operation, which may be implemented by hardware, software, or a combination of hardware and software. 
       FIG. 1  is a side cross-sectional view illustrating a structure of a light modulator  100  according to an example embodiment. 
     Referring to  FIG. 1 , the light modulator  100  according to the example embodiment is a reflective device in which incident light Li is incident on and output light Lo is output from the same plane which may be the upper surface of the light modulator  100 . The light modulator  100  includes a substrate  110 , a heating element  120  provided on the substrate  110 , a first reflective structure  130  provided on the heating element  120 , a cavity layer  140  provided on the first reflective structure, and a second reflective structure  150  provided on the cavity layer  140 . An upper layer  160  formed of a transparent material may be provided on the second reflective structure  150 . 
     The substrate  110  may include a metal. For example, the metal included in the substrate  110  may include gold (Au), aluminum (Al), or silver (Ag), but is not limited thereto. The substrate  110  including the metal may serve as a reflective layer which reflects light passing through a resonator. Accordingly, reflectivity of the light modulator  100  with respect to the incident light Li may be higher than that when the substrate  110  does not include the metal. However, embodiments are not limited thereto, and the substrate  110  may not include a metal. An area of the substrate  110  may be larger than an area of the resonator. For example, a width P SLM  of the substrate  110  may be larger than a width W heating element  of the heating element  120  inducing a change in refractive index in the resonator. 
     The heating element  120  may include a metal wire, and heating temperature may vary according to the intensity of current supplied to the metal wire. 
     The first reflective structure  130 , the cavity layer  140 , and the second reflective structure  150  form a Fabry-Perot resonator together. In this case, the reflectivity of the second reflective structure  150  may be designed to be lower than the reflectivity of the first reflective structure  130  such that light resonating between the first reflective structure  130  and the second reflective structure  150  may be output to the outside of light modulator  100  via the second reflective structure  150 . 
     A total optical thickness L C  which is a value obtained by multiplying a physical thickness thereof by the refractive index of a material layer of the cavity layer  140  may be set by the following Equation 1 for resonance between the first reflective structure  130  and the second reflective structure  150 . 
       2π/λ×2 L   C +φ 1 +φ 2 =2π m   [Equation 1]
 
     Here, φ 1  is a reflection phase experienced by light waves incident on the first reflective structure  130  from the cavity layer  140 , φ 2  is a reflection phase experienced by light waves incident on the second reflective structure  150  from the cavity layer, and m is an integer representing the order of resonance. 
     The first reflective structure  130  is a grating reflector. In the grating reflector, gratings  131  may be arranged spaced apart from each other at certain intervals. The reflectivity and transmissivity of light may be controlled by adjusting, for example, the interval P HCG  between the gratings  131  and a height t HCG , width W HCG  of the gratings  131 . of the gratings  131 . The gratings  131  may have a refractive index greater than a refractive index of a peripheral material  170 . For example, the gratings  131  may be formed of Si. 
     The second reflective structure  150  is a multilayer reflector in which first material layers  151  and second material layers  152  having different refractive indices are repeatedly and alternately stacked, and at least one of locations at which the first material layers  151  are to be provided is replaced with a third material layer  153 . The third material layer  153  has a refractive index different from the refractive indices of the first material layers  151  and the second material layers  152 . For example, the first material layer  151  may be formed of Si, the second material layer  152  may be formed of silicon oxide (SiO 2 ), and the third material layer  153  may be formed of tin oxide (TiO 2 ). The refractive index n Si  of Si is approximately 3.5, the refractive index n TiO2  of TiO 2  is approximately 2.0, and the refractive index n SiO2  of SiO 2  is approximately 1.5. 
     All the first material layers  151  have the same optical thickness, and an optical thickness of the third material layer  153  is different from that of the first material layers  151 . For example, the first material layers  151  may have the same physical thickness t DBR1 , and a physical thickness t DBR3  of the third material layer  153  may be the same as the thickness t DBR1  of the first material layers  151 . Specifically, both the first material layers  151  and the third material layer  153  may have a physical thickness which is an odd-number multiple of λ/(4*n 1 ). Here, A represents the wavelength of incident light and n 1  represents refractive index of the first material layer. For example, when an operation wavelength is 1.55 μm, the first material layer  151  may be formed of Si and have a thickness of 110.7 nm corresponding to λ/(4n Si ), and the third material layer  153  may be formed of TiO 2  and have a thickness of 110.7 nm which is the same as the first material layer  151 . However, embodiments are not limited thereto, and the third material layer  153  may have a thickness different from that of the first material layer  151 . 
     Similarly, the second material layers  152  may have the same physical thickness t DBR2 . Specifically, the second material layers  152  may have a thickness which is an odd-number multiple of λ/(4*n 2 ). Here, A represents the wavelength of incident light and n 2  represents a refractive index of the second material layer  152 . For example, the second material layer  152  may be formed of SiO 2  and have a thickness of 258.3 nm corresponding to λ/(4 n SiO2 ). 
     Furthermore, the first material layers  151  and the second material layers  152  may have the same optical thickness. In this case, only the third material layer  153  may have an optical thickness different from that of the first material layers  151 . For example, when the operation wavelength is 1.55 μm, the first material layers  151  and the second material layers  152  may have an optical thickness of 397.5 nm corresponding to λ/4, and the third material layer  153  may have an optical thickness of 221.4 nm. 
     When incident light Li having a certain wavelength is incident on the second reflective structure  150 , reflection may occur at an interface between the first material layers  151  and the second material layers  152  and an interface between the second material layers  152  and the third material layer  153 . 
     In this case, when only the first material layers  151  and the second material layers  152  are alternately stacked, the differences in phase between all reflected light are the same, and thus a constructive interference may occur between the reflected light. When the second reflective structure  150  includes first material layers  151 , the second material layers  152 , and the third material layer  153 , the differences in phase between all reflected light may not be the same, and thus a constructive interference may be prevented from occurring between some of the reflected light, thereby reducing reflectivity. 
       FIG. 2  is a graph showing the relationship between reflectivity of a reflective multilayer structure and the number of third material layers  153 .  FIG. 3  is an enlarged graph showing a section of  FIG. 2  with reflectivity of 95% to 100%.  FIG. 4  illustrates a result of sorting, in descending order, various reflectivities obtained from various combinations of a first material layer  151 , a second material layer  152 , and a third material layer  153 . 
     The graphs of  FIGS. 2 to 4  show results obtained when an operation wavelength was 1.55 μm, first material layers  151  were formed of Si having a thickness of 110.7 nm, second material layers  152  were formed of SiO 2  having a thickness of 258.3 nm, and third material layers  153  were formed of TiO 2  having a thickness of 110.7 nm. 
     In  FIGS. 2 and 3 , DBR # represents the number of unit multilayer structures. Here, the unit multilayer structures may be a pair of the first material layer  151  and the second material layer  152  or a pair of the third material layer  153  and the second material layer  152 . Here, solid lines represent reflectivity when the number of third material layer  153  is zero, where the distributed Bragg reflector consisting of only the first material layers  151  and the second material layers  152 . 
     Referring to  FIGS. 2 and 3 , when the number of unit multilayer structures is the same, reflectivity when the number of material layer  153  is one is lower than reflectivity when the number of third material layer  153  is zero. Furthermore, reflectivity when the number of third material layers  153  is two is lower than reflectivity when the number of third material layer  153  is one. Generally, reflectivity increases as the number of unit multilayer structures increases. 
     As described above, it may be understood that reflectivity decreases due to loss caused as the third material layer  153  interferes with constructive interference between the reflected light in the resonator. 
     In  FIG. 4 , horizontal dotted lines represent discrete and discontinuous reflectivity obtained when the third material layer  153  is not provided. For example, referring to both  FIGS. 3 and 4 , horizontal dotted line A represents reflectivity of about 99.55 when the number of unit multilayer structures each consisting of a pair of the first material layer  151  and the second material layer  152  is four, and horizontal dotted line B represents reflectivity of about 99.92 when the number of unit multilayer structures each consisting of a pair of the first material layer  151  and the second material layer  152  is five. A reflectivity between about 99.05 and about 99.92 cannot be achieved in the case of where a multilayer reflector does not include the third material layer  153 . 
     In contrast, when a multilayer reflector is formed by various combinations of the first material layer  151 , the second material layer  152 , and the third material layer  153 , there may be seven reflectivities between the horizontal dotted line A and the horizontal dotted line B. In other words, as the third material layer  153  is employed, the reflectivity of the second reflective structure  150  may be adjusted very precisely. 
     As described above, the reflectivity of the second reflective structure  150  should be lower than that the reflectivity of the first reflective structure  130 , such that light resonating between first reflective structure  130  and the second reflective structure  150  may be output to the outside through the second reflective structure  150 . In addition, in order to secure high resonance efficiency, the reflectivities of the first reflective structure  130  and the second reflective structure  150  should be as high as possible. The light modulator  100  according to the example embodiment may employ the third material layer  153  to adjust reflectivity very precisely, thereby outputting light while securing a relatively high resonance efficiency. 
     Next, the principle of adjusting a reflection phase by an external signal of the light modulator  100  of the example embodiment will be described.  FIG. 5  illustrates a change in a refractive index of silicon (Si) with temperature. The refractive index of Si may vary according to Equation 2 below. 
     
       
         
           
             
               
                 
                   
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                         10 
                         
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     Here, T is a temperature and n is a refractive index. According to Equation 2 above, when a temperature changes by about 300 degrees, the refractive index of Si changes by about 0.1. 
     When the refractive index of Si changes, a reflection phase of the gratings  131  of the first reflective structure  130  changes, thus changing a resonance wavelength. As a result, an operation wavelength is out of the resonance wavelength. This is called detuned resonance. A reflection phase may be adjusted using detuned resonance. 
     In the example embodiment, the first reflective structure  130  is positioned adjacent to the substrate  110  and the second reflective structure  150  is positioned adjacent to the upper layer  160 , but the positions of the first reflective structure  130  and the second reflective structure  150  may be switched. 
     Although a case in which the heating element  120  is positioned between the substrate  110  and the first reflective structure  130  is described according to an example embodiment, embodiments are not limited thereto. For example, the heating element  120  may be positioned above or below the second reflective structure  150  to change the reflection phase of the second reflective structure  150 . In this case, the heating element  120  may be formed of a material transparent with respect to the incident light Lithe output light L o . 
       FIG. 6  is a side cross-sectional view illustrating a structure of a light modulator  200  according to another example embodiment. Referring to  FIG. 6 , the light modulator  200  according to the example embodiment is a transmissive device in which incident light L i  is incident on a lower surface of the light modulator  200  and output light L o  is output via an upper surface of the light modulator  200 . The light modulator  200  includes a substrate  210  formed of a transparent material, a heating element  220  which is provided on the substrate  210  and transparent with respect to incident light Li, a first reflective structure  230  provided on the heating element  220 , a cavity layer  240  provided on the first reflective structure  230 , and a second reflective structure  250  provided on the cavity layer  240 . An upper layer  260  formed of a transparent material may be provided on the second reflective structure  250 . The second reflective structure  150  may be a multilayer reflector in which first material layers  251  and second material layers  252  having different refractive indices are repeatedly and alternately stacked, and at least one of locations at which the first material layers  251  are to be provided may be replaced with a third material layer  253 . The positions of the first reflective structure  230  and the second reflective structure  250  may be switched. 
     The light modulator  200  of the example embodiment may be substantially the same as the light modulator  100  of the above-described example embodiment with reference to  FIG. 1 , except that the substrate  210  or the heating element  220  is formed of a transparent material with respect to the incident light Li. 
     A case in which the heating element  220  is positioned between the substrate  210  and the first reflective structure  230  is described as an example in the example embodiment, but the heating element  220  may be positioned on or below the second reflective structure  250 . 
       FIG. 7  is a side cross-sectional view illustrating a structure of a light modulator  300  according to another example embodiment. Referring to  FIG. 7 , the light modulator  300  according to the example embodiment includes a substrate  310 , a heating element  320  provided on the substrate  310 , a first reflective structure  330  provided on the heating element  320 , a cavity layer  340  provided on the first reflective structure  330 , and a second reflective structure  350  provided on the cavity layer  340 . An upper layer  360  formed of a transparent material may be provided on the second reflective structure  350 . The light modulator  300  may be a reflective type or transmissive type. The light modulator  300  of the example embodiment may be substantially the same as the light modulators  100  and  200  of the above-described embodiments with reference to  FIGS. 1 and 6 , except the structure of the second reflective structure  350 . 
     The reflective structure  350  is a multilayer reflector in which first material layers  351  and  353  and second material layers  352  having different refractive indices are stacked repeatedly and alternately. A first material layer may include a thickness-changed first material layer  353  having a thickness t DBR3  that is different from a thickness t DBR1  of the other first material layers  351 . 
     The thickness t DBR1  of the other first material layers  351  is an odd-number multiple of λ/(4*n 1 ), and the thickness t DBR3  of the thickness-changed first material layer  353  may satisfy a condition in Equation 3 below. 
         t   DBR1 −0.75*λ/(4* n   1 )≤ t   DBR3   &lt;t   DBR1  or  t   DBR1   &lt;t   DBR3   ≤t   DBR1 +λ/(4* n   1 )  [Equation 3]
 
     Here, λ represents the wavelength of incident light and n 1  represents a refractive index of the first material layers. 
     For example, when the thickness t DBR1  of the other first material layers  351  is λ/(4*n 1 ), and the thickness t DBR3  of the thickness-changed first material layer  353  satisfies a condition in Equation 4 below. 
       0.25*λ/(4* n   1 )≤ t   DBR3 &lt;λ/(4* n   1 ) or λ/(4* n   1 )&lt; t   DBR3 ≤2*λ/(4* n   1 )  [Equation 4]
 
     Each of the second material layers  352  may have the same physical thickness. For example, the second material layers  352  may have a thickness which is an odd-number multiple of λ/(4*n 2 ). Here, A is the wavelength of incident light and n 2  is a refractive index of the second material layer  352 . 
     In addition, the other first material layers  351  and the second material layers  352  may have the same optical thickness. In this case, only the thickness-changed first material layer  353  may have an optical thickness different from that of the other first material layers  351  and the second material layers  352 . 
     As in the example embodiment described above with reference to  FIG. 1 , constructive interference may be prevented from occurring between some of reflected light, and reflectivity may be reduced due to the thickness-changed first material layer  353 . As the third material layer  353  is employed, the reflectivity of the second reflective structure  350  may be adjusted very precisely. 
     A case in which the heating element  320  is positioned between the substrate  310  and the first reflective structure  330  is described as an example in the example embodiment, but the heating element  320  may be positioned on or below the second reflective structure  350  to change a reflection phase of the second reflective structure  350 . 
       FIG. 8  is a side cross-sectional view illustrating a structure of a light modulator  400  according to another example embodiment. Referring to  FIG. 8 , the light modulator  400  according to the example embodiment includes a substrate  410 , a heating element  420  provided on the substrate  410 , a first reflective structure  430  provided on the heating element  420 , a cavity layer  440  provided on the first reflective structure  430 , and a second reflective structure  450  provided on the cavity layer  440 . An upper layer  460  formed of a transparent material may be provided on the second reflective structure  450 . The light modulator  400  may be of a reflective type or transmissive type. The light modulator  400  of the example embodiment is substantially the same as the light modulators  100  and  200  of the example embodiments described above with reference to  FIGS. 1 and 6 , except that the first reflective structure  430  is a distributed Bragg reflector. 
     The second reflective structure  450  may be a multilayer reflector in which first material layers  451  and second material layers  452  having different refractive indices are stacked repeatedly and alternately, and at least one of locations at which the first material layers  451  are to be provided may be replaced with a third material layer  453 . The reflective structure  350  may be a multilayer reflector in which first material layers  451  and  453  and second material layers  452  having different refractive indices are stacked repeatedly and alternately. and The first material layers may include a thickness-changed first material layer  453  having a thickness that is different from a thickness of the other first material layers  451 . 
     The first reflective structure  430  is a distributed Bragg reflector formed by repeatedly and alternately stacking fourth material layers  431  and fifth material layers  432  having different refractive indices. For example, the fourth material layers  431  may be formed of Si and the fifth material layers  432  may be formed of SiO 2 . The fourth material layers  431  and the fifth material layer  432  may have the same optical thickness. 
     As in the example embodiment described above with reference to  FIG. 1 , when a temperature changes, the refractive index of Si changes, and thus the reflection phase of the first reflective structure  430  changes, thus changing a resonance wavelength. As a result, an operation wavelength is out of the resonant wavelength, thereby adjusting the reflection phase. 
     According to the example embodiment, the first reflective structure  430  may be a distributed Bragg reflector, but embodiments are not limited thereto. Similar to the second reflective structure  450 , the first reflective structure  430  may be configured by repeatedly and alternately stacking the fourth material layers  431  and the fifth material layers  432  having different refractive indices such that at least one of locations at which the fourth material layers  431  are to be provided is replaced with a sixth material layer having a refractive index different from that of the fourth material layers  431  or has a thickness different from that of the other first material layers  431 . 
     A case in which the heating element  420  is positioned between the substrate  410  and the first reflective structure  430  is described as an example in the example embodiment, but the heating element  420  may be positioned on or below the second reflective structure  450  to change a reflection phase of the second reflective structure  450 . 
       FIG. 9  is a side cross-sectional view illustrating a structure of a light modulator  500  according to another example embodiment. 
     Referring to  FIG. 9 , the light modulator  500  according to the example embodiment includes a substrate  510 , a first reflective structure  530  provided on the substrate  510 , a cavity layer  540  provided on the first reflective structure  530 , and a second reflective structure  550  provided on the cavity layer  540 . An upper layer  560  formed of a transparent material may be provided on the second reflective structure  550 . The light modulator  500  may be of a reflective type or transmissive type. The light modulator  500  of the example embodiment is substantially the same as the light modulators  100 ,  200 ,  300 , and  400  of the above-described embodiments with reference to  FIGS. 1, 6, 7, and 8 , except that an electrical signal is used as a stimulus instead of heat. 
     The cavity layer  540  may be an active layer, the physical properties of which change according to electrical conditions thereof. The permittivity or refractive index of the cavity layer  540  may vary according to the electrical conditions associated with the cavity layer  540  and surrounding regions thereof. A change in the permittivity/refractive index of the cavity layer  540  may be due to a change of a charge concentration of one or more inner regions of the cavity layer  540 . The permittivity/refractive index of the cavity layer  540  may change due to a change in the charge concentration of the one or more inner regions of the cavity layer  540 . The permittivity/refractive index of the cavity layer  540  may vary according to an electric field or voltage applied to the cavity layer  540 . The cavity layer  540  may include, for example, a transparent conductive oxide (TCO), such as indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), gallium zinc oxide (GZO), aluminum gallium zinc oxide (AGZO), and gallium indium zinc oxide (GIZO). Alternatively, the cavity layer  540  may include a transition metal nitride (TMN) such as titanium nitride (TiN), zirconium nitride (ZrN), hafnium nitride (HfN), or tantalum nitride (TaN). In addition, the cavity layer  540  may include an electro-optic (EO) material, the effective permittivity of which changes when an electrical signal is applied thereto. The EO material may include, for example, a crystalline material, such as lithium niobium trioxide (LiNbO 3 ), lithium tantalite (LiTaO 3 ), potassium tantalate niobate (KTN), or lead zirconate titanate (PZT), or various polymers having electro-optic properties. The cavity layer  540  may be a conductor or a semiconductor. 
     The light modulator  500  may further include a first electrode  571  in contact with the first reflective structure  530  and a second electrode  572  in contact with the second reflective structure  550 . When the first reflective structure  530  and the second reflective structure  550  have conductivity, the first electrode  571  and the second electrode  572  may be electrode pads in contact with the first reflective structure  530  and the second reflective structure  550 . When the first reflective structure  530  and the second reflective structure  550  do not have conductivity, the first electrode  571  and the second electrode  572  may include transparent electrode layers on upper or lower portions of the first reflective structure  530  and the second reflective structure  550 . 
     When a certain voltage is applied between the first electrode  571  and the second electrode  572  from an external power source, the refractive index of the cavity layer  540  may change. Accordingly, the phase of light resonating between the first reflective structure  530  and the second reflective structure  550  changes. Accordingly, the phase of the incident light L i  and the phase of the output light L o  may be different from each other. 
     Furthermore, when a certain voltage is applied between the first electrode  571  and the second electrode  572  from an external power source, a part of the resonating light is absorbed in the cavity layer  540  due to electroabsorption. Accordingly, the intensities of the incident light L i  and the output light L o  may be different from each other. 
     A case in which the cavity layer  540  is an active layer, the physical properties of which change according to electrical conditions is described as an example embodiment, but embodiments are not limited thereto. For example, at least one of the layers of the first reflective structure  530  or the second reflective structure  550  may be an active layer, the physical properties of which change according to electrical conditions. For example, at least one layer of the first reflective structure  530  may be an EO material layer including an EO material, the effective permittivity of which changes when an electrical signal is supplied thereto. Accordingly, when power is supplied from an external power source, the refractive index of the EO material layer of the first reflective structure  530  changes and the phase of light resonating between the first reflective structure  530  and the second reflective structure  550  changes. 
     Heat and voltage have been described above as examples of an external stimulus in the above-described example embodiments, but embodiments are not limited thereto. The external stimulus may be current or a magnetic field, and at least one of the cavity layer  540 , the first reflective structure  530  or the second reflective structure  550  of the resonator may be formed of a material, the refractive index of which is changed by current or a magnetic field. 
       FIG. 10  is a plan view illustrating a structure of an optical device according to an example embodiment. Referring to  FIG. 10 , a spatial light modulator  600 A with a plurality of light modulators M 1  may be provided. The spatial light modulator  600 A is an example embodiment of an optical device. The plurality of light modulators M 1  may be arranged in a plurality of rows and columns in an X-axis direction and a Y-axis direction. Each of the light modulators M 1  may be understood as a pixel or a channel, and may have various shapes such as a tetragonal shape, a circular shape, a polygonal shape, etc. The reflection phases or transmission phases of the light modulators M 1  may be independently adjusted, and thus, the spatial light modulator  600 A may have different phase distributions in units of pixels or channels. 
       FIG. 11  is a plan view illustrating a structure of an optical device according to another example embodiment. Referring to  FIG. 11 , a spatial light modulator  600 B including a plurality of light modulators M 2  may be provided. The spatial light modulator  600 B is an example of an optical device. The plurality of light modulators M 2  may have a line shape when viewed from the top. For example, the plurality of light modulators M 2  may extend in the Y-axis direction and arranged spaced apart from each other in the X-axis direction. 
     The arrangements of the optical devices shown in  FIG. 10 or 11  are examples, but embodiments are not limited thereto. In addition, three or more unit regions may be arranged in various ways. 
       FIG. 12  is a plan view illustrating a structure of an optical device according to another example embodiment. 
     Referring to  FIG. 12 , a beam scanning device  700  may include a spatial light modulator  730  configured to modulate the phase of light, and a phase mask  740  configured to emit light from the spatial light modulator  730 . The beam scanning device  700  is an example of an optical device. 
     The spatial light modulator  730  may include light modulators  720  arranged in units of pixels on a substrate  710 . The spatial light modulator  730  may modulate the phase of light for each of the light modulators  720  corresponding to each pixel. Each of the light modulators  720  may be a unit capable of independently driving the spatial light modulator  730 . Each of the light modulators  720  may be a unit capable of modulating a phase. The light modulators  720  may be disposed on the substrate  710  to be spaced apart from each other in units of pixels. A certain phase may be controlled according to input value, such as voltage, current, heat, or a magnetic field, which is input to the spatial light modulator  730 . The phase of light emitted to the outside may be adjusted by changing resonance characteristics by an external signal supplied to the spatial light modulator  730 . 
     The phase mask  740  may include a support plate  741  and a plurality of nanostructures  742  arranged on the support plate  741 . The support plate  741  may be a transparent plate that transmits light. 
     The nanostructures  742  may include nanosized structures. A nanostructure may include, for example, a columnar structure. The phase mask  740  may include a nanostructure array in a region corresponding to the pixels of the spatial light modulator  730 . For example, the phase mask  740  may have a different nanostructure arrangement for each pixel. At least one of the size, number, shape, or arrangement intervals of the nanostructures  742  may be differently set according to pixels. The nanostructures  742  may be arranged periodically or aperiodically. The phase of transmitted waves may be adjusted by arranging the nanostructures  742  by using a combination of the shape, arrangement intervals, and size of the nanostructures  742 . 
     Accordingly, the beam scanning device  700  may be configured by combining the spatial light modulator  730  and the phase mask  740 . The beam scanning device  700  may operate as a reflection type or a transmission type according to the combination of the spatial light modulator  730  and the phase mask  740 . In the beam scanning device  700  which is of a reflective type, light may be incident toward the phase mask  740  and the light incident on the spatial light modulator  730  through the phase mask  740  may resonate in the spatial light modulator  730  and be emitted via the phase mask  740 . In the beam scanning device  700  which is of a transmission type, light may be incident toward the spatial light modulator  730 , resonate in the spatial light modulator  730 , and be emitted via the phase mask  740 . 
     An optical wavefront of emission light L o  emitted from the beam scanning device  700  has a phase distribution according to the combination of the spatial light modulator  730  and the phase mask  740 , and thus a traveling direction of the emission light L o  may be controlled according to the phase distribution. 
     The beam scanning device of the above-described example embodiment may be employed in a system, for example, a three-dimensional (3D) sensor such as a vehicle LiDAR sensor or a depth sensor used in a 3D camera, to increase the accuracy of the system. The beam scanning device of the above-described example embodiment may be used for not only LiDAR for a vehicle but also LiDAR for a robot, LiDAR for a drone, a security system for intruder detection, a subway screen door obstacle detection system, a depth sensor, a user face recognition sensor for a mobile phone, augmented reality (AR), motion recognition and object profiling in a TV or entertainment equipment, and the like. 
     For example,  FIG. 13  is a block diagram illustrating a structure of an electronic apparatus  800  including an optical device according to example an embodiment. 
     Referring to  FIG. 13 , an electronic apparatus  800  according to an embodiment may include a light source  810  emitting light, a beam scanning device  820  configured to adjust a traveling direction of light incident from the light source  810 , a photodetector  840  configured to sense light emitted from the beam scanning device  820  and reflected from an object, and a controller  830  configured to control the beam scanning device  820 . 
     The light source  810  may include, for example, a light source that emits visible light or a laser diode (LD) or a light-emitting diode (LED) that emits near-infrared ray of a wavelength band of about 800 nm to about 1500 nm. 
     The beam scanning device  820  may include the example embodiments described above with reference to  FIGS. 10 to 12 . The beam scanning device  820  may adjust a traveling direction of an optical beam by modulating a phase of the optical beam according to at least one input of voltage, current, heat, temperature, or a magnetic field for each pixel. The beam scanning device  820  is capable of performing scanning with a wide viewing angle through a spatial light modulator and a phase mask. Although  FIG. 13  illustrates an example in which the light source  810  is provided separately from the beam scanning device  820 , the light source  810  may be provided in the beam scanning device  820 . 
     The controller  830  may control operations of the beam scanning device  820 , the light source  810 , and the photodetector  840 . For example, the controller  830  may control on/off operations of the light source  810  and the photodetector  840  and a beam scanning operation of the beam scanning device  820 . In addition, the controller  830  may calculate information about an object, based on a measurement result of the photodetector  840 . 
     The electronic apparatus  800  may periodically emit light with respect to a plurality of surrounding regions by using the beam scanning device  820  to obtain information about objects at a plurality of locations in the vicinity. 
       FIG. 14  is a diagram schematically illustrating an electronic apparatus  900  including an optical device according to an embodiment. 
     As illustrated in  FIG. 14 , the electronic apparatus  900  may include a beam scanning device  910  and a photodetector  930  for detecting light steered by the beam scanning device  910  and reflected from an object. The electronic apparatus  900  may further include a circuit unit  920  connected to at least one of the beam scanning device  910  or the photodetector  930 . The circuit unit  920  may include an operation unit for obtaining and calculating data and further include a driver, a controller, and the like. The circuit unit  920  may further include a power supply and a memory. 
     Although  FIG. 14 , illustrates a case in which the electronic apparatus  900  includes the beam scanning device  910  and the photodetector  930 , the beam scanning device  910  and the photodetector  930  may be provided separately in different apparatuses. The circuit unit  920  may be connected to the beam scanning device  910  or the photodetector  930  not by wire but by wireless communication. In addition, the configuration of  FIG. 14  may be variously changed. 
     The beam scanning device according to the example embodiment described above is applicable to various types of systems. For example, the beam scanning device is applicable to a LiDAR device. The LiDAR device may be a phase-shifted device or a time-of-flight (TOF) device. The LiDAR device is applicable to self-driving vehicles, flying objects such as drones, mobile devices, small-sized walking devices, for example, bicycles, motorcycles, strollers, boards, etc., robots, human/animal assistance devices, for example, canes, helmets, accessories, clothing, watches, bags, etc., Internet-of-Things (IoT) devices/systems, security devices/systems, and the like. 
       FIG. 15  is a side view illustrating an example in which an electronic apparatus according to an example embodiment is applied to LiDAR for a vehicle.  FIG. 16  is a plan view illustrating an example in which a beam scanning device according to an example embodiment is applied to LiDAR for a vehicle. 
       FIGS. 15 and 16  are conceptual diagrams illustrating cases in which a LiDAR device, including a beam scanning device, according to an example embodiment is applied to a vehicle.  FIG. 15  is a side view and  FIG. 16  is a top view. 
     Referring to  FIG. 15 , a LiDAR device  1100  is applicable to a vehicle  1000  and information about a subject  1200  may be obtained using the LiDAR device  1100 . The vehicle  1000  may be a vehicle having a self-driving function. The LiDAR device  1100  may be used to detect the subject  1200  that is, for example, an object or a human, in a traveling direction of the vehicle  1000 . In addition, a distance to the subject  1200  may be measured using information such as a time difference between a transmitted signal and a detected signal. In addition, as illustrated in  FIG. 16 , information about the subject  1200  in a near distance within a scan range and a subject  1300  in a far distance may be obtained. 
     Beam scanning devices according to various example embodiments are applicable to various types of systems, as well as LiDAR devices. For example, 3D information about spaces and subjects may be obtained by scanning through beam scanning devices according to various example embodiments, and thus the beam scanning devices are applicable to 3D image obtaining apparatuses, 3D cameras, etc. The beam scanning devices are applicable to holographic display devices and structured light generating devices. The beam scanning devices are applicable to various types of optical devices such as a hologram generating devices, light coupling devices, variable focus lenses, depth sensors, and the like. The beam scanning devices are applicable to various fields using a metasurface or a metastructure. In addition, a beam scanning device and a system including the same according to example embodiments are applicable to various optical and electronic fields for various purposes. 
     A light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device according example embodiments as described above are only examples and it will be apparent to those of ordinary skill in the art that various modifications may be made and other equivalent embodiments may be derived. Therefore, the scope of the present disclosure should be defined by the appended claims. 
     In a light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device according to the example embodiments set forth herein, a reflective multilayer structure may have precisely adjusted reflectivity or transmittance. 
     In a light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device according to the example embodiments set forth herein, pixels may be reduced in size. 
     In a light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device according to the example embodiments set forth herein, a reflective multilayer structure may be reduced in thickness. 
     In a light modulator, an optical device including the light modulator, and an electronic apparatus including the optical device according to the example embodiments set forth herein, a distribution of reflection phases or transmission phases may be precisely controlled. 
     It should be understood that example embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
     While example embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.