Patent Publication Number: US-2023137953-A1

Title: Narrowband sensors based on plasmonic metasurfaces integrated on piezoelectric plates

Description:
BACKGROUND 
     A metasurface is an artificially made material, also referred to as a metamaterial, that includes structures of symmetrically arranged geometric patterns having sub-wavelength dimensions with respect to a portion of the electromagnetic spectrum. A plasmonic metasurface is a type of metasurface that exhibits negative real permittivity and, under conditions of electromagnetic excitation, can create surface charge-density oscillations known as surface plasmon-polaritons (SPPs). Plasmonic metasurfaces are formed by metals or metal-like materials, such as a combination of metallic and dielectric materials, and contain subwavelength-scaled structures that are distributed on or under the surface. The structures may have similar or different geometries and may be repeated and spaced across a layer to alter the behavior of electromagnetic waves, thereby generating the SPPs. For example, the structures may be separated circular, square or cross-like metal patches that are placed on a dielectric layer. A plasmonic metasurface can be designed to interact with an electromagnetic wave in a certain light spectrum, such as visible or infrared (IR) light, to absorb or reflect light at a certain wavelength or frequency. 
     SUMMARY 
     In accordance with at least one example of the description, an apparatus includes a first metal layer, a piezoelectric layer on the first metal layer, a second metal layer on the piezoelectric layer, a plasmonic metasurface on the second metal layer, the plasmonic metasurface including a dielectric layer and metal structures, and the apparatus further including a voltage detector coupled to the first metal layer and the second metal layer. 
     In accordance with another example of the description, an optical device includes a plasmonic metasurface configured to absorb an incident light having a frequency spectrum and modulated in time, a first metal layer coupled to the plasmonic metasurface, a piezoelectric layer coupled to the first metal layer, a second metal layer coupled to the plasmonic metasurface, and a voltage detector coupled to the first metal layer and second metal layer, the voltage detector configured to detect an amplitude of an electrical signal modulated in time according to the modulated incident light. 
     In accordance with another example of the description, an optical detector system includes a light source configured to emit light having a frequency spectrum and modulated in time, and an optical detector configured to detect an intensity of the light at a wavelength range within the frequency spectrum, the optical detector including a piezoelectric layer, a first metal layer coupled to a first surface of the piezoelectric layer, a second metal layer coupled to a second surface of the piezoelectric layer, a plasmonic metasurface coupled to the first metal layer and configured to absorb the light at the wavelength range, the plasmonic metasurface including metal structures and a dielectric layer disposed on the first metal layer, and the optical detector system further including a voltage detector coupled to the first metal layer and the second metal layer, the voltage detector configured to detect a voltage at a frequency of the modulated light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of a light detector system, in accordance with various examples. 
         FIG.  2    is a graph of light intensity of emitted light in the light detector system of  FIG.  1   , in accordance with various examples. 
         FIG.  3    is a diagram of an optical device, in accordance with various examples. 
         FIG.  4    is a graph of absorption of incident light on the optical device of  FIG.  3   , in accordance with various examples. 
         FIG.  5    is a graph of measured voltage responsive to incident light on the optical device of  FIG.  3   , in accordance with various examples. 
         FIG.  6 A  is a diagram showing a top view of an optical device including multiple sensor units, in accordance with various examples. 
         FIG.  6 B  is a diagram showing a cross section view of the optical device of  FIG.  6 A , in accordance with various examples. 
         FIG.  7 A  is a diagram showing a top view of an optical device including multiple sensor units, in accordance with various examples. 
         FIG.  7 B  is a diagram showing a cross section view of the optical device of  FIG.  7 A , in accordance with various examples. 
         FIG.  8    is a graph of absorption of incident light on an optical device including multiple sensor units, in accordance with various examples. 
         FIG.  9    is a diagram of a cross section of a sensor unit anchored at two opposite sides, in accordance with various examples. 
         FIG.  10    is a graph of voltage to temperature sensitivity of the sensor unit of  FIG.  9   , in accordance with various examples. 
         FIG.  11    is a diagram of a cross section of a sensor unit anchored at four sides, in accordance with various examples. 
         FIG.  12    is a graph of voltage to temperature sensitivity of the sensor unit of  FIG.  11   , in accordance with various examples. 
         FIG.  13    is a circuit diagram of an optical detector system including multiple sensor units, in accordance with various examples. 
         FIG.  14    is a graph of multiple light absorption profiles for an optical device including multiple sensor units, in accordance with various examples. 
         FIG.  15    is a flow diagram of a method for light detection in an optical detector system, in accordance with various examples. 
         FIG.  16    is a block diagram of a hardware architecture for processing signal data, in accordance with various examples. 
     
    
    
     DETAILED DESCRIPTION 
     Optical detectors, which may also be referred to as light detectors, are types of devices that detect light at a specific frequency or wavelength range. The detection includes absorbing a portion of light radiation that illuminates a surface of the detector and converting it into a signal, such as an electrical signal, which can be measured and analyzed. Light that illuminates or is projected onto a surface may also be referred to as incident light. Analysis of the measured electrical signal is useful to infer the characteristics of a sample exposed to the light radiation. The characteristics of the sample may include the type, composition, or density of substances in the sample. For example, optical detectors can be useful as gas or fluid detectors with one or more light sources having a frequency spectrum, such as infrared, visible light, or ultraviolet laser sources. Optical detectors may include various materials and layers designed for specific detection applications. 
     An optical detector may include a plasmonic metasurface that is engineered according to the application. This may involve the plasmonic metasurface achieving a peak in absorption at a wavelength of light and substantially low or no absorption away from that wavelength. Such response is referred to as a resonance response, and the wavelength at the peak absorption is referred to as the resonance wavelength. The resonance response may provide a filtering effect of the incident light where light may be absorbed within a relatively narrow wavelength range with respect to the frequency spectrum of the emitted light. Plasmonic metasurface design includes determining the spacing and size of structures dispersed across the plasmonic metasurface, for instance in the form of a two-dimensional (2D) array. To achieve detection, the plasmonic metasurface may be combined with other materials and layers that are stacked over one another and anchored with low thermal coupling to a base, such as a silicon (Si) substrate or other form of substrate. 
     Responsive to projecting incident light on the plasmonic metasurface, the incident light radiation may interact with the plasmonic metasurface causing energy oscillation at the surface. The energy oscillation may be referred to as a plasmon. The energy generated at the plasmonic metasurface which undergoes the plasmon effect can propagate into sublayers in the detector and is useful for detection. The metamaterial may be coupled to a piezoelectric layer and metal layers on the substrate. The piezoelectric layer is an active layer that expands or contracts, responsive to absorbing the light radiation from the plasmonic metasurface, and converts the thermally-induced deformation into an electric signal. 
     The description provides various examples of combining plasmonic metasurfaces with a piezoelectric layer in an optical detector. The combination may provide for light detection with multi-sensor capability which is useful for characterizing multiple samples. The various examples of the optical detector design may detect multiple gas concentrations in a medium with a broadband light source. The design can have a compact form factor, and therefore may have a reduced cost and energy consumption in comparison to other approaches. The optical detector may include multiple plasmonic metasurfaces disposed on a piezoelectric layer and metal layers. Each plasmonic metasurface on the piezoelectric layer may operate as a sensor unit in the optical detector and may be designed for absorbing a broadband incident light at a corresponding resonance wavelength. The optical detector may also include contacts that anchor each sensor unit to the surrounding components in the optical detector package. The anchor contacts may provide structural support with limited thermal coupling of the sensor units. 
     The broadband light from the light source, which may have an IR or visible frequency spectrum, may be modulated at a certain frequency within the spectrum. The modulated light may be absorbed by the optical detector, and cause the piezoelectric layer to contract and expand periodically in time due to light modulation. Light energy absorption in the piezoelectric layer may provide an electric signal which may be measured in the form of voltage. The amplitude of the measured voltage may be proportional to the amount of absorbed light at the resonance wavelength of each plasmonic metasurface of a sensor unit in the optical detector. 
     The sensor units can be coupled in parallel through respective transimpedance amplifier circuits, and a multiplexer may couple the amplifier circuits to a processor to enable analyzing the voltage at the piezoelectric layer. The analysis may be based on the relation between the voltage amplitude detected from the piezoelectric layer and the temperature reached by the piezoelectric layer caused by light absorption. Thus, the detected voltage level can be linked to the amplitude of the incident light that is absorbed by the optical detector, and a change of measured voltage can be correlated with a change of temperature and therefore light amplitude. The change in light amplitude at the optical detector can be attributed to a characteristic of the sample, such as volume or concentration of a gas, exposed to the light before absorption. Further, the voltage to temperature sensitivity can be increased by adding more anchor contacts to the sensor units, as described below. 
       FIG.  1    is a block diagram of a light detector system  100 , in accordance with various examples. The light detector system  100  may include a light source  110  and an optical detector  120  separated by a space  121 . The light source  110  may be any light emitting device that emits a light beam  134  directed toward the optical detector  120 . For example, the light source  110  may be a laser that emits the light beam  134  in the visible spectrum or the infrared (IR) spectrum. The optical detector  120  may be positioned in front of the light source  110  to detect at least a portion of the emitted light beam  134  that is incident on the surface of the optical detector  120 . The optical detector  120  may be designed to absorb a portion of the light beam  134  within a wavelength or frequency range which falls in the light spectrum of the light source  110 . The intensity or amplitude of the absorbed portion of the light beam  134  may be detected by the optical detector  120 . The components of the optical detector  120  may be encased in a package to protect the optical detector  120 . 
     A sample  135  to be analyzed may be disposed in the space  121  between the light source  110  and the optical detector  120  such that the sample  135  is exposed to the light beam  134 . The absorbed portion of the light beam  134  may be collected at the optical detector  120  and converted into an electric signal, such as a voltage, which may be measured and analyzed to infer characteristics of the sample  135 . For example, the sample  135  may be a fluid, a gas, or multiple gases. The characteristics of the sample  135  may include the chemical composition, density, concentration, or molecular size of the sample  135 . The light detector system  100  may also include a chamber  140  for holding or containing the sample  135 . The chamber  140  may include openings in front of the light source  110  and the optical detector  120  to allow the passing of the light beam  134  from the light source  110  to the optical detector  120  through the chamber  140 . In an example, the chamber  140  may include a first opening  142  and a second opening  143 , separated by a distance (d). The first opening  142  may be an inlet for injecting the sample  135  into the chamber  140 , and the second opening  143  may be an outlet for releasing the sample  135  from the chamber  140 . The chamber  140  may include a lens  145  for aligning and projecting the light beam  134  toward the optical detector  120 . 
     The light detector system  100  may also include a processing system  150  electrically coupled to the optical detector  120 . The processing system  150  may receive electrical signals from the optical detector  120 , responsive to the optical detector  120  absorbing a portion of the light beam  134  from the light source  110 . The electrical signals may be measured, such as via a voltage detector, and analyzed to determine the characteristics of the sample  135 . The processing system  150  may include a processor (not shown) for processing the electrical signals based on stored data or models for characterizing the sample  135 . For example, the processing system  150  may be a computer system including a processing chip (not shown) and a storage medium (not shown). 
       FIG.  2    is a graph  200  of light intensity of emitted light in the light detector system  100 , in accordance with various examples. In the graph  200 , the x-axis represents a range of wavelengths of the emitted light in micrometers (μm) and the y-axis represents a relative scale of the power values of the emitted light beam  134  by the light source  110 . The wavelength range of the x-axis is between approximately 1 and 7 μm, which may be part of the IR spectrum. The light intensity is represented by a curve  201  and corresponds to the radiation power of the light beam  134  emitted by the light source  110  in the IR spectrum. The wavelength range may overlap at least partially with a wavelength region  202  centered at approximately the resonance wavelength (at approximately 4.5 μm) of a plasmonic metasurface in the optical detector  120 . Accordingly, the optical detector  120  may absorb a portion of the light beam  134  that is useful for detection of the light in the wavelength region  202 . 
       FIG.  3    is a diagram of an optical device  300 , in accordance with various examples. The optical device  300  may be a light sensing device capable of absorbing incident light  301  on a surface of the optical device  300 . Absorbing a portion of the incident light  301  may be useful for detecting the intensity or amplitude of the incident light  301  in the wavelength region  202 . For example, the optical device  300  may be part of the optical detector  120  and may provide an electrical signal responsive to absorbing a portion of the incident light  301 . The incident light  301  may be emitted from a light source  302  having a frequency spectrum in the IR region, such as the light source  110  which emits the light beam  134 . The amplitude of the incident light  301  in the IR spectrum may also be modulated in time by varying the power of the light source  302  between different power levels according to a time wave pattern or by switching the light source  302  on and off periodically in time. 
     The optical device  300  may include multiple layers and materials designed to increase the absorbed amount or portion of the incident light  301  within a certain wavelength range. Increasing the amount of absorbed light may increase the signal-to-noise ratio in a light detection system and therefore allow for more accurate detection results. The optical device  300  may include a patched metal layer as a plasmonic metasurface  310 , a piezoelectric layer  330 , a first metal layer  332  between the plasmonic metasurface  310  and the piezoelectric layer  330 , and a second metal layer  332  under the piezoelectric layer  330 . 
     The plasmonic metasurface  310  may include a one-dimensional (1D) or 2D array of structures  342  disposed on or in a dielectric layer  344 . For example, the structures  342  may correspond to a grid of metal patches equally spaced on the dielectric layer  344 . In another example, the plasmonic metasurface  310  may include a grid of equally spaced gaps in a metal layer (not shown). The gaps may be empty gaps or may be filled with a dielectric or other material or a combination thereof. The structures  342  may have various geometric patterns, sizes, and spacing. For example, the geometric patterns may include square, round, slit, or cross patterns. The spacing of the structures  342  may determine the resonance response of the plasmonic metasurface  310  responsive to absorbing a portion of the incident light  301  at a wavelength range narrower than the IR spectrum. The dielectric layer  344  may include a dielectric material transparent to the incident light  301  in at least a portion of the IR spectrum. For example, the dielectric layer  344  may be a silicon oxide (SiO 2 ) layer or any other suitable dielectric material. 
     The piezoelectric layer  330  may be formed from a piezoelectric material. An example of a piezoelectric material is a crystal material capable of converting mechanical energy into electrical energy. For example, the piezoelectric layer  330  may be an aluminum nitride (AlN) layer and the first and second metal layers  332  may be molybdenum (Mo) layers. The piezoelectric layer  330  may deform in the contour or thickness directions responsive to absorbing the light energy from the plasmonic metasurface  310 , which may provide heat energy in the piezoelectric layer  330 . The heat energy may be converted by the piezoelectric layer  330  into electric energy. The electric energy may be collected by coupling electrodes  307  and  309  to the first and second metal layers  332 , respectively. The transferred electric energy may be measured as an electrical signal, such as a voltage  311 . For example, the electrodes  307  and  309  may be coupled to the first metal layer  332  between the plasmonic metasurface  310  and the piezoelectric layer  330 , and to the second metal  332  under the piezoelectric layer  330 . The optical device  300  may also include contacts (not shown) that anchor the plasmonic metasurface  310 , the metal layer  320 , and the piezoelectric layer  330  to the surrounding substrate in a component package that includes or encases the optical device  300 . The anchor contacts may provide structural support for the layers of the optical device and may work as signal routers to connect the first and second metal layers  332  to the coupling electrodes  307  and  309 . 
       FIG.  4    is a graph  400  of absorption of incident light  301  on the optical device  300 , in accordance with various examples. In the graph  400 , the x-axis represents a frequency range of the incident light  301  and the y-axis represents the absorption values of incident light  301 . The absorption values reflect the amplitude of the absorbed light at the plasmonic metasurface  310  as a percentage. The frequency range of the x-axis is represented in terahertz (THz), which corresponds to the IR spectrum. The light absorption is represented by a curve  401  that shows percentages of the absorbed light at the plasmonic metasurface  310  within the frequency range. The frequency range of light absorption may include a resonance frequency (ω 0 ) of the plasmonic metasurface  310 . The absorbed light in this frequency range may be converted via the piezoelectric layer  330  into electric energy which can be measured as a voltage  311 . 
       FIG.  5    is a graph  500  of measured voltage responsive to incident light  301  on the optical device  300 , in accordance with various examples. In some examples, the voltage  311  is measured at the piezoelectric layer  330 . In the graph  500 , the x-axis represents time in units of milliseconds (ms) and the y-axis represents a relative scale of the amplitude values of the measured voltage in units of volts (V). The measured voltage is represented by a modulated voltage signal  501  that includes the voltage values over a time range. The modulated voltage signal  501  may be measured responsive to absorbing, at the plasmonic metasurface  310 , the incident light  301 , which may be modulated in time. Accordingly, the modulated voltage signal  501  may vary periodically in time between two values having a voltage difference (ΔV) proportional to the amplitude value of the absorbed light at wo, as described above with respect to  FIG.  4   . 
       FIGS.  6 A and  6 B  show a top view and a cross section view, respectively, of an optical device  600  including multiple sensor units, in accordance with various examples. The optical device  600  may include a reference sensor unit  610 , a first sensor unit  620 , a second sensor unit  630 , and a first surrounding surface  642  on a second surrounding surface  644 , which is disposed on a substrate  650 . The reference sensor unit  610 , the first sensor unit  620  and the second sensor unit  630  may be separated by a gap  660  from the substrate  650 . The gap  660  may provide thermal isolation between the reference sensor unit  610 , the first sensor unit  620 , the second sensor unit  630 , and the substrate  650 . 
     Each one of the reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630  may correspond to a separate surface of the optical device  600 . The reference sensor unit  610 , first sensor unit  620 , and second sensor unit  630  enable the optical device  600  to absorb incident light  301  on the surface of the optical device  300  at multiple frequency or wavelength ranges, each corresponding to one of the reference sensor unit  610 , first sensor unit  620 , and second sensor unit  630 . For example, the light beam  134  may be emitted from the light source  110  having a frequency spectrum in the IR region and may be modulated in time by varying the power of the light source  110 . The optical device  600  may be part of the optical detector  120  and may provide an electrical signal responsive to the optical device  600  absorbing the portion of the light beam  134  at the multiple wavelength ranges. 
     The reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630  may have a rectangle or square surface geometry. In other examples, the reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630  may have other surface geometries. The reference sensor unit  610  may include a reference sensor unit surface  662 , a reference sensor unit piezoelectric layer  663 , a first reference sensor unit metal layer  664  between the reference sensor unit surface  662  and the reference sensor unit piezoelectric layer  663 , and a second reference sensor unit metal layer  665  on the gap  660  facing surface of the reference sensor unit piezoelectric layer  663 . The reference sensor unit surface  662  may include a uniform metal surface  666  on a reference sensor unit dielectric layer  667 . The first sensor unit  620  may include a first plasmonic metasurface  672 , a first sensor unit piezoelectric layer  673 , a first sensor unit first metal layer  674  between the first plasmonic metasurface  672  and the first sensor unit piezoelectric layer  673 , and a first sensor unit second metal layer  675  on the gap  660  facing surface of the first sensor unit piezoelectric layer  673 . The first plasmonic metasurface  672  may include a first 2D array of first structures  676  on a first sensor unit dielectric layer  677 . The second sensor unit  630  may include a second plasmonic metasurface  682 , a second sensor unit piezoelectric layer  683 , a second sensor unit first metal layer  684  between the second plasmonic metasurface  682  and the second sensor unit piezoelectric layer  683 , and a second sensor unit second metal layer  685  on the gap  660  facing surface of the second sensor unit piezoelectric layer  683 . The second plasmonic metasurface  682  may include a second 2D array of second structures  686  on a second sensor unit dielectric layer  687 . The first structures  676  and second structures  686  may have patterns of various geometries and sizes. For example, the first structures  676  and second structures  686  may be square patches. The spacing between the patches of the first structures  676  and second structures  686  may be different, causing a different resonance response and resonance frequency for absorbing the incident light  301  in the IR spectrum. 
     The uniform metal surface  666  of the reference sensor unit  610  may not have a resonance response for absorbing the incident light  301  around a resonance frequency, and may have lower absorption than the first plasmonic metasurface  672  of the first sensor unit  620  and the second plasmonic metasurface  682  of the second sensor unit  630 . Therefore, the reference sensor unit  610  may be useful to detect a base absorption level of incident light  301  as a reference level to the absorption levels of the first sensor unit  620  and the second sensor unit  630 . 
     The first surrounding surface  642 , the reference sensor unit dielectric layer  667 , the first sensor unit dielectric layer  677 , and the second sensor unit dielectric layer  687  may include the same dielectric layer. The second surrounding surface  644 , the reference sensor unit piezoelectric layer  663 , the first sensor unit piezoelectric layer  673 , and the second sensor unit piezoelectric layer  683  may include the same piezoelectric layer. The reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630  may be separated from the first surrounding surface  642 , and second surrounding surface  644  on the substrate  650  by gaps  660  etched in the layers of the optical device  600 . This separation may provide thermal isolation between the reference sensor unit  610 , the first sensor unit  620 , the second sensor unit  630 , and the surrounding components and layers on the substrate  650 . 
     The reference sensor unit  610  may be anchored to the first surrounding surface  642  and the second surrounding surface  644  through two contacts  691  on opposite sides of the reference sensor unit  610 . The contacts  691  may be composed of the reference sensor unit dielectric layer  667  and the reference sensor unit piezoelectric layer  663 , which extend from the reference sensor unit  610  to the first surrounding surface  642  and the second surrounding surface  644 . Similarly, the first sensor unit  620  may be anchored to the first surrounding surface  642  and the second surrounding surface  644  through two contacts  692  on opposite sides of the first sensor unit  620 . The contacts  692  may be composed of the first sensor unit dielectric layer  677  and the first sensor unit piezoelectric layer  673 , which extend from the first sensor unit  620  to the first surrounding surface  642  and the second surrounding surface  644 . The second sensor unit  630  may be anchored to the first surrounding surface  642  and the second surrounding surface  644  through two contacts  693  on opposite sides of the second sensor unit  630 . The contacts  693  may be composed of the second sensor unit dielectric layer  687  and the second sensor unit piezoelectric layer  683 , which extend from the second sensor unit  630  to the first surrounding surface  642  and the second surrounding surface  644 . 
       FIGS.  7 A and  7 B  show a top view and a cross section view, respectively, of an optical device  700  that includes multiple sensor units, in accordance with various examples. The sensor units may correspond to separate surfaces which enable the optical device  700  to absorb incident light on the surface of the optical device  700  at multiple frequency or wavelength ranges. For example, the optical device  700  may be part of the optical detector  120  and may provide an electrical signal responsive to the optical device  700  absorbing the incident light  301  at the multiple wavelength ranges. The incident light  301  may be a portion of the light beam  134  emitted from the light source  110  having a frequency spectrum in the IR region and may be modulated in time by varying the power of the light source  110 . 
     The optical device  700  may include multiple layers and materials similar to the optical device  600 . The optical device  700  may include a reference sensor unit  710 , a first sensor unit  720 , a second sensor unit  730 , and a first surrounding surface  742  on a second surrounding surface  743 , which is disposed on a substrate  744 . As shown in  FIGS.  7 A and  7 B , the reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730  are separated by a first surrounding surface  742  and a second surrounding surface  743 . The reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730  may also be separated by respective gaps  745 ,  746 , and  747  from the substrate  744 . The gaps  745 ,  746 , and  747  may provide thermal isolation between the reference sensor unit  710 , the first sensor unit  720 , the second sensor unit  730 , and the substrate  744 . 
     The reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730  may have a rectangle or square surface geometry. In other examples, the sensor units may have other surface geometries. The reference sensor unit  710  may include a reference sensor unit surface  762 , a reference sensor unit piezoelectric layer  763 , a first reference sensor unit metal layer  764  between the reference sensor unit surface  762  and the reference sensor unit piezoelectric layer  763 , and a second reference sensor unit metal layer  765  on the gap  745  facing surface of the reference sensor unit piezoelectric layer  763 . The reference sensor unit surface  762  may include a uniform metal surface  766  on a reference sensor unit dielectric layer  767 . The first sensor unit  720  may include a first plasmonic metasurface  772 , a first sensor unit piezoelectric layer  773 , a first sensor unit first metal layer  774  between the first plasmonic metasurface  772  and the first sensor unit piezoelectric layer  773 , and a first sensor unit second metal layer  775  on the gap  746  facing surface of the first sensor unit piezoelectric layer  773 . The first plasmonic metasurface  772  may include a first 2D array of first structures  776  on a first sensor unit dielectric layer  777 . The second sensor unit  730  may include a second plasmonic metasurface  782 , a second sensor unit piezoelectric layer  783 , a second sensor unit first metal layer  784  between the second plasmonic metasurface  782  and the second sensor unit piezoelectric layer  783 , and a second sensor unit second metal layer  785  on the gap  747  facing surface of the second sensor unit piezoelectric layer  783 . The second plasmonic metasurface  782  may include a second 2D array of second structures  786  on a second sensor unit dielectric layer  787 . The first structures  776  and second structures  786  may have patterns of various geometries and sizes. For example, the first structures  776  and second structures  786  may be square patches. The spacing between the patches in first structures  776  and second structures  786  may be different, causing a different resonance response and resonance frequency for absorbing the incident light  301  in the IR spectrum. 
     The uniform metal surface  766  of the reference sensor unit  710  may not have a resonance response for absorbing the incident light  301  around a resonance frequency, and may have lower absorption than the first plasmonic metasurface  772  of the first sensor unit  720  and the second plasmonic metasurface  782  of the second sensor unit  730 . Therefore, the reference sensor unit  710  may be useful to detect a base absorption level of incident light  301  as a reference level to the absorption levels of the first sensor unit  720  and the second sensor unit  730 . 
     The first surrounding surface  742 , the reference sensor unit dielectric layer  767 , the first sensor unit dielectric layer  777 , and the second sensor unit dielectric layer  787  may be composed of the same dielectric layer. The second surrounding surface  743 , the reference sensor unit piezoelectric layer  763 , the first sensor unit piezoelectric layer  773 , and the second sensor unit piezoelectric layer  783  may be a same (e.g., contiguous) piezoelectric layer. As shown in  FIG.  7 A , the sides of the reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730  may be separated from the first surrounding surface  742  and second surrounding surface  743  by gaps  745 ,  746 , and  747  etched in the layers of the optical device  700 , which may provide thermal isolation between the reference sensor unit  710 , the first sensor unit  720 , the second sensor unit  730 , and the surrounding components and layers on the substrate  744 . 
     Each of the reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730  in the optical device  700  may be anchored by four contacts on four sides of the sensor units. The reference sensor unit  710  may be anchored to the first surrounding surface  742  and the second surrounding surface  743  through four contacts  791  on four sides of the reference sensor unit  710 . The contacts  791  may include the reference sensor unit dielectric layer  767  and the reference sensor unit piezoelectric layer  763 , which extend from the reference sensor unit  710  to the first surrounding surface  742  and the second surrounding surface  743 . Similarly, the first sensor unit  720  may be anchored to the first surrounding surface  742  and the second surrounding surface  743  through four contacts  792  on four sides of the first sensor unit  720 . The contacts  792  may include the first sensor unit dielectric layer  777  and the first sensor unit piezoelectric layer  773 , which extend from the first sensor unit  720  to the first surrounding surface  742  and the second surrounding surface  743 . The second sensor unit  730  may also be anchored to the first surrounding surface  742  and the second surrounding surface  743  through four contacts  793  on four sides of the second sensor unit  730 . The contacts  793  may include the second sensor unit dielectric layer  787  and the second sensor unit piezoelectric layer  783 , which extend from the second sensor unit  730  to the first surrounding surface  742  and the second surrounding surface  743 . 
       FIG.  8    is a graph  800  of absorption of incident light on an optical device including multiple sensor units, such as the optical device  600  or the optical device  700 , in accordance with various examples. In  FIG.  8   , the x-axis represents a frequency range of the incident light and the y-axis represents the absorption values of incident light. The absorption values reflect the amplitude of the absorbed light at the sensor units of the optical device as a percentage. The frequency range of the x-axis is represented in THz, which corresponds to the IR spectrum. The sensor units of the optical device may include a reference sensor unit, a first plasmonic metasurface and a second plasmonic metasurface. For example, the sensor units may correspond to the reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630 , or may correspond to the reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730 . The light absorption is represented by the curves  801 ,  802 , and  803  that show percentages of the absorbed light at the surfaces of the reference sensor unit, the first plasmonic metasurface, and the second plasmonic metasurface, respectively. 
     The curve  801  shows an absence of a peak in absorption in the frequency range, which may be caused by the absence of a resonance response for light absorption at the reference sensor unit of the optical device. The curve  802  shows a peak in absorption in the frequency range at a first resonance frequency (ω 1 ) of the first plasmonic metasurface of the optical device. The curve  803  shows a second peak in absorption in the frequency range at a second resonance frequency (ω 2 ) of the second plasmonic metasurface of the optical device. The first and second resonance frequencies may be useful for detecting two samples with a broadband IR light source. The peaks in absorptions in the curves  802  and  803  of the first and second plasmonic metasurfaces may be at approximately equal amplitude percentages (as shown in  FIG.  8   ) or may be at different amplitude levels above the reference level in curve  801 . The absorption percentages in the curve  801  may be useful as a reference level for light absorption compared to the absorption percentages in the curves  802  and  803 . Because the measured voltages responsive to light absorption at the reference sensor unit  610  or  710 , the first sensor unit  620  or  720 , and the second sensor unit  630  or  730  in the optical device  600  or  700  may be proportional to light absorption at the sensor units, the measured voltage for the reference sensor unit  610  or  710  may also be used as a base voltage level to the measured voltages for the first sensor unit  620  or  720  and second sensor unit  630  or  730 . Accordingly, a relative voltage value may be calculated for each of the first sensor unit  620  or  720  and the second sensor unit  630  or  730  with respect to the measured base voltage for the reference sensor unit  610  or  710 . The relative voltage values may be useful to quantify the respective amount of light absorption at the first plasmonic metasurface  672  or  772  and the second plasmonic metasurface  682  or  782 . 
       FIG.  9    is a diagram of a cross section  900  of a sensor unit  910  anchored at two opposite sides, in accordance with various examples. In  FIG.  9   , the x-axis represents a length or width of the sensor unit  910  in μm, and the y-axis represents the depth or thickness in μm. The sensor unit  910  may have a rectangle or square or other surface geometry with four sides and may be anchored by two contacts  920  coupled to two opposite sides of the four sides. The cross section  900  shows one of the contacts  920  on one side of the sensor unit  910 . For example, the sensor unit  910  may be any of the reference sensor unit  610 , first sensor unit  620 , or second sensor unit  630  in the optical device  600  that is anchored through two corresponding contacts  691 ,  692 , or  693  to the surrounding surfaces  642 ,  644  on the substrate  650 .  FIG.  9    shows a bending in the sensor unit  910  responsive to a deformation across the piezoelectric and dielectric layers that form the sensor unit  910 . The contracting and expanding movement of the piezoelectric layer may be responsive to absorbing a time modulated incident light on the sensor unit  910 . 
       FIG.  10    is a graph  1000  of voltage to temperature sensitivity of the sensor unit  910  of  FIG.  9   , in accordance with various examples. In the graph  1000 , the x-axis represents a measured voltage in units of V and the y-axis represents the detectable temperature change (ΔT) in kelvin (K). The voltage-to-temperature sensitivity of the sensor unit  910  is represented by a curve  1010  that relates V to ΔT for the piezoelectric layer of the sensor unit  910 . ΔT may be proportional to the absorbed light energy in the sensor unit  910 . The slope of the curve  1010  may be useful as a metric for quantifying the voltage-to-temperature sensitivity of the piezoelectric layer in the sensor unit  910 . 
       FIG.  11    is a diagram of a cross section  1100  of a sensor unit  1110  anchored at four sides, in accordance with various examples. In  FIG.  11   , the x-axis represents a length or width of the sensor unit  1110  in μm and the y-axis represents the depth or thickness in μm. The sensor unit  1110  may have a rectangle or square or other surface geometry with four sides and may be anchored by four contacts  1120  coupled to the four sides. The cross section  1100  shows two of the four contacts  1120  on two opposite sides of the sensor unit  1110 . In various examples, the sensor unit  1110  may be any of the reference sensor unit  710 , first sensor unit  720 , or second sensor unit  730  of the optical device  700  that is anchored through four corresponding contacts  791 ,  792 , or  793  to the surrounding surfaces  742 ,  743  on the substrate  744 .  FIG.  11    shows thickness expansion in the sensor unit  1110  responsive to absorbing the light energy. In comparison to the sensor unit  910  that is anchored with two contacts  920  on two opposite sides, the sensor unit  1110  may exhibit reduced bending responsive to absorbing the light energy, and thus provide more stability, as a result of its anchoring with the four contacts  1120  on the four sides of the sensor unit  1110 . 
       FIG.  12    is a graph  1200  of voltage to temperature sensitivity of the sensor unit  1110  of  FIG.  11   , in accordance with various examples. In the graph  1200 , the x-axis represents a measured voltage in units of V, and the y-axis represents ΔT in K. The voltage-to-temperature sensitivity of the sensor unit  1110  is represented by a curve  1210  that relates V to ΔT for the sensor unit  1110 . ΔT may be proportional to the absorbed light energy in the sensor unit  1110 . The slope of the curve  1210  may be useful as a metric for quantifying the voltage-to-temperature sensitivity of the sensor unit  1110 . In comparison to the curve  1010  for the sensor unit  910 , the curve  1210  for the sensor unit  1110  shows increased voltage-to-temperature sensitivity responsive to the increase in number of contacts  1120  for anchoring and resultant thickness expansion of the sensor unit  1110 . The increased number of contacts  1120  for anchoring the sensor unit  1110  may increase thermal coupling and reduce thermal isolation with the surrounding layers. 
       FIG.  13    is a circuit diagram of an optical detector system  1300  including multiple sensor units  1310 , in accordance with various examples. The multiple sensor units  1310  may be combined in an optical device  1311  of the optical detector system  1300 . The optical device  1311  may be enabled to absorb and detect an incident light beam, such as for characterizing one or more samples, in accordance with the examples described above. For example, the optical detector system  1300  may be part of or correspond to the optical detector  120  in the light detector system  100 . The optical detector system  1300  may include a multiplexer  1312  (MUX) coupled to the multiple sensor units  1310  through respective transimpedance amplifiers  1320 , a controller  1322  coupled to the multiplexer  1312  through an analog-to-digital converter (ADC)  1340  and an antenna  1360  coupled to the controller  1322 . 
     Each sensor unit  1310  may absorb incident light on the surface of the optical device  1311  in the optical detector system  1300  within a frequency or wavelength range, as described in the examples above. For example, the sensor unit  1310  may be one of the reference sensor unit  610 , first sensor unit  620 , or second sensor unit  630  in the optical device  600 , or one of the reference sensor unit  710 , first sensor unit  720 , or second sensor unit  730  in the optical device  700 . Each sensor unit  1310  may include a plasmonic metasurface, a piezoelectric layer, and one or more metal layers coupled to the piezoelectric layer. 
     The transimpedance amplifier  1320  may provide high-to-low impedance transformation to maintain or amplify the voltage level collected from the respective sensor unit  1310  to a voltage level detectable by other components of the optical detector system  1300 . The combination of the transimpedance amplifier  1320  and the respective sensor unit  1310  may model an amplifier circuit  1370  with multiple capacitance sources. For example, the modeled amplifier circuit  1370  may include a current source  1371  representing the current flow from the sensor unit  1310 , a first capacitor  1372  representing sensor unit intrinsic capacitance, a second capacitor  1373  representing parasitic capacitance, and a third capacitor  1374  representing the input capacitance of the transimpedance amplifier  1310 . To maintain or increase the measured voltage level from the transimpedance amplifier  1320 , the capacitances of the second capacitor  1373  and third capacitor  1374  may be substantially smaller than the capacitance of the first capacitor  1372  in the modeled amplifier circuit  1370 . 
     The multiplexer  1312  may collect the amplified voltage levels from the respective sensor units  1310  via the respective transimpedance amplifiers  1320 , multiplex the voltage signals in a time sequence and send the signals as an analog electric signal to the ADC  1340 . The ADC  1340  may convert the analog electric signal from the multiplexer  1312  into a digital signal that can be processed by the controller  1322 . The controller  1322  may receive and process the signal to detect the amplitude of absorbed light in the sensor units  1310  and determine the characteristics of the one or more samples. The controller  1322  may determine which sensor unit  1310  will be read at each time by selecting the corresponding input in the multiplexer  1312  and may provide time modulation of the light source  110 . The antenna  1360  may establish wireless connections to enable the controller  1322  to send or receive signals. For example, the optical detector system  1300  may be a stand-alone or mountable device that detects light, measures a resulting voltage, characterizes a sample accordingly, and sends the data in a wireless connection to a central device or system (not shown). In other examples, the optical detector system  1300  may detect light, measure a resulting voltage, and send this information to another device or system (not shown) for analysis. 
       FIG.  14    is a graph  1400  of multiple light absorption profiles for an optical device including multiple sensor units, in accordance with various examples. In the graph  1400 , the x-axis represents a range of wavelengths of the incident light in nanometers (nm) and the y-axis represents the absorption values of incident light. The absorption values reflect the amplitude of the absorbed light at the multiple sensor units of the optical device as a percentage. The wavelength range is between approximately 7000 and 12000 nm, which corresponds to the IR spectrum for a broadband light source. The graph  1400  shows the light absorption profiles represented by curves  1401 ,  1402 , and  1403  for three respective sensor units  1410 ,  1420 , and  1430 , which may be combined in an optical detector. For example, the sensor units  1410 ,  1420 , and  1430  may be combined in the optical detector  120  of the light detector system  100  or in the optical detector system  1300 . The sensor units  1410 ,  1420 , and  1430  may be configured similar to the reference sensor unit  610 , the first sensor unit  620 , and the second sensor unit  630 , respectively, in the optical device  600  or the reference sensor unit  710 , the first sensor unit  720 , and the second sensor unit  730 , respectively, in the optical device  700 . Each sensor unit  1410 ,  1420 , and  1430  includes a plasmonic metasurface designed to absorb light at a certain resonance frequency or wavelength. 
     The curves  1401 ,  1402 , and  1403  represent the resonance responses for the sensor units  1410 ,  1420 , and  1430 , respectively. The curves  1401 ,  1402 , and  1403  show percentages of the absorbed light at the plasmonic metasurfaces of the sensor units  1410 ,  1420 , and  1430  in the IR wavelength range of the broadband light source. The resonance responses of the sensor units  1410 ,  1420 , and  1430  include three resonance wavelengths at approximately 9300 nm, 9900 nm, and 10800 nm, respectively. By integrating the three sensor units  1410 ,  1420 , and  1430  in the optical device, the resonance response, and therefore the detection capability of the optical detection system, can be increased to multiple types of samples over a broadband spectrum. 
       FIG.  15    is a flow diagram of a method  1500  for light detection in an optical device, in accordance with various examples. The optical device may include a plasmonic metasurface, a piezoelectric layer, and one or more metal layers coupled to the piezoelectric layer. For example, the method  1500  may be performed by a light detector system such as the light detector system  100  or the optical detector system  1300 . The light detector system may include an optical device such as the optical devices  300 ,  600 , or  700 , or another optical device designed in accordance with the optical devices described above. At step  1501 , a light source having a frequency spectrum may be modulated in time. For example, the frequency spectrum of the emitted light may be an IR spectrum or a visible light spectrum. At step  1502 , one or more plasmonic metasurfaces of a light detector system may absorb, at one or more respective resonance wavelengths of the plasmonic metasurfaces, the light emitted by the light source that strikes the plasmonic metasurfaces as incident light. The one or more plasmonic metasurfaces may correspond to one or more sensor units in the optical device and may each absorb a portion of the incident light within a corresponding resonance wavelength range. At step  1503 , an amplitude of a voltage across a piezoelectric layer of the light detector may be measured. The measured voltage may be modulated in time in accordance with the modulated incident light on the optical device. The piezoelectric layer may be coupled to the one or more plasmonic metasurfaces and may convert the energy of the absorbed light into a measurable voltage. The voltage may be measured in one or more sensor units with a time multiplexing scheme. At step  1504 , an amplitude of the voltage, and accordingly a corresponding change in the intensity of the incident light, may be determined. The light intensity may be determined based on the amplitude and analyzed by a microprocessor or a processing system to infer characteristics of one or more samples exposed to the light beam after emission by the light source and before absorption of light by the optical device. 
       FIG.  16    is a block diagram of a hardware architecture  1600  of a processing system, in accordance with various examples. The hardware architecture  1600  includes hardware components that may be part of the processing system. For example, the hardware architecture  1600  may correspond to the processing system  150  in the light detector system  100 . As shown in  FIG.  16   , the hardware architecture  1600  may include one or more processors  1601  and one or more memories  1602 . In some examples, the hardware architecture  1600  may also include one or more transceivers  1603  and one or more antennas  1604  for establishing wireless connections. These components may be connected through a bus  1605  or in any other suitable manner. In  FIG.  16   , an example in which the components are connected through a bus  1605  is shown. 
     The processor  1601  may be configured to read and execute computer-readable instructions. For example, the processor  1601  may be configured to invoke and execute instructions stored in the memory  1602 , including the instructions  1606 . The processor  1601  may support one or more global systems for wireless communication. Responsive to the processor  1601  sending a message or data, the processor  1601  drives or controls the transceiver  1603  to perform the sending. The processor  1601  also drives or controls the transceiver  1603  to perform receiving, responsive to the processor  1601  receiving a message or data. Therefore, the processor  1601  may be considered as a control center for performing sending or receiving, and the transceiver  1603  is an executor for performing the sending and receiving operations. 
     In an example, the memory  1602  may be coupled to the processor  1601  through the bus  1605  or an input/output port. In another example, the memory  1602  may be integrated with the processor  1601 . The memory  1602  is configured to store various software programs and/or multiple groups of instructions, including instructions  1606 . For example, the memory  1602  may include a high-speed random-access memory and may include a nonvolatile memory such as one or more disk storage devices, a flash memory, or another nonvolatile solid-state storage device. The memory  1602  may store an operating system such as ANDROID, IOS, WINDOWS, or LINUX. The memory  1602  may further store a network communications program. The network communications program is useful for communication with one or more attached devices, one or more user equipment, or one or more network devices, for example. The memory  1602  may further store a user interface program. The user interface program may display content of an application through a graphical interface, and receive a control operation performed by a user on the application via an input control such as a menu, a dialog box, or a physical input device (not shown). The memory  1602  may be configured to store the instructions  1606  for implementing the various methods and processes provided in accordance with the various examples of this application. 
     The antenna  1604  may be configured to convert electromagnetic energy into an electromagnetic wave in free space or convert an electromagnetic wave in free space into electromagnetic energy in a transmission line. The transceiver  1603  may be configured to transmit a signal that is provided by the processor  1601  or may be configured to receive a wireless communications signal received by the antenna  1604 . In this example, the transceiver  1603  may be considered a wireless transceiver. 
     The hardware architecture  1600  may also include another communications component such as a Global Positioning System (GPS) module, a BLUETOOTH module, or a WI-FI module. The hardware architecture  1600  may also support another wireless communications signal such as a satellite signal or a short-wave signal. The hardware architecture  1600  may also be provided with a wired network interface or a local area network (LAN) interface to support wired communication. 
     In accordance with various examples, the hardware architecture  1600  may further include an input/output device (not shown), such as an audio input/output device, a key input device, a display, and the like. The input/output device may be configured to implement interaction between the hardware architecture  1600  and a user/an external environment, and may include the audio input/output device, the key input device, the display, and the like. The input/output device may further include a camera, a touchscreen, a sensor, and the like. The input/output device may communicate with the processor  1601  through a user interface. 
     The hardware architecture  1600  shown in  FIG.  16    is a possible implementation in various examples of this application. During actual application, the hardware architecture  1600  may include more or fewer components. This is not limited herein. 
     The term “couple” is used throughout the specification. The term may cover connections, communications or signal paths that enable a functional relationship consistent with this description. For example, if device A provides a signal to control device B to perform an action, in a first example device A is coupled to device B or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal provided by device A. 
     A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. 
     A device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described device. For example, a structure described as including one or more elements (such as structures or layers) and/or one or more sources (such as voltage and/or current sources) may instead include only the elements within a single physical device (e.g., the structures and layers in the device) and may be adapted to be coupled to at least some of the sources to form the described structure or system either at a time of manufacture or after a time of manufacture, such as by an end-user and/or a third-party. 
     While certain components may be described herein as being of a particular process technology, these components may be exchanged for components of other process technologies. Structures and designs described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement. 
     Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means+/−10 percent of the stated value. Modifications are possible in the described examples, and other examples are possible within the scope of the claims.