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Abstract
Detection of aldehyde carbonyl radiation plays an essential role in guaranteeing the safety of fried food. However, the radiation of low-content aldehyde carbonyl is always weak and includes polarized light. Focusing the weak radiation with polarization-sensitive configurations provides an efficient way to improve the signal-to-noise ratio of detection. The advent of dynamic metasurfaces based on phase-change materials (PCMs) have demonstrated superiorities over their traditional counterparts in tunability and miniaturization. In this paper, we propose two reflected varifocal metasurfaces, which combine Ge2Sb2Se4Te1 (GSST) with two materials that have close optical constants with amorphous and crystalline GSST. The first one realizes a four-spot focal system with linearly-polarized incidence based on polarization multiplexing. It adds a new polarization degree of freedom compared with traditional varifocal metasurfaces. Compared with traditional spatial-multiplexing method, our second metasurface enables the independent control of the polarization and phase profiles of circularly-polarized light. Remarkably, it reduces energy loss and crosstalk. We believe the novel scenarios of combing GSST with similar materials provide a new direction for tunable metasurfaces based on PCMs.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Carbonyl value is a sensitive indicator of the thermal fission of fried food. It reflects the degree of oil rancidity and the content of ketones, aldehydes, and other harmful substances, which are of great significance for food safety testing. However, the current detection instruments are generally bulky and nonadjustable. Metasurface has emerged as a powerful platform for optical applications, including beam shaping, focusing, and holography [1–3]. It has the capability of controlling light scattering properties via the interaction between electromagnetic waves and subwavelength structures [3–8]. However, there are many limitations in metasurfaces. For example, once the structure of traditional metasurfaces is fixed, they can only enable static functionalities [9–14]. At present, tunable metasurfaces have been investigated in exploiting phase-change materials (PCMs) and found many interesting applications [15–23]. PCMs provide an effective platform for realizing tunable metasurfaces with many advantages: fast phase transition speed, easy phase transition conditions, and large differences in optical properties before and after phase transition. Ge2Sb2Se4Te1 (GSST), a representative of nonvolatile PCMs, has a large difference in refractive index between amorphous GSST (a-GSST) and crystalline GSST (c-GSST) (Δn > 1). More importantly, although GSST atoms need a high temperature to be rearranged during phase transition, their properties can still be effectively maintained at room temperature. In addition, the imaginary part of the refractive index of GSST in the mid-infrared is almost zero, which greatly reduces its optical loss [17].
Many multifunctional metadevices have been achieved with GSST, such as metalens and beam-steering devices. However, tunable metasurfaces have limited degrees of freedom. Previously reported works of varifocal metasurfaces only achieved two focal spots on basis of linear polarizations before and after the phase transition. Besides, there always brings in deviation in phase profiles in traditional methods when discretizing the phase profiles at two states of the PCMs [16]. Notably, independent control of circular polarizations at two focal spots, before and after the phase transition of GSST, has never been achieved without spatial multiplexing.
According to Kirchhoff's law of thermal radiation, the absorptivity and emissivity of any substance under thermal equilibrium are identical. However, the radiation intensity of the aldehyde carbonyl is too weak to detect because of its low content in fried food. To realize dynamically tunable detection of aldehyde carbonyl whose absorption peak at 5.8 µm, we proposed two reflected varifocal metasurfaces in this work. We use two materials CdGeP2 and PbSe which possess similar refractive indices with a-GSST and c-GSST to design the metasurfaces. The first metasurface realizes a tunable system of four focal spots for linearly-polarized (LP) light based on polarization multiplexing by combining the inner and outer elliptical cylinders. The second metasurface realizes the transformation of focal spots and polarization of incident light before and after phase transition. It greatly reduces the crosstalk of LCP and right-circularly-polarized (RCP) lights compared with the traditional method based on spatial multiplexing [24–27].
2. Design and method
Since most organic molecules have asymmetry or chirality, the radiation includes polarized component [28]. Therefore, the polarization-dependent tunable focusing metasurface design can further separate the signal light from background to improve the signal-to-noise ratio. With this consideration, we design two polarization-dependent tunable metasurfaces. The schematic of the first reflected varifocal metasurface operating at the wavelength of 5.8 µm is shown in Fig. 1(a). In the state of a-GSST, x-/y-polarized light is respectively focused at fa-x = 140 µm/fa-y = 180 µm. In the state of c-GSST, x-/y-polarized light is focused at fc-x = 160 µm (1/3 of the left of the x-axis)/fc-y = 160 µm (1/3 of the right of the x-axis).
Fig. 1. Design of the first varifocal metasurface (a) Schematic of the focal position of x-/y-polarized incident light with a-GSST and c-GSST. (b) Schematic of a meta-atom. (c) Cross sectional view of a meta-atom (p = 2.5 µm, hPbSe = hGSST = hCdGeP2 = 900 nm, hCaF2 = 500 nm, hAu = 300 nm). (d-e) Phase and reflectance database of meta-atoms as functions of r1 and r2 for x-polarized light in case of a-GSST at the operating wavelength of 5.8 µm. (f-g) Phase and reflectance database of meta-atoms as functions of r1 and r2 for x-polarized light in case of c-GSST at the operating wavelength of 5.8 µm.
The schematic metasurface and structural parameters are shown in Figs. 1(b-c). The period is 2.5 µm. In the top layer, materials from inner to outer are PbSe, GSST, and CdGeP2, respectively. The refractive indices of the a-GSST and c-GSST at 5.8 µm are 3.2 and 4.7, respectively, which are optically the same to CdGeP2 and PbSe [17].
Due to the little difference in refractive indices of PbSe and c-GSST, the effective refractive index (neff) of elliptic cylinder were calculated through linear interpolation of complex permittivity [29].
where s is the volume fraction (from 0 to 100%) of PbSe in the combined elliptic cylinder, nc-GSST and nPbSe stand for the refractive indices of c-GSST and PbSe. With calculation, neff is 4.74 and s is 40%. We obtained 2π phase coverage and high-reflectance (average 97.5%) via sweeping the semi-axes r1 and r2 of the elliptic cylinder before and after phase transition as shown in Figs. 1(d-g). The reflectance of nearly all meta-atoms reaches 1 since the refractive index of the material selected for the design has no imaginary part. Finite Difference Time Domain (FDTD) method is used to conduct the simulations. Each meta-atom is placed at z = 0 µm and the x-/y-polarized light is incident at z = 18 µm along the negative z-axis. The FDTD simulation area is set from −1.25 µm to 1.25 µm in the x- and y-directions, and from −6 µm to 30 µm in the z-direction. Periodic boundary conditions are applied along x- and y-directions while perfectly matched layer (PML) condition is adopted along z-directions. 2D Z-normal monitor is added at z = 26 µm to record average phase and reflectance.Target phases required for focusing follows Eq. (2) [23]. To ensure the size matching of the inner and outer elliptic cylinder, we use a set of optimization algorithms to select the meta-atoms.
3. Results and discussions
We arranged the first metasurface with 75*75 meta-atoms at z = 0 µm and the x-/y-polarized light is incident at z = 18 µm along negative z-axis. The FDTD simulation area is set from −95 µm to 95 µm in the x- and y-directions, and from −6 µm to 200 µm in the z-direction. PML conditions are applied along x-, y- and z-directions. We added a 2D Y-normal monitor at y = 0 µm to observe the information of focal spots. The vertical four focal spots on x-z planes are shown in Figs. 2(a-d), respectively. The contrasts between each focal spot and its background are remarkable. Among them, the fa-x/fa-y/fc-x/fc-y are 130/170/152/152 µm, which have little deviation from the predefined positions.
Fig. 2. Characterizations of the first varifocal metasurface operating at the wavelength of 5.8 µm. (a-b) Normalized intensity distributions of the two focal spots (fa-x = 140 µm, fa-y = 180 µm) on x-z plane in the state of a-GSST. (c-d) The x-z intensity distributions of two focal spots (fc-x = 160 µm (1/3 of the left of the x-axis), fc-y = 160 µm (1/3 of the right of the x-axis)) in the state of c-GSST, respectively. (e-f) Horizontal intensity distributions for the fa-x and fa-y spots in the state of a-GSST. (g-h) In the state of c-GSST, each plot represents the intensity distribution for the fc-x and fc-y focal planes. The theoretical diffraction-limited values are denoted by black dashed curves.
We evaluate the quality of focal spots by comparing the actual focal spot profiles with the theoretical diffraction limit. It can be seen that the simulated results are in well agreement with theoretical values. Figures 2(e-f) show the normalized intensity comparison with theoretical diffraction-limited values (black dashed curve). The full width at half maximum (FWHM) of the focal spot at fa-x (fa-y) is 6.06 µm (7.39 µm), while the theoretical value is 3.64 µm (4.73 µm). The deviation can be attributed to the phase mismatches of meta-atoms since the periodic boundary conditions in metaatoms simulation are not kept for metadevice. Red solid curve denotes the actual focal spot profile and blue solid curve shows the “phantom” focal one.
Remarkably, crosstalk is another critical metric since it may result in ghost image information and thus severely degrades image quality. In order to address this issue between x- and y-polarized light, we define the polarization contrast ratio (CR) [16]. In the state of a-GSST, CR is defined as:
Figures 2(g-h) illustrate the comparison of intensity profiles with the red solid curve (actual focal spot profile) and blue solid curve (“phantom” focal spot) with those of theoretical diffraction limit (black dashed curve). The FWHM of the focal intensity at fc-x (fc-y) is 6.06 µm (5.39 µm), while the theoretical value is 4.36 µm (4.36 µm). In the state of c-GSST, we define CR as:
The second reflected varifocal metasurface operating at the wavelength of 5.8 µm is designed for circularly-polarized (CP) incident light as shown in Fig. 3(a). It combines GSST with similar refractive materials to realize the co-polarization focusing in the state of a-GSST and the cross-polarization focusing in the state of c-GSST for LCP incidence, respectively. The reflected focusing is at fa = 120 µm (LCP) and fc = 180 µm (RCP), respectively.
Fig. 3. Design of the second varifocal metasurface. (a) The schematic of the focal position of the LCP incident light in the state of a-GSST (LCP light reflected) and in the state of c-GSST (RCP light reflected). (b) Schematic of a meta-atom, the parameters are the same as Fig. 1(c) (p = 2.5 µm, hPbSe = hGSST = hCdGeP2 = 900 nm, hCaF2 = 500 nm, hAu = 300 nm). (c) Curves of the phase and reflectance of the structure as a function of the cylinder radius(r) at the operating wavelength of 5.8 µm, the black dotted line represents the position of r = 750 nm.
The meta-atom shown in Fig. 3(b) is the same to that of first metasurface rather than the inner PbSe cylinder. In our design, the reflected light of co-polarized requires the same phase shift in x- and y-directions, while the reflected cross-polarized light needs phase difference of π between x- and y-polarizations. In the state of a-GSST, the optical response of structure only depends on the size of PbSe cylinder, enabling the focusing of the LCP light with co-polarization. In the state of c-GSST, the optical response of structure only depends on the half-wave plate (PbSe + c-GSST, neff = 4.73, r1 = 1080 nm, r2 = 750 nm) and allows the focusing of the LCP light with cross-polarization at the other position.
Represented as the red curve in Fig. 3(c), phase coverage of 2π is obtained through sweeping the radius (r) of the PbSe cylinder. The corresponding reflectance is denoted by the blue line in Fig. 3(c) and the average reflectance of the meta-atoms is 97.2%. The required meta-atoms are selected and arranged according to the focusing formula (2).
The second metasurface is also composed of 75*75 meta-atoms. We added two sources at z = 18 µm to obtain LCP light. One with phase and polarization angle set to 90° and 0°, respectively. The other with phase and polarization angle set to 0° and 90°. Other settings are the same as the first meta-surface mentioned above. The intensities on x-z plane of two focal spots are shown in Figs. 4(a-b). The fa/fc are 114/168 µm, which slightly deviate from the theoretical positions. We evaluate the quality of focal spots by comparing the actual focal spot profiles with theoretical diffraction-limited values. Figures 4(c-d) plot the normalized intensity with the red solid curve (actual focal spot profile) and blue solid curve (“phantom” focal spot) across the focal spots along with theoretical diffraction-limited values (black dashed curve). The FWHM of the focal intensity at fa (fc) is 4.47 µm (5.66 µm), while the theoretical value is 3.23 µm (4.72 µm). It should be noted that the normalized intensity at the actual focal plane is very close to the theoretical values.
Fig. 4. Characterizations of the second varifocal metasurface operating at the wavelength of 5.8 µm. (a-b) The x-z plane normalized intensity images of the two focal spots (fa = 120µm fc = 180µm) in the state of a-GSST and c-GSST, respectively. (c-d) In the state of a-GSST and c-GSST, each plot contains the focal spot intensity distributions for the fa and fc focal planes. The theoretical diffraction limit is marked with black dashed curves.
Notably, we take crosstalk into account between the state of a-GSST and c-GSST, which is characterized by the switching contrast ratio CR. We define:
It’s necessary to have further consideration on the experimental feasibility. As to the proposed structure, the bottom Au and CaF2 layers can be deposited with accurate thickness through electron beam evaporation [30]. The top layer can be fabricated through electron beam lithography (EBL), standard plasma etching, and sputtering methods [16]. Specifically, a layer of CdGeP2 needs to be deposited first, and then the GSST and PbSe parts should be etched away through the process of spin-coating, exposure, and development [31]. Afterwards, PbSe can be etched after depositing a layer of GSST in the same processes. The PbSe layer should be deposited at the end to complete the preparation of the top layer [32]. It’s also worth noting that the alignment of PbSe and GSST is also challenging. With the state-of-the-art EBL process, the alignment accuracy is within 100 nm [33]. Detecting aldehyde carbonyl is a potential application of our proposed metasurface as it can dynamically manipulate polarized light and enhance weak signals by focusing. In this work, we mainly focus on the design concept and light modulation effect of the proposed metasurface over other scenarios in miniaturization and dynamical polarization engineering. As to the high-temperature fluctuations which are required to trigger the phase transition of GSST may affects the normal work of sensing applications, the metasurface would be placed a certain distance from the detector in real implementation. Besides, the phase transition of GSST can be precisely controlled by applied voltage without exposing the whole detection system to high temperature environment [17,34]. Notably, GSST is a non-volatile phase change material, whose optical properties after phase change can be maintained at room temperature so as not to affect the normal operation of the sensor.
4. Conclusion
In conclusion, we propose two reflected varifocal metasurfaces based on GSST. It allows the switchable manipulation of phase profiles with different polarized light. A systematic design approach by combing the GSST with two materials that have close optical constants was presented. The first metasurface realizes a switchable system of four focal spots based on linear polarization multiplexing. Compared with previous studies, it adds a new polarization degree of freedom to design varifocal metasurface. All focal spots are diffraction-limited and have significant polarization contrasts with CRa = 16.83 dB and CRc = 35.85 dB. By switching the state of GSST from amorphous to crystalline, the second metasurface can selectively control the polarization and focal position of the reflected light. Remarkably, our design overcomes the large energy loss and crosstalk of the traditional method of spatial multiplexing. The switching contrast is 17.03 dB. Our work exhibits great potential in application of tunable detection of aldehyde carbonyl and it also provides a novel and flexible method for tunable metasurfaces based on PCMs.
Funding
Shanghai Municipal Science and Technology Major Project (2019SHZDZX01); Natural Science Foundation of Zhejiang Province (LR22F050004); Science and Technology Commission of Shanghai Municipality (20JC1416000, 22JC1402900, 22ZR1472700); Shanghai Rising-Star Program (20QA1410400); Strategic Priority Research Program of Chinese Academy of Sciences (XDB43010200); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2017285); National Natural Science Foundation of China (61875218, 61991440, 91850208); National Key Research and Development Program of China (2018YFA0306200).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
The data that support the findings of this study are available on request from the corresponding author on reasonable request.
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