Reconfigurability analysis of an all-dielectric thermal microfluidic-based metasurface

Metasurface tuning is performed using different ways for a wide range of applications. This study presents the design of a thermally-tuned all-dielectric reconfigurable metasurface. A microfluidic channel, filled with different concentrations of tellurium–selenium (Te-Se) alloy, is added on the top of the elliptical dielectric resonator (EDR) unit cell of the considered metasurface. The electrical properties of used semiconductor alloy are varied in the range of 400 to 700 ◦ C (steps size of 100 ◦ C). The impact of thermal tuning on the reflection and transmission characteristics of the designed metasurface is analyzed in the frequency range of 20–30 GHz using COMSOL Multiphysics. Obtained results demonstrated that the realized metasurface exhibits reconfigurable behavior in terms of variations in the reflection and transmission characteristics with a change in either temperature or concentrations of selenium and tellurium. The wider bands with high reflection and low transmission frequency bands are obtained with lower concentrations of selenium and tellurium for all operating temperatures.


INTRODUCTION
Metasurfaces, composed of unit cells or resonators, have been widely used in numerous RF and microwave applications. The periodic arrangement of designed sub-wavelength resonators brings a change in the wave fronts of the incident electromagnetic waves. [1][2][3][4] This principle led to the development of numerous applications using metasurfaces such as metalenses, 5,6 broadband polarization converters, [7][8][9] THz sensors, 2 holograms, 10,11 beam steering, 12,13 and absorbers, 7,14,15 to name a few. All-dielectric metasurfaces, providing additional advantages of low losses, have gained a lot of attention for the development of reconfigurable metasurfaces. [2][3][4][16][17][18] The dynamic control of the variations in transmission and reflection properties of the meta-atom unit cells is possible in reconfigurable metasurfaces. This is possible due to conventional approaches including the integration of active components such as diodes, integrated circuits (IC), and MEMS. [7][8][9][19][20][21][22] In particular, the electrical properties of some alloys and fluid metals were varied to obtain the reconfigurable nature of the realized broadband metasurfaces. 15,23,24 Recent studies 1,16,[25][26][27][28][29][30] showed that the tunable nature of metasurfaces can also be obtained by manipulating the thermal properties of the used materials. The thermal tuning of a terahertz (THz) metasurface using Indium antimonide (InSb) alloy is presented in Reference 1. A comprehensive analysis of the thermal tuning of the realized metasurface using different semiconductor materials of group IV-VI is presented in Reference 30. Chen et al. employed a split ring resonator to demonstrate the temperature dependence of a THz metasurface response based on YBa 2 Cu 3 O 6.05 (YBCO) alloy. 25 The same alloy was used for the analysis of the nonlinear thermal properties of a THz metasurface. 26 The study of Reference 16 confirmed that electrical and magnetic resonances of an all-dielectric metasurface change with the change in the temperature or ambient refractive index of employed materials. The authors in Reference 16 suggested that the loading of an all-dielectric optical metasurface structure with metal enhances its thermal emissions for radiative cooling applications. Authors in Reference 27 vary the temperature in the range of 80-873 K to demonstrate the frequency-shift characteristics of metasurface based on silicon and germanium semiconductors. Recently, Wang et al., 28 demonstrated the cloaking operation of a flexible copper-graphene-based thermal metasurface. Table 1 compares the reviewed studies.
In this work, switchable transmission and reflection characteristics of an elliptical dielectric resonator (EDR) based thermal metasurface are obtained. Figure 1 shows the realized EDR-based metasurface. The EDR is made of Rogers RO3210 substrate (ε = 10.2 and tan δ = 0.03). A microfluidic channel is created on the top of the EDR resonator which is then filled with liquid semiconductor alloy of Te 1−x − Se x . The liquid alloy is made by combining selenium and tellurium above their melting points. The permittivity and permeability of the used semiconductor liquid selenium-tellurium alloy are 10 and 27, respectively. 31,32 The study analyzes the performance of the realized metasurface under two cases: (i) changing the concentration of the materials to form eight different semiconductor alloys of Te 1−x − Se x for 0 ≤ x ≤ 0.7 for fixed temperature; (ii) varying the operating thermal condition of each formed alloy of Te 1−x − Se x for 0 ≤ x ≤ 0.7 in the range of 400 to 700 • C (steps size of 100 • C). The characterization for the aforementioned 32 case studies (combination of both "x" and temperature) is done by analyzing the two-port S-parameters of the structure in the frequency range of 20-30 GHz. The COMSOL Multiphysics software is used for the full-wave numerical analysis of each case study. The performed analysis suggests that the role of the thermal conditions and joining concentrations of combining materials is vital in the realized reconfigurable operating nature of the proposed metasurface. The simplicity of design and ease of tuning of the switchable transmission and reflection characteristics of the realized metasurface makes it a better choice as compared to legacy approaches. [7][8][9]15,[19][20][21]23,24 The main contribution of the study is demonstrating that changes in the semiconductor alloy in the microfluidic channel vary the dynamic transmission and reflection characteristic of all-dielectric metasurface operating in microwave frequencies. To the best of our knowledge, the proposed study is a first of the kind that presents a comprehensive analysis of the impact of variations in the Te-Se semiconductor alloy concentrations on the reconfigurable properties of an all-dielectric metasurface (based on Roger substrate) in the microwave band.
The details of the metasurface design, numerical analysis setup, case studies conditions, and discussion about the obtained results are given in the following sections.

ALL-DIELECTRIC ELLIPTICAL METASURFACE DESIGN
An all-dielectric elliptical metasurface is designed in which a unit call has the shape of the EDR. Figure 1 depicts the schematic of the realized metasurface. The EDR unit cell constitutes Rogers RO3210 having a relative permittivity of 10.2 and loss tangent of 0.003. The optimized structure of EDR has a thickness (g) of 2.56 mm, minor axis radius (a) of 2.5 mm, and major axis radius (b) of 2a mm. On the top of elliptical EDR wings, a microfluidic channel of 100 × 100 μm is created which is covered by polydimethylsiloxane (PDMS). The width of this created microfluidic channel is 0.5 mm and it has a thickness (d) of 1.1 mm. Figure 1B illustrates the dimensions and port definitions of the dielectric resonator unit cell. The dimensions of the EDR are Lx = 9.9 mm (Lx < ), Ly = 9.9 mm (Ly < ), and Lz = 16 . The value is calculated at the highest analyzed frequency of 30 GHz. For the periodic infinite arrangement of EDR resonators, each cell is connected with the next one through a length of (Lx-2a/2) and a width of 2.56 mm (g).
The installed microfluidic channel in the EDR resonator is filled with semiconductor liquid selenium-tellurium alloy of Te 1−x − Se x and the temperature of the alloy was changed to analyze the impact of the temperature variations on the transmission and reflection parameters of the realized metasurface. The analysis was performed using COMOSOL 5.4 Multiphysics software in the frequency range of 20-30 GHz.
The periodic port is used as port definition and the excitation of the structure is performed through normal plane wave excitation in the z-direction. The periodic port is used because the realized metasurface is itself a periodic structure that constitutes an array of unit cells.
The two-port S-parameters are recorded for the entire analysis frequency band to observe the variations in the transmission and reflection properties of the realized metasurface under different thermal conditions.

NUMERICAL ANALYSIS
The use liquid semiconductor alloy used in this study is liquid selenium-tellurium Te 1−x − Se x . 33 The electrical properties (conductivity) of liquid semiconductors vary with the change in thermal conditions. 33,34 Perron 33 did an extensive analysis of the variations in the electrical conductivity of different concentrations of selenium-tellurium liquid alloys. Based on the performed analysis of Reference 33, different case studies are devised to analyze the thermal reconfigurability of the proposed EDR-based all-dielectric metasurface. Table 2 lists the analyzed case studies as per the presented results in Reference 33. The melting points of tellurium and selenium are 450 and 220 • C, respectively. 33 The chosen case study temperature are based on their melting points to ensure that proper alloy is made when both tellurium and selenium are combined with different concentrations (i.e., x). The change of "x" in the liquid alloy of Te 1−x − Se x produces different variants of semiconductor alloy with the change in concentrations of combined liquid materials. Each variant, for example, x = 0.05 produce a liquid alloy of Te 1−0.05 − Se 0.05 that have different electrical conductivity based on thermal conditions. Table 2 summarizes the changes in the electrical conductivity of the numerous versions of Te 1−x − Se x under different thermal conditions. Table 2 is derived from the presented Figure 1 Table 2 for other variants of Te 1−x − Se x for different temperatures.
The conductivity of the filled Te 1−x − Se x alloy in the microfluidic channel of Figure 1 EDR resonator is varied for all the mentioned cases in Table 2. For each case, two-port S-parameter results are recorded to analyze the variations in the EDR-metasurface properties which are discussed in the next section.
It can be noted from the Figure 2 waveforms that the designed metasurface is operating in mode 2, that is, high transmission and low reflection from 20-21.5 GHz. The operational behavior of the realized structure changes to low

Without microfluidics in channel
First, the performance of the designed elliptical metasurface is analyzed without any fluid in the microfluidic channel. The transmission and reflection characteristics of Figure 2 waveforms can be divided into two main modes as illustrated in Table 3. Mode 1 presents the high reflection and low transmission behavior corresponding to the region where |S11| ≥ −5 dB and |S21| ≤ −10 dB. On the contrary, the frequency bands where |S11| ≤ −10 dB and |S21| ≥ −5 dB are referred to the as high transmission and low reflection or mode 2 in this study. These modes are defined for the brevity of analysis in this study. It can be noted from the Figure 2 waveforms that the designed metasurface is operating in mode 2, that is, high transmission and low reflection from 20-21.5 GHz. The operational behavior of the realized structure changes to low transmission and high reflections from 21.5 GHz till around 23 GHz as for this region |S21| ≤ −10 dB. The next bands where the mode 1 operation can be noted are 26.2-27.7 and 28.3-29.5 GHz, respectively. These results depict that the metasurface is continuously shifting among the low and high transmission regions in the analyzed frequency range of 20 to 30 GHz.

With Te 1−x − Se x micro-fluid in channel
This section details the results of the different case studies of Table 2  The observed waveforms of liquid alloy Te 1−0 − Se 0 for the thermal conditions of 600 and 700 • C are almost the same as shown in Figure 3. This is because of no significant variations in the conductivity of the material for all cases it's in the range of (× 105) with the change in operating temperature. This shows that the operating nature of the realized dielectric metasurface is quite insensitive to the change in operating temperature when the alloy is made by combining Te 1−0 and Se 0 .

4.2.2
Te 1−0.05 − Se 0.05 with different operating temperatures The second analyzed case for the inserted alloy is the mixture of liquid selenium-tellurium for x = 0.05 to form Te 1−0.05 − Se 0.05 . As like Te 1−0 − Se 0 , for this case analysis of the realized metasurface is performed for four different operating temperatures of 400, 500, 600, and 700 • C, respectively. The comparison of reflection and transmission parameters results is shown in Figure 4. The electric conductivity of the inserted liquid alloy is set to 7.05E5 to meet the analyzed operating condition for 400 • C. As the electric conductivity of the material is similar to the electric conductivity of alloy for x = 0, the waveforms of Figure 4 at 400 • C look similar to Figure 3 results at 400 • C.
At 400 • C, the high transmission and low reflection characteristic are noted in the frequency band of 20-20.15532 GHz which are similar to the mode 2 bands of Te 1−0 − Se 0 alloy with a slight increase in the bandwidth of mode 2 from 1503 to 1553.1 MHz in this case.
The mode 1 conditions corresponding to high reflection and low transmission are observed from 22.6554-23.8578, 26.5632-26.964, and 27.6654-28.1163 GHz, respectively. These three bands of Figure 4 waveform at 400 • C are similar to the first three bands corresponding temperature results in Figure 3.
The waveforms of S11 and S21 curve when the operating temperature of the Te 1−0.05 − Se 0.05 is changed to 500 • C are shown in Figure 4 (pink color waveforms). Although the waveform pattern looks similar to earlier ones, for this case, The further increase in the operating temperature to 500 • C for this alloy configuration varies the transmission and reflection parameters and tunes mode 1 and mode 2 to different bands as shown in Figure 5. These results are similar to Figure 3, that is, for Te 1−0 − Se 0 at 500 • C. This shows that changing of temperature to 500 • C for Although these two cases have different electrical conductivities for the same temperature, the change in the material concentrations in the semiconductor alloy produces similar results for these two cases. The same trend of similar operating bands of both modes (1 and 2) is noticed for this case and Te 1−0 − Se 0 when the operating temperature is changed to 600 and 700 • C, respectively, as depicted in Figure 5. This affirms the reconfigurable nature of the proposed structure with the change in temperature and material concentrations.

4.2.4
Te 1−0.2 − Se 0.2 with different operating temperatures  Figure 8 depicts the S-parameter results for this case. At Temp. = 400 • C, we observed that the metasurface is only operating in mode 2 in the frequency band of 20-20.952 GHz. As the conductivity of the alloy for this case is the same as for the alloy concentrations with x = 0.3 at 500 • C, good similarities between this case waveforms for this can be noted by comparing Figures 7 and 8, respectively. The change of conductivity to 1.9E3 for the operating temperature of 500 • C did not produce a major change in the reflection and transmission parameters of the metasurface as compared in Figure 7. However, the further increase of the temperature to 600 • C produces different results for S11 and S21 as compared to the earlier two cases due to the increase in the conductivity to 2.0E4 s/m.
The bands covering mode 1 for 600 • C configurations are 26.5632-26.964 GHz (400.8 MHz), 27.6153-28.1163 GHz (450.9 MHz), and 29.5191-29.6193 GHz (100.2), respectively. The mode 2 operation is noted from 20 to 21.5532 GHz. It can be observed that for the same operating conditions, the earlier case of alloy for x < = 0.3 produces wider frequency bands for mode 1 and 2 operations.

4.2.7
Te 1−0.5 − Se 0.5 with different operating temperatures Figure 9 presents the variations in the reflection and transmission parameters of the metasurface when the liquid alloy is changed to Te 1−0.5 − Se 0.5 for different analyzed operating temperatures. The conductivity of the semiconductor alloy drops to 200 s/m for 400 • C which brought the changes in the dynamic behavior of this case as compared to earlier analyzed results. When the metasurface is operating at 400 • C, the mode 2 operating band covers only 300 MHz ranging from 20-20.3007 GHz which is lowest for the all analyzed configurations of Te 1−x − Se x so far. Only two bands (21.0522-22.0041 and 21.0522-22.0041 GHz) depicting the mode 1 characteristics can be noted from the S-parameter characteristic curves of the 400 • C case. The bandwidth of these aforementioned modes 1 is also lower as compared to earlier cases for the same temperature and different concentrations of materials.
The operating frequencies for mode 1 are only 21.0522 to 22.0041 GHz for the same concentration alloy with an operating temperature of 500 • C. While the mode 2 bands remain unchanged with the change in temperature to 500 from 400 • C.
Due to the increase in the conductivity of the alloy for 600 • C, the number of frequency regions corresponding to mode 1 characteristics increased as can be noted from Figure 9  The further increase of the temperature to 600 • C increases the high transmission and low reflection band to 300.6 MHz (20-20.3007 GHz) from 200.4 MHz for the earlier two cases of this alloy concentration. This increase in the operating range of mode 2 can be noted in Figure 10 results. The variations in the bands representing the mode 1 operation can also be noticed for 600 • C waveforms by comparing its curves with 400 and 500 • C configurations, respectively.
The last analyzed case study is for the operating temperature of 700 • C which changes the conductivity of the alloy to 750 s/m. The analyzed results depict that only mode 2 operation exists in this case. The observed band reflecting high transmission and low reflection region is from 20-20.7516 GHz. Table 3 presents a summary of high reflection and low transmission (mode 1) and high transmission and low reflection (mode 2) for all analyzed case studies of Table 2. The comparison reflects that mode 1 is the dominant mode of operation for the realized metasurface across all case studies. The mode 1 characteristics are observed in different bands for each case study. On the contrary, the mode 2 operation has a relatively fixed band in the frequency range of 20-22.5 GHz for all analyzed cases of different concentrations of alloys.

COMPARISON AND DISCUSSION
For x = 0, the impact of temperature on both dominant operating modes is almost negligible. The same thing can be observed for other cases of x till x = 0.7. These changes in the concentrations of the joining materials (x) to form different variants of semiconductor alloy vary the operating bands of both modes. That reflects the tunable and reconfigurable nature of the proposed metasurface.
For a fixed operating temperature of 400 • C, the change in the concentrations of the semiconductor alloy Te 1−x − Se x from 0 to 0.2 enhances the operating bandwidth of mode 2 from 1503 to 2254.5 MHz. The same trend can also be noticed for almost all operating bands corresponding to mode 1 for the fixed operating temperature of 400 • C and 0 ≤ x ≤ 0.2. The reason for this is the decrease in the conductivity of the alloy from 1.5 × 10 5 (x = 0) to 5 10 3 (x = 0.2) at a temperature before the melting point of tellurium (Te). After that, although the temperature is fixed, the role of combined metal concentrations becomes important and it results in a decrease of the operating bandwidths of both mode 1 and mode 2 for the same fixed operating temperature of 400 • C.
The overall wider frequency bands for both modes' operations are observed for the lower concentrations of both materials. The increase in the concentrations of both selenium and tellurium (e.g., for x = 0.5 and x = 0.7) results in two things: (i) an overall decrease in the bandwidths of both modes, (ii) a decrease in the number of operating bands for mode 1. The reason for this is the reduction in the overall conductivity of the alloy for these cases as compared to the other cases. The nature of the used alloy becomes more insulator (lesser conductive) with the increase in the concentration of both selenium-tellurium. This reduces the both number of operating modes and their respective bandwidths for these cases. This shows that even for a fixed temperature, the switchable nature of the realized metasurface could be obtained by just varying the materials combinations.
Another important aspect of the realized metasurface is that nearly similar transmission and reflection characteristics can be tuned for different The reason for such similar characteristics is the comparable conductivities of the alloy for these cases.
The comparison of Table 3 and Figures 3-10 depicts that the operating characteristics of the realized metasurface can be easily tuned for different bands of high reflection and low transmission and vice versa by playing either with the operating temperature or the joining concentrations of metals to form different version of inserted alloy in the microfluidic channel.
Analysis of the electromagnetic field distribution in Figures 11 and 12 reveals that the magnitude of the electric field in the microfluidic channel increase with increasing the ratio x of Selenium within the alloy at the selected frequencies.
F I G U R E 11 Mode (1) and mode (2)  This increase is more significant for T = 400 • C than T = 600 • C. This behavior may be attributed to the large increase in hole mobility with atomic ratio x of selenium as explained in Reference 33 and the trend to the metallic state at high temperature. 33 We can also note from Figures 11 and 12, the presence of two different configurations for both electric and magnetic fields at all the selected frequencies, which confirm the presence of two resonating modes within the structure.
However, the magnetic field in the microfluidic channel tunnels seems to be not affected by the changes in Selenium ratio x or temperature, which means there are no magnetic properties of the alloy in the studied frequency band 20-30 GHz.
It can be observed from the conducted comprehensive analysis that the operational characteristics (high transmission/low reflection) of the realized all-dielectric metasurface vary significantly in the microwave range by just changing the operating temperature of the added microfluid for a fixed concentration. The electrical properties of the semiconductor vary with the temperature change and thus play a role in the transmission characteristics of the structure. The realization of tunable metasurfaces by just changing the thermal properties of the microfluidic provides added advantages of simple operation and a compact size as compared to conventional suggested approaches of adding active components such as ICs and diodes and MEMS as in [19][20][21][22] The findings of this study could be used as a guideline for the design of thermally tuned dielectric metasurfaces. The analysis of the transmission/reflection characteristics of the realizing metasurface for different temperature conditions can be performed efficiently using the proposed technique. The obtained results using the proposed method could be beneficial for the fabrications of the designed metasurface as the optimized design parameters and concentrations of the used semiconductor alloy will reduce the cost of fabrication of different fabricated versions.

CONCLUSION
The work has presented an extensive analysis of the impact of the operating temperature (400-700 • C with a step size of 100 • C) and concentrations of the materials to form a semiconductor alloy (Te 1−x − Se x for 0 ≤ x ≤ 0.7) on the transmission and reflection properties of an all-dielectric EDR-based metasurface. The performed analysis demonstrated that realized metasurface mainly operates in the high reflection and low transmission mode for the wider frequency bands as compared to low reflection and high transmission frequency ranges for all analyzed 32 case studies. The tuning of the mode's operation was noticed for both the change in material combinational concentrations and varying thermal conditions. The switching properties are more sensitive to the change in material combinations or the fixed thermal condition, that is, the operating temperature of the metasurface. The performance, although depicts reconfigurable nature, decorates in terms of the mode's operating bandwidths with the increase in materials concentrations after x = 0.5. This deduces that better performance of the proposed EDR-based metasurface can be obtained with lower concentrations of both Te and Se materials in the used alloy in the microfluidic channel of the studied metasurface.

CONFLICT OF INTEREST STATEMENT
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.