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Abstract
Recently, optically-transparent metasurface based on indium tin oxide (ITO) film has attracted wide attention due to its remarkable optical and electromagnetic characteristics. However, most previous researches on the ITO film mainly focus on the absorption because of its prominent loss-resistance property, but neglecting the further exploration on programmable functions. Here, we present a programmable metasurface based on an optically-transparent ITO glass, on which varactors are integrated to achieve flexible amplitude manipulation range of about 25 dB. More importantly, the presented programmable design can be applied for direct modulation on the carrier incident wave with the desired pre-designed analog wave-form. Within the 10 MHz modulation speed, both programmable amplitude manipulation and analog information modulation are demonstrated in the measurements, showing good agreement with theoretical analysis and simulations. Combining both optical transparency and programmable modulation capability, the presented metasurface will promote the potential applications in wireless communication, internet of things and other smart scenarios.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials are artificially-composite structures with extraordinary physical properties that natural materials do not have. Through the orderly design of structure on the key physical scale of materials, we can break through the limitations of some apparent natural laws, and obtain extraordinary material functions beyond the inherent ordinary properties of nature. Metamaterials are used to realize negative phase velocities [1–3], perfect lenses [4], or invisibility cloaking [5,6]. Regarded as the two-dimensional version of traditional 3D metamaterials, metasurfaces can realize flexible and effective control of amplitude [7,8], phase [9–12], polarization mode [13,14] and propagation mode [15] of electromagnetic (EM) wave.
In order to eliminate electromagnetic waves in an ultrathin profile, metasurface absorber has become an important research direction [16,17]. Metamaterial absorbers usually convert the incident electromagnetic wave energy into the loss of other energy forms like a thermal loss. To achieve tunable and flexible absorbing performance, reconfigurable absorbers have been proposed [18–26]. The reconfiguration methods of metasurface absorbers include voltage regulation [18], mechanical reconfiguration [19], liquid crystal [20], phase change material [21], diode [22], varactor [23], lumped components [24,25], and water [26] as substrate materials. In recent researches, indium tin oxide (ITO) film has been applied to the realization of absorber [27]. ITO is an n-type oxide semiconductor, with designable conductivity and good transparency [28]. Metamaterials based on ITO thin films have various interesting optical and electrical properties, which can be used in many applications, such as broadband absorption [29], two-dimensional beam control [30], phase and amplitude modulation [31], broadband resonance [32]and multispectral stealth [33]. In addition, the use of ITO films in metasurface has more advantages on CMOS compatibility comparing to previous approaches using precious metals [34,35]. Some metasurfaces based on ITO thin film have been proposed [36,37]. However, these researches cannot achieve flexible amplitude manipulation and analog information modulation, they are still limited in the fixed or limited functionalities, which is urgent to break through the existing limitations to achieve more advanced functions.
In this paper, we propose a tunable metasurface absorber based on ITO films to demonstrate the first steps towards analog information modulation with transparent ITO metasurfaces. A varactor is integrated on the element structure to tune the reflective magnitude of EM wave by applying specific voltage. Such a modulation mechanism is further extended to a flexible analog modulator for information transmission. Combining optical-transparent ITO on glass and flexible EM-wave amplitude modulation, the presented work will have diverse potential applications in imaging display [38], wireless communication, scenarios of internet of things (IoT) and smart city.
2. Principle and results
We first introduce an metasurface for flexible EM manipulation using an optical-transparent ITO glass, as shown in Fig. 1. To achieve the programmable real-time control, varactors are integrated on the metasurface, which are controlled by a field programmable gate array (FPGA). When the bias-voltage changes, the reflected amplitude of a normal incidence in specific polarization alters distinctively. Such a mechanism can be easily applied for spatial propagating wave flexible modulation. For instance, when FPGA implements a time-varying control signal on the varactors of the metasurface, the reflected energy level of EM wave changes in real-time as well. Since the capacitance of the varactor is continuously tunable with applied voltage, a near-analog modulation can be achieved on the spatially propagating wave as FPGA executes specific control voltage.
Fig. 1. The schematic of programmable metasurface for real-time electromagnetic (EM) wave modulation. When under the digital signal control of the Field Programmable Gate Array (FPGA), the metasurface modulates the spatial EM incidence into an analog-modulated EM signal.
To realize the illustrated functionality, we design a programmable meta-atom based on the ITO film on glass, integrated with a varactor. A three-dimensional (3D) diagram of the unit cell is shown in Fig. 2(a). The structure of the proposed element has three layers. From the top to the bottom, there are a symmetrical ITO structure (depicted in light green color), a soda-lime glass (a dielectric constant of 7.75 and loss tangent of 0.004) substrate with thickness h = 2.5mm and an ITO layer. A varactor is welded between two planar symmetrical ITO films (thickness of about 0.01 mm and surface resistivity of about 3 Ω/Sq.) printed on the glass substrate with a dielectric constant of 7.75 and loss tangent of 0.004. The model number of the varactors is Skyworks SMV2019. The ITO layer, which is used as an electrical ground, covers the bottom of the metasurface completely. The bias voltage of the varactor is provided by ITO wires on both sides of each patch. The period of the unit cell is l = 12.5mm. The more detailed dimensions of the element are designed as below: a=9mm, b = 3.1mm, c = 3.3mm, d = 2mm, e = 4.4mm, g1 = 0.5mm and g2 = 0.2 mm. In order to explore highly efficient tunability, we use the commercial software, CST Microwave Studio, to simulate and optimize the engineered structure and the capacitance of the varactor. In the simulation, periodic boundary conditions are used to optimize the element, in which y-polarized normal incident plane wave is served as the excitation. Generally, the varactor is regarded as resistor-inductor-capacitor (RLC) model to ensure the simulated results more accurate and its equivalent circuit is exhibited in Fig. 2(a), where R=2.2Ω, L=0.4nH and C varies from 0.2pF to 2pF.
Fig. 2. The sketch of the element and its amplitude responses with different surface resistivity of ITO film. (a) The detailed structure of the designed element and the equivalent circuit of the varactor diode. (b)The variation of amplitude with resistance and capacitance at 4.8 GHz. (c) and (d) The reflective amplitude response of unit cell with R_ITO=3 or 10Ω/Sq, as the capacitance of the varactor diode C changes from 0.2 to 2pF. (e) and (f) The reflective phase response of unit cell with R_ITO=3 or 10Ω/Sq, as the capacitance of the varactor diode C changes from 0.2 to 2pF. (g), (h), (i), (j), (k) and (l) The electric field distributions of C=0.2, 0.38, 0.56, 0.74, 0.92, and 2 pF.
To analyze the reflection response of the element, the capacitance of varactors and the conductivity of ITO films are mainly researched. Figure 2(b) shows the variation of amplitude with resistance and capacitance at 4.8 GHz. When R_ITO = 10, 20 or 50 Ω/Sq, the amplitude response of the unit does not change significantly with the increase of capacitance. Figures 2(c) and 2(d) show the simulated results of the reflected amplitude responses are given when the ITO film is applied with different surface resistivity. The purpose of Fig. 2(c) and 2(d) is to explore the effect of ITO resistance on the electromagnetic response of the unit. As shown in Fig. 2(c), when the ITO resistance is fixed, we change the capacitance of the varactor to analyze the change of reflected amplitude. It is obvious that the tunable range of the reflection amplitude of the unit cell is the largest at 4.8 GHz. Similarly, when R_ITO = 20Ω/Sq, the absorption performance is mediocre, and the tunable amplitude range is small in Fig. 2(d). Therefore, according to Figs. 2(b)-2(d), when R_ITO = 3 Ω/Sq, the element has the best reflection absorption and great tunable performance at 4.8 GHz. Figures 2(e) and (f) are the reflected phase responses of the element when the R_ITO = 3 and 20 Ω/Sq, in which the phase responses (when the R_ITO = 3 Ω/Sq) are almost the same when the capacitance of the varactor changes. It should be noted that all the elements on the metasurface share the same bias voltage, resulting in their same phase responses. Consequently, the amplitude of reflected beam in the normal reflection direction is only related to the amplitude response of the elements, according to the scattering theory of metasurface [39].
The mechanism of this work is that the capacitance of varactor can be changed by varying the voltage, thus changing the unit reflection equivalent circuit, and then affecting the reflection amplitude. From the perspective of propagating EM wave, the equivalent spatial impedance of the metasurface is changed which affects the reflection amplitude of metasurface element. To further explain and demonstrate this mechanism, the electric field distributions of the unit under different capacitance values at 4.8 GHz is plotted in Figs. 2(g)-2(l). For ITO films, the electric field is mainly concentrated on the ITO films along x-axis. With the increase of the varactor capacitance, the current intensity at the junction of two planar symmetrical ITO film decreases, while the current intensity on the ITO films along x-axis increases apparently.
Based on the above-mentioned elements, we propose a tunable metasurface with 30×30 elements. The capacity of varactors in the same row is coincident. The specific bias voltage is provided by the ITO bias line, so that the capacitance value of the same row of varactor can be modulated. The state of the varactor, which determines the amplitude response of the metasurface, can be changed by controlling the bias voltage generated by FPGA. The full-waves simulations were performed in the CST Microwave Studio, and the results for metasurface are given in Fig. 3. We intend to fully show the relationship between the reflected beam amplitude and the volume control of varactor, which is directly related to the amplitude modulation of the presented metasurface. And we use these discrete modulation states of distinct capacitance values in the later beam direct-control. To exhibit the manipulation more obviously, we analyze six control states of varactor capacitances with C=0.2, 0.38, 0.56, 0.74, 0.92, and 2 pF. The far-field scattering results of metasurface with different capacitances of the varactor at a working frequency of 4.8 GHz are illustrated in Figs. 3(a)–3(f), respectively. It can be seen from Fig. 3 that the amplitude of the main beam at 0° decreases with the increase of the varactor capacitance. Therefore, we can directly modulate the reflected amplitude of the propagating wave by controlling the varactor state within the presented metasurface design.
Fig. 3. The simulated far field result of the metasurface at 4.8 GHz when R_ITO=3Ω/Sq. (a) The scattering field for metasurface with C=0.2pF. (b) The scattering field for metasurface with C=0.38pF. (c) The scattering field for metasurface with C=0.56pF. (d) The scattering field for metasurface with C=0.74pF. (e) The scattering field for metasurface with C=0.92pF. (f) The scattering field for metasurface with C=2pF.
In order to observe the simulation performance of the measured metasurface, as shown in Fig. 4(a), we designed a rotatable table, metasurface sample, feed source and receiver to demonstrate the experiment. Both the metasurface sample and the feed source are fixed on the rotatable table, and their relative positions are completely fixed, so the incident wave always illuminates on the metasurface normally. When the rotatable table rotates, the two-dimensional far-field data can be measured. The feed source is 1 meter away from the metasurface sample, and the receiver is 10 meters away from the metasurface sample. The feed source is a broadband antenna (with the gain of about 12 dB at 5 GHz) connected with a signal generator. The feed power is 20 dB in experiments. The receiver is a rectangular horn with the gain of about 22 dB at 5 GHz. Since the size of transmitting antenna is small, and the receiving antenna is far away from the metasurface and its excitation source, so the transmitting antenna has little influence on the measurement process. Both the feed source and receiver are rectangular horn antennas. Since we mainly focus on the amplitude modulation at the single frequency point in the normal reflection direction, we perform the experimental demonstration for the five control states. Figures 4(b)–4(g) present the measured results in the far-field of the different capacitance of the varactor diode. The measurement frequency is 4.87 GHz, which has a little deviation compared to the simulation. The reasons of this frequency deviation include: (1) the error of the simulation equivalent circuit model itself; (2) welding and machining errors in making physical objects. Figures 4(b)–4(g) respectively present the measured data of C=0.2, 0.38, 0.56, 0.74, 0.92, and 2 pF. In order to clearly show the performance, we added simulated data to the diagram for comparison. In the Figs. 4(b)–4(g), we can observe that the measurement data and the simulation results have a good consistency. The experimental results are marked in red and the simulation results are shown in blue. The deviation between the measured and simulated results may be due to the following reasons: (1) fabrication error due to the limitation of manufacturing precision, (2) manual placement of antennas and metasurfaces can lead to angle and measurement errors;(3) the component error between varactor diodes. In addition, there are some slight stray radiations in the measurement, which may be caused by other components near the metasurface.
Fig. 4. The measurement configuration and the measured results in the far-field of the different capacitance of the varactor. (a) The measurement configuration. (b), (c), (d), (e), (f) and (g) The compared results in far-field of C=0.2, 0.38, 0.56, 0.74, 0.92, and 2 pF.
More significantly, the metasurface can undertake direct modulation on spatially propagating wave within such simple architecture. With further integration improvement, this work can be applied in wireless communication and smart repeating indoor. Here we provide a concept of proof to exhibit the modulation performance, as shown in Fig. 5(a). When a normal incidence (consists of sine signal) illuminates on the surface on the window, the reflected wave is directly modulated with specific information. It should be noted that the reflected wave is also perpendicular to the metasurface. In realistic applications, the amount of modulation states is limited, which means digitization is made on the desired analog wave-form. The fabricated sample is also shown in Fig. 5(a). The ITO film used in this paper achieves about 88% transparency. It should be noted that the varactors used in is optically opaque, but in this proof of concept, we intend to demonstrate the direct modulation performance of the presented metasurface. In addition, the varactor itself is relatively small and does not affect the field of vision much. In the future improvement design, we can use a smaller volume encapsulation varactor to reduce the impact of this defect. In this work, five distinct states are selected with the magnitude ranging from about 14 dB to -4 dB, as marked in Fig. 5(b), where the relationship curve between varactors capacitance and bias-voltage is given. Five control states are selected in the amplitude modulation as: 0 V (2 pF), 3.1 V (0.92 pF), 4.6 V (0.74 pF), 6 V (0.58 pF), 9.7 V (0.38 pF). As we employ the varactors, whose inverse bias state almost has no current. Hence, the ITO film also has no current, resulting in no voltage consumption. In Fig. 5(c), a sine function with four periods is illustrated, considering as the desired wave-form. A digitized wave-form with five states is obtained in Fig. 5(d), in which each state lasts for s certain time to satisfy the period length. This time-varying wave-form is the final control voltage signal, applied on the all varactors. Because of the relation between bias-voltage and reflected energy, the reflected wave is directly modulated as the wave-form in Fig. 5(e), where the incidence is regarded as a carrier wave. A blue box is marked to show that the carrier wave is a normal sine signal. It should be mentioned that the results in Fig. 5(c)-(e) are all theoretically calculated results.
Fig. 5. The programmable real-time modulation achieved by the surface. (a) The application we assumed in which the presented metasurface acted as an surface on the window. (b) The relationship between bias-voltage and the capacitance of varactors, where five states are selected for modulation. (c) The designed wave-form to modulate on the normal incidence. (d) The digitization of the designed wave-form with five states. (e) The wave-form final modulated wave.
To experimentally verify the above idea, we perform a measurement of two modulation wave-forms, as shown in Fig. 6(a) and (c). The whole system is composed of a signal generator, a pair of antennas as transmitter and receiver, and an oscilloscope to detect the wave-form in the time domain. The feed source is set at the distance of 20 cm away from the metasurface, while the receiver is at 50 cm away from the metasurface to collect the reflected wave. It should be noted that the source/detector distances changed for the measurement of the modulated waveforms. In order to receive the detection modulation waveform more clearly, we set the transmitting and receiving antenna at a relatively close distance, to reduce the interference of background noise. The maximum modulation speed of the fabricated sample is 10 MHz in experiments. In Fig. 6(a), a normal sine function marked in blue is exhibited as the desired wave-form, while the digitized waveform is presented in red. The final modulated signal is measured as presented in Fig. 6(b), in which we clearly observe the outline amplitude of the carrier wave is modulated. Similarly, in Fig. 6(c), we design another function, which is a combination of sin(t) and sin(-t). The desired and digitized wave-form is exhibited in blue and red colors respectively. The measured wave-form in Fig. 6(d) shows a great agreement with our design. It should be noted that the measured results are all normalized into 1. The distortion of the measured results is mainly resulted from the error in the measurement system, including cables, connectors and environment interference. The digitization of the wave-form has deviation as the theoretical analysis in Fig. 5. The reasons of this difference are due to: (1) The processing method of metasurface is PCB processing, which leads to the processing error;(2) the error caused by the inconsistency of component welding; (3) the error comes from the difference between the measured and simulated component characteristics; (4) manual installation leads to the installation measurement deviation.
Fig. 6. The measured results of the EM wave modulation. (a) The desired wave-form of sine function and its digitization. (b) The measured results of the modulated wave-form. (c) The desired wave-form of a combination function and its digitization. (d) The measured results of the related wave-form.
3. Conclusion
In summary, we design and experimentally demonstrate an and programmable metasurface based on ITO glass, to realized flexible reflected amplitude manipulation and analog information direct modulation. A meta-atom consisted of ITO glass and varactor is designed, simulated and experimentally verified. By applying different bias-voltage, the reflected amplitude of a linear-polarized incidence can be changed as wish. Comparing to some tunable metasurfaces within traditional PCB fabrication [23], we use ITO film with good optical transparence as conductive structure, whose electromagnetic response will shift in amplitude and frequency with a larger modulation range, when the capacitance value changes. Furthermore, under the control of the time-varying signal from FPGA, the metasurface can directly modulate the spatial propagating wave with the desired analog wave-form. The measurement results have good consistency with our simulation, verifying our design. The design idea of this paper can be further improved. By optimizing the structure of multi-frequency modulation, designing independent tunable unit, and combining with phase control, this work can be extended to multi-frequency and multi-incidence-angle applications. Besides, graphene is optically transparent and behaves similarly to ITO [40,41]. However, the varactors can hardly be integrated on the graphene film to realize amplitude modulation. Also, the modulation range presented in [40,41] is limited. In our work, we presented a flexible information modulation with a large amplitude control range. And this method can be easily achieved and friendly for integrating with active components like varactors. We believe this work will have various applications in next generation communication and smart home [42]. Take indoor wireless communication scenarios as an example, the presented optically transparent metasurfaces can be easily integrated on the glass of windows to undertake as a signal modulator and transmitter. By developing the advanced modulation methods such as phase and polarization [43–45], such metasurface architecture can be further applied in the next-generation wireless communication for deep channel optimization and fusion.
Funding
SHIEP Foundation (K2014-054, Z2015-086); National Natural Science Foundation of China (11404207); The Local Colleges and Universities Capacity Building Program of the Shanghai Science and Technology Committee, China (15110500900).
Disclosures
The authors declare no conflicts of interest.
References
1. S. N. Burokur, A. Sellier, B. Kante, and A. de Lustrac, “Symmetry breaking in metallic cut wire pairs metamaterials for negative refractive index,” Appl. Phys. Lett. 94(20), 201111 (2009). [CrossRef]
2. A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater. 8(11), 867–871 (2009). [CrossRef]
3. N. Yu and F. Capasso, “Flat optics with designer metasurfaces,” Nat. Mater. 13(2), 139–150 (2014). [CrossRef]
4. Z. Roosta, A. Keshavarz, and G. Honarasa, “Gaussian beam focusing in metamaterial with the second harmonic generation effect as a perfect lens using paraxial group transformation,” Optik 174, 648–654 (2018). [CrossRef]
5. S. S. Islam, M. M. Hasan, and M. R. I. Faruque, “A new metamaterial-based wideband rectangular invisibility cloak,” Appl. Phys. A 124(2), 160 (2018). [CrossRef]
6. F.-Y. Meng, Y. Liang, Q. Wu, and L.-W. Li, “Invisibility of a metamaterial cloak illuminated by spherical electromagnetic wave,” Appl. Phys. A 95(3), 881–888 (2009). [CrossRef]
7. L. Chen, Q. Ma, H. B. Jing, H. Y. Cui, Y. Liu, and T. J. Cui, “Space-Energy Digital-Coding Metasurface Based on an Active Amplifier,” Phys. Rev. Appl. 11(5), 054051 (2019). [CrossRef]
8. Q. Ma, L. Chen, H. B. Jing, Q. R. Hong, H. Y. Cui, and Y. Liu, “Controllable and Programmable Nonreciprocity Based on Detachable Digital Coding Metasurface,” Adv. Opt. Mater. 7(24), 1901285 (2019). [CrossRef]
9. X. Hong, S. Feng, H. Guo, and C. Li, “High-efficiency full-phase modulation of a terahertz wave based on a dielectric metasurface,” Laser Phys. Lett. 16(7), 076202 (2019). [CrossRef]
10. Q. Ma, Q. R. Hong, X. X. Gao, H. B. Jing, and T. J. Cui, “Smart sensing metasurface with self-defined functions in dual polarizations,” Nanophotonics 9(10), 3271–3278 (2020). [CrossRef]
11. Q. Ma, G. D. Bai, H. B. Jing, C. Yang, L. Li, and T. J. Cui, “Smart metasurface with self-adaptively reprogrammable functions,” Light: Sci. Appl. 8(1), 1–12 (2019). [CrossRef]
12. L. Chen, Y. Ruan, and H. Y. Cui, “Liquid metal metasurface for flexible beam-steering,” Opt. Express 27(16), 23282–23292 (2019). [CrossRef]
13. Y. Xie, C. Yang, Y. Wang, Y. Shen, X. Deng, and X. Luo, “The Influence of Incident Modes for polarization conversion in a terahertz metasurface,” Opt. Commun. 435, 341–344 (2019). [CrossRef]
14. Q. Ma, Q. R. Hong, G. D. Bai, H. B. Jing, R. Y. Wu, L. Bao, Q. Cheng, and T. J. Cui, “Editing Arbitrarily Linear Polarizations Using Programmable Metasurface,” Phys. Rev. Appl. 13(2), 021003 (2020). [CrossRef]
15. X. Wan, Y. B. Li, B. G. Cai, and T. J. Cui, “Simultaneous controls of surface waves and propagating waves by metasurfaces,” Appl. Phys. Lett. 105(12), 121603 (2014). [CrossRef]
16. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
17. H. Tao, N. I. Landy, C. M. Bingham, X. Zhang, R. D. Averitt, and W. J. Padilla, “A metamaterial absorber for the terahertz regime: Design, fabrication and characterization,” Opt. Express 16(10), 7181–7188 (2008). [CrossRef]
18. T. Aghaee and A. A. Orouji, “Reconfigurable multi-band, graphene-based THz absorber: Circuit model approach,” Results Phys. 16, 102855 (2020). [CrossRef]
19. J. Kim, H. Jeong, and S. Lim, “Mechanically actuated frequency reconfigurable metamaterial absorber,” Sens. Actuators, A 299, 111619 (2019). [CrossRef]
20. S. Yin, “Reconfigurable Chiral Metasurface Absorbers Based on Liquid Crystals,” IEEE Photonics J. 10(6), 1–9 (2018). [CrossRef]
21. H. Jeong, J.-H. Park, Y.-H. Moon, C.-W. Baek, and S. Lim, “Thermal Frequency Reconfigurable Electromagnetic Absorber Using Phase Change Material,” Sensors 18(10), 3506 (2018). [CrossRef]
22. W. Xu and S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). [CrossRef]
23. B. Ma, S. Liu, X. Kong, Y. Jiang, J. Xu, and H. Yang, “A novel wide-band tunable metamaterial absorber based on varactor diode/graphene,” Optik 127(5), 3039–3043 (2016). [CrossRef]
24. H. Zhai, C. Zhan, L. Liu, and Y. Zang, “Reconfigurable wideband metamaterial absorber with wide angle and polarisation stability,” Electron. Lett. 51(21), 1624–1626 (2015). [CrossRef]
25. H. Zhai, B. Zhang, K. Zhang, and C. Zhan, “A stub-loaded reconfigurable broadband metamaterial absorber with wide-angle and polarization stability,” J. Electromagnetic Waves Appl. 31(4), 447–459 (2017). [CrossRef]
26. Y. Pang, Y. Shen, Y. Li, J. Wang, Z. Xu, and S. Qu, “Water-based metamaterial absorbers for optical transparency and broadband microwave absorption,” J. Appl. Phys. 123(15), 155106 (2018). [CrossRef]
27. G. Dayal and S. A. Ramakrishna, “Broadband infrared metamaterial absorber with visible transparency using ITO as ground plane,” Opt. Express 22(12), 15104–15110 (2014). [CrossRef]
28. Z. Yin, Y. Lu, S. Gao, J. Yang, W. Lai, and Z. Li, “Optically transparent and single-band metamaterial absorber based on indium-tin-oxide,” Int. J. RF Microw. Comput. Aided Eng. 29(2), e21536 (2019). [CrossRef]
29. B. Yu, Y. Zhao, J. Chen, Y. Ge, and X. Chen, “Broadband transparent metamaterial absorber in wireless communication band based on indium tin oxide film,” Int. J. RF Microw. Comput. Aided Eng. 29(12), 21955 (2019). [CrossRef]
30. J. McKenna, “Notable advances in photonics: the JOPT Highlights of 2016,” J. Opt. 19(4), 040401 (2017). [CrossRef]
31. L. Bao, Q. Ma, G. D. Bai, H. B. Jing, R. Y. Wu, and X Fu, “Design of digital coding metasurfaces with independent controls of phase and amplitude responses,” Appl. Phys. Lett. 113(6), 063502 (2018). [CrossRef]
32. K. Zhou, Q. Cheng, J. Song, L. Lu, Z. Jia, and J. Li, “Broadband perfect infrared absorption by tuning epsilon-near-zero and epsilon-near-pole resonances of multilayer ITO nanowires,” Appl. Opt. 57(1), 102–111 (2018). [CrossRef]
33. S. Zhong, L. Wu, T. Liu, J. Huang, W. Jiang, and Y. Ma, “Transparent transmission-selective radar-infrared bi-stealth structure,” Opt. Express 26(13), 16466–16476 (2018). [CrossRef]
34. J. K. Pradhan, S. A. Ramakrishna, B. Rajeswaran, A. M. Umarji, and A. Ghosh, “High contrast switchability of VO2 based metamaterial absorbers with ITO ground plane,” Opt. Express 25(8), 9116–9121 (2017). [CrossRef]
35. S. Shrestha, W. Yu, O. Adam, L. Ming, S. Aaron, and D. N. Luca, “Indium Tin Oxide Broadband Metasurface Absorber,” ACS Photonics 5(9), 3526–3533 (2018). [CrossRef]
36. G. Dayal and S.A. Ramakrishna, “Broadband infrared metamaterial absorber with visible transparency using ITO as ground plane,” Opt. Express 22(12), 15104–15110 (2014). [CrossRef]
37. Q. Zhou, X. Yin, F. Ye, R. Mo, Z. Tang, X. Fan, and L. Zhang, “Optically transparent and flexible broadband microwave metamaterial absorber with sandwich structure,” Appl. Phys. A 125(2), 131 (2019). [CrossRef]
38. T. Zhan, J. Xiong, J. Zou, and S.-T. Wu, “Multifocal displays: review and prospect,” PhotoniX 1(1), 10 (2020). [CrossRef]
39. L. Chen, H. L. Ma, Y. Ruan, and H. Y. Cui, “Dual-manipulation on wave-front based on reconfigurable water-based metasurface integrated with PIN diodes,” J. Appl. Phys. 125(2), 023107 (2019). [CrossRef]
40. Z. H. Chen, J. Tao, J. H. Gu, J. Li, D. Hu, Q. L. Tan, F. C. Zhang, and X. G. Huang, “Tunable metamaterial-induced transparency with gate-controlled on-chip graphene metasurface,” Opt. Express 125(2), 131 (2019). [CrossRef]
41. V. Sorathiya, S.K. Patel, and D. Katrodiya, “Tunable graphene-silica hybrid metasurface for far-infrared frequency,” Opt. Mater. 91, 155–170 (2019). [CrossRef]
42. Q. Ma and T. J. Cui, “Information Metamaterials: bridging the physical world and digital world,” PhotoniX 1(1), 1–32 (2020). [CrossRef]
43. R. Zhao, L. Huang, and Y. Wang, “Recent advances in multi-dimensional metasurfaces holographic technologies,” PhotoniX 1(1), 20 (2020). [CrossRef]
44. L. Chen, Q. Ma, Q. F. Nie, Q. R. Hong, H. Y. Cui, Y. Ruan, and T. J. Cui, “Dual-polarization programmable metasurface modulator for near-field information encoding and transmission,” Photonics Res. 9(2), 116–124 (2021). [CrossRef]
45. Z. Qiao, Z. Wan, G. Xie, J. Wang, L. Qian, and D. Fan, “Multi-vortex laser enabling spatial and temporal encoding,” PhotoniX 1(1), 13 (2020). [CrossRef]
