Reconfigurable Terahertz Metasurface Pure Phase Holograms

Abstract

of applications. Spatial light modulators, which can impose programmable modification of the spatial intensity and/or phase distribution of an optical beam, are commonly used in applications, such as optical information processing, imaging, and communications. However, in the THz frequency range, the development of spatial light modulators remains a great challenge due to the lack of electrooptical materials, although they are urgently requested with the ongoing advancement of THz technology and the identification of a variety of potential applications. [5] While a few spatial THz intensity modulators have already been proposed, [6][7][8][9] a pure phase spatial modulation can greatly increase the immunity to noise, therefore is more robust and applicable in THz systems where the intensity of THz radiation is typically low. Steinbusch et al. used the photogenerated graded index grating to actively steer the THz beam; the phase modulation was considered. [10] However, the phase modulation is caused by the change of reflection index of the thin semiconductor film pumped by the visible light, which is quite small. Other attempts used semiconductor quantum well structures at cryogenic temperatures and liquid crystals with low modulation speed. [11,12] More recently, metamaterials/ metasurfaces integrated with semiconductor and graphene have allowed more efficient modulation of THz phase. [13,14] However, the range of THz phase modulation needs further improvement, and one should also address the drawback of accompanied intensity modulation associated with the resonant response.Despite these limitations to overcome, the emergence of metasurfaces has provided an excellent opportunity to accomplish a variety of functional THz devices. [15][16][17] Metasurfaces are constructed from carefully designed discrete metallic or dielectric elements and have spatially varying characteristics across its surface, [18] enabling effective manipulation of the amplitude, phase, and polarization state of electromagnetic waves with continuously broadband achromatism for ultrathin planar lenses, [19,20] complex light field generation, [21] spin orbital angular momentum coupling, [22,23] and metasurface holograms. [24,25] Activating these functionalities will no doubt expand the application scope and create a greater societal impact of metasurfaces, which necessitates the integration of functional materials within the metasurface structures, including most widely used semiconductors, phase transition materials, and Terahertz (THz) radiation has many potential applications. However, comparing with the rapid development of THz sources and detectors, functional devices for THz modulation, especially the spatial modulation devices, are still insufficient. Here, a novel approach for generating arbitrary wavefronts of a THz beam is presented. By dynamically creating metasurface structures through illuminating a thin silicon wafer with femtosecond laser, which is spatially modulated, an array of reconfigurable subwavelength resonators is generate...

Terahertz Metasurfaces: Toward Multifunctional and Programmable Wave Manipulation

“…In the terahertz and optical spectrum, tiny dimensions and immature technics result in the functional multiplexing facing great difficulties. Although more hardships are confronted compared to merely achieving function tuning, some active metasurfaces with dynamically functional multiplexing have still been implemented [91][92][93][94][95][96].…”

“…Measured results of focus shifting and the vertical astigmatism are respectively depicted in Figures 10B,C. For further verifying the mechanical robustness of the lens, the strain field was applied more than 1,000 cycles and no image degradation was observed. More recently, a novel photoexcited approach has been presented and applied to a thin silicon wafer for arbitrary terahertz wavefronts shaping [95], apart from the above electrically controllable metasurfaces. In such a work, inhomogeneity of the metasurface was caused by a local IMT in the silicon wafer, and Femtosecond laser was chosen as the photoexcitation.…”

Real-time terahertz meta-cryptography using polarization-multiplexed graphene-based computer-generated holograms

“…Since the intrinsic loss of graphene layers, the full reflection phase span of 360°has not been achieved; however, the phase span of 315°is sufficient to provide good performance in holographic applications [67]. Moreover, such a wide phase modulation range has been accomplished without need to any imposed polarization conversion seen in the conventional architectures [68]. The DC voltage needed for tuning the chemical potential of graphene is additionally studied in Supplementary Information B to ensure no breakdown occurs in the structure.…”

A Review of THz Modulators with Dynamic Tunable Metasurfaces

“…Recently, Chen et al proposed a new method for generating wavefronts of arbitrary THz beams, as shown in Figure 8b. The hologram and zoom lens can be realized in real time by dynamically controlling the direction of the resonator [72]. Except as the dielectric metasurface provided, the metal metasurface also has outstanding performance in holographic imaging [73,74].…”