Active manipulation of terahertz (THz) beam deflection and intensity is highly desired for possible applications in wireless communication, radar, and remote sensing. Here, by integrating the phase-gradient metasurfaces and tunable liquid crystal materials, we demonstrate an active THz beam deflection device based on polarization mode conversion. The resonant modes in the photonic cavity formed by the double-layer metasurface and the tunable anisotropic liquid crystal material in the cavity not only improve the polarization conversion efficiency of the device, but also actively regulate the resonance matching conditions. As a consequence, a beam deflection of 47.5° with 50% diffraction intensity at 0.69 THz is achieved in the x-to-y polarization conversion mode, and this beam can be actively modulated with an ultrahigh modulation depth of 99.6% by rotating the anisotropic optical axis of liquid crystals. Moreover, the proposed device can also work as the deflection of 32.5° in the y-to-x polarization conversion mode at 0.94 THz with a maximum diffraction intensity of 38% and an intensity modulation depth of 97.8%. This work provides a new approach based on liquid crystal photonic devices for wavefront manipulation and active modulation for THz waves.
Metasurfaces transform the wavefront by spatially varying the amplitude or phase of the incoming beam. Instead of encoding such variation by subwavelength unit cells, it is achievable over diffraction engineering of supercell structures, which outperforms the unit-cell method when the spatial gradient is large. In addition to tight focusing, here we apply this method to achieve plane wave-to-Bessel beam transformation and point-to-point focusing at terahertz frequencies. The Bessel beam has a small beam waist (0.57λ) and long depth of focus (9.1λ) for subwavelength-resolution imaging over a long distance. The point-to-point focusing changes the divergence angle from 16° to 70°. Both devices are validated by numerical simulations and experimental results with good agreement.
Metasurface-based beam splitters attracted huge interest for their superior properties compared with conventional ones made of bulk materials. The previously reported designs adopted discrete metasurfaces with the limitation of a discontinuous phase profile. In this paper, we propose a dual-band beam splitter, based on an anisotropic quasi-continuous metasurface, by exploring the optical responses under x-polarized (with an electric field parallel to the direction of the phase gradient) and y-polarized incidences. The adopted metasurface consists of two identical trapezoidal silicon antenna arrays with opposite spatial variations that lead to opposite phase gradients. The operational window of the proposed beam splitter falls in the infrared and visible region, respectively, for x- and y-polarized light, resulting from the different mechanisms. When x-polarized light is incident, the conversion efficiency and total transmission of the beam splitter remains higher than 90% and 0.74 within the wavelength range from 969 nm to 1054 nm, respectively. In this condition, each array can act as a beam splitter of unequal power. For y-polarized incidence, the maximum conversion efficiency and transmission reach approximately 100% and 0.85, while the values remain higher than 90% and 0.65 in the wavelength range from 687 nm to 710 nm, respectively. In this case, each array can be viewed as an effective beam deflector. We anticipate that it can play a key role in future integrated optical devices.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.