Active terahertz spin state and optical chirality in liquid crystal chiral metasurface

Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to −30.7% of spin-locked circular dichroism at 0.76 THz and −98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

Figure 1

(a) Optical microscope photo and geometric parameters of the metasurface. (b) Schematic diagram of dynamically controllable polarization conversion. (c) Schematic diagram of the spin-locked states and spin-flipped states. (d) Schematic diagram of the THz-TDPS system. (e) Experimental setup for polarization and phase measurement.

Figure 2

Schematic diagram of the geometric symmetry in the propagation direction and the transmission of CP light for the single asymmetric metasurface (a), the single LC layer (b), and the composite LC chiral metasurface (c).

Figure 3

The experimental RCP and LCP transmission spectra (a) and the ellipticity (c) of the asymmetric metasurface when illuminated by an $x$- or $y$-LP light. The simulated RCP and LCP transmission spectra (b) and the ellipticity (d) of the asymmetric metasurface when illuminated by an $x$- or $y$-LP light.

Figure 4

The experimental transmission spectra (a) and the CD (c) of the spin-locked states and spin-flipped states when illuminated by RCP or LCP light. The simulated transmission spectra (b) and the CD (d) of the spin-locked states and spin-flipped states when illuminated by RCP or LCP light.

Figure 5

Experimental THz time-domain signals of the +45° components (a) and −45° components (b) with the bias voltage increase from 0 to 300 V when illuminated by a +45°-LP light. Experimental THz time-domain signals of the +45° components (c) and −45° components (d) with the bias voltage increase from 0 to 300 V when illuminated by a −45°-LP light.

Figure 6

The experimental transmission spectra $T_{RR}$ (a), $T_{LR}$ (b), $T_{LL}$ (c), and $T_{RL}$ (d) with the bias voltage increase from 0 to 300 V when illuminated by RCP or LCP light.

Figure 7

The simulated transmission spectra $T_{RR}$ (a), $T_{LR}$ (b), $T_{LL}$ (c), and $T_{RL}$ (d) with the angle increase from 0° to 90° when illuminated by RCP or LCP light.

Figure 8

The ellipticity of the composite LC chiral metasurface with the bias voltage increase from 0 to 300 V when illuminated by RCP light (a) or LCP light (b). (c) Output polarization states at 0.71 and 1.0 THz with the bias voltage increase from 0 to 300 V when illuminated by RCP light. (d) Output polarization states at 0.48 and 1.0 THz with the bias voltage increase from 0 to 300 V when illuminated by LCP light.

Figure 9

The spin-locked CD (a) and spin-locked OA (c) with the bias voltage increase from 0 to 300 V when illuminated by RCP or LCP light. The spin-flipped CD (b) and spin-flipped OA (d) with the bias voltage increase from 0 to 300 V when illuminated by RCP or LCP light.