Chirality Driven Ratchet Currents in Two-Dimensional Tellurene with an Asymmetric Grating

TL;DR

The study demonstrates a THz helicity-driven ratchet current in a 2D tellurene sheet bearing an asymmetric grating, yielding a DC current along the chiral axis that reverses with light helicity. It combines a semiclassical Boltzmann-kinetic framework with device-level experiments at room temperature, showing gate-tunable circular photocurrents that depend on carrier dispersion (Weyl-like vs nearly parabolic) and disorder. The key contribution is a quantitative link between the lateral asymmetry parameter Xi and the circular ratchet amplitude gamma, explaining the observed gate- and frequency-dependent current, its E^2 scaling, and helicity reversal. The work highlights potential for room-temperature THz rectification in chiral 2D materials and points to plasmonic design and miniband engineering as routes to enhance performance.

Abstract

The emergence of the terahertz (THz) ratchet effect is a rapidly expanding field of research that utilizes broken spatial symmetry in low-dimensional materials to rectify alternating current (AC) induced by THz fields into direct current (DC). This mechanism is highly promising for next-generation, room-temperature terahertz applications, particularly in high-speed, sensitive detection and imaging. In this work, we explore a ratchet effect generated in two dimensional tellurene, a novel promising semiconductor material consisting of helical atomic chains, creating a structure with inherent chirality. As a key result, the DC circular ratchet current flowing in the chiral axis direction $c$ is determined by the helicity of the radiation and can be reversed by switching the helicity from right to left handed. The circular ratchet effect excited by THz laser radiation is demonstrated for room temperature. The effect is demonstrated at various gate voltages when the Fermi level lies in vicinity of the Weyl point in the conduction band, in the band gap, and in the valence band with almost parabolic energy dispersion. The results are described by the developed microscopic theory based on the Boltzmann kinetic equation approach.

Figures (12)

• Figure 1: Panels (a) and (b) show an optical micrograph and a cross-section of the ratchet structure. Tellurene flake is deposited on top of $p+$ Si substrate covered by 90 nm SiO$_2$ layer. Nickel contacts were fabricated by electron beam evaporation on the short sides of the rectangular flakes, which were $46 \times 23~\mu$m$^2$. To cap the Te flakes, we deposited a 15-nm-thick layer of Al$_2$O$_3$. The asymmetric grating consists of supercells, each formed by two Ti stripes having widths $d_1 = 1.5$ and $d_2 = 0.5$$\mu$m, spacings $a_1 = 0.5$ and $a_2=2.5$$\mu$m, and thickness 50 nm. The supercell is repeated $8$ times forming the asymmetric grating with the period $d = d_1 + a_1 +d_2 +a_2$. Panel (c) shows the two-point resistance as a function of the back gate voltage. Panel (d) sketches the experimental set-up.
• Figure 2: The normalized photocurrent, $J/P_{\rm s}$, as a function of the lambda-quarter rotation angle, $\varphi$. The data are shown for a radiation frequency $f=1.07$ THz and several effective back gate voltages ranging from hole- ($U_{\rm G} < 0$) to electron-conductivity ($U_{\rm G} > 0$). The solid lines represent the corresponding fits according to Eq. \ref{['phi']}. The fitting coefficients are plotted in Fig. \ref{['fig4']} and Fig. S4 of SI. Vertical arrows indicate the right- ($\sigma^+$) and left-handed ($\sigma^-$) circular polarizations, and the ellipses on top illustrate the polarization states at various angles $\varphi$.
• Figure 3: Phase angle $\varphi$ and effective gate voltage dependencies of the color-coded photocurrent strength $(J-J_0)/P_{\rm s}$ as a function of the angle $\varphi$. The data are obtained for $f=1.07$ THz.
• Figure 4: Dependencies of the circular photocurrent excited by radiation with frequencies $f=1.07$ and $f=2.02$ THz on the effective gate voltage $U_{\rm G}$. Insets sketches indirect (Drude-like) optical transitions in the valence (left, negative gate voltages) and conduction (right, positive gate voltages) energy bands.
• Figure 5: Dependencies of the photocurrent (colored symbols) excited by radiation with $f=2.02$ THz on the rotation angle $\varphi$ of the $\lambda/4$ wave plate. The data are shown for several effective back gate voltages ranging from hole ($U_{\rm G} < 0$) to electron conductivity ($U_{\rm G} > 0$). The photoresponse is normalized to the laser power $P_{\rm s}$ incident on the sample. The vertical arrows in each panel mark the right-handed ($\sigma^+$) and left-handed ($\sigma^-$) circular polarization states, respectively. The solid lines represent the corresponding fits according to Eq. \ref{['phi']}. The fit coefficients are plotted in Fig. \ref{['fig4']} and Fig. S5 of SI.