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

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

Problem and Motivation
The emergence of the terahertz (THz) ratchet effect represents a significant avenue for room-temperature THz applications, particularly in high-speed detection and imaging. This effect relies on breaking spatial symmetry in low-dimensional materials to rectify alternating currents (AC) induced by THz fields into direct currents (DC). While bulk tellurium (Te) and two-dimensional (2D) tellurene are established chiral materials exhibiting chirality-dependent optoelectronic phenomena (such as circular photogalvanic effects), there is a need to explore how extrinsic chirality—created by superimposing asymmetric structures on 2D materials—can drive ratchet currents. Specifically, this work addresses the generation of a DC circular ratchet current in 2D tellurene, a material characterized by helical atomic chains and inherent chirality, when subjected to THz radiation and an asymmetric lateral grating.

Methodology
The study employs a combination of device fabrication, experimental characterization, and microscopic theoretical modeling:

• Device Fabrication: Researchers fabricated asymmetric metal interdigitated gratings on 2D tellurene flakes (approx. 30 nm thick) synthesized via hydrothermal growth. The tellurene was transferred to a p+ Si substrate with a 90 nm SiO₂ layer (serving as a back gate) and capped with a 15 nm Al₂O₃ layer for electrical isolation. The asymmetric grating consists of supercells with two Ti stripes of differing widths ($d_1 = 1.5$ µm, $d_2 = 0.5$ µm) and spacings ($a_1 = 0.5$ µm, $a_2 = 2.5$ µm), creating a period $d$.
• Experimental Setup: Experiments were conducted at room temperature. The devices were excited by normal-incidence pulsed THz laser radiation at frequencies of 1.07 THz and 2.02 THz. Radiation helicity was controlled using a $\lambda/4$ plate, varying the polarization from linear to right- ($\sigma^+$) and left-handed ($\sigma^-$) circular.
• Measurement: Induced photocurrents were detected as voltage drops across 50 $\Omega$ load resistors. The back gate voltage ($U$) was swept to modulate the carrier concentration and type (electrons or holes), with the charge neutrality point (CNP) identified at approximately -3 V.
• Theoretical Approach: The authors developed a microscopic theory based on the Boltzmann kinetic equation. The model considers the simultaneous action of a static electrostatic force (from the grating potential) and a dynamic force (from the near-field diffraction of the THz radiation). The theory accounts for both parabolic energy dispersion (valence band) and linear dispersion near Weyl points (conduction band), as well as short-range and long-range (Coulomb) disorder scattering.

Key Results

• Observation of Circular Ratchet Current: The experiments demonstrated a DC photocurrent that reverses direction when the helicity of the THz radiation is switched from right- to left-handed. This current flows along the chiral axis ($c$) of the tellurene.
• Role of Asymmetry: Reference samples without the asymmetric grating showed no circular photocurrent, confirming that the effect arises from the ratchet mechanism driven by the broken spatial inversion symmetry of the grating.
• Gate Voltage Dependence: The circular ratchet current was observed across a range of gate voltages:
  • In the vicinity of the Weyl point in the conduction band (positive gate voltages).
  • In the band gap (near the CNP).
  • In the valence band with almost parabolic dispersion (negative gate voltages).
  • The current direction reverses at small negative effective gate voltages.
• Frequency and Intensity Dependence: The current scales linearly with radiation intensity ($J \propto E^2$). Measurements at 2.02 THz showed a dominance of linear photocurrent components over the circular ones compared to the 1.07 THz data, though circular effects remained detectable.
• Theoretical Agreement: The developed Boltzmann-based theory successfully describes the experimental data. It attributes the current to the modulation of the electron distribution by the combined static and dynamic fields. The theory explains the sign reversal of the current near the CNP as a consequence of the carrier charge changing sign (electrons vs. holes) and the different scattering mechanisms (short-range vs. Coulomb) affecting the ratchet coefficient $\gamma$ in parabolic versus linear dispersion regimes.

Significance and Claims
The paper claims to demonstrate a THz-driven circular ratchet effect in 2D tellurene at room temperature, unifying the physics of chiral materials with the ratchet effect. The primary significance lies in:

1. Material Discovery: Establishing 2D tellurene as a promising platform for chirality-dependent optoelectronic phenomena, specifically utilizing its inherent helical atomic structure.
2. Mechanism Validation: Providing experimental evidence that an asymmetric grating on a chiral 2D material can generate a helicity-driven DC current, a mechanism distinct from intrinsic circular photogalvanic effects.
3. Theoretical Framework: Offering a microscopic theory that links the ratchet current to the specific energy dispersion (Weyl vs. parabolic) and scattering mechanisms of the charge carriers, accurately predicting the gate voltage and frequency dependencies.
4. Technological Potential: The observation of this effect at room temperature suggests a pathway for developing novel, high-speed, sensitive THz detectors and imaging systems.

The authors modestly note that while the current is detected, its magnitude could potentially be enhanced by optimizing the lateral asymmetry parameter ($\Xi$) or tuning the structure to plasmonic resonances. They also suggest that future work could explore quantum-mechanical regimes by reducing the structure period below the mean free path.