THz ratchet effect in HgTe interdigitated structures

TL;DR

This paper reports the observation of THz-induced ratchet effects in HgTe dual-gate structures, revealing how band structure and quantum well thickness influence the magnitude and sign of the ratchet currents, with potential for THz applications.

Contribution

It demonstrates the presence of linear, circular, and polarization-independent ratchet effects in HgTe structures and links these effects to band structure properties and quantum well thickness.

Findings

Ratchet effects are observed in HgTe-based dual-gate structures.

Signal magnitude increases with decreasing quantum well width, peaking at Dirac fermions.

Gate voltage oscillations cause sign changes and are linked to valence band complexity.

Abstract

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in HgTe-based dual-grating-gate structures hosting different band structure properties. Applying polarized terahertz laser radiation we detected linear and polarization independent ratchets, as well as an radiation-helicity driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness we observed that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observed sign-alternating oscillations for gate voltages corresponding to p-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for n-type conducting QWs. The oscillations and the signal enhancement are shown to be caused by the complex valence band structure of HgTe-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of THz applications.