Detection of thermodynamic "valley noise" in monolayer semiconductors: access to intrinsic valley relaxation timescales

TL;DR

This paper introduces a passive, noise-based optical method to measure intrinsic valley relaxation timescales in monolayer transition-metal dichalcogenides, revealing long relaxation times crucial for valleytronic applications.

Contribution

It demonstrates a novel, non-perturbative approach using valley-specific optical Faraday rotation to detect spontaneous valley fluctuations in 2D semiconductors.

Findings

Valley noise spectra show narrow Lorentzian lineshapes indicating long relaxation times.

The method validates relaxation times obtained from conventional pump-probe techniques.

Provides a new way to measure intrinsic valley dynamics without external excitation.

Abstract

Together with charge and spin degrees of freedom, many new 2D materials also permit information to be encoded in an electron's valley degree of freedom - that is, in particular momentum states in the material's Brillouin zone. With a view towards future generations of valley-based (opto)electronic technologies, the intrinsic timescales of scattering and relaxation between valleys therefore represent fundamental parameters of interest. Here we introduce and demonstrate an entirely passive, noise-based approach for exploring intrinsic valley dynamics in atomically-thin transition-metal dichalcogenide (TMD) semiconductors. Exploiting the valley-specific optical selection rules in monolayer TMDs, we use optical Faraday rotation to detect, under conditions of strict thermal equilibrium, the stochastic thermodynamic fluctuations of the valley polarization in a Fermi sea of resident carriers. Frequency spectra of this spontaneous "valley noise" reveal narrow Lorentzian lineshapes and therefore long exponentially-decaying intrinsic valley relaxation. Moreover, the valley noise signals are shown to validate both the relaxation times and the spectral dependence of conventional (perturbative) pump-probe measurements. These results provide a viable route toward quantitative measurements of intrinsic valley dynamics, free from any external perturbation, pumping, or excitation.