Nonlinear Optical studies of the Transient Coherence in the Quantum Hall System

TL;DR

This paper reviews femtosecond nonlinear optical responses in quantum Hall systems, highlighting how Coulomb correlations and magnetoplasmon excitations influence transient coherence and produce distinctive four-wave mixing signals.

Contribution

It introduces a combined experimental and theoretical approach to study interband and intraband coherences in 2DEG under quantum Hall conditions, revealing the role of many-particle interactions.

Findings

Coulomb correlations significantly affect nonlinear optical signals.

Intraband many-particle coherences dominate femtosecond response.

Observation of large off-resonant four-wave-mixing signals at low carrier densities.

Abstract

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors $\nu \sim 1$. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical responce. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.