Coherent Control for a Two-level System Coupled to Phonons

TL;DR

This paper demonstrates how coherent control techniques can manipulate four-wave mixing signals in a two-level semiconductor system coupled to phonons, revealing interference effects and phonon-induced spectral features.

Contribution

It provides an exact evaluation of four-wave mixing in a two-level phonon-coupled system, highlighting the effects of coupling strength and pulse timing on interference and spectral features.

Findings

Oscillations at phonon frequency in weak coupling regime

Sharp peaks at multiples of phonon period for strong coupling

Splitting of phonon peaks into a doublet due to destructive interference

Abstract

The interband polarizations induced by two phase-locked pulses in a semiconductor show strong interference effects depending on the time tau_1 separating the pulses. The four-wave mixing signal diffracted from a third pulse delayed by tau is coherently controlled by tuning tau_1. The four-wave mixing response is evaluated exactly for a two-level system coupled to a single LO phonon. In the weak coupling regime it shows oscillations with the phonon frequency which turn into sharp peaks at multiples of the phonon period for a larger coupling strength. Destructive interferences between the two phase-locked pulses produce a splitting of the phonon peaks into a doublet. For fixed tau but varying tau_1 the signal shows rapid oscillations at the interband-transition frequency, whose amplitude exhibits bursts at multiples of the phonon period.