Multiple Rabi rotations of trions in InGaAs quantum dots observed by photon echo spectroscopy with spatially shaped laser pulses

TL;DR

This study demonstrates multiple Rabi rotations in trions within InGaAs quantum dots using photon echo spectroscopy with spatially shaped laser pulses, revealing insights into coherence and dephasing mechanisms.

Contribution

It introduces flattop laser pulses for uniform excitation and analyzes the effects of charging and phonon coupling on Rabi rotations in quantum dots.

Findings

Robust Rabi rotations observed up to 5.5π pulse area

Photo-induced charging reduces neutral quantum dots under intense excitation

Acoustic phonons are identified as the main decoherence mechanism

Abstract

We study Rabi rotations arising in intensity-dependent photon echoes from an ensemble of self-assembled InGaAs quantum dots. To achieve a uniform distribution of intensities within the excited ensemble, we introduce flattop intensity profiles of picosecond laser pulses. This allows us to overcome the damping of Rabi rotations imposed by the spatial inhomogeneity of Rabi frequencies by a Gaussian laser profile. Using photon echo polarimetry, we distinguish between the coherent optical responses from exciton and trion ensembles. Here, we demonstrate that a photo-induced charging of the quantum dots leads to a significant reduction of the number of neutral quantum dots under resonant excitation with intensive optical pulses with areas exceeding $\frac{\pi}{2}$. The trion ensemble shows robust Rabi rotations when the area of the refocussing pulse is increased up to 5.5$\pi$. We analyze the remaining attenuation of Rabi rotations by theoretical modeling of excitation induced dephasing, inhomogeneity of dipole moments, and coupling to acoustic phonons. The latter is identified as the dominating mechanism resulting in a loss of optical coherence during the action of the involved optical pulses.