Coherent dynamics of localized excitons and trions in ZnO/(Zn,Mg)O quantum wells studied by photon echoes

TL;DR

This study investigates the coherent optical behavior of excitons and trions in ZnO/(Zn,Mg)O quantum wells at cryogenic temperatures using photon echo techniques, revealing how localization affects coherence times and energy relaxation.

Contribution

It provides the first detailed measurement of coherence times and population dynamics of localized excitons and trions in ZnO-based quantum wells using time-resolved four-wave mixing.

Findings

Localized trions have coherence times up to 60 ps.

Neutral excitons exhibit coherence times up to 4.5 ps.

Energy relaxation from B to A states occurs rapidly (<1 ps).

Abstract

We study optically the coherent evolution of trions and excitons in a $\delta$-doped 3.5 nm-thick ZnO/Zn$_{0.91}$Mg$_{0.09}$O multiple quantum well by means of time-resolved four-wave mixing at temperature of 1.5~K. Employing spectrally narrow picosecond laser pulses in the $\chi^{(3)}$ regime allows us to address differently localized trion and exciton states, thereby avoiding many-body interactions and excitation-induced dephasing. The signal in the form of photon echoes from the negatively charged A excitons (T$_\text{A}$, trions) decays with coherence times varying from 8 up to 60~ps, depending on the trion energy: more strongly localized trions reveal longer coherence dynamics. The localized neutral excitons decay on the picosecond timescale with coherence times up to $T_2=4.5$~ps. The coherent dynamics of the X$_\text{B}$ exciton and T$_\text{B}$ trion are very short ($T_2<1$~ps), which is attributed to the fast energy relaxation from the trion and exciton B states to the respective A states. The trion population dynamics is characterized by the decay time $T_1$, rising from 30~ps to 100~ps with decreasing trion energy.