Spin Dynamics in the Second Subband of a Quasi Two Dimensional System Studied in a Single Barrier Heterostructure by Time Resolved Kerr Rotation

TL;DR

This study investigates spin dynamics in the second subband of a quasi-two-dimensional system using time-resolved Kerr rotation, revealing sign reversal, different decoherence times, and resonant spin exchange effects.

Contribution

It demonstrates a novel method to distinguish spin dynamics of subbands via Kerr rotation sign reversal and uncovers resonant spin exchange coupling effects at specific wavelengths.

Findings

Sign reversal of Kerr rotation distinguishes subband spin dynamics.

The second subband exhibits a decoherence time ten times longer than the first at 803nm.

Resonant spin exchange coupling causes abrupt changes in spin coherence times and g-factors at specific wavelengths.

Abstract

By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as the absorption layer, the spin dynamics for both of the first and second subband near the AlAs barrier are examined. We find that when simultaneously scanning the photon energy of both the probe and pump beams, a sign reversal of the Kerr rotation (KR) takes place as long as the probe photons break away the first subband and probe the second subband. This novel feature, while stemming from the exchange interaction, has been used to unambiguously distinguish the different spin dynamics ($T_2^{1*}$ and $T_2^{2*}$) for the first and second subbands under the different conditions by their KR signs (negative for $1^{st}$ and positive for $2^{nd}$). In the zero magnetic field, by scanning the wavelength towards the short wavelength, $T_2^{1*}$ decreases in accordance with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm, $T_2^{2*}$(450ps) becomes ten times longer than $T_2^{1*}$(50ps). However, the value of $T_2^{2*}$ at 803nm is roughly the same as the value of $T_2^{1*}$ at 815nm. A new feature has been disclosed at the wavelength of 811nm under the bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times ($T_2^{1*}$ and $T_2^{2*}$) and the effective $g^*$ factors ($|g^*(E1)|$ and $|g^*(E2)|$) all display a sudden change, due to the "resonant" spin exchange coupling between two spin opposite bands.