Layer-Dependent Spin Properties of Charge Carriers in Vertically Coupled Telecom Quantum Dots

Abstract: We investigate the spin properties of charge carriers in vertically coupled InAs/InAlGaAs quantum dots grown by molecular beam epitaxy, emitting at telecom C-band wavelengths, with a silicon $\delta$-doped layer. Using time-resolved pump-probe Faraday ellipticity measurements, we systematically study single-, two-, and four-layer quantum dot (QD) configurations to quantify how vertical coupling affects key spin-coherence parameters. Our measurements reveal distinct layer-dependent effects: (1) Adding a second QD layer flips the resident charge from electrons to holes, consistent with optically induced electron tunneling into lower-energy dots and resultant hole charging. (2) Starting from the four-layer sample, the pump-probe signal develops an additional non-oscillating, decaying component absent in single- and two-layer samples, attributed to multiple layer growth changing the strain environment, which reduces heavy-hole and light-hole mixing. (3) With four-layers or more, hole spin mode locking (SML) can be observed, enabling quantitative extraction of the hole coherence time $T_2 \approx 13$\,ns from SML amplitude saturation. We also extract longitudinal spin relaxation ($T_1$) and transverse ($T_2^*$) spin dephasing times, g-factors, and inhomogeneous dephasing parameters for both electrons and holes across all layer configurations. The hole spin dephasing times $T_2^*$ remain relatively constant (2.26-2.73\,ns) across layer counts, while longitudinal relaxation times $T_1$ decrease with increasing layers (from 1.03\,$\mu$s for single-layer to 0.31\,$\mu$s for four-layer samples). These findings provide potential design guidelines for engineering spin coherence in telecom-band QDs for quantum information applications.