Spatial mapping of quantum-dot dynamics across multiple timescales at low temperature using remote asynchronous optical sampling

Abstract

Quantum dots (QDs) offer significant potential for applications in quantum information and optoelectronic devices; however, conventional time-resolved spectroscopy cannot generally simultaneously extract both long-lived relaxation dynamics and short-lived quantum beats from ensemble measurements. This limitation arises from the inherent trade-off between temporal resolution and total acquisition time. Here, we demonstrate that asynchronous optical sampling based on a fiber-delivered frequency comb enables simultaneous observation of QD dynamics across multiple timescales. By integrating a galvanometric scanner, we achieve spatial mapping over a $1 \times 1$-\si{\milli\meter}$^2$ area at 441 discrete points in 30.1~min, a measurement that would otherwise require more than 12~days. At each location, both quantum beats and relaxation lifetimes are resolved, giving physical insights into QD ensembles that were previously inaccessible and paving the way for rapid feedback in device fabrication.

Figures (8)

• Figure 1: (a) Cross-sectional schematic of the sample: a 50-period InAs QD stack embedded between upper and lower distributed Bragg reflectors (DBRs), forming an optical resonator. (b) PL spectrum of the sample (black), dominated by the exciton ground-state transition, along with the spectra of the pump (red) and probe (blue) combs after band-pass filtering.
• Figure 2: Schematic of asynchronous optical sampling. Definitions: $f_{\mathrm{rep,probe}}$ and $f_{\mathrm{rep,pump}}$, repetition frequencies of the probe and pump pulse combs; $\Delta t$, incremental pump--probe delay for each pulse pair.
• Figure 3: Experimental setup for spatial mapping of quantum-dot dynamics using OPOP-ASOPS. Definitions: DCF, dispersion-compensating fiber; EDFA, erbium-doped fiber amplifier; PC, polarization controller; PD, photodetector; Col., collimating lens; Pol., polarizer; PBS, polarizing beam splitter; BPF, band-pass filter; GS, galvanometric scanner; LPF, low-pass filter.
• Figure 4: Temporal pulse widths measured by intensity autocorrelation immediately before the sample (after the band-pass filter): (a) probe comb and (b) pump comb.
• Figure 5: Waveform measured at $X=0.5$ mm, $Y=0.5$ mm: (a) full trace up to 15ns with the baseline $c$ removed, showing the fitting function $I_{\text{longitudinal}}(t)$ as an orange line; (b) enlarged view of the $\pm500ps$ window with the fitting curve $I_{\text{longitudinal}}(t)+I_{\text{beat}}(t)+d$ (inset: schematic of exciton energy levels in the QDs); (c) quantum-beat signal obtained from the baseline-corrected data after subtracting the slow exponential decay component $I_{\text{longitudinal}}(t)$ and the residual offset $d$. The orange curve shows the fitted $I_{\text{beat}}(t)$.