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.

• The paper introduces an ASOPS-based method that simultaneously resolves sub-nanosecond quantum beats and nanosecond relaxation in quantum dots.
• It achieves sub-10 ps temporal resolution over a 16.2 ns window, mapping key parameters like T1, δ, and T2,sub with high throughput across 441 spatial points.
• The approach reveals spatial correlations in QD dynamics, informing optimization strategies for quantum light sources and scaling of quantum devices.

Spatially Resolved, Multi-Timescale Quantum Dot Dynamics via Asynchronous Optical Sampling

Introduction

This work presents a technique for spatially resolved mapping of quantum dot (QD) dynamics at cryogenic temperatures using asynchronous optical sampling (ASOPS) with a remotely delivered frequency comb source. The study directly addresses the fundamental limitation in conventional optical pump–optical probe (OPOP) time-resolved spectroscopy: the inability to simultaneously resolve both sub-nanosecond quantum beat phenomena and nanosecond-scale relaxation dynamics across large sample areas due to an inherent trade-off between temporal resolution, time window, and mechanical scan throughput.

Methodology

The system leverages two phase-locked, erbium-doped fiber frequency combs with a controlled repetition frequency detuning to realize asynchronous sampling. This architecture obviates mechanical delay lines, thus allowing rapid automatic time-delay sweeps with sub-10 ps resolution over multi-nanosecond time windows. The comb light is delivered over a 419-meter telecom single-mode fiber, enabling spatial separation of the comb source and cryogenic measurement apparatus, significantly expanding experimental flexibility.

Spatial mapping is accomplished via a galvanometric scanner, which rasterizes the probe across a 1 × 1 mm² sample region with 50 μm steps (441 points). At each pixel, a complete time-domain OPOP trace is acquired with just 2.56 seconds of integration—orders of magnitude faster than traditional approaches.

Experimental Details

The sample comprises a multilayer InAs quantum dot ensemble embedded in an InP-based distributed Bragg reflector cavity, with an areal QD density near 10¹² cm⁻². The PL spectrum displays a ground state exciton emission at 1567 nm with 18 nm inhomogeneous broadening, aligning well with the 1550 nm operation band of the employed frequency combs. Polarization control enables excitation of coherent superpositions of the QD bright-exciton doublet states, necessary for quantum beat detection.

The effective experimental time resolution is determined to be ~6 ps, dictated by the detector bandwidth and pulse durations. The system achieves a measurement window of 16.2 ns, capturing both quantum coherence phenomena ($T_{2,sub}$, $\delta$) and population relaxation ($T_1$) in single, uninterrupted scans.

Results

Simultaneous Multiscale Dynamics

At each spatial coordinate, the extracted time-domain trace reveals both rapidly oscillating quantum beats (damped within $\sim$100 ps) and nanosecond-scale exponential relaxation, accessible due to the high temporal resolution and large observation window. From fits to the experimental data, the key parameters extracted per position are:

• Longitudinal relaxation time $T_1$ (mean: 1.28 ns, CV 0.78%)
• Exciton fine-structure splitting $\delta$ (mean: 61.2 μeV, CV 1.18%)
• Ensemble dephasing time $T_{2,sub}$ (mean: 24.5 ps, CV 3.1%)
• Peak differential transmission $\Delta T/T$ (mean: $3.45 \times 10^{-2}$, CV 5.4%)

The method demonstrates robust discrimination between relaxation and coherence metrics, with sub-ps and sub-μeV measurement precision (low standard deviations in extracted parameters across the map).

High-Throughput Spatial Mapping

ASOPS enables acquisition of the full spatial map (441 points) in just 30.1 minutes. The same task with a mechanical delay line and lock-in averaging—assuming a modest 1 s per time point—would require $\sim$12.5 days. This high throughput opens practical routes to statistically and spatially relevant studies of sample inhomogeneity, a critical aspect in quantum device evaluation.

Inhomogeneity and Correlations

Spatial maps and histograms reveal small but statistically significant variations in $\delta$0, $\delta$1, $\delta$2, and $\delta$3, attributed to local variations in strain, defects, or cavity detuning. Correlation analysis shows:

• A positive correlation between $\delta$4 and $\delta$5 ($\delta$6, $\delta$7), implying regions of large fine-structure splitting also exhibit longer coherence times, consistent with local structural variations.
• Negative correlations of $\delta$8 with $\delta$9 and $T_1$0 (–0.23 and –0.21, both significant), indicating that increased structural asymmetry shortens relaxation time and reduces pump/probe coupling.

The magnitude and sign of these correlations persist under Monte Carlo resampling and variations of analysis windows, supporting their physical relevance.

Implications and Future Developments

This work establishes ASOPS as a powerful tool for rapid, high-resolution, multiscale mapping of QD ensemble dynamics at cryogenic temperatures. By decoupling the frequency comb source from the low-temperature probe site via telecommunication fiber, it paves the way for integration with advanced material platforms and the centralization of costly comb resources.

From a device-engineering perspective, this technique enables routine, high-throughput, spatially resolved feedback on QD inhomogeneity, a key parameter space for optimization in quantum light sources, quantum gate operations, and nanophotonics. The robust identification of systematic spatial correlations between fine-structure splitting and dynamical parameters provides a framework for diagnosing growth-induced and process-induced variability. These insights are critical for the deterministic control and scalability of quantum photonic devices.

The approach is broadly generalizable to other systems requiring both high temporal resolution and spatial mapping—such as complex perovskite nanomaterials, hybrid quantum wells, or emergent two-dimensional quantum materials. Expansion to ultrafast studies in the terahertz regime and implementation with higher repetition rate combs or larger area scanners are promising routes for further extension.

Conclusion

ASOPS-based OPOP enables simultaneous, spatially resolved measurement of coherent and incoherent exciton dynamics in quantum dot ensembles at low temperature. The approach bridges a longstanding gap between throughput, time-resolution, and spatial mapping in quantum material characterization, with direct application to the development of quantum devices. The observed parameter distributions and interdependencies provide useful feedback for QD fabrication, and the demonstrated architecture suggests a general roadmap for multiscale spatial mapping experiments in diverse nanomaterial systems.

(2604.03041)