Optical Properties of Nanowire-based Quantum Dots across the Near-Infrared

Abstract

Ideal single-photon sources are key elements to realizing quantum technologies such as quantum information processing and cryptography, using photons as the smallest unit to carry information. Solid-state two-level emitters are considered promising candidates for generating single photons with high efficiency, high single-photon purity and a high degree of indistinguishability. In particular, quantum dots, made of binary semiconductor (e.g. II-VI, III-V) materials, are artificial atoms that exhibit outstanding optical performance. A well-studied example is the InGaAs/GaAs self-assembled quantum dot system. However, these dots are typically formed through random nucleation on a sample substrate. To overcome this barrier, a position-controlled nanowire-based quantum dot system, made of InAsP/InP will be investigated in this work. This thesis will first introduce the growth technique of embedding a disk-shaped quantum dot (InAsP) in a nanowire waveguide (InP). The optical properties of three different samples with tunable emission wavelengths are demonstrated. A tuning range from 890 nm to 1530 nm is achieved using two approaches. By varying the size and the As composition of the dot, the emission wavelengths can reach up to ∼1 µm. Furthermore, devices can be made to operate in the telecom C-band by employing a dot-in-a-rod structure. Time-resolved photoluminescence measurements and second-order auto-correlation measurements are conducted under different excitation schemes (above-band, p-shell and resonant excitation) to investigate the optical performance of these samples. For the first time, strictly resonant excitation was applied to the nanowire quantum dot system. Photoluminescence spectra, coherent control, and auto-correlation measurements demonstrated that the pump laser’s overlap with the quantum dot emission can be effectively suppressed using a polarization-rejection optical setup. Additionally, experimental results suggest that resonant excitation significantly reduces the excitation timing jitter compared to above-band excitation. Lastly, this work focuses on the coherence properties and two-photon indistinguishability of the photons emitted from different samples. Linewiths were measured using high-resolution photoluminescence spectroscopy with a Fabry-Pérot etalon and the two-photon indistinguishability was investigated with Hong-Ou-Mandel interferometry. Both measurements were performed under above-band and resonant excitation. The linewidths and the visibilities of the two-photon interference consistently demonstrated the impact of excitation methods on the coherence properties and aligned well with findings from other quantum dot systems.