Nanowire optics

This was my PhD project at the Nanotechnology Institute of Lyon, carried out between 2010 and 2013. The project focused on experimental investigation of the optical properties of InAs/InP nanowire (NW) heterostructures using photoluminescence (PL) and micro-PL (μPL) spectroscopy. A first series of experiments addressed the influence of growth conditions and substrate on the optical emission of InP NWs. We showed that wurtzite InP/InAs/InP core-shell NWs grown on silicon substrates emit at telecommunication wavelengths, and that exciton lifetimes depend sensitively on growth temperature. We also demonstrated that transferring NWs onto foreign host substrates introduces strain through thermal expansion mismatch, which shifts and broadens the PL emission — establishing substrate choice as a critical parameter for optical device integration.

A major focus of the project was the polarization of light emitted and absorbed by InAs/InP NW heterostructures containing quantum rods (QRods) and radial quantum wells (QWells). Single-NW μPL measurements revealed strong linear polarization along the NW axis, consistent with dielectric confinement, and these polarization properties were found to be independent of temperature, in agreement with theoretical simulations. We showed that the polarization anisotropy of NW ensembles can be quantitatively linked to the properties of individual NWs through knowledge of their orientations on the substrate, allowing ensemble-level measurements to yield single-NW information. Using this approach, polarization was characterized as a function of excitation wavelength and temperature, and shown to persist up to room temperature in both quantum dot (QD) and QRod NW configurations.

Finally, we investigated two effects that directly affect the emission efficiency and spectral properties of the heterostructures. Using a PL setup coupled to an integrating sphere, we measured the absolute PL quantum efficiency of InAs/InP QRod and QWell NW heterostructures grown on silicon substrates as a function of excitation power. We also reported evidence of a strain-induced piezoelectric field in wurtzite InAs/InP QRod NWs, arising from the lattice mismatch between InAs and InP. This internal electric field causes a quantum-confined Stark effect that redshifts the emission spectrum, and we showed it can be screened by photogenerated carriers at high excitation power or suppressed by increasing temperature — a behaviour confirmed by simulations of the wurtzite NW band structure.