Elliptical micropillars for efficient generation and detection of coherent acoustic phonons

Coherent acoustic phonon generation and detection assisted by optical resonances are at the core of efficient optophononic transduction processes. However, when dealing with a single optical resonance, the optimum generation and detection conditions take place at different laser wavelengths, i.e. different detunings from the cavity mode. In this work, we theoretically propose and experimentally demonstrate the use of elliptical micropillars to reach these conditions simultaneously at a single wavelength. Elliptical micropillar optophononic resonators present two optical modes with orthogonal polarizations at different wavelengths. By employing a cross-polarized scheme pump-probe experiment, we exploit the mode splitting and couple the pump beam to one mode while the probe is detuned from the other one. In this way, at a particular micropillar ellipticity, both phonon generation and detection processes are enhanced. We report an enhancement of a factor of ~3.1 when comparing the signals from elliptical and circular micropillars. Our findings constitute a step forward in tailoring the light-matter interaction for more efficient ultrahigh-frequency optophononic devices

Morphology Tailoring and Growth Mechanism of Indium-Rich InGaN/GaN Axial Nanowire Heterostructures by Plasma-Assisted Molecular Beam Epitaxy

We investigate the growth mechanism of axially heterostructured InGaN/GaN nanowires (NWs) as a function of the flux conditions. The InGaN heterostructure morphology critically depends on the In/Ga flux ratio affecting the local V/III ratio at the NW growth front. Locally N-rich conditions are associated with tapered island-like morphologies, while metal-rich conditions, leading to the formation of a stable Indium adsorbed layer at the NW growth front, promote the growth of heterostructures with a disk-like shape. Based on experimental results and theoretical predictions, we demonstrate that this indium ad-layer acts as a surfactant inducing a modification of the InGaN heterostructure growth mode. The impact of flux conditions and strain relaxation on the Indium incorporation are also addressed. The resulting insertions present abrupt interfaces and a homogeneous In distribution for In contents up to 40%