Emission Wavelength Instability with Temperature in Low-Dimensional Optical Cavities: The Role of Geometry, Orientation and Interface.

Within an optical nanocavity, the temperature dependence of the emission wavelength defines its emission energy stability. Thermal management, therefore, is crucial in controlling wavelength drift, and/or radiative recombination rates, especially in electrically-driven light sources. Our results show that heat accumulation in a nanocavity directly depends on its geometry, orientation and interface area with the underlying substrate. We use the temperature-dependence of the bandgap to compare the heat retention in nanocavities with lateral and standing orientations. For this purpose, we compare lateral nanofins (fins), and free-standing nanopillars, a.k.a., nanowires (NWs). Hyperspectral cathodoluminescence (CL) microscopy is used to monitor the temperature dependence of the bandgap. Results show that the emission energy of a lateral nanocavity is less dependent on temperature variation compared to that of a standing one. This variation in temperature dependence is attributed to the extent of heat accumulation in the lattice. It is shown that the retained heat in a nanocavity depends on its interface area with the underlying substrate and its surface-to-volume ratio, where the former is the predominant factor. Theoretical modeling shows that introducing a short heat pulse to a standing-nanostructure results in a rapid temperature increase. The initial heating is due to the ultrafast phonon-phonon couplings and build-up of lattice heat. By increasing the interface area between the nanostructure and the substrate the temperature spike disappeared. Results show the interface area has a far more important contribution than a larger surface-to-volume ratio in heat dissipation. These findings are particularly significant in thermal management of electrically-driven optical nanocavities, where operating temperatures could exceed 350 K. As such, while the design and fabrication of efficient nanocavities are essential, controlling their thermal interactions with the surrounding are equally important. Interfacial engineering thus plays a vital role in developing built-in mechanisms for realization of efficient and stable nanoscale light sources.