P.-O. Chapuis

In a thermophotonic device used in an energy-harvesting configuration, a hot light-emitting diode (LED) is coupled to a photovoltaic (PV) cell by means of electroluminescent radiation in order to produce electrical power. Using fluctuational electrodynamics and the drift-diffusion equations, we optimize a device made of an AlGaAs PIN LED and a GaAs PIN PV cell with matched bandgaps. We find that the LED can work as an efficient heat pump only in the near field, where radiative heat transfer is increased by wave tunneling. A key reason is that non-radiative recombination rates are reduced compared to radiative ones in this regime. At 10 nm gap distance and for 100 cm s−1 effective surface recombination velocity, the power output can reach 2.2 W cm−2 for a 600 K LED, which highlights the potential for low-grade energy harvesting.

Understanding how thermal-phonon paths can be shaped is key for controlling heat dissipation at the nanoscale. Thermophononic crystals are periodic porous nanostructures with thermal conductivity deviating from effective medium theory, which is possible if the characteristic sizes are of the order of phonon mean free paths and/or if phonons are forced to flow in privileged directions. We investigate suspended silicon nanomembranes with a periodic array of partially perforated holes of original paraboloid shape, with all characteristic lengths below 100 nm. Results from scanning thermal microscopy, a thermal sensing technique derived from atomic force microscopy, indicate that partial perforation of the membranes impacts heat conduction moderately, with the holey crystals showing a thermal conductivity reduction by a factor 6 in comparison to the bulk and a factor 2.5 in comparison to the non-perforated membrane. The impact of the phononic shapes is analyzed in light of a complementary Monte Carlo ray-tracing estimate of the effective phonon mean free paths that include multiple phonon reflection and highlights phonon backscattering.