One of the main challenges in nanophononics, i.e. the engineering of acoustic phonons at the nanoscale, is the absence of standard transducers to generate, control and detect acoustic phonons. The most used experimental techniques to study phonon dynamics are thus based on optical techniques. During the project, a considerable part of my team research time was devoted to setting up three complementary experiments to study phonon dynamics at ultra-high frequencies, essential for the achievement of the main objectives.
-High-resolution Brillouin/Raman scattering setup: this spectroscopy technique relies on a continuous-wave laser probing the phonon population in a sample by inelastic scattering. We coupled a double spectrometer and optimized the full optical setup to measure in the 15-1000 GHz range. The most important improvements in the experiment were the development of a novel stray light filtering technique based on the diffracted light from a single micropillar that allowed us to measure Brillouin scattering at 40 GHz from a micron-sized object, the coupling of an etalon filter, and the investigation of novel measuring geometries enhancing our laser-filtering efficiency such as the use of single mode optical fibers to spatially filter the signals, or the polarization rotation as an alternative technique to access signal down to 18 GHz from the laser.
-Coherent Phonon Generation in a Pump-Probe scheme: using ps/fs pulsed lasers we study the phonon dynamics in multilayered structures. We implemented a scanning technique that allowed us to spatially map the intensity of the generated phonon wave-package around the excitation spot, as well as accessing transport of high frequency acoustic phonons in complex nano- and microstructures.
-Micro-Photoluminescence at 4 K: a microPL experiment optimized to work with optical (mechanical) micropillars embedding quantum dots was set up. This set up is now coupled to a closed-cycle cryostat which is integrated into an optical table, and allows for an electrical control of the quantum emitter. Time correlation experiments were added, to measure the g2 of a single quantum dot coupled to an optophononic microcavity.
Likewise, we developed simulation tools to account for the measured signals. We developed a simulation code based on the transfer matrix method that allows us to numerically simulate the time and spectral domain experimental results. In addition, we developed a three-dimensional model based on finite element methods, that accounts for the three-dimensional confinement effects.
We worked on several techniques to control the propagation of acoustic phonons in multilayered structures. In particular, we engineered i) an adiabatic cavity that has the potential to be more robust under lateral etching in the fabrication of micropillars, ii) a topological cavity that is robust against a disorder that does not change the topological phases of the constituent superlattices, and iii) a general method to mimic with multilayered structures the effects of an effective acoustic potential at the nanoscale. Moreover, in an optical micropillar, a high-frequency acoustic resonator can replace the spacer, and both the optical cavity and acoustic cavity resonances can be independently tuned. We have developed a strategy to measure confined acoustic phonons in the 300 GHz range, using optical micropillars, and accounting for the thermal effects induced enhanced by the optical confinement.
