Ultrafast Dynamics of Photoexcited Carriers and Phonons in Tailored 1D Acoustic Phonon Potentials

Above bandgap photoexcitation of a semiconductor absorber results in kinetically energetic electrons and holes, collectively called hot carriers. Hot carriers lose their excess energy through a plethora of inelastic scatterings among themselves and through interactions with lattice vibrations-phonons. Frustrating one or more carrier energy loss mechanisms is a key challenge for realizing novel devices like hot carrier solar cells, which promise efficiencies beyond the Shockley-Quiesser limit. Despite evidence of hot carrier solar cell operation, experimental demonstration of performance requires control of the electron-phonon interactions, necessitating developing a deeper understanding of these processes. Controlling phonon properties directly through nanostructures should allow insights into electron-phonon interactions and phonon dynamic processes such as phonon bottleneck. Acoustic cavities spatially confine acoustic phonons, analogous to optical cavities for photons. Since the wavelengths of visible light and low-frequency (GHz) phonon modes are of the same order, the confinement of both photons and phonons is possible in III-V semiconductor superlattices. Superlattices realized by stacking two dissimilar materials give rise to phonon minibands at the zone centre resulting in phonon distributed Bragg Reflector (DBR). Recently, a more compact cavity design has been proposed to achieve arbitrary phononic potential by smoothly varying the thicknesses of the unit cell's constituent layers, keeping the unit cell's length constant in the superlattice. We have demonstrated phonon cavities operating at 96GHz with tailored 1D phonon potentials realized using GaAs/AlAs multilayers. Room temperature ultrafast vibrational spectroscopy showed long-lived coherent acoustic phonon modes compared to DBRs. By also performing time-resolved photoluminescence, the impact of the phonon confinement on photogenerated carriers' energy loss in the phonon potential and DBR has been studied. The carriers' excess energy, estimated using a full spectrum photoluminescence fitting, reveals a significantly slower carrier energy loss rate in the phononic cavity.