High Q-Factor Room Temperature GaAs/AlAs Phononic Nanocavities

The development of semiconductor physics in the last century allowed the development of semiconductor nanostructures which in turn enabled us to spatially control both electrons and photons for a myriad of applications. Phonons directly interact with both electrons and photons and thus provide another avenue to control the nanoscale properties of matter. By manipulating the lattice vibrations, direct control of the material’s macroscopic thermal properties becomes possible.<br/>Acoustic cavities are structures that can spatially confine acoustic phonons and are a mechanical analog to optical cavities for photons. The wavelengths of visible light and of low-frequency GHz phonon modes are on the same order, and thus both phonons and photons can be practically confined in III-V semiconductor superlattices. When heterostructures of two different materials are periodically stacked in a superlattice, phonon minibands are formed analogously to the electronic bands of the corresponding structure. The superlattices can be utilized as Distributed Bragg Reflectors to confine certain phonon modes by forming a phononic Fabry-Perot resonator structure. A more compact design was recently proposed in which the periodicity of the superlattice is gradually altered by smoothly varying the superlattice unit cell length [1], [2]. This adiabatic potential tuning approach mitigates the need for thick complex structures which offers prospects for practically achievable large Q-factor phononic cavities.<br/>We apply the ultrafast transient reflection pump-probe spectroscopy on a range of GaAs/AlAs GHz acoustic phonon nanocavities designed using the novel adiabatic potential tuning method to excite and detect the confined phonon modes. A number of working phononic cavities with a versatile distribution of confined phonon modes demonstrate the ability to finely engineer the phononic properties of semiconductors.<br/>The grown structures possess high Q-factors at room temperature. This is particularly evidenced by comparing acoustic phonon lifetimes of conventional superlattices and our phononic cavities where the latter possesses an order of magnitude longer acoustic phonon lifetimes than the former, reaching even up of several nanoseconds.<br/>The time-resolved pump-probe measurement not only demonstrates the long acoustic phonon lifetimes in high-quality resonators, but also offers prospects to observe the non-linear phonon dynamics. Thus, we also focus on the possible detection of anharmonic phonon processes by observing the transients of multiple phonon modes. As the transient reflection signal simultaneously contains information about the electron dynamics, in addition to the phonon dynamics, it is possible to test if the confined thermal acoustic phonon energy can be harvested to reheat electrons. This electron reheating can be a beneficial mechanism to realize a number of novel devices including a hot carrier solar cell device.<br/>[1] O. Ortíz, M. Esmann, and N. D. Lanzillotti-Kimura, “Phonon engineering with superlattices: Generalized nanomechanical potentials,” <i>Phys. Rev. B</i>, vol. 100, no. 8, p. 085430, Aug. 2019, doi: 10.1103/PhysRevB.100.085430.<br/>[2] F. R. Lamberti <i>et al.</i>, “Nanomechanical resonators based on adiabatic periodicity-breaking in a superlattice,” <i>Appl. Phys. Lett.</i>, vol. 111, no. 17, p. 173107, Oct. 2017, doi: 10.1063/1.5000805.