Confinement of Acoustical Vibrations in a Semiconductor Planar Phonon Cavity

Abstract

Extending the idea of optical microcavities to sound waves, we propose a phonon cavity consisting of two semiconductor superlattices enclosing a spacer layer. We show that acoustical phonons can be confined in such layered structures when the spacer thickness is an integer multiple of the acoustic half-wavelength at the center of one of the superlattice folded minigaps. We report Raman scattering experiments that, taking profit of an optical microcavity geometry, demonstrate unambiguously the observation of a phonon-cavity confined acoustical vibration in a GaAs/AlAs based structure. The experimental results compare precisely with photoelastic model calculations of the Raman spectra.

Phononic Molecules Studied by Raman Scattering

“…This peak, marked with an asterisk, corresponds to one of the split acoustic confined modes. It is clearly shifted to higher energies from the uncoupled nanocavity mode (20.2 cm -1 ) [2]. The feature indicated with a vertical dotted line is related to the lower energy mode, shifted to lower energies.…”

“…The experiments were performed at room temperature under double optical resonance condition [8]. The use of an optical microcavity gives access to the zone-center excitations through a wave vector conserving contribution, thus allowing the observation of confined acoustic modes [2]. However, since the coupled modes present a different symmetry with respect to the center of the structure, only one of them will be Raman active [4].…”

“…These peaks arise from contributions of acoustic modes distributed in the whole structure (SL mirror modes). The simulations were performed using a photoelastic model, taking full account of the acoustic and optical confined fields [2,4]. The spectrum was gaussian convoluted to take into account the experimental resolution (0.15 cm -1 ).…”

“…In particular, two identical acoustic nanocavities are the phononic equivalent of a hydrogen molecule [1]. Single acoustic nanocavities have been experimentally studied by both Raman scattering [2] and coherent phonon generation techniques [3,4]. Acoustic molecules and acoustic coupling effects have been theoretically investigated [1,5].…”

“…We report the observation of the acoustic mode splitting of a phononic molecule by using high-resolution Raman scattering. By embedding the phononic molecule in an optical microcavity we performed Raman scattering experiments under double optical resonance [2,8], allowing the observation of one of the two confined modes.…”