arXiv subjects
Publications and source records attributed to A. Fainstein.
We demonstrate that the parity-time symmetry for sound is realized in the laser-pumped multiple-quantum-well structure. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly-selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters.
The engineering of phononic resonances in ferroelectric structures appears as a new knob in the design and realization of novel multifunctional devices. In this work we experimentally study phononic resonators based on insulating (BaTiO3, SrTiO3) and metallic (SrRuO3) oxides. We experimentally demonstrate the confinement of acoustic waves in the 100 GHz frequency range in a phonon nanocavity, the time and spatial beatings resulting from the coupling of two different hybrid nanocavities forming an acoustic molecule, and the direct measurement of Bloch-like oscillations of acoustic phonons in a system formed by 10 coupled resonators. By means of coherent phonon generation techniques we study the phonon dynamics directly in the time-domain. The metallic SrRuO3 introduces a local phonon generator and transducer that allows for the spatial, spectral and time-domain monitoring of the complex generated waves. Our results introduce ferroelectric cavity systems as a new tool for the study of complex wave localization phenomena at the nanoscale.
Using resonant Raman scattering spectroscopy with 25 different laser lines, we describe the Raman scattering spectra of mono- and multi-layers 2H-molybdenum diselenide (MoSe$_2$) as well as the different resonances affecting the most pronounced features. For high-energy phonons, both A- and E- symmetry type phonons present resonances with A and B excitons of MoSe$_2$ together with a marked increase of intensity when exciting at higher energy, close to the C exciton energy. We observe symmetry dependent exciton-phonon coupling affecting mainly the low-energy rigid layer phonon modes. The shear mode for multilayer displays a pronounced resonance with the C exciton while the breathing mode has an intensity that grows with the excitation laser energy, indicating a resonance with electronic excitations at energies higher than that of the C exciton.
Polariton-mediated light-sound interaction is investigated through resonant Brillouin experiments in GaAs/AlAs multiple-quantum wells. Photoelastic coupling enhancement at exciton-polariton resonance reaches 105 at 30 K as compared to a typical bulk solid room temperature transparency value. When applied to GaAs based cavity optomechanical nanodevices, this result opens the path to huge displacement sensitivities and to novel ultrastrong-coupling cavity phenomena with optomechanical couplings g0 in the range of 100 THz.
Resonant photoelastic coupling in semiconductor nanostructures opens new perspectives for strongly enhanced light-sound interaction in optomechanical resonators. One potential problem, however, is the reduction of the cavity Q-factor induced by dissipation when the resonance is approached. We show in this letter that cavity-polariton mediation in the light-matter process overcomes this limitation allowing for a strongly enhanced photon-phonon coupling without significant lifetime reduction in the strongly-coupled regime. Huge optomechanical coupling factors in the PetaHz/nm range are envisaged, three orders of magnitude larger than the backaction produced by the mechanical displacement of the cavity mirrors.
We report on confined exciton resonances of acoustic and folded acoustic phonon light scattering in a GaAs/AlAs multi-quantum-well. Significant variations of the line shifts and widths are observed across the resonance and quantitatively reproduced in terms of the polariton dispersion. This high resolution Brillouin study brings new unexpectedly detailed informations on the polariton dynamics in confined systems.
We studied the intensity of resonant Raman scattering due to optical phonons in a planar II-VI-type semiconductor microcavity in the regime of strong coupling between light and matter. Two different sets of independent experiments were performed at near outgoing resonance with the middle polariton (MP)branch of the cavity. In the first, the Stokes-shifted photons were kept at exact resonance with the MP, varying the photonic or excitonic character of the polariton. In the second, only the incoming light wavelength was varied, and the resonant profile of the inelastic scattered intensity was studied when the system was tuned out of the resonant condition. Taking some matrix elements as free parameters, both independent experiments are quantitatively described by a model which incorporates lifetime effects in both excitons and photons, and the coupling of the cavity photons to the electron-hole continuum. The model is solved using a Green's function approach which treats the exciton-photon coupling nonperturbatively.
We demonstrate semiconductor superlattices or nanocavities as narrow band acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics experiments in the transmission geometry with pump and probe incident on opposite sides of the thick substrate, phonon generation and detection processes are fully decoupled. Generating with the semiconductor device and probing on the metal, we show that both superlattices and nanocavities generate spectrally narrow wavepackets of coherent phonons with frequencies in the vicinity of the zone center and time durations in the nanosecond range, qualitatively different from picosecond broadband pulses usually involved in picosecond acoustics with metal generators. Generating in the metal and probing on the nanoacoustic device, we furthermore evidence that both nanostructured semiconductor devices may be used as very sensitive and spectrally selective detectors.
We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.
Extending the idea of optical microcavities to sound waves, we propose a {\it phonon cavity} consisting of two semiconductor superlattices enclosing a spacer with thickness determined by the acoustic wavelength at the center of the first zone-center folded minigap. We show that acoustical phonons can be confined in these layered structures, and propose Raman experiments which are able to probe these novel excitations. The Raman experiments take profit of an optical microcavity scattering geometry that, through the forward-scattering contribution, gives access to the zone-center excitations. We report experimental results of Raman scattering in a structure based in GaAs/AlAs materials that demonstrate unambiguously the observation of phonon cavity confined acoustical vibrations. The experimental results compare precisely with photoelastic model calculations of the Raman spectra of the proposed phonon-cavity embedded optical microcavity.