Micropillar Resonators for Optomechanics in the Extremely High 19-95-GHz Frequency Range.

Strong confinement, in all dimensions, and high mechanical frequencies are highly desirable for quantum optomechanical applications. We show that GaAs/AlAs micropillar cavities fully confine not only photons but also extremely high frequency (19-95 GHz) acoustic phonons. A strong increase of the optomechanical coupling upon reducing the pillar size is observed, together with record room-temperature Q-frequency products of 10^{14}.

Polariton Resonances for Ultrastrong Coupling Cavity Optomechanics in GaAs/AlAs Multiple Quantum Wells.

Polariton-mediated light-sound interaction is investigated through resonant Brillouin scattering experiments in GaAs/AlAs multiple-quantum wells. Photoelastic coupling enhancement at exciton-polariton resonance reaches 10(5) 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 ultrastrong coupling regimes in cavity optomechanics with couplings g(0) in the range of 100 GHz.

Towards GHz-THz cavity optomechanics in DBR-based semiconductor resonators.

Resonators based on acoustic distributed Bragg reflectors (DBRs) were optimized to work in the GHz-THz regime, and grown by molecular beam epitaxy. We show that in structures made of GaAlAs alloys a simultaneous optimal confinement of light in the visible range and phonons in the tens of GHz range can be achieved. We report time resolved differential optical reflectivity experiments performed with fs-ps laser pulses.

Low-temperature Raman fingerprints for few-quintuple layer topological insulator Bi2Se3 films epitaxied on GaAs.

Topological insulators (Bi2Se3) of single- and few-quintuple-layer (few-QLs) films were investigated by Raman spectroscopy and epitaxied on a GaAs substrate. At a measurement temperature of 80 K, we observed the emergence of additional A2u and Eu modes (Raman inactive in the bulk crystal) below 9-QLs film thicknesses, assigned to the crystal-symmetry breakdown in ultrathin films. Furthermore, the out-of-plane A1g modes changed in width, frequency, and intensity for decreasing numbers of QL, while the in-plane Eg mode split into three Raman lines, not resolved in previous room temperature experiments.

Fabry-Pérot-multichannel spectrometer tandem for ultra-high resolution Raman spectroscopy.

We present a novel ultra-high resolution Raman spectroscopy technique based in a Fabry-Pérot/triple spectrometer tandem with multichannel acquisition. We describe the system, detail the calibration process, and experimentally test the technique, showing that effective finesses in excess of 1000 are possible. The technique is specifically tailored for low intensity, complex and spectrally extended Raman spectra, providing shorter acquisition times with respect to similar tandem systems with monochannel detectors.

Strong optical-mechanical coupling in a vertical GaAs/AlAs microcavity for subterahertz phonons and near-infrared light.

We show that distributed Bragg reflector GaAs/AlAs vertical cavities designed to confine photons are automatically optimal to confine phonons of the same wavelength, strongly enhancing their interaction. We study the impulsive generation of intense coherent and monochromatic acoustic phonons by following the time evolution of the elastic strain in picosecond-laser experiments. Efficient optical detection is assured by the strong phonon backaction on the high-Q optical cavity mode.

Bloch oscillations of THz acoustic phonons in coupled nanocavity structures.

Nanophononic Bloch oscillations and Wannier-Stark ladders have been recently predicted to exist in specifically tailored structures formed by coupled nanocavities. Using pump-probe coherent phonon generation techniques we demonstrate that Bloch oscillations of terahertz acoustic phonons can be directly generated and probed in these complex nanostructures. In addition, by Fourier transforming the time traces we had access to the proper eigenmodes in the frequency domain, thus evidencing the related Wannier-Stark ladder.

Enhancement and inhibition of coherent phonon emission of a Ni film in a BaTiO3/SrTiO3 cavity.

We report pump-probe time resolved reflectivity experiments in a hybrid air-Ni metal-BaTiO(3)/SrTiO(3) oxide mirror phonon cavity. We demonstrate that the generated coherent acoustic phonon spectra of the impulsively excited metallic film can be inhibited or enhanced in the phonon cavity with respect to a Ni film directly grown on a SrTiO(3) substrate. The experiments are compared with simulations that highlight the role of the phonon density of states in the coherent acoustic emission, extending concepts at the base of the optical Purcell effect to the field of phononics.

Lifetime of THz acoustic nanocavity modes.

We present an ultrahigh resolution Raman study of the lifetime of 1 THz acoustic phonons confined in nanocavities. We demonstrate that the cavity Q factor can be controlled by design. Anharmonicity contributes only marginally to limit the cavity phonon lifetime, even at room temperature, while thickness fluctuations in the scale of 1/10 of a unit cell are the main limitation for the performance of THz phonon cavities.

Coherent generation of acoustic phonons in an optical microcavity.

Ultrafast coherent generation of acoustic phonons is studied in a semiconductor optical microcavity. The confinement of the light pulse amplifies both the generation and the detection of phonons. In addition, the standing wave character of the photon field modifies the generation and detection phonon bandwidth.

From spin flip excitations to the spin susceptibility enhancement of a two-dimensional electron gas.

The g-factor enhancement of the spin-polarized two-dimensional electron gas was measured directly over a wide range of spin polarizations, using spin flip resonant Raman scattering spectroscopy on two-dimensional electron gases embedded in Cd(1-x)Mn(x)Te semimagnetic quantum wells. At zero Raman transferred momentum, the single-particle spin flip excitation, energy Z*, coexists in the Raman spectrum with the spin flip wave of energy Z, the bare giant Zeeman splitting. We compare the measured g-factor enhancement with recent spin-susceptibility enhancement theories and deduce the spin-polarization dependence of the mass renormalization.

Selective optical generation of coherent acoustic nanocavity modes.

Femtosecond pump-probe experiments on a Ga0.85In0.15As nanocavity enclosed by two Ga(0.

Subterahertz phonon dynamics in acoustic nanocavities.

We report a direct determination of the dynamic behavior of confined acoustic phonons in nanocavities by picosecond acoustics. We provide the broadband, high resolution transmission amplitude curve in the subterahertz range, and we give evidence of resonant transmission peaks in three successive stop bands, in quantitative agreement with acoustic simulations. We furthermore demonstrate transit times in the nanosecond range at the cavity peaks reflecting the strong confinement of resonant phonons within the cavity layer.

Spin excitations of the spin-polarized electron gas in semimagnetic quantum wells.

Collective and single-particle spin-flip excitations of a two-dimensional electron gas in a semimagnetic Cd(1-x)Mn(x)Te quantum well are observed by resonant Raman scattering. Application of a magnetic field splits the spin subbands and a spin polarization is induced in the electron gas. Above 1 T the collective modes, which disperse with the in-plane wave vector, dominate the spectra.

Confinement of acoustical vibrations in a semiconductor planar phonon cavity.

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.

Standing optical phonons in finite semiconductor superlattices studied by resonant Raman scattering in a double microcavity.

We report optical double resonant enhancement of Raman scattering in a new double microcavity geometry. The design allows almost backscattering geometries, providing easy access to the excitations' in-plane dispersion. The cavity is used to study the phonon spectra of a finite GaAs/AlAs superlattice.

Resonant mechanisms of inelastic light scattering by low-dimensional electron gases.

The contribution of elementary excitations in low-dimensional electron gases to resonant inelastic light scattering is found to be determined by interband transitions involving states at specific wave vectors. In modulation-doped GaAs/GaAlAs quantum wells, we detect only the single-particle excitations (SPE) at resonances with electron-hole transitions at the Fermi wave vector, and only plasmons at resonances with zone-center excitons. The plasmon cross section is comparable to the SPE when double electronic resonance is achieved by tuning the plasmon energy to a valence subband separation.