To achieve full control over acoustic phonons at ultrahigh frequencies, it is essential to characterize the phonon transport properties. Recent works have shown that surface acoustic waves at gigahertz frequencies can propagate over micrometer distances in different nanostructures such as nanowires, nanogratings, and nanoantennas. Here, we aim at investigating acoustic phonon transport by designing a GaAs/AlAs optophononic waveguide. Along the vertical direction of the waveguide, a Fabry-Perot cavity ensures an efficient confinement of acoustic phonons that has been demonstrated in planar and micropillar structures. In the lateral direction, the interface of air and semiconductor serves as an acoustic mirror to reflect phonons in the waveguide. We perform pump-probe experiments to generate coherent acoustic phonons at one position and detect them remotely on the waveguide. We analyze the signals originated by phonons generated in the pump position reaching the remote probe location, revealing a clear indication of phonon transport at room temperature. Our findings have potential applications in quantum technologies and data processing.
Accessing acoustic phonons at high frequencies in nanostructures becomes more and more essential in nanoelectronics, nano- and opto-mechanics and quantum technologies, as phonons can strongly interact with electrons and photons at the nanoscale. In spontaneous Brillouin scattering processes, the scattered photons energy, direction and polarization are constrained by selection rules for a given input state. These selection rules are usually considered as intrinsic material properties in crystalline solids and the polarization of the scattered photons depends on the polarization of the excitation. In this work, we use elliptical optophononic micropillar resonators to control these optical polarization selection rules. The degeneracy of the optical cavity modes of circular micropillars is lifted due to the elliptical cross-section of the micropillars, leading to two cavity modes orthogonally polarized and split in energy. The optical field polarization state will depend on both orthogonal cavity modes and their associated polarization states. Therefore, an incident laser beam linearly polarized along the diagonal axis of the elliptical pillar undergoes a wavelength dependent polarization rotation. By choosing the polarization and wavelength of the incident laser, we demonstrate that the polarization state of the incident and reflected laser and the Brillouin scattering signal are different. In this way, background-free spontaneous Brillouin scattering spectra can be efficiently measured in a cross-polarization scheme down to 18 GHz. Here, we theoretically and experimentally explore the optimal conditions for the polarization and wavelength of the incident laser, and the ellipticity of the micropillars, to improve the polarization-based filtering applied to Brillouin spectroscopy.
GaAs/AlAs heterostructures constitute a unique platform for the conception, engineering, and implementation of opto-phononic systems. In addition to all the accumulated know-how inherited from the optoelectronics industry, a unique coincidence in the contrasts of the optical and acoustic impedances, and the speeds of light and sound, enable a perfect colocalization of the optical electric and acoustic displacement fields. We present the design principles for GaAs/AlAs opto-phononic heterostructures supporting topological interface modes and further analyse the performance of these structures in the optical and the acoustic domain.
Metallic nanoantennas are able to spatially localize far-field electromagnetic waves on a few nanometer length scale in the form of surface plasmon excitations 1-3. Standard tools for fabricating bowtie and rod antennas with sub-20 nm feature sizes are Electron Beam Lithography or Ga-based Focused Ion Beam (FIB) Milling. These structures, however, often suffer from surface roughness and hence show only a limited optical polarization contrast and therefore a limited electric field localization. Here, we combine Ga- and He-ion based milling (HIM) for the fabrication of gold bowtie and rod antennas with gap sizes of less than 6 nm combined with a high aspect ratio. Using polarization-sensitive Third-Harmonic (TH) spectroscopy, we compare the nonlinear optical properties of single HIM-antennas with sub-6-nm gaps with those produced by standard Ga-based FIB. We find a pronounced enhancement of the total TH intensity of more than three in comparison to Ga-FIB antennas and a highly improved polarization contrast of the TH intensity of 250:1 for Heion produced antennas 4. These findings combined with Finite-Element Method calculations demonstrate a field enhancement of up to one hundred in the few-nanometer gap of the antenna. This makes He-ion beam milling a highly attractive and promising new tool for the fabrication of plasmonic nanoantennas with few-nanometer feature sizes.
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