Optophononic resonators based on GaAs/AlAs multilayer structures can confine near-infrared photons and sub-terahertz phonons. Previous works have studied the generation and detection of coherent acoustic phonons in planar and circular micropillar cavities. However, these structures exhibit only one optical cavity mode, which makes it challenging to reach the efficient generation and sensitive detection simultaneously in a standard pump-probe experiment. Here, we propose the use of elliptical micropillars to reach an improved condition of these two processes. The elliptical cross-section of the microcavity lifts the degeneracy of the fundamental optical modes, related to each of its major and minor axes. By tuning the pump pulses in resonance with one optical mode, the generation efficiency is enhanced by maximizing the electromagnetic field inside the cavity. Meanwhile, the probe pulses at the same wavelength and with orthogonal polarization detects phonons at the slope of the other mode, where it is sensitive to reflectivity changes. We experimentally demonstrated that the phonon amplitude is enhanced by introducing the ellipticity compared to the circular micropillar. This improvement is promising for future developments in constructing efficient phonon transducers.
Acoustic interface states have been evidenced in superlattices with frequencies at tens to hundreds of GHz. A recently demonstrated scheme to generate interface states in one-dimensional superlattices is based on the principle of band inversion, obtained by concatenating two periodic lattices with inverted spatial mode symmetries around the bandgap. Most of the realizations exploit a given bandgap for which there is one symmetry inversion. In this work, we present high-order topological nanophononic interface states in multilayered structures, achieved by concatenating two superlattices with different bandgap orders centered around the same frequency. In this case, inverted mode symmetries are not required to achieve the interface state, which is enabled by modifying the unit cells of the two superlattices. We showcase designs for versatile topological devices where interface states can be simultaneously created across a wide frequency range. The topological modes can be experimentally accessed in Brillouin or pump-probe experiments. The ability to explore higher bandgap order acoustic interface states in the GHz range is unique in nanophononic superlattices due to the linear dispersion relation of acoustic phonons. The demonstrated systems can thus be exploited to investigate schemes that are difficult or impossible to study in electronics or optics, due to their non-linear dispersion relations.
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.
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