Quartic scaling of sound attenuation with frequency in vitreous silica

Several theoretical approaches to disordered media predict that acoustic waves should undergo a quartic increase in their attenuation coefficient with increasing frequency in the sub-terahertz region. Such Rayleigh-type scattering would be related to the anomalous low-temperature plateau in the thermal conductivity and to the so-called boson peak, i.e. an excess of vibrational modes above the Debye density of states at around 1 THz. Brillouin scattering of light allows the measurement of sound absorption and velocity dispersion up to about 0.1 THz while inelastic x-ray scattering is limited to frequencies larger than about 1 THz. We take advantage of the advent of ultrafast optical techniques to explore the acoustical properties of amorphous SiO2 layers in the difficult but crucial frequency region within this gap. A quartic scaling law with frequency is clearly revealed between 0.2 and 0.9 THz, which is further shown to be independent of temperature. This strongly damped regime is accompanied by a decrease in the sound velocity already starting from about 0.5 THz, in line with theories. Our study assists to clarify the anomalous acoustical properties in glasses at frequencies entering the boson peak region.Comment: 4 figures, 11 page

Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals

Abstract Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry–Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately. Accordingly, we inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump–probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples

Fabrication and Characterization of Nanoscale Pillar Arrays With Engineered Phononic and Photonic Properties

This dissertation reports the results of the investigation of nanoscale pillar structures with engineered phononic and photonic properties. It has been proven that phonon states in semiconductors can be tuned by external boundaries, either as a result of phonon confinement effects in individual nanostructures or as a result of artificially induced external periodicity, e.g., phononic crystals. The possibility of engineering acoustic phonon spectrum is of ultimate importance from scientific and application perspectives since they are the main heat carriers in nonmagnetic semiconductor and insulator materials. The change in acoustic phonon dispersion would affect the heat transport properties, alter the electron-phonon interactions, and affect the optical properties of the material system. It has been recently suggested that periodic structures with properly tuned dimensions can act simultaneously as phononic and photonic crystals, strongly affecting the light-matter interactions. In this dissertation research, I prepared nanoscale pillar structure samples and investigated their properties. The “pillar with hat” structures were fabricated using electron beam lithography on silicon substrates followed by inductively-coupled plasma cryogenic dry etching. The hats of the pillars were created with a unique design to have exactly the same orientation plane as the substrate. The structures were inspected with scanning electron microscopy. I used Brillouin-Mandelstam spectroscopy to measure the dispersion of acoustic phonons with energies in the range from 2 GHz up to 20 GHz through the entire second and higher order Brillouin zones. I analyzed the spectral signatures resulting from the surface ripple mechanism which dominates the light scattering in these specific samples. I found clear signatures of the phonon spectrum modification in the appearance of localized phonon sub-bands at energies between 2˗20 GHz. The variable angle ellipsometry measurements indicated modification of light scattering due to nanostructuring. The experimental data confirmed the dual functionality of the structure with engineered phononic-photonic properties. The results obtained in this dissertation research have important implications for the next generation of photonic and optoelectronic device technologies

Coherent phonon optics in a chip with an electrically controlled active device

Phonon optics concerns operations with high-frequency acoustic waves in solid media in a similar way to how traditional optics operates with the light beams (i.e. photons). Phonon optics experiments with coherent terahertz and sub-terahertz phonons promise a revolution in various technical applications related to high-frequency acoustics, imaging, and heat transport. Previously, phonon optics used passive methods for manipulations with propagating phonon beams that did not enable their external control. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the coherent phonon flux by an external electrical bias applied to the active layer. Phonon optics using external control broadens the spectrum of prospective applications of phononics on the nanometer scale

Mechanical properties of nanoporous metallic ultrathin film: A paradigmatic case

Nanoporous ultrathin films, constituted by a slab less than 100 nm thick and a certain void volume fraction provided by nanopores, are emerging as a new class of systems with a wide range of possible applications, including electrochemistry, energy storage, gas sensing and supercapacitors. The film porosity and morphology strongly affect nanoporous films mechanical properties, the knowledge of which is fundamental for designing films for specific applications. To unveil the relationships among the morphology, structure and mechanical response, a comprehensive and nondestructive investigation of a model system was sought. In this review, we examined the paradigmatic case of a nanoporous, granular, metallic ultrathin film with comprehensive bottom-up and top-down approaches, both experimentals and theoreticals. The granular film was made of Ag nanoparticles deposited by gas-phase synthesis, thus providing a solvent-free and ultrapure nanoporous system at room temperature. The results, bearing generality beyond the specific model system, are discussed for several applications specific to the morphological and mechanical properties of the investigated films, including bendable electronics, membrane separation and nanofluidic sensing

Third-order Optical Nonlinearities for Integrated Microwave Photonics Applications

The field of integrated photonics aims at compressing large and environmentally-sensitive optical systems to micron-sized circuits that can be mass-produced through existing semiconductor fabrication facilities. The integration of optical components on single chips is pivotal to the realization of miniature systems with high degree of complexity. Such novel photonic chips find abundant applications in optical communication, spectroscopy and signal processing. This work concentrates on harnessing nonlinear phenomena to this avail. The first part of this dissertation discusses, both from component and system level, the development of a frequency comb source with a semiconductor mode-locked laser at its heart. New nonlinear devices for supercontinuum and second-harmonic generations are developed and their performance is assessed inside the system. Theoretical analysis of a hybrid approach with synchronously-pumped Kerr cavity is also provided. The second part of the dissertation investigates stimulated Brillouin scattering (SBS) in integrated photonics. A fully-tensorial open-source numerical tool is developed to study SBS in optical waveguides composed of crystalline materials, particularly silicon. SBS is demonstrated in an all-silicon optical platform