Interferometric detection of extensional modes of GaN nanorods array

Coherent Phonon Transport Measurement and Controlled Acoustic Excitations Using Tunable Acoustic Phonon Source in GHz-sub THz Range with Variable Bandwidth

We experimentally demonstrated a narrowband acoustic phonon source with simultaneous tunabilities of the centre frequency and the spectral bandwidth in the GHz-sub THz frequency range based on photoacoustic excitation using intensity-modulated optical pulses. The centre frequency and bandwidth are tunable from 65 to 381 GHz and 17 to 73 GHz, respectively. The dispersion of the sound velocity and the attenuation of acoustic phonons in silicon dioxide (SiO₂) and indium tin oxide (ITO) thin films were investigated using the acoustic phonon source. The sound velocities of SiO₂ and ITO films were frequency-independent in the measured frequency range. On the other hand, the phonon attenuations of both of SiO₂ and ITO films showed quadratic frequency dependences, and polycrystalline ITO showed several times larger attenuation than those in amorphous SiO₂. In addition, the selective excitation of mechanical resonance modes was demonstrated in nanoscale tungsten (W) film using acoustic pulses with various centre frequencies and spectral widths.

Gigahertz Acoustic Vibrations of Elastically Anisotropic Indium-Tin-Oxide Nanorod Arrays

Active control of light is important for photonic integrated circuits, optical switches, and telecommunications. Coupling light with acoustic vibrations in nanoscale optical resonators offers optical modulation capabilities with high bandwidth and small footprint. Instead of using noble metals, here we introduce indium-tin-oxide nanorod arrays (ITO-NRAs) as the operating media and demonstrate optical modulation covering the visible spectral range (from 360 to 700 nm) with ∼20 GHz bandwidth through the excitation of coherent acoustic vibrations in ITO-NRAs. This broadband modulation results from the collective optical diffraction by the dielectric ITO-NRAs, and a high differential transmission modulation up to 10% is achieved through efficient near-infrared, on-plasmon-resonance pumping. By combining the frequency signatures of the vibrational modes with finite-element simulations, we further determine the anisotropic elastic constants for single-crystalline ITO, which are not known for the bulk phase. This technique to determine elastic constants using coherent acoustic vibrations of uniform nanostructures can be generalized to the study of other inorganic materials.

Nondestructive Complete Mechanical Characterization of Zinc Blende and Wurtzite GaAs Nanowires Using Time-Resolved Pump-Probe Spectroscopy

We have developed and demonstrated an experimental method, based on the picosecond acoustics technique, to perform nondestructive complete mechanical characterization of nanowires, that is, the determination of the complete elasticity tensor. By means of femtosecond pump-probe spectroscopy, coherent acoustic phonons were generated in an ensemble of nanowires and their dynamics was resolved. Specific phonon modes were identified and the detection mechanism was addressed via wavelength dependent experiments. We calculated the exact phonon dispersion relation of the nanowires by fitting the experimentally observed frequencies, thus allowing the extraction of the complete elasticity tensor. The elasticity tensor and the nanowire diameter were determined for zinc blende GaAs nanowires and were found to be in a good agreement with literature data and independent measurements. Finally, we have applied this technique to characterize wurtzite GaAs nanowires, a metastable phase in bulk, for which no experimental values of elastic constants are currently available. Our results agree well with previous first principle calculations. The proposed approach to the complete and nondestructive mechanical characterization of nanowires will allow the efficient mechanical study of new crystal phases emerging in nanostructures, as well as size-dependent properties of nanostructured materials.

Gigahertz coherent guided acoustic phonons in AlN/GaN nanowire superlattices

The generation of guided acoustic phonons in the GHz range in GaN/AlN superlattices grown atop a GaN nanowire is presented. Combined with a femtosecond laser, ultrafast pump-probe spectroscopy allows the generation and detection of guided acoustic phonons at different frequencies in the nanowire superlattices. The capability of the nanowire superlattices to be excellent detectors of acoustic phonons at specific frequencies is then used to observe the strong dispersion, as a result of nanoconfinement, of guided acoustic phonons after their propagation in the nanowire. The generation of high frequency coherent guided acoustic phonons could be useful not only to realize an acoustic transducer with a nanolateral size but also as a source to understand the thermal behavior of nanowires.