Ultrafast Acoustics Based on Piezoelectric Semiconductors
1Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan
* Presenter:CHI-KUANG SUN
Thanks to ultrafast acoustics, a better understanding of acoustic dynamics on a short time scale has been obtained and new characterization methods at the nanoscale have been developed. Among the materials that were studied during the development of ultrafast acoustics, nitride-semiconductor-based heterostructures play a particular role due to their piezoelectric properties and the possibility to generate coherent acoustic phonons with a femtosecond pulsewidth and with over-1THz bandwidth. In this presentation, we will review some of the ultrafast acoustics work performed using piezoelectric semiconductors, with a focus on THz phonon spectroscopy and nanoscopy. First, we will present a brief description of the theory and experiment of coherent acoustic phonon generation by piezoelectric heterostructure. From this starting point, we then present the applications of terahertz acoustics and femtosecond acoustics.
With a bandwidth over 1 THz, THz acoustics is ready to answer the long debate regarding the ballistic heat transport in the “collisionless” regime in crystals and in the Boson peak regime in glass, the so-called solid water. By converting a femtosecond optical pulse into a femtosecond acoustic pulse, one can also take advantage of the ultraslow sound velocity. It can be noted that within 100 fs, the femtosecond acoustic pulse travels only a distance of 3 angstroms, assuming a 3000 m/s velocity. Femtosecond acoustics can thus provide a spatial resolution down to 1.5 angstroms supposing an acoustic round trip path and 100 fs temporal resolution. Our study supported the fact that femtosecond acoustics can indeed noninvasively monitor the dynamic changes of physical structures longitudinally at a solid/water interface and the sub-surface area with an sub-atomic-level resolution, thus providing the capability to reveal key physical information on reactions occurring at solid/liquid interfaces, ideal for next generation energy conversion and electrochemical energy storage device developments as well as for the study of early-stage corrosion or oxidation processes.

Keywords: Nitride, terahertz acoustics, ultrafast acoustics, femtosecond acoustics, interface