The novel and versatile physical properties of nanowires have stimulated large fundamental and technological interests during the last decades. The appearance of novel boundary conditions, compared to bulk materials, drastically modifies phonon propagation and scattering in these materials. As a consequence nanowires has been intensively studied for the various implications of such modifications: control of thermal transport at the nanoscale, thermoelectricity or nano-sized acoustic transducer.
Ultrafast pump probe spectroscopy is perfectly suited to study phonons in such structures. The absorption of a femtosecond pulse leads to the generation of high frequency coherent acoustic phonons and their evolution can be time resolved thanks to the high temporal resolution of this technique.
Here, I will discuss some of our most recent studies of coherent acoustic phonons in semiconductor nanowires. I will first briefly describe the modifications of light and sound behaviour due to the 1D structure. I will then present observations of both confined and propagating acoustic phonons in nanowires. Finally, I will show how from the observation of the phonons, complete elastic characterization of nanowires can be achieved. After demonstrating the capabilities of this method on well-known materials, it is applied to the wurtzite phase of GaAs, a metastable phase in bulk, and measured for the first time its elasticity tensor. These results, in addition to the characterization capabilities, also provide an understanding of the thermal transport at the nanoscale and opens new possibilities for light control at the nanoscale.
Pierre-Adrien Mante, "Phonons in semiconductor nanowires (Conference Presentation)," Proc. SPIE 10530, Ultrafast Phenomena and Nanophotonics XXII, 105300M (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2287822.5751512416001.
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