Phonons are the major heat carriers in most solid materials. In bulk materials, the phonon transport is purely diffusive and governed by Fourier’s Law. In nanostructures, phonons can be scattered at boundaries, interfaces and nanoparticles, as well as quantum confinement effects become dominant controlling factors, making their dynamics strikingly different from that in the bulks. Due to their ultrafast nature, it is very challenging to obtain a profound understanding of fundamental phonon transport in complex nanostructures. We study the phonon dynamics in nanomaterials with ultrafast phonon spectroscopy and molecular dynamics simulations, aiming to gain knowledge of individual phonons in complex nanostructures, and facilitate the design of new materials with desirable phonon properties.
Accurate characterization of thermal properties in complex nanostructures is nontrivial. Direct-contact measurements usually suffer from the entangling of thermal resistance across the contacts with thermal properties of the sample. Characterizing thermal properties with optical techniques have multiple advantages: non-invasive, simple sample preparation, and highly sensitive to small values. We utilize time domain thermo-reflectance (TDTR) technique to measure thermal conductivity in nanostructures and across interfaces with high spatial resolution.
With the rapid progress of quantum functions in nano-devices, probing quantum dynamics of carriers (electrons, holes, plasma, etc.) with high temporal resolution has become crucial for further advances in nano science and technology. Because the lifetimes of these carriers usually fall into femtosecond (10-15 s) to picosecond (10-12 s) range, a fast "CAMERA" is necessary to capture their dynamics. We have set up several ultrafast spectrometers ( fast CAMERAs) to study the carrier dynamics in nanostructures, including femsecond pump-probe spectrometer, white light continuum probing spectrometer, and optical emission spectrometer. All these spectrometers are powered by our three femtosecond lasers: Spectra Physics Tsunami (10nJ pulse energy, 75MHz repetition rate and 30 fs pulse width), Spectra Physics Spitfire (1.2mJ pulse energy, 5KHz repetition rate and 35 fs pulse width) and Light Conversion Topas Prime (tunable laser wavelength from 290nm to 2700nm).
We are enthusiastic about developing novel optical techniques to solve new problems. Currently we are developing two spectrometers a) Transient thermal grating spectrometer to measure thermal properties and phonon dynamics in 2D materials and b) Ultrafast optical transmission spectrometer to study the plasma chemistry in plasma-assisted carbon disassociation. (collaborated with Dr. Halil Berberugolu's group).