Phonon Transport

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. 

Thermal Property Characterization 

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.

Carrier Dynamics

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).

Phonon Transport

Phonons are quantized lattice vibrations, and the major heat carriers in most solid materials. Our scientific understanding of phonons lags that of electrons and photons, mainly because of the difficulties in measuring and manipulating individual phonons. In bulk materials, the phonon transport is purely diffusive and governed by Fourier’s Law. In most bulk materials at room temperature, the main scattering mechanisms affecting thermal transport result from phonon-phonon scattering and phonon-impurity scattering. However, when the size of the materials is reduced down to micro- or nano- meter scales, phonon conduction can travel ballistically across the device. In this limit, boundary scattering becomes more important and phonon lifetime can become much shorter. In nanostructures with characteristic sizes comparable to the wavelength of dominant phonons, or with lower dimensions (2D, 1D, 0D), the phonon structure can be drastically changed, which affects not only phonon lifetime, but also phonon velocity and frequency [Tamura, et al. 1999].  In materials containing several different nanostructures, which is very common in many applications, phonon dynamics becomes exceptionally complicated. 

In most cases, the atoms in crystals vibrate randomly and phonons are generally incoherent. Under some extreme conditions, e.g. when illuminated with intensive ultrafast laser pulses, atoms can vibrate collectively at the same frequency, and with well-defined phase with each other, from which coherent phonons are generated. Coherent phonons provide a good physical model, and perhaps the only way, to study the dynamics of individual phonon mode.

Coherent Phonons usually only last for several picoseconds, too fast to be captured by traditional transport measurements. Coherent Phonon Spectroscopy (CPS) with femtosecond laser pulses is a powerful tool to generate and detect coherent phonons in a variety of materials. Coherent optical phonons can be generated via impulsively stimulated Raman scattering (ISRS) or displacive excitation of coherent phonons (DECP) mechanisms. The left figure below shows the layout of CPS and a typical reflectivity signal revealing coherent optical phonon oscillations. Coherent acoustic phonons can be excited through various approaches. As shown in the right figure below, ultrafast optical pulses locally heat the near-surface layer, which expands, creating a coherent acoustic phonon wave propagating away from the surface. The propagation and scattering of the coherent acoustic phonons can be detected using the ultrafast phonon spectroscopy due to changes in the reflectivity when the coherent acoustic phonon wave is reflected from the back surface of the sample or from the film-substrate interface.

Pump ProbePicture

Thermal Property Characterization

Time Domain Thermo-Reflectance (TDTR) Technique

Frequency-doubled Nd:Yd laser pulses ( 532nm, ~ 10ns ) are focused onto the sample and work as a heating source. Surface temperature of the sample will rise up rapidly, which affects the reflectivity of the sample. A much weaker continuous pulse (He-Ne, 632nm) is focused at the center of pump beam and detect the reflectivity change in time domain. Reflectivity change in time domain is proportional to  the surface temperature, which is related to the thermal properties of the sample. A 1-D thermal conduction model is also used to simulate the surface temperature change. Thermal properties are extracted by fitting the experimental results with simulations. 

Nanosecond Pump Probe

Carrier Dynamics

- MoS2 is a typical material of transiton metal dichalcogenide family,  which exhibits unique properties when thickness reduces to monolayer.
- Strong photoluminescence and high carrier mobility makes it a promising candidate for furture photonic and FET applications.
- Our ultrafast measurement utilizes optical 400nm-pump 800nm-probe spectroscopy to reveal the relaxation dynamics of photo-excited carriers in both bulk and monolayer MoS2.
- Measurement is carried out at ambient pressure for different pump fluences.

Ultrafast Optics