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.