When the Al transducer is illuminated by the laser pulse, its temperature increases rapidly, and decreases gradually as the heat becomes dissipated into the deeper side of the sample. We monitor the temperature variation of the Al transducer by measuring the thermoreflectance, a complex-valued relative reflectivity change, in the time domain (time-domain thermoreflectance (TDTR) measurement), and determine physical parameters involved with the cross-plane thermal transport.

Nanoscale thermal transport

We recently improved our TDTR setup to focus on the nanoscale thermal transport; a fast amplitude modulation(~10 MHz) of the pulsed pump laser beam generates a fast oscillating complex temperature profile on the Al transducer, and the heat flows into the underneath material with a nanoscale thermal penetration depth so that the measurement sensitivity to the nanoscale thermal transport is greatly improved.

In the nanoscale length, non-equilibrium transport phenomena occur between phonons themselves or between phonon and other heat carriers like electron or spin-wave. These non-equilibrium states give time-domain phases of the complex temperature largely deviated from trivial Fourier's diffusion law. We determine fundamental energy coupling constants between heat carriers, and also examine quasi-ballistic phonon transport behavior beyond the simple diffusion model.

Acoustic phonon transmission spectroscopy

Phonon is a quantized quasi-particle of lattice waves in solids, and its transmission through the interface is a crucial factor for the nano-scale thermal transport through thermal interfaces. We can experimentally determine the transmittance of each phonon mode by solving a frequency-dependent phonon BTE-based algorithm to fit TDTR experiment data.

From this, we expect to deepen our understanding of the nanoscale thermal transport and contribute to solve the heat dissipation problem in nanoscale devices.