Resolve optical and acoustic phonon thermal nonequilibrium in 2D materials under photon excitation

In the last two decades, Raman spectroscopy has been widely used to measure the thermal conductivity and interface thermal conductance of 2D materials with simultaneous laser heating and Raman excitation. The cascading energy transport and resulting temperature difference between optical phonons (OPs) and acoustic phonons (APs) in 2D materials is one critical problem in Raman-based energy transport characterization. This talk will cover our work over the last four years on new physics development and experimental characterization to distinguish the temperatures of OPs and APs, evaluate their energy coupling factor, and determine the thermal conductivity and interface thermal conductance of 2D materials with high confidence by using our time and frequency domains energy transport state-resolved Raman (ET-Raman). Both suspended and supported 2D materials will be covered in this talk with their thickness down to a few nm. Also outlook will be given on challenges in this area and the urgent need for high-level physics development and understanding.