Frequency-domain thermoreflectance with beam offset without the spot distortion for accurate thermal conductivity measurement of anisotropic materials

The measurement of thermal conductivities of anisotropic materials and atomically thin films is pivotal for the thermal design of next-generation electronic devices. Frequency-domain thermoreflectance (FDTR) is a pump-probe technique that is known for its accurate and straightforward approach to determining thermal conductivity and stands out as one of the most effective methodologies. Existing research has focused on advancing a measurement system that incorporates beam-offset FDTR. In this approach, the irradiation positions of the pump and probe lasers are spatially offset to enhance sensitivity to in-plane thermal conductivity. Previous implementations primarily adjusted the laser positions by modifying the mirror angle, which inadvertently distorted the laser spot. Such distortion significantly compromises measurement accuracy, which is especially critical in beam-offset FDTR, where the spot radius has a crucial impact on measured values. This study introduces an advanced FDTR measurement system that realizes probe laser offset without inducing spot distortion, utilizing a relay optical system. The system was applied to measure the thermal conductivities of both isotropic standard materials and anisotropic samples, including highly oriented pyrolytic graphite and graphene. The findings corroborate those of prior studies, validating the measurement's reliability in terms of sensitivity. This development of a beam-offset FDTR system without laser spot distortion establishes a robust basis for accurate thermal conductivity values of anisotropic materials via thermoreflectance methods.