THEORY OF PHOTOACOUSTIC CHARACTERIZATION OF THERMOPHYSICAL PROPERTIES OF MULTILAYERED NANOFILMS

摘要

In this work, the use of the photoacoustic technique for probing thermophysical properties of multi-layered nanofilms is explored. In the photoacoustic measurement of nanofilms, the thermal diffusion length must match with the thickness of the thin film, and the modulating frequency of the incident light should be much higher than usual so that to enhance the thermal-mechanical coupling and the spatial waviness of photoacoustic signal in both photoacoustic cell and sample itself. The current photoacoustic technique cannot fulfill the requirement of measuring thermophysical properties of multi-layered nanofilms. In present study, we develop a general photoacoustic model which takes thermal, mechanical, and optical properties, the geometry of a multilayer film, as well as the thermal contact resistances between layers intoconsideration. The frequency of the photoacoustic effect is extended to the ultrasonic region. The possibility of taking advantage of the resonance in the photoacoustic cell to overcome the decrease of the photoacoustic signal with the frequency increase is also explored. It is suggested that at high frequencies, the photoacoustic signals are obtained close to the resonance peak, which also contains the information ofmaterial’s properties. Resonance frequencies, sensitivity and the frequency range with the highest sensitivity for different types of materials are analyzed. As an example, calculations of the photoacoustic signals of nanofilms of SiO2 and Ni show that increasing the frequency can improve the precision and accuracy of the photoacoustic measurement, and the photoacoustic technique is promising for depth-profiling ofthermophysical properties of multi-layered nanofilms.