Verification of complex acoustic mismatch model in sub-THz regime

An acoustic mismatch model is used to describe the reflection and transmission of acoustic waves at interfaces of two materials with mismatched impedances. Acoustic attenuation is typically ignored for calculating acoustic impedance, and the parameters in the acoustic mismatch model are thus all real numbers instead of complex numbers. However, acoustic attenuation up to the THz regime is significant. The validity and potential applications of the complex acoustic mismatch model have not been investigated in the sub-THz regime. We report the verification experiments of this model in a complex form. We experimentally measured the amplitude and phase of subpicosecond acoustic pulses, reflected from the interface of GaN and silica. Based on the acoustic mismatch model, the frequency dependence of complex acoustic impedance of silica, which includes the information of sound velocity and attenuation, was experimentally obtained in the sub-THz regime. The acoustic properties of silica were compared with the conventional method, which analyzes the acoustic pulses traveling inside the same silica film. Agreement of sound velocity from two methods was confirmed. The attenuations of the silica film, obtained through using the acoustic mismatch model to analyze the reflected acoustic pulses from silica, were validated by a typical method up to 0.3 THz. This work demonstrates potential applications for characterizing the sub-THz acoustic properties of thick and highly damped materials, which could be challenging by using conventional methods.