Sub-terahertz photoacoustic effect detectable with a microphone

We demonstrate the generation of photoacoustic signals by irradiating aqueous samples with modulated continuous sub-terahertz waves, which can be detected with a standard microphone. The use of millimeter-waves for excitation and acoustic waves for detection allows us to inspect a sample inside a container through its wall in a non-contact manner. We confirm that the frequency response of the photoacoustic signals is dependent on the sound velocity in the sample, demonstrating its potential for non-contact inspection. As a proof of concept, we demonstrate that the stiffness of gelatin samples can be discerned with the proposed method.