Generating in vivo continuous ultrasound based on sub-terahertz photoacoustic effect

Non-contact ultrasound excitation based on the photoacoustic effect using short optical pulses has been widely used for biomedical and industrial inspections. However, generating and detecting photoacoustic signals in water or aqueous samples requires careful choice of the excitation wavelength. Here, we show that continuous-wave (CW) ultrasound can be directly generated in aqueous samples by irradiating them with the CW sub-terahertz waves modulated at acoustic frequencies, even when the stress confinement condition is not satisfied. The ultrasound generated at resonance can be detected even in the air using a microphone. The sub-terahertz waves exhibit a water absorption coefficient akin to peak near-infrared wavelengths while offering transmittance through diverse materials. Leveraging recent advances in high-frequency electronics, we develop a compact experimental system with the potential for further miniaturization. To demonstrate the potential of the proposed method, we present proof-of-concept applications of bulk modulus measurement of gelatin gels and in vivo anatomical imaging of human hands.