Study of ultrasound transducer which produces second harmonic superimposed signal

Cavitation is the formation of gas cavities in a liquid, in the consequence of rarefactional pressure acting upon the liquid. Producing such negative pressure exceeding the cavitation threshold in a travelling wave mode is hard to achieve due to nonlinear propagation followed by focal phase shift. Superimposing the second harmonic to fundamental is an alternative way to solve the problem while able to efficiently form and control the cavitation bubbles which expedite the treatment with high intensity focused ultrasound (HIFU). However, an air-backed transducer in conventional design is not suitable to generate both the second harmonic and fundamental at the same time. In order to overcome this problem, we propose a high impedance matching layer approach. We further explore the possibility of a double matching layer design with an additional lighter layer, to increase the performance of the transducer. Numerical simulation and experimental measurement have shown that by using high impedance matching layer, efficiently generating both the second harmonic (2 MHz) and fundamental (1 MHz) at the same time is possible. Numerical simulation also showed that further study is needed for the double matching layer approach to significant improve the performance.