The enhancement of second harmonic generation in ultrasonic microscopic observation by triple transition

The high power density of acoustic waves in the focal region of an ultrasonic microscope makes nonlinear effects significant. The generation of second harmonic waves is mainly due to the nonlinear parameter of the material at the focal region, in general. So the ultrasonic microscope may become a tool for scanning the B/A value of samples at the focus. We found an approach to enhance the second harmonic generation greatly by reflection. The receiving signal of the direct arriving wave reached maximum as the transducer was located near the focus. As the receiving transducer was moved close to the lens, at certain position, the acoustic waves (f₀ and 2f₀) were reflected by the transducer, reflected again by the lens, and finally received by the transducer, the received second harmonic signal was about 16 dB larger than that at the direct focusing maximum. The experimental results agree with the theoretical calculations for both direct focusing and triple transition