Piezoelectric semiconductor with heterostructure can be treated as a piezoelectric transducer for the generation of acoustic waves with wavelength less than 10 nm (nano-acoustic waves) by optical technique. This optical piezoelectric transducer has also been utilized for the detection of the nano-acoustic waves (NAW). In this paper, we discuss the generation, detection, and propagation of nano-acoustic waves in piezoelectric semiconductors. We demonstrate that the acoustic frequency of the NAW can be tuned by an optical control technique. Besides, we have also developed an acoustic sensor with THz bandwidth which can be used to study NAW propagation control devices such as nano-phononic bandgap crystal. We demonstrated that the roughness of an interface can be evaluated by the NAW with a resolution less than 1 nm through the acoustic phasefront distortion effect. With the optical piezoelectric transducer, nano-ultrasonics, which is analogous to typical ultrasonics but on the nanometer scale, has been successfully developed.
Multiple quantum well (MQW) structure piezoelectric semiconductor can be treated as a piezoelectric transducer to generate nanometer wavelength and THz frequency acoustic waves. The generation mechanism of nano acoustic wave (NAW) in quantum wells induced by femtosecond optical pulses can be modeled by a macroscopic elastic continuum theory. The absorption of the MQW's modulated by NAW's through quantum confined Franz-Keldysh (QCFK) effect allows another femtosecond optical probe pulses to monitor the propagating NAW. Many applications in typical ultrasonics can be achieved by NAWs. The simultaneous waveform synthesis is demonstrated by an optical coherent control technique. The phase of the totally reflected NAW is studied. Acoustic coherent control can be achieved by designing the thickness of the cap layer on the MQW. We also demonstrated the feasibility to apply THz NAWs to acoustically control an electronic device with higher operation speed and spatial accuracy. Seismology, which is the first step toward ultrasonic imaging, was also demonstrated. The arrival time of the echo is obtained by processing the transient transmission changes of the probe. Ultrafast technique and nano technology are ready for nano ultrasonics.