|Budget Amount *help
¥11,900,000 (Direct Cost : ¥11,900,000)
Fiscal Year 1990 : ¥1,900,000 (Direct Cost : ¥1,900,000)
Fiscal Year 1989 : ¥10,000,000 (Direct Cost : ¥10,000,000)
An acoustic microscope has been developed which makes use of pressurized superfluid helium as an acoustic coupling medium. By pressurizing superfluid ^4He, the efficiency of antireflection coating increases because of the increase of the acoustic impedance of the liquid ^4He, and the power level of saturation caused by nonlinearity of the superfluid ^4He is raised. A good signal-to-noise ratio of imaging is thus obtained even at 1.9K, relatively high temperature. And for the frequency 850MHz, under 13atm at 0.3K, the resolution of about 0.25mum was obtained by nonlinear effects. It exceeds the diffraction limit. We have measured the power dependence of the propagation of coherent focused acoustic pulse in superfluid ^4He underseveral pressures. At the low pressure region where the dispersion relation is anomalous, the simple saturation and the weak depletion were observed. On the other hand, at the higher pressure region where the dispersion relation becomes normal, the completely new
nonlinear phenomena have been observed. That is, when the input power increases, the received power increases monotonously. Saturates and rapidly depletes. As the input power is increased further, the received power increases and depletes again. To explain this phenomenon, we propose a mixed nonlinear acoustic process, which consists of two different nonlinear processes. One is the 4-phonon decay process and the self-amplification of its decay products, and the other is the harmonic generation and its recovery to fundamental wave by the property of the focused beam. To verify this model. We have observed the second harmonics and shown that the second harmonics is generated in superfluid ^4He. Moreover, we have measured the resolution of acoustic microscope and observed its improvement beyond the diffraction limit at nonlinear region. Therefore, it shows that the fundamental wave recovered from harmonics exists in superfluid ^4He, too. This model for the nonlinear phenomena in pressurized superfluid ^4He is thus verified and the essential part of the phenomena is understood.