Koichi Karaki

Nonlinear resolution improvement and second‐harmonic generation of a pressurized superfluid 4He acoustic microscope

We observed the input power dependence of the second‐harmonic generation of a 370‐MHz focused acoustic beam in pressurized superfluid 4He at about 300 mK. We also measured the resolving power of a reflection mode acoustic microscope using pressurized superfluid 4He as a coupling liquid under 11 atm (anomalous dispersion region) and 21 atm (normal dispersion region). At the nonlinear power region of each pressure, the resolution was improved beyond the diffraction limit. Results of these two experiments confirmed that the higher harmonics are generated and they are converted to the fundamental as the wave propagates through the focal point. The higher harmonics play an important role in recently observed exotic nonlinear phenomenon in pressurized superfluid 4He.

Acoustic microscope using pressurized superfluid 4He

An acoustic microscope has been developed which makes use of pressurized superfluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure superfluid 4He under saturated vapor pressure at 0.1 K and under 25 atm at 0.1 and 1.9 K, for the frequency of 380 MHz. By pressurizing liquid 4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, 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. The entire imaging instrument set is within a closed cell. The focusing mechanism is composed of two voice coils of superconducting wire. Using a servocontrol mechanism with a sensor and drive coil, Z‐direction stability was better than 0.05 μm. The manipulation sensitivity is 0.5 μm/mV, and the whole stroke is about 2 mm.

New nonlinear phenomena in focused acoustic beam in pressurized superfluid 4He

We have studied the propagation properties of a focused acoustic beam in superfluid 4 He at 300 mK under several pressures and observed new nonlinear phenomena under pressures higher than 18 atm. Under these high pressures (normal dispersion region), the received power is increased in proportion to the power input for the low-power region, but it is depleted rapidly after the saturation for higher power input. It exhibits several sharp dips for further increased power. This nonlinear phenomenon is a completely new one, so there is as yet no explanatory theory. We propose a possible mechanism to explain it which consists of two nonlinear processes.

Low temperature acoustic microscopy for material characterization

To characterize the mechanical properties of materials at low temperatures, a novel variable low-temperature scanning acoustic microscopy (VLTSAM) has been developed. The temperature of the sample can be continuously varied between 30 degrees C and -94 degrees C in a methanol coupler. As a demonstration of the VLTSAM, frozen onion cells were imaged at quenched and slowly cooled states. Defects in a 0.6-mm-thick epoxy layer were also observed at -30 degrees C with much more sensitivity than at ambient temperatures. As a tool to analyze subsurface images in VLTSAM, the V(z) curve theory for SAM was extended to describe time-resolved subsurface echoes.<<ETX>>

Pressurized Superfluid 4He Acoustic Microscopy

An acoustic microscope has been developed which makes use of pressurized super fluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure super fluid 4He under saturated vapor pressure at 0.2 K and under 25 atm at 0.2 K and 1.9 K, for the frequency of 400 MHz. By pressurizing liquid 4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, and the power level of saturation caused by nonlinearity of the super fluid 4He is raised. A good signal-to-noise ratio of imaging is thus obtained.

Self-defocusing effect in a focused acoustic beam in superfluid 4He

Nonlinear propagation of the focused acoustic beam in superfluid 4He was experimentally investigated. We measured the amplitude of an, acoustic wave propagating through superfluid 4He as a function of input power and reflector-to-lens distance under pressure range of 17-24 atm using a reflection mode acoustic microscope. In the high-pressure region, a new type of nonlinear behavior was observed in which the effective focal length becomes larger with increasing input power (self-defocusing effect). Discussion is presented regarding the origin of this nonlinear behavior.

Detailed study of nonlinear wave front distortion of focused sound in superfluid4He

We have investigated a nonlinear phenomenon which appears in a focused sound in superfluid4He under pressure higher than 18 atm. Wave front distortion of the focused ultrasound by nonlinear effect was obtained by the Fourier transform of the transducer output as a function of the defocusing length. The wave was found to suffer discontinuous wave front distortion for the input power above a certain value. This distortion is well represented by the picture that a second wave whose phase is shifted by approx. π develops, and interferes with the original wave. The amplitude of this second wave decreases suddenly as the pressure is lowered below 18 atm and the nonlinear wave front distortion also disappears. The possible mechanism of this second wave generation are discussed.

Amplitude dependence of first sound velocity in superfluid 4He

Abstract Precise sound velocity measurement of first sound in superfluid 4He as a function of sound amplitude was made at 430 MHz. An anomalous sound velocity increase was observed in the pressure region higher than 18 atm, as the sound amplitude was raised above the threshold value. The effect became more conspicuous with increasing pressure and input power density, and at 24 atm δc/c reached as much as 3 × 10−4 at the input power density of 50 W/m2. This phenomenon showed a large pressure dependence, but the threshold amplitude was almost independent of pressure. We also performed a numerical simulation to give the functional form of velocity change versus input amplitude.

Pressure dependence of quantum zero sound attenuation in normal liquid3He

In the Fermi liquid theory Landau predicted that sound quanta are absorbed and emitted by directly creating and annihilating quasiparticle-quasihole pairs in normal liquid3He, when the angular frequency of ultrasound satisfies the condition Ћω>kT. In this regime sound attenuation remains finite at absolute zero temperature. We studied this quantum absorption limit as a function of pressure using an ultrasound of 389.1 MHz and have verified that this quantum zero sound absorption does exist.

Pressure dependent self-defocusing effect in a convergent ultrasonic beam in superfluid4He

Abstract We have observed a self-defocusing effect of the finite amplitude converging 400MHz acoustic beam in superfluid4He in the high pressure region. In order to clarify the mechanism of the defocusing, we experimentally investigated the transverse profile of the ultrasonic beam after it passes through the focal point and found that the central part of the beam has π phase shift in the high input power region at 24atm. This π-shift cannot be explained by ordinary nonlinear effects such as a conversion to higher harmonics.