Development of High Resolution Acoustic Microscope Using Superfluid ^4He

研制了一种利用加压超流氦作为声耦合介质的声学显微镜。通过加压超流体^4He，增加了液体^4He的声阻抗，从而提高了减反射膜的效率，并提高了由超流体^4He的非线性引起的饱和功率级。因此，即使在1.9K、温度相对较高的情况下，也能获得良好的成像信噪比。在频率为850 MHz、温度为13atm、温度为0.3K的情况下，利用非线性效应获得了约0.25微米的分辨率。它超过了衍射极限。我们测量了几种压力下相干聚焦声脉冲在超流中传播的功率依赖性。在色散关系反常的低压区，观察到简单饱和和弱耗尽现象。另一方面，在色散关系变得正常的高压区，全新的…观察到了更多的非线性现象。也就是说，当输入功率增加时，接收功率单调增加。饱和并迅速耗尽。随着输入功率的进一步增加，接收功率增加并再次耗尽。为了解释这一现象，我们提出了一个混合非线性声学过程，它由两个不同的非线性过程组成。一种是四声子衰变过程及其衰变产物的自放大，另一种是利用聚焦光束的特性产生谐波并恢复为基波。来验证这个模型。我们观察到了二次谐波，表明二次谐波是在超流~4He中产生的。此外，我们还测量了声学显微镜的分辨率，观察到其在非线性区超过衍射极限的改善。由此可见，从谐波中恢复的基波也存在于超流体~4He中。从而验证了加压超流~4He中非线性现象的模型，理解了现象的本质。较少

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 … More 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. Less