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。通过对超流体^4He加压，提高了液体^4He的声阻抗，提高了增透涂层的效率，提高了超流体^4He非线性引起的饱和功率。因此，即使在1.9K，相对较高的温度下，也能获得良好的成像信噪比。在850MHz频率下，在13atm、0.3K条件下，通过非线性效应获得了约0.25mum的分辨率。它超过了衍射极限。我们测量了相干聚焦声脉冲在不同压力下在超流体^4He中传播的功率依赖性。在色散关系异常的低压区，观察到简单饱和和弱耗竭。另一方面，在色散关系趋于正常的高压区，观察到全新的、更多的非线性现象。即当输入功率增大时，接收功率单调增大。饱和并迅速消耗。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。一是4声子衰减过程及其衰减产物的自放大，二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，并证明了二次谐波是在超流体^4He中产生的。此外，我们还测量了声显微镜的分辨率，并观察到其在非线性区域的衍射极限以外的改进。由此可见，超流体^4He中也存在由谐波恢复的基波。从而验证了加压超流体^4He非线性现象的模型，并了解了该现象的本质部分。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