Nunez I, Negreira C
Inst. de Fisica, Fac. de Ciencias, Montevideo, Uruguay.
IEEE Trans Ultrason Ferroelectr Freq Control. 1998;45(2):511-9. doi: 10.1109/58.660160.
Acoustical perturbation by targets smaller than the wavelength can generate evanescent waves, which decay with the distance of propagation. By putting such targets immediately under the free surface of water, the diffracted acoustical field can excite the surface membrane before evanescence, and it produces a static ripple because of the radiation pressure. A collimated beam of light reflects at the perturbed surface, and it becomes modulated in phase. In this work we show experimental results where we achieve an optical image of the acoustical evanescent waves produced by an array-like target. Arising from the theory, we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from the target in order to verify the experimental results. With this method, we look for a spatial resolution smaller than acoustical wavelength, for normal incidence and plane waves. In our experimental device, we use a sound wavelength in water greater than 1.5 mm, generated by a PZT transducer. We can resolve an array of 1.0 mm of periodicity.
小于波长的目标产生的声学扰动可生成倏逝波，其会随着传播距离而衰减。将此类目标直接置于水面之下，衍射声场可在消逝之前激发表面膜，并且由于辐射压力而产生静态波纹。准直光束在受扰动表面反射，其相位会受到调制。在这项工作中，我们展示了通过阵列状目标产生的声学倏逝波获得光学图像的实验结果。基于该理论，我们对目标衍射的超声场产生的光学图像进行了数值计算，以验证实验结果。使用此方法，对于垂直入射和平行波，我们寻求小于声学波长的空间分辨率。在我们的实验装置中，我们使用由PZT换能器产生的水中声波长大于1.5毫米。我们能够分辨出周期为1.0毫米的阵列。
