Ultrasonic-photonic local probe microscopy

本文研究了利用斩波激光和超声振动的光外差效应对材料热弹性性质进行纳米级分辨率成像的超声-光子局域探针显微术，从理论上探讨了光外差法的可行性，其中，将高频热弹性振动转换为比原子力显微镜（AFM）的基本共振频率低的频率振动，悬臂梁，并在实验上证明了图案化的多层纳米结构，由沉积在（100）Si衬底上的100 nm Cr膜的区域组成。样品在Cr和Si之间含有150 nm的二氧化硅膜。我们得到了几个图像的光外差信号（振幅和相位）的样品，并与理论结果进行了比较。一个一维模型的热弹性响应的结构，假设表面光吸收正确预测的光外差振幅图像应显示小的对比度。然而，在相位图像中观察到的对比度（140度）比预测的大大约10倍。这种差异可能是由于模型中没有考虑边界热阻的影响，我们增加了激励超声频率以提高显微镜的分辨率。我们配备了带有超声换能器的常规原子力显微镜，对样品施加射频（频率f_1〜170 MHz，振幅<1 nm）的超声振动。超声换能器采用厚度为50 μm和20 μm的LiNbO_3单晶。超声力显微镜（UFM）使我们能够知道是否成功地将超声振动施加到样品上。UFM实验的结果表明，我们能够激发和检测的超声波振动的Ge量子点上的硅衬底高达170 MHz的激发频率。

We studied ultrasonic-photonic local probe microscopy, designed to image thermoelastic properties of material with nanomater resolution, by exploiting the optical heterodyne of chopped laser illumination and ultrasonic vibration.We investigated theoretically the possiblity of the optical heterodyne method, in which a high frequency thermoelastic vibration was converted into a lower frequemcy vibration than the fundamental resonance of the atomic force microscope (AFM) cantilever, and demonstrated experimentally for a patterned multilayer nanostructure, consisting of a region of a 100-nm Cr film deposited on a (100) Si substrate. The sample contained a 150-nm silica film between the Cr and the Si. We obtained several images of the optical heterodyne signals (amplitude and phase) of the sample and compared them with theoretical results. A one-dimensional model of the thermoelastic response of the structure, assuming surface optical absorption correctly predicts that the optical heterodyne amplitude image should show litte contrast. The observed contrast in the phase image, 〜40 degrees, is however, about 10 times larger than that predicted. The discrepancy may be due to a thermal boundary resistance not accounted for in the model.We increased excitation ultrasonic frequency in order to improve the resolution of the microscope. We equipped a conventional atomic force microscope with an ultrasonic transducer to apply ultrasonic vibration of radio frequency (frequency f_1〜170 MHz, amplitude<1 nm) to the sample. LiNbO_3 single crystals of the thickness 50 μm and 20 μm were used for ultrasonic transducers. Ultrasonic force microscopy (UFM) allowed us to know if the ultrasonic vibration was successfully applied to the sample. The results of UFM experiments showed that we were able to excite and detect ultrasonic vibration on Ge quantum dots on a Si substrate up to 170 MHz excitation frequencies.