Ultrasonic-photonic local probe microscopy

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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.