Self-oscillating electronics for improving scanning speed in AF/PSTM

An atomic Force Photon Scanning Tunneling Microscope (AF/PSTM) is reported here,which uses a shift in resonance frequency to control the distance between a sample and a fiber probe.For self-oscillating tapping mode AF/PSTM applications,a sharp end and large conical angle optical fiber tip with an apex diameter smaller than 100 nm and a cone angle of 60～90° is fabricated.The tip is mounted on a piezo,then the piezo is placed in a positive feedback circuit to excite the probe to oscillate.The oscillation frequency of the tip is demodulated by a PLL,and its value is maintained a constant by controlling the negative feedback loop of the Z piezo to track the sample surface.Both theory and experiment are carried out to compare the externally excitation mode and self-oscillating mode.Results show that the self-oscillation mode reduces the response time of the tip in theory,and the bandwidth of self-oscillating mode in proposed AF/PSTM is 50 Hz in experiments,which is higher an order of magnitude than that of externally excitation mode response.A sample of grating is scanned by proposed instrument with a scanning rate of 1 Hz,and results demonstrate that the topography and optical images obtained by self-excitation mode have clearer contrast than those obtained by externally excitation mode.Besides faster response,the self-oscillating mode can improve the system resolution by increasing Q without effecting on the system response speed.