"Scanning Frequency Comb Microscopy--A new method of Scanning Probe Microscopy", a post-deadline poster at M&M-2018: Microscopy & Microanalysis, a conference Aug. 5-9, 2018, Baltimore, MD.
My group is studying Scanning Frequency Comb Microscopy (SFCM) as a tool for improved resolution and greater stability for feedback control in nano-scale imaging. In SFCM, a mode-locked laser focused onto the tip-sample junction of a Scanning Tunneling Microscope (STM) generates currents at integer multiples of the laser pulse repetition frequency by optical rectification. Simulations suggest that these harmonics can be used as an observable in the feedback control of the tip-sample distance in place of the tunneling current that is used in an STM.
An attowatt level microwave signal with a signal-to-noise ratio of 20-dB is used in place of the noisy nanoampere tunneling current for feedback control of the tip-sample distance in a scanning tunneling microscope. Each harmonic sets the present state-of-the-art for a narrow-linewidth microwave source to enable a signal-to-noise ratio greater than 20-dB even at attowatt power levels. Ohmic heating in the tip-sample junction is less than with an STM because of the low duty-cycle of the laser. Simulations suggest that feedback control is more stable and images may have finer resolution than with an STM. This new method appears to be especially appropriate for measurements with resistive samples. We are pursuing applications to the nanoscale carrier profiling of semiconductors. This material is based upon work supported by the National Science Foundation under Grant 1648811.