Abstract
A method for superimposing a microwave frequency comb (MFC) on the DC tunneling current in a scanning tunneling microscope (STM) is described in which a mode-locked laser is focused on the tunneling junction. The MFC is caused by optical rectification of the regular sequence of laser pulses due to the nonlinear current-voltage characteristics of the tunneling junction. Hundreds of harmonics, at integer multiples of the laser pulse-repetition frequency, are generated with a metal tip and sample. However, the harmonics have less power with a resistive sample due to the loss in its spreading resistance. The microwave power is greatest at a tip-sample distance that is unique for each sample resistivity. This distance may be set by using different pairs of the applied DC bias and the set-point for the DC tunneling current. However, the laser, and not the applied DC bias or the DC tunneling current, is the source of energy for the MFC so they are not required. Feedback control of the tip-sample distance may be based on maximizing the attowatt-level microwave power of the harmonics, which have a signal-to-noise ratio of 20 dB. This method shows promise for nondestructive carrier profiling of semiconductors with true sub-nanometer resolution which is essential in the continued progress below the 40-nm technology node. It may enable carrier profiling with 2-D materials such as graphene, and it also shows promise for finer resolution in images of biological materials or other resistive samples.
Highlights
A mode-locked laser, focused on the tunneling junction of an scanning tunneling microscope (STM) having a metal tip and sample, superimposes currents at hundreds of harmonics of the laser pulse-repletion frequency on the DC tunneling current to create a microwave frequency comb (MFC).[1]
We present the first analysis to determine the mechanism for generating the harmonics with resistive samples, such as semiconductors, and introduce a new method for Scanning Probe
A mode-locked laser focused on a tunneling junction with a metal tip and metal sample with sub-nm spacing generates hundreds of microwave harmonics at integer multiples of the laser pulserepetition frequency
Summary
A mode-locked laser, focused on the tunneling junction of an STM having a metal tip and sample, superimposes currents at hundreds of harmonics of the laser pulse-repletion frequency on the DC tunneling current to create a microwave frequency comb (MFC).[1]. Spectrum analyzers measure the power spectral density (PSD, in dBm/Hz) over a chosen frequency range. These measurements are made with a specified resolution bandwidth (RBW) where the signal acquisition time varies inversely with the RBW. For the first 200 harmonics the measured microwave power varies as the inverse square of the frequency.[1] We attribute this roll-off to capacitive shunting of the tunneling junction by the leads. There is a long history of using different means to generate microwave radiation by laser-assisted Scanning Tunneling Microscopy,[3] but we are not aware of previous measurements showing such a wide range of range of frequencies in which the roll-off is attributed to an equivalent circuit instead of thermal processes. Microscopy which does not require an applied DC bias or a DC tunneling current
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