Scanning Frequency Comb Microscopy--A new method for Scanning Probe Microscopy

by Prof. Mark J. Hagmann (Department of Electrical and Computer Engineering, University of Utah)

Thursday Dec 5, 2013, 3:30 PM → 4:30 PM Pacific/Honolulu

112 (Watanabe Hall)

The spectrum of a mode-locked ultrafast laser is a series of narrow spectral lines at harmonics which are integer multiples of the pulse repetition frequency (PRF), which is called an “optical frequency comb” (OFC). Numerous applications of OFC include arbitrary waveform synthesis and precise metrology in frequency and distance. We have shown that when the laser is focused on the tunneling junction of a scanning tunneling microscope (STM) optical rectification causes a microwave frequency comb (MFC) to be superimposed on the dc tunneling current. The MFC was measured by focusing a 15 fs pulse train with a PRF of 74.254 MHz from a passively mode-locked Ti:Sapphire laser on the tunneling junction of a STM (UHV700, RHK Technology) operated in air. The harmonics of the MFC were measured with a spectrum analyzer connected to the sample circuit of the STM by means of a bias-T. Hundreds of harmonics have been measured where the Nth harmonic of the MFC has a frequency of N x PRF. For example, the 200th harmonic, at 14.851 GHz, has a linewidth of less than 1 Hz (setting the present state-of-the-art for narrow linewidth at microwave frequencies) and a signal to noise ratio of 20 dB. In Scanning Frequency Comb Microscopy (SFCM) the magnitude at one or more harmonics of the MFC are measured as the tip is scanned across the surface of the sample. Thus, if an oxide or other material having greater resistivity than the sample, the resistivity at the surface may be measured with sub-nm resolution (limited by the radius of the tunneling junction). We have modeled, but not yet tested, the use of SFCM for dopant profiling in semiconductors, where the dc bias of the STM may be adjusted to control the depth of a depletion region at the surface of the semiconductor. Fourier analysis shows that within the tunneling junction there is no significant roll-off in the magnitude of the current at successive harmonics until N ≈ 10,000, so the frequency comb continues to terahertz frequencies. However, the sampled MFC has a roll-off that is consistent with the model of an ideal current source located within the tunneling junction that is shunted by the junction capacitance and the impedance of the spectrum analyzer. Thus, SFCM may be used for time and frequency metrology at nanoscale, as well as a localized multi-frequency source.