A cryogenic scanning near-field optical microscope with shear-force gapwidth control

We present a scanning near-field optical microscope designed for nanoscale optical imaging and spectroscopy as well as simultaneous tuning fork shear-force topographic imaging at cryogenic temperatures. The whole setup is immersed in superfluid helium (T=1.8 K). In this medium we observe resonance frequency fluctuations of the tuning fork sensor with an amplitude of Δν≈5%–10% of the full width at half-maximum of the resonance. Possible reasons for the occurrence of the frequency fluctuations are discussed. A stable gapwidth feedback can still be achieved if the set value of the frequency shift is chosen slightly larger than the fluctuation amplitude. As an example we demonstrate shear-force topographic imaging of a silicon grating in superfluid helium.