Cryogenic scattering-type scanning near-field optical microscopy (s-SNOM) system, based on a contact and dynamic force AFM with parabolic optics for tip illumination and collection of scattered near-field light. (a) s-SNOM schematic. The arrow in the center of the vacuum chamber indicates the magnetic field direction; the copper block marked with T is the heater and temperature sensor connected to the flow cryostat. (b) Cross-sectional view of core components. The AFM consists of two positioners for independent tip and sample spatial control. Light is focused by an off-axis parabolic mirror, mounted on a 2-axis goniometer. All components are held by a titanium housing mounted on a continuous flow cryostat, with optical access through its base for epi-illumination and detection. The dimensions are minimized such that insertion into a high-field superconducting magnet (not shown) is possible. (c) Photo of the s-SNOM apparatus, with AFM inside the test chamber (cut away view).
s-SNOM approach curves on a Au surface at the first three tip harmonic demodulations show a near-field localization <30 nm (a). Panels (b)–(d) display the topography and associated s-SNOM optical signal for a Au-Si interface using optical excitation from a CO2 laser (λ = 10.6 μm). An optical dielectric material contrast is observed in near-field amplitude (c), but only negligible in phase (d) due to the non-resonant properties of Au and Si/SiO2 at the excitation wavelength.
The topography (a), (c) and near-field response taken with a CO2 laser at 10.6 μm (b), (d) of a VO2 crystal at 340 K (a), (b) and 355 K (c), (d). Due to the difference in optical conductivity between the metallic and insulating phases, the insulating phase appears dark and the metallic phase appears bright. (e) s-SNOM signal together with topography line-scan along the dashed line in (c), (d).
Cryogenic micro-Raman (a), and mid-IR s-SNOM images of doped V2O3 (taken with a CO2 laser at 10.6 μm) (c) with corresponding topography (b). Metallic phases (red) with spatial variation on 10's nm are seen to nucleate near defects (large islands are a dielectric surface impurity).
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