Cryogenic optical near-field experiments
This protocol is extracted from research article:
Internal strain tunes electronic correlations on the nanoscale
Sci Adv, Dec 14, 2018; DOI: 10.1126/sciadv.aau9123

For near-field experiments, the sapphire substrates were mounted on the sample holder via a conductive silver paste that acts as a good heat conductor as well. When the glue had dried, the sample holder was inserted into a quick-entry load lock for entry into the ultrahigh vacuum (10−10 mbar) environment of the microscope chamber. After insertion to the sample holder, the sample was cooled to low temperature by a flexible copper braid connected to a liquid helium flow cryostat (Janis Research Company); precise temperature control is afforded by an integrated heater and temperature sensor (silicon diode) operated with a temperature controller (Lake Shore Cryotronics). At each temperature of interest, the sample was micropositioned and raster scanned (attocube micropositioning hardware) below the sharp probe tip of the integrated AFM, which is operated in noncontact feedback. Coherent radiation from a CO2 gas laser (910 cm−1 ≈ 113 meV) was directed through an infrared-transparent ZnSe window in the microscope chamber to an off-axis parabolic mirror micropositioned to focus directly onto the metallized tip of the AFM probe. Back-scattered radiation associated with the localized (<20 nm resolved) probe-sample near-field interaction (compare the sketch in Fig. 1F) was collimated by the same optic, detected outside the chamber at a liquid nitrogen–cooled mercury cadmium telluride photodetector and discriminated by pseudoheterodyne interferometry (47). More details on the technical realization of near-field spectroscopy in cryogenic circumstances can be found in (3, 4, 9), for instance.

The sample was cooled in the microscope at a rate of 1 K/min from room temperature down to 140 K, and the last few kelvins above TCO were ramped slowly (0.05 K/min) to avoid undershooting the transition. Sample resistance was monitored in situ to establish the transition temperature. After reaching each temperature set point, the temperature was stabilized for approximately 5 min to ensure thermal equilibrium on the sample before initiating each near-field measurement (typically 500 by 500 pixels), requiring about 2 hours for each map. Near-field maps of several single crystals were recorded with typical results, as presented in Figs. 2, 3, and 6 (D and E).

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