Near-Field Infrared Imaging at Cryogenic Temperatures
The characterizations of rich optical phenomena in materials under conditions of extreme temperature and pressure has conventionally relied on far-field spectroscopy techniques. In contrast, the fundamental features of many phase transitions can only be revealed at characteristically nanometer length scales, where the diffraction limit prohibits investigation by far-field spectroscopy instruments, proving the need for tools capable of nanoscale optical characterization of transitions in systems such as superconductors, correlated oxides, and Bose-Einstein condensates of surface polaritons. Nanoscale infrared investigations of these interesting systems have yet to be performed at their transition temperatures. For this reason, our lab has developed a cryogenic scanning near-field optical microscope (Cryo-SNOM) capable of infrared imaging at temperatures down to 40K and pressures of 1e-8 mbar. Current samples of interest for characterization with this instrument include the low-temperature transition oxides V2O3 and Fe3O4, magnetically doped semiconductors like GaMnAs, as well as single-layer graphene subjected to ultra-high vacuum conditions.
View of the exterior of the Cryo-SNOM chamber: The AFM head is positioned at the lighted area in the center of the chamber, with an optical inspection camera positioned above. Laser illumination enters from a ZnSe window (yellow, left inside the chamber), and focused onto the AFM tip by the parabolic mirror (black, foreground inside chamber).
Detailed view of the chamber interior: CT = cryostat (cold finger), L = lens, AT1 = attocube stack 1 (Attocube), B1 = braid connector, B2 = braid connector, PM = parabolic mirror, AT2 = piezo positioning stack 2 (Attocube), PT = platform, GR = glass rod coupling. For cooling the sample, flexible copper braids (not shown in this schematic) are attached from CT to B1 and B2.