Infrared and THz Optical Nanoscopy of High-Tc Superconductor Devices

We use cryogenic atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to study thin films and nano-constriction devices in La_2-xSr_xCuO₄ high-Tc cuprate superconductors. Using light in the mid-infrared region we demonstrate that our customized AFM-SNOM setup can provide three dimensional dielectric characterization of devices fabricated by tightly focused Helium ion beams. Light demodulation up to the 4^th harmonic of the AFM tapping frequency allows us to observe in the dielectric response irradiation induced amorphization effects which extend on length scales that are orders of magnitude larger than the size of the focused ion beam. We ascribe this widespread damage to a Helium depth distribution substantially modified by internal device interfaces. Low-temperature data in the THz range enables detection of superconducting fluctuations in a 13 nm thick La_2-xSr_xCuO₄ film with a superconducting critical temperature T_c ~ 31.5 K. Our results show that AFM-SNOM is a powerful technique for probing and characterizing superconducting interfaces and devices with nanometer scale resolution.