Development of a broadband cryogenic magnetic scanning near-field optical microscope

We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme condition nanoscope operation and obtain the first cryogenic magneto time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K and magnetic fields up to 5 T simultaneously. Our cryogenic magneto scanning near-field optical microscopy, or cm-SNOM, instrument comprises: a broadband spectrum ranging from THz to mid-IR, a 5 T split pair magnetic cryostat with a custom-made insert, and an atomic force microscope (AFM) unit that accepts ultrafast excitation. We demonstrate the ultrafast and ultrabroadband cm-SNOM operation with a CW IR laser source, tunable femtosecond IR pulses, and single-cycle THz pulses. We apply the cm-SNOM to obtain measurements of superconducting, topological, and magneto-plasmonic materials. The new capabilities for studying these quantum materials, which require the extreme environment of cryogenic operation and applied magnetic fields simultaneously in nanometer space, femtosecond time, across broadband energy scales, represent a leap forward in near-field microscopy.