Scattering-Type Scanning Near-Field Optical Microscopy with Akiyama Piezo-Probes in High Magnetic Fields

Recent developments of the scattering-type scanning near-field optical microscope at cryogenic temperatures (cryo-SNOM) have led to many breakthroughs in the studies of low energy excitations in quantum materials. However, the simultaneous demands on vibration isolation, low base temperature, precise nano-positioning, and optical access make the construction of a cryo-SNOM a daunting task. Adding to the overhead space required for a cryo-SNOM is the atomic force microscopy (AFM) control, which predominantly utilizes a laser-based detection scheme for determining the cantilever tapping motion. Here we provide an alternative and simplified route for performing s-SNOM using metal-coated Akiyama probes, where the cantilever tapping motion is detected through a piezoelectric signal. The Akiyama-based cryo-SNOM attains high spatial resolution, good near-field contrast, and can perform imaging with a significantly more compact system compared to other cryo-SNOM techniques. By combining this system with a 7 T magnetic field, we have directly imaged Dirac magnetoplasmons in charge-neutral monolayer graphene with subwavelength resolution. These magnetoplasmons manifest as edge interference patterns in the optical signal and as interesting edge currents in the photocurrent signal.