Few-layer hexagonal boron nitride as a shield of brittle materials for cryogenic s-SNOM exploration of phonon polaritons

Surface phonon polaritons (SPhPs) in van der Waals (vdW) materials are of great interest in fundamental and applied research fields. Probing the characteristics of vdW SPhPs at cryogenic temperatures is an essential task for their implementation in low-temperature physics. However, the most commonly used characterization technique of vdW SPhPs - scattering-type scanning near-field optical microscopy (s-SNOM) operating in a tapping mode (an intermittent-contact mode) - can be problematic at low temperatures because the sample being tested may become brittle and fragile. Therefore, high fracture toughness is desired for the samples under intermittent-contact s-SNOM scanning at low temperatures. In this work, by taking α-phase molybdenum trioxide (α-MoO3) as an example, we first confirm the potential surface deterioration induced by tip-sample interactions at low temperatures. Then, we propose to use few-layer hexagonal boron nitride as a mechanically tough yet optically passive cladding layer to enhance the surface stability of α-MoO3. Finally, we demonstrate the validity of our surface reinforcement strategy by probing the previously unexplored temperature dependence of SPhPs within the third Reststrahlen band of α-MoO3. Our method allows a sustained operation of tapping mode s-SNOM at cryogenic temperatures with negligible effect on intrinsic properties of SPhPs.