WSe2 as Transparent Top Gate for Infrared Near-Field Microscopy

N. C. H. Hesp, M. K. Svendsen, K. Watanabe, T. Taniguchi, K. S. Thygesen, I. Torre and F. H. L. Koppens

Nano Letters 22, 6200 (2022)
Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe2 can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy. We carry out nanoimaging of plasmons in a bilayer graphene heterostructure tuning the plasmon wavelength using a trilayer WSe2 gate, achieving a density modulation amplitude exceeding 2 × 1012 cm–2. The observed ambipolar gate–voltage response allows us to extract the energy gap of WSe2, yielding a value of 1.05 eV. Our results provide an additional tuning knob to cryogenic near-field experiments on emerging phenomena in 2D materials and moiré heterostructures.