Terahertz Surface Emission from MoSe2 at the Monolayer Limit

Abstract

Surface-charge region of bulk and monolayer MoSe2 is analyzed directly by terahertz (THz) surface emission spectroscopy in a non-destructive way. Both surface nonlinear optical polarization and surface field induced photocurrent contribute to the THz radiation in both bulk and monolayer MoSe2. The first THz emission mechanism is due to the surface optical rectification and the second one is due to the photo-generated carriers accelerated by the surface depletion field. The THz radiation contribution from the surface optical rectification is basically the same for both bulk and monolayer MoSe2 due to the same symmetry at the surface. However, the contribution from the surface field induced photocurrent is ~94.2% in bulk MoSe2 and it goes down to 74.5% in monolayer MoSe2. This is due to the larger surface depletion field in bulk MoSe2 (~2.54 × 107 V/m) compared with that in monolayer MoSe2 (~5.42 × 105 V/m). As such THz emission from bulk is approximately four times larger than that from monolayer MoSe2. This work not only proves clear THz radiation mechanism from MoSe2 crystals but also affords a THz technology for the surface characterization of two-dimensional materials.