Terahertz Surface Emission from MoSe at the Monolayer Limit.

The surface-charge region of bulk and monolayer MoSe is analyzed directly by terahertz (THz) surface emission spectroscopy in a nondestructive way. Both surface nonlinear optical polarization and surface field-induced photocurrent contribute to the THz radiation in both bulk and monolayer MoSe. The first THz emission mechanism is due to the surface optical rectification and the second one is due to the photogenerated 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 MoSe because of the same symmetry at the surface. However, the contribution from the surface field-induced photocurrent is ∼94.2% in bulk MoSe and it goes down to 74.5% in monolayer MoSe. This is due to the larger surface depletion field in bulk MoSe (∼2.54 × 10 V/m) compared with that in monolayer MoSe (∼5.42 × 10 V/m), as such THz emission from the bulk is approximately four times larger than that from monolayer MoSe. This work not only proves the clear THz radiation mechanism from MoSe crystals but also affords a THz technology for the surface characterization of two-dimensional materials.