Atomic Layer-controlled Nonlinear Terahertz Valleytronics in Semi-metal and Semiconductor PtSe₂

As a two-dimensional (2D) material for terahertz (THz) applications, platinum diselenide (PtSe2) can be uniquely tuned from a semiconductor in the near infrared to a semimetal with the number of atomic layers, in contrast to other transition metal dichalcogenides (TMDs). Consequently, the material has unique photonic properties at THz frequencies that can be enhanced by atomic layer engineering. Here, we demonstrate that a controlled THz nonlinearity - tuned from monolayer to bulk PtSe2 - can be realized in wafer size PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering is highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when PtSe2 becomes a semimetal and when the K-valleys can be excited. As well as showing that PtSe2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics to harmonic generation.