Fig. 3: THz emission spectroscopy and second harmonic generation of 1 T’ MoSSe, 1 T’ MoS₂, 2H MoSSe and 2H MoS₂.

a Left plot shows a schematic of THz emission setup. 1 T’ MoSSe shows a dramatically enhanced THz emission signal compared with three other types of monolayer TMDs (right plot). b Left plot shows a schematic of polarization analysis of THz emission, which shows the emission contains a major in-plane component with a possible out-of-plane contribution (right plot). c Dependence of peak THz field on excitation polarization. No polarizers were used for the emitted THz signal. d Scaling of THz emission shows a linear dependence to incident power at low fluences and saturation at higher fluences. e Left plot shows a schematic of second harmonic generation with normally incident 800-nm excitation. The white arrow marks the incident 800-nm beam and the black arrow marks the SHG light. The right plot shows SHG intensity of five different flakes in each sample and shows SHG is enhanced in 1 T’ MoSSe and 2H MoSSe compared with 1 T’ MoS₂ and 2H MoS₂. f The left plot shows the schematic of the angle-resolved SHG setup that measures out-of-plane dipole. The white arrow marks the incident 800-nm beam and the black arrow marks the SHG light. The beam position at the objective back aperture is scanned perpendicular to the incident beam direction with a motorized stage, which tunes the incident angle θ accordingly. The right plot shows the angle-dependent SHG intensity ratio between p and s polarization (I_p and I_s) in 1 T’ MoSSe, 2H MoSSe, and 2H MoS₂. In the 1 T’ MoSSe and 2H MoSSe, the I_p/I_s ratio increases at non-normal incidence angles, indicative of out-of-plane dipoles. In 2H MoS₂, almost no change is observed as the incident angle varies.