THz modulation of monolayer WSe ₂ -silicon hybrid structure and its performance after oxidation

In recent years, THz modulators have been improved by 2D materials, yet facing a conflict between the demand for high modulation depth and the limitation of low pump power. Previously, by pumping continuous-wave laser on a highresistivity silicon wafer, we achieved modulation depth >95% in 0.3-1.5 THz, demanding continuous-wave pump power of 11.8 W. In this work, we added a mono-layer WSe ₂ on the high-resistivity silicon wafer by mechanical exfoliation, raising the modulation depth of THz pulse from 20% to 58% under 0.05 W femtosecond laser pump. The modulation depth can be further enhanced by raising pump power. The modulation behavior is most significant from 1.3 THz to >1.5 THz. This modulation enhancement is due to the interface state between WSe ₂ and silicon, as well as the direct bandgap of mono-layer WSe ₂ . If exposed to the air, WSe ₂ starts oxidization at a low exciting power of <1 mW. Experiments shows that even if WSe2 is partly oxidized to WO ₃ , the modulation depth is only slightly weakened, yet still better than the bare silicon substrate. Also, even if WSe ₂ does not fully cover the range of THz wave, or if the pump laser beam does not fully cover the range of THz wave, the modulation enhancement is also partly effective. Our work suggests a lowcost way to improve the efficiency of all-optical THz modulators.