Waveguide-integrated mid-IR photodetector and all-optical modulator based on interlayer excitons absorption in a WS/HfS heterostructure.

Novel 2D van der Waals semiconductors facilitate the formation of heterostructures and thus support bandgap engineering for atomically thin modern photonic applications. When these heterostructures form a type II band structure, interlayer excitons (ILEs) are formed as a result of the ultrafast charge transfer between the layers. Here, we present for the first time a waveguide-coupled, mid-IR photodetector and modulator based on the ILE absorption. The device consists of a heterostructure of a single layer of tungsten disulfide (WS) and a few layers of hafnium disulfide (HfS) integrated to a silicon waveguide on a sapphire substrate. We measure broadband mid-IR photodetection (3.8-5.5 µm) with responsivity in the order of tens of µA/W and with no significant effect on the waveguide's transmission. Additionally, we demonstrate waveguide-integrated, mid-IR, all-optical modulation by controlling the ILE population with the interband transition of the individual layers of the heterostructure.