van der Waals (vdW) heterojunctions enable arbitrary combinations of different layered semiconductors with unique band structures, offering distinctive band engineering for photonic and optoelectronic devices with new functionalities and superior performance. Here, an interlayer photoresponse of a few-layer MoSe 2 /WSe 2 vdW heterojunction is reported. With proper electrical gating and bias, the heterojunction exhibits highsensitivity photodetection with the operation wavelength extended up to the telecommunication band (i.e. 1550 nm). The photoresponsivity and normalized photocurrent-to-dark current ratio reach up to 127 mA W −1 and 1.9 × 10 4 mW −1 , respectively. The results not only provide a promising solution to realize high-performance vdW telecommunication band photodetectors, but also pave the way for using sub-bandgap engineering of two-dimensional layered materials for photonic and optoelectronic applications.

Bandgap engineering with two-dimensional layered materials based heterostructures provides a new method for designing high-performance broadband photodetectors, modulators and lasers.

Thanks for all your contributions to discussion, data analyzing, measurements, as well as suggestions. Finally, I would like to thank my parents for their encouragement, support, and everlasting love.

Monolayer transition-metal dichalcogenides show strong optical nonlinearity with great potential for various emerging applications. Here we demonstrate the gate-tunable interband resonant four-wave mixing and sum-frequency generation in monolayer MoS 2 . Up to 80% modulation depth in four-wave mixing is achieved when the generated signal is resonant with the A exciton at room temperature, corresponding to an effective third-order optical nonlinearity |χ (3) eff | tuning from (∼12.0 to 5.45) × 10 –18 m 2 /V 2 . The tunability of the effective second-order optical nonlinearity |χ (2) eff | at 440 nm C-exciton resonance wavelength is also demonstrated from (∼11.6 to 7.40) × 10 –9 m/V with sum-frequency generation. Such a large tunability in optical nonlinearities arises from the strong excitonic charging effect in monolayer transition-metal dichalcogenides, which allows for the electrical control of the interband excitonic transitions and thus nonlinear optical responses for future on-chip nonlinear optoelectronics.