Optoelectronic properties of bottom gate-defined in-plane monolayer WSe_2 p–n junction

Monolayer transition-metal dichalcogenides （TMDs） are considered to be fantastic building blocks for a wide variety of optical and optoelectronic devices such as sensors, photodetectors, and quantum emitters, owing to their direct band gap, transparency, and mechanical flexibility. The core element of many conventional electronic and optoelectronic devices is the p-n junction, in which the p- and n-types of the semiconductor are formed by chemical doping in different regions. Here, we report a series of optoelectronic studies on a monolayer WSe2 in-plane p-n photodetector, demonstrating a low- power dissipation by showing an ambipolar behavior with a reduced threshold voltage by a factor of two compared with the previous results on a lateral electrostatically doped WSe2 p-n junction. The fabrication of the device is based on a polycarbonates （PC） transfer technique and hence no electron-beam exposure induced damage to the monolayer WSe2 is expected. Upon optical excitation, the photodetector demonstrates a photoresponsivity of 0.12 mA.W-1 and a maximum external quantum efficiency of 0.03%. Our study provides an alternative platform for a flexible and transparent two- dimensional photodetector, from which we expect to further promote the development of next-generation optoelectronic devices.

Gap plasmon-enhanced photoluminescence of monolayer MoS_2 in hybrid nanostructure

Monolayer transition-metal dichalcogenides（TMDs） have attracted a lot of attention for their applications in optics and optoelectronics.Molybdenum disulfide（MoS2）,as one of those important materials,has been widely investigated due to its direct band gap and photoluminescence（PL） in visible range.Owing to the fact that the monolayer MoS2 suffers low light absorption and emission,surface plasmon polaritons（SPPs） are used to enhance both the excitation and emission efficiencies.Here,we demonstrate that the PL of MoS2 sandwiched between 200-nm-diameter gold nanoparticle（Au NP） and 150-nm-thick gold film is improved by more than 4 times compared with bare MoS2 sample.This study shows that gap plasmons can possess more optical and optoelectronic applications incorporating with many other emerging two-dimensional materials.

Enhancement of the second harmonic generation from monolayer WS_(2) coupled with a silica microsphere

Monolayer transition metal dichalcogenides(TMDs) are widely used for integrated optical and photoelectric devices.Owing to their broken inversion symmetry, monolayer TMDs have a large second-order optical nonlinearity. However, the optical second-order nonlinear conversion efficiency of monolayer TMDs is still limited by the interaction length. In this work, we theoretically study the second harmonic generation(SHG) from monolayer tungsten sulfide(WS2) enhanced by a silica microsphere cavity. By tuning the position, size, and crystal orientation of the material, second-order nonlinear coupling can occur between the fundamental pump mode and different second harmonic cavity modes, and we obtain an optimal SHG conversion efficiency with orders of magnitude enhancement. Our work demonstrates that the microsphere cavity can significantly enhance SHG from monolayer 2D materials under flexible conditions.

Spectrally multiplexed and bright entangled photon pairs in a lithium niobate microresonator

On-chip bright quantum sources with multiplexing ability are extremely high in demand for integrated quantum networks with unprecedented scalability and complexity.Here,we demonstrate a bright and broadband biphoton quantum source with spectral multiplexing generated in a lithium niobate microresonator system.Without introducing the conventional domain poling,the on-chip microdisk produces photon pairs covering a broad bandwidth promised by natural phase matching in spontaneous parametric down conversion.Experimentally,the multiplexed photon pairs are characterized by 30 nm bandwidth limited by the filtering system,providing over 40 multiplexing channels with a 0.8 nm channel spacing.Meanwhile,the generation rate reaches 5.13 MHz/μW with a coincidence-to-accidental ratio up to 804,and the quantum source manifests a high purity with a heralded single photon correlation g^((2))_(H)(0)=0.0098±0.0021.Furthermore,the energy-time entanglement is demonstrated with an excellent interference visibility of 96.5%±2%.Such a quantum source at the telecommunication band paves the way for high-dimensional entanglement and future integrated quantum information systems.

On-chip path encoded photonic quantum Toffoli gate

The quantum Toffoli gate is one of the most important three-qubit gates,but it is challenging to construct a chip according to the complicated traditional circuit.Using the optimized 3D configuration with an overpass waveguide to reduce the circuit complexity,we successfully fabricate an on-chip path encoded photonic quantum Toffoli gate enabled by the 3D capability of the femtosecond laser direct writing(FLDW)for the first time to our knowledge,whose truth-table fidelity is higher than 85.5%.Furthermore,a path encoded four-qubit controlled-controlled-controlled NOT gate is written to confirm the scalability of this resource-saving technique.This work paves the way for the FLDW of more complex and powerful photonic quantum computation chips.

Generation of multiphoton quantum states on silicon

Multiphoton quantum states play a critical role in emerging quantum technologies and greatly improve our fundamental understanding of the quantum world.Integrated photonics is well recognized as an attractive technology offering great promise for the generation of photonic quantum states with high-brightness,tunability,stability,and scalability.Herein,we demonstrate the generation of multiphoton quantum states using a single-silicon nanophotonic waveguide.The detected four-photon rate reaches 0.34 Hz even with a low-pump power of 600μW.This multiphoton quantum state is also qualified with multiphoton quantum interference,as well as quantum state tomography.For the generated four-photon states,the quantum interference visibilities are greater than 95%,and the fidelity is 0.78±0.02.Furthermore,such a multiphoton quantum source is fully compatible with the on-chip processes of quantum manipulation,as well as quantum detection,which is helpful for the realization of large-scale quantum photonic integrated circuits(QPICs)and shows great potential for research in the area of multiphoton quantum science.