Graphene/silicon hybrid photodetector operating at communication wavelength has attracted enormous attention recently due to its potential to realize bandwidth larger than 100 GHz. However, the responsivity is intrinsically limited by the low absorption from the atomic-thick graphene monolayer, which imposes significant obstacles towards its practical application. Although plasmonic structures has been widely applied to enhance the responsivity, it may induce the metallic absorption thus limit the responsivity lower than 0.6 A/W. Twisted bilayer graphene (TBG) has been reported to hold the ability to dramatically enhance the optical absorption due to the unique twist-angle-dependent van Hove singularities. In this article, we present a design of a silicon/TBG hybrid photodetector with a responsivity higher than 1 A/W and bandwidth exceeding 100 GHz. The enhanced responsivity is achieved by tuning the twisted angle of TBG to increase the absorption within the 1550 nm as well as utilizing the silicon slot waveguide to boost the mode overlap with TBG. The fabrication process of proposed design is also discussed demonstrating the advantages of low fabrication complexity. The proposed silicon/TBG photodetector could not only exhibit superior performance compared to previously reported silicon/monolayer graphene photodetector, but also pave the way for the practical application of graphene-based silicon optoelectronic devices.

Pulse delay generators are ubiquitous in laboratories to coordinate and control the timing between different devices in applications that include lasers, mass spectrometers, and other scientific instruments. The most important required characteristics are precision, to control time exactly, and low-jitter, to minimize uncertainty in experiments. Here, we introduce a new design of a high precision and low-jitter digital delay generator based on a Field Programmable Gate Array (FPGA). The final delay is composed of steps of 4.2 ns (coarse delay) with fine steps of 16 ps (fine delay). The coarse delay is generated by a 240 MHz pulse sequence from the FPGA with a 50 MHz clock. An embedded time-to-digital conversion unit is used to measure the interval between the external trigger and the clock signal, which, together with an integrated delay generator, is used to realize the fine delay. Jitter compensation is achieved through a measurement-and-feedback module. A computer interface is designed to control the system through a Nios II processor. Measurements confirm a time resolution of 16 ± 2 ps with jitter below 450 ± 20 ps (at 24 °C) with a maximum delay of 1 s. The whole system is simple in structure and low in cost.

A cryogenic beam apparatus for studying neutral clusters has been built and tested. The lowest beam temperature reaches less than 9 K at a repetition rate of 20 Hz. Mechanical decoupling from the refrigerator avoids misalignment during temperature ramping. Adopting a permanent magnet based magnetic deflector eliminates the hysteresis and electric noise of the traditional electromagnet and offers excellent reproducibility of the applied magnetic field. The mass spectrometer can operate in either Mass Spectroscopy Time-Of-Flight mode or Position-Sensitive Time-Of-Flight mode with spatial resolution better than 7  μm. Its performance is demonstrated with niobium and cobalt clusters.

We propose a novel physical model-based QoT estimation with achieved OSNR estimation error ≤ 1.5 dB in primary paths and OSNR prediction error ≤ 1.76 dB in unestablished lightpaths for both super-C bands and super-C+L bands for the first time.