Tingge Dai

Ultracompact optical modulator based on graphene-silica metamaterial

We present an ultracompact electro-optic modulator with a length of 15 nm by utilizing a permittivity-tunable metamaterial channel, which is composed of alternative layers of graphene and silica. Optical waves can propagate through the metamaterial channel only if its permittivity is tuned to be near zero. The finite-difference time-domain (FDTD) simulations show the insertion loss is roughly -0.27  dB while the 3 dB extinction ratio is obtained with a 0.8 V gate voltage. The devices footprint is as small as 0.01 μm2. This modulator consumes low power and can potentially be ultrafast.

Proposal for a 2

We propose a new method to change the resonant status of the microring by modifying the light-absorption ability of the waveguide. A conception of a 2 × 2 microring switch based on the graphene-silicon-waveguide is presented by inserting a single graphene flake inside the lateral slot waveguide. The resonance procedure of the microring is thoroughly blocked when the graphene is tuned to be a zero-permittivity material, which significantly enhances the waveguides loss. Neither a high quality factor nor a large driving voltage of the microring is needed. According to the numerical analysis, an extinction ratio for is obtained in the drop port with a broad bandwidth. The extinction ratio in the through port is 34 dB. The radius of the microring is 5.1 μm and the driving voltage is 2.38 V.

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A three-mode (de)multiplexer based on two cascaded asymmetric Y junctions is proposed and experimentally demonstrated on a silicon-on-insulator platform for mode-division multiplexing applications. Within a bandwidth from 1537 to 1566 nm, the best demultiplexing crosstalk of the fabricated device, composed of a three-mode multiplexer, a multimode straight waveguide, and a three-mode demultiplexer, is up to -31.5  dB, while in the worst case it is -9.7  dB. The measured maximum insertion loss is about 5.7 dB at a wavelength of 1550 nm. The mode crosstalk and insertion loss can be further improved by high-quality fabrication processes.

2 Optical Switch Based on Graphene-Silicon-Waveguide Microring

Controlling the light transmission in the epsilon-near-zero channel, which is covered by the perfect-electric-conductor (PEC) walls, has been demonstrated in [L. Yang, T. Hu, A. Shen, C. Pei, B. Yang, T. Dai, H. Yu, Y. Li, X. Jiang, and J. Yang, Opt. Lett. 39, 1909 (2014)]. In this work, we investigate the possibility of replacing the PEC by some practical metal materials. Based on it, an ultracompact plasmonic switch with the length of 15 nm is presented. Its highest extinction ratio is more than 13 dB, while the 3-dB extinction ratio is obtained with a 0.41 V gate voltage.

Silicon three-mode (de)multiplexer based on cascaded asymmetric Y junctions

We proposed a nonvolatile optical waveguide switch by utilizing a floating-gate (FG) configuration whose FG layer is a single-layer graphene. The switching signal can be removed after the optical switching is accomplished. The propagation state of light then can be retained by charges trapped in the graphene layer until the next erasing signal. Depending on waveforms of driving signals, the device can work as either a phase shifter or an intensity switch. In the phase shifter mode, a 646-μm-long device can achieve a phase shift of π. Corresponding energy consumptions to program/erase, the π phase shift is 82.8 and 118.2 pJ, respectively. In the intensity switch mode, a 328-μm-long device is able to attenuate the light by 20 dB. Energies consumed by the programming and the erasing operations are 35.7 and 45.4 pJ, respectively.

Ultracompact plasmonic switch based on graphene-silica metamaterial

An ultra-compact silicon bandpass filter with wide bandwidth tunability is proposed and experimentally demonstrated. The filter architecture is based on a multiple micro-ring resonator-cascaded structure. A wide bandwidth tunability (from 75 to 300 GHz) can be achieved by controlling the resonant frequency of the microring resonators when a good shape factor (0.24-0.44) is held. The filter has a wide free spectral range (about 1.2 THz). The center wavelength can be tuned over several nanometers linearly. The footprint is only 0.053  mm2.

Graphene-Based Floating-Gate Nonvolatile Optical Switch

An integrated high-resolution ratio-metric wavelength monitor (RMWM) is demonstrated on SOI platform. The device consists of a reconfigurable demultiplexing filter based on cascaded thermally tunable microring resonators (MRRs) and Ge-Si photodetectors integrated with each drop port of the MRRs. The MRRs are supposed to achieve specific resonant wavelength spacing to form the “X-type” spectral response between adjacent channels. The ratio of the two drop power between adjacent channels varies linearly with the wavelength in the “X-type” spectral range, thus the wavelength can be monitored by investigating the drop power ratio between two pre-configured resonant channels. The functional wavelength range and monitor resolution can be adjusted flexibly by thermally tuning the resonant wavelength spacing between adjacent rings, and an ultra-high resolution of 5 pm or higher is achieved while the resonant spacing is tuned to 1.2nm. By tuning the resonant wavelength of the two MRRs synchronously, the monitor can cover the whole 9.6nm free spectral range (FSR) with only two ring channels. The power consumption is as small as 8 mW/nm. We also demonstrate the multi-channel monitor that can measure multi-wavelength-channel simultaneously and cover the whole FSR by presetting the resonant wavelengths of every MRR without any additional power consumption. The improvements to increase the resolution are also discussed.

Bandwidth and wavelength tunable optical passband filter based on silicon multiple microring resonators

A broadband tunable silicon filter has been demonstrated on silicon-on-insulator platform. The device is based on the loop of multimode anti-symmetric waveguide Bragg grating. A wide bandwidth tunability about 1.455 THz (0.117-1.572 THz) is achieved. The device, functions like a ring, can realize the bandwidth tunable of the drop port and the through port. And, its feature has simultaneous wavelength tuning and no free space ranges limitation. A high out-of-band contrast of 30 dB is achieved with a bandwidth of 1.572 THz (Δλ = 13 nm). The out-of-band contrast is 18 dB at the minimum bandwidth 0.117 THz (Δλ = 1.0 nm).

Microring based ratio-metric wavelength monitor on silicon

We propose and demonstrate an effective method for mode analysis of a silicon-based two-mode waveguide using time-domain scanning low-coherence interferometry (LCI). An on-chip offset launch technique is implemented to motivate and couple out the two modes. From the low-coherence signal, it is convenient to simultaneously perform the mode identification and analyze the differential modal group delay in a silicon-based multimode waveguide. Our experimental results show good agreement with the simulation results. The low-coherence interferogram shows that only short samples of waveguide (<;220 μm) is sufficient for the LCI to do the mode analysis. This letter provides a feasible method for the mode identification and the transmission channel analysis in silicon-based multimode waveguide used for 2 × 2 on-chip mode-division multiplexing.

Broadband tunable filter based on the loop of multimode Bragg grating

We quantitatively investigate the main source of the intermodal crosstalk of a silicon-based bent multimode waveguide by experiment. The measurement is performed through time-domain scanning low-coherence interferometry. From the measurement results, one can not only calculate the modal crosstalk, but can also locate the position where the crosstalk appears. The results indicate that the modal mismatch at the points where the curvature of the waveguide changes is the main origin of the modal crosstalk. For a two-mode waveguide with a bending radius of 5 μm at 1310 nm, the crosstalk is as high as -20 and -16  dB for the fundamental and first-order mode, respectively. This work gives us a deep insight into how the guided modes actually propagate through the bent waveguide.