Changyun Zhao

Ultracompact plasmonic switch based on graphene-silica metamaterial

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.

An all-optical modulation method in sub-micron scale

We report a theoretical study showing that by utilizing the illumination of an external laser, the Surface Plasmon Polaritons (SPP) signals on the graphene sheet can be modulated in the sub-micron scale. The SPP wave can propagate along the graphene in the middle infrared range when the graphene is properly doped. Graphenes carrier density can be modified by a visible laser when the graphene sheet is exfoliated on the hydrophilic SiO2/Si substrate, which yields an all-optical way to control the graphenes doping level. Consequently, the external laser beam can control the propagation of the graphene SPP between the ON and OFF status. This all-optical modulation effect is still obvious when the spot size of the external laser is reduced to 400 nm while the modulation depth is as high as 114.7 dB/μm.

Low-Coherence Method for Mode Analysis of a Silicon-Based Two-Mode Waveguide

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.

Wavelength Tunable Cavity Mirror for Silicon Micro-Ring-Based Hybrid Integrated Lasers

Two types of wavelength tunable cavity mirrors have been presented and silicon-on-insulator (SOI) platform. Both cavity mirrors show high reflectivity and large Q magnitude. They are used for realizing the silicon hybrid integrated lasers. The Q magnitude of the mirrors is about 6 × 104, and the reflectivity is more than 95%.We use a suspended edge coupler to connect the III-V semiconductor optical amplifier chip and the cavity mirrors. The measured highest laser output power is 6.1 mW in uncooled condition. A wide bandwidth tenability (the whole FSR) can be achieved, the tuning efficiency is about 6.7 mW/nm, and over 35-dB side mode suppression ratio is obtained.

Measurement of the intermodal crosstalk of a bent multimode waveguide.

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.

Ultrahigh-Resolution Ratio-Metric Wavelength Monitors Based on Tunable Microrings on Silicon

An ultrahigh-resolution ratio-metric wavelength monitor based on microring resonators (MRRs) is demonstrated on silicon. The theoretical wavelength resolution is related to the functional wavelength range and the quality (Q)-factor of the microring. We analyze the relationship and experimentally demonstrate that the functional range and the resolution can be adjusted by thermally tuning the resonance spacing of the MRRs. The resolution is also limited by the noise introduced in the measurements. An ultrahigh experimental resolution of 1.5 pm is obtained within a 0.72 nm functional range and an ultrahigh theoretical extreme resolution of ~0.4

Temperature-fluctuation-sensitive accumulative effect of the phase measurement errors in low-coherence interferometry in characterizing arrayed waveguide gratings

pm can be expected considering of the intrinsic systems noise only. The causes of the difference between the experimental and theoretical resolution and the measures to reduce the difference are also discussed.

Ultra-sensitive silicon photonic current sensor using a ring resonator

We investigate the accumulative effect of the phase measurement errors in characterizing optical multipath components by low-coherence interferometry. The accumulative effect is caused by the fluctuation of the environment temperature, which leads to the variation of the refractive index of the device under test. The resulting phase measurement errors accumulate with the increasing of the phase difference between the two interferometer arms. Our experiments were carried out to demonstrate that the accumulative effect is still obvious even though the thermo-optical coefficient of the device under test is quite small. Shortening the measurement time to reduce the fluctuation of the environment temperature can effectively restrain the accumulative effect. The experiments show that when the scanning speed increases to 4.8 mm/s, the slope of the phase measurement errors decreases to 5.52×10(-8), which means the accumulative effect can be ignored.

Fano-resonance-based ultra-high-resolution ratio-metric wavelength monitor on silicon.

We proposed and experimentally investigated a compact and ultra-sensitive integrated photonic current sensor based on a silicon ring resonator in this paper. The current flowing through the integrated resistive TiN heater produces the Joules heat and changes the temperature, which results in the change of refractive index and physical dimensions of the ring. An optical spectrum analyzer is used to monitor the resonant wavelength shift of the ring. The experiment results show that the sensor achieves an ultra-high sensitivity of 6.8 × 104 nm A−2 and good linearity between real-time current and wavelength shift in the test range of 0–10 mA.