Research

 

 
 

Our group is integrating silicon nanophotonic devices with CMOS circuitry as illustrated in this artistic interpretation of a future chip optoelectronic chip. We envisage that “wires for light” integrated with systems of resonators will enable the complete electronic control of the properties of light.

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Advanced Modulation Formats

..... We have demonstrated for the first time the generation of high bit-rate optical signals using an Optical Time Division Multiplexer (OTDM). The OTDM is fully integrated on a Si chip, is compact (1mm footprint) and consumes no power. From a 5 Gbit/s source we have demonstrated the generation of both 20 Gbit/s and 40 Gbit’s data streams, and can be extended to the generation of 100’s of Gbit/s signals. Our OTDM approach has the distinct advantage that inherently slow modulators can be used to achieve very high overall bit-rates at a single channel, in turn enabling high performance and robust optical interconnects. [PDF]

A top-view scanning electron microscopy (SEM) image for 20 Gb/s OTDM. Magnified images of the spiral with length ∆L = 3.694mm and the 1:4 Y-splitter are shown (left). Experimental setup used to test the devices with fiber-ring mode locked laser schematic. (right).
The input stream of pulses at 5 Gb/s from a fiber-ring mode locked laser. 20 Gb/s TDM signal at the output of the device (left). Schematic of full OTDM multiplexer with the integration of EO modulator in each channel. Each EO modulator is used to switch the pulses on/off. The modulated pulses are recombined at an effectively higher bit-rate (right).

 

..... We have demonstrated the generation of amplitude-shift-keying (ASK) optical signals using a system of parallel microring resonators. By independently modulating two symmetric microring resonators arranged in a Mach-Zehnder configuration, we realize the generation of three levels. The proposed scheme can be extended to any number of logic levels, which effectively increases the data rate of an optical link using slower modulators. Here, we separately utilize thermo-optic and ultrafast all-optical modulation schemes to generate ASK signals on a silicon photonic chip. [PDF]

(a) SEM image of the fabricated device. (b) Optical microscope image of the device with heaters.

Thermo-Optic modulation of both resonators generates three amplitude levels on a single carrier (left). Ultra-fast response of the system with three temporal levels is obtained by opticlally switching two ring resonators with a 200 ps delay (right).

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Backend Compatible Silicon Photonics Platforms

..... We have demonstrated broadband all-optical modulation in low loss hydrogenated amorphous silicon waveguides. Significant modulation occurs with a device of only 15 µm in stark contrast to an identical crystalline silicon waveguide. We attribute the enhanced modulation to the significantly larger free-carrier absorption effect of a-Si:H, estimated here to be ∆α = 1.63E-16∙∆N (1/cm). In addition, we measured the modulation time to be only τc ~ 400 ps, which is comparable to the recombination rate measured in nanosized crystalline silicon waveguides, illustrating the strong dominance of surface recombination in nanosized a-Si:H waveguides. Consequently, a-Si:H could serve as a high performance platform for backend integrated CMOS photonics.[PDF]

All-optical modulation in a-Si:H using a pump-probe scheme. The carriers undergo a rapid thermalization before undergoing non-radiative recombination (~ 400 ps).

..... We have experimentally measured the optical nonlinearities in hydrogenated-amorphous silicon (a-Si:H) waveguides through the transmission of ultra-short pulses. The measured two-photon absorption coefficient β is 4.1 cm/GW and we obtain a 3.5π nonlinear phase shift at 4.1 W coupled input power corresponding to a nonlinear refractive index n2 of 4.2∙10-13 cm2/W. The measured nonlinear coefficient γ = 2003 (1/W∙m) is at least 5 times the value in crystalline silicon. The measured free carrier absorption coefficient σ = 1.9∙10-16 cm2 agrees with the values predicted from the Drude-Lorenz model. It is seen that a-Si:H exhibits enhanced nonlinear properties at 1550 nm and is a promising platform for nonlinear silicon photonics.[PDF]

   
The output power (black squares) as a function of coupled input power for a-Si:H waveguide. The blue line indicates the fit to the measured data based on solving the nonlinear equations (left). Measured spectral broadening in a-Si:H waveguides at different coupled powers due to self-phase modulation and simulation fits based on solving nonlinear Schrödinger equations (right).

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Controlled Storage of Light in Silicon Cavities

..... We have experimentally demonstrated a tunable delay element on a silicon chip that achieves up to 300 ps of delay. It is capable of arbitrarily storing and releasing a pulse of light through dynamic tuning of a system of microcavities. The inherent storage time is more than 32 times the duration of the stored pulse. [PDF]

Scanning electron microscope image of the fabricated device with three ring resonators and a schematic of the capture and release process of the three ring system.
Different delays are measured through changing the time between the store and the release pulses. The data fitting yields an intrinsic decay time of ~160ps.