• Ultra High Speed Silicon Electro-Optic Modulator
• On-Chip Nonlinear Optics
• High Performance on-chip Ge Photodetectors
• Networks on-chip
• Ultra Small Mode Volumes
• Novel Photonic Structure Design Using an Evolutionary Algorithm
• Slow/Fast Light on Chip
• Slot-Waveguide for Strong Confinement of Light in Low-Index Materials
• Photonics for Sensing

Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures not previously possible. We have experimentally demonstrated NRZ modulation of our electro-optic PIN modulator at 18 Gbps. Computer simulations show that this device can have bit rates scalable to 40 Gbps in this device.

Simulations of the PINIP structure show that the device exhibits a carrier injection time of 10 ps and an extraction time of 15 ps enabling 100 GHz operation.

Top view schematic of the PIN electro-optic ring modulator with the inset showing a cross-setion of the PIN configuration.

Schematic of the PINIP electro-optic resonator.

Measured optical transmission of the PIN electro-optic modulator operating at 18 Gbps (NRZ)..

We demonstrated net on/off gain over a wavelength range of 28nm through the optical process of phase-matched four-wave mixing in suitably designed SOI channel waveguides. We also demonstrated wavelength conversion in the range 1511–1591nm with peak conversion efficiencies of 15.2 dB, which represents more than 20 times improvement on previous four-wave-mixing efficiencies in SOI waveguides. These advances allow for the implementation of dense wavelength division multiplexing in an all-silicon photonic integrated circuit. Additionally, all-optical delays, all-optical switches, optical signal regenerators and optical sources for quantum information technology, all demonstrated using fourwave mixing in silica fibres, can now be transferred to the SOI platform.

We have also demonstrated optical 2R regeneration in an integrated silicon device consisting of an 8-mm-long nanowaveguide followed by a ring resonator bandpass filter. The regeneration process is based on nonlinear spectral broadening in the waveguide and subsequent spectral filtering through the ring resonator. We measured the nonlinear power transfer function for the device and find an operating peak power of 6 W. Measurements indicate that the output pulse width is determined only by the bandwidth of the bandpass filter. Numerical modeling of the nonlinear process including free-carrier effects shows that this device can be used for all-optical regeneration at telecommunication data rates.

We have achieved parametric wavelength conversion via fourwave mixing using ultra-low peak pump powers of a few milliwatts in a micrometer-scale silicon device. The response time of our device is 100 ps allowing for implementation in high-bandwidth optical communications. At these ultra-low power levels and microscale sizes, it should be possible to realize hundreds of these devices operating simultaneously on a single chip.

We recently proposed a new technique to realize an optical time lens for ultrafast temporal processing that is based on four-wave mixing in a silicon nanowaveguide. The demonstrated time lens produces more than 100 pi of phase shift, which is not readily achievable using electro-optic phase modulators. Using this method we
demonstrate 20x magnification of a signal consisting of two 3 ps pulses, which allows for temporal measurements using a detector with a 20 GHz bandwidth. Our technique offers the capability of ultrafast temporal characterization and processing in a chip-scale device.

We have also developed a technique to measureoptical waveforms with 220-fs resolution over lengths greater than 100 ps, which represent the largest record-length-to-resolution ratio (>450) of any single-shot-capable picosecond waveform measurement technique. Our implementation allows for single-shot measurements and uses only highly developed electronic and optical materials of complementary metal-oxide-semiconductor (CMOS)-compatible silicon-on-insulator technology and single-mode optical fibre. The mature silicon-on-insulator platform and the ability to integrate electronics with these CMOS-compatible photonics offer great promise to extend this technology into commonplace bench-top and chip-scale instruments.

Four-wave mixing with a degenerate pump. The four-wave mixing process involves the conversion of two pump photons to a signal photon and an idler photon (top). By suitable design of the waveguide, momentum conservation (that is, phase-matching) is also satisfied and amplification of the signal occurs (bottom).

The microring resonator, shown in this SEM prior to being clad, is used for performing the parametric nonlinear optical process of four-wave mixing. Here we demonstrate low-power frequency conversion of an input signal, Is (in), to an output idler, Ii (out). Inset: Schematic of cross-section of waveguide.

Measured conversion efficiency (in the device shown above) as a function of wavelength for several ring resonances (red) and transmission spectrum (blue) in a 50-μm radius ring. A 15-dB enhancement of the FWM process due to the resonant cavity is observed. Inset: Sample on-resonance FWM spectrum with pump (P), signal (S), converted idler of interest (C1), and additional converted idler (C2).

We demonstrate germanium photodetectors integrated on submicron silicon waveguides fabricated with a low temperature ( ≤ 400C) wafer bonding and ion-cut process. The devices shows a low dark current of ~100 nA, a fiber accessed responsivity of > 0.4 A/W and an estimated quantum efficiency of above 90%. We are currently working on Ge detectors with performances > 40 GHz.

Schematics of (a) the integrated Ge photodetector on a silicon waveguide, and (b) the device cross section. (c) TE mode profiles of the input silicon waveguide and the two modes excited in the photodetector region.

(a) SEM top view image of the silicon waveguide without cladding. (b) SEM cross sectional image of the ion-cut Ge layer. (c) SEM top view image of the Ge pad with tapers on top of the silicon waveguide. The relatively low contrast between the silicon and SiO2 trenches is caused by the 40 nm SiO2 cladding layer on top of silicon. The Ge pad still has E-beam resist left. (d) Optical image of the fabricated Ge photodetector before contact pads.

Photonic Networks-on-Chip (NoC) have recently become popular as a viable option for increasing the bandwidth, lowering the latency and reducing the power in chip multiprocessors (CMPs). By utilizing an optical network to link multiple processors the full capability of large on-chip parallel systems can be achieved.

We demonstrated a spatially non-blocking optical 4x4 router with a footprint of 0.07 mm^2 for use in integrated photonic interconnection networks. The device was dynamically switched using thermo-optically tuned silicon microring resonators with a wavelength shift to power ratio of 0.25nm/mW. It was possible to route four optical inputs to four outputs with individual bandwidths of up to 38.5 GHz. All tested configurations successfully routed a single-wavelength laser and provided a maximum extinction ratio larger than 20 dB.

We have also shown the simultaneous all-optical switching of 20 continuous wave wavelength channels in a microring resonator-based silicon broadband 1x2 comb switch. Ssingle-channel power penalty measurements were performed during the active operation of the switch at both the through and the drop output ports. A statistical characterization of the drop-port insertion losses and extinction ratios of both ports showed broad spectral uniformity, and bit-error-rate measurements during passive operation indicated a negligible increase in signal degradation as the number of wavelength channels exiting the drop port are scaled from one to 16, with peak powers of -6 dBm per channel. A high-speed broadband switching device, such as the one used for this comb switch is a crucial element for the deployment of interconnection networks based on silicon photonic integrated circuits.

A 4x4 Gb/s microring modulator cascade, which can directly convert data from a parallel electrical bus to a
multiple-wavelength optical signal in a single silicon-on-insulatorwaveguide, was also demonstrated and characterized. The integrity of the modulated optical signal was verified using Q-factor extrapolations. In addition, the frequency characteristics and crosstalk, in terms of total harmonic distortion, were quantified. A transparent translator from electronics to optics such as this is crucial for the development of large-scale high-bandwidth interconnects based on photonic integrated circuits.

Microscope image showing heaters that are used to fine-tune the resonances of ring resonators. Electrical contact is made to the gold contact pads.

Microring resonators are used as individual routing devices. (a) An active switching element switches light at an intersection (insert shows switches in off state). (b) A simple 4x4 routing device highlighting a contention state where a red and a blue signal have used the same physical path.

Diagram of the device structure and experimental setup used for active measurements.

Schematic of space-parallel electronic (dotted wires) to wavelength parallel photonic (solid waveguides) translator.	Q-factor extrapolations from time-window measurements for the four modulated signals. The measured Q-factor (solid squares) is plotted against the fraction of the eye sampling window with linear curve fits. The curves appear from lowest (darkest) to highest wavelength (lightest). Eye diagrams for the four ptical signals are given from lowest (left) to highest (right) wavelength.

One of the exciting applications of integrated photonic devices is the controlled enhance light-matter interactions. Using resonant cavities one can increase on-chip light emission, sensing and non-linear phenomena. The figure of merit describing a resonant cavity’s ability to enhance these phenomena is the ratio of the cavity quality factor (Q) to the effective volume of the resonant mode (Veff). Veff is a measure of how concentrated the electric field is at its peak. Thus the ratio of Q/Veff describes how much the light matter interaction is enhanced in the region of the peak electric field. While much research has been focused on increasing the cavity quality factor, little progress has been shown in decreasing Veff. This is largely due to the belief that the smallest possible Veff is a cubic half wavelength. We have recently shown it is possible to reduce Veff to several orders of magnitude below what was once considered the fundamental limit. Utilizing the electric field discontinuity perpendicular to a dielectric interface, we can increase the value of the electric field by an order of magnitude in a low index slot imbedded a high index resonant cavity. This localized increase in the electric field results in a decrease in mode volume to several orders of magnitude smaller than a cubic half-wavelength. This could allow for enhanced nonlinearity and light emission from material placed inside these cavities.

(a) Topography of the resonant cavity as measured by an atomic force microscope. Inset shows a scanning electron micrograph corresponding to the dashed box. Arrows show the direction of light propagation. (b) Measured transmission through the cavity recorded simultaneously with the topography in (a). (c) Measured and calculated relative change in transmission (T'/T) and corresponding change in radiative lifetime as a function of the source-probe separation taken along the dashed lines in (b) and (d), respectively. (d) Calculated change in transmission as a function of probe position.

AFM image of the scanned SOI waveguide. (b) Simultaneously recorded TraNSOM image of the fundamental quasi-TM mode. (c) Simulated major component (|Ey|^2) of the fundamental quasi-TM mode. Dashed lines show the outline of the probe at three positions. Bold arrows show the path of the probe convolution. (d) Solid line shows the measured TraNSOM signal taken along the solid line in (b). Dashed line shows the simultaneously measured topography. Dotted line shows the probe-field convolution for all three polarization components.

We demonstrate an algorithm which evolves devices from a random start. The devices possess periodical patterns and demonstrate unprecedented geometry and ultra low modal volume of 0.112(lambda/2n)^3. The evolved structure indicates that periodicity is principal condition to effective control of distribution of light.

Evolution of a planar resonator in an evolutionary algorithm; (a) 1st generation, completely random device; (b) 100th generation, the bowtie shape is defined, (c) 200th generation, the bowtie shape is well defined and grating like structure begins to emerge; (d) 800th generation, the bowtie and the grating like structure are cleanly defined; each image is 120x120 pixels, each pixel represents 40x40nm square peg 250nm high in a 3D finite difference time domain simulation.

(a) Resonant mode amplitude (Ey) in a planar bowtie cavity; (b) radiative mode amplitude (E) in a bowtie, the dotted line represents the curvature of radiating field; insets, dielectric geometry of the respective devices, white: low index oxide 1.45, black: high index silicon 3.45.

Optical integration on chip has shown great progress in recent years with various optical devices being demonstrated on silicon. However, a high performance optical buffer on chip, a necessary component for optical information processing, remains to be demonstrated. In order to buffer optical information on chip, where the device dimensions are required to be small, the speed of light has to be significantly reduced. We show optical delays in a passive integrated structure where the fundamental tradeoff between the transmission and the delay is not present. The structure is composed by a double-ring resonator (middle figure below), whose spectrum has a narrow transparency peak with low group velocity analogous to that in electromagnetically induced transparency.

We also work on devices which demonstrate tunable superluminal propagation of light in silicon. We have shown tunable negative delays of up to 85 ps and effective group indices tunable between -1158 and -312.

Left: Experimental setup for coupling into the SOI planar waveguide. Right: Schematic configuration for generating and measuring delay. The signal and pump pulses are overlapped in time going into the waveguide.

Top view microscope image of the device to show Tunable Superluminal Propagation.

Guiding light in low-index materials such as air is thought to be prohibited in conventional waveguides based on total internal reflection (TIR). We demonstrate that the field can be enhanced and confined in a low-index material even when light is guided by TIR in a structure called slot-waveguide. This strong confinement in low-index materials relies on E-field discontinuity at high-index-contrast interfaces. This discontinuity can be used to strongly enhance and confine light in a nanometer-wide region of low-index material (see mode profile below). Since the guiding mode is indeed an eigenmode, the structure is fundamentally lossless and has very low wavelength sensitivity. Furthermore, we show that this structure is compatible with highly integrated photonics technology: we have fabricated and successfully characterized ring resonators based on the slot-waveguide, as shown in the SEM picture on the right.

SEM picture (top view) of a ring resonator based on the slot waveguide.

3D mode profile in the slot waveguide as obtained through Full-Vectorial Finite-Difference Mode Solver simulations.

Schematic representation of the structures of a single (a1) and a triple slot waveguide (b1). The normalized optical field (|E|^2) distributions are simulated using a numerical mode solver based on finite-difference time-domain (FDTD) methods. Their corresponding crosssectional SEM images to the right of the schematic drawings (a2) and (b2) show that the layered structures and each layer thickness are well controlled in fabrication.

There are many current research efforts to produce optical biosensors. Just as with other photonic devices, there are several advantages to shrinking down these devices and integrating them for use in high throughput Lab-on-a-chip applications such as single molecule detection and DNA sequencing. Some of our efforts involve the development of completely integrated optical biosensors using a variety of related technologies including fluorescence detection, refractive index changing sensors, and photonic bandgap devices. We are also exploring new biosensors applications based on new transducing methods using the manipulation of optical fields at sub-wavelength dimensions.

We also demonstrated an integrated microfluidic/photonic architecture for performing dynamic optofluidic trapping and transport of particles in the evanescent field of solid core waveguides. Our architecture consists of SU-8 polymer waveguides combined with soft lithography defined poly(dimethylsiloxane) (PDMS) microfluidic channels. The forces exerted by the evanescent field result in both the attraction of particles to the waveguide surface and propulsion in the direction of optical propagation both perpendicular and opposite to the direction of pressuredriven flow. Velocities as high as 28 μm/s were achieved for 3 μm diameter polystyrene spheres with an estimated 53.5 mW of guided optical power at the trapping location. The particle-size dependence of the optical forces in such devices is also characterized.

We also demonstrated a chip-scale photonic system for the room temperature detection of gas composition and pressure using a slotted silicon microring resonator. We measure shifts in the resonance wavelength due to the presence and pressure of acetylene gas and resolve differences in the refractive index as small as 0.0001 in the near-IR. The observed sensitivity of this device (enhanced due to the slot-waveguide geometry) agrees with the expected value of 490 nm/refractive index unit.

	Schematic of trapping experiment. The optical waveguide propulsion is perpendicular to the direction of the pressure driven flow in the channel.
Click the 3 links to view videos of opto-fluidic trapping: Movie1, Movie2, Movie3.

(a) Photograph of the gas cell used for on-chip gas sensing, which is affixed to the silicon photonic chip. The dotted line shows the path of the light through the waveguide and the circle denotes the approximate location of the microring. (b) Schematic of the experimental setup which was used to measure the resonant wavelength of the microring under different gaseous environments.