Bradley Schmidt

Micrometre-scale silicon electro-optic modulator

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Broad-band optical parametric gain on a silicon photonic chip

Developing an optical amplifier on silicon is essential for the success of silicon-on-insulator (SOI) photonic integrated circuits. Recently, optical gain with a 1-nm bandwidth was demonstrated using the Raman effect, which led to the demonstration of a Raman oscillator, lossless optical modulation and optically tunable slow light. A key strength of optical communications is the parallelism of information transfer and processing onto multiple wavelength channels. However, the relatively narrow Raman gain bandwidth only allows for amplification or generation of a single wavelength channel. If broad gain bandwidths were to be demonstrated on silicon, then an array of wavelength channels could be generated and processed, representing a critical advance for densely integrated photonic circuits. Here we demonstrate net on/off gain over a wavelength range of 28 nm through the optical process of phase-matched four-wave mixing in suitably designed SOI channel waveguides. We also demonstrate wavelength conversion in the range 1,511–1,591 nm with peak conversion efficiencies of +5.2 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 four-wave mixing in silica fibres, can now be transferred to the SOI platform.

Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides

The ability to manipulate nanoscopic matter precisely is critical for the development of active nanosystems. Optical tweezers are excellent tools for transporting particles ranging in size from several micrometres to a few hundred nanometres. Manipulation of dielectric objects with much smaller diameters, however, requires stronger optical confinement and higher intensities than can be provided by these diffraction-limited systems. Here we present an approach to optofluidic transport that overcomes these limitations, using sub-wavelength liquid-core slot waveguides. The technique simultaneously makes use of near-field optical forces to confine matter inside the waveguide and scattering/adsorption forces to transport it. The ability of the slot waveguide to condense the accessible electromagnetic energy to scales as small as 60 nm allows us also to overcome the fundamental diffraction problem. We apply the approach here to the trapping and transport of 75-nm dielectric nanoparticles and λ-DNA molecules. Because trapping occurs along a line, rather than at a point as with traditional point traps, the method provides the ability to handle extended biomolecules directly. We also carry out a detailed numerical analysis that relates the near-field optical forces to release kinetics. We believe that the architecture demonstrated here will help to bridge the gap between optical manipulation and nanofluidics.

Tailored anomalous group-velocity dispersion in silicon channel waveguides.

We present the first experimental demonstration of anomalous group-velocity dispersion (GVD) in silicon waveguides across the telecommunication bands. We show that the GVD in such waveguides can be tuned from -2000 to 1000 ps/(nm*km) by tailoring the cross-sectional size and shape of the waveguide.

Generation of correlated photons in nanoscale silicon waveguides

.We experimentally study the generation of correlated pairs of photons through four-wave mixing (FWM) in embedded silicon waveguides. The waveguides, which are designed to exhibit anomalous group-velocity dispersion at wavelengths near 1555 nm, allow phase matched FWM and thus efficient pair-wise generation of non-degenerate signal and idler photons. Photon counting measurements yield a coincidence-to-accidental ratio (CAR) of around 25 for a signal (idler) photon production rate of about 0.05 per pulse. We characterize the variation in CAR as a function of pump power and pump-to-sideband wavelength detuning. These measurements represent a first step towards the development of tools for quantum information processing which are based on CMOS-compatible, silicon-on-insulator technology.

Photolithographic patterning of organic electronic materials

Hydrofluoroethers are shown to be benign solvents to a wide variety of organic electronic materials, even at extreme conditions such as boiling temperature. Coupled with fluorous functional photoresist-acidsensitive semi-perfluoroalkyl resorcinarene, they open new frontiers for photolithographical patterning for organic electronic systems. Summary of Research: Organic electronics is emerging as a promising technology to enable mechanically flexible devices through solution processing of organic materials [1]. As with traditional electronics, organic devices require active functional materials to be tailored into micropatterned and multi-layered device components. While the former relies on photolithographic patterning techniques, the latter is restricted from adopting such robust, high-resolution and high-throughput techniques because of the chemical compatibility issue between organic materials and patterning agents [3]. Namely, deterioration of materials’ performance occurs during the photoresist deposition and removal stages due to aggressive organic solvents, as well as in the pattern development steps by aqueous base solutions. In our search for universal, materials-friendly solvents, we have identified environmentally benign fluorous solvents combined with specifically tailored patterning materials as a possible solution to this complex problem. Fluorous solvents are poor solvents for non-fluorinated organic materials [2]. Among the variety of fluorous solvents, segregated hydrofluoroethers (HFEs) attracted our attention because of their nonflammability, zero ozone-depletion potential and low toxicity for humans [3]. We tested the impact of HFEs solvents on wellcharacterized and commercially available organic electronic materials. We demonstrated that HFE solvents do not damage or alter electronic and optoelectronic properties of wide class of organic electrnic materials, including: organic semiconductors (pentacene and poly-3-hexylthiophene (P3HT)), conducting polymer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and organic light emitting polymers and small molecule compounds (polyfluorenes, and [Ru(bpy)3] (PF6 –)2 complex). To further demonstrate the orthogonality of HFEs to active organic material as well as to organic/metal interface we used aforementioned organic materials to make organic light emitting diodes (OLEDs) and thin film transistors (TFTs) which we characterized before and after exposing to HFE [4]. We found HFE did not significantly change the characteristic of tested devices even at elevated temperatures. For example, Figure 1 shows [Ru(bpy)3] (PF66 –)2 based electroluminescent device [5] in boiling HFE 7100 (61°C). We operated the device in the boiling HFE for one hour and did not observe any substantial change in its performance. This new dimension in solvent orthogonality which is enabled by the use of HFEs offers unique opportunities for the chemical processing of organic electronic materials. One example is in the area of photolithographic processing: One can use a photoresist that is properly fluorinated to be processable in HFEs [6]. We have successfully demonstrated this approach

Optofluidic trapping and transport on solid core waveguides within a microfluidic device

In this work we demonstrate 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 pressure-driven flow. Velocities as high as 28 mum/s were achieved for 3 mum 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.

High Speed Carrier Injection 18 Gb/s Silicon Micro-ring Electro-optic Modulator

We experimentally demonstrate electrooptic modulation in silicon at 18 Gbps (NRZ) in a micro-ring of 12 micron diameter using a pre-emphasis technique. Device simulations indicate that this technique can extend the bit rate to 40 Gbps.

Compact electro-optic modulator on silicon-on-insulator substrates using cavities with ultra-small modal volumes.

We experimentally demonstrate a micron-size electro-optic modulator using a high-index-contrast silicon Fabry-Perot resonator cavity. This compact device consists of a 1-D cavity formed within a single mode silicon channel waveguide and an embedded p-i-n junction on a silicon-oninsulator platform. The entire device is 6.0 microns in length. We demonstrate modulation depths as large as 5.87 dB at speeds of 250 Mbps limited only by fabrication imperfections, with optimized theoretical speeds of several Gbps.

Ultrafast all-optical modulation on a silicon chip

We experimentally demonstrate ultrafast all-optical modulation using a micrometer-sized silicon photonic integrated device. The device transmission is strongly modulated by photoexcited carriers generated by low-energy pump pulses. A p-i-n junction is integrated on the structure to permit control of the generated carrier lifetimes. When the junction is reverse biased, carriers are extracted from the device in a time as short as 50 ps, permitting greater than 5 Gbit/s modulation of optical signals on a silicon chip.