• Compact Fiber to Waveguide Couplers
• Micrometer-scale Silicon Electro-Optic Modulator
• Ultra Small Mode Volumes
• Novel Photonic Structure Design Using an Evolutionary Algorithm
• Slow Light on Chip
• Subwavelength Light Confinement in Integrated Metal Slot Waveguide on Silicon
• Slot-Waveguide for Strong Confinement of Light in Low-Index Materials
• Rare Earth Doped GaN Powder and Applications
• Quantum Dot Photonics
• Photonics for Biosensing
• All-optical Switching in Silicon

Published papers: Nano-taper for Compact Mode Conversion, Optics Letters

High refractive index contrast allows the fabrication of nanophotonic structures such as waveguides and photonic crystals. Coupling to and from these devices usually involves high losses (more than 7 dB) due to the large index and mode dimensions mismatches between the fiber and the waveguide structure which induces coupling to radiation modes and back-reflection.

Up to date, most of the on-chip structures suggested to alleviate this coupling problem suffer from one of the following drawbacks: very long (hundreds of micrometers), presence of strong back reflections, or difficulty of fabrication. Here we propose a set of micrometer-long devices leading to extremely low coupling losses between an optical fiber and a high-index contrast waveguide.

The structures are composed of high index contrast materials, and consist of one or multiple nano-size tips tapered to the waveguide dimensions. They convert both the mode size and the effective index of a waveguide to that of a fiber. Using structures based on Si/SiO₂, we numerically demonstrate coupling efficiency of more than 90% in a 40 micron long device.

We fabricated and characterized such couplers and obtained enhancement of coupling efficiency of about one order of magnitude, for both TE and TM-like modes over the 1520 - 1620 nm range.

Published papers:	Micrometre-scale silicon electro-optic modulator, Nature, 19 May 2005. Micrometer-scale all-optical wavelength converter on silicon, Opt. Lett., Oct. 2005. Ultrafast all-optical modulation on a silicon chip , Opt. Lett., Nov. 2005.

Metal interconnections are expected to become the bottleneck of performance of electronic systems as transistors continue to scale. Optical interconnections, implemented at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections could enable low power dissipation, low latencies and high bandwidths. The realization of such small-scale optical interconnections relies on the ability to integrate micro-optical devices with the microelectronics chip. We experimentally demonstrated the first high-speed electro-optical modulator in highly 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 microns 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 not previously possible.

Published papers: Ultrasmall Mode Volumes in Dielectric Optical Microcavities, Phys. Rev. Lett.

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.

Published papers: Spontaneous Emergence of Periodic Patterns in a Biologically Inspired Simulation of Photonic Structures, Phys. Rev. Lett.

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.

Published papers:

Direct measurement of tunable optical delays on chip analogue to electromagnetically induced transparency, Optics Express.

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 (first figure below)), whose spectrum has a narrow transparency peak with low group velocity analogous to that in electromagnetically induced transparency. Direct time-domain measurement of tunable optical delay in a silicon resonating structure is presented. Effective group indices from 90 to 290 are obtained by tuning the resonator thermally. The measurements agree well with the theoretical analysis.

Published papers:	Guiding and Confining Light in Void Nanostructure, Opt. Lett. Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material, Opt. Lett.

Guiding light in low-index materials such as air is thought to be prohibited in conventional waveguides based on total internal reflection (TIR). We propose and 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 (see SEM picture below, left). 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 proposed 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.

Corresponding publications:

Q. Xu, V. R. Almeida and M. Lipson, Time-resolved study of Raman gain in highly confined silicon-on-insulator waveguides, Optics Express, Vol. 12, No. 19, 4437-4442, 20 September 2004.

Q. Xu, V. R. Almeida and M. Lipson, Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material, Optics Letters, Vol. 29, No. 14, 1626, July 2004.

Published papers:	Photoluminescence and cathodoluminescence analyses of GaN powder doped with Eu Green emission from Er-doped GaN powder

Group III nitrides have wide applications in optoelectronic devices, such as light-emitting diodes, ultraviolet or blue lasers, and full color displays.The band gaps of III nitrides range from the ultraviolet to the near infrared. Furthermore, these semiconductor materials show very promising applications as electroluminescent devices when they are doped with rare earth ions.Wide band gap semiconductors, like gallium nitride GaN, doped with rare earth elements are especially interesting because of the decreased quenching effects of the luminescence at room temperature. GaN:RE powder enables the possibility of hybrid integration with a variety of optically inactive materials because of the flexible powder form.

Published papers: Photoluminescence enhancement of colloidal quantum dots embedded in a monolithic microcavity, Apl. Phys. Lett.

Photonic crystals exhibit novel features, arising from our ability to tailor the photon density of states in a prescribed manner. When embedding active materials in the photonic crystals important quantum electrodynamics (QED) effects are expected to be observed. These include for example modification of radiative lifetime, collective switching and thresholdless lasing. We intend to achieve strong light-matter interaction of colloidal Quantum Dots in photonic crystals. Sub-micron size devices are possible using light confining structures, since the effective optical interaction is enhanced by orders of magnitude. These systems could be integrated into current technologies for devices applications. This research will introduce a new class of photonic devices, where basic atomic properties such as energy levels and radiative lifetime could be controlled externally.

Published papers:	Subwavelength Confinement in Integrated Metal Slot Waveguide on Silicon, Opt. Lett.

There is a growing research interest in optical circuits at the nanometer scale for future integration. For this goal, however, the typical dimensions of conventional dielectric waveguides are dictated by diffraction, therefore limiting dense on-chip integration. Plasmonic waveguides such as nanoparticle chains, nanorods, in contrast, guide light through the interaction of photon and electron oscillation around the metal surface, and are potential candidates for nanoscale optical elements with sizes much smaller than the diffraction limit. Here we demonstrate a low loss plasmonic waveguide with predicted confinement substantially below the optical wavelength (~1.55 um). We also demonstrate efficient integration of the plasmonic waveguide with dielectric silicon wire waveguides using very compact tapers. The realization of deep subwavelength confinement and efficient coupling with standard dielectric silicon waveguides has attractive applications in nanoscale circuits and on-chip integration of optical, optoelectronic and electronic devices.

Published papers:	Nano-cavity in a Silicon waveguide for ultra sensitive detection, Appl. Phys. Lett. Dendrimer-Scaffold-based electron-beam patterning of biomolecules , Adv. Mat.

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.