Roberto R. Panepucci

All-optical control of light on a silicon chip

Photonic circuits, in which beams of light redirect the flow of other beams of light, are a long-standing goal for developing highly integrated optical communication components. Furthermore, it is highly desirable to use silicon—the dominant material in the microelectronic industry—as the platform for such circuits. Photonic structures that bend, split, couple and filter light have recently been demonstrated in silicon, but the flow of light in these structures is predetermined and cannot be readily modulated during operation. All-optical switches and modulators have been demonstrated with III–V compound semiconductors, but achieving the same in silicon is challenging owing to its relatively weak nonlinear optical properties. Indeed, all-optical switching in silicon has only been achieved by using extremely high powers in large or non-planar structures, where the modulated light is propagating out-of-plane. Such high powers, large dimensions and non-planar geometries are inappropriate for effective on-chip integration. Here we present the experimental demonstration of fast all-optical switching on silicon using highly light-confining structures to enhance the sensitivity of light to small changes in refractive index. The transmission of the structure can be modulated by up to 94% in less than 500 ps using light pulses with energies as low as 25 pJ. These results confirm the recent theoretical prediction of efficient optical switching in silicon using resonant structures.

Nanotaper for compact mode conversion.

We propose and demonstrate an efficient coupler for compact mode conversion between a fiber and a submicrometer waveguide. The coupler is composed of high-index-contrast materials and is based on a short taper with a nanometer-sized tip. We show that the micrometer-long silicon-on-insulator-based nanotaper coupler is able to efficiently convert both the mode field profile and the effective index, with a total length as short as 40 microm. We measure an enhancement of the coupling efficiency between an optical fiber and a waveguide by 1 order of magnitude due to the coupler.

Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material

We experimentally demonstrate a novel silicon waveguide structure for guiding and confining light in nanometer-wide low-refractive-index material. The optical field in the low-index material is enhanced because of the discontinuity of the electric field at high-index-contrast interfaces. We measure a 30% reduction of the effective index of light propagating in the novel structure due to the presence of the nanometer-wide low-index region, evidencing the guiding and confinement of light in the low-index material. We fabricate ring resonators based on the structure and show that the structure can be implemented in highly integrated photonics.

All-optical switching on a silicon chip

We present an experimental demonstration of fast all-optical switching on a silicon photonic integrated device by employing a strong light-confinement structure to enhance sensitivity to small changes in the refractive index. By use of a control light pulse with energy as low as 40 pJ, the optical transmission of the structure is modulated by more than 97% with a time response of 450 ps.

Electrooptic modulation of silicon-on-insulator submicrometer-size waveguide devices

In this paper, we propose and analyze an electrically modulated silicon-on-insulator (SOI) submicrometer-size high-index-contrast waveguide. The geometry of the waveguide provides high lateral optical confinement and defines a lateral p-i-n diode. The electrooptic structure is electrically and optically modeled. The effect of the waveguide geometry on the device performance is studied. Our calculations indicate that this scheme can be used to implement submicrometer high-index-contrast waveguide active devices on SOI. As an example of application, a one-dimensional microcavity intensity modulator is predicted to exhibit a modulation depth as high as 80% by employing a dc power consumption as low as 14 /spl mu/W.

Compact silicon tunable Fabry-Perot resonator with low power consumption

We demonstrate a 20-/spl mu/m-long tunable optical resonator integrated on a silicon-on-insulator waveguide. The microresonator consists of a planar Fabry-Perot microcavity defined by deep Si/SiO/sub 2/ Bragg reflectors with a high finesse of 11.2. The device is electrically driven and shows a modulation depth as high as 53% for a power consumption of only 20 mW.

Photonic crystals in polymers by direct electron-beam lithography presenting a photonic band gap

Direct lithography in electron-beam sensitive polymers was investigated to fabricate 2D-slab polymer-based photonic crystal structures. Polymethyl-methacrylate (PMMA) doped with azo dye Disperse Red 1 (DR1) chromophores was used as a test material to produce optimized low-index contrast photonic crystals presenting a photonic band gap for TE polarization. Extensive computational simulations of the full 3D-slab modes guided the design and fabrication strategy through optimization of the lattice structure, lattice parameter, hole size, and slab thickness. An exposure strategy that takes advantage of 100kV beam energy for deep lithography, and exposure control for multilevel pattern definition is presented, resulting in the high aspect ratio and verticality required to achieve a strong band gap effect. Finally, a method that enables a high-quality air-clad PMMA-DR1 to be fabricated and integrated with optical waveguides for characterization is presented, enabling successful observation of a photonic bandgap ...

Light Guiding in Low Index Materials using High-Index-Contrast Waveguides

We propose a novel high-index-contrast waveguide structure capable of light strong confinement and guiding in low-refractive-index materials. The principle of operation of this structure relies on the electric field (E-field) discontinuity at the interface between high-indexcontrast materials. We show that by using such a structure the E-field can be strongly confined in a 50-nm-wide low-index region with normalized average intensity of 20 µm -2 . This intensity is approximately 20 times higher than that can be achieved in SiO2 with conventional rectangular or photonic crystal waveguides.

Novel SU-8 optical waveguide microgripper for simultaneous micromanipulation and optical detection

A novel microgripper device structure with optical waveguides as arms is presented. The devices shown here have waveguide dimensions of 50×30μm2. The device fabrication, using SU-8 polymer, is described. Its capability to perform micromanipulation while simultaneously carrying out optical detection is demonstrated.

Waveguide Microgripper Power Distribution

We studied the factors affecting the power distribution across the waveguide facet of novel microgrippers. Knife edge measurements, microscopy imaging and 2D-FDTD numerical simulations are carried to study the effects of specifications of series microgrippers.