Vilson R. Almeida

Guiding and confining light in void nanostructure

We present a novel waveguide geometry for enhancing and confining light in a nanometer-wide low-index material. Light enhancement and confinement is caused by large discontinuity of the electric field at high-index-contrast interfaces. We show that by use of such a structure the field can be confined in a 50-nm-wide low-index region with a normalized intensity of 20 microm(-2). This intensity is approximately 20 times higher than what can be achieved in SiO2 with conventional rectangular waveguides.

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.

Optical bistability on a silicon chip

We demonstrate, for the first time to our knowledge, optical bistability on a highly integrated silicon device, using a 5-microm-radius ring resonator. The strong light-confinement nature of the resonator induces nonlinear optical response with low pump power. We show that the optical bistability allows all-optical functionalities, such as switching and memory with microsecond time response and a modulation depth of 10 dB, driven by pump power as low as 45 microW. Silicon optical bistability relies on a fast thermal nonlinear optical effect presenting a 500-kHz modulation bandwidth.

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.

Time-resolved study of Raman gain in highly confined silicon-on-insulator waveguides

We show time-resolved measurement of Raman gain in Silicon submicron-size planar waveguide using picosecond pump and probe pulses. A net nonlinear gain of 6 dB is obtained in a 7-mm long waveguide with 20.7-W peak pump power. We demonstrate an ultrafast all-optical switch based on the free-carrier dispersion effect in the silicon waveguide, whose transmission is enhanced by more than 13 dB due to the Raman effect.

Micrometer-scale all-optical wavelength converter on silicon

We demonstrate a highly integrated micrometer-scale low-power wavelength converter based on the free-carrier dispersion effect in silicon. The conversion is achieved through all-optical modulation of a silicon ring resonator by use of modulated cw control light. The ring resonator has a radius of 5 microm and a Q of approximately 10,000. Both inverted and noninverted modulation are achieved at a bit rate of 0.9 Gbits/s with a control power of 4.5 mW. The scaling of the required control power for operation with respect to the characteristics of the ring resonator is established.

Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide

We show high Raman gain in a silicon submicrometer-size planar waveguide. Using high-confinement structures and picosecond pump pulses, we show 3.1-dB net internal gain with 2.8-W peak pump power in a 7-mm-long waveguide. We also analyze experimentally and theoretically the effect of free-carrier absorption on the Raman gain.