Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses.

We characterize the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared between 3.3 µm and 4 µm using femtosecond optical pulses. We estimate the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index from the experimental measurements. The effect of multiphoton absorption on the optical nonlinear Kerr response is evaluated and the nonlinear figure of merit estimated providing some guidelines for designing nonlinear optical devices in the mid-IR. Finally, we compare the impact of free-carrier absorption at mid-infrared wavelengths versus near-infrared wavelengths for these ultra-short pulses.

Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared.

We have investigated the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared wavelength range from 3.25- 4.75µm using picosecond optical pulses. We observed and measured the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index. The dynamics of the spectral broadening suggests that, in addition to multiphoton absorption, the corresponding higher order nonlinear refractive phenomena also needs to be included when high optical pulse intensities are used at mid-infrared wavelengths in this material.

Amorphous silicon nanowires combining high nonlinearity, FOM and optical stability.

We demonstrate optically stable amorphous silicon nanowires with both high nonlinear figure of merit (FOM) of ~5 and high nonlinearity Re(γ) = 1200W(-1)m(-1). We observe no degradation in these parameters over the entire course of our experiments including systematic study under operation at 2 W coupled peak power (i.e. ~2GW/cm(2)) over timescales of at least an hour.

Slow-light dispersion engineering of photonic crystal waveguides using selective microfluidic infiltration.

We experimentally demonstrate dispersion engineering of slow light photonic crystal (PhC) waveguides using selective infiltration of the first two rows of air holes with high index ionic liquids. The infiltrated PhC waveguide exhibits a dispersion window of 3 nm with a nearly constant group velocity of ~c/80 that depends on the liquid physical properties. We investigate how the effective refractive index changes in time due to the dynamics of the liquids in the holes. This demonstration highlights the versatility, flexibility, and tunability offered by optofluidics in PhC circuits.

Reconfigurable photonic crystal waveguides created by selective liquid infiltration.

We experimentally demonstrate reconfigurable photonic crystal waveguides created directly by infiltrating high refractive index (n≈2.01) liquids into selected air holes of a two-dimensional hexagonal periodic lattice in silicon. The resulting effective index contrast is large enough that a single row of infiltrated holes enables light propagation at near-infrared wavelengths. We include a detailed comparison between modeling and experimental results of single line defect waveguides and show how our infiltration procedure is reversible and repeatable. We achieve infiltration accuracy down to the single air hole level and demonstrate control on the volume of liquid infused into the holes by simply changing the infiltration velocity. This method is promising for achieving a wide range of targeted optical functionalities on a "blank" photonic crystal membrane that can be reconfigured on demand.

Four-wave mixing in slow light engineered silicon photonic crystal waveguides.

We experimentally investigate four-wave mixing (FWM) in short (80 µm) dispersion-engineered slow light silicon photonic crystal waveguides. The pump, probe and idler signals all lie in a 14 nm wide low dispersion region with a near-constant group velocity of c/30. We measure an instantaneous conversion efficiency of up to -9dB between the idler and the continuous-wave probe, with 1W peak pump power and 6 nm pump-probe detuning. This conversion efficiency is found to be considerably higher (>10 × ) than that of a Si nanowire with a group velocity ten times larger. In addition, we estimate the FWM bandwidth to be at least that of the flat band slow light window. These results, supported by numerical simulations, emphasize the importance of engineering the dispersion of PhC waveguides to exploit the slow light enhancement of FWM efficiency, even for short device lengths.

Optical signal processing on a silicon chip at 640Gb/s using slow-light.

We demonstrate optical performance monitoring of in-band optical signal to noise ratio (OSNR) and residual dispersion, at bit rates of 40Gb/s, 160Gb/s and 640Gb/s, using slow-light enhanced optical third harmonic generation (THG) in a compact (80microm) dispersion engineered 2D silicon photonic crystal waveguide. We show that there is no intrinsic degradation in the enhancement of the signal processing at 640Gb/s relative to that at 40Gb/s, and that this device should operate well above 1Tb/s. This work represents a record 16-fold increase in processing speed for a silicon device, and opens the door for slow light to play a key role in ultra-high bandwidth telecommunications systems.

Liquid crystal dynamics in a photonic crystal cavity created by selective microfluidic infiltration.

A microfluidic double heterostructure cavity is created in a silicon planar photonic crystal waveguide by selective infiltration of a liquid crystal. The spectral evolution of the cavity resonances probed by evanescent coupling reveals that the liquid crystal evaporates, even at room temperature, despite its relatively low vapor pressure of 5 × 10(-3) Pa. We explore the infiltration and evaporation dynamics of the liquid crystal within the cavity using a Fabry-Perot model that accounts for the joint effects of liquid volume reduction and cavity length variation due to liquid evaporation. While discussing how the pattern of the infiltrated liquid can be optimized to restrict evaporation, we find that the experimental behavior is consistent with basic microfluidic relations considering the small volumes of liquids and large surface areas present in our structure.

A proposal for enhancing four-wave mixing in slow light engineered photonic crystal waveguides and its application to optical regeneration.

In this paper, we investigate both analytically and numerically four-wave mixing (FWM) in short (80 microm) dispersion engineered slow light photonic crystal waveguides. We demonstrate that both a larger FWM conversion efficiency and an increased FWM bandwidth (approximately 10 nm) can be achieved in these waveguides as compared to dispersive PhC waveguides. This improvement is achieved through the net slow light enhancement of the FWM efficiency (almost 30dB as compared to a fast nanowire of similar length), even in the presence of slow light increased linear and nonlinear losses, and the suitable dispersion profile of these waveguides. We show how such improved FWM operation can be advantageously exploited for designing a compact 2R and 3R regenerator with the appropriate nonlinear power transfer function.

Dispersion engineering of slow light photonic crystal waveguides using microfluidic infiltration.

We present a technique based on the selective liquid infiltration of photonic crystal (PhC) waveguides to produce very small dispersion slow light over a substantial bandwidth. We numerically demonstrate that this approach allows one to control the group velocity (from c/20 to c/110) from a single PhC waveguide design, simply by choosing the index of the liquid to infiltrate. In addition, we show that this method is tolerant to deviations in the PhC parameters such as the hole size, which relaxes the constraint on the PhC fabrication accuracy as compared to previous structural-based methods for slow light dispersion engineering.

Strain, size, and composition of InAs quantum sticks embedded in InP determined via grazing incidence x-ray anomalous diffraction.

We have used x-ray anomalous diffraction to recover the model-independent Fourier transform (x-ray structure factor) of InAs quantum sticklike islands embedded in InP. The average height of the quantum sticks, as deduced from the width of the structure factor profile, is 2.54 nm. The InAs out-of-plane deformation, relative to InP, is 6.1%. Diffraction anomalous fine structure provides evidence of pure InAs quantum sticks. Finite difference method calculations reproduce well the diffraction data, and give the strain along the growth direction. The chemical mixing at interfaces is also analyzed.