Cross-phase modulation-induced spectral and temporal effects on co-propagating femtosecond pulses in silicon photonic wires.

By performing time-resolved experiments and power-dependent measurements using femtosecond pulses inside submicron cross-section Si photonic-wire waveguides, we demonstrate strong cross-phase modulation (XPM) effects. We find that XPM in Si wires can be significant even for low peak pump powers, i.e., ~15 mW for pi phase shift. Our experimental data closely match numerical simulations using a rigorous coupled-wave theoretical treatment. Our results suggest that XPM is a potentially useful approach for all-optical control of photonic devices in Si wires.

Transmission of slow light through photonic crystal waveguide bends.

The spectral dependence of the bending loss of cascaded 60 degrees bends in photonic crystal (PhC) waveguides is explored in a slab-type silicon-on-insulator system. An ultralow bending loss of (0.05 +/- 0.03) dB/bend is measured at wavelengths corresponding to the nearly dispersionless transmission regime. In contrast, the PhC bend is found to become completely opaque for wavelengths corresponding to the slow-light regime. A general strategy is presented and experimentally verified to optimize the bend design for improved slow-light transmission.

Coupling into the slow light mode in slab-type photonic crystal waveguides.

Coupling external light signals into a photonic crystal (PhC) waveguide becomes increasingly inefficient as the group velocity of the waveguiding mode slows down. We have systematically studied the efficiency of coupling in the slow light regime for samples with different truncations of the photonic lattice at the coupling interface between a strip waveguide and a PhC waveguide. An inverse power law dependence is found to best fit the experimental scaling of the coupling loss on the group index. Coupling efficiency is significantly improved up to group indices of 100 for a truncation of the lattice that favors the appearance of photonic surface states at the coupling interface in resonance with the slow light mode.

Ultrafast-pulse self-phase modulation and third-order dispersion in Si photonic wire-waveguides.

By propagating femtosecond pulses inside submicron-crosssection Si photonic-wire waveguides with anomalous dispersion, we demonstrate that the pulse-propagation dynamics is strongly influenced by the combined action of optical nonlinearity and up to third-order dispersion with minimal carrier effects. Because of strong light confinement, a nonlinear phase shift of a few pi due to self-phase modulation is observed at a pulse peak-power of just ~250 mW. We also observe soliton-emitted radiation, fully supported by theoretical analysis, from which we determine directly the third-order dispersion coefficient, beta(3) = -0.73 +/- 0.05 ps(3)/m at 1537 nm.

Active control of slow light on a chip with photonic crystal waveguides.

It is known that light can be slowed down in dispersive materials near resonances. Dramatic reduction of the light group velocity-and even bringing light pulses to a complete halt-has been demonstrated recently in various atomic and solid state systems, where the material absorption is cancelled via quantum optical coherent effects. Exploitation of slow light phenomena has potential for applications ranging from all-optical storage to all-optical switching. Existing schemes, however, are restricted to the narrow frequency range of the material resonance, which limits the operation frequency, maximum data rate and storage capacity. Moreover, the implementation of external lasers, low pressures and/or low temperatures prevents miniaturization and hinders practical applications. Here we experimentally demonstrate an over 300-fold reduction of the group velocity on a silicon chip via an ultra-compact photonic integrated circuit using low-loss silicon photonic crystal waveguides that can support an optical mode with a submicrometre cross-section. In addition, we show fast (approximately 100 ns) and efficient (2 mW electric power) active control of the group velocity by localized heating of the photonic crystal waveguide with an integrated micro-heater.

Spontaneous Raman scattering in ultrasmall silicon waveguides.

We report spontaneous Raman scattering at 1550 nm in ultrasmall silicon-on-insulator (SOI) strip waveguides of 0.098-microm2 cross-sectional area. The submicrometer-scale dimensions provide tight optical confinement and, hence, highly efficient Raman scattering with milliwatt-level cw pump powers. The prospect of Raman amplification in such a deeply scaled-down waveguide device in the presence of various loss mechanisms, particularly free-carrier loss that arises from two-photon absorption, is discussed, and the feasibility of high-gain SOI-based fully integrated optical amplifiers is shown.

Observation of surface states in a truncated photonic crystal slab.

The effect of lattice termination on the surface states in a two-dimensional truncated photonic crystal slab is experimentally studied in a high-index-contrast silicon-on-insulator system. A single-mode silicon strip waveguide that is separated from the photonic crystal by a trench of variable width is used to evanescently couple to surface states in the surrounding lattice. It is demonstrated that the dispersion of the surface states depends strongly on the specific termination of the lattice.