Jerry I. Dadap

Raman amplification in ultrasmall silicon-on-insulator wire waveguides.

We measure stimulated Raman gain at 1550 nm in an ultrasmall SOI strip waveguide, cross-section of 0.098 microm2. We obtain signal amplification of up to 0.7 dB in the counter-propagating configuration for a sample length of 4.2 mm and using a diode pump at 1435 nm with powers of <30 mW. The Raman amplifier has a figure-of-merit (FOM) of 57.47 dB/cm/W. This work shows the feasibility of ultrasmall SOI waveguides for the development of SOI-based on-chip optical amplifiers and active photonic integrated circuits.

C-band wavelength conversion in silicon photonic wire waveguides

We demonstrate C-band wavelength conversion in Si photonic-wire waveguides with submicron cross-section by means of nonresonant, nondegenerate four-wave mixing (FWM) using low-power, cw-laser sources. Our analysis shows that for these deeply scaled Si waveguides, FWM can be observed despite the large phase mismatch imposed by strong waveguide dispersion. The theoretical calculations agree well with proof-of-concept experiments. The nonresonant character of the FWM scheme employed allows to demonstrate frequency tuning of the idler from ~ 20 GHz to > 100 GHz thus covering several C-band DWDM channels.

Theory of optical second-harmonic generation from a sphere of centrosymmetric material: small-particle limit

The electromagnetic theory of optical second-harmonic generation from small spherical particles comprised of centrosymmetric material is presented. The interfacial region where the inversion symmetry is broken provides a source of the nonlinearity. This response is described by a general surface nonlinear susceptibility tensor for an isotropic interface. In addition, the appropriate weak bulk terms for an isotropic centrosymmetric medium are introduced. The linear optical response of the sphere and the surrounding region is assumed to be isotropic, but otherwise arbitrary. The analysis is carried out to leading order in the ratio of (a/λ), the particle radius to the wavelength of the incident light, and can be considered as the Rayleigh limit for second-harmonic generation from a sphere. Emission from the sphere arises from both induced electric dipole and electric quadrupole moments at the second-harmonic frequency. The former requires a nonlocal excitation mechanism in which the phase variation of the pump beam across the sphere is considered, while the latter is present for a local-excitation mechanism. The locally excited electric dipole term, analogous to the source for linear Rayleigh scattering, is absent for the nonlinear case because of the overall inversion symmetry of the problem. The second-harmonic field is found to scale as (a/λ)3 and to be completely determined by two effective nonlinear susceptibility coefficients formed as a prescribed combination of the surface and bulk nonlinearities. Characteristic angular and polarization selection rules resulting from the mechanism of the radiation process are presented. Various experimental aspects of the problem are examined, including the expected signal strengths and methods of determining the nonlinear susceptibilities. The spectral characteristics associated with the geometry of a small sphere are also discussed, and distinctive localized plasmon resonances are identified.

Engineering nonlinearities in nanoscale optical systems: physics and applications in dispersion-engineered silicon nanophotonic wires

The nonlinear optics of Si photonic wires is discussed. The distinctive features of these waveguides are that they have extremely large third-order susceptibility χ(3) and dispersive properties. The strong dispersion and large third-order nonlinearity in Si photonic wires cause the linear and nonlinear optical physics in these guides to be intimately linked. By carefully choosing the waveguide dimensions, both linear and nonlinear optical properties of Si wires can be engineered. We review the fundamental optical physics and emerging applications for these Si wires. In many cases, the relatively low threshold powers for nonlinear optical effects in these wires make them potential candidates for functional on-chip nonlinear optical devices of just a few millimeters in length; conversely, the absence of nonlinear optical impairment is important for the use of Si wires in on-chip interconnects. In addition, the characteristic length scales of linear and nonlinear optical effects in Si wires are markedly different from those in commonly used optical guiding systems, such as optical fibers or photonic crystal fibers, and therefore guiding structures based on Si wires represent ideal optical media for investigating new and intriguing physical phenomena.

Ultrahigh-Bandwidth Silicon Photonic Nanowire Waveguides for On-Chip Networks

An investigation of signal integrity in silicon photonic nanowire waveguides is performed for wavelength-division-multiplexed optical signals. First, we demonstrate the feasibility of ultrahigh-bandwidth integrated photonic networks by transmitting a 1.28-Tb/s data stream (32 wavelengths times 40-Gb/s) through a 5-cm-long silicon wire. Next, the crosstalk induced in the highly confined waveguide is evaluated, while varying the number of wavelength channels, with bit-error-rate measurements at 10 Gb/s per channel. The power penalty of a 24-channel signal is 3.3 dB, while the power penalty of a single-channel signal is 0.6 dB. Finally, single-channel power penalty measurements are taken over a wide range of input powers and indicate negligible change for launch powers of up to 7 dBm.

Asymmetric Y junctions in silicon waveguides for on-chip mode-division multiplexing.

Silicon waveguide asymmetric Y junction mode multiplexers and demultiplexers are demonstrated for applications in on-chip mode-division multiplexing (MDM). We measure demultiplexed crosstalk as low as -30 dB, <-9 dB over the C band, and insertion loss <1.5 dB for multimode links up to 1.2 mm in length. The frequency response of these devices is shown to depend upon Y junction angle and multimode interconnect length. Interference effects are shown to be advantageous for low-crosstalk MDM, even while using compact Y junctions designed to be outside the mode-sorting regime.

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.

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.

Nonlinear-optical phase modification in dispersion-engineered Si photonic wires

The strong dispersion and large third-order nonlinearity in Si photonic wires are intimately linked in the optical physics needed for the optical control of phase. By carefully choosing the waveguide dimensions, both linear and nonlinear optical properties of Si wires can be engineered. In this paper we provide a review of the control of phase using nonlinear-optical effects such as self-phase and cross-phase modulation in dispersion-engineered Si wires. The low threshold powers for phase-changing effects in Si-wires make them potential candidates for functional nonlinear optical devices of just a few millimeters in length.

Optical Third-Harmonic Generation in Graphene

We report strong third-harmonic generation in monolayer graphene grown by chemical vapor deposition and transferred to an amorphous silica (glass) substrate; the photon energy is in threephoton resonance with the exciton-shifted van Hove singularity at the M point of graphene. The polarization selection rules are derived and experimentally verified. In addition, our polarization- and azimuthal-rotation-dependent third-harmonic-generation measurements reveal in-plane isotropy as well as anisotropy between the in-plane and out-of-plane nonlinear optical responses of graphene. Since the third-harmonic signal exceeds that from bulk glass by more than 2 orders of magnitude, the signal contrast permits background-free scanning of graphene and provides insight into the structural properties of graphene.