Large molecular third-order optical nonlinearities in polarized carotenoids.

Garito and co-workers have suggested a mechanism to dramatically increase the second hyperpolarizability, gamma, in linear pi-electron-conjugated molecules. Polarization is introduced that leads to a difference between the dipole moments of the molecule's ground state and excited state. Here a series of carotenoids was examined that had increasing intramolecular charge transfer (ICT) from the polyenic chain to the acceptor moiety in the ground state, and gamma was measured for these compounds as a function of wavelength by third-harmonic generation. The compound with the greatest ICT exhibited a 35-fold enhancement of gammamax (the gamma measured at the peak of the three-photon resonance) relative to the symmetric molecule beta-carotene, which itself has one of the largest third-order nonlinearities known. Stark spectroscopic measurements revealed the existence of a large difference dipole moment, Delta mu, between the ground and excited state. Quantum-chemical calculations underline the importance of interactions involving states with large Delta mu.

Optical spatial solitons at the interface between two dissimilar periodic media: theory and experiment.

Discrete spatial solitons traveling along the interface between two dissimilar one-dimensional arrays of waveguides were observed for the first time. Two interface solitons were found theoretically, each one with a peak in a different boundary channel. One evolves into a soliton from a linear mode at an array separation larger than a critical separation where-as the second soliton always exhibits a power threshold. These solitons exhibited different power thresholds which depended on the characteristics of the two lattices. For excitation of single channels near and at the boundary, the evolution behavior with propagation distance indicates that the solitons peaked near and at the interface experience an attractive potential on one side of the boundary, and a repulsive one on the opposite side. The power dependence of the solitons at variable distance from the boundary was found to be quite different on opposite sides of the interface and showed evidence for soliton switching between channels with increasing input power.

Optical modes at the interface between two dissimilar discrete meta-materials.

We have studied theoretically and experimentally the properties of optical surface modes at the hetero-interface between two meta-materials. These meta-materials consisted of two 1D AlGaAs waveguide arrays with different band structures.

Observation of two-dimensional surface solitons.

We report the first experimental observation of two-dimensional surface solitons at the boundaries (edges or corners) of a finite optically induced photonic lattice. Both in-phase and gap nonlinear surface self-trapped states were observed under single-site excitation conditions. Our experimental results are in good agreement with theoretical predictions.

Power thresholds of families of discrete surface solitons.

We have investigated both theoretically and experimentally the power threshold of discrete Kerr surface solitons at the interface between a discrete one-dimensional (1D) (waveguide array) and a continuous 1D (slab waveguide) AlGaAs medium. Decreasing power thresholds were predicted and measured for soliton trapping at sites with increasing distance from the boundary. The thresholds approached asymptotically the power required for a discrete soliton of equivalent width in an infinite lattice. The minimum threshold coincided with a minimum in the interchannel coupling strength.

Observation of discrete surface solitons.

We report the first observation of discrete optical surface solitons at the interface between a nonlinear self-focusing waveguide lattice and a continuous medium. The effect of power on the localization process of these optical self-trapped states at the edge of an AlGaAs waveguide array is investigated in detail. Our experimental results are in good agreement with theoretical predictions.

All-optical switching and multifrequency generation in a dual-core photonic crystal fiber.

We demonstrate all-optical switching at 1550 nm between two weakly coupled cores in a photonic crystal fiber for intensities up to 0.5 TW/cm2. Spectrum analysis at higher intensities reveals that the output was dominated by continuum generation primarily towards shorter wavelengths.

Stability properties of multiwavelength, incoherent, dissipative spatial solitons.

We have investigated the interaction between two dissipative spatial solitons of different frequencies in periodically patterned semiconductor optical amplifiers. The experimental results are in good agreement with the theory. Simulations suggest that multiwavelength interactions do not produce stable bound solitons unless the system's modeling equations are completely symmetric.

Spatial ultrafast switching and frequency conversion in lithium niobate waveguide arrays.

We demonstrate phase-insensitive, ultrafast, all-optical spatial switching and frequency conversion in quadratically nonlinear waveguide arrays in periodically poled lithium niobate. Routing of milliwatt signals with wavelengths in the communication band (1550 nm) is achieved without pulse distortions by parametric interaction with a control beam with 10-W power and wavelengths near 775 nm.

Beam interactions with a blocker soliton in one-dimensional arrays.

We investigate experimentally and numerically the interaction of a highly localized, single-channel discrete soliton (blocker) with a wide, tilted beam in a one-dimensional AlGaAs array. In agreement with theory the blocker is observed to discretely shift its position by multiple channels, depending on the intensity and relative phase of the tilted beam.

Incoherent blocker soliton interactions in Kerr waveguide arrays.

We have observed the incoherent interaction between a highly confined (blocker) soliton and wide, moving signal beams of a different wavelength in a one-dimensional discrete Kerr medium. Digital switching of the blocker solitons to successive adjacent channels was measured with increasing signal power via both one and two cascaded interactions in an AlGaAs waveguide array, operations equivalent to a reconfigurable three-output router.

Modulational instability of a strip beam in a bulk type I quadratic medium.

The evolution of a strip (one-dimensional) fundamental beam with propagation distance owing to spatial modulational instabilities was analyzed in a quadratic medium near type I phase matching. We obtained the gain coefficient for the modulational instability and showed that the wave evolves into a clean periodic sequence of solitary waves and does not reproduce the incident beam.

Two-dimensional bright spatial solitons stable over limited intensities and ring formation in polydiacetylene para-toluene sulfonate.

Spatial soliton formation in the bulk single-crystal polydiacetylene para-toulene sulfonate has been studied experimentally. The unique nonlinearity of this material at 1600 nm leads to stable solitons below an intensity maximum, and for higher intensities the input beams form a ring and a soliton.

Nonlinear surface plasmons guided by thin metal films.

We have derived and solved numerically the dispersion relations for waves guided by a thin metal film bounded on one or both sides by media with intensity-dependent refractive indices. Depending on the conditions, the usual two modes can be cut off and can undergo radical changes in their field distributions, and new modes can exist above certain power threshholds.

Spatial ring emission and filament formation in an optical fiber with a saturable nonlinear cladding.

We present numerical simulations of Gaussian beam propagation in an optical fiber with a linear core and a saturable self-focusing cladding. Cylindrical nonlinear waves are emitted from the core into the cladding and are initially localized only in the radial direction, forming rings. These rings can break their cylindrical symmetry through a transverse instability, which yields filaments localized both azimuthally and radially, most of which stay close to the core for long propagation distances.

Soliton switching in an erbium-doped nonlinear fiber coupler.

A numerical study of the all-optical switching properties of solitons in an erbium-doped nonlinear fiber coupler is presented. The small gain provided by the erbium both lowers and sharpens the switching threshold, while producing pulse-compression factors of 3 to 9. As a result, we show that it is possible to achieve all-optical switching, amplification, and compression of short pulses in a single device.

Excitation of solitons with cascaded chi((2)) nonlinearities.

Mutual trapping of the fundamental and second-harmonic waves propagating in a material with cascaded second-order nonlinearity is investigated. Formation of solitonlike waves under a variety of excitation conditions is numerically observed, consistent with the robustness hypothesis of solitons.

One-dimensional quadratic walking solitons.

The properties of one-dimensional quadratic walking solitons were investigated in planar lithium niobate waveguides near the type I phase-matching condition for second-harmonic generation. Wave propagation was studied under different conditions of phase matching, walk-off angle, and incident fundamental power.

Four-photon absorption in the single-crystal polymer bis(paratoluene) sulfonate.

Strong four-photon absorption corresponding to Im[x((7)) (-?; ?, -?, ?, -?, ?, -?, ?)] has been measured for the first time to the authors' knowledge with 100-fs pulses at 1600 nm in the single-crystal polymer poly[bis (p-toluene sulfonate)] of 2, 4-hexadiyne-1, 6-diol. The transition involved is from the even-symmetry ground state into the vibronic subband of the dominant, even-symmetry, excited two-photon state at an energy of 2.7 eV.

Measurement of modulational instability gain of second-order nonlinear optical eigenmodes in a one-dimensional system.

We have investigated the amplification of a spatially periodic perturbation applied to a wide fundamental beam launched near phase matching for second-harmonic generation in a lithium niobate film waveguide. We measured the gain coefficient for the modulational instability of quadratic eigenmodes as a function of periodicity, intensity, and wave-vector mismatch. Excellent agreement with theory was obtained.