The ultrafast response of a high-reflectivity GaAs∕AlAs Bragg mirror to optical pumping is investigated for all-optical switching applications. Both Kerr and free carrier nonlinearities are induced with 100 fs, 780 nm pulses with a fluence of 0.64 and 0.8kJ∕m2. The absolute transmission of the mirror at 931 nm increases by a factor of 27 from 0.0024% to 0.065% on a picosecond time scale. These results demonstrate the potential for a high-reflectivity ultrafast switchable mirror for quantum optics and optical communication applications. A design is proposed for a structure to be pumped below the band gaps of the semiconductor mirror materials. Theoretical calculations on this structure show switching ratios up to 2200 corresponding to switching from 0.017% to 37.4% transmission.

A bipartite multiphoton entangled state is created through stimulated parametric down-conversion of strong laser pulses in a nonlinear crystal. It is shown how detectors that do not resolve the photon number can be used to analyze such multiphoton states. Entanglement of up to 12 photons is detected using both the positivity of the partially-transposed density matrix and a newly derived criteria. Furthermore, evidence is provided for entanglement of up to 100 photons. The multiparticle quantum state is such that even in the case of an overall photon collection and detection efficiency as low as a few percent, entanglement remains and can be detected.

Band-gap structure of periodic waveguide arrays is investigated in the linear and the nonlinear regimes. Excitation of array modes belonging to high bands is demonstrated. Floquet-Bloch solitons are demonstrated experimentally and shown to be a generalization of discrete solitons.

We investigated the interaction of discrete solitons with defect states fabricated in arrays of coupled waveguides. We achieved attractive and repulsive defects by decreasing and increasing, respectively, the spacing of one pair of waveguides in an otherwise uniform array. Linear and nonlinear propagation in the same samples show distinctly different properties. The role of the Peierls–Nabarro potential in the interaction of the soliton with the defect is discussed.

Multiphoton fluorescence is used for the direct observation of a new class of breathers in waveguide arrays, which are a coherent superposition of Floquet-Bloch solitons of different bands. These Floquet-Bloch breathers oscillate along their spatial propagation axis, and possess several novel properties. Some behavior of these breathers is readily understood intuitively in terms of the band structure of the waveguide array and the properties of discrete solitons.

We overview theoretical and experimental results on spatial optical solitons excited in arrays of nonlinear waveguides. First, we briefly summarize the basic properties of the discrete nonlinear Schrodinger (NLS) equation frequently employed to study spatially localized modes in arrays, the so-called discrete solitons. Then, we introduce an improved analytical model that describes a periodic structure of thin-film nonlinear waveguides embedded into an otherwise linear dielectric medium. Such a model of waveguide arrays goes beyond the discrete NLS equation and allows studying many new features of the nonlinear dynamics in arrays, including the complete bandgap spectrum, modulational instability of extended modes, different types of gap solitons, the mode oscillatory instability, the instability-induced soliton dynamics, etc. Additionally, we summarize the recent experimental results on the generation and steering of spatial solitons and diffraction management in waveguide arrays. We also demonstrate that many effects associated with the dynamics of discrete gap solitons can be observed in a binary waveguide array. Finally, we discuss the important concept of two-dimensional (2-D) networks of nonlinear waveguides, not yet verified experimentally, which provides a roadmap for the future developments of this field. In particular, 2-D networks of nonlinear waveguides may allow a possibility of realizing useful functional operations with discrete solitons such as blocking, routing, and time gating.

We investigate the propagation of short, intense laser pulses in arrays of coupled silica waveguides, in the anomalous dispersion regime. The nonlinearity induces trapping of the pulse in a single waveguide, over a wide range of input parameters. A sharp transition is observed for single waveguide excitation, from strong diffraction at low powers to strong localization at high powers.

An AlGaAs waveguide array below the half-bandgap is used to investigate experimentally basic dynamic features of discrete systems. In particular, nonlinear locking of a discrete soliton to its input waveguide was observed for certain input conditions. We also investigated the soliton dynamics as a function of the position of the initial excitation and found that small shifts from the centers of symmetries of the structure could be greatly enhanced. Both effects depend on the geometry of the array and on the beam size.

We investigate the generation of discrete spatial solitons in arrays of coupled waveguides. Light was launched into the center of the array, and different beam sizes and array geometries were tested. At low power, the propagating field spreads as it couples to more waveguides. When the intensity is increased, localization is observed around the input waveguides, leading to the formation of a discrete soliton. For wide input beams, exciting a few waveguides, soliton splitting, which is due to instability induced by multiphoton absorption, is observed. All of these effects are described well by a coupled-mode formalism.

We observed simultaneous focusing in both space and time for light pulses propagating in a planar waveguide. In particular, 60 fs pulses with a width of 170 μm were injected into a planar glass waveguide in the anomalous dispersion regime. Output pulses as short as 30 fs and as narrow as 20 μm were measured. The results suggest that multiphoton absorption and intrapulse stimulated Raman scattering arrest the spatiotemporal contraction. The results were compared to the pulse evolution in zero and normal dispersion regimes and were shown to be significantly different. All of the experimental results were reproduced by a numerical model.

We show that two regimes of diffraction exist in arrays of waveguides, depending upon the input conditions. At higher powers, normal diffraction leads to self-focusing and to the formation of bright solitons through the nonlinear Kerr effect. By slightly changing the input conditions, light experiences anomalous diffraction and is nonlinearly defocused. For the first time, self-focusing and self-defocusing have been achieved for the same medium, structure, and wavelength.

By operating an AlGaAs waveguide array at the half band gap, we experimentally demonstrate basic features of discrete systems, which do not appear in the corresponding continuous counterpart - the slab waveguide. Under selected initial conditions, we observed nonlinearly induced locking of an initially moving soliton to the input waveguide. If the power is even higher, originally locked beams start to move again, but in a direction opposite to the initial tilt.

By using a nonlinear waveguide array we experimentally demonstrate dynamic features of solitons in discrete systems. Spatial solitons do not exhibit these properties in continuous systems. We experimentally recorded nonlinearly induced locking of an initially moving soliton at a single waveguide. We also show that discrete solitons can acquire transverse momentum and propagate at an angle with respect to the waveguide direction, when the initial excitation is not centered on a waveguide. This is to our knowledge the first time that the effect of the Peierls-Nabarro potential has been observed in a macroscopic system.

We experimentally demonstrate the occurrence of optical Bloch oscillations in a waveguide array with linearly growing effective index of the individual guides. We monitored the output profiles for varying propagation lengths and observed a periodic transverse motion of the field and a complete recovery of the initial excitation. The action of the focusing nonlinearity leads to a loss of recovery, symmetry breaking, and power-induced beam spreading.

We experimentally investigate the linear and nonlinear optical properties of a nonuniform waveguide array. By reducing the width of a single waveguide, we decrease its effective index and induce waveguiding along the defect. Due to the positive nonlinearity, the index difference is reduced for increasing power levels with the result that the field escapes. Waveguiding is suppressed by the action of the nonlinearity.

We report the numerical observation of edge-type phase defects which occur during the self-focusing of ultrashort pulses. We identified two distinct kinds of defects, formed at the temporal or spatial edges of the pulse. We show the effect of phase defect creation on the dynamics and the eventual arrest of the self-focusing process. Phase defects often lead to the inversion of the phase fronts curvature, transforming contracting pulses into expanding pulses.

We report the observation of discrete spatial optical solitons in an array of 41 waveguides. Light was coupled to the central waveguide. At low power, the propagating field spreads as it couples to more waveguides. When sufficient power was injected, the field was localized close to the input waveguides and its distribution was successfully described by the discrete nonlinear Schrödinger equation.

In an experiment on thin flat strips falling through a fluid in a vertical cell, two fundamental motions are observed: side-to-side oscillation (flutter) and end-over-end rotation (tumble). At high Reynolds number, the dimensionless similarity variable describing the dynamics is the Froude number Fr, being the ratio of characteristic times for downward motion and pendular oscillations. The transition from flutter to tumble occurs at Fr$_c$ = 0.67 \pm 0.05. We propose a phenomenological model including inertial drag and lift which reproduces this motion, and directly yields the Froude similarity.

Third harmonic generation near the focal point of a tightly focused beam is used to probe microscopical structures of transparent samples. It is shown that this method can resolve interfaces and inhomogeneities with axial resolution comparable to the confocal length of the beam. Using 120 fs pulses at 1.5 μm, we were able to resolve interfaces with a resolution of 1.2 μm. Two-dimensional cross-sectional images have also been produced.