We describe a novel cold neutron spectrometer under development at NIST optimized for wave vector resolved spectroscopy with incident energies between 2.1 meV and 20 meV and energy resolution from 0.05 meV (E i = 2.1 meV) to 3.0 meV (E i = 20 meV). By using a 1428 cm 2 double focusing PG (0 0 2) monochromator close to the National Institute of Standards and Technology (NIST) cold neutron source the instrument provides up to 5 × 10 8 neutrons cm −2 s −1 on a 8 cm 2 sample area. The measured performance is consistent with Monte Carlo simulations. The monochromating system, which includes radial collimators, three filters and a variable beam aperture, offers considerable flexibility in optimizing Q-resolution, energy resolution and intensity. The detector system will consist of an array of 20 channels which combined will subtend a solid angle of 0.2 sr. This is approximately a factor of 40 more than a conventional triple axis spectrometer. Each detector channel contains a vertically focusing double crystal analyzer system (DXAL) actuated by a single stepping motor. We find identical integrated reflectivity at approximately 10% coarser energy resolution for the 130 mosaic double bounce analyzer as compared to a conventional 25 analyzer at the same energy. The vertical focusing of the DXAL allows for smaller detectors for enhanced signal to noise with 8 • vertical acceptance. Options for post sample collimators and filters provide flexibility in the choice of scattered beam energy and wavevector resolution.

Experimental evidence of scattering of second-harmonic light from the surface of spherical particles of optical dimensions is presented. This mechanism for second-harmonic generation is observed in a suspension of monodisperse spherical colloidal particles, ordered in a centrosymmetric crystalline lattice. In this periodic structure the mechanism of phase matching is provided by the bending of the photon dispersion curve near the Bragg reflection band. A simple theoretical analysis based on the Rayleigh-Gans scattering approximation shows that constructive interference of the second-harmonic light scattered from different portions of a singlesphere surface leads to a nonvanishing field with a quadrupolar distribution intensity pattern. ͓S1050-2947͑97͒06506-2͔

We report what is believed to be the first experimental demonstration of the azimuthal self-breaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.

Phase matched second harmonic generation is observed experimentally in a centrosymmetric crystalline lattice of dielectric spheres of optical dimensions. The inversion symmetry is broken locally at the surface of each sphere in such a way that the scattered second harmonic light interferes constructively leading to a nonvanishing macroscopic field. Phase matching of the fundamental and second harmonic waves in such periodic lattice is observed to be naturally provided by the bending of the photon dispersion curve at the edge of the Bragg reflection band of a given set of lattice planes.

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We investigate light propagation through materials with periodically modulated gain/loss profile in both transverse and longitudinal directions, i.e. in material with twodimensional modulation in space. We predict effects of self-collimation (diffraction-free propagation) of the beams, as well as superdiffusion (spatial frequency filtering) of the beams in depending on the geometry of the gain/loss lattice, and justify the predictions by numerical simulations of the paraxial wave propagation equations.PACS numbers: 42.55.Tv, 42.25.Fx, It is well known, that the materials with the refractive index modulated in space on the wavelength scale, i.e. the so-called photonic crystals (PCs), bring about a significant modification of the propagation properties of waves, both in time and space domains. In time domain the usual (temporal) dispersion is modified, and photonic band gaps appear in the frequency spectra [1,2]. The photonic bandgaps are, perhaps, the most celebrated property of photonic crystals. In the space domain, the spatial dispersion (diffraction)can also be modified. This leads, analogously to the temporal case, to the appearance of bandgaps in terms of the propagation direction (i.e. in the propagation wavevector domain). The character of diffraction in the propagation bands is also modified: it can

We predict and experimentally observe the enhancement by three orders of magnitude of phase mismatched second and third harmonic generation in a GaAs cavity at 650 and 433 nm, respectively, well above the absorption edge. Phase locking between the pump and the harmonics changes the effective dispersion of the medium and inhibits absorption. Despite hostile conditions the harmonics resonate inside the cavity and become amplified leading to relatively large conversion efficiencies. Field localization thus plays a pivotal role despite the presence of absorption, and ushers in a new class of semiconductor-based devices in the visible and uv ranges.Since it was discovered by Franken in the 1960s, second harmonic ͑SH͒ generation has been one of the most studied phenomena in nonlinear optics ͓1͔. To date most efforts have been directed at improving the efficiency of the process by developing new materials with high effective nonlinear coefficients, accompanied by phase and group velocity matching ͓2-10͔. Consequently, most studies have been concerned with maximizing conversion efficiencies, generally achievable at or very near phase matching ͑PM͒ conditions, ensuring maximum energy transfer from the fundamental beam to the harmonics. A special effort was focused toward engineering new artificial materials capable of compensating material dispersion, for example, using quasiphase matching techniques ͓11,12͔ or structured materials ͓13͔. Outside of PM conditions, which generally coincide with low conversion efficiencies ͓3͔, the only relevant processes that have been investigated are cascaded parametric processes that can produce phase-modulation of the fundamental beam ͓14͔, pulse breaking ͓15͔ or nonlinear diffraction ͓16͔. This has caused other possible working conditions to remain largely unexplored. A relevant feature is that in all these previous studies the nonlinear material was assumed to be transparent for both fundamental and harmonics beams, since conventional wisdom holds that an absorptive material will reabsorb any generated harmonic signal.More recently, an effort was initiated to systematically study the behavior of SH and third harmonic ͑TH͒ fields in transparent and opaque materials under conditions of phase mismatch ͓17-19͔. Briefly, when a pump pulse crosses an interface between a linear and a nonlinear medium there are always three generated SH ͑and/or TH͒ components. One component is generated backward into the free space, due the presence of the interface, and the remaining components are forward moving. These components may be understood on the basis of the mathematical solution of the homogeneous and inhomogeneous wave equations at the SH frequency ͓4͔. Continuity of the tangential components of all the fields at the boundary leads to generation of the two forwardpropagating components that interfere in the vicinity of the entry surface and give rise to Maker fringes ͓2,20͔ and to energy exchange between the fundamentals and SH and/or TH beams. It turns out that while the homogeneous component tr...

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