Second-harmonic Light Research Articles

Vectorial and Octupolar Components of the First-Order Hyperpolarizability in Push-Pull Benzopyranic Series

Vectorial (weight 1) and octupolar (weight 3) components of the first-order hyperpolarizability are computed using the PM3/RHF/FF method following two different decomposition schemes. These calculations evidence the importance of the weight 3 component of the β tensor even in push-pull molecules. The β values, output from the two decomposition schemes, are different but show the same evolutionary trend within benzopyranic molecule series. The square root of a macroscopic square average quantity 〈β,〉2)1/2 (which is usually determined in elastic second-harmonic light scattering experiment) is then computed and compared to the norm of the molecular β cartesian tensor.

Second-harmonic generation and the conservation of orbital angular momentum with high-order Laguerre-Gaussian modes

Laguerre-Gaussian modes of various order are frequency doubled. The azimuthal phase structure of the second-harmonic light is measured directly by interfering the beam with its mirror image. We show that the orbital angular momentum per photon is doubled, so conserving the orbital angular momentum in the light beam. The frequency-doubled output beam is shown to have a Gegenbauer-Gaussian amplitude distribution at the beam waist. The beam can be described as a summation of Laguerre-Gaussian modes that interfere so that it changes form with propagation, but the distribution at the beam waist is reproduced in the far field.

Ultrabroadband phase-matched optical parametric generation in the ultraviolet by use of guided waves

We present what is believed to be the first experimental demonstration of guided-wave phase-matched frequency mixing and harmonic conversion in gases. Broad-bandwidth ultrafast pulses, tunable around 270 nm, were generated from an ultrafast Ti:sapphire amplifier system using 2? + 2? - ? parametric wave mixing in a capillary waveguide. We achieved nonresonant phase matching by coupling both the fundamental and the second-harmonic light into the lowest-order mode. The output 3? pulses have an energy of >4muJ at a 1-kHz repetition rate. Simple extensions of this method can generate higher-energy 10-20-fs pulses tunable throughout the vacuum ultraviolet.

Second-harmonic specular and scattered generated light: application to the experimental study of zinc-sulfide thin films

We present measurements of second-harmonic generation with zinc-sulfide thin films. Both scattered light and specular light are investigated in linear optics as well as in second-harmonic generation. We show that second-harmonic generation is a powerful tool for understanding the nonlinear properties of thin films; it allows the study of the anisotropy found in the scattered and specular second harmonic. Using the symmetry of susceptibility tensors, we show that the films cannot be considered homogeneous when the crystallites are large. Finally, we outline nonlinear scattering measurements, which bring out the usefulness of second-harmonic light in probing the structure of thin films.

Quasi-permanent all-optical encoding of noncentrosymmetry in azo-dye polymers

We present an original poling technique that uses a purely optical excitation process. The experiment consists of a seeding-type process. Writing and probing periods are alternated. Writing periods correspond to simultaneous irradiation of the sample by the coherent superposition of the 1064-nm fundamental and the 532-nm second-harmonic light of a picosecond-pulsed Nd:YAG laser. The sample is a spin-coated film of a poly(methyl methacrylate) copolymer onto which the azo-dye molecule Disperse Red 1 is grafted. We demonstrate efficient and quasi-permanent poling of the molecules with a spatial period that satisfies the phase-matching condition for second-harmonic generation. The influence of seeding parameters such as the relative phase and the relative intensities between the writing beams is studied both theoretically and experimentally. Tensorial properties and the spatial profile of the photoinduced χ(2) are analyzed from a microscopic point of view. Dark and photostimulated relaxation processes are investigated from a chemical-physics point of view. The physical origin of the photoinduced molecular orientation process is discussed. A minimal model involving the relevant experimental parameters is developed. Numerical simulations are in agreement with the experiment.

A large aperture laser triggered intensified charge coupled device using second-harmonic laser light triggering

For application to a ruby laser Thomson scattering system, we have developed a laser triggered intensified charge coupled device (CCD) with 80 mm aperture, two stages of intensification, and 80 ns gating. To improve the dynamic range, the CCD is cooled and read out slowly (1 s). To obtain a good extinction ratio (>1.1×107), the zoom electrode of the first intensifier is gated using a ∼10 kV laser triggered spark gap. The stability of this spark gap has been greatly improved by frequency doubling the laser trigger light.

Scattering of second-harmonic light from small spherical particles ordered in a crystalline lattice

Scattering of second-harmonic light from small spherical particles ordered in a crystalline lattice

Second-harmonic generation from a strongly rough metal surface

We discuss experimental observations of diffusely scattered second-harmonic light from a silver surface with rms roughness comparable to the illumination wavelength. The surface produces a backscattering peak in its linear scattering distribution as is expected, but presents a distinct minimum there in the second harmonic. The minimum is attributed to multiple scattering within surface valleys.

Domain and domain wall contributions to optical second harmonic generation in thin magnetic films

A symmetry analysis of nonlinear-optical imaging of domains and domain walls in thin magnetic films is presented. Different contributions to the second harmonic (SH) response depending on the local magnitude of the film magnetization M as well as on its spatial derivatives are calculated. The polarization of the SH light is shown to be a sensitive function of the relative weight of different magnetization-induced contributions, the film symmetry, and the type of the domain wall.

Stationary walking solitons in bulk quadratic nonlinear media

We study the mutual trapping of fundamental and second-harmonic light beams propagating in bulk quadratic nonlinear media in the presence of Poynting vector beam walk-off. We show numerically the existence of a two-parameter family of (2 + 1)-dimensional stationary, spatial walking solitons. We have found that the solitons exist at various values of material parameters with different wave intensities and soliton velocities. We discuss the differences between (2 + 1) and (1 + 1)-dimensional walking solitons and between walking and non-walking solitons.

Observations of second-harmonic generation from randomly rough metal surfaces

The angular distributions of second-harmonic light scattered from metal surfaces with weak random roughness are studied experimentally. The power spectrum of the roughness has a rectangular form centered on the surface plasmon polariton wave number at the fundamental frequency, producing strong excitation of these surface waves. The scattering distributions exhibit a pair of distinct peaks at angles consistent with the nonlinear interaction of the incident wave with fundamental plasmon polaritons. The controlled experiments allow a number of other scattering processes to be identified that include, for example, the nonlinear excitation of surface plasmon polaritons at the harmonic frequency. A peak in the second-harmonic distribution, predicted to appear in a direction perpendicular to the mean surface, is not observed.

Planar geometry thin-film all-optical programmable switch

We describe an all-optical programmable switch that can perform logic gate functions. This switch consists of a planar geometry germanium-doped silica waveguide, a Q-switched and mode-locked Nd:YAG laser, and the means of coupling laser light into different waveguiding modes of the thin film at the fundamental and second-harmonic frequencies. By the application of the appropriate optical programming sequence, the film-generated second-harmonic light can be made to perform the functionalities of various gates. In particular, a single waveguide was optically programmed to perform the Or function and was then made to perform the And function with little change to the experimental arrangement.

Observations of backscattering effects in second-harmonic generation from a weakly rough metal surface

We present experimental observations of diffusely scattered second-harmonic light from a silver surface with weak random roughness. The roughness provides coupling of the incident wave to counterpropagating surface plasmon polaritons in the second harmonic. Rather than observing enhancement in the backscattering direction, a distinct minimum in the angular distribution is seen.

Surface-state-induced second-harmonic generation

We propose a simple and intuitively clear method for analysing the second-harmonic generation induced by surface states. The two-band model is used to calculate the wave functions and Green's function so that the second-harmonic current density and polarization can be evaluated. Assuming that the polarization is concentrated on the surface, we calculated the reflection intensity of the second-harmonic light. We apply our formalism to the L-gap surface states on the silver (110) surface and obtain results that agree reasonably well with experiments.

Phase of second-harmonic light self-generated in a glass fiber

Focusing an intense laser beam and its second harmonic into a glass fiber transforms the fiber into a frequency doubler. We measure the temporal evolution of both the amplitude and the phase of the second-harmonic light produced by a germanium-doped fiber and so determine the initial phase of the second-harmonic light to be Deltatheta= -71 degrees +/- 3 degrees . We demonstrate that the fiber-produced green light can exceed the seeding green light even if these two beams are 90 degrees out of phase. We also show that cross-phase modulation in the fiber can limit the maximum useful interaction length and consequently the ultimate efficiency of second-harmonic generation in fibers.

Second-harmonic generation in second-harmonic fiber Bragg gratings

We consider the production of second-harmonic light in gratings resonant with the generated field, through a Green's function approach. We recover some standard results and obtain new limits for the uniform grating case. With the extension to nonuniform gratings, we find the Green's function for the second harmonic in a grating with an arbitrary phase shift at some point. We then obtain closed form approximate expressions for the generated light for phase shifts close to π/2 and at the center of the grating. Finally, comparing the uniform and phase-shifted gratings with homogeneous materials, we discuss the enhancement in generated light and the bandwidth over which it occurs, and the consequences for second-harmonic generation in optical fiber Bragg gratings.

Soliton excitation and mutual locking of light beams in bulk quadratic nonlinear crystals

The mutual trapping and locking of intense fundamental and second-harmonic continuous-wave light beams propagating in bulk crystals with large second-order nonlinearities is investigated. We report the outcome of a comprehensive series of numerical experiments to study the dynamics of the excitation of solitons with Gaussian input beams under different material and excitation conditions in terms of input powers, wave-vector mismatch, and linear walk-off between the fundamental and the second-harmonic waves. We show the dynamics of the mutual trapping and study how solitons emerge from the input beams in a wide variety of conditions that are not necessarily close to those given by stationary solutions of the governing equations. Solitons also emerge with inputs that fall far from those solutions. We specifically investigate the dynamics of soliton excitation with only the fundamental beam at the input face of the nonlinear crystal and in configurations with a moderately large phase mismatch. We find large oscillations in the amplitude of the beams with potentially important implications. We also study the mutual dragging of the beams in the presence of linear walk-off and in particular of phase-matching geometries with a nonsmall, but moderate, walk-off.

Second-Harmonic Generation in Vertical-Cavity Surface-Emitting Laser

A concept for a short-wavelength compact laser is proposed, in which second-harmonic coherent light is produced by converting fundamental light lased in a vertical-cavity surface-emitting laser. A layer is incorporated inside the laser cavity particularly for efficient second-harmonic generation. This layer consists of second-order optical nonlinear crystals which are preferably III–V- or II–VI-system compound semiconductors epitaxially grown with crystal orientation tilted from <100>. Simulation indicates that the device will produce several hundred µ W with AlAs/GaAs alternating layers for second-harmonic generation and an InGaAs active layer for lasing.

Modal properties of second-harmonic generation in doped-silica planar waveguides

Planar waveguides of germania-doped silica have been optically modified to allow second-harmonic generation in various waveguiding modes. After optical modification, the propagation of only the fundamental wave produced film-generated second-harmonic light in the same waveguide mode with which it had been prepared. The relative efficiencies of several mode combinations were all measured to be within an order of magnitude of one another and in reasonable agreement with theoretical predictions. Furthermore, we discuss the discrepancy between the theoretical calculations and experimental observations of the modal properties of second-harmonic generation in fibers.

Compact 10 mW all-solid-state 491 nm laser based on frequency doubling a master oscillator power amplifier laser diode

We report on continuous-wave blue-green light generation by direct frequency doubling a monolithically integrated master oscillator power amplifier laser diode in a potassium niobate (KNbO 3) crystal. 10 mW of diffraction limited ( M 2 ≤ 1.1) second-harmonic light at 491 nm is generated with a fundamental power of 760 mW incident on a 17 mm long crystal. The blue-green laser radiation is 100% polarised and single-longitudinal mode with a frequency linewidth of ∼33 GHz corresponding to a coherence length of ∼6 mm. The root-mean-square fluctuations of the second-harmonic output power is as low as 0.1% and the power drift is less than 3% over six hours.