Third harmonic generation of the surface wave in the semiconductor-metal slowing-down structure

A nonlinear process is considered of the surface wave third harmonics generation in a slowing-down semiconductor-metal structure. The process is conditioned by non-parabolicity of the charge carrier dispersion law. It is shown that in narrow-gap semiconducting materials it is necessary to account for the process together with the surface wave second harmonics generation conditioned by nonlinearity of quasi-hydrodynamics and the Maxwell equations. The conclusion is made that the third harmonies amplitude in narrow-gap semiconductors may exceed substantially the signal amplitude at the 3w frequency in a gas plasma and be of the same order with the surface waves second harmonies amplitude.

Phase-matching control of high-order harmonic generation in a two-color laser field

The phase-matching condition of high-order harmonic generation driven by intense few-cycle pulses could be controlled by adding second-harmonic pulses to change the ionization fraction of the gaseous medium. The harmonic generation efficiency could be improved by moving the phase-matching point with an all-optical control of the ionization fraction or a proper change of the confocal parameter. A specific order of harmonics could be easily controlled to reach phase matching at a fixed higher gas pressure by adding second-harmonic pulses with a suitable intensity. Such an all-optical phase-matching control was demonstrated to be dependent upon the temporal delay between the fundamental-wave and second harmonic pulses.

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications. (c) 2006 Elsevier GmbH. All rights reserved.

Generation of the high efficiency high peak-power femtosecond blue optical pulse

The analysis and calculation of the compensation for the phase mismatch of the frequency-doubling using the frequency space chirp introduced from prisms are made. The result shows that suitable lens can compensate the phase mismatch in a certain extent resulting from wide femtosecond spectrum when the spectrum is space chirped. By means of this method, the experiment of second harmonic generation is carried out using a home-made femtosecond KLM Ti:sapphire laser and BBO crystal. The conversion efficiency of SHG is 63 %. The average output power of blue light is 320 mW. The central wavelength is 420 nm. The spectrum bandwidth is 5.5 nm. It can sustain the pulse width of 33.6 fs. The tuning range of blue light is 404-420 nm,when the femtosecond Ti:sapphire optical pulse is tuned using the prisms in the cavity.

High-order harmonic generation and multi-photon ionization of Na-2 in laser fields

In this paper high-order harmonic generation (HHG) spectra and the ionization probabilities of various charge states of small cluster Na-2 in the multiphoton regimes are calculated by using time-dependent local density approximation (TDLDA) for one-colour (1064 nm) and two-colour (1064 nm and 532 nm) ultrashort (25 fs) laser pulses. HHG spectra of Na2 have not the large extent of plateaus due to pronounced collective effects of electron dynamics. In addition, the two-colour laser field can result in the breaking of the symmetry and generation of the even order harmonic such as the second order harmonic. The results of ionization probabilities show that a two-colour laser field can increase the ionization probability of higher charge state.

Generation and control of ultrafast pulse trains for quasi-phase-matching high-harmonic generation

Two techniques are demonstrated to produce ultrashort pulse trains capable of quasi-phase-matching high-harmonic generation. The first technique makes use of an array of birefringent crystals and is shown to generate high-contrast pulse trains with constant pulse spacing. The second technique employs a grating-pair stretcher, a multiple-order wave plate, and a linear polarizer. Trains of up to 100 pulses are demonstrated with this technique, with almost constant inter-pulse separation. It is shown that arbitrary pulse separation can be achieved by introducing the appropriate dispersion. This principle is demonstrated by using an acousto-optic programmable dispersive filter to introduce third-and fourth-order dispersions leading to a linear and quadratic variation of the separation of pulses through the train. Chirped-pulse trains of this type may be used to quasi-phase-match high-harmonic generation in situations where the coherence length varies through the medium. (C) 2010 Optical Society of America

Influence of multiple ionization thresholds on harmonic generation from Ar+

We apply time-dependent R-matrix theory to investigate harmonic generation from ground-state Ar+ with M = 0 at a wavelength of 390 nm. Contributions associated with the different 3s(2)3p(4) ionization thresholds are assessed, including the interference between these. The dominant contribution originates from the second ionization threshold, 3s(2)3p(4 1)D. Changes to the harmonic yields arising from the higher 3s3p(5) thresholds are also assessed. We further confirm that Ar+ has a higher harmonic yield than He for the same laser pulse, despite having a higher ionization threshold. 

Generation of triangular pulses in normally dispersive fibre

In this scheme, nonlinearity and dispersion in the NDF lead to various reshaping processes of an initial, conventional pulse according to the chirping value and power level at the input of the fibre. In particular, we have observed that triangular-shaped pulses can be generated for sufficiently high energies and a positive initial chirp parameter. In our experiments, 2.8 ps-FWHM, transform-limited pulses generated from a mode-locked fibre laser source at a repetition rate of 1.25 GHz were pre-chirped by propagating the pulses through different lengths of standard mono-mode fibre. The chirped pulses were then amplified to different power levels before being launched into a 2.3 km section of True Wave fibre (TWF). The corresponding numerically calculated pulse temporal intensity profile and numerical and experimental second-harmonic generation frequency-resolved optical gating (SHG FROG) spectrograms were also derived. In conclusion, we have presented numerical modelling results which show the system design parameters required for the generation of triangular-shaped pulses in a nonlinear NDF, and experimentally demonstrated triangular pulse shaping in conventional NDF.

SHG in a low-loss orientation patterned GaAs waveguide

We demonstrate second harmonic generation at 1621 nm in a low-loss orientation-patterned GaAs waveguide pumped by an optical parametric oscillator system. The losses were estimated to be 2.12 dB/cm.

Probing second harmonic components of pH-sensitive Redox processes in a mesoporous TiO2-Nafion film electrode with Fourier-transformed large-amplitude sinusoidally modulated voltammetry

Electrochemical processes in mesoporous TiO2-Nafion thin films deposited on indium tin oxide (ITO) electrodes are inherently complex and affected by capacitance, Ohmic iR-drop, RC-time constant phenomena, and by potential and pH-dependent conductivity. In this study, large-amplitude sinusoidally modulated voltammetry (LASMV) is employed to provide access to almost purely Faradaic-based current data from second harmonic components, as well as capacitance and potential domain information from the fundamental harmonic for mesoporous TiO2-Nafion film electrodes. The LASMV response has been investigated with and without an immobilized one-electron redox system, ferrocenylmethyltrimethylammonium+. Results clearly demonstrate that the electron transfer associated with the immobilized ferrocene derivative follows two independent pathways i) electron hopping within the Nafion network and ii) conduction through the TiO2 backbone. The pH effect on the voltammetric response for the TiO2 reduction pathway (ii) can be clearly identified in the 2nd harmonic LASMV response with the diffusion controlled ferrocene response (i) acting as a pH independent reference. Application of second harmonic data derived from LASMV measurement, because of the minimal contribution from capacitance currents, may lead to reference-free pH sensing with systems like that found for ferrocene derivatives.

Resonant second harmonics generation of the submillimeter surface wave in the semiconductor superlattice bounded by a metal

In this study, the process of the resonant second harmonics generation of the submillimeter (SM), which is of interest for design of the semiconductor frequency multipliers is evaluated. Particularly, the possibility to use the semiconductor superlattice-metal structures as an effective second harmonics generator is demonstrated.

Growth and physical properties of a new chiral open-framework crystal for NLO applications: Cesium hydrogen l-malate monohydrate

Cesium hydrogen l-malate monohydrate, CsH(C4H4O5)·H2O, is a new chiral open-framework semi-organic crystalline material with a second-harmonic generation efficiency one order of magnitude greater than KDP. Single crystals of this new material have been grown by the conventional slow cooling technique from aqueous solution. Grown crystals display both platy and prismatic morphologies depending on the imposed supersaturation. Hardness values measured using Vickers hardness indenter show considerable anisotropy. The resistivity behavior at room temperature and above, places the crystal between an ionic conductor and a dielectric. The single-crystal SHG efficiency estimated through Maker fringes experiment gives deff which is 4.24 times that of KDP. Single and multiple shot experiments performed on the grown crystals for the fundamental and second harmonic of pulsed Nd:YAG laser (1064 and 532 nm) show that it exhibits a high laser damage threshold which is a favorable property for nonlinear optical applications.

Third harmonic generation through shaped superconductors

This paper describes a methodology of obtaining only third harmonic along with the fundamental using shaped superconductors. It also indicates how one can design a nonlinear superconducting resistor with the required current versus resistance variation.

Relaxation behavior of twin nonlinear optical chromophores: Effect of the spacer length

A new series of twin nonlinear optical (NLO) molecules, having two 4-nitrophenol chromophores that are linked via a flexible polymethylene spacer of varying length [(CH2)(n), n = 1-12], were synthesized. Powder second harmonic generation measurements of these twin samples indicated a pronounced odd-even oscillation, with the odd twins exhibiting a high SHG value while the even ones gave no measurable SH signal. This behavior reflects the crystal packing preferences in such twin NLO systems that have odd and even numbers of atoms linking them - the even ones appear to prefer a centrosymmetric packing arrangement. The orientational/disordering dynamics of these twin NLO molecules, doped in a polymer (poly(methyl methacrylate)) matrix, has also been studied using SHG in electric field poled samples. Interestingly, the maximum attainable SH signal, chi((2)), in, the poled samples also showed an odd-even oscillation; the odd ones again having a higher value of chi((2)) This unprecedented odd-even oscillation in such molecularly doped systems is rationalized as being due to the intrinsically greater ease of a parallel alignment of the two chromophores in the twins with an odd spacer than in those with an even one. Further, the temporal stability of the SHG intensity at 70 degrees C, after the removal of the applied corona, was also studied. The relaxation of all the twin chromophores followed a biexponential decay; the characteristic relaxation time (tau(2)) for the slow decay component suggests that while the twin with a single methylene unit relaxes relatively slowly, the relaxation is significantly faster in cases where n = 2 and 3. In the twins with even longer spacer segments, the relaxation again becomes slower and reaches a saturation value. The observed minimum appears to reflect the interplay of two competing factors that affect the chromophore alignment in such twin systems, namely, the electrostatic repulsion between neighboring oriented dipoles and the intrinsic flexibility of the spacer.