X-wave mediated instability of plane waves in Kerr media

Plane waves in Kerr media spontaneously generate paraxial X-waves (i.e. non-dispersive and non-diffractive pulsed beams) that get amplified along propagation. This effect can be considered a form of conical emission (i.e. spatio-temporal modulational instability), and can be used as a key for the interpretation of the out of axis energy emission in the splitting process of focused pulses in normally dispersive materials. A new class of spatio-temporal localized wave patterns is identified. X-waves instability, and nonlinear X-waves, are also expected in periodical Bose condensed gases.Comment: 4 pages, 6 figure

Absolute Frequency Measurements of the Hg^+ and Ca Optical Clock Transitions with a Femtosecond Laser

The frequency comb created by a femtosecond mode-locked laser and a microstructured fiber is used to phase coherently measure the frequencies of both the Hg^+ and Ca optical standards with respect to the SI second as realized at NIST. We find the transition frequencies to be f_Hg=1 064 721 609 899 143(10) Hz and f_Ca=455 986 240 494 158(26) Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg^+ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of |(df_Ca/dt)/f_Ca| < 8 x 10^{-14} yr^{-1}, and |(df_Hg/dt)/f_Hg|< 30 x 10^{-14} yr^{-1}.Comment: 6 pages, including 4 figures. RevTex 4. Submitted to Phys. Rev. Let

Policiklički aromatski ugljikovodici u zraku okolice plinskog polja Molve

Small-volume air samples (~7 m3 per 24 h) of airborne PM10 particle fraction were collected on quartz fibre filters at two measuring sites in the vicinity of the gas field Molve in April and July 2006. It took five to seven days for each sample to collect and one month to collect five to seven samples. Mass concentrations of PM10 fractions were determined by gravimetry while PAHs were analysed using a HPLC with a fluorescent detector. The analysis included fluoranthene (Flu), pyrene (Pyr), benzo(a)anthracene (BaA), chrysene (Cry), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), benzo(ghi)perylene (BghiP) and indene(1,2,3-cd)pyrene (Ind). Average concentrations of all PAHs in April at site A were slightly higher than at site B while in July they were lower and similar on both sites. Average BaP concentration measured at site A in April was 0.156 ng m-3 and at site B 0.129 ng m-3, while July BaP averages were 0.022 ng m-3 at both sites. In both months, the mass concentration of BaP was lower than the limit value (1 ng m-3) and well below the tolerant value (2 ng m-3) set by a Croatian regulation of 2005. This suggests that the air near gas field Molve was of acceptable quality in respect to BaP at the time of the measurement.Uzorci PM10 frakcije lebdećih čestica skupljani su prosisavanjem malog volumena zraka (~7 m3 za 24 h) kroz filtre od kvarcnih vlakana. Svaki je uzorak skupljan tijekom pet do sedam dana na dva mjerna mjesta u okolici plinskog polja Molve. Uzorkovanje je provedeno tijekom travnja i srpnja 2006. godine. Masene koncentracije PM10 frakcije određene su gravimetrijski, dok je analiza policikličkih aromatskih ugljikovodika (PAU) rađena tekućinskom kromatografijom visoke djelotvornosti (HPLC) s fluorescentnim detektorom. Mjereni su sljedeći PAU: fluoranten, piren, benzo(a)antracen, krizen, benzo(b)fluoranten, benzo(k)fluoranten, benzo(a)piren, benzo(ghi)perilen i indeno(1,2,3-cd)piren. Srednje vrijednosti masenih koncentracija svih PAU u travnju na mjernome mjestu A bile su neznatno više od onih na mjernome mjestu B, dok su u srpnju na oba mjerna mjesta bile vrlo slične. Srednja vrijednost koncentracije BaP izmjerenog u travnju na mjernome mjestu A bila je viša od vrijednosti izmjerene na mjernome mjestu B. U srpnju su srednje vrijednosti bile znatno niže i na oba mjerna mjesta iznosile su 0.022 ng m-3. Iz rezultata dobivenih mjerenjem vidljivo je da je koncentracija benzo(a)pirena bila znatno niža od granične vrijednosti (1 ng m-3) i tolerantne vrijednosti (1 ng m-3, Uredba iz 2005.), pa možemo zaključiti da je zrak u okolici plinskog polja Molve tijekom istraživanog razdoblja bio prve kategorije kakvoće

Ultra-precise measurement of optical frequency ratios

We developed a novel technique for frequency measurement and synthesis, based on the operation of a femtosecond comb generator as transfer oscillator. The technique can be used to measure frequency ratios of any optical signals throughout the visible and near-infrared part of the spectrum. Relative uncertainties of $10^{-18}$ for averaging times of 100 s are possible. Using a Nd:YAG laser in combination with a nonlinear crystal we measured the frequency ratio of the second harmonic $\nu_{SH}$ at 532 nm to the fundamental $\nu_0$ at 1064 nm, $\nu_{SH}/\nu_0 = 2.000 000 000 000 000 001 \times (1 \pm 7 \times 10^{-19})$.Comment: 4 pages, 4 figure

On the Properties of Two Pulses Propagating Simultaneously in Different Dispersion Regimes in a Nonlinear Planar Waveguide

Properties of two pulses propagating simultaneously in different dispersion regimes, anomalous and normal, in a Kerr-type planar waveguide are studied in the framework of the nonlinear Schroedinger equation. Catastrophic self-focusing and spatio-temporal splitting of the pulses is investigated. For the limiting case when the dispersive term of the pulse propagating in the normal dispersion regime can be neglected an indication of a possibility of a stable self-trapped propagation of both pulses is obtained.Comment: 18 pages (including 15 eps figures

Modulational instability of bright solitary waves in incoherently coupled nonlinear Schr\"odinger equations

We present a detailed analysis of the modulational instability (MI) of ground-state bright solitary solutions of two incoherently coupled nonlinear Schr\"odinger equations. Varying the relative strength of cross-phase and self-phase effects we show existence and origin of four branches of MI of the two-wave solitary solutions. We give a physical interpretation of our results in terms of the group velocity dispersion (GVD) induced polarization dynamics of spatial solitary waves. In particular, we show that in media with normal GVD spatial symmetry breaking changes to polarization symmetry breaking when the relative strength of the cross-phase modulation exceeds a certain threshold value. The analytical and numerical stability analyses are fully supported by an extensive series of numerical simulations of the full model.Comment: Physical Review E, July, 199

Modulation Instability of Ultrashort Pulses in Quadratic Nonlinear Media beyond the Slowly Varying Envelope Approximation

We report a modulational instability (MI) analysis of a mathematical model appropriate for ultrashort pulses in cascaded quadratic-cubic nonlinear media beyond the so-called slowly varying envelope approximation. Theoretically predicted MI properties are found to be in good agreement with numerical simulation. The study shows the possibility of controlling the generation of MI and formation of solitons in a cascaded quadratic-cubic media in the few cycle regimes. We also find that stable propagation of soliton-like few-cycle pulses in the medium is subject to the fulfilment of the modulation instability criteria

Ultrashort filaments of light in weakly-ionized, optically-transparent media

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure

Cloning and characterization of a 9-lipoxygenase gene induced by pathogen attack from Nicotiana benthamiana for biotechnological application

<p>Abstract</p> <p>Background</p> <p>Plant lipoxygenases (LOXs) have been proposed to form biologically active compounds both during normal developmental stages such as germination or growth as well as during responses to environmental stress such as wounding or pathogen attack. In our previous study, we found that enzyme activity of endogenous 9-LOX in <it>Nicotiana benthamiana </it>was highly induced by agroinfiltration using a tobacco mosaic virus (TMV) based vector system.</p> <p>Results</p> <p>A <it>LOX </it>gene which is expressed after treatment of the viral vectors was isolated from <it>Nicotiana benthamiana</it>. As the encoded LOX has a high amino acid identity to other 9-LOX proteins, the gene was named as <it>Nb-9-LOX</it>. It was heterologously expressed in yeast cells and its enzymatic activity was characterized. The yeast cells expressed large quantities of stable 9-LOX (0.9 U ml^-1cell cultures) which can oxygenate linoleic acid resulting in high yields (18 μmol ml^-1cell cultures) of hydroperoxy fatty acid. The product specificity of Nb-9-LOX was examined by incubation of linoleic acid and Nb-9-LOX in combination with a 13-hydroperoxide lyase from watermelon (Cl-13-HPL) or a 9/13-hydroperoxide lyase from melon (Cm-9/13-HPL) and by LC-MS analysis. The result showed that Nb-9-LOX possesses both 9- and 13-LOX specificity, with high predominance for the 9-LOX function. The combination of recombinant Nb-9-LOX and recombinant Cm-9/13-HPL produced large amounts of C₉-aldehydes (3.3 μmol mg^-1crude protein). The yield of C₉-aldehydes from linoleic acid was 64%.</p> <p>Conclusion</p> <p>The yeast expressed Nb-9-LOX can be used to produce C₉-aldehydes on a large scale in combination with a <it>HPL </it>gene with 9-HPL function, or to effectively produce 9-hydroxy-10(<it>E</it>),12(<it>Z</it>)-octadecadienoic acid in a biocatalytic process in combination with cysteine as a mild reducing agent.</p