Filamentational Instability of Partially Coherent Femtosecond Optical Pulses in Air

The filamentational instability of spatially broadband femtosecond optical pulses in air is investigated by means of a kinetic wave equation for spatially incoherent photons. An explicit expression for the spatial amplification rate is derived and analyzed. It is found that the spatial spectral broadening of the pulse can lead to stabilization of the filamentation instability. Thus, optical smoothing techniques could optimize current applications of ultra-short laser pulses, such as atmospheric remote sensing.Comment: 8 pages, 2 figures, to appear in Optics Letter

Tester for space micro-accelerometer

AbstractMicro-accelerometer MAC04 has been developed in order to measure very low accelerations such as those caused to satellites by atmospheric drag and other non-gravitational forces. The instrument uses a cubic proof-mass inside a small cavity. In an open loop the change of capacitance between the cube and 12 electrodes on the inner cavity surface is a measure of the applied acceleration. It is very difficult to ground test and calibrate such a device due to gravity. The tester simulates the change of capacitances (base capacitance 13,5pF, changes in a range of ±1.5pF). Complete closed loop system is presented

Non-Gaussian Statistics of Multiple Filamentation

We consider the statistics of light amplitude fluctuations for the propagation of a laser beam subjected to multiple filamentation in an amplified Kerr media, with both linear and nonlinear dissipation. Dissipation arrests the catastrophic collapse of filaments, causing their disintegration into almost linear waves. These waves form a nearly-Gaussian random field which seeds new filaments. For small amplitudes the probability density function (PDF) of light amplitude is close to Gaussian, while for large amplitudes the PDF has a long power-like tail which corresponds to strong non-Gaussian fluctuations, i.e. intermittency of strong optical turbulence. This tail is determined by the universal form of near singular filaments and the PDF for the maximum amplitudes of the filaments

Enhanced second harmonic generation from resonant GaAs gratings

We study second harmonic generation in nonlinear, GaAs gratings. We find large enhancement of conversion efficiency when the pump field excites the guided mode resonances of the grating. Under these circumstances the spectrum near the pump wavelength displays sharp resonances characterized by dramatic enhancements of local fields and favorable conditions for second harmonic generation, even in regimes of strong linear absorption at the harmonic wavelength. In particular, in a GaAs grating pumped at 1064nm, we predict second harmonic conversion efficiencies approximately five orders of magnitude larger than conversion rates achievable in either bulk or etalon structures of the same material.Comment: 8 page

Light Filaments Without Self Guiding

An examination of the propagation of intense 200 fs pulses in water reveals light filaments not sustained by the balance between Kerr-induced self-focusing and plasma-induced defocusing. Their appearance is interpreted as the consequence of a spontaneous reshaping of the wave packet form a gaussian into a conical wave, driven by the requirement of maximum localization, minimum losses and stationarity in the presence of non-linear absorption.Comment: Submitted to Phys. Rev. Lett. on July 7th, 200

Genetically Determined Folate Deficiency Is Associated With Abnormal Hepatic Folate Profiles in the Spontaneously Hypertensive Rat

Increased levels of plasma cysteine are associated with obesity and metabolic disturbances. Our recent genetic analyses in spontaneously hypertensive rats (SHR) revealed a mutated Folr1 (folate receptor 1) as the quantitative trait gene associated with diminished renal Folr1 expression, lower plasma folate levels, hypercysteinemia, hyperhomocysteinemia and metabolic disturbances. To further analyse the effects of the Folr1 gene expression on folate metabolism, we used mass spectrometry to quantify folate profiles in the plasma and liver of an SHR-1 congenic strain, with wild type Folr1 allele on the SHR genetic background, and compared them with the SHR strain. In the plasma, concentration of 5-methyltetrahydrofolate (5mTHF) was significantly higher in SHR-1 congenic rats compared to SHR (60±6 vs. 42±2 nmol/l, P<0.01) and 5mTHF monoglutamate was the predominant form in both strains (>99 % of total folate). In the liver, SHR-1 congenic rats showed a significantly increased level of 5mTHF and decreased concentrations of dihydrofolate (DHF), tetrahydrofolate (THF) and formyl-THF when compared to the SHR strain. We also analysed the extent of folate glutamylation in the liver. Compared with the SHR strain, congenic wild-type Folr1 rats had significantly higher levels of 5mTHF monoglutamate. On the other hand, 5mTHF penta- and hexaglutamates were significantly higher in SHR when compared to SHR-1 rats. This inverse relationship of rat hepatic folate polyglutamate chain length and folate sufficiency was also true for other folate species. These results strongly indicate that the whole body homeostasis of folates is substantially impaired in SHR rats compared to the SHR-1 congenic strain and might be contributing to the associated metabolic disturbances observed in our previous studies

Determination of the Carrier-Envelope Phase of Few-Cycle Laser Pulses with Terahertz-Emission Spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond time scale. In order to take full advantage of such pulses, it is crucial to measure and stabilise their carrier-envelope (CE) phase, i.e., the phase difference between the carrier wave and the envelope function. We introduce a novel approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range via plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows for a determination of the phase by measurement of the amplitude and polarity of the THz pulse

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