Coherent shear phonon generation and detection with ultrashort optical pulses

Using an optical technique we generate and detect picosecond shear and quasishear coherent acoustic phonon pulses in the time domain. Thermoelastic and piezoelectric generation are directly achieved by breaking the sample lateral symmetry using crystalline anisotropy. We demonstrate efficient detection in isotropic and anisotropic media with various optical incidence geometries.     

THz-wave generation by optical rectification of ultrashort laser pulses with tilted amplitude front

The optical rectification of ultrashort laser pulses with a titled amplitude front in LiNbO₃ crystal is studied theoretically. It is shown that the results of calculation are in reasonable agreement with experimental data.     

Nonlinear source for the generation of high-energy few-cycle optical pulses

We investigate the evolution of optical pulses in a hollow waveguide filled with noble gas at pulse intensities for which tunnel ionization dominates the nonlinear response of the gas. A numerical analysis reveals that the spectral chirp generated by the plasma nonlinearity is to a good approximation linear over the whole pulse spectrum and can be compensated in a dispersive delay line. Our calculations predict the generation of 3-4-fs optical pulses with energies of a few milijoules. To our knowledge, these energies are an order of magnitude greater than the pulse energies that have been realized to date in hollow-fiber compressors based exclusively on the nonlinear Kerr effect.     

Temporal broadening and scintillations of ultrashort optical pulses

The purpose of this paper is to study the temporal broadening and temporal scintillations of ultrashort optical pulses. We present the general formulation of the temporal broadening and scintillation index of ultrashort (femtosecond) space-time Gaussian pulses propagating through weak optical turbulence and derive analytic approximations for the near- and far-field zones. We then apply the results to cross-link, uplink and downlink communication channels in a laser satellite communication system. We show that both the temporal broadening and scintillations depend mainly on the strength of optical turbulence and the outer scale and increase with these parameters. We also show that 10-30 fs pulses broaden significantly more and have greater scintillations than wide pulses. Furthermore, the horizontal and vertical/slant path temporal broadening are identical and the temporal scintillation indices exhibit similar behaviour.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Nonlinear polarization evolution of ultrashort pulses in microstructure fiber

We present experimental and numerical results for nonlinear polarization evolution of femtosecond pulses during propagation in microstructure fiber. Numerical modeling shows that fiber dispersion permits a long interaction length between the components polarized along the two principal axes, thereby enhancing the effective nonlinear polarization evolution in microstructure fiber.     

Tunable THz pulse generation by optical rectification of ultrashort laser pulses with tilted pulse fronts

Optical rectification of ultrashort near-IR laser pulses with tilted pulse fronts and pulse energies of a few J in Mg-doped stoichiometric LiNbO₃ cooled to low temperature is a powerful technique for efficient generation of THz pulses. The pulse energy critically depends on the Mg doping (necessary for preventing photorefractive damage) and can be easily increased by a factor of three if the MgO content is reduced. Pulse energies up to 400 pJ at repetition rates of 200 kHz and 3.4% quantum conversion efficiency are achieved at 77 K. At 10 K, changing the tilt angle of the pump pulse front results in continuous tuning of the frequency across the 1.0–4.4 THz range. The temporal pulse shapes measured by electro-optic sampling are in good agreement with the signal calculated by a simple theory. This model predicts tunability on a considerably broader range and narrower spectra even at room temperature if GaSe is used instead of LiNbO₃. The advantages of the velocity matching technique utilizing tilted pulse fronts are analyzed in comparison with quasi-phase-matching in periodically poled LiNbO₃ crystals. The first method provides a ten times higher pulse energy conversion efficiency. PACS 42.65.Ky; 42.70.Mp; 42.72.Ai     

Matched second-harmonic generation of ultrashort laser pulses in photonic crystals

It is shown that one-dimensional photonic bandgap structures are capable of simultaneously satisfying the phase and group-velocity matching conditions for second-harmonic generation involving extremely short light pulses. When these conditions are satisfied, an optical frequency doubler utilizing photonic bandgap structures provides a means for increasing the rate of growth of the second-harmonic signal as a function of the nonlinear-optical interaction length relative to structures designed for quasi-matched interactions and affords possibilities for enhancing the frequency doubling efficiencies independently of the matching length in the bulk nonlinear material. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 800–805 (25 December 1999)     

Nonlinear absorption of ultrashort laser pulses in thin metal films

Self-consistent simulations of the ultrafast electron dynamics in thin metal films were performed. A regime of nonlinear oscillations was observed that corresponds to ballistic electrons bouncing back and forth against the films' surfaces. When an oscillatory laser field is applied to the film, the field energy is partially absorbed by the electron gas. Maximum absorption occurs when the period of the external field matches the period of the nonlinear oscillations, which, for sodium films, lies in the infrared range. Possible experimental implementations are discussed.     

New laser crystals for the generation of ultrashort pulses

Since the beginning of the 1990s, titanium sapphire has become the crystal of choice for the development of ultrashort laser systems producing very short and powerful pulses using the Chirped Pulse Amplification technique. In parallel to these developments leading to commercial products, new laser crystals have been studied in order to reach directly other wavelength ranges and to overcome the need to develop cw or pulsed green laser to pump the titanium sapphire crystal. In order to be able to directly pump the crystals with a very efficient and high-power semiconductor laser, new crystals doped with chromium or ytterbium ions have been developed. This article will review the latest development in this research field with the best performances obtained in terms of pulse duration. To cite this article: F. Druon et al., C. R. Physique 8 (2007).     

Spatiotemporal control of ultrashort optical pulses by refractive-diffractive-dispersive structured optical elements

Structured optical elements that control the spatial and temporal characteristics of femtosecond light pulses are analyzed and synthesized. We show that unique spatiotemporal effects can be attained based on the diffraction, refraction, and dispersive effects that appear in the femtosecond regime. We argue that the design requirements for ultrafast optics are beyond the achromatization considerations that are usually applied to incoherent illumination because of the need to consider coherent effects. Despite fundamental limitations in the space-time control of ultrashort pulses, we show the potential of this technique to improve simultaneously the spatial and the temporal resolution of a lens and to generate ultrafast pulse sequences.     

Measurement of ultrashort optical pulses by two-photon photoconductivity techniques

Two-photon conductivity in GaAs and CdS_0·5Se_0·5 were investigated with modelocked Nd:glass laser pulse excitation, with a view to determine their suitability for picosecond pulsewidth measurement. The contrast ratio using GaAs as twophoton conductor was 1.8 and it increased to 2.4 when CdS_0·5Se_0·5 was used. The improved contrast ratio in the case of CdS_0·5Se_0·5 was partly due to the improved resolution of the crystal (2 ps) and partly due to the extended square-law region.     

Tunable second harmonic generation by phase-modulated ultrashort laser pulses

We report on the generation of tunable light around 400?nm by frequency-doubling ultrashort laser pulses whose spectral phase is modulated by a sum of sinusoidal functions. The linewidth of the ultraviolet band produced is narrower than 1?nm, in contrast to the 12?nm linewidth of the non-modulated incident spectrum. The influence of pixellation of the liquid crystal spatial light modulator on the efficiency of the phase-modulated second harmonic generation is discussed.     

Waves in nonlinear lattices: ultrashort optical pulses and Bose-Einstein condensates

The nonlinear Schr?dinger equation i (partial differential)(z)A(z,x,t)+(inverted Delta)(2)(x,t)A+[1+m(kappax)]|A|2A=0 models the propagation of ultrashort laser pulses in a planar waveguide for which the Kerr nonlinearity varies along the transverse coordinate x, and also the evolution of 2D Bose-Einstein condensates in which the scattering length varies in one dimension. Stability of bound states depends on the value of kappa=beamwidth/lattice period. Wide (kappa>1) and kappa=O(1) bound states centered at a maximum of m(x) are unstable, as they violate the slope condition. Bound states centered at a minimum of m(x) violate the spectral condition, resulting in a drift instability. Thus, a nonlinear lattice can only stabilize narrow bound states centered at a maximum of m(x). Even in that case, the stability region is so small that these bound states are "mathematically stable" but "physically unstable."     

Efficient generation of high-power VUV ultrashort pulses by four-wave mixing

We report the generation of ultra-short pulses in the VUV (λ = 173.5 nm) using a four-wave mixing technique in magnesium vapour. Using a mode-locked ruby laser as the original source, conversion efficiencies of up to 0.2% and powers of 0.3 MW were obtained.     

Sliding pump method for generation and amplification of ultrashort light pulses

A method for the generation of an ultrashort Raman pulse train with peak power considerably in excess of the pumping one is proposed and theoretically developed. It is shown that it can be used for obtaining tunable picosecond pulses in Raman fiber laser.     

High-average-power 2-kHz laser for generation of ultrashort x-ray pulses

We describe a Ti:sapphire-based laser-x-ray system specifically designed for generation of ultrafast x-ray pulses in the tenths-of-nanometers spectral range at a 2-kHz repetition rate. To obtain high-contrast laser pulses we divide the laser system into a section for generation of microjoule, high-contrast pulses with pulse cleaning and a subsequent section for chirped-pulse amplification and pulse compression. This laser section operates in conjunction with an x-ray-generation section based on a moving copper wire in a He atmosphere. The high reliability of the entire system permits maintenance-free production of x-ray pulses over tens of hours. Average x-ray fluxes of 10(13) photons/(s 4pi sr 1 keV) at 3 keV and 10(9) photons/(s 4pi sr) above 5 keV of photon energy are produced.     

Attosecond burst and high-harmonic generation in molecular ionization by ultrashort laser pulses

It is shown that the efficiency of attosecond pulse and high-harmonic generation in the ionization of excited molecular structures by a powerful femtosecond optical pulse can appreciably exceed the efficiency of analogous processes in atomic systems. This is due to the presence of a delocalized electron wave-packet component in the nonequilibrium molecular states, resulting in an increase of the number of particles that are effectively involved in the bremsstrahlung generation in the course of recollisions of laser-accelerated electrons with molecular core. Calculations suggest that, by optimizing the nonlinear response of molecular systems in the ionization process, one can develop compact sources of coherent vacuum ultraviolet and X-ray radiation with luminance at a level that is presently achieved only at large-scale accelerator facilities with free-electron lasers.     

Coherence of supercontinua generated by ultrashort pulses compressed in optical fibers

Femtosecond fiber lasers together with nonlinear fibers are compact, reliable, all-fiber supercontinuum sources. Maintaining an all-fiber configuration, however, necessitates pulse compression in an optical fiber, which can lead to nonlinearities for subhundred femtosecond, nanojoule pulses. In this work we show that using large-mode-area fibers for pulse compression mitigates the nonlinearity, resulting in compressed pulses with significantly reduced satellite pulses. Consequently, supercontinua generated with these pulses are shown to have as much as a 10 dB increase in coherence fringe contrast. By using a hybrid highly nonlinear fiber-photonic crystal fiber, the continuum can be extended to visible wavelengths while still maintaining high coherence.