Arnaud Couairon

Multi-octave supercontinuum generation from mid-infrared filamentation in a bulk crystal

In supercontinuum generation, various propagation effects combine to produce a dramatic spectral broadening of intense ultrashort optical pulses. With a host of applications, supercontinuum sources are often required to possess a range of properties such as spectral coverage from the ultraviolet across the visible and into the infrared, shot-to-shot repeatability, high spectral energy density and an absence of complicated pulse splitting. Here we present an all-in-one solution, the first supercontinuum in a bulk homogeneous material extending from 450 nm into the mid-infrared. The spectrum spans 3.3 octaves and carries high spectral energy density (2 pJ nm−1–10 nJ nm−1), and the generation process has high shot-to-shot reproducibility and preserves the carrier-to-envelope phase. Our method, based on filamentation of femtosecond mid-infrared pulses in the anomalous dispersion regime, allows for compact new supercontinuum sources.

Pulse self-compression to the single-cycle limit by filamentation in a gas with a pressure gradient

We calculate pulse self-compression of a 30 fs laser pulse traversing gas with different pressure gradients. We show that an appropriate density profile brings significant improvement to the self-compression by filamentation. Under an optimal pressure gradient, the pulse duration is reduced to the single optical cycle limit over a long distance, allowing easy extraction into an interaction chamber.

Self-compression of ultra-short laser pulses down to one optical cycle by filamentation

Theoretical studies of filamentation of ultra-short near-IR laser pulses propagating in a noble gas predict near single-cycle pulses with the intensity being clamped to the field ionization threshold. Experimental results show that this method is carrier envelope offset phase preserving and provides a very simple source for generating few-cycle intense laser pulses. This suggests a very simple design for the generation of ultra-short, sub-femtosecond XUV optical pulses.

Forward THz radiation emission by femtosecond filamentation in gases: theory and experiment

A transition-Cherenkov electromagnetic emission by a femtosecond laser pulse propagating in a self-induced plasma channel in air has been very recently proposed as mechanism for production of terahertz (THz) radiation in the forward direction. In this paper, we study in detail the theory of the transition-Cherenkov process. The theoretical model is developed and compared with recent experimental results for several gases.

Spatio-temporal characterization of few-cycle pulses obtained by filamentation

Intense sub-5-fs pulses were generated by filamentation in a noble gas and subsequent chirped-mirror pulse compression. The transversal spatial dependence of the temporal pulse profile was investigated by spatial selection of parts of the output beam. Selecting the central core of the beam is required for obtaining the shortest possible pulses. Higher energy efficiency is only obtained at the expense of pulse contrast since towards the outer parts of the beam the energy is spread into satellite structures leading to a double-pulse profile on the very off-axis part of the beam. Depending on the requirements for a particular application, a trade-off between the pulse duration and the pulse energy has to be done. The energy of the sub-5-fs pulses produced was sufficient for the generation of high order harmonics in Argon. In addition, full simulation is performed in space and time on pulse propagation through filamentation that explains the double-pulse structure observed as part of a conical emission enhanced by the plasma defocusing.

Conical emission, pulse splitting, and X-wave parametric amplification in nonlinear dynamics of ultrashort light pulses.

The precise observation of the angle-frequency spectrum of light filaments in water reveals a scenario incompatible with current models of conical emission (CE). Its description in terms of linear X-wave modes leads us to understand filamentation dynamics requiring a phase- and group-matched, Kerr-driven four-wave-mixing process that involves two highly localized pumps and two X waves. CE and temporal splitting arise naturally as two manifestations of this process.

Sharply autofocused ring-Airy beams transforming into non-linear intense light bullets

Controlling the propagation of intense optical wavepackets in transparent media is not a trivial task. During propagation, low- and high-order non-linear effects, including the Kerr effect, multiphoton absorption and ionization, lead to an uncontrolled complex reshaping of the optical wavepacket that involves pulse splitting, refocusing cycles in space and significant variations of the focus. Here we demonstrate both numerically and experimentally that intense, abruptly autofocusing beams in the form of accelerating ring-Airy beams are able to reshape into non-linear intense light-bullet wavepackets propagating over extended distances, while their positioning in space is extremely well defined. These unique wavepackets can offer significant advantages in numerous fields such as the generation of high harmonics and attosecond physics or the precise micro-engineering of materials.

ABSOLUTE AND CONVECTIVE INSTABILITIES, FRONT VELOCITIES AND GLOBAL MODES IN NONLINEAR SYSTEMS

Abstract We study the existence of self-sustained saturated solutions of the real Ginzburg-Landau equation subject to a boundary condition at x = 0; such solutions are called nonlinear global (NG) modes. The NG instability referring to the existence of these solutions is rigorously determined and the scaling behavior of the NG modes close to threshold is derived. The NG instability is first compared to the linear concept of convective/absolute (C/A) instability characterizing whether the impulse response of an unstable flow in an infinite domain is asymptotically damped or amplified at a fixed location. NG modes are shown to exist while at the same time the flow may be linearly stable, convectively unstable, or absolutely unstable. The growth size of the NG modes is shown to be proportional to ϵ − 1 2 when NG and A instabilities exist simultaneously, ϵ being the criticality parameter, whereas a ln ( 1 ϵ ) scaling is found when the NG instability occurs while the flow is C unstable or linearly stable. The nonlinear convective/absolute (NC/NA) instability defined Chomaz (1992) by considering, in infinite homogeneous domains, whether the front separating a bifurcated state from the basic state moves downstream or upstream, is determined using van Saarloos and Hohenberg (1992) results for the selected front velocity. Remarkably, the NA domain and the NG domain are shown to coincide. Similar results are presented for supercritical bifurcating systems, for the “van der Pol-Duffing” system, and for a transcritical model. In all the cases, the A instability is only a sufficient condition for the existence of an NG mode, and these simple models demonstrate that a system may be nonlinearly absolutely unstable whereas it is linearly convectively unstable. This property should be generic if one accepts the conjecture that the selected front velocity is always larger than the linear front velocity. Response to a constant forcing applied at the origin is also studied. It is shown that in the NG region, the system possesses intrinsic dynamics which cannot be removed by the forcing. By contrast, the behavior of a nonlinear spatial amplifier is observed in a domain larger than the NC region. NC instability is only a sufficient condition to trigger the system with forcing.

Against the wind

two generic cases are discussed: a supercritical case in which the instability is absolute and a subcritical case in which the instability is solely convective. The subcritical case gives a mathematical example of a bypass transition due to transient growth. The supercritical case allows a fully quantitative comparison, including the effect of the domain size, with results obtained by

Time-resolved refractive index and absorption mapping of light-plasma filaments in water

By means of a quantitative shadowgraphic method, we performed a space-time characterization of the refractive index variation and transient absorption induced by a light-plasma filament generated by a 120 fs laser pulse in water. The formation and evolution of the plasma channel in the proximity of the nonlinear focus were observed with a 23 fs time resolution.