Tight focusing of electromagnetic fields by large-aperture mirrors.

We derive nonparaxial input conditions for simulations of tightly focused electromagnetic fields by means of unidirectional nonparaxial vectorial propagation equations. The derivation is based on the geometrical optics transfer of the incident electric field from significantly curved reflecting surfaces such as parabolic and conical mirrors to the input plane, with consideration of the finite thickness of the focusing element and large convergence angles, making the propagation vectorial and nonparaxial. We have benchmarked numerical solutions of propagation equations initiated with the nonparaxial input conditions against the solutions of Maxwell equations obtained by vectorial diffraction integrals. Both transverse and longitudinal components of the electric field obtained by these methods are in excellent agreement.

Transformation of ring-Airy beams during efficient harmonic generation.

We theoretically study the evolution of ring-Airy beams during harmonic generation with the focus on the regime of pump energy depletion. We demonstrate that in this regime, ring-Airy beams still preserve their abrupt autofocusing properties, while transforming to a multiple ring-Airy structure. A similar transformation is observed on the beam of the generated harmonic. We suggest a simple analytical model that explains and predicts with precision our numerical findings.

Accessing Extreme Spatiotemporal Localization of High-Power Laser Radiation through Transformation Optics and Scalar Wave Equations.

Although tightly focused intense ultrashort laser pulses are used in many applications from nano-processing to warm dense matter physics, their nonparaxial propagation implies the use of numerical simulations with vectorial wave equations or exact Maxwell solvers that have serious limitations and thus have hindered progress in this important field up to now. Here we present an elegant and robust solution that allows one to map the problem on one that can be addressed by simple scalar wave equations. The solution is based on a transformation optics approach and its validity is demonstrated in both the linear and the nonlinear regime. Our solution allows accessing challenging problems of extreme spatiotemporal localization of high power laser radiation that remain almost unexplored theoretically until now.

Controlling high-power autofocusing waves with periodic lattices.

We show numerically that by using radial symmetric lattices, the focal spot characteristics and filament peak intensity of high-power autofocusing waves can be controlled. The lattice induced diffraction is able to isolate the main on-axis peak and control the focus position. In addition, at higher power the lattice can control and stabilize the peak intensity of the filament over an extended distance.

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.

Eutectic epsilon-near-zero metamaterial terahertz waveguides.

We present and analyze the unique phenomena of enhanced THz transmission through a subwavelength LiF dielectric rod lattice embedded in an epsilon-near-zero KCl host. Our experimental results in combination with theoretical calculations show that subwavelength waveguiding of terahertz radiation is achieved within an alkali-halide eutectic metamaterial as result of the coupling of Mie-resonance modes arising in the dielectric lattice.

Cavitation dynamics and directional microbubble ejection induced by intense femtosecond laser pulses in liquids.

We study cavitation dynamics when focusing ring-shaped femtosecond laser beams in water. This focusing geometry reduces detrimental nonlinear beam distortions and enhances energy deposition within the medium, localized at the focal spot. We observe remarkable postcollapse dynamics of elongated cavitation bubbles with high-speed ejection of microbubbles out of the laser focal region. Bubbles are ejected along the laser axis in both directions (away and towards the laser). The initial shape of the cavitation bubble is also seen to either enhance or completely suppress jet formation during collapse. In the absence of jetting, microbubble ejection occurs orthogonal to the laser propagation axis.

Observation and optical tailoring of photonic lattice filaments.

We demonstrate, for the first time, that photonic lattices support a new type of laser filaments, called lattice filaments (LF). The LF attributes (length, width, and intensity) can be tailored by both varying the photonic lattice properties and also dynamically through the interaction between filaments. This opens the way for extensive all-optical control of the nonlinear propagation of intense ultrafast wave packets. Our approach is generic and applicable to all transparent media, with potential strong impact on various photonic applications.

Measuring easily electron plasma densities in gases produced by ultrashort lasers and filaments.

We present an easy way to calibrate the simple plasma conductivity (PCo) technique for measuring electron plasma densities in gases. We show that calibration can be achieved using a single absolute plasma density measurement through an independent analytical technique, in our case the in-line holographic microscopy (i-HOM). We show the validity and power of the method by comparing the calibrated PCo with results from i-HOM over an extended range of experimental parameters.

Intense dynamic bullets in a periodic lattice.

Femtosecond filamentation inside a periodic lattice in air is numerically shown to form intense dynamic bullets. The long propagation distance of the bullet structure is primarily attributed to the effect of the lattice that regulates the competition between linear and nonlinear spatiotemporal effects in the region of normal dispersion.

Strong terahertz emission enhancement via femtosecond laser filament concatenation in air.

We investigate the emission of terahertz (THz) radiation from two laser filaments in air. An increase by 1 order of magnitude of the overall THz power is found when the two filaments are coherently linked on-axis, leading to a single longer concatenated filament. The observed enhancement is found to be the same for the cases of single-color and two-color filamentation approaches.

Terahertz pulse emission optimization from tailored femtosecond laser pulse filamentation in air.

We study the generation of intense terahertz pulses produced by two-color laser pulse filamentation in air. We tailor the filamentation process and the produced plasma strings and study how the generated terahertz field is modified. An important terahertz pulse shortening is found for plasma strings with uniform electron density.

Long spatio-temporally stationary filaments in air using short pulse UV laser Bessel beams.

The formation of long stationary filaments resulting in uniform high density plasma strings in air using short pulse UV laser Bessel beams is shown. The length and the electron density of the plasma strings can be easily tuned by adjusting the conical Bessel wavefront angle. It is shown that in this regime the length of the plasma string can be extended over meter-long scales without any compromise in the string uniformity or any temporal evolution of the filamented laser pulse.

Efficient third-harmonic generation through tailored IR femtosecond laser pulse filamentation in air.

The process of third-harmonic generation during the filamentation of intense IR femtosecond laser pulses in air is investigated experimentally. It is shown that the introduction of a thin plasma string created by another femtosecond pulse, perpendicularly to the filament's path, dramatically reshapes the third-harmonic beam into a Bessel-like far-field distribution, while at the same time significantly enhances, up to 250 times, its conversion efficiency.

Linear X-wave generation by means of cross-phase modulation in Kerr media.

We exploit cross-phase modulation by a strong driving pulse onto a weaker probe pulse at a different wavelength to induce the formation of an X wave possessing the typical nondispersive and nondiffractive propagation properties.

Spontaneous emergence of pulses with constant carrier-envelope phase in femtosecond filamentation.

We study the possibility to obtain high-intensity pulses that maintain a constant Carrier Envelope Phase (CEP) during propagation in dispersive media, i.e. pulses such that the carrier-wave offset with respect to the main intensity peak remains fixed. Our numerical experiments strongly suggest that pulse splitting and X-wave formation within femtosecond laser pulse filamentation leads to the formation of "constant-CEP" within well-defined regions inside the filament. We study the creation of "constant- CEP" pulses in both gasseous and condensed media showing that this is a generic feature of filaments.

Kerr-induced spontaneous Bessel beam formation in the regime of strong two-photon absorption.

We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.

Plasma strings from ultraviolet laser filaments drive permanent structural modifications in fused silica.

Long self-trapped femtosecond ultraviolet filaments created in the bulk of pure fused silica are used to induce permanent structural changes in the medium. We monitor the laser pulse propagation as a filamentary structure and the plasma string left at its trail. We investigate and demonstrate the link of the filament-induced plasma to the permanent structural changes left in the medium. Specific electron density thresholds are found for the induced modifications.

Long-range filamentary propagation of subpicosecond ultraviolet laser pulses in fused silica.

We report on what is to our knowledge the first observation of subpicosecond ultraviolet laser pulse filamentation in transparent solid materials. Robust filaments were created in fused silica and observed over distances that exceed 3 cm in length. Intensities as high as 10(13) W/cm2 were found to be transported in the filamented beam without material damage in the bulk of the fused-silica sample.

Picosecond time-resolved X-ray absorption spectroscopy of ultrafast aluminum plasmas.

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.