Laguerre-Gaussian laser filamentation for the control of electric discharges in air.

We study the use of Laguerre-Gaussian (LG) femtosecond laser filament with multi GW peak power to guide electric sparks in the atmosphere. We demonstrate that an LG beam with a vortex phase or with 6 azimuthal phase steps generates a filamentation regime, where a longer and more uniform energy deposition is produced compared to a normal beam with a flat phase. Such filaments can guide electric discharges over much longer distances. This technique could significantly extend the guiding range of laser filaments for lightning control and other long-range atmospheric experiments involving filamentation.

Revealing Local Temporal Profile of Laser Pulses of Intensity above 1014 W/cm2.

We demonstrated a method for in situ temporal characterization of an intense femtosecond laser pulse around its focus where the laser intensity exceeds 1014 W/cm2. Our method is based on the second harmonic generation (SHG) by a relatively weak femtosecond probe pulse and the intense femtosecond pulses under analysis in the gas plasma. With the increase in the gas pressure, it was found that the incident pulse evolves from a Gaussian profile to a more complicated structure featured by multiple peaks in the temporal domain. Numerical simulations of filamentation propagation support the experimental observations of temporal evolution. This simple method can be applied to many situations involving femtosecond laser-gas interaction, when the temporal profile of the femtosecond pump laser pulse with an intensity above 1014 W/cm2 cannot be measured in traditional ways.

4D spatio-temporal electric field characterization of ultrashort light pulses undergoing filamentation.

We present an experimental method capable of capturing the complete spatio-temporal dynamics of filamenting ultrashort laser pulses. By employing spatially resolved Fourier transform spectrometry in combination with the capability to terminate the filament at any length, we can follow the nonlinear dynamics in four dimensions, i.e. the transverse domain, time and filament length. Our method thus not only enables the full characterization of the filamentation process throughout its evolution, but also allows to identify and select laser pulses with desired parameters.

Nonlinear Propagation and Filamentation on 100 Meter Air Path of Femtosecond Beam Partitioned by Wire Mesh.

High-intensity (∼1 TW/cm2 and higher) region formed in the propagation of ∼60 GW, 90 fs Ti:Sapphire laser pulse on a ∼100 m path in air spans for several tens of meters and includes a plasma filament and a postfilament light channel. The intensity in this extended region is high enough to generate an infrared supercontinuum wing and to initiate laser-induced discharge in the gap between the electrodes. In the experiment and simulations, we delay the high-intensity region along the propagation direction by inserting metal-wire meshes with square cells at the laser system output. We identify the presence of a high-intensity region from the clean-spatial-mode distributions, appearance of the infrared supercontinuum wing, and occurrence of the laser-induced discharge. In the case of free propagation (without any meshes), the onset of the high-intensity zone is at 40-52 m from the laser system output with ∼30 m extension. Insertion of the mesh with 3 mm cells delays the beginning of the high-intensity region to 49-68 m with the same ∼30 m extension. A decrease in the cell size to 1 mm leads to both delay and shrinking of the high-intensity zone to 71-73 m and 6 m, respectively. Three-dimensional simulations in space confirm the mesh-induced delay of the high-intensity zone as the cell size decreases.

Efficient second-harmonic generation of a high-energy, femtosecond laser pulse in a lithium triborate crystal.

We demonstrate the highest efficiency (∼80%) second harmonic generation of joule level, 27 fs, high-contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate gives a maximum efficiency of 26%. LBO thus offers high-intensity (>1018-19W/cm2), ultra-high contrast femtosecond pulses, which have great potential for high energy density science and applications, particularly with nanostructured targets.

Postcompression of picosecond pulses into the few-cycle regime.

In this work, we demonstrate postcompression of 1.2 ps laser pulses to 13 fs via gas-based multipass spectral broadening. Our results yield a single-stage compression factor of about 40 at 200 W in-burst average power and a total compression factor >90 at reduced power. The employed scheme represents a route toward compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

In-line Spectral Interferometry in Shortwave-Infrared Laser Filaments in Air.

We investigate the nonlinear propagation of intense, two-cycle, carrier-envelope phase (CEP) stable laser pulses at 1.7 µm center wavelength in air. We observe CEP-dependent spectral interference in the visible part of the forward-propagating white light generated on propagation. The effect is robust against large fluctuations of the input pulse energy. This robustness is enabled by rigid clamping of both the peak optical field and the phase of the propagating waveform, which has been revealed by numerical simulations. The CEP locking can enhance the yield of the CEP-dependent strong-field processes in gaseous media with long-wavelength drivers, while the observed spectral interference enables single-shot, stand-off CEP metrology in the atmosphere.

DNA Base Modifications Mediated by Femtosecond Laser-Induced Cold Low-Density Plasma in Aqueous Solutions.

Applications based on near-infrared femtosecond laser-induced plasma in biological materials involve numerous ionization events that inevitably mediate physicochemical effects. Here, the physical chemistry underlying the action of such plasma is characterized in a system of biological interest. We have implemented wavefront shaping techniques to control the generation of laser-induced low electron density plasma channels in DNA aqueous solutions, which minimize the unwanted thermo-mechanical effects associated with plasma of higher density. The number of DNA base modifications per unit of absolute energy deposited by such cold plasma is compared to those induced by either ultraviolet or standard ionizing radiation (γ-rays). Analyses of various photoinduced, oxidative, and reductive DNA base products show that the effects of laser-induced cold plasma are mainly mediated by reactive radical species produced upon the ionization of water, rather than by the direct interaction of the strong laser field with DNA. In the plasma environment, reactions among densely produced primary radicals result in a dramatic decrease in the yields of DNA damages relative to sparse ionizing radiation. This intense radical production also drives the local depletion of oxygen.

Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids.

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1-50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities-a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids-a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium.

Second-order cascading-assisted filamentation and controllable supercontinuum generation in birefringent crystals.

We experimentally investigate filamentation and supercontinuum generation in a birefringent medium (BBO crystal), in the self-focusing regime where intrinsic cubic nonlinearity is either enhanced or reduced by the second-order cascading due to phase-mismatched second harmonic generation. We demonstrate that the supercontinuum spectral extent is efficiently controlled by varying the phase mismatch parameter. In the range of negative phase mismatch, we achieve full control of the blue-shifted spectral broadening, which is very robust and independent on the input pulse energy. In the range of positive phase mismatch, both the blue-shifted and the red-shifted spectral broadenings are controlled simultaneously, however showing a certain dependence on the input pulse energy. The results are interpreted in terms of complex interplay between the self-phase-matched second harmonic generation, which is a process inherent to narrow ultrashort pulsed laser beams and concurrent self-steepening processes which arise from cubic and cascaded-quadratic nonlinearities.

Cell viability and shock wave amplitudes in the endothelium of porcine cornea exposed to ultrashort laser pulses.

PURPOSE: Some forms of keratoplasty assisted by ultrashort-pulse lasers require performing laser cuts close to the endothelium, which requires the knowledge of "safe" values concerning incision depth and pulse energy preserving endothelial cell viability. Our study aims to determine the thresholds for cell death in porcine corneas exposed to ultrashort laser pulses, in terms of laser pulse energy and nearness of the impacts to the endothelium. METHODS: Using a laboratory laser set-up, lamellar cuts were induced while varying pulse energies and distances from the endothelium. A fluorescent staining protocol was used to determine the percentage of surviving endothelial cells. Numerical simulations of the Euler equations for compressible fluids provided pressure level and axial and radial pressure gradient estimates at the endothelium. RESULTS: Ninety percent of the endothelial cells survived when using 16.5 µJ pulses no closer than 200 µm to the endothelium, or pulses not exceeding 2 µJ at a distance of 50 µm. The comparison of the observed percentage of surviving cells with the estimates of the shock wave amplitudes and gradients generated by the laser pulses yielded cell death thresholds at amplitudes in the megapascal range, or gradients of the order of 108 Pa/m. CONCLUSIONS: Our results provide limits in terms of pulse energy and distance of the incision from the endothelium within which endothelial cell viability is preserved. Current forms of corneal laser surgery are compatible with these limits. However, these limits will need to be considered for the development of future laser routines working in close proximity to the endothelium.

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses.

Acoustic signals generated by filamentation of ultrashort terawatt laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.

Laser-assisted guiding of electric discharges around objects.

Electric breakdown in air occurs for electric fields exceeding 34 kV/cm and results in a large current surge that propagates along unpredictable trajectories. Guiding such currents across specific paths in a controllable manner could allow protection against lightning strikes and high-voltage capacitor discharges. Such capabilities can be used for delivering charge to specific targets, for electronic jamming, or for applications associated with electric welding and machining. We show that judiciously shaped laser radiation can be effectively used to manipulate the discharge along a complex path and to produce electric discharges that unfold along a predefined trajectory. Remarkably, such laser-induced arcing can even circumvent an object that completely occludes the line of sight.

Light trajectory in Bessel-Gauss vortex beams.

We investigate the early stage of propagation of Bessel-Gauss vortex beams where a transition regime shows a progressive lateral expansion of the main intensity ring before reaching a diffraction-free regime. The eikonal equation is used to characterize the beam structure. The beam is featured by a family of hyperboloids with variable waists, generating a tapered tubular caustic. Our analytical results are in excellent agreement with numerical and experimental results. We show the transition regime can be well eliminated by using hollow input beams.

Tubular filamentation for laser material processing.

An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 10(13) W/cm(2) has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.

Plasma luminescence from femtosecond filaments in air: evidence for impact excitation with circularly polarized light pulses.

Filaments produced in air by intense femtosecond laser pulses emit UV luminescence from excited N(2) and N(2)(+) molecules. We report on a strong dependence at high intensities (I≥1.4×10(14) W/cm(2)) of this luminescence with the polarization state of the incident laser pulses. We attribute this effect to the onset of new impact excitation channels from energetic electrons produced with circularly polarized laser pulses above a threshold laser intensity.

Superfilamentation in air.

The interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be 1 order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments.

Backward Lasing of Air plasma pumped by Circularly polarized femtosecond pulses for the saKe of remote sensing (BLACK).

Recently, S. Mitryukovskiy et al. presented experimental evidence showing that backward Amplified Spontaneous Emission (ASE) at 337 nm can be obtained from plasma filaments in nitrogen gas pumped by circularly polarized 800 nm femtosecond pulses (Opt. Express, 22, 12750 (2014)). Here, we report that a seed pulse injected in the backward direction can be amplified by ~200 times inside this plasma amplifier. The amplified 337 nm radiation can be either linearly or circularly polarized, dictated by the seeding pulse, which is distinct from the non-polarized nature of the ASE. We performed comprehensive measurements of the spatial profile, optical gain dynamics, and seed pulse energy dependence of this amplification process. These measurements allow us to deduce the pulse duration of the ASE and the amplified 337 nm radiation as well as the corresponding laser intensity inside the plasma amplifier. It indicates that the amplification is largely in the unsaturated regime and that further improvement of laser energy is possible. Moreover, we observed optical gain in plasma created in ambient air. This represents an important step towards future applications exploiting backward lasing for remote atmospheric sensing.

Ultrabroadband supercontinuum and third-harmonic generation in bulk solids with two optical-cycle carrier-envelope phase-stable pulses at 2 µm.

We report on the generation of ultrabroadband supercontinuum (SC) by filamentation of two optical-cycle, carrier-envelope phase-stable pulses at 2 µm in fused silica, sapphire, CaF2 and YAG. The SC spectra extend from 450 nm to more than 2500 nm, and their particular shapes depend on dispersive properties of the materials. Prior to spectral super-broadening, we observe third-harmonic generation, which occurs in the condition of large phase and group velocity mismatch and consists of free and driven components. A double-peaked third-harmonic structure coexists with the SC pulse as demonstrated by the numerical simulations and verified experimentally. The SC pulses have stable carrier envelope phase with short-term rms fluctuations of ∼ 300 mrad, as simultaneously measured in YAG crystal by f-2f and f-3f interferometry, where the latter makes use of intrinsic third-harmonic generation.

Wavelength scaling of terahertz generation by gas ionization.

Low-frequency currents induced by ultrashort laser-driven ionization can emit extremely broadband, single-cycle terahertz pulses. We present a model that predicts a strong wavelength dependence of the THz emission in good agreement with our experimental study. This reveals that the combined effects of plasma currents rising proportionally to the square of the pump wavelength and wavelength-dependent focusing conditions lead to 30 times higher THz emission at 1800 nm compared to an 800 nm wavelength. Unrivaled single-cycle electric field strengths of 4.4 MV/cm are achieved with this compact table-top setup.