Ultrabroadband conical emission generated from the ultraviolet up to the far-infrared during the optical filamentation in air

Multiharmonic Spanning Nonlinear X-wave: An Asymptotic State of Extreme Long Wave Infrared Dispersive Shock Regularization

We predict the emergence of novel X-waves emitted as a consequence of extreme dispersive shock regularization of an intense long wave few cycle pulse propagating through a weakly dispersive medium. This robust propagation-invariant solution to Maxwell's equations appears as the asymptotic state in the high harmonic conversion when the pump propagates in a strongly nonlinear weakly dispersive regime, while the weakly nonlinear conical emission is dominated by chromatic dispersion.

Exploring the Femtosecond Filamentation Threshold in Liquid Media Using a Mach-Zehnder Interferometer

We experimentally studied the supercontinuum induced by femtosecond filamentation in different liquid media. Using a Mach-Zehnder interferometer, we determined the relative filamentation thresholds (Pth) of these media. Research has shown that the value of the filamentation threshold is greater than that of Pcr (critical power for self-focusing), which can mainly be attributed to the strong dispersion effect. Changing the focal length of the focusing lens affects filamentation dynamics, thereby affecting the measured results regarding the filamentation threshold. With shorter focal lengths, the linear focusing (i.e., geometrical focusing) regime dominates, and the measured values of Pth for different liquid media are almost the same; as the focal length becomes larger, self-focusing starts to play a role, making the values of Pth for different media different from each other. This study presents an efficient method for investigating the femtosecond filamentation phenomenon in liquid media, helpful to provide further insights into the physical mechanism of supercontinuum generation via femtosecond filamentation in liquid media.

Elongation of filamentation and enhancement of supercontinuum generation by a preformed air density hole

The filamentation of the femtosecond laser pulse in air with a preformed density hole is studied numerically. The result shows that density-hole-induced defocusing effect can relieve the self-focusing of the pulse, and by changing the length of the density hole and relative delay time, the filamentation length, intensity, spectral energy density and broaden region can be effectively controlled. When a short density hole with millisecond delay time is introduced, a significant elongation of the filamentation and enhancement of supercontinuum intensity can be obtained. This study provides a new method to control filamentation by pulse sequence.

Generation of annular femtosecond few-cycle pulses by self-compression and spatial filtering in solid thin plates

Annular-shaped femtosecond few-cycle pulses are generated by 40fs laser pulses propagating through 6 solid thin plates in numerical simulations as well as in experiments. The generation of such pulses takes advantage of the conical emission caused by plasma effect, which introduces continuously varying off-axis plasma density along the radial direction of the propagating beam. The negative dispersion induced by the plasma causes the pulse at particular radial location to be self-compressed and to form an annular beam of short pulse, which can be extracted simply by spatial filtering. Meanwhile, by adjusting the input pulse energy and position of each thin plate relative to the laser focus, we control the plasma density in thin plates which changes the ratio between ionization and effects providing positive dispersion, and obtain a higher compression ratio indicating that the scheme of solid thin plates has the flexibility to regulate the laser intensity so as to plasma density, thus the negative dispersion the pulse experiences during propagation. Few-cycle pulses as short as 8.8 fs are generated in experiments, meanwhile the shortest pulse duration found in the simulations is 5.0 fs, which corresponds to two optical cycles at its central wavelength 761 nm. This method has great potential in high-power few-cycle pulse generation.

Formation of spiraling infrared emission patterns by controlled interaction of optical filaments

We analyzed the formation of mid-infrared conical emission patterns possessing spiral and half-ring shaped wavelength contours from a beam of a few optical filaments. The complex patterns were generated and modified experimentally by adaptive wavefront shaping of the femtosecond laser pulse. Mutual interactions between co-propagating filaments can induce curvature in their paths, and the spiral and half-ring emissions were shown to be a direct consequence of this angular deflection. Based on our experimental and computational results, the spirals form in the far-field due to self-interference of conical emission from a helically moving filament. The presented findings will advance the tailoring of spatial conical emission patterns potentially beneficial for spectroscopic applications and terahertz generation.

Can amplified spontaneous emission produce intense laser guide stars for adaptive optics?

Adaptive optics is a key technology for ground-based optical and infrared astronomy, providing high angular resolution and sensitivity. Systems employing laser guide stars can achieve high sky coverage, but their performance is limited by the available return flux. Amplified spontaneous emission could potentially boost the intensity of beacons produced by resonant excitation of atomic or molecular species in the upper atmosphere. This requires the production of a population inversion in an electronic transition that is optically thick to stimulated emission. Mesospheric metals have insufficient column density for amplified spontaneous emission, but atomic oxygen and nitrogen are potential candidates. They could potentially be excited by a high-energy chirped femtosecond pulsed laser, making visible-wavelength transitions accessible. Such lasers can also generate a white-light supercontinuum in the atmosphere. In addition to providing high intensity, the broadband emission from such a source could facilitate the sensing of the tilt component of atmospheric turbulence.

Short-pulse lasers for weather control

Filamentation of ultra-short TW-class lasers recently opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of 0.1-1 mm in diameter, spanning over hundreds of meters in length, and producing a low density plasma (10¹⁵-10¹⁷ cm^-3) along their path. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or non-linear polarization saturation. While non-linearly propagating in air, these filamentary structures produce a coherent supercontinuum (from 230 nm to 4 µm, for a 800 nm laser wavelength) by self-phase modulation (SPM), which can be used for remote 3D-monitoring of atmospheric components by Lidar (Light Detection and Ranging). However, due to their high intensity (10¹³-10¹⁴ W cm^-2), they also modify the chemical composition of the air via photo-ionization and photo-dissociation of the molecules and aerosols present in the laser path. These unique properties were recently exploited for investigating the capability of modulating some key atmospheric processes, like lightning from thunderclouds, water vapor condensation, fog formation and dissipation, and light scattering (albedo) from high altitude clouds for radiative forcing management. Here we review recent spectacular advances in this context, achieved both in the laboratory and in the field, reveal their underlying mechanisms, and discuss the applicability of using these new non-linear photonic catalysts for real scale weather control.

Emissions from single filaments in air triggered by tailored background energy flows

We investigated emission patterns from single or few filaments in air created by femtosecond laser pulses with spatially modulated wavefronts. For 800 nm filaments, spiral emissions can be obtained in the infrared. Time-resolved analysis of the corresponding dynamics of the energy surrounding the filament reveals stable energy flows with angular momentum that interact with the filament and with each other. Changing this energy distribution by modulation of the wavefront of the initial laser pulse directly affects the emission behavior of the filament.

Measurements of the nonlinear refractive index of air, N2, and O2 at 10 μm using four-wave mixing

We report on measurements of the nonlinear index of refraction of air, N₂, and O₂ at a wavelength close to 10 μm by collinear four-wave mixing of a 200 MW CO₂ laser beat-wave. The use of a 200 ps long beat-wave comprising radiation amplified on the 10P20 and 10R16 lines of the CO₂ laser provides a sensitive method to measure the small nonlinearities characteristic of the gas phase in a spectral region where no such data exists.

Polarization evolution of ultrashort pulses in air

Measurements of polarization of filamenting light pulses at 800 nm are presented. Electronic nonlinearity, molecular alignment and nonlinear losses all contribute to modify the polarization of a femtosecond filamenting pulse. The polarization is modified in each stage of preparation, filamentation and divergence after the filament.

Optical Spectroscopy Using Gas-Phase Femtosecond Laser Filamentation

Femtosecond laser filamentation occurs as a dynamic balance between the self-focusing and plasma defocusing of a laser pulse to produce ultrashort radiation as brief as a few optical cycles. This unique source has many properties that make it attractive as a nonlinear optical tool for spectroscopy, such as propagation at high intensities over extended distances, self-shortening, white-light generation, and the formation of an underdense plasma. The plasma channel that constitutes a single filament and whose position in space can be controlled by its input parameters can span meters-long distances, whereas multifilamentation of a laser beam can be sustained up to hundreds of meters in the atmosphere. In this review, we briefly summarize the current understanding and use of laser filaments for spectroscopic investigations of molecules. A theoretical framework of filamentation is presented, along with recent experimental evidence supporting the established understanding of filamentation. Investigations carried out on vibrational and rotational spectroscopy, filament-induced breakdown, fluorescence spectroscopy, and backward lasing are discussed.

Kilometer range filamentation

We demonstrate for the first time the possibility to generate long plasma channels up to a distance of 1 km, using the terawatt femtosecond T&T laser facility. The plasma density was optimized by adjusting the chirp, the focusing and beam diameter. The interaction of filaments with transparent and opaque targets was studied.

Efficient spectral-step expansion of a filamenting laser pulse

We report an efficient transfer of 800 nm energy into both the ultraviolet and the far infrared (IR) during the filamentation in air of an appropriately shaped laser pulse. The multiorder enhancement of the IR supercontinuum in the 3-5 μm atmospheric transmission windows was achieved thanks to spectral-step cascaded four-wave mixing occurring within the spectrum of the shaped femtosecond laser pulse. These results also point out the limit of the self-phase modulation model to explain the spectral broadening of a filamenting laser pulse.

Ultrafast laser spectroscopy and control of atmospheric aerosols

We review applications of ultrafast laser pulses for aerosol analysis via linear and non-linear spectroscopy, including the most advanced techniques like coherent control of molecular excited states. We also discuss the capability of such pulses to influence the nucleation of atmospheric aerosols by assisting condensation of water in air.

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.

Broadband coherent light sources are crucial for numerous applications, such as imaging and spectroscopy. Using filamentation of mid-infrared laser pulses in bulk crystals, Silva et al. generate supercontinuum spectra over three octaves, from 4.5 μm to 450 nm, with carrier-envelope phase stability.

The effect of propagation in air on the filament spectrum

Filamentation studies traditionally start from letting a beam focus in air. We present filament studies with control over the preparation propagation, in air or vacuum, using an aerodynamic window. The spectral content of the filament strongly depends on its preparation medium.

Conical emission from laser filaments and higher-order Kerr effect in air

We numerically investigate the conical emission (CE) from ultrashort laser filaments, both considering and disregarding the higher-order Kerr effect (HOKE). While the consideration of HOKE has almost no influence on the predicted CE from collimated beams, differences arise for tightly focused beams. This difference is attributed to the different relative contributions of the nonlinear focus and of the modulational instability over the whole filament length.

Infrared generation by filamentation in air of a spectrally shaped laser beam

We demonstrated the generation of infrared radiation by filamentation of a spectrally shaped femtosecond laser beam. The spectrum is divided in two distinctive parts using an acousto-optic programmable dispersive filter (AOPDF) as a pulse shaper, resulting in two pulses of different colors. One pulse is frequency doubled and the beams are then focused to produce an optical filament. Efficient infrared generation occurred in the filament zone through the four-wave mixing interaction. This in-line setup allowed perfect spatial overlap of the pulses, fine control of the relative delay and the remote control of the infrared spectral distribution through spectral shaping of the initial femtosecond laser beam via the AOPDF.

Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.

Femtosecond laser filamentation for atmospheric sensing

Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation.

Emission of spiral patterns from filaments in the infrared

We investigated the spectral and spatial properties of the supercontinuum emission of single filaments in air in the infrared (1.5 µm-5.3 µm). The infrared emission of the filament was controlled by modulating the spatial phase of the femtosecond driver pulse with a deformable mirror. Filaments with a characteristic spiral emission pattern in the infrared were generated for a variety of different wavefront profiles of the femtosecond pulse. The properties of this novel class of emission were analyzed more closely. Further understanding of the corresponding emission dynamics of the filament will help to refine current models of filament propagation.

Conical third-harmonic generation in normally dispersive media

It is shown, both theoretically and experimentally, that in normally dispersive media under tight-focusing conditions third harmonic is generated by six-wave mixing rather than via common third-order frequency tripling. Though far-field pattern of third-harmonic signal was an axially symmetric ring for a wide range of the material wave vector mismatch and laser beam focusing conditions, in some cases the generation of more complex beams has been found possible. Results of simulations of the proposed model qualitatively correspond well with experimental data for the calculated values of third- and fifth-order nonlinear susceptibilities of sodium.

Ultraviolet-visible conical emission by multiple laser filaments

We characterized the angular distribution of the supercontinuum emission from multiple infrared laser filaments propagating in air over long distances, from the infrared (1080 nm) to ultraviolet (225 nm). These experimental data suggest that the X-Waves modeling or Cerenkov emission, rather than phase matching of four-wave mixing, could explain the conical emission. We also estimate the total light conversion efficiency from the original laser wavelength into the white-light continuum.