Mid-infrared laser filamentation in molecular gases

Control of spectral shift, broadening, and pulse compression during mid-IR self-guiding in high-pressure gases and their mixtures

Precise control of the nonlinear optical phenomena is the limiting factor for the spectral broadening and pulse compression techniques for high-power laser systems. Here we demonstrate that generation of the blue and red components under filamentation of 4.55-μm mid-IR pulses can be easily adjusted independently through the use of inert and molecular gases, while uniform broadening up to 1-μm bandwidth at the 1/e² level relies on the proper choice of gas mixture and its compounds partial pressure. Such synthesized media provide a feasible route for the free of damage control of pulse spectral broadening and compression for gigawatt peak power laser systems operating in the mid-IR. Additional management of a generated spectrum can be realized through the adjustment of focusing conditions. The resulted pulse is compressed by a factor of 2.6 down to 62 fs pulse duration (4.1 optical cycles) with additional dispersion compensation. Controllable nonlinear compression down to four optical cycles keeping the millijoule energy level of a mid-IR laser pulse provides direct access to extreme nonlinear optics.

High-harmonic-driven inverse Raman scattering

Spectral analysis of high-order harmonics generated by ultrashort mid-infrared pulses in molecular nitrogen reveals well-resolved signatures of inverse Raman scattering, showing up near the frequencies of prominent vibrational transitions of nitrogen molecules. When tuned on a resonance with the v^'=0→v^''=0 pathway within the B³Π_g→C³Π_u second positive system of molecular nitrogen, the eleventh harmonic of a 3.9 µm, 80 fs driver is shown to acquire a distinctive antisymmetric spectral profile with red-shifted bright and blue-shifted dark features as indicators of stimulated Raman gain and loss. This high-harmonic setting extends the inverse Raman effect to a vast class of strong-field light-matter interaction scenarios.

Molecular quantum wakes for clearing fog

High intensity laser filamentation in air has recently demonstrated that, through plasma generation and its associated shockwave, fog can be cleared around the beam, leaving an optically transparent path to transmit light. However, for practical applications like free-space optical communication (FSO), channels of multi-centimeter diameters over kilometer ranges are required, which is extremely challenging for a plasma based method. Here we report a radically different approach, based on quantum control. We demonstrate that fog clearing can also be achieved by producing molecular quantum wakes in air, and that neither plasma generation nor filamentation are required. The effect is clearly associated with the rephasing time of the rotational wave packet in N₂.Pump excitation provided in the form of resonant trains of 8 pulses separated by the revival time are able to transmit optical data through fog with initial extinction as much as -6 dB.

Optical Carrier-Wave Subcycle Structures Associated with Supercritical Collapse of Long-Wavelength Intense Pulses Propagating in Weakly Anomalously Dispersive Media

We predict the emergence of attosecond-duration structures on an optical carrier wave when intense, long-wavelength pulses propagate through bulk media with weak anomalous dispersion. Under certain conditions, these structures can undergo a new type of carrier-resolved supercritical collapse, forming infinite spatiotemporal gradients in the field. The mathematical conditions for the onset of this singularity are briefly overviewed, and we demonstrate with a full 3D+time (3+1) simulation that such structures persist under realistic conditions for a 10 micron laser pulse propagating in air.

Near-octave intense mid-infrared by adiabatic down-conversion in hollow anti-resonant fiber

We show that adiabatic down-conversion can be made the dominant four-wave mixing process in an anti-resonant hollow-core fiber for nearly a full octave of mid-infrared bandwidth with energy exceeding 10 μJ, allowing the generation of energetic and shapeable two-cycle pulses. A numerical study of a tapered fiber with an applied gas pressure gradient predicts the efficient conversion of a 770-860 nm near-infrared frequency band to 3-5 μm, while a linear transfer function allows pre-conversion pulse shaping and simple dispersion management. Our proposed system may prove to be useful in diverse research topics employing nonlinear spectroscopy or strong light-matter interactions.

Filamentation of mid-IR pulses in ambient air in the vicinity of molecular resonances

Properties of filaments ignited by multi-millijoule, 90 fs mid-infrared pulses centered at 3.9 μm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25  m) to loose (f=7  m) focusing, we observe a shift from plasma-assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the nonlinear Raman frequency downshift, which leads to the overlap with the CO₂ resonance in the vicinity of 4.2 μm. The dynamic CO₂ absorption in the case of 3.9 μm filaments with their low plasma content is the main mechanism of energy losses and, either alone or together with other nonlinear processes, contributes to the arrest of intensity.

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.

Self-Channeling of High-Power Long-Wave Infrared Pulses in Atomic Gases

We simulate and elucidate the self-channeling of high-power 10  μm infrared pulses in atomic gases. The major new result is that the peak intensity can remain remarkably stable over many Rayleigh ranges. This arises from the balance between the self-focusing, diffraction, and defocusing caused by the excitation induced dephasing due to many-body Coulomb effects that enhance the low-intensity plasma densities. This new paradigm removes the Rayleigh range limit for sources in the 8-12  μm atmospheric transmission window and enables transport of individual multi-TW pulses over multiple kilometer ranges.

Multi-millijoule few-cycle mid-infrared pulses through nonlinear self-compression in bulk

The physics of strong-field applications requires driver laser pulses that are both energetic and extremely short. Whereas optical amplifiers, laser and parametric, boost the energy, their gain bandwidth restricts the attainable pulse duration, requiring additional nonlinear spectral broadening to enable few or even single cycle compression and a corresponding peak power increase. Here we demonstrate, in the mid-infrared wavelength range that is important for scaling the ponderomotive energy in strong-field interactions, a simple energy-efficient and scalable soliton-like pulse compression in a mm-long yttrium aluminium garnet crystal with no additional dispersion management. Sub-three-cycle pulses with >0.44 TW peak power are compressed and extracted before the onset of modulation instability and multiple filamentation as a result of a favourable interplay between strong anomalous dispersion and optical nonlinearity around the wavelength of 3.9 μm. As a manifestation of the increased peak power, we show the evidence of mid-infrared pulse filamentation in atmospheric air.

Exploring strong-field laser interaction requires pulses that are both energetic and short. Here, the authors demonstrate a mid-IR soliton-like pulse compression in a mm-long YAG crystal, reaching the multi-millijoule energy range and showing pulse filamentation in atmospheric air.

Interaction-induced nonlinear refractive-index reduction of gases in the midinfrared regime

The nonlinear optical response of a dilute atomic gas to ultrashort high-intensity midinfrared pulse excitation is calculated fully microscopically. The optically induced polarization dynamics is evaluated for the interacting many-electron system in a gas of hydrogen atoms. It is shown that the many-body effects during the excitation distinctly influence not only the atomic ionization dynamics, but also the nonlinear polarization response in the midinfrared regime. The delicate balance between the Kerr focusing and the ionization-induced defocusing is dramatically modified and a significant decrease of the nonlinear refractive index is predicted for increasing wavelength of the exciting pulse.

Measurement of the nonlinear refractive index of air constituents at mid-infrared wavelengths

We measure the nonlinear refractive index coefficients in N₂, O₂, and Ar from visible through mid-infrared wavelengths (λ=0.4-2.4  μm). The wavelengths investigated correspond to transparency windows in the atmosphere. Good agreement is found with theoretical models of χ((3)). Our results are essential for accurately simulating the propagation of ultrashort mid-infrared pulses in the atmosphere.

Mid-infrared-to-mid-ultraviolet supercontinuum enhanced by third-to-fifteenth odd harmonics

A high-energy supercontinuum spanning 4.7 octaves, from 250 to 6500 nm, is generated using a 0.3-TW, 3.9-μm output of a mid-infrared optical parametric chirped-pulse amplifier as a driver inducing a laser filament in the air. The high-frequency wing of the supercontinuum spectrum is enhanced by odd-order optical harmonics of the mid-infrared driver. Optical harmonics up to the 15th order are observed in supercontinuum spectra as overlapping, yet well-resolved peaks broadened, as verified by numerical modeling, due to spatially nonuniform ionization-induced blue shift.

Mid-infrared laser filaments in the atmosphere

Filamentation of ultrashort laser pulses in the atmosphere offers unique opportunities for long-range transmission of high-power laser radiation and standoff detection. With the critical power of self-focusing scaling as the laser wavelength squared, the quest for longer-wavelength drivers, which would radically increase the peak power and, hence, the laser energy in a single filament, has been ongoing over two decades, during which time the available laser sources limited filamentation experiments in the atmosphere to the near-infrared and visible ranges. Here, we demonstrate filamentation of ultrashort mid-infrared pulses in the atmosphere for the first time. We show that, with the spectrum of a femtosecond laser driver centered at 3.9 μm, right at the edge of the atmospheric transmission window, radiation energies above 20 mJ and peak powers in excess of 200 GW can be transmitted through the atmosphere in a single filament. Our studies reveal unique properties of mid-infrared filaments, where the generation of powerful mid-infrared supercontinuum is accompanied by unusual scenarios of optical harmonic generation, giving rise to remarkably broad radiation spectra, stretching from the visible to the mid-infrared.

Intense Cr:forsterite-laser-based supercontinuum source

Supercontinuum pulses covering the range from 1100 to 1700 nm with energies >1.0  mJ and excellent beam quality are generated via nonlinear spectral broadening of Cr:forsterite (1240 nm, 110 fs) pulses in pressurized molecular nitrogen. Our spectra, which extend over more than half an octave, offer an attractive alternative to intense few-cycle pulse synthesis in the 1-2 μm range and lend themselves as an important add-on to Cr:forsterite laser technologies.

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.