Femtosecond Laser Filament In Air Research Articles

Pressure scaling of femtosecond laser filamentation in air: Prospects for long-range atmospheric propagation

The principal trend of the last decades is the use of laser systems generating ultrashort (femtosecond) laser pulses of gigawatt and terawatt power in various spheres of human activity, in particular, in atmospheric researches. One of the important challenges within this problem is the use of the unique nonlinear optical phenomena manifesting in the atmosphere as pulse self-focusing, light-induced ionization of the medium, plasma generation and laser filamentation for remote laser diagnostics of the aerosol-gas constituents and efficient delivery of concentrated optical energy over long distances in the atmosphere. This brings to the forefront the problem of increasing the actual length of the nonlinear interaction of an ultrashort laser pulse with medium to kilometer distances whereas real physical length of most existing optical paths is usually limited by several hundred meters. We propose and theoretically substantiate an effective way to solve the problem of long-range atmospheric filamentation by transforming the propagation medium itself using short laboratory traces and optical cuvettes with gases of increased (over-atmospheric) pressure. Following the scaling laws of the optical characteristics of pressurized medium, the coefficients describing the interaction between optical wave and medium increase along with the increase in gas pressure. By the numerical simulations in the framework of the nonlinear Schrodinger equation, we show that this makes it possible to emulate kilometers-long atmospheric traces for laser beams of centimeter diameter and terawatt pulse power on the laboratory scale (tens of meters).

A Retrospective Study: Are the Multi-Dips in the THz Spectrum during Laser Filamentation Caused by THz–Plasma Interactions?

During the process of terahertz (THz) wave generation via femtosecond laser filamentation in air, as well as through the mixing of THz waves with externally injected plasma filaments, THz waves engage in interactions with the plasma. A characteristic feature of this interaction is the modulation of the THz radiation spectrum by the plasma, which includes the generation of THz spectral dips. This information is essential for understanding the underlying mechanisms of THz–plasma interactions or for inferring plasma parameters. However, a current debate exists on the number of THz spectral dips observed after the interaction, with different opinions of single versus multiple dips, thus leaving the interaction mechanisms still ambiguous. In this work, we retrospectively analyzed the experimental appearance of multiple dips in the THz spectrum and found that the current observations of such dips are predominantly a result of the water vapor absorption with a low spectral resolution. Additionally, we observed that altering the acquisition width of the temporal THz signal also influenced the dips’ number. Hence, in future research, simultaneous attention should be paid to the following two aspects of THz–plasma interactions: (1) It is necessary to ensure a sufficiently wide time-domain window to accurately represent the spectral dip characteristics. (2) The spectral dips should be carefully distinguished from the water absorption lines before being further studied. On the other hand, for the case of a single dip in the THz spectrum, we also put forward a new viewpoint of the resonance between surface plasmon waves and THz waves, which should also be taken into consideration in future studies.

Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level. We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy. This was achieved by applying an external DC electric field to the air plasma filament. Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold. The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Pulse repetition rate effect on the plasma inside femtosecond laser filament in air

Pulse repetition rate effect on the plasma inside femtosecond laser filament in air

Fluorescence and lasing of neutral nitrogen molecules inside femtosecond laser filaments in air: mechanism and applications.

High power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studied. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered "air lasing" effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented.

Spectral response of chirp-dependent femtosecond laser filamentation in air

Spectral response of chirp-dependent femtosecond laser filamentation in air

Nitrogen fluorescence emission induced by femtosecond vortex beams in air

Fluorescence emission is a typical nonlinear phenomenon occurring during the femtosecond laser filamentation in air. The characteristics of nitrogen fluorescence emission induced by femtosecond vortex beams in air are experimentally studied. It is found that the topological charge has a great influence on the nitrogen fluorescence emission induced by a femtosecond filament: the fluorescence signals from nitrogen molecules (e.g., 337, 357 and 380 nm spectral lines) and molecular nitrogen ions (e.g., 391 and 428 nm spectral lines) differ much in the intensity distribution along the propagation direction. The discrepancy is explained by revisiting the generation mechanisms for nitrogen fluorescence, and it is found that the intersystem crossing scheme can account for it, indicating that the intersystem crossing scheme plays the main role in the formation of This work is helpful to the understanding of the physical mechanism of fluorescence emission induced by the filamentation of femtosecond vortex beams in air.

Toward 3D imaging of femtosecond laser filament in air by a CCD within a single exposure.

We experimentally demonstrated the 3D propagation of laser filament in air by an Fabry-Pérot (F-P) cavity assisted imaging within a single exposure. The F-P cavity was composed of two parallel mirrors with certain reflectivity and transmission at filament laser, so that the beam was reflected and refracted multiple times between the two mirrors. The cross-sectional intensity patterns at different longitudinal positions along filament within a single exposure of CCD (Charge-coupled Device) were recorded. When keeping the incident angle of the F-P cavity as a constant and reducing its spacing distance, a better longitudinally resolved evolution of cross-sectional filament intensity patterns was obtained. The intensity evolution along laser filament by the F-P cavity assisted imaging method was consistent with the filament fluorescence measurement from the side. As an application, the transition of laser propagation from linear to nonlinear was unveiled by the F-P cavity assisted 3D imaging.

Detection of 1.4 μg/m3 Na+ in aerosol at a 30m distance using 1 kHz femtosecond laser filamentation in air.

An optimized remote material detection scheme based on the laser filament-induced plasma spectroscopy and light detection and ranging (FIPS-LIDAR) is proposed in this work. The elemental composition and concentration of aerosol are measured by FIPS-LIDAR. By focusing the femtosecond laser with a large aperture (Φ41 cm) concave mirror and coaxial fluorescence collection scheme, the remote detection of aerosol in air at μg/m3 level has been realized at a distance of 30 m. The limit of detection for Na+ in aerosol droplets is 8 ppm (3 μg/m3 in air), which is the lowest detection limit that has been reported using millijoule femtosecond laser pulse (4.4 mJ). Furthermore, using spectral preprocessing and optimization of the proposed significance of peak (SOP) algorithm, feature peak signals are extracted from weak signals and the limit of detection can be further decreased to 1.4 μg/m3.

High directional aerosol fluorescence distribution affected by Mie scattering during femtosecond laser filamentation in air

Backward angular distributions of the sodium fluorescence in NaCl aerosols induced by femtosecond laser filamentation in air with a distance of 10 m are measured. The recorded signals appear as strong angular dependence. Further study indicates that it could be associated with the Mie scattering of the filamentation-induced “aerosol lasing” in the forward direction. These results will benefit the remote-sensing of air pollutants by femtosecond laser filamentation-based LIDAR (light detection and ranging).

Pulse repetition-rate effect on the intensity inside a femtosecond laser filament in air

Abstract As intense, ultrashort, kHz-repetition-rate laser systems become commercially available, pulse cumulative effects are critical for laser filament-based applications. In this work, the pulse repetition-rate effect on femtosecond laser filamentation in air was investigated both numerically and experimentally. The pulse repetition-rate effect has negligible influence at the leading edge of the filament. Clear intensity enhancement from a high-repetition pulse is observed at the peak and tailing edge of the laser filament. As the repetition rate of the laser pulses increases from 100 to 1000 Hz, the length of the filament extends and the intensity inside the filament increases. A physical picture based on the pulse repetition-rate dependent ‘low-density hole’ effect on filamentation is proposed to explain the obtained results well.

Effect of laser repetition rate on the fluorescence characteristic of a long-distance femtosecond laser filament.

In this paper, the effect of the laser repetition rate on the long-distance femtosecond laser filament in air is investigated by measuring the fluorescence characteristic of the filament. A femtosecond laser filament emits fluorescence due to the thermodynamical relaxation of the plasma channel. Experimental results show that as the repetition rate of femtosecond laser increases, the fluorescence of the filament induced by a single laser pulse weakens, and the position of the filament moves away from the focusing lens. These phenomena may be attributed to the slow hydrodynamical recovery process of air after being excited by a femtosecond laser filament, whose characteristic time is on the millisecond time scale and comparable to the inter-pulse duration of the femtosecond laser pulse train. This finding suggests that at a high laser repetition rate, to generate an intense laser filament, the femtosecond laser beam should scan across the air to eliminate the adverse effect of slow air relaxation, which is beneficial to laser filament remote sensing.

Beam Wander Restrained by Nonlinearity of Femtosecond Laser Filament in Air.

The filamentation process under atmospheric turbulence is critical to its remote-sensing application. The effects of turbulence intensity and location on the spatial distribution of femtosecond laser filaments in the air were studied. The experimental results show that the nonlinear effect of the filament can restrain the beam wander. When the turbulence intensity was , the mean deviation of the wander of the filament center was only 27% of that of the linear transmitted beam. The change in turbulence location would lead to a change in the standard deviation of the beam centroid drift. Results also show that the filament length would be shortened, and that the filament would end up earlier in a turbulent environment. Since the filamentation-based LIDAR has been highly expected as an evolution multitrace pollutant remote-sensing technique, the study promotes our understanding of how turbulence influences filamentation and advances atmospheric remote sensing by applying a filament.

Propagation distance-resolved characteristics of copper plasma emission induced by axicon-focused femtosecond laser filamentation in air.

It is well known that Bessel beams have non-diffractive characteristics, which can be generated by Gaussian beams focused by an ideal axicon. In general, the length of filament generated by Bessel beams is longer than that by Gaussian beams and the electron density in the filament generated by Bessel beams is more uniform. This paper experimentally studied the propagation distance-resolved characteristics of copper plasma emission induced by axicon-focused femtosecond laser filamentation in the air. The evolution of the spectral intensity, plasma temperature, and electron density with the filament propagation path was obtained. The experiment results showed that when the base angle of the axicon was 5.0°, the spectral intensity along with the filament propagation path was more stable than that the base angle of the axicon was 0.5°. The changes in the plasma temperature and electron density along the filament propagation path were consistent with the change in the spectral intensity. This work provides a demonstration for the applications of filament-induced breakdown spectroscopy (FIBS), such as long-distance detection.

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

The propagation of femtosecond laser filaments in air with continuously varying pressures

The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power Pin=5Pcr. The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0.3∼1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy Ein=1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p=0.3∼1 atm is still longer than that of p=1∼0.3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components.

THz birefringence inside femtosecond laser filament in air

Large birefringence of THz wave inside a femtosecond laser filament in air has been found in this work. By varying the laser pulse energy, the filament length is changed, inducing variation in the phase delay between two perpendicularly polarized THz pulses. The birefringence could be explained by the delayed rotational molecular wave packet of air molecules inside the femtosecond laser-induced filament. The newly discovered THz birefringence is essentially attributed to the strong confinement of the THz pulse inside the filament. This phenomenon has opened a new way to freely control the polarization of THz pulses emitted from an optical filament and significantly benefits polarization-sensitive THz spectroscopy and control of matter.

External focusing dependence of spatial distribution of air lasers during femtosecond laser filamentation in air

The spatial distribution of the forward-propagating amplified spontaneous emission (ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore, the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.

Spatial distribution of nitrogen fluorescence emission induced by femtosecond laser filamentation in air

As a major component in the air, nitrogen emits fluorescence when it interacts with intensive laser field. The fluorescence comes from the first negative band system (<inline-formula><tex-math id="M7">\begin{document}${{\rm{B}}^{{2}}}\Sigma _{\rm{u}}^{{ + }} \to {{\rm{X}}^{{2}}}\Sigma _{\rm{g}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M7.png"/></alternatives></inline-formula> transition) of <inline-formula><tex-math id="M8">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M8.png"/></alternatives></inline-formula> and the second positive band system (<inline-formula><tex-math id="M9">\begin{document}${{\rm{C}}^{{3}}}\Pi _{\rm{u}}^{{ + }} \to {{\rm{B}}^{{3}}}\Pi _{\rm{g}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M9.png"/></alternatives></inline-formula> transition) of <inline-formula><tex-math id="M10">\begin{document}${{\rm{N}}_{{2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M10.png"/></alternatives></inline-formula>. Under the action of high-intensity femtosecond laser, <inline-formula><tex-math id="M11">\begin{document}${{\rm{N}}_{{2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M11.png"/></alternatives></inline-formula> can be directly photo-ionized into <inline-formula><tex-math id="M12">\begin{document}${\rm{N}}_{{2}}^{{ + }}{{(}}{{\rm{B}}^{{2}}}\Sigma _{\rm{u}}^{{ + }})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M12.png"/></alternatives></inline-formula>, which results in fluorescence emission of <inline-formula><tex-math id="M13">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M13.png"/></alternatives></inline-formula>. In the process of femtosecond laser filament formation, the dynamic processes such as ionization and excitation of nitrogen molecules are affected by the laser intensity distribution and laser polarization direction. The products show different distributions in the propagation direction and radial space, which, in turn, affects its light emission. Therefore, it is necessary to further ascertain its generation mechanism through the spatial distribution of nitrogen fluorescence. In this experiment, the spatial distribution of the nitrogen fluorescence emission generated by linearly polarized femtosecond laser pulse filaments in air is measured. By changing the polarization direction of the laser to study the distribution of nitrogen fluorescence in the radial plane, it is found that the fluorescence emission of <inline-formula><tex-math id="M14">\begin{document}${\rm{N}}_2^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M14.png"/></alternatives></inline-formula> is more intense in the direction perpendicular to the laser polarization, while it is weaker in the direction parallel to the laser polarization. The nitrogen fluorescence emission has the same intensity in all directions. The ionization probability of a linear molecule depends on the angle between the laser polarization direction and the molecular axis, which is maximum (minimum) when the angle is <inline-formula><tex-math id="M15">\begin{document}${{{0}}^{\rm{o}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M15.png"/></alternatives></inline-formula>(<inline-formula><tex-math id="M16">\begin{document}${{9}}{{{0}}^{\rm{o}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M16.png"/></alternatives></inline-formula>). The <inline-formula><tex-math id="M17">\begin{document}${{\rm{N}}_{{2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M17.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M17.png"/></alternatives></inline-formula> gas is more likely to be ionized in the laser polarization direction, the nitrogen molecular ions <inline-formula><tex-math id="M18">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M18.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M18.png"/></alternatives></inline-formula> and electrons are separated in the direction parallel to the laser polarization. Therefore, more ions (<inline-formula><tex-math id="M19">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M19.png"/></alternatives></inline-formula>) are generated in the direction parallel to the laser polarization, and the fluorescence emission of <inline-formula><tex-math id="M20">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M20.png"/></alternatives></inline-formula> is more intense. Along the propagation direction of the laser, it is found that the fluorescence of <inline-formula><tex-math id="M21">\begin{document}${{\rm{N}}_{{2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M21.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M21.png"/></alternatives></inline-formula> appears before the fluorescence of <inline-formula><tex-math id="M22">\begin{document}${\rm{N}}_2^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M22.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M22.png"/></alternatives></inline-formula> and disappears after the fluorescence of <inline-formula><tex-math id="M23">\begin{document}${\rm{N}}_{{2}}^{{ + }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M23.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M23.png"/></alternatives></inline-formula> has vanished. This is due to the fact that <inline-formula><tex-math id="M24">\begin{document}${{\rm{N}}_{{2}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M24.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M24.png"/></alternatives></inline-formula> can be ionized into <inline-formula><tex-math id="M25">\begin{document}${\rm{N}}_{{2}}^{{ + }}{{(}}{{\rm{B}}^{{2}}}\Sigma_{\rm{u}}^{{ + }})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M25.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M25.png"/></alternatives></inline-formula> at the position of high enough laser intensity, thus emitting fluorescence of <inline-formula><tex-math id="M26">\begin{document}${\rm{N}}_2^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M26.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M26.png"/></alternatives></inline-formula>. However, the laser energy is not enough to ionize nitrogen at the beginning and end of laser transmission, but it can generate <inline-formula><tex-math id="M27">\begin{document}${\rm{N}}_2^ * $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M27.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M27.png"/></alternatives></inline-formula>, which emits nitrogen fluorescence through the process of intersystem crossing <inline-formula><tex-math id="M28">\begin{document}${\rm{N}}_2^*\xrightarrow{{{\rm{ISC}}}}{{\rm{N}}_2}({{\rm{C}}^3}\Pi _{\rm{u}}^ + )$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M28.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M28.png"/></alternatives></inline-formula>. The spatial distribution of nitrogen fluorescence emission during femtosecond laser filament formation shows that in the case of short focal length, the intersystem crossing scheme can explain the formation of <inline-formula><tex-math id="M29">\begin{document}${{\rm{N}}_{{2}}}{{(}}{{\rm{C}}^{{3}}}\Pi _{\rm{u}}^{{ + }})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M29.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20201704_M29.png"/></alternatives></inline-formula>. This research is helpful in understanding the mechanism of nitrogen fluorescence emission.

Generation of visible-wavelength-tunable ultrashort laser pulses through femtosecond laser filamentation in air: strong external focusing effect

Wavelength-tunable ultrashort laser pulses in the visible spectral range are generated in a femtosecond laser filament in air through four-wave mixing of an intense 808-nm near-infrared (NIR) laser pulse and a weak tunable infrared (IR: 1.3–2.2 µm) laser pulse. We found that the external focusing geometries strongly influence the four-wave mixing efficiency in air filament. The measurements of the resultant four-wave mixing laser intensity reveal that changing both the tilting angle and the focal length of the focal lens can optimize the four-wave mixing signal. We interpret the process based on the strong influence of the external focusing on the plasma density and the volume of the filament during the filament formation.