Filamentation Of Ultrashort Laser Pulses Research Articles

Air-Lasing: High-Resolution Spectroscopy for Atmospheric Remote Sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as the gain medium, featuring significant advantages such as high collimation, high coherence and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-laser can be induced remotely which provides an ideal light source for atmospheric remote sensing and chemical specie-resolved detection. Thanks to the coherent atomic/molecular excitation process accompanying the generation of air laser, remote sensing based on air-laser has high spectral resolution and high detection sensitivity, which is recently proved to be a powerful tool for important applications such as trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air laser is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Air-lasing high-resolution spectroscopy for atmospheric remote sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as gain medium, featuring high collimation, high coherence, and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-lasing can be induced remotely, providing an ideal light source for atmospheric remote sensing and chemical species-resolved detection. Owing to the coherent atomic/molecular excitation process accompanied with the generation of air laser, remote sensing based on air-lasing has high spectral resolution and high detection sensitivity, which recently proved to be a powerful tool for important applications such as in trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air lasing is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation.

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Diffraction-ray tube analysis of ultrashort high-intensity laser pulse filamentation in air

The results of theoretical simulation of femtosecond Ti:Sapphire laser pulse propagation in air in the self-focusing and filamentation regimes are presented. The self-focusing of pulsed radiation was analyzed based on the diffraction-ray tracing method, within which the beam power propagates within specific light structures, known as the diffraction-ray tubes. These tubes do not intersect in space and do not exchange their energy, but the changes in their shape and cross-section reflect the physical effects occurring with the radiation during its propagation through the medium. This allows discovering the formation of specific light structures in a laser beam during its self-focusing. One of these structures is the energy-replenishing diffraction-ray tube (ERT), which provides the filamentation domain with the necessary light energy and exists also in the form of a high-intensity light channel during the post-filamentation propagation of a pulse. The dependences of the radius and power of this energy-replenishing tube on the initial beam radius and peak radiation power at a fixed pulse length are derived. It is revealed that the radiation energy expenditure for filamentation decreases as the beam radius increases. The peak power in ERT does not exceed the critical self-focusing power for a Gaussian beam during the post-filamentation propagation of a pulse and weakly depends on the initial pulse parameters.

Transient setting of relativistic ponderomotive non-linearity and filamentation of ultra-short laser pulses in collisionless plasmas

AbstractWe study the setting up of relativistic ponderomotive non-linearity in an under-dense collisionless cold plasma. Using the fluid model, coupled system of equations of the laser beam and electron plasma oscillations has been derived. We present the numerical simulation for this coupled system of equations, when the coupling arises through relativistic ponderomotive non-linearity. The filamentation of the laser beam has been found to vary appreciably with perturbation wave number. The results show that with time, localized structures become more complex and the plasma oscillation frequency spectra have several harmonic peaks at terahertz frequencies when the electron plasma frequency is in terahertz range and laser frequency is around 2.35 × 1015 rad/s. We also present the semi-analytical model to capture the underlying physics.

Influence of air humidity on 248-nm ultraviolet laser pulse filamentation.

At first glance, the amount of water molecules naturally contained in humid air is negligibly small to affect filamentation of ultrashort laser pulses. However, here we show, both experimentally and numerically, that for ultraviolet laser pulses with 248nm wavelength this is not true. We demonstrate that with increase of air humidity the plasma channels generated by the ultraviolet laser pulses in air become longer and wider, while the corresponding electron density in humid air can be up to one order of magnitude higher compared to dry air.

Optimal wavelength for two-color filamentation-induced terahertz sources.

We theoretically study the generation of terahertz (THz) radiation by two-color filamentation of ultrashort laser pulses with different wavelengths. We consider wavelengths in the range from 0.6 to 10.6 μm, thus covering the whole range of existing and future powerful laser sources in the near, mid and far-infrared. We show how different parameters of two-color filaments and generated THz pulses depend on the laser wavelength. We demonstrate that there is an optimal laser wavelength for two-color filamentation that provides the highest THz conversion efficiency and results in generation of extremely intense single cycle THz fields.

Phase control algorithms and filamentation of ultrashort laser pulses in a scattering medium

The scattering nature of biological systems restricts the non-linear propagation of coherent light in media of biological interest. Using a spatial phase shaping technique, we optimize the filamentation of femtosecond laser pulses in an ethanol solution containing a fluorescent dye, while the laser beam has first traveled through 1 cm of scattering aqueous solution. Following three-photon absorption process, the fluorescence signal of the Coumarin 440 dye is used as a control parameter by genetic and partitioning algorithms. Applied to the incident wavefront, the partitioning algorithm successfully enhances the dye fluorescence signal in a medium with a scattering coefficient ( $$\mu _{\text {s}}$$ ) varying from 1.24 to $$3.71\hbox { cm}^{-1}$$ and gives rise to the production of a single laser filament several millimeters inside the observation cell that contains an ethanol solution. Interestingly, the optimal enhancement for the highest scattering medium is obtained with the partitioning algorithm at the lowest resolution. On the other hand, no significant enhancement is obtained when applying the genetic algorithm.

Two-dimensional photoacoustic imaging of femtosecond filament in water

We report a first-of-its-kind optoacoustic tomography of a femtosecond filament in water. Using a broadband (~100 MHz) piezoelectric transducer and a back-projection reconstruction technique, a single filament profile was retrieved. Obtained pressure distribution induced by the femtosecond filament allowed us to identify the size of the core and the energy reservoir with spatial resolution better than 10 µm. The photoacoustic imaging provides direct measurements of the energy deposition into the medium under filamentation of ultrashort laser pulses that cannot be obtained by existing techniques. In combination with a relative simplicity and high accuracy, photoacoustic imaging can be considered as a breakthrough instrument for filamentation investigation.

Near- and mid-IR ultrashort laser pulse filamentation in a molecular atmosphere: a comparative analysis

Self-focusing and filamentation of high-power pulsed laser radiation along a spectrally selective absorbing air path are numerically simulated in three spectral ranges centered at 0.8, 3.9, and 10.6 μm. A detailed comparative analysis of the main pulse parameters in the filamentation area is performed to examine the potential of various laser sources for atmospheric optics. We found that the radiation of mid-IR lasers (3.9 and 10.6 μm) forms the longest filamentation region and the widest supercontinual spectrum in comparison with near-IR radiation. Filamentation of the 3.9 μm laser pulse results in the best spatially continued plasma channel, while the 10.6 μm pulse retains the widest spectral composition under the conditions of strong molecular absorption in the atmosphere.

Filamentation of ultrashort laser pulses of different wavelengths in argon

We investigate the filaments formed by the ultrashort laser pulses with different wavelengths of 400 nm, 586 nm and 800 nm propagating in argon. Numerical results show that, when the input power or the ratio of the input power to the critical power is given, the pulse with 400 nm wavelength has the largest on-axis intensity, as well as the narrowest filament and the most stable beam radius. These results indicate that the pulse with shorter wavelength is more suitable for the long-range propagation in argon.

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.

Role of coherent resonant nonlinear processes in the ultrashort KrF laser pulse propagation and filamentation in air

Recent experiments on multiple filamentation of sub-picosecond terawatt-level KrF laser pulse in air and multi-photon ionization of air revealed an extremely low electron density in filaments, which is out of the conventional filamentation model considering Kerr self-focusing and plasma de-focusing. We propose here the coherent resonant scattering and ionization processes at the pulse durations significantly less than the polarization relaxation time to be possible explanation of the observed filamentation peculiarities. Namely, we argue that the plasma production results from the resonance enhanced (2+1)-photon ionization of the oxygen molecules through the two-photon excitation of the 3s metastable Rydberg state. Coherent Raman self-scattering at rotational transitions of nitrogen molecules provides self-induced focusing of the ultrashort UV laser pulse and filament formation.

Remote electrical arc suppression by laser filamentation

We investigate the interaction of narrow plasma channels formed in the filamentation of ultrashort laser pulses, with a DC high voltage. The laser filaments prevent electrical arcs by triggering corona that neutralize the high-voltage electrodes. This phenomenon, that relies on the electric field modulation and free electron release around the filament, opens new prospects to lightning and over-voltage mitigation.

Filamentation of ultrashort laser pulses in silica glass and KDP crystals: A comparative study

Ionizing 800-nm femtosecond laser pulses propagating in silica glass and in potassium dihydrogen phosphate (KDP) crystal are investigated by means of a unidirectional pulse propagation code. Filamentation in fused silica is compared with the self-channeling of light in KDP accounting for the presence of defect states and electron-hole dynamics. In KDP, laser pulses produce intense filaments with higher clamping intensities up to 200 TW/cm$^2$ and longer plasma channels with electron densities above $10^{16}$ cm$^{-3}$. Despite these differences, the propagation dynamics in silica and KDP are almost identical at equivalent ratios of input power over the critical power for self-focusing.

Generation of high harmonics and attosecond pulses with ultrashort laser pulse filaments and conical waves

Results illustrating the nonlinear dynamics of ultrashort laser pulse filamentation in gases are presented, with particular emphasis on the filament properties useful for developing attosecond light sources. Two aspects of ultrashort pulse filaments are specifically discussed: (i) numerical simulation results on pulse self-compression by filamentation in a gas cell filled with noble gas. Measurements of high harmonics generated by the pulse extracted from the filament allows for the detection of intensity spikes and subcycle pulses generated within the filament. (ii) Simulation results on the spontaneous formation of conical wavepackets during filamentation in gases, which in turn can be used as efficient driving pulses for the generation of high harmonics and isolated attosecond pulses.

Mid-infrared laser filamentation in molecular gases

We observed the filamentation of mid-infrared ultrashort laser pulses (3.9 μm, 80 fs) in molecular gases. It efficiently generates a broadband supercontinuum over two octaves in the 2.5-6 μm spectral range, with a red-shift up to 500 nm due to the Raman effect, which dominates over the blue shift induced by self-steepening and the gas ionization. As a result, the conversion efficiency into the Stokes region (4.3-6 μm) 65% is demonstrated.

Effects of higher-order Kerr nonlinearity and plasma diffraction on multiple filamentation of ultrashort laser pulses in air

The effect of higher-order Kerr nonlinearity on channel formation by, and filamentation of, ultrashort laser pulses propagating in air is considered. Filament patterns originating from multiphoton ionization of the air molecules with and without the higher-order Kerr and molecular-rotation effects are investigated. It is found that diverging multiple filaments are formed if only the plasma-induced defocusing effect is included. In the presence of the higher-order Kerr effects, the light channel can exist for a long distance. The effect of noise on the filament patterns is also discussed.

Supercontinuum generation and filamentation of ultrashort laser pulses in hybrid silicate nanocomposite materials on the basis of polysaccharides and hyperbranched polyglycidols

The possibility of using hybrid silicate nanocomposite materials on the basis of polysaccharides and hyperbranched polyglycidols for efficient conversion of pulsed radiation of a femtosecond Ti : sapphire laser at the wavelength into supercontinuum in the range from is experimentally demonstrated. It is established that the addition of a small concentration of Au and CdS nanoparticles to the studied materials essentially affects the efficiency of conversion of the laser radiation energy into the supercontinuum spectrum.

Experimental study of filamentation of high-power ultrashort laser pulses with initial angular divergence in air

Experimental study of the nonlinear propagation of near-IR gigawatt femtosecond laser pulses in air in the self-focusing and filamentation regimes have been performed in open air and in laboratory. The influence of the initial geometric divergence (both positive and negative) of the laser beam with an irregular intensity profile on the transverse light energy distribution at the end of the path is studied. It is shown experimentally that the displacement of the filamentation region due to geometric focusing or defocusing, makes it possible to control the number and spatial location of light energy density peaks in the receiving plane. The conditions under which a light filament can be reconstructed after the beam transmission through a linear focal waist are determined. A semi-empirical threshold relation is obtained for the beam focusing force and the beam power, when light beam undergoes filamentation behind the geometric focus of the optical system.