Femtosecond filamentation in transparent media

We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations.

Mechanisms of femtosecond laser nanosurgery of cells and tissues

Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials,...

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration and peak powers exceeding several terawatt (TW). When such pulses propagate through optically transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid on the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Far-field spectra provide promising diagnostics. Attention is also paid to the multifilamentation instability of broad beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy and the possibility of guiding electric discharges in air are finally addressed on the basis of experimental results.

https://iopscience.iop.org/article/10.1088/0034-4885/70/10/R03/pdf

Ultrashort filaments of light in weakly ionized, optically transparent media

Most long-path remote spectroscopic studies of the atmosphere rely on ambient light or narrow-band lasers. High-power femtosecond laser pulses have been found to propagate in the atmosphere as dynamically self-guided filaments that emit in a continuum from the ultraviolet to the infrared. This white light exhibits a directional behavior with enhanced backward scattering and was detected from an altitude of more than 20 kilometers. This light source opens the way to white-light and nonlinear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionization inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.

https://science.sciencemag.org/content/sci/301/5629/61.full.pdf

White-light filaments for atmospheric analysis.

We have investigated experimentally and numerically the influence of the initial temporal phase modulation of a pulse on the spatiotemporal intensity distribution and the frequency-angular spectrum of femtosecond laser pulses with self-channelling in a condensed medium. We have detected a decrease in the intensity of divergent anti-Stokes frequency components during filamentation of radiation under conditions of normal group-velocity dispersion (GVD) and strong phase modulation. In the zero-GVD regime under conditions of the phase modulation of radiation, the spatiotemporal transformation of the pulse is similar to that in the normal-GVD regime, which leads to a qualitative change in the supercontinuum spectrum. In the anomalous-GVD regime, a sequence of 'light bullets' is formed in the filament for both a phase-modulated and a transform-limited pulse.

http://iopscience.iop.org/article/10.1070/QE2014v044n06ABEH015453/pdf

Filamentation of a phase-modulated pulse under conditions of normal, anomalous and zero group velocity dispersion

We study, both experimentally and numerically, the variations of the refractive index at the filamentation of a femtosecond laser pulse in a fused silica. Using the polarigraphy and shadowgraphy techniques, we investigate the spatio-temporal variations of the refractive index induced by Kerr and plasma nonlinearities. The induced variations of the refractive index play the role of dynamic lenses: focusing for Kerr nonlinearity and defocusing for plasma nonlinearity. We show that intensity saturation occurs at the distance, where the total optical force of Kerr and plasma nonlinear lenses reaches its maximum.

Dynamic lenses in femtosecond filament

Propagation of high-power femtosecond laser pulse through random media is accompanied by transverse spatial distortions of the laser beam. Occurrence of high-intensity small-scale perturbations due to atmospheric disturbance may result in beam breakup and filament generation. Coherent scattering on random ensemble of aerosol particles result in these disturbance. On the other hand, plasma formation, owing to focusing of light into aerosol microdroplet, may hinder filamentation. The purpose of this paper is to numerical study the propagation of a femtosecond laser pulse through water aerosol. In particular, we will find the transverse intensity distribution resulting from coherent scattering of a 800 nm 45 fs pulse with 10 - 60 GW peak power on the ensemble of water droplets with the atmospheric size distribution and the density 100 cm-3. The forward scattering on aerosol particles takes into account the phase of the scattered radiation. The transverse distribution of the laser field behind the aerosol layer is calculated as the result of the interference of light fields formed by each particular particle. In the numerical simulations the input radius of a Gaussian beam was a = 2.5 mm. The average size of an aerosol particle was 4 μ. The length of a propagation path was set to the half of the diffraction length: 0.5.ka2. As the result we have shown that in aerosol medium it is possible forming of several hot spots containing approximately 1 critical power for self-focusing.

V.P. Kandidov

Papers

Filamentation of a phase-modulated pulse under conditions of normal, anomalous and zero group velocity dispersion

Dynamic lenses in femtosecond filament

Beam breakup and filament initiation induced by femtosecond pulse transmission through water aerosol

Femtosecond laser filamentation in turbulent atmosphere

Light bullet dynamics in uniform dielectrics