Femtosecond filamentation in transparent media

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

We have extended the tunneling ionization model of Ammosov-Delone-Krainov (ADK) for atoms to diatomic molecules by considering the symmetry property and the asymptotic behavior of the molecular electronic wave function. The structure parameters of several molecules needed for calculating the ionization rates using this molecular ADK model have been obtained. The theory is applied to calculate the ratios of ionization signals for diatomic molecules with their companion atoms that have nearly identical binding energies. The origin of ionization suppression for some molecules has been identified. The predicted ratios for pairs with suppression $({\mathrm{D}}_{2}:\mathrm{Ar},$ ${\mathrm{O}}_{2}:\mathrm{Xe})$ and pairs without suppression $({\mathrm{N}}_{2}:\mathrm{Ar},$ CO:Kr) are in good agreement with the measurements. However, the theory predicts suppression for ${\mathrm{F}}_{2}:\mathrm{Ar},$ which is in disagreement with the experiment. The ionization signals of NO, ${\mathrm{S}}_{2},$ and of SO have also been derived from the experimental data, and the results are also shown to be in agreement with the prediction of the present molecular ADK theory.

/pdf/theory-of-molecular-tunneling-ionization-1yoxqhwye5.pdf

Theory of molecular tunneling ionization

Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

https://iopscience.iop.org/article/10.1088/0957-0233/12/11/305/pdf

Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses

Frequency mixing the fundamental-and second-harmonic fields of an ultrafast laser in any one of a number of materials can generate radiation at terahertz frequencies. A better understanding of this process leads to a brighter source of light at these very useful wavelengths.

Coherent control of terahertz supercontinuum generation in ultrafast laser–gas interactions

We measured the optical breakdown threshold (OBT) in dielectrics with different band gaps for single and double 25-fs 800-nm transform-limited laser pulses. Our pump-probe double pulse measurements indicate that the plasma energy in dielectrics experiences ultrafast decay which lasts only $\ensuremath{\sim}100\mathrm{fs}$ and does not follow an exponential decay curve. Therefore, a decay term must be included in the electron density rate equation. Our double pulse measurements also demonstrate that the OBT is temperature dependent. The OBT in dielectrics was determined using a novel technique, which eliminates the ambiguity in its definition and also allows real-time data acquisition.

Ultrafast electron dynamics in femtosecond optical breakdown of dielectrics

High-precision ion yields are taken with 30-fs ultrashort laser pulses in various species of doubly ionized diatomic molecules, ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2},$ including metastable, charge symmetric, and asymmetric dissociating channels. The experimental results indicate that the detailed electronic structure is a critical factor influencing strong field nonsequential ionization. The study of ellipticity effects of nonseqential ionization is also extended, for the first time to our knowledge, to the molecular system.

Nonsequential double ionization of molecular fragments

We have developed a compact dispersion-free TG
(transient-grating) FROG (frequency-resolved optical gating) by
utilizing a mask that separates the input beam into three distinct
beams focused into fused silica to create the FROG signal. Two of
the beams are reflected off the same set of mirrors to ensure identical
optical paths, eliminating the difficulty in establishing zero time
delay between the beams. In addition, the use of only reflective
optics avoids material dispersion in the FROG except for the mixing
crystal. This TG FROG is capable of operating with an intensity of
1 × 1011 W/cm2 and has resolutions less
than 0.5 and 1.3 fs for 25- and 10-fs input pulses,
respectively.

Dispersion-free transient-grating frequency-resolved optical gating.

Using a new double pulse technique, we have observed for the first time that charge asymmetric dissociation in diatomic molecules leaves one of the fragments in an electronically excited state. For example, we observed the reaction ${\mathrm{I}}_{2}+(\mathrm{pulse}1)\ensuremath{\rightarrow}({\mathrm{I}}_{2}^{2+}{)}^{**}\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+({\mathrm{I}}^{2+}{)}^{*}+(\mathrm{pulse}2)\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+{\mathrm{I}}^{3+}$, demonstrating that the ${\mathrm{I}}^{2+}$ fragment must have been in an excited state. More generally, just as asymmetric dissociation implies that the initial molecular ion is in an excited electronic state, the observation of asymmetric channels in the postdissociation ionization shows that the ionic fragments are themselves electronically excited.

Direct Observation of Excited State Fragments Following Molecular Ionization and Dissociation in Strong Fields

It is shown that mode matching, aberration compensation, control of beam overlap, and feedback are critical issues in designing multipass amplifiers for ultrashort pulse amplification. We have developed a Ti:sapphire amplifier system capable of producing aberration-free millijoule-level 25-fs pulses at 1 kHz. This system has a unique five-mirror ring design, which is capable of compensating second-order aberrations, and the flexibility to mode match the input beam and the pump laser. A unique compressor, which is a part of the amplification system, was designed to eliminate feedback.

Flexible aberration-free multipass amplifier and compressor for ultrashort-pulse amplification

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