F A Ilkov

Beam filamentation and the white light continuum divergence

Abstract We investigate the spatial characteristics of the white light continuum generated in water by femtosecond Ti:sapphire laser pulses with peak power well above the self-focusing threshold. The wavelength dependence of the divergence of the white light generated in small-scale filaments was measured. The observed dependence was interpreted, using self-phase modulation theory, as resulting from the radial intensity distribution in the small-scale filaments. Based on this interpretation the intensity profile in the filament was reconstructed.

The tunnel ionization of atoms, diatomic and triatomic molecules using intense 10.6 mu m radiation

A broad collection of experimental tunnel ionization data obtained using intense 10.6 mu m CO2 laser radiation, is presented. The variety of species studied includes three rare gas atoms (Xe, Kr and Ar), three homonuclear diatomic molecules (H2, O2 and N2), two heteropolar diatomic gases (CO and NO) and one triatomic molecule (CO2). The ionization behaviour going from the neutral particle to its associated singly charged ionic state, for all of the above species, is compared with a quasistatic tunnel ionization model. Good agreement between this theoretical model and the complete range of experimental results is found.

Tunnel ionization of diatomic molecules by an intense CO2 laser

The non-resonant interaction of an intense nanosecond CO2 laser pulse with Hg2, N2, and H2 shows that the molecules are tunnel ionized as if they were atoms with the same ionization potentials as the molecules. Moreover, dissociation of the neutral molecule appears to be much less probable even though the dissociation energies are lower than the ionization potentials.

Experimental study of the supercontinuum spectral width evolution in CO2 gas

Abstract Experimental results on the evolution, as a function of pressure, of the supercontinuum spectrum generated in CO 2 gas (1–40 atm) by weakly focussed 1 ps pulses at 593 nm are described. They are adequately fitted by a self-phase modulation model including the effect of self-steepening of the pulse. Above a given pressure optical breakdown appears in the gas. Optical breakdown during supercontinuum generation affects differently the behaviour of the red and blue side broadening since the blue side stops broadening after a certain pressure while the red side keeps increasing.

Supercontinuum generation in CO2 gas accompanied by optical breakdown

An experiment on the supercontinuum generation in high pressure CO2 (1-40 atm) by weakly focused 1 ps pulses at 593 nm is described. While changing the gas pressure and the laser power, the authors observed self-focusing and optical breakdown in the gas. They studied the evolution of the supercontinuum spectral width and efficiency after self-focusing. Their results show that: (i) the spectral width of the supercontinuum spectrum before optical breakdown can be explained in terms of self-phase modulation by only taking into account the cubic non-linearity of the gas and the increased laser intensity due to self-focusing; (ii) the plasma contribution to the supercontinuum generation was, in their experiment, a self-limiting type, owing to the shortening of the interaction length as the electron density increased.

The dynamical behaviour of and in a strong, femtosecond, titanium:sapphire laser field

Detailed measurements of and dissociation fragment kinetic energy dependences on laser intensity, using 150 fs, 800 nm pulses, are presented. The yields for both molecular and atomic ions are also given. The observed three-peak kinetic energy spectrum carries within it the signature of the different stages of the interaction. The two lower energy peaks are a product of bond softening (and above threshold) dissociation of the molecular ion from Franck - Condon populated vibrational levels. The third higher-energy peak results from enhanced ionization of the dissociating molecular ions. No light-induced vibrational trapping need be invoked to interpret the higher-energy fragments.

Self-action of ultrashort intense laser pulses in dense gases: self-focusing, optical breakdown, supercontinuum generation

Self-action during the propagation of ultrashort intense laser pulses through dense gases is practically unavoidable and leads to a dramatic change of pulse characteristics such as the beam size and frequency spectrum. The authors report results of experiments on propagation of ultrashort intense laser pulses in high pressure (1-40 atm) gas media. Self-focusing (SF) was observed under various conditions of gas pressure and laser power, accompanied by such phenomena as supercontinuum (SC) generation, conical emission (CE) and optical breakdown (OB). Supercontinuum generation (i.e. a nearly white spectrum generated upon propagation of intense short laser pulses through nonlinear media) has been observed in gases only recently, with appearance a new generation of powerful ultrashort lasers. The authors measured for the first time the spectral evolution of the supercontinuum radiation as a function of incident laser power and gas pressure. The single shot SC spectrum was deeply modulated, with this modulation changing with gas pressure and laser power. Beyond the SC generation threshold the authors also observed an intense conical emission with sharp spectral and spatial maxima.

Coulomb expansion of a two-component ion bunch: influence on high-field ionization experiments

A new aspect of the space-charge effect is investigated. We have observed Ar+ double peaks in the time-of-flight mass spectra recorded during the tunnel ionization of the Xe-Ar gas mixture using CO2 laser radiation. It is shown theoretically that this effect results from the Coulomb expansion of the two-component ion bunch produced in the focal volume. Calculations based on the proposed qualitative theoretical model agree well with the experimental data. This phenomenon is shown to be important in high-field ionization experiments in which multi-component ion bunches are produced.

Our personal and scientific encounter and collaboration with Delone (or a story of the development of multiphoton and tunnel ionization)

Each of us, (SLC) and (FI), met N.B. Delone under different circumstances. Nevertheless his scien tific work and personal demeanor had profound effect on both of us. Thanks to Delone we were able to work together for a number of years, and that impelled us to form a duet in relating memories of our encounter with Delone’s work and life. In what follows we are stepping in front of each other rushing to convey our stories, mixing first and third person narrative modes, trying to reach through years and continents in an attempt to do the impossi ble recreate one’s personal live. We admit our failure before hand to stress the fact that such happening as the life of Nikolay Delone is unprecedented and any slice and projection that we claim to remember are just flimsy rendering of the phenomenon.