Conical emission from laser plasma interactions in the filamentation of powerful ultrashort laser pulses in air

We performed detailed experimental and numerical investigations of the conical emission (CE) accompanying the filamentation of powerful ultrashort laser pulses in air. It was found that the CE originates from self-phase modulation in the plasma produced by the pulse during propagation. The experiment and the simulation agree on the essential features of the CE: The CE angle decreases with increasing wavelength and is independent of the position along the filament, and there is no CE at Stokes-shifted wavelengths.

Moving focus in the propagation of ultrashort laser pulses in air

The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.

Generation of H(2), O(2), and H(2)O(2) from water by the use of intense femtosecond laser pulses and the possibility of laser sterilization

An intense femtosecond Ti-sapphire laser pulse was focused into water, leading to self-focusing. Apart from generating a white light (supercontinuum), the intense laser field in the self-focusing regions of the laser pulse dissociated the water molecules, giving rise to hydrogen and oxygen gas as well as hydrogen peroxide. Our analysis shows that the formation of free radicals O, H, and OH preceded the formation of the stable products of H(2), O(2), and H(2)O(2). Because O radicals and H(2)O(2) are strong oxydizing agents, one can take advantage of this phenomenon to design a laser scheme for sterilization in medical and biological applications.

Operational stabilization of a high-power ultrashort dye oscillator-amplifiers laser chain

Stable operation of a high-power ultrashort dye laser chain is achieved with an inexpensive heating technique. The femtosecond oscillator and its pumping laser are separately covered with boxes to isolate them from external temperature fluctuations. Both laser-isolating boxes are lined with a heating pellicle that permanently and uniformly warms the lasers to preset operating temperatures. The power, spectrum, and duration of the laser-chain output pulse remain constant from day to day. This technique could be applied to other ultrafast laser devices that require long-term performance stability.

Supercontinuum generation versus optical breakdown in CO(2) gas

Threshold powers for optical breakdown (OB) and supercontinuum (SC) generation in high-pressure CO(2) gas have been measured at different focusing conditions. It has been found that by varying the focal length of the focusing lens, it is possible to pass from the case of OB without SC generation, through OB with SC generation, and into SC without OB. The experimental results demonstrate that plasma formation is not an essential mechanism for SC generation, whereas self-focusing causes SC generation characteristics to be dependent on conditions of external focusing.

Spectral modulation of two coherently separated femtosecond laser pulses

A train of 150-fs 76-MHz pulses is directed into a Michelson interferometer with a path difference larger than the pulse spatial length. A modulated spectrum with a modulation frequency that increases with the path difference is observed. The consequence is discussed.

Spot-size measurement of an intense CO(2) laser beam

The determination of a high-power CO(2) laser beam spot size is described. The method consists of measuring burn pattern radii in holographic emulsion as a function of laser pulse energy.

Stable operation of a powerful 3-Hz line tunable TEA CO(2) oscillator-amplifiers system

We communicate results of the stable operation of a TEA CO(2) oscillator-amplifiers system, producing pulses of 10(14) W/cm(2) at best focus, furnished with a simple provision for retropulse isolation.

Nonlinear refraction in CS(2) at 10.6 microm

The nonlinear refractive index n(2) of CS(2) at 10.6 microm, the value of which is a subject of a recent controversy, is measured by an external self-focusing method. We found that n(2) = (2.1 +/- 0.7) x 10(-11) esu, similar to the accepted value of n(2) in the visible. Self-bending of spatially asymmetric CO(2) laser beam in CS(2) due to an ultrafast Kerr effect is observed, and the deduced value of n(2) is in agreement with our results.

Simultaneous nonlinear absorption and index effects in the propagation of intense TEA CO2 laser pulses through CDF3

Changes in the beam profile of the CO2 laser 10R(26) line, caused by transmission through, and absorption by, CDF3 were studied using an array of pyroelectric detectors. During the propagation of the laser beam through CDF3, nonlinear absorption and self-defocusing of the beam have both been determined from measurements of the effect on the exit beam of fluence, radiant energy, CDF3 pressure, transmission cell length, and distance from the exit of the cell to the detector array.

Single-pulse photoacoustic technique for measuring IR multiphoton absorption by polyatomic molecules

The single-pulse photoacoustic technique was used to measure cross sections for IR multiphoton absorption. A discussion of this technique and the applicable pressure range is given. The source of the signals from the microphone was identified. The first peak is proportional to the energy absorbed by the gas in the focal region, while the subsequently arriving signals correspond to a mixture of the signal from the entrance and exit volumes and the ringings of the microphone and the gas cell.

Self-focusing of CO(2) laser pulses in low-pressure SF(6)

We point out the importance of intensity-dependent refractive-index effects when measuring transmission or absorption characteristics of gases with high-power TEA CO(2) laser pulses. An example is given showing how self-focusing in low-pressure SF(6) gas severely affected the transmission of a Gaussian beam. This effect increased rapidly with pressure but remained unchanged when the TEA CO(2) fluence varied from 0.3 to 1.0 J/cm(2).