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In this work we present experiments by focusing 42 femtosecond laser pulses in air using three different focal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is a weak plasma column, is observed. When the shorter focal length lens is used, a high density plasma is generated near the geometrical focus and coexists with a weak plasma channel of the filament. Under the tightest focusing condition, filamentation is prevented and only a strong plasma volume appears at the geometrical focus.

Multiphoton ionization of Mg was studied with an excimer laser (XeCl, λ = 308 nm). The laser-intensity dependence of the number of ions indicates that the lowest-order two-photon process, which has enough energy to reach the first ionization limit, fails to dominate the ionization of Mg. The slope is 3 when a low-enough extracting field is applied and increases with the latter. A three-photon resonance with some autoionizing states is proposed as a partial explanation of this abnormal phenomenon, but the variation of the slope of the log N versus log I curve as a function of a dc electric field remains unexplained.

Since the IR multiple-photon decomposition probability of polyatomic molecules is a function of radiative fluence, the measured fraction of the dissociated molecules could be affected by the spatial distribution of radiation. In this paper the influence of the radiative distribution of a Gaussian beam on measured probability is investigated in detail. The main conclusions are that, for two special cases, namely, cylindrical beam and double-cone beam, one will be able to get information of the fluence dependence of real decomposition probability from measured energy dependence of average probability by means of a log–log plot. This article will be helpful for consideration of experimental conditions, summarizing of data, and comparison of the results of different experiments to avoid possible confusion.

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.

Changes in the beam profile of the CO_2 laser 10R(26) line, caused by transmission through, and absorption by, CDF_3 were studied using an array of pyroelectric detectors. During the propagation of the laser beam through CDF_3, 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, CDF_3 pressure, transmission cell length, and distance from the exit of the cell to the detector array.

We measure the nonlinear refractive index of SF6 at three different CO2 laser lines. Higher terms than the cubic nonlinearity are found and the behavior of the refractive index as a function of the laser intensity depends strongly on the frequency used, reflecting the characteristic SF6 absorption dynamics.

Based on results of TEA-CO2 laser-frequency tuning by an intracavity dispersive prism, the feasibility of CO2 laser-frequency chirping by a nonlinear intracavity prism or due to self-bending effects is discussed. The proposed scheme is shown to be superior to other nonlinear optics methods for CO2 laser-frequency chirping.

Self-bending of a transversely nonuniform CO_2 laser beam owing to the Kerr effect in CS_2 is studied. The effect is used to measure the nonlinear refractive index n_2 of CS_2 at 10.6 μm. Self-bending can be used for ultrafast all- optical light-beam manipulation such as light-by-light deflection and nonlinear-optical switching/power limiting and for an optical streak camera.

We observed self-focusing and self-defocusing of a TEA CO2 laser pulse in CDF3 vapor under different conditions. The experimental parameters we varied are the pressure inside the interaction cell, the frequency of the laser, the energy and the temporal length of the pulse. We have shown that it is possible to pass from self-focusing to self-defocusing by only increasing the intensity of the laser pulse. We propose a physical model that can explain these experimental results. This model is different from that used to explain the selffocusing of a CO2 laser in SF6.