Propagation dynamics of high-intensity femtosecond laser pulses in argon

Abstract

. lntroductlonWe present experimental measurements that map out thestructure ofaself-focusing 800-nm ultrashort pulse propagating in air. Self-focusing of ultrashort pulses in air was first reported as a phenomenon by Braun, et al. ' We measure self-focusing with measurements taken over a maximum propagation distance of about 24 m that include shot-to-shot recordings of the pulse-energy, beam-profile images, autocorrelation data, and spectra. The data are taken at three propagation distances, 5.3 m, 11.3 m, and 24.3 m. At each of these positions along the beam path, the pulseenergy is varied from about 0.2 m) to about 3.5 ml. By varying the energy in the laser pulse, and recording data for each pulse, a high-resolution mapping of the pulse evolution in the spatial, temporal, and spectral domains, is obtained as a function of pulse energy. The data is of sufficient precision for benchmarking self-focusing theory calculations, including the prediction of the nonlinear index, n2, as well as dispersion in air. Experlmental SetupFor these experiments. a chirped~pulse amplihcalion TkSapphire laser system is made 10 output pulsesaflessthan33GW (4m1,120fs).Thepath of the pulses is outlined in Figure 1. Before the laser beam enters the compressor, the beam is sent through a fused-silica apodizing aperture to impose a sub-Gaussian, but smooth, profile on the beam. The output from the compressor is split by a beam-splitter. The beam-splitter transmits 5% of the pulse energy, which is used for the energy measurement. Tne remaining 95% of the beam energy is sent 10 a mite of diagnostics (inset, Fig. I ) located at a prescribed distance, as noted above. Spatial, temporal, and spectral in-formation about the beam collapse is obtained with this diagnostic package. Within the diagnostic package a thin ( I mm) fused-silica plate is held at extreme grazing-incidence (87.25 degrees) in order to split the beam. The reflected portion is incident onto an alumina screen and is imaged by a video camera to obtain beam-profile images. The grazing-incidence angle geometrically attenu a t e~ the transmitted beam to below the threshold of nonlinear interaction within the fused-sili a plate. In order to ensure against nonlinear interaction within the beam splitter, the angle of incidence is varied while the spectrum of the transmitted beam is monitored for no change. The transmitted beam goes through 3-500-pm pinhole to elect the central, highly nonlinear portion of the beam. Finally, the laser beam is attenuated further by a sapphire plate before being routed to a spectrometer and aurocorrelat~r far single-shot acquisition. Data is sent to a campuler in real-time with a rep-rate of 5 HI. ResultsA set of data runs, obtained at 5.3 meters over various days, is illustrated in Fig. 2. For the purpose of checking data reproducibility, the critical power necessary for whole beam self-focusing is defined to occur when the pulse width sharply decreases. The theoretical basis for this experimental critical power can be found in the work of Silberberg...