Experimental observation of a traveling plasma grating formed by two crossing filaments in gases

The spatial motion and effective duration of a traveling plasma grating formed by two interfering femtosecond laser filaments in gases is characterized by its Doppler effect imparted on a probe pulse. The shift velocity determined experimentally agrees with the theoretical calculations. Nonlinear filamentation propagation of intense femtosecond (10 -15 s) pulses in transparent materials, such as ambient air and water, gives rise to many spectacular phenomena, including supercontinuum generation, formation of long plasma channels, pulse selfcompression and terahertz radiation [1, 2]. This nonlinear propagation process has attracted many research interests. The reason is not only the rich physics involved, but also its various potential applications, such as remote sensing of atmospheric pollution [3], pulse compression down to few cycle regime [4], triggering and guiding of electric discharge [5], remote THz pulse generation [6], to name a few. In the last few years, the interaction of two filaments in gases and solids has triggered new research interests [713]. It was found that the interaction of two filaments can enhance the yield of third order harmonic from a filament in air by 2 orders of magnitude [8]. A stationary plasma grating formed in ambient air, at the overlap region of two laser filaments of same frequencies, can be useful for high intensity laser applications and possibly the formation of lattice solitons [9-11], while a traveling plasma grating was recently proposed by Liu et al [14] to explain the efficient energy exchange between two crossing femtosecond filaments of different frequencies. Up to now, the presence of this traveling plasma grating has not yet been confirmed experimentally. Furthermore, its basic properties such as its traveling velocity and effective duration are still largely unknown. In this letter, we first capture the spatial profile of the traveling plasma grating. It is found that the plasma grating fringes do not wash out in a time-integrated picture even though they travel with a velocity as high as 7.8% of the light velocity, as will be shown below. Next, the traveling nature of this plasma grating is demonstrated by the Doppler shift that it imparts on a 400 nm probe pulse. The velocity of this grating derived from the measurement agrees with the calculation. In the experiment, a commercial chirped-pulse-amplification (CPA) laser system is employed (Alpha 100, Thales laser). This laser delivers 15 mJ, 40 fs laser pulses with central wavelength at 800 nm at a repetition rate of 100 Hz. The laser beam with 15 mm diameter is first splitted by a 10/90 beam splitter, with the 10% beam used as a probe pulse. The 90% pump beam is further split in two beams with a 50/50 beam splitter. The two pump pulses are focused by two identical convex lenses of f = 1000 mm in a gas  Corresponding author: [email protected] ha l-0 08 52 00 1, v er si on 1 19 A ug 2 01 3 Author manuscript, published in "Applied Physics Letters 98, 12 (2011) 121110" DOI : 10.1063/1.3568888