Mid-infrared laser filaments in air at a kilohertz repetition rate

Abstract

Laser filamentation overcomes diffraction over a highly extended distance, making itself a powerful tool for long-range stand-off detection and light detection and ranging (LIDAR) applications. Mid-infrared (mid-IR) wavelengths are optimal for detecting biochemicals and air pollutants due to molecular fingerprints. Here, we demonstrate mid-IR laser filamentations in ambient air at a kilohertz repetition rate for the first time. Laser filaments significantly longer than the linear confocal parameter are generated with a pump power exceeding the critical power in air using a kilohertz, 2.1 μm, femtosecond, multi-millijoule optical parametric chirped-pulse amplifier. Odd-harmonic generation up to the ninth order at ultraviolet and the mid-IR spectral extensions up to 3.5 μm are observed. The highest third and fifth harmonic efficiencies from ambient air are obtained, to our knowledge, thanks to the extended interaction length within the filaments. Numerical simulations reproduce the harmonic generation with good agreement and confirm that the plasma effect dominates over the higherorder Kerr effect as the main defocusing mechanism of laser filamentation in our experiment. The detection of atmospheric CO 2 is demonstrated via mid-IR absorption spectroscopy. High-flux ultrabroadband mid-IR filaments are useful for the fast and sensitive detection of multiple chemical species in air.Laser filamentation is a highly nonlinear process that occurs as the self-focusing from the optical Kerr effect is balanced by diffraction, plasma defocusing, and other nonlinear mechanisms. The laser peak power needs to be higher than the critical power that scales with λ 2 to reach the filamentation regime, where λ is the laser wavelength. Filamentation has been widely studied in solids and high-pressure gases for various applications, such as supercontinuum generation, few-cycle pulse compression, and high-harmonic generation [1,2]. Filamentation in air has especially attracted considerable attention for applications in remote sensing, LIDAR, and laser-induced breakdown spectroscopy [3,4]. Compared to the traditional LIDAR and spectroscopy based on linear laser beam propagation, filamentation overcomes diffraction over much longer distances, and, additionally, the back scattered light is much more directional [2]. This makes laser filamentation a powerful tool for long-range stand-off detection. Moreover, the broadband supercontinuum from the filamentation covers a large spectral range, enabling us to access multiple single-wavelength traces simultaneously in a single-shot measurement. However, most experimental work on filamentation in ambient air has been limited to the visible and near-infrared (near-IR) range, where high-energy, ultrafast laser sources are readily available. By extending the spectral range to the mid-IR, one could explore higher-order nonlinear processes, as the odd harmonics in the visible and near-IR regions propagate well in the atmosphere through filaments [5][6][7]. More importantly, mid-IR filamentation in air ...

Molecular quantum wakes for clearing fog

“…This discovery opens new perspectives in the application of the method for real, atmospheric scale operation like FSO. For instance, intense mid-IR lasers, which provide only moderate ionization as they propagate in air [38][39][40][41], are now potentially very attractive candidates for fog clearing applications. It was recently reported in particular that TW peak powers CO 2 lasers can provide wide diameter self-guiding channels over kilometric distances [24], fulfilling the requirements for FSO between earth and satellites.…”

Nonlinear transparency window for ultraintense femtosecond laser pulses in the atmosphere

“…With the MIR central wavelength, the conversion efficiency to the supercontinuum increases due to weak dispersion. With propagation the distinctly pronounced harmonics up to the 15th [19,22,23] introduce strong spectral intensity modulation smoothed over by the supercontinuum [24,25]. The Stokes side of the continuum reveals a robust well-separated hump [26][27][28][29], which continuously downshifts its central frequency and consumes up to ten percent of the pulse energy [27,30,31] with propagation.…”

“…MIR frontiers of femtosecond laser manufacturing exploit the optima in parametric [18] and chirped pulse amplification laser systems. Only a few existing laser setups exceed the critical power for self-focusing P cr in the mid-and far-infrared range: at 2.1 μm [19], 3.3-3.9 μm [20], 3.3 μm [21], 3.9 μm [8,16], and 10.6 μm [17]. With the MIR central wavelength, the conversion efficiency to the supercontinuum increases due to weak dispersion.…”

Nonlinear rovibrational polarization response of water vapor to ultrashort long-wave infrared pulses

“…This phenomenon has possible applications in longrange free-space optical communication, remote sensing [4,5], synthesis of ultrashort pulses by higher harmonics generation [6,7], and lightning guiding [8][9][10], to name but a few. The atmospheric transparency windows in the mid-wave infrared (MWIR) (3-5 μm) [11,12] and long-wave infrared (LWIR) (8-12 μm) [13,14] regimes are of particular interest. Studies of nonlinear responses in molecular gases have so far been limited to the response of the electrons (Kerr effect) and the increased polarizability of the electrons due to a rotational response of the nuclei (rotational Raman effect).…”