High-brightness switchable multiwavelength remote laser in air

Abstract

Remote laser in air based on amplified spontaneous emission (ASE) has produced rather well-collimated coherent beams in both backward and forward propagation directions [1-3], opening up possibilities for new remote sensing approaches [4,5]. The remote ASEbased lasers were shown to enable operation either at ~391 and 337 nm using molecular nitrogen or at ~845 nm using molecular oxygen as gain medium, depending on the employed pump lasers. To date, a multi-wavelength laser in air that allows for dynamically switching the operating wavelength has not yet been achieved, although this type of laser is certainly of high importance for detecting multiple hazard gases. In this Letter, we demonstrate, for the first time to our knowledge, a harmonic-seeded switchable multi-wavelength laser in air driven by intense mid-infrared femtosecond laser pulses. Furthermore, population inversion in the multi-wavelength remote laser occurs at an ultrafast time-scale (i.e., less than ~200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons, which is fundamentally different from the previously reported pumping mechanisms based either on electron recombination of ionized molecular nitrogen or on resonant two-photon excitation of atomic oxygen fragments resulting from resonant two-photon dissociation of molecular oxygen. The bright multi-wavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear spectroscopy [6].

Optical gain in rotationally excited nitrogen molecular ions

“…Note that in contrast to the previous studies, e.g., [5], the broadband pedestal due to the third-harmonic emission that seeds stimulated emission at 428 nm is not visible in the spectrum. That is because our spectrometer has a much higher resolution compared to the spectrometers used in the earlier studies.…”

“…Various schemes of turning air into an active laser medium are being investigated [3][4][5][6]. One of the most promising approaches is based on pumping atmospheric nitrogen by ultraintense femtosecond laser pulses propagating in the filamentation regime in air [7].…”

Alignment-Dependent Fluorescence Emission Induced by Tunnel Ionization of Carbon Dioxide from Lower-Lying Orbitals

“…Therefore, establishment of a technique that allows direct observation of tunnel ionization from individual lower-lying orbitals will undoubtedly be of great importance. In this Letter, we show that such an ambition can be realized by employing the fact that the intensities of the fluorescence lines directly reflect the ionization probabilities from lower-lying molecular orbitals [14,15]. Particularly, we have recently observed that tunnel ionization from lower-lying molecular orbitals can produce not only molecular ions at excited states, but also population inversion in the ionized molecules by which lasing can be initiated [14].…”

“…In this Letter, we show that such an ambition can be realized by employing the fact that the intensities of the fluorescence lines directly reflect the ionization probabilities from lower-lying molecular orbitals [14,15]. Particularly, we have recently observed that tunnel ionization from lower-lying molecular orbitals can produce not only molecular ions at excited states, but also population inversion in the ionized molecules by which lasing can be initiated [14]. Thus, we expect that by deconvolution of the measured alignment dependence of the fluorescence, we can 5 obtain the alignment dependence of the ionization probability of HOMO-1 and HOMO-2 orbitals of molecules.…”

Natural population inversion in a gaseous molecular filament

“…In the latter work, amplified spontaneous emission from N 2 and N + 2 was measured. Several recent works also observed gain in N 2 and N + 2 in air [8,9] . However, the mechanism of population inversion in N + 2 has not been completely understood.…”