Remotely pumped stimulated emission at 337 nm in atmospheric nitrogen

Summary form only given. The realization of remotely pumped laser-like sources in the common air is an enabling step towards the development of new standoff sensing schemes in the atmosphere [1]. Although laser sources generating light in the opposite direction relative to that of the pumping radiation are particularly desirable, forward-emitting sources may also be useful. Pulsed backward-propagating laser emission at 845nm wavelength from atmospheric oxygen has been recently demonstrated [2]. The lasing scheme involved pumping by nanosecond deep-UV laser pulses at 226nm. An intense research effort is currently under way that targets the development of viable lasing schemes in another major constituent of air, nitrogen. Remote lasing in the atmospheric nitrogen under external laser pumping has been analyzed numerically and found feasible [3,4]. In 2003, it has been suggested that dilute plasma filaments that are produced through self-focusing of ultra-intense femtosecond infrared laser pulses in air emit weak directional laser-like radiation at the 357nm and 391nm transitions of neutral and singly-ionized nitrogen molecules, respectively [5]. Since 2003, those findings have not been independently confirmed. More recently, the observation of forward-propagating emission on various UV lines of singly ionized nitrogen molecules has been reported, under the conditions of pumping by tightly focused, tunable mid-IR femtosecond laser pulses [6]. The emission was found to be seeded by the spectral tail of the fifth harmonic of the pump.In this contribution, we report the demonstration of transient UV nitrogen lasing at a standoff distance of 2 meters. Our lasing scheme is based on pumping by the Comet laser system at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. The pump laser beam at 1,053nm wavelength with up to 12 Joules of energy per pulse and pulse duration tunable from 0.5ps to ~20ps was weakly focused into the ambient air. Forward-propagatin- emission at 337nm wavelength was detected by a spectrometer and its energy was characterized by a pyroelectric energy meter. Our data is summarized in the Fig. 1. The emitted radiation at 337nm wavelength was polarized parallel to the polarization of the pump beam, suggesting that the emission was seeded by the spectral tail of the third harmonic of the pump. Optical gain at the 337nm wavelength was estimated as 1.5cm-1. The measured maximum pulse energy at that wavelength was 0.4mJ.