Remotely pumped stimulated emission at 337 nm in atmospheric nitrogen

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.