  Remote laser in air based on amplified spontaneous emission (ASE) has
produced rather well-collimated coherent beams in both backward and forward
propagation directions, opening up possibilities for new remote sensing
approaches. The remote ASE-based 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.
