  We report on the observation of backward lasing at 391.4 nm of nitrogen ions
pumped by linearly polarized intense femtosecond pulses at 800 nm. The strongly
enhanced spectral intensity at 391.4 nm, as well as the amplification of an
externally injected backward seeding pulse, confirm that the backward 391.4 nm
signal is due to optical amplification in the gas plasma. Compared to the
forward emission at 391.4 nm, the optimal backward emission is achieved at a
lower gas pressure around 10 mbar, which is due to asymmetry of the backward
and forward directions rooted in the traveling excitation geometry. This
method, using the widely available 800 nm femtosecond pulses as a pump laser,
provides a promising scheme for the generation of backward air laser, which
holds a unique potential for optical remote sensing.
