49 research outputs found
Impulsive rotational Raman scattering of N2 by a remote "air laser" in femtosecond laser filament
We report on experimental realization of impulsive rotational Raman
scattering from neutral nitrogen molecules in a femtosecond laser filament
using an intense self-induced white-light seeding "air laser" generated during
the filamentation of an 800 nm Ti: Sapphire laser in nitrogen gas. The
impulsive rotational Raman fingerprint signals are observed with a maximum
conversion efficiency of ~0.8%. Our observation provides a promising way of
remote identification and location of chemical species in atmosphere by
rotational Raman scattering of molecules.Comment: 4 pages, 4 figure
Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields
We carry out a combined theoretical and experimental investigation on the
population distributions in the ground and excited states of tunnel ionized N2
molecules at various driver wavelengths in the near- and mid-infrared range.
Our results reveal that efficient couplings (i.e., population exchanges)
between the ground state and the excited states occur in strong laser fields.
The couplings result in the population inversion between the ground and the
excited states at the wavelengths near 800 nm, which is verified by our
experiment by observing the amplification of a seed at ~391 nm. The result
provides insight into the mechanism of free-space nitrogen ion lasers generated
in remote air with strong femtosecond laser pulses.Comment: 18 pages, 4 figure
Remote creation of strong and coherent emissions in air with two-color ultrafast laser pulses
We experimentally demonstrate generation of strong narrow-bandwidth emissions
with excellent coherent properties at ~391 nm and ~428 nm from molecular ions
of nitrogen inside a femtosecond filament in air by an orthogonally polarized
two-color driver field (i. e., 800 nm laser pulse and its second harmonic). The
durations of the coherent emissions at 391 nm and 428 nm are measured to be
~2.4 ps and ~7.8 ps respectively, both of which are much longer than the
duration of the pump and its second harmonic pulses. Furthermore, the measured
temporal decay characteristics of the excited molecular systems suggest an
"instantaneous" population inversion mechanism that may be achieved in
molecular nitrogen ions at an ultrafast time scale comparable to the 800 nm
pump pulse.Comment: 19 pages, 4 figure
Real-time observation of dynamics in rotational molecular wave packets by use of "air laser" spectroscopy
Molecular rotational spectroscopy based on strong-field-ionization-induced
nitrogen laser is employed to investigate the time evolution of the rotational
wave packet composed by a coherent superposition of quantum rotational states
created in a field-free molecular alignment. We show that this technique
uniquely allows real-time observation of the ultrafast dynamics of the
individual rotational states in the rotational wavepacket. Our analysis also
shows that there exist two channels of generation of the nitrogen laser,
shedding new light on the population inversion mechanism behind the air laser
generated by intense femtosecond laser pulses.Comment: 23 pages, 6 figure
Self-induced white-light seeding laser in a femtosecond laser filament
We report, for what we believe to be the first time, on the generation of
remote self-seeding laser amplification by using only one 800 nm Ti:Sapphire
femtosecond laser pulse. The laser pulse (~ 40 fs) is first used to generate a
filament either in pure nitrogen or in ambient air in which population
inversion between ground and excited states of nitrogen molecular ions is
realized. Self-induced white light inside the filament is then serving as the
seed to be amplified. The self-induced narrow-band laser at 428 nm has a pulse
duration of ~2.6 ps with perfect linear polarization property. This finding
opens new possibilities for remote detection in the atmosphere.Comment: 18 pages, 5 figure