32 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
High-brightness switchable multi-wavelength remote laser in air
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.Comment: 18 pages, 5 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
Direct generation of intense extreme-ultraviolet supercontinuum with 35-fs, 11-mJ pulses from a femtosecond laser amplifier
We report on the generation of an intense extreme-ultraviolet (EUV) supercontinuum with photon energies spanning from 35 to 50 eV (i.e., supporting an isolated attosecond pulse with a duration of similar to 271 as) by loosely focusing 35 fs, 11 mJ pulses from a femtosecond laser amplifier into a 10-mm-long gas cell filled with krypton gas. The dramatic change in spectral and temporal properties of the driver pulses after passing through the gas cell indicates that propagation effects play a significant role in promoting the generation of the EUV supercontinuum
Second harmonic generation in a centrosymmetric gas medium with spatiotemporally focused intense femtosecond laser pulses
We demonstrate unexpectedly strong second harmonic generation (SHG) in Argon
gas by use of spatiotemporally focused (SF) femtosecond laser pulses. The
resulting SHG by the SF scheme at a 75 cm distance shows a significantly
enhanced efficiency than that achieved with conventional focusing scheme, which
offers a new promising possibility for standoff applications. Our theoretical
calculations reasonably reproduce the experimental observations, which indicate
that the observed SHG mainly originates from the gradient of nonuniform plasma
dynamically controlled by the SF laser field.Comment: 14 pages, 4 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