35 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
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
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
Harmonic-seeded remote laser emissions in N2-Ar, N2-Xe and N2-Ne mixtures: a comparative study
We report on the investigation on harmonic-seeded remote laser emissions at
391 nm wavelength from strong-field ionized nitrogen molecules in three
different gas mixtures, i.e., N2-Ar, N2-Xe and N2-Ne. We observed a decrease in
the remote laser intensity in the N2-Xe mixture because of the decreased
clamped intensity in the filament; whereas in the N2-Ne mixture, the remote
laser intensity slightly increases because of the increased clamped intensity
within the filament. Remarkably, although the clamped intensity in the filament
remains nearly unchanged in the N2-Ar mixture because of the similar ionization
potentials of N2 and Ar, a significant enhancement of the lasing emission is
realized in the N2-Ar mixture. The enhancement is attributed to the stronger
third harmonic seed, and longer gain medium due to the extended filament.Comment: 10 pages, 5 figure
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
Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses
We experimentally demonstrate ultrafast dynamic of generation of a strong
337-nm nitrogen laser by injecting an external seed pulse into a femtosecond
laser filament pumped by a circularly polarized laser pulse. In the pump-probe
scheme, it is revealed that the population inversion between the excited and
ground states of N2 for the free-space 337-nm laser is firstly built up on the
timescale of several picoseconds, followed by a relatively slow decay on the
timescale of tens of picoseconds, depending on the nitrogen gas pressure. By
measuring the intensities of 337-nm signal from nitrogen gas mixed with
different concentrations of oxygen gas, it is also found that oxygen molecules
have a significant quenching effect on the nitrogen laser signal. Our
experimental observations agree with the picture of electron-impact excitation.Comment: 9 pages, 5 figure
Driving forces and typologies behind household energy consumption disparities in China: A machine learning-based approach
Establishing an intuitive link between driving factors of household energy consumption activities and inequalities is important for the understanding of household heterogeneity in energy consumption behaviours. This paper proposes a novel typology framework based on machine learning approaches and data from 3637 Chinese households in 2014 from 85 cities. Activity-based energy consumption was measured, highlighting inequalities across activities, regions and household types. The results showed significant energy consumption disparities between urban/rural and north/south households, especially in cooking, space heating and vehicle activities. By identifying driving factors of energy consumption, a new household typology classified samples into 6 (all), 6 (urban) and 7 (rural) types. Within these types, households with similar demographic structures, lifestyles and energy consumption habits were clustered. Demographic structure, region, and primary energy demand were used as the basis for the typology. The findings demonstrated how household lifestyle differences explained the cause and underlying driving factors of urban-rural energy consumption inequalities and provided suggestions for city-by-city and type-by-type measurements to support effective low-carbon transformation in cities