ArticlePDF Available

Laser-induced water condensation in air


Abstract and Figures

Triggering rain on demand is an old dream of mankind, with a huge potential socio-economical benefit. To date, efforts have mainly focused on cloud-seeding using silver salt particles. We demonstrate that self-guided ionized filaments generated by ultrashort laser pulses are also able to induce water-cloud condensation in the free, sub-saturated atmosphere. Potential contributing mechanisms include photo-oxidative chemistry and electrostatic effects. As well as revealing the potential for influencing or triggering water precipitation, laser-induced water condensation provides a new tool for the remote sensing of nucleation processes in clouds.
Content may be subject to copyright.
Laser-induced water condensation in air
Philipp Rohwetter1,Je
ˆme Kasparian2*, Kamil Stelmaszczyk1, Zuoqiang Hao3, Stefano Henin2,
¨lle Lascoux3, Walter M. Nakaema1,YannickPetit
2, Manuel Queißer1, Rami Salame
Estelle Salmon3, Ludger Wo
¨ste1and Jean-Pierre Wolf2
Triggering rain on demand is an old dream of mankind, with a huge potential socio-economical benefit. To date, efforts
have mainly focused on cloud-seeding using silver salt particles. We demonstrate that self-guided ionized filaments
generated by ultrashort laser pulses are also able to induce water-cloud condensation in the free, sub-saturated
atmosphere. Potential contributing mechanisms include photo-oxidative chemistry and electrostatic effects. As well as
revealing the potential for influencing or triggering water precipitation, laser-induced water condensation provides a new
tool for the remote sensing of nucleation processes in clouds.
Global warming and stratospheric ozone depletion have
demonstrated that human activities can significantly alter
the climate of Earth. However, the potential to locally alter
or even control the weather is still the subject of intensive
debate1,2. There have been long-standing efforts dedicated to
seeding clouds3with silver salt particles to encourage precipitation.
Here, we demonstrate that self-guided ionized filaments4–8 gener-
ated by ultrashort laser pulses are also able to induce water cloud
condensation in the free, sub-saturated atmosphere. In additional
laboratory experiments under both saturated and sub-saturated con-
ditions, we estimate a water uptake rate of up to 5 mg cm
the active volume of the filament-induced plasma channels. We
briefly discuss possible mechanisms that could contribute to this
observed laser-induced water condensation, although further inves-
tigations are needed to fully clarify their roles. Laser-based conden-
sation provides a new tool for the remote sensing of nucleation
processes in clouds and may even open up the possibility of influen-
cing or triggering water precipitation.
Self-guided laser filaments result from a nonlinear propagation
regime of ultra-short laser pulses. Beyond a critical power (P
3 GW in air at a wavelength of 800 nm), the beam self-focuses due
to the optical Kerr effect until its intensity is sufficient to allow
multiphoton ionization of air molecules, generating a cold plasma.
At this point, the released free electrons (typically 10
and the negative higher-order Kerr terms9tend to defocus the
beam and dynamically balance Kerr self-focusing. As a result, one
or several self-guided filaments10 with a diameter of 100 mmare
generated over distances much longer than the Rayleigh length,
up to hundreds of metres11. Filaments can be initiated at predefined
remote distances12 and propagate through adverse conditions
including fog and clouds10, turbulence13,14 or reduced pressures15.
They are therefore well suited for atmospheric applications4,7,even
in perturbed atmospheres.
We recently demonstrated that laser filaments can trigger corona
discharges within thunderclouds16, opening the way to lightning
control applications. With the present demonstration of water conden-
sation, self-guided filaments also raise new hopes that laser-assisted
local weather modification may be achieved, which, in contrast to
cloud-seeding using rockets, could be operated continuously and
would be free of environmental side effects.
As detailed in the Methods, experiments were conducted both in the
free atmosphere and under controlled conditions in a diffusion
cloud chamber filled with ambient air. A bundle of 20 to 30 self-
guided filaments was generated by the Teramobile femtosecond-
terawatt laser17, which provided 220-mJ pulses with a duration of
60 fs (3.5 TW peak power) at a central wavelength of 800 nm and
a repetition rate of 10 Hz. The filamentation onset was adjusted
by providing a negative chirp to the emitted laser pulse, so that
the group velocity dispersion (GVD) in the air recompressed the
pulse at a distance chosen for the interaction with the air mass
under investigation, either in the atmospheric cloud chamber or
in the free atmosphere.
Results and discussion
Highly reproducible filament-induced water condensation trails
were observed with the naked eye (see Fig. 1a,b and
Supplementary Movie) when the filaments were launched into the
atmospheric cloud chamber at a saturation of S¼2.3+0.7 (that
is, a relative humidity, RH¼230+70%) and a temperature
T¼224 8C. In ten experiments, we were able to confirm this quali-
tative observation by recording the corresponding evolution of
droplet density and size distribution using a Malvern Spraytec
aerosol particle sizer (see typical result in Fig. 1c,d). The particle
sizer gave access to particles greater than 2.4 mm only, so the con-
densation nuclei (CN) and cloud condensation nuclei (CCN)
could not be detected. The initial size distribution featured three
modes at diameters of 4, 50 and 250 mm. The droplet density in
each size class fluctuated significantly due to the residual air turbu-
lence in the chamber and the corresponding inhomogeneous distri-
bution of the pre-existing aerosols. However, a Student t-test
comparing the measured signals before and after the laser shot
confirmed that the observed effect has a statistical significance of
.0.9995. After the laser was fired, the average diameter of
the small particles grew to 6 mm, and their density dropped by
half, a change well beyond the fluctuations recorded before the
laser pulse. The total water content of this smaller mode therefore
remained almost constant. The decrease of this mode is most prob-
ably due to the coalescence of droplets, particularly the bigger ones.
This coalescence process is sustained by the mutual attraction of
1Teramobile, Institut fu¨r Experimentalphysik, Freie Universita
¨t Berlin, Arnimallee 14, D-14195 Berlin, Germany, 2Teramobile, GAP, Universite
´de Gene
20 rue de l’Ecole de Me
´decine, CH-1211 Gene
`ve 4, Switzerland, 3Teramobile, Universite
´Lyon 1; CNRS; LASIM UMR 5579, ba
ˆtiment A. Kastler, 43 Boulevard
du 11 novembre 1918, F-69622 Villeurbanne Cedex, France. *e-mail:
PUBLISHED ONLINE: 2 MAY 2010 | DOI: 10.1038/NPHOTON.2010.115
© 2010 Macmillan Publishers Limited. All rights reserved.
particles bearing opposite net charges generated in amounts of
charges cm
by the filaments18.
Simultaneously, the density of the mode around 50 mm doubled.
Figure 1c clearly shows that this mode does not develop from
smaller droplets, because these two modes remain distinct through-
out the growth sequence. Rather, the sudden rise in the medium
mode shortly after the laser fired probably stems from the laser-
induced fragmentation of droplets from the larger mode at
250 mm. The 50-mm droplets then grew to 80 mm within 3 s.
Simultaneously, the biggest mode also grew towards 400 mm. As a
consequence of this growth, the total atmospheric content of con-
densed water, as determined by integrating the volume of the dro-
plets over the measured size distribution and averaging over the
beam, increased by half (þ70 mgcm
). Considering that the
100-mm filaments only occupy 0.5% of the laser beam volume,
the local increase within the filament active volume amounted to a
factor of 100 (15 mg cm
), that is, 5 mg cm
over the 3 s of
growth time. Such results provide clear evidence of filament-assisted
condensation. The final droplet diameter of 80 mm was twice as big
as that predicted by a diffusive growth model for pure water under
thermodynamically stable conditions19 (that is, growth limited by
the local depletion of water vapour). Their growth rate of
10 mms
was four times faster, probably due to Wilson-type20
enhancement of the growth rate of the droplets charged by their
exposure to the high-charge density generated by the filaments.
However, the Wilson mechanism is not the only mechanism
that can explain laser-induced water vapour condensation,
because a dramatic and highly reproducible effect was also observed
in sub-saturated conditions (Fig. 2). We varied the relative humidity
in the chamber between 70 and 90% and the temperature between
20 and 60 8C. The observation of an increase in probe light scatter-
ing was governed by thewater content of the atmosphere rather than
by the relative humidity. Condensation was observed only when this
was above 80 mgcm
provided the relative humidity exceeded
75%. In such conditions, a 30-s series of 300 multifilamenting
laser pulses resulted in an immediate rise in the scattering signal
by a factor of 10, followed by a slower increase up to a factor of
25 with a time constant
4 s (Fig. 2). Such a steep rise again pro-
vides clear evidence of an increase in droplet size and number
density in the chamber, consistent with observations with the naked
eye. Moreover, saturation of the growth rate is typical of a process
limited by vapour depletion and the diffusion rate of vapour into
the beam region. In contrast, lower water vapour concentrations
did not allow substantial condensation even at RH ¼95%. The
requirement for condensable water before a visible effect is seen
shows that it is water vapour condensation indeed that is observed.
This condensation is not affected by the heat deposited by the
laser filaments into the air. A typical filament5–8 with an intensity
of 50 GW cm
, diameter of 100 mm and pulse duration of 100 fs
carries 0.4 mJ of energy. Even if this energy was totally absorbed
over a 10-m length of air, the specific heat of 1 kJ kg
the density of 1.2 kg m
would yield a temperature increase
limited to 3.3 K. In fact, only a small fraction of the filament
energy is absorbed, so heating of the air can be neglected in the
analysis of our results, and the ambient temperature can be con-
sidered as representative of the conditions within the filaments.
Time relative to laser pulse (s)
2.4 7.2 18 101 240 48042
Diameter (µm)
Droplet density (cm
/size class)
Total condensed mass (µg cm−3)
5 mm
5−2 −1 0 1 2 3 4
Time relative to laser pulse (s)
Total condensed mass
Small droplets (<20µm)
Medium droplets
(20−100µm) (×50)
Droplet density (cm−3)
5 mm
Figure 1 |Laser-induced condensation in an atmospheric cloud chamber (T5224 88888CandRH5230%). a,b, View inside the chamber before (a)andafter
(b) firing a set of three laser shots at 100-ms intervals. The laser filaments induce macroscopically visible droplet condensation, as is evident from the
massive increase in light scattering (see also the Supplementary Movie). c,d, Effect of a pair of laser shots launched in the chamber at t¼0andt¼0.1 s.
Temporal evolution of the particle size distribution (c) and the amount of small and medium droplets (d, left axis) as well as the total condensed water mass
(d, right axis) per unit volume. The arrows indicate the correspondence between the modes of the size distribution on panel c, and the curves displaying
their mode-integrated droplet density on panel d.
© 2010 Macmillan Publishers Limited. All rights reserved.
This negligible heating of the air contrasts with the high electron
temperature in the plasma filament, which can reach up to
6,000 K (refs 5–8). It should be noted that, due to the small trans-
verse dimension of the filaments, the slight local heating of the air
results in large temperature gradients, leading to the development
of a shockwave21. The corresponding expansion of the air might
contribute to the condensation process.
Note also that the fragmentation of pre-existing water droplets
into smaller ones increases the scattering signal. We quantified
this effect by modelling the laser-induced droplet fragmentation,
considering that each mother droplet absorbs an amount of
energy proportional to its cross-section, part of which is turned
into additional surface energy during fragmentation. A one-step
fragmentation model based on a maximum entropy principle
results in a Poissonian size distribution of the fragments of each
mother droplet22. By treating this distribution as continuous, even
in cases where it contained only a few tens of daughter particles23,
we found that the contribution of laser-induced droplet fragmenta-
tion to the effect observed in the sub-saturated cloud chamber was
marginal, regardless of whether nonlinear absorption or linear
absorption were considered. Instead, the observed condensation
may be understood by remembering that the atmospheric cloud
chamber is filled with ambient air. Therefore, in the urban
environment of the laboratory (including aerosols and gaseous
pollutants) and in the presence of the high-intensity laser
field, photochemically or charge-assisted mechanisms contribute
significantly to droplet formation24–26.
To provide a definitive demonstration of the capability of laser
filaments to trigger condensation, not only in controlled laboratory
conditions but also in real atmospheric conditions, we performed
open-field experiments (Fig. 3a) in the late autumn of 2008 in
Berlin, Germany, under conditions of polar air mass, providing a
high relative humidity (RH ¼90–93%) together with low level of
background aerosols (70 km horizontal visibility). The laser was
launched vertically into the atmosphere, at a repetition rate of
5 Hz. The filaments were most active between heights of 45 and
75 m. Their strength then decreased over a few tens of metres
beyond this range. The aerosol content of the atmosphere was mon-
itored by LIDAR (light detection and ranging)27 using a low-power
frequency-doubled Nd:YAG laser at 10 Hz repetition rate. This
allowed the performance of differential measurements of the
changes induced by the terawatt laser pulses preceding the LIDAR
pulses (Fig. 3a). The LIDAR return signals provide range-resolved
measurements of the total volume backscattering coefficient
which comprises a molecular contribution (Rayleigh scattering is
subtracted in the data processing) and an aerosol contribution
(Mie scattering). This aerosol backscattering coefficient
defined as27
Mie =1
where N(r) is the number density of droplets of size r, and
=pis the size- and refractive index-dependent back-
scattering differential cross-section of the particles.
therefore pro-
vides information averaged over all aerosol types and sizes within
the probed volume.
The LIDAR measurements were taken 1 ms after firing the tera-
watt laser pulses. This time delay, much shorter than typical droplet
growth times, was imposed by a lateral wind sweeping the air
ionized by the filaments out of the detection volume. As already
mentioned above, the filaments occupy a fraction of only 2.5 ×10
of the air volume probed by the LIDAR. Despite those difficulties
and atmospheric fluctuations, the beam-averaged value of
the height of the filaments was up to 0.5% higher when following a
filamenting laser pulse than without filaments (Fig. 3b). This
increase corresponds to a local enhancement of Mie scattering by a
factor of 20 within the filaments, from
. The latter value is typical of haze27,inspiteofa
growth time of 1 ms, much shorter than the signal rise time identified
in the sub-saturated chamber (Fig. 2b). Because
is a measure of all
kinds of aerosols, one could argue that only sulphate and nitrate CN or
CCN were observed. However, the results of the experiment in the
sub-saturated chamber discussed above show that the laser filaments
also cause the subsequent condensation of water droplets provided
enough water vapour is available in the atmosphere.
The statistical significance of the observed effect was assessed by
a Mann–Whitney U-test, comparing the sets of LIDAR signals fol-
lowing a filamenting pulse with the reference LIDAR signals. The
null-hypothesis of this non-parametric test is that the two samples
are drawn from a single population, so their probability distri-
butions are equal. It therefore makes no assumption about the
shape of the underlying distribution(s) and is insensitive to outliers.
The Mann–Whitney test can be seen as assessing for differences in
medians of the considered distributions. Statistically significant
results (
,0.01, where 1 2
is the confidence level) were obtained
between 6:00 and 6:30, when temperature and relative humidity
were 2.9 8C and 90%, respectively.
Afterwards, the meteorological conditions changed. A reduction
in the visibility, a slow increase in the relative humidity up to 93%
over 2 h and a rise in the absolute value of the LIDAR signal
suggested an increase in the background concentration of water
aerosols. Correlatively, the effect of filament-induced condensation
on the backscattering signal faded into the background. The fact
Normalized counts
0 5 10 15 20 25 30
1 50 100 150 200 250 300
Time after first shot (s)
Laser shot no.
Mean normalized counts
Time (s)
Laser ON time
Scattering signal
Exponential fit, τ=4s
Figure 2 |Laser-induced condensation in a sub-saturated atmospheric
cloud chamber (T560 88888C, RH 575–85%), observed through the
scattered signal at 9088888.a, High reproducibility of the effect over repeated
laser on/off cycles of 300 laser shots each. b, Rise time (time constant
4 s) of the light scattered by the growing droplets, averaged over the 17
cycles of panel a.
© 2010 Macmillan Publishers Limited. All rights reserved.
that the observed effects depend on the weather conditions excludes
a systematic experimental flaw. Furthermore, we can exclude direct
contribution of the filament plasma to the LIDAR signal, because
the lifetime of the plasma generated by the filaments does not
exceed the microsecond timescale, well below the millisecond
interval between the pump and the probe pulses28.
As in the cloud chamber experiment, we checked that the
enhancement of the LIDAR signal by the laser filaments could
not be explained by laser-induced aerosol fragmentation. First, the
observed effect decreases when the background LIDAR signal
increases, that is, when more water droplets are available for frag-
mentation. The above-described model of droplet fragmentation
quantitatively confirmed this qualitative argument. Based on very
high-visibility conditions and air-mass back trajectories29, we con-
sidered an initial maritime haze size distribution30. The visibility
provided the water droplet concentration, which was equal to
136 mm
. Alternative rural, remote continental and urban size
distributions were also considered, without affecting the result
qualitatively. Between 8 and 400 fragments per mother droplet
were considered, with refractive indices in the range 1.3–1.5 com-
monly encountered in hazes31. Even if an overestimation of the fila-
ment number (100) and diameter (200 mm) were taken into account
in the calculations, we found that fragmentation could increase the
Mie backscattering coefficient by at most 0.1–0.2%. Thus, fragmen-
tation does not provide the dominant contribution to the observed
effect in the atmospheric experiments.
Systematic parametric measurements would be required to better
understand and optimize the complex processes at play in our
observations. Although such a study is beyond the scope of the
present work, some important facts have to be considered. Each fila-
ment generates a cold plasma5–8 with 10
electrons cm
(ref. 5), that is, an average charge generation rate of 10
charges cm
at a repetition rate of 10 Hz. Most of these elec-
trons attach to ions within a few picoseconds while, typically,
0.0 0.2 0.4 0.6
Relative increase of
Mie backscattering (%)
Height (m)
800 nm, 220 mJ,
120 fs, 5 Hz
LIDAR transmitter:
532 nm, 5 mJ, 7 ns, 10 Hz
Ref. PD
light Filamenting
Fibre bundle
Filter +
1 ms
Figure 3 |Laser-induced condensation experiment in the atmosphere. a, Experimental set-up. The Teramobile laser (red) is fired 1 ms before the LIDAR
pulse (green) measuring the aerosol content of the atmosphere. b, Time-averaged relative increase of the Mie backscattering coefficient
between 6:00 and 6:30 with and without firing the Teramobile laser. The signal enhancement at the height of the filaments (the most active filamenting
region at 45–75 m is shaded) is a clear indication for filament-induced condensation.
Height in chamber (cm)
Laser position
Saturation ratio
T (K)
Figure 4 |Atmospheric diffusion chamber for laboratory experiments. a, Experimental set-up (schematic). A, Teramobile beam; B, particle sizer laser beam;
C, particle sizer receiver unit; D, imaging charge-coupled device camera; E, heat and vapour source; F, cold bottom plate. b, Measured vertical temperat ure
(blue, right scale) and derived supersaturation profile (red, left scale) in the chamber. The dashed lines correspond to the worst-case combinations of 2
temperature and chamber top saturation measurement errors. The grey region indicates the position of the laser beam.
© 2010 Macmillan Publishers Limited. All rights reserved.
of them attach to O
molecules to form O
ions, which
last for several microseconds5. These oxygen ions can form Wilson
clusters24 with high efficiency due to the extremely high initial con-
centration of O
, even without photoactivation. Direct multipho-
ton water photolysis25 can further contribute to this process
because the intensity of 50 GW cm
in the filaments allows pro-
cesses that are usually only considered for UV lasers. However, ther-
mally driven chemical reactions are not to be considered, because, as
discussed above, the plasma generated by the filaments is cold5–8.
The exceptionally high charge density could also result in a pro-
duction rate of oxidizing molecules such as O
OH radicals
that is orders of magnitude above the natural rates found in the
atmosphere. The resulting
OH, together with the O
ions men-
tioned above, will rapidly produce Wilson clusters25, oxidize SO
(refs 26,32) and NO
into H
and HNO
, respectively, and
assist their heterogeneous nucleation as well as that of volatile
organic species. Such processes are indeed compatible with the
millisecond timescale of the observed process. Most of these aero-
sols are highly hygroscopic and will later serve as CCN, allowing
the growth of water droplets in adequate humidity conditions, as
demonstrated in the sub-saturated cloud chamber experiment.
However, at the time of the probe pulse, the scattering efficiencies
are still low because the droplets have not had time to grow.
As a conclusion, we have experimentally demonstrated that self-
guided filaments generated by ultrashort laser pulses can assist
water condensation, even in an undersaturated free atmosphere.
Potential contributing mechanisms include photo-oxidative chem-
istry and electrostatic effects. The phenomenon provides a new
and attractive tool for remote characterization of the humid atmos-
phere and cloud formation. In addition, it may even provide the
potential to influence or trigger water precipitation using continu-
ously operating lasers rather than rockets.
The Teramobile femtosecond–terawatt laser17 used to generate the filaments
provided 220-mJ pulses of 60-fs duration at 800 nm, with a repetition rate of 10 Hz.
The beam formed a bundle of 20–30 filaments. In laboratory experiments, it was
launched through a diffusion chamber33 (Fig. 4a) filled with ambient air, as single
shots or in bursts of up to 30 s in duration. The filaments typically started a few
metres before the chamber, which was positioned 15 m away from the laser output.
A strong temperature gradient was maintained in the chamber volume of
0.6 ×0.3 ×0.2 m3by using a cold base plate that was kept at 260 8C by indirect
contact with a liquid-nitrogen reservoir, and a thermostated circulator heated to
10 8C at the top of the chamber. Water vapour from a reservoir at the top of the
chamber diffused towards the bottom plate. We estimated the relative humidity at
the top of the chamber to be 42+10% by measuring the evaporation time of
51+1 min of sessile water drops with an initial volume of 2 ml (ref. 34). The typical
vertical temperature profile, measured with a K-type thermocouple, is shown in
Fig. 1b. This profile was used to estimate the supersaturation profile, assuming steady
state35 and a zero water vapour concentration at the bottom of the chamber. The
supersaturation was estimated to be S4 near the bottom of the chamber,
consistent with the fact that we operated the chamber slightly below the threshold of
sensitivity to cosmic rays36. The resulting supersaturation in the interaction region
was S¼2.3+0.7 at a temperature of T224 8C. Sub-saturated conditions were
obtained by installing a water reservoir that was maintained above 100 8C at the top
of the cloud chamber while its bottom was circulator cooled to þ11 8C. The relative
humidity at the position of the beam was monitored with a capacitance hygrometer.
It ranged between 75 and 85% (S¼0.75–0.85) at a local temperature of 60 8C.
The water aerosol density in the atmospheric cloud chamber was observed with
the naked eye and monitored by launching a low-powercontinuous wave (c.w.) laser
(Nd:YAG, 532 nm, 10 mW) across the path of the pump beam and observing the
scattering at 908or 458. Scattering is a signature of water droplets, because Rayleigh
scattering by air molecules within the cell is negligible at atmospheric pressure over
the considered metre scale. Alternatively, the particle size distribution was
monitored by measuring the angular distribution of the scattered light of a He:Ne
laser in the forward direction using a Malvern Spraytec particle sizer. The data were
inverted using Mie theory, while considering the dominant particles to be spherical
water droplets.
The atmospheric experiment was performed at night in late autumn of 2008 in
Berlin (5282724′′ N, 1381738′′ E, 55 m above sea level), under conditions of
incoming arctic cold air (2.9 8C) and initially strong westerly winds at an
atmospheric pressure of 995 hPa, with a relative humidity initially stable at 90%
over 1 h and then slowly rising to 93% over 2 h. The horizontal visibility and wind
speed and direction were measured 33 and 39 m above ground, respectively, 1,140 m
east of the experimental location. Temperature and relative humidity were recorded
upwind at 620 m in the east–south–east direction away from the experimental site.
The initial horizontal standard visibility was 70 km, indicating an exceptionally
low background of aerosol scatterers. This value was used to calibrate the
aerosol-related fraction of the atmospheric backscattering coefficient from the
LIDAR signals.
In this experiment (Fig. 3), the Teramobile beam was expanded to a diameter of
10 cm and launched vertically into the free atmosphere at a repetition rate of 5 Hz.
The pulses were chirped and the beam slightly focused to maximize the strength of
multiple filamentation at a distance of 60 m. Backscattering from the atmosphere
was probed with a LIDAR using a 5-mJ YAG laser beam at 532 nm, pulsed at 10 Hz,
emitted collimated with a beam diameter of 4 cm, and overlapped with the
Teramobile beam on a dichroic mirror. This overlap was checked at 0 and 60 m by
folding the beams horizontally, ensuring that the strongly filamenting region was
effectively superposed with the probe beam.
The probe beam alternately measured the backscattering 1 ms following a
Teramobile shot and then in unaffected atmosphere 100 ms following the shot. The
horizontal wind speed of 2.5–5 m s
ensured that each pulse (resp. pulse pair)
interacted with a fresh air column. Single-shot LIDAR transients were collected with
a 11.4-cm-diameter, f¼500 mm telescope (4 mrad field of view), 20 cm off the axis
of the laser beam, detected by a photomultiplier tube equipped with a narrowband
(1 nm bandwidth full-width at half-maximum (FWHM) at 532 nm) interference
filter, and recorded on a digital oscilloscope used as a transient recorder (500 MHz
bandwidth). Each individual LIDAR signal was normalized by the pulse energy of
the probe pulses, as recorded using a high-speed photodiode. The inclination
between the axes of the laser beams and of the telescope provided 100% overlap
around 60 m, in the filamenting region. We integrated the LIDAR signals generated
by probe pulses over the most active filamenting region, between altitudes of 45 and
75 m (shaded region of Fig. 3), and compared those following a Teramobile pulse
with reference pulses.
Received 11 November 2009; accepted 8 March 2010;
published online 2 May 2010
1. Qiu, J. & Cressey, D. Taming the sky. Nature 453, 970–974 (2008).
2. US National Research Council. Critical Issues in Weather Modification Research
(National Academies, 2003).
3. Langmuir, I. Growth of particles in smokes and clouds and the production of
snow from supercooled clouds. Science 106, 505 (1947).
4. Kasparian, J. et al. White-light filaments for atmospheric analysis. Science 301,
61–64 (2003).
5. Couairon A. & Mysyrowicz, A. Femtosecond filamentation in transparent media.
Phys. Rep. 44, 47–189 (2007).
6. Berge
´, L. Skupin, S., Nuter, R., Kasparian, J. & Wolf, J.-P. Ultrashort filaments of
light in weakly-ionized, optically-transparent media. Rep. Prog. Phys. 70,
1633–1713 (2007).
7. Kasparian, J. & Wolf, J.-P. Physics and applications of atmospheric nonlinear
optics and filamentation. Opt. Express 16, 466–493 (2008).
8. Chin, S. L. et al. The propagation of powerful femtosecond laser pulses in optical
media: physics, applications and new challenges. Can. J. Phys. 83,
863–905 (2005).
9. Be
´jot, P. et al. Higher-order Kerr terms allow ionization-free filamentation in air.
Phys. Rev. Lett. 104, 103903 (2010).
10. Me
´jean, G. et al. Multifilamentation transmission through fog. Phys. Rev. E. 72,
026611 (2005).
11. La Fontaine, B. et al. Filamentation of ultrashort pulse laser beams resulting from
their propagation over long distances in air. Phys. Plasma 6, 1615–1621 (1999).
12. Rodriguez, M. et al. Kilometer-range non-linear propagation of femtosecond
laser pulses. Phys. Rev. E 69, 036607 (2004).
13. Chin, S. L. et al. Filamentation of femtosecond laser pulses in turbulent air. Appl.
Phys. B 74, 67–76 (2002).
14. Salame
´, R., Lascoux, N., Salmon, E., Kasparian, J. & Wolf, J.-P. Propagation of
laser filaments through an extended turbulent region. Appl. Phys. Lett. 91,
171106 (2007).
15. Me
´chain, G. et al. Propagation of fs-TW laser filaments in adverse atmospheric
conditions. Appl. Phys. B 80, 785–789 (2005).
16. Kasparian, J. et al. Electric events synchronized with laser filaments in
thunderclouds. Opt. Express 16, 5757–5763 (2008).
17. Wille, H. et al. Teramobile: a mobile femtosecond–terawatt laser and detection
system. Eur. Phys. J.—Appl. Phys. 20, 183–190 (2002).
18. Kasparian, J., Sauerbrey, R. & Chin, S. L. The critical laser intensity of self-guided
light filaments in air. Appl. Phys. B 71, 877–879 (2000).
19. Pruppacher, H. R. & Klett, J. D. Microphysics of Clouds and Precipitation
(Kluwer Academic Publishing, 1997).
© 2010 Macmillan Publishers Limited. All rights reserved.
20. Wilson, C. T. R. On a method of making visible the paths of ionising particles
through a gas. Proc. R. Soc. Lond. A 85, 285–288 (1911).
21. Yu, J. et al. Sonographic probing of laser filaments in air. Appl. Opt. 42,
7117–7117 (2003).
22. Cohen, R. D. Shattering of a liquid drop due to impact. Proc. R. Soc. Lond. A 435,
483–503 (1991).
23. Villermaux, E. Fragmentation. Annu. Rev. Fluid Mech. 39, 419–446 (2007).
24. Byers Brown, W. Photonucleation of water vapour in the presence of oxygen.
Chem. Phys. Lett. 235, 94–98 (1995).
25. Clark, I. D. & Noxon, J. F. Particle formation during water-vapor photolysis.
Science 174, 941–944 (1971).
26. He, F. & Hopke, P. K. SO
oxidation and H
binary nucleation by
radon decay. Aerosol Sci. Technol. 23, 411–421 (1995)
27. Measures, R. M. Laser Remote Sensing—Fundamentals and Applications (Wiley
Interscience, 1984).
28. Tzortzakis, S., Prade, B., Franco, M. & Mysyrowicz, A. Time evolution of the
plasma channel at the trail of a self-guided IR femtosecond laser pulse in air.
Opt. Commun. 181, 123–127 (2000).
29. Aeronet project, NASA, back trajectory data for stations Leipzig (D), Hamburg
(D), and Belsk (Pl),
30. Jaenicke, R. Tropospheric aerosol, in Aerosol–Cloud–Climate (Hobbs, P. V., ed.)
(Academic Press, 1993).
31. Quentzel, H., Ruppersberg, G. H. & Schellhase, R. Calculations about the
systematic error of the visibility-meters measuring scattered light. Atmos.
Environ. 9, 587–601 (1975).
32. Caffrey, P. et al. In-cloud oxidation of SO
by O
and H
: cloud chamber
measurements and modelling of particle growth. J. Geophys. Res. 106,
27587–27601 (2001).
33. Langsdorf, A. Jr. A continuously sensitive diffusion cloud chamber. Rev. Sci.
Instrum. 10, 91–103 (1939).
34. Scho¨nfeld, F., Graf, K. H., Hardt, S. & Butt, H. J. Evaporation dynamics of sessile
liquid drops in still air with constant contact radius? Int. J. Heat Mass Transfer
51, 3696–3699 (2008).
35. Saavedra, I. On the theory of the diffusion cloud chamber. Nucl. Instrum. 3,
85–89 (1958).
36. Tohmfor, G. & Volmer, M. Die keimbilding unter dem einfluß elektrischer
ladungen. Annalen der Physik 425, 109–131 (1938).
The authors would like to acknowledge J. Kirkby of CERN for fruitful discussions,
I. Sorge of Institut fu
¨r Meteorologie, FU-Berlin, Germany, for providing weather data,
and T. L. Kucsera (GEST) at NASA/Goddard for back-trajectories (available at the website). This work was supported by the Deutsche
Forschungsgemeinschaft, Agence Nationale de la Recherche (ProjectANR-05-Blan-0187),
the Fonds National Suisse de la Recherche Scientifique (FNS, grant nos. 200021-116198 and
200021-125315), and the Swiss Secre
´tariat d’E
´tat a
´ducation et a
`la Recherche in the
framework of the COST P18 project ‘The Physics of Lightning Flash and its Effects’.
Author contributions
All authors contributed extensively to the work presented in this paper. More specifically,
P.R., J.K., K.S., L.W. and J.-P.W. conceived and designed the study. P.R., K.S., Z.H., S.H.,
N.L., W.N., Y.P., M.Q., R.S. and E.S. perf ormed the experiments.P.R., J.K. and K.S. analysed
the data, and J.K., L.W. and J.-P.W. wrote the paper.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper at
Reprints and permission
information is available online at
Correspondence and requests for materials should be addressed to J.K.
© 2010 Macmillan Publishers Limited. All rights reserved.
... Phase explosion in confined liquid volumes has recently gained interest because of its connection with thriving research areas like x-ray liquid crystallography 1 , x-ray holography 2,3 , extreme UV light, and plasma generation 4 . A better understanding of the interaction of high-power lasers with small liquid particles is also relevant in laser-based atmospheric monitoring techniques 5,6 or in optical atomisation techniques that can be applied to the production of airborne transported micro-drops used as drug carriers 7 . At the heart of all of these research fields is the injection of high-power photons into a small liquid sample, the initiation of phase transition from liquid to vapour, the rapid pressure fluctuations, and the successive complex fluid mechanics driven by this impulsive energy input. ...
Full-text available
In this work, we present experiments and simulations on the nucleation and successive dynamics of laser-induced bubbles inside liquid droplets in free-fall motion, i.e. a case where the bubbles are subjected to the influence of a free boundary in all directions. The droplets of a millimetric size are released from a height of around 20\,cm and acquire a velocity of around 2\,m/s at the moment the bubble is nucleated. Within this droplet, we have investigated the nucleation of secondary bubbles induced by the rarefaction wave that is produced when the shock wave emitted by the laser-induced plasma reflects at the drop surface. Interestingly, three-dimensional clusters of cavitation bubbles are observed. Their shape is compared with the negative pressure distribution computed with a CFD model and allows us to estimate a cavitation threshold value. High-speed recordings of the drop/bubble dynamics are complemented by the velocity and pressure fields simulated for the same initial conditions. The effect of the proximity of a curved free surface on the jetting dynamics of the bubbles was qualitatively assessed by classifying the cavitation events using a non-dimensional stand-off parameter which depends on the drop size, the bubble maximum radius and the relative position of the bubble inside the drop. Here, we found that the curvature of the free surface does not play a determinant role on the jet dynamics, being the distance to the surface the dominant parameter. The oscillation of the laser-induced bubbles promote the onset of Rayleigh-Taylor and Rayleigh-Plateau instabilities, observed on the drop's surface. The specific mechanisms leading to the destabilisation of the droplet surface were identified through a careful inspection of the high speed images.
... Since its discovery a few decades ago [1], femtosecond laser filamentation, which is formed by a dynamic competition between the self-focusing of the laser beam and the plasma defocusing caused by the ionization of air molecule or its major components (N 2 and O 2 ), has been continuously advancing our understanding of molecular ionization, dissociation, excitation and other dynamical processes through intriguing emissions such as fluorescence [2], lasing action [3][4][5][6], supercontinuum [7], and terahertz [8,9]. Among them, the prototype nitrogen fluorescence emissions from the excited N 2 and N 2 + molecules with a predominate line at 337 nm and 391 nm, respectively, can provide valuable clues to identify the formation mechanism of the molecular excited states for increasing the efficiency of population inversion, and thus the intensity of the remote cavity-free laser which holds great potential for the remote sensing and weather control [10][11][12]. ...
Full-text available
An experimental investigation on nitrogen fluorescence emissions pumped by a single 800 nm fs optical vortex (OV) beam with different topological charges (TC) is reported. The intensity of the two prototype emission lines from neutral nitrogen molecules (N2) and molecular nitrogen ions (N2 +), i.e., 337 nm and 391 nm, respectively, shows different TC dependences: the former slightly decreases as the TC increases, while the latter sharply decreases as the TC changes from zero to a non-zero value. The dependences of the 337-nm and 391-nm emission intensity on pulse energy are also different: the former shows a linear variation for different TCs, while the latter has an abrupt change in the slope when changing the TC. Furthermore, the dependence of the 337-nm emission intensity on gas pressure exhibits a plateau which is universal for different TCs. In contrast, the dependence of the 391-nm emission intensity on gas pressure shows an apparent hump which is respective of the TC. These actual new results will facilitate further theoretical study on the formation dynamics of the nitrogen fluorescence emissions induced by the OV beam, and inspire that such beam can been taken as a unique pure optical tool to manipulate the transitions between different electronic-vibrational states.
... Filaments can deliver high intensities to kilometer scale distances 1-3 , propagate through turbulence 4 , and reform after interacting with aerosols 5 . These properties, among others, make filaments an excellent candidate for applications like microwave guiding 6,7 , white light LIDAR 8 , standoff laser induced breakdown spectroscopy (LIBS) [9][10][11] , cloud condensation 12,13 , and cloud cutting 14 . Propagation to or at high altitudes may broaden the application space for several of those processes listed above. ...
Full-text available
Filamentation is favorable for many long-range outdoor laser applications, some of which require propagation to or at high altitudes. Understanding how the filamentation process and filament properties are impacted by the low pressure conditions present at high altitudes is essential in designing effective applications. The scaling of filament preconditions with pressure is considered. An increase in critical power and decrease in transition numerical aperture (NA) is predicted to occur with a drop in pressure, indicating that nonlinear pulse propagation and filamentation at high altitudes requires higher energy and a longer assisted focal length than sea level filamentation. A summary of pressure-scaled filament properties is also presented. New simulations demonstrate filamentation at pressures as low as 0.0035 atm (38.5 km altitude) is possible.
... During the transmission of intense femtosecond laser pulse in the transparent medium, a stable plasma channel will be formed when the Kerr self-focusing and plasma defocusing effects reach a dynamic balance, which is also called filamentation [1][2][3]. Since the first experimental observation of femtosecond filamentation [4], it has aroused researchers' great interest due to its promising application prospects in remote sensing [5,6], lightning protection [7,8], and rainmaking [9,10] etc. Femtosecond filamentation is a complex nonlinear process, in which self-steepening, self-compression, self-splitting, space-time defocusing and other nonlinear effects play key roles, making the plasma density and light intensity change in the time domain dramatically, and thus resulting in self-phase modulation (SPM) effect which is usually recognized as the dominant mechanism for spectral broadening [2,11,12]. During femtosecond filamentation, the spectrum is greatly broadened, ranging from ultraviolet wavelength-band to far-infrared one [13,14], which is called white-light continuum or supercontinuum (SC). ...
Full-text available
Supercontinuum (SC) generation is a typical nonlinear phenomenon that occurs during femtosecond filamentation in transparent media. The interference of the SC induced by femtosecond filament in water is explored by using interferometry with the aid of a Mach-Zehnder interferometer (MZI). In the low pulse energy case (single filament is formed), the MZI is used to precisely determine the actual value of filamentation threshold Pth. It is found that the value of Pth is much higher than that of the critical power for self-focusing Pcr. In the higher pulse energy case, by blocking one arm of the MZI, self-interference resulted from SC emitted by femtosecond filaments is studied. By analyzing the interference patterns, we can acquire information on femtosecond filament, such as filament number and filament spacing. The energy range for the generation of single, double and triple filaments in water is also determined. Diffraction effect will distort and even mask the interference patterns, and to eliminate its influence on the results, the spectral signals at shorter wavelength are selected. This work provides an effective approach to study the complex femtosecond filamentation process intuitively and conveniently
... Filamentation refers to the phenomenon of the plasma channel generated by the dynamic balance between the optical Kerr selffocusing effect and the plasma defocusing effect caused by the ionization of neutral molecules when an ultrashort pulsed high-intensity laser propagates in a transparent medium such as air [1][2][3][4] . Femtosecond laser filamentation opens up many potential applications involving lightning control [5,6] , atmospheric condensation and precipitation [7,8] , remote sensing [9][10][11] , THz emission [12,13] , spectral broadening, and pulse compression [14] . Thus, the diagnosis of the filament, such as plasma channel size and electron density, is of great significance for understanding nonlinear propagation and applications of femtosecond laser pulses. ...
The temporal evolutions of electron density and plasma diameter of 1 kHz femtosecond laser filament in air are experimentally investigated by utilizing a pump-probe longitudinal diffraction method. A model based on scalar diffraction theory is proposed to extract the spatial phase shift of the probe pulse from the diffraction patterns by the laser air plasma channel. The hydrodynamic effect on plasma evolution at 1 kHz filament is included and analyzed. The measured initial peak electron density of ∼10 18 cm −3 in our experimental conditions decays rapidly by nearly two orders of magnitude within 200 ps. Moreover, the plasma channel size rises from 90 μm to 120 μm as the delay time increases. The experimental observation is in agreement with numerical simulation results by solving the rate equations of the charged particles.
Full-text available
Cumulative effects are crucial for applications of laser filaments, such as for the remote transfer of energy and the control of electric discharges. Up to now, studies of cumulative effects in the air of high-repetition-rate pulse trains have been performed at lower rates than 10 kHz. Herein, the nonlinear effects associated with short plasma filaments produced by pulses of moderate energy (0.4 mJ per pulse) and repetition rates up to 100 kHz are experimentally characterized. With increasing repetition rate, a decrease in absorption, fluorescence emission, and breakdown voltage and concurrently an increase in peak intensity and third-harmonic-generation efficiency are observed. Hydrodynamic simulations of the heated gas show that the observed decreases are directly related to a quasi-stationary state of reduced gas density in the filament. However, further investigations are required to fully understand the physics underpinning the observed sharp reduction of the breakdown voltage at 100 kHz repetition rates. The results may prove relevant for energy and information delivery applications by laser-induced air waveguide or electric discharge and lightning control.
The filamentation of the femtosecond vortex beam has attracted much attention because of the unique filamentation characteristics, such as annular distribution and helical propagation, and related applications. The critical power for self-focusing of the femtosecond vortex beams is a key parameter in the filamentation process and applications. But until now, there is no quantitative determination of the critical power. In this work, we experimentally determine the self-focusing critical power of femtosecond vortex beams in air by measuring fluorescence using a photomultiplier tube. The relation between the self-focusing critical power and the topological charge is further obtained. Our work provides a simple method to determine the self-focusing critical power not only for vortex beams but also for Airy, Bessel, vector, and other structured laser beams.
Full-text available
We experimentally investigated clean optical emissions from multiple combustion intermediates including free radicals C 2 , CH, and CN at multiple wavelengths induced by ultrashort 1,030-nm laser pulses. We systematically study the evolution of the fluorescence emissions induced by the femtosecond laser filament in the combustion field with the parameters such as the laser pulse energy, pulse duration, and focal length. Compared with the previous work, we promote that the fluorescence emissions of the combustion product can be manipulated effectively by controlling the femtosecond laser characteristics including pulse energy, duration, and the focusing conditions. This process helps to optimize its signal-to-noise ratio, which provides a further application of the femtosecond laser pulses to sense the combustion intermediates.
Full-text available
Based on energy and entropy principles, a statistical model describing the shattered state of a single spherical liquid drop after being subjected to a relatively sudden but uniform (over the whole surface area of the drop) impact is developed. The problem is addressed from a fundamental standpoint, with the intention of providing a predictive framework for the various modes of breakup and the size and number of droplets produced. Upon neglecting viscous effects, several results in terms of the energy of impact, non-dimensionalized with respect to the surface energy of the drop before impact, are derived. The model is quite simple and straightforward, yet it appears to predict in a fairly consistent manner certain experimental observations that have been made repeatedly in relation to drop breakup in stirred dispersions, by collision, and exposure to shocks.
Full-text available
Due to a typesetting error, a factor 0.367 is missing in the denominator of the collapse distance evaluated for focused Gaussian beams from the Marburger formula [see equation (50)]. The complete correct version of this formula is given in the PDF.
The oxidation of SO2 by ·OH radical produced by radon decay and binary nucleation of H2SO4H2O were studied using a thermal diffusion cloud chamber. A kinetic model was developed to examine the oxidation of SO2 by ·OH to H2SO4 to estimate airborne concentration of the critical species. The kinetic modelling calculations suggested that ·OH radical production is the rate control step among the gas-phase reactions proposed. The experimental results showed that nucleation rates increase with increasing radon and SO2 concentrations. Two distinct types of nucleation, mist and rain, were observed at different super-saturation in the thermal diffusion cloud chamber. The experimental observations suggest that both SO2 and H2SO4 can cause binary nucleation with water vapor. It is necessary to further distinguish between these two nucleation mechanisms in the future in order to obtain a better understanding of this combined oxidation and nucleation mechanism.
Publisher Summary This chapter presents some important aspects of tropospheric aerosols, including their sources, geographic distributions, sizes, chemical properties, and their transport and residence times. The emphasis is placed on the physical and chemical properties of tropospheric aerosols. There are two major sources of atmospheric aerosols: widespread surface sources and spatial sources. Widespread surface sources means sources at the base of the atmospheric volume (for example, oceans and deserts). Spatial sources refer to those within the atmospheric volume (for example, GPC and clouds). Additional point sources, such as volcanoes, are globally important in their influence on the stratosphere. Otherwise, because of short tropospheric residence times, aerosols from point sources affect mainly regional and local scales. The extreme variability of tropospheric aerosols gives rise to many problems and questions. Because of the short residence time of tropospheric aerosols (the residence time of water is comparable to that of aerosols), measuring networks with the density of precipitation networks are necessary to understand atmospheric aerosols. Sound strategies are needed to obtain the most useful and reliable measurements, particularly as dense measuring networks are unlikely to become available.
This book provides the most comprehensive treatment to date of the microphysical processes which lead to cloud and precipitation formation. Emphasis is placed on presenting a quantitative description of the various mechanisms (e.g., nucleation, diffusional growth and evaporation, collisional growth and breakup) which lead to the formation of cloud and precipitation particles, individually and in populations.
A revision is made of Shutt's theory by using more accurate expressions describing the transport phenomena in the chamber. This leads to a new expression for the temperature gradient. The importance of the correction thus introduced depends upon the particular components of the binary mixture considered. (auth)
Controlled cloud chamber experiments were conducted to measure particle growth resulting from the oxidation of SO2 by O3 and H2O2 in cloud droplets formed on sulfuric acid seed aerosol. Clouds were formed in a 590 m3 environmental chamber with total liquid water contents ranging from 0.3-0.6 g m-3 and reactant gas concentrations <10 ppbv for SO2 and H2O2 and <70 ppbv for O3. Aerosol growth was measured by comparison of differential mobility analyzer size distributions before and after each 3-4 min cloud cycle. Predictions of aerosol growth were then made with a full microphysical cloud model used to simulate each individual experimental cloud cycle. Model results of the H2O2 oxidation experiments best fit the experimental data using the third-order rate constant of Maass et al. [1999] (k = 9.1 × 107 M-2 s-1), with relative aerosol growth agreeing within 3% of measured values, while the rate of Hoffmann and Calvert [1985] produced agreement within 4-9%, and the rate of Martin and Damschen [1981] only within 13-18%. Simulation results of aerosol growth during the O3 oxidation experiments were 60-80% less than the measured values, confirming previous results [Hoppel et al., 1994b]. Experimental results and analyses presented here show that the SO2 - O3 rate constants would have to be more than 5 times larger than currently accepted values to explain the measured growth. However, unmeasured NH3 contamination present in trace amounts (<0.2 ppb) could explain the disagreement, but this is speculative and the source of this discrepancy is still unknown.
Instruments determining the visibility from scattered light give different results, depending on the differing size distribution and refractive index of the atmospheric aerosol particles. Sixteen instruments are tested in a reproduceable way by calculating the differences, which would occur over a great variety of size distributions and refractive indices. Two hundred and thirty seven size distributions found at different times and places around the world are used.All instruments show a small jump in scale reading at the change-over from haze to cloud or fog, which occurs between about 0.6 and 1 km standard visibility. The measured values of the integrating nephelometers vary by a factor of 1.26 and less, around an average value for haze or cloud and fog. The results from backscatter-instruments that measure at a scattering angle of exactly 180° vary by a factor of 1.44 (Lidar: 1.68) and less but only when the refractive index of aerosol particles stays constant. Variations in the refractive index lead to much larger deviations. Backscatter-instruments that do not measure at exactly 180° give a larger range of values around a different average even when the refractive index remains constant. Values derived experimentally often show a smaller spread than those derived from our calculations. The reasons for this discrepancy can only be guessed at.The ratio of the backscattering coefficient to the scattering coefficient for the monostatic Rb-lidar systems lies between 0.013 and 0.036 for haze and between 0.035 and 0.083 for clouds and fog. The values are similar for Nd-lidar systems. The values from backscatter-instruments that do not measure at exactly 180° are up to an order of magnitude smaller for clouds and fog.