Laser-induced ozone production at 100 TW

Observations of the ozone production from filament-free beam confirmed the sub-stantial contribution of the photon bath in the low-intensity regime.

We checked that no filaments were present below 330 GW/cm², where the sono-metric setup [48], already described in Sec. 1.3.2, detected no ionization of the air.

Ozone generation clearly featured a dual-component dependence on the average beam intensity, as displayed in Fig. 5.12. Increasing the fluence at a fixed pulse duration (red curve) resulted in a steep rise of ozone concentration; on the other hand, for a constant fluence (blue), the intensity had no influence on the ozone yield as long as no filaments were generated. The latter trend was observed also

Figure 5.11: (T =8−12^◦C and 75−95% RH). Dependence of laser-assisted particle production on the beam-average incident intensity for a constant incident fluence of 23 mJ/cm². The black dashed lines mark the edges of the linear scaling range of the filaments number. Reprint from [28]

Figure 5.12: Ozone production from filament-free ultrashort laser pulses. The vertical black dashed line marks the threshold for detectable ionization by the sonometric setup. Reprint from [28]

5.3 Impact of the laser energy on the aerosol and ozone production

for nanoparticles generation from the photon bath of the ’DRACO’ laser beam (Fig. 5.11). Therefore, for a filament-free beam, the ozone generation was gov-erned by the energy; conversely, at high intensities, it rised non-linearly with the incident intensity.

5.3.4 Discussion

Laser-induced condensation therefore appears to stem from three contributions.

The first one originates from filaments, and is responsible for the linear scaling of generated particles with their number. The photon bath yields both a non-linear intensity dependent term, and a linear fluence-driven one, which is observed at high fluence and low intensity, i.e. for long pulses.

Owing to the limited number of experimental data points, the precise order of the non-linear contribution of the photon bath could not be retrieved, although it seems to lie between 5 and 8. At 800 nm, fifth-order and eight-order pro-cesses may respectively be identified as the multi-photon dissociation and the multi-photon ionization of oxygen. Together with nitrogen, whose multi-photon dissociation at 800 nm is a seventh-order process [145], oxygen is at the root of laser-induced condensation, via ozone formation [110, 146]. As it resulted from the sonometric detection, the photon bath alone yielded no ionization of the air molecules; therefore, dissociation rather than ionization is more likely to be at the basis of the observed non-linearity.

Such relevant contribution of the photon bath to a non-linear process at extreme average incident intensity is not a unique peculiarity of the laser-assisted particle production. Indeed, it has also been observed in the case of white-light continuum generation in the multi-TW regime [27].

The linear, fluence-dependent contribution of the photon bath to the condensa-tion process at low intensity may be due to mid-IR atmospheric photochemistry;

possible pathways involve the excitation of organic peroxyl radicals (RO2) [147], or water-cluster mediated chemistry [148]. These radicals, which originate from the volatile organic compounds present in the atmosphere, subsequently generate hydroxyl (OH) or hydroperoxyl (HO2) radicals which are known to contribute to ozone formation. Other mechanisms with time constants well above the pulse du-ration, like thermal- or shockwave- related processes, may also be activated by the laser illumination. A detailed study of these incoherent processes, already active at low intensities, will be crucial to anticipate the relevance of the high-fluence, picosecond pulses contribution, which potentially offers alternative ways to opti-mize the laser conditions for atmospheric experiments.

5.4 Conclusions

We have investigated the production of nanoparticle assisted by the laser at the multi-TW level of input power. The efficient particle production from the photon bath at such extreme intensities [28], which adds up to the filaments contribution, provides an unexpected perspective on laser-assisted water vapor condensation;

indeed, the significantly increased yield of individual laser shots pushes the laser effect towards larger scales.

Including the photon bath, the total active volume of the laser beam is typically 10³−10⁴times larger than that of the filaments only. Such wider activated volume allows for a more efficient use of the available water vapor in the atmosphere. The particle production from the photon bath is free from the geometric constraints that limit the filament number at high intensities [25], and scales faster than lin-early with the incident energy.

Therefore, these results demonstrates that the highest possible laser power and energy improve the prospects for macroscopic effect of laser pulses on water con-densation; thus, the implementation of laser-based weather modulation techniques can’t prescind from the use of such extremely powerful lasers.

State-of-the-art multi-Terawatt or Petawatt laser systems require fixed installations in large facilities, therefore not suitable for atmospheric mobile applications, both for logistic and economical reasons. However, the ultrashort lasers technology is knowing an extremely fast development since few decades; it is thus reasonable to expect that high power portable laser systems will be available in the future, making more realistic the perspective of cost-effective large scale atmospheric applications of such lasers.

5.5 Outlook: potential applications to laser-induced weather modulation

Weather modification is one of the challenges that humankind is facing over the last decades, aimed at attaining the control of some local atmospheric events, such as precipitations, thunderstorms, or lightnings. It has enormous potential conse-quences both social and economical; agriculture and safety are just few of the fields upon which rain or lightning control can have far implications.

The most known weather control technique is by far cloud seeding [149], which entails spraying small solid particles such as silver iodides (AgI), or dry ice or other salts onto a cloud in order to provide condensation nuclei (CCN); owing to the high humidity, which is a fundamental requirement for this technique to succeed, they can thus be activated and grow to rain droplets via water vapor con-densation and coalescence. Precipitations could be thus induced, and many people

5.5 Outlook: potential applications to laser-induced weather modulation

claimed to have succeeded.

Nevertheless, the efficiency of this method is still debated [150, 151], since it is difficult to demonstrate a posteriori to which extent the seeding action was neces-sary to trigger the rain. Moreover, the toxicity of AgI salts still raises doubts.

A way to depict laser-induced water condensation is to think of it as an alternative approach to cloud seeding, which does not require any solid particle to be injected in the atmosphere; only light is used, as an excitation to generate the missing CCN.

The potential advantages of the laser-based cloud seeding can be discerned even if we are far from its realization. First of all, it is non-polluting. Second, a full temporal and spatial control can be achieved thanks to the possibility to direct a laser beam and to initiate filaments at the desired position and time; a well defined air volume can thus be spanned. Conversely, the dispersed salts are completely out of control and can be spread over large areas by the wind.

One should however keep in mind that so far the observations only demonstrated the first part of the rain droplet formation process, i.e. the activation of aerosols up to micrometric size in a sub-saturated atmosphere with relative humidity higher than 70%. This fundamental requirement limits the applicability of this technique to specific situations where the concerned air mass contains enough water vapor.

Moreover, the growth of the aerosols in our experiments was limited by the avail-able amount of nitric acid required for the stabilization of the droplets. Indeed, it was shown in Section 3.4 that the mass ratio between nitric acid and water must be kept constant. This represents another limiting factor: in order to make such droplets able to reach the size of a typical rain droplet, one should provide much more relevant amount of HNO₃in sub-saturated conditions. Apart from being ex-tremely difficult from a technical point of view, this would also result in strongly acid droplets.

This limitation could however be overcome, if the temperature decreases and the relative humidity thus exceeds 100%, as it can occur, for example, in a raising air mass. Such dynamics is typical of cloud formation by adiabatic cooling of hu-mid air. This would automatically activate the generated aerosol which could then grow by water uptake and form cloud and rain droplets; the nitric acid dissolved in the resulting droplets would then be highly diluted. According to this scenario, triggering of precipitations is achieved through the combined action of the artifi-cial laser-induced condensation of CCN and the natural processes leading to their activation, resulting in clouds formation.

This technique could be efficiently employed in situations when the local atmo-spheric conditions (temperature, humidity and air mass flow) are spontaneously evolving towards a raining event, but the cloud condensation nuclei concentration is too low to yield a raining cloud processing. The laser-generated aerosols may provide such missing CCN. In this context, the term ’weather control’ is mislead-ing since it can induce the reader to imagine a scenario where the laser can trigger

atmospheric events on demand, regardless of the atmospheric conditions and other natural components.Weather modulationis a more appropriate definition, since it better highlights the role of the laser in the overall process; the laser indeed mod-ulates or, in other words, slightly perturbs the environment in a way that alters the subsequent evolution of the system.

At the present stage, we cannot by far speak of an already working weather mod-ulation technique; rather, the laser-induced water condensation could potentially be at the basis of a new method to trigger precipitations, provided that a proper strategy to extend the laser action to a much larger volume is found and imple-mented.

Conclusions

Drawing its inspiration from Wilson experiments on the photo-nucleation of wa-ter vapor, the GAP Biophotonics group, as a partner of the Teramobile project, opened a new line of research in the field of atmospheric applications of lasers.

Indeed, we conceived the idea of using the laser as an active tool to assist new particle formation in the real atmosphere.

We demonstrated that infrared filaments can efficiently act as a localized source of new stable aerosols in the lower troposphere and, even more surprisingly, promote the direct gas-to-particle conversion in presence of a virtually clean air. The most remarkable feature of the laser-generated particles is their thermodynamical equi-librium with the surrounding, which preserves them from evaporation even in a sub-saturated atmosphere; they provide, thus, suitable sites for the water conden-sation. In other words, they possess the potentiality to act as Cloud Condensation Nuclei.

The stability of such particles is ensured by their chemical composition: our result suggest, indeed, that they are not pure aqueous droplets, but rather solutions that feature a lower saturation vapor pressure, made of water and other compounds such as nitric, sulphuric, or organic acids. While nitric acid is produced via the oxidation of laser-generated nitrates, the other chemicals result from the oxida-tion of atmospheric trace gases, such as sulphates and VOCs, by highly reactive species generated in large amounts within the filament volume. The reactants, such as ozone, NOx and OH-radicals, are produced via photo-dissociation of air molecules.

Therefore, the laser action can be depicted as a ’photo-chemical’ trigger of a po-tentially broad variety of reactions leading to condensing vapours, whose yield can also be expected to depend on the abundance of atmospheric trace gases, hence the considered environment (rural, industrial, maritime, etc..).

The laser-induced modulation of the thermodynamic equilibrium between air and atmospheric particles may lead to amazing consequences: by seeding the air with condensation sites, a cloud may form if the air mass subsequently cools down, eventually evolving in a precipitation event. This scenario would open attractive perspectives in view of an all-optical version of cloud seeding, which would

ex-hibit several potential advantages with respect to the existing techniques, specifi-cally in terms of controllability and pollution issues.

Furthermore, our research also focused on the effects of laser irradiation on sim-ulated cirrus clouds, which are typically formed in the upper troposphere. We reported a remarkable phenomenon: the laser, indeed, proved very efficient in modulating the optical properties of cirrus clouds by multiplying pre-existing ice crystals. This finding may be at the basis of a new strategy in the context of airborne climate engineering, since the transformed clouds are made of smaller particles and can reverse their contribution to the global radiative balance of the atmosphere.

Our experimental investigations therefore showed that ultrafast lasers can suc-cessfully be employed to actively modify the local concentration of atmospheric aerosols and ice crystals, suggesting spectacular practical uses in the context of weather modulation. However, these striking effects have been observed at the scale of filaments volume, i.e. less than 0.1 cm³. It is therefore straightforward that, for the weather modulation scenario to be realistic, the laser-induced particle generation must be active on a macroscopic volume.

Increasing the laser irradiation appears to be a promising approach to that issue: as we demonstrated by experimentally investigating the laser effect with a 100 TW laser, the nano-particle yield scales faster than linearly with the laser power. In parallel, the GAP Biophotonics group is currently working on different strategies to increase the yield of individual laser shots, by exploring this phenomenon at UV wavelengths and by implementing a closed-loop optimization of the optical pulse shape.

Although we aim at up-scaling the laser effect with respect to the volume occu-pied by filaments, its action would be extended to a macroscopic scale that, in the meteorological context, is however considered local. A rough estimation indicates that, by steering the beam, a single laser source located at ground may irradiate a surface of few km²at an altitude of 2 km.

The choice of the expressionweather modulation, as a possible practical applica-tion that may benefit from these findings, is extremely emblematic. We strongly intend to avoid misleading interpretations that may induce the reader to prefig-ure science-fiction scenarios such as a laser being able to trigger raining events on-demand, irrespective of the atmospheric state, and modify the weather on a global scale. For this reason, we believe that a term such as ’weather control’

may be unappropriate. The key concepts that we want to highlight are, first of all, that clouds formation or modification are potentially expected to happen at a meteorologic local scale; second, but not less important, favourable atmospheric conditions are an essential requirement for the laser action to be effective.

In particular, the atmospheric environment has to lie close but slightly below the threshold for the spontaneous occurrence of the desired macroscopic event. The

5.5 Outlook: potential applications to laser-induced weather modulation

role of the laser would therefore consist in applying a modulation to the equilib-rium state, in order to push the atmosphere beyond that barrier. Further work is in progress in order to evaluate this potential.

Appendix: High-energy mobile source of THz radiation

The TeraHertz (THz) portion of the electromagnetic spectrum - 1 THz corre-sponds to a wavelength of 300µm - is potentially of great interest both for basic research and for applied science. Our group is currently involved in the generation of high-energy THz radiation in nonlinear crystals, with the purpose of develop-ing a platform for THz nonlinear spectroscopy.

In the context of the general topic this work focuses on, using THz waves might be of interest as a tool to probe the water vapor concentration via an absorption measurement. Fig. 13 displays the experimental data on water vapor absorption, measured by time-domain linear spectroscopy, obtained by Van Exter et al. [29].

They observed single absorption lines at 0.557 THz and 0.752 THz, as well as multiple stronger lines around 1 THz, that well agree with theoretical predictions found in literature.

In our setup, THz is emitted through optical rectification by a Mg-doped LiNbO₃ crystal, pumped by a femtosecond front-tilted pulse at 800 nm. Besides having realized this generation setup in our laboratories, we also embedded it in the Ter-amobile stand-alone container, in order to upscale the output energy by increasing the laser pump to 160 mJ. We reported of a record-breaking generation of THz pulses with peak energy of 50µJ, with a broad spectrum centered at 0.2 THz and extending up to 1 THz [152].

In addition to the experimental activity on the laser-assisted aerosol generation, I also actively participated in the development of the THz platform, both in the laboratory, for table-top experiments, and in the stand-alone configuration. Our published work on the THz source based on the Teramobile laser is attached in the following.

Figure 13: Amplitude absorption coefficient of a THz wave propagating in air at 8% RH and a temperature of 20.5^◦C. Reprint from [29]

Appl Phys B (2010) 101: 11–14 DOI 10.1007/s00340-010-4186-4

Mobile source of high-energy single-cycle terahertz pulses

A.G. Stepanov·S. Henin·Y. Petit·L. Bonacina· J. Kasparian·J.-P. Wolf

Received: 15 July 2010 / Published online: 20 August 2010

© Springer-Verlag 2010

Abstract The Teramobile laser facility was used to real-ize the first mobile source of high-power THz pulses. The source is based on a tilted-pulse-front pumping THz gen-eration scheme optimized for application of terawatt laser pulses. Generation of 50-µJ single-cycle electromagnetic pulses centered at 0.19 THz with a repetition rate of 10 Hz was obtained for incoming 700-fs 120-mJ near-infrared laser pulses. The corresponding laser-to-THz photon con-version efficiency is approximately 100%.

1 Introduction

THz waves have attracted considerable interest in recent years owing to their prospective applications in different scientific and industrial fields [1, 2]. Some of these appli-cations require ultrashort THz pulses of high peak power, such as for nonlinear optics and spectroscopy in the THz frequency range and for recently developed time-resolved spectroscopy with THz pump [3–7]. To date, the high-est THz peak power (100 MW) has been achieved with accelerator-based sources [3]. These sources have a number of obvious disadvantages typical for large-scale facilities.

Several table-top techniques based on femtosecond lasers have been tested for obtaining high-power near-single-cycle THz pulses, including photoconductive switches, optical

A.G. Stepanov

Institute for Spectroscopy RAS, Fizicheskaya Str. 5, Troitsk, Moscow Region 142190, Russia

e-mail:andrei_g_stepanov@yahoo.com

S. Henin·Y. Petit·L. Bonacina·J. Kasparian·J.-P. Wolf (⁾

GAP-Biophotonics, Université de Genève, Rue de l’École de Médecine 20, Genève 1211, Switzerland

rectification and, more recently, four-wave mixing in air/gas plasma [8]. For most of these techniques the generation of THz pulses with an average frequency of ∼1 THz and peak power of more than 1 MW is problematic owing to the low laser-to-THz conversion efficiency and the

rectification and, more recently, four-wave mixing in air/gas plasma [8]. For most of these techniques the generation of THz pulses with an average frequency of ∼1 THz and peak power of more than 1 MW is problematic owing to the low laser-to-THz conversion efficiency and the