Ultrafast-Laser Control over Local Atmospheric Properties

Clouds, fog, snowfalls and rain showers have a great impact on economy and human wellbeing. They affect visibility, disturb traffic and paralyze free-space optical communication. On the other hand, an absence of rain leads to droughts and shortage of drinking water. Nowadays, triggering rain and clearing of the sky is done by disposing a large amount of chemicals from airplanes, as it was suggested almost a century ago. This method has inadequate selectivity, high operational costs, poor efficiency and ambiguous ecological impact. In 2003 a group of scientists from Teramobil project had suggested to use laser filaments for local control over water precipitation. Laser filaments are narrow (0.1-1 mm) and long (up to 1 km) channels of light with extremely high intensity, maintaining its own size and structure due to modification of optical properties of a medium, in which the light propagates. A generation of filaments in air is possible when ultrashort laser pulses (coherent flashes of light with a duration of several tens of femtoseconds (fs)) have gigawatt-level peak powers. One can understand those numbers in a following way: the age of the Universe is 13.8x109 years, what is ~436x1015 seconds. One fs is in 1015 times shorter than a second (s). Hence, 436 s (~7 minutes) are as long to compare to 1 fs, as the lifetime of the Universe to 1 s. One GW in turn is comparable to a power produced by a single nuclear reactor, however the nuclear reactor generates this power continuously, while laser pulse lasts only tens of fs. Nevertheless, because of such a short time interval intensity of light in filaments is so high, that it is sufficient to ionize and excite molecules of air and trigger complex photochemical processes, leading to a formation of hydrophilic molecules. In humid air, those molecules became centers of nucleation and facilitate production of aerosols microscopic precursors of droplets. Moreover, the energy transferred from laser light to molecules via ionization or excitation leads to the local heating of the air. The heated narrow region explosively expands, what creates a shock wave, pushing water droplets out from the laser path, clearing the path for transmission of optical signals through the fog or clouds. In our project, we suggest to use mid-infrared lasers as drivers for filamentation. Mid-infrared wavelengths (>2500 nm) are a few times longer, than the reddest color, which can be percept by human eye (~740 nm), and are very important in atmospheric photochemistry. Beside the major compounds (O 2, N2, Ar, CO2), Earth atmosphere contains various trace gases, some of which are involved in aerosol formation. We propose excitation of the molecules of the trace gases with resonant mid-infrared light, which is a way to stimulate precipitation. In addition, we will optimize laser parameters (wavelength, pulse duration and energy) for efficient path clearing in clouds enabling optical communication between the Earth and satellites.

Clouds, fog, snowfalls and rain showers have a great impact on economy and human wellbeing. They affect visibility, disturb traffic and paralyze free-space optical communication. An absence of rain leads to droughts and a shortage of drinking water. Nowadays, triggering rain and clearing of the sky is done by disposing of chemicals from airplanes, as was suggested almost a century ago. This method has inadequate selectivity, high operational costs, poor efficiency and ambiguous ecological impact. In 2003 a group of scientists suggested to use of laser filaments for local control over water precipitation. Laser filaments are narrow (0.1-1 mm) and long (up to 1 km) channels of light with extremely high intensity, maintaining its size and structure due to modification of optical properties of a medium, in which the light propagates. A generation of filaments in the air is possible when ultrashort laser pulses (coherent flashes of light with a duration of several tens of femtoseconds (fs)) have gigawatt-level peak powers. One can understand those numbers in the following way: the age of the Universe is 13.8x10^9 years, what is ~436x10^15 seconds. One fs is in 10^15 times shorter than a second (s). Hence, 436 s (~7 minutes) are as long to compare to 1 fs, as the lifetime of the Universe to 1 s. One GW in turn is comparable to a power produced by a single nuclear reactor, however the nuclear reactor generates this power continuously, while laser pulse lasts only tens of fs. Because of such a short time interval intensity of light in filaments is so high, that it is sufficient to ionize and excite molecules of air and trigger complex photochemical processes, leading to a formation of hydrophilic molecules, which become centers of nucleation and facilitate production of aerosols - microscopic precursors of droplets. Moreover, the energy transferred from laser light to molecules via ionization or excitation leads to the local heating of the air. The heated narrow region explosively expands, creating a shock wave, pushing water droplets out from the laser path, and clearing the path for transmission of optical signals through the fog or clouds. In our project, we suggest using mid-infrared lasers as drivers for filamentation. Mid-infrared wavelengths are a few times longer than the reddest color, which can be percept by human eye, and is very important in atmospheric photochemistry. Besides the major compounds, Earth's atmosphere contains various trace gases, some of which are involved in aerosol formation. We propose excitation of the molecules of the trace gases with resonant mid-infrared light to stimulate precipitation.