Collaborators working at labs in Russia, Austria, and the United States have succeeded in pumping more than 200 gigawatts of power into a 0.1-millimeter-wide filament formed in the ambient air by a laser. In a paper published in the 17 February issue of Scientific Reports, they describe how they created laser pulses in the mid-infrared part of the spectrum. By making them 100 femtoseconds (10-13 s) long, they could pack sufficient energy in these pulses to carve out a filament in the air several meters long. The researchers claim to have achieved a world record; but, at the same time, they say that they have hit up against a limit in the amount of power that can be transmitted via the filament because of the presence of CO2 in the atmosphere.
When lasers first appeared during the 1960s, expectations were high that laser guns would eventually replace rifles. However laser guns were soon relegated to the world of science fiction because even needle-thin rays produced by lasers quickly lose their punch because the photons spread out. “If you have a freely propagating laser beam, it will diffract,” says Alexey Zheltikov, a physicist who holds research positions at both Moscow State University and Texas A&M University. “This is a force of nature and there is nothing you can do as long as you are working in linear optics,” says Zheltikov, the scientist who led the research team.
By increasing the intensity of the laser pulses, nonlinear effects become dominant, Zheltikov explains. Because the beam, at high intensities, modifies the refractive index of air, and because the intensity of the beam is higher in the middle than at the edges, it creates a narrow tube like path with a higher refractive index in the middle, bending the light inwards.
But if it weren't for another nonlinear effect, there would be no filamentation.
This other effect occurs at even higher intensities, when the beam ionizes the air in its path. The electron density is the highest at the center of the beam. It is there that the beam lowers the refraction index and forms a negative lens, or plasma lens. The laser pulses focus and defocus simultaneously, maintaining the stability of the filament. “Filamentation is the balance between two nonlinear phenomena, self-focusing and self-defocusing due to the plasma lens,” says Zheltikov.
To achieve self-focusing, the pulses must carry a minimum amount of power; but too much power results in too much ionization, which disrupts the delicate balance struck by the plasma lens. This excess ionization is more likely to occur with pulses of shorter wavelengths, where photons have more energy and more ionizing power. The researchers found that the amount of power that can be transmitted through a filament created with 1-micrometer pulses is much lower than the power that can be transmitted with 4-µm (mid-infrared) pulses. In fact, the power that can be transmitted through a filament is proportional to the square of the wavelength, so 4-µm pulses can transmit 16 times as much power as 1-µm pulses.
Naturally, Zheltikov and his colleagues wanted the most powerful pulses possible. But pulses with wavelengths over 4 µm were quickly ruled out because CO2 in the atmosphere starts blocking the light. Still, at 4 µm, they could go for laser pulses packing more than 200 GW of power—16 times the power that achieved filamentation with a previous experiment by the group with a 1 µm laser.
However, suitable lasers that could produce 200 GW pulses were simply not available. The researchers solved this problem by starting out with a powerful 1-micrometer laser then downconverting the wavelength and amplifying the pulses via several stages of optical parametrical amplification. They ultimately obtained the required 100-femtosecond pulses of over 200 GW of power. “This is how we were able to achieve an unprecedented peak power in the mid infrared,” says Zheltikov. Unfortunately, because light absorption by CO2 rules out higher-power pulses, this the “ultimate experiment in the atmosphere,” Zheltikov adds.
Notwithstanding this limitation, there still is room for interesting applications such as remote sensing. It should be possible to focus the laser pulses in such a way that the filamentation occurs several hundreds of meters up in the atmosphere. There, analysis of the light emitted by the filament would allow specific molecules, such as air pollutants, to be identified.
And there is the "laser in the sky." In experiments with high-pressure gases, the researchers observed lasing of light within the filament, and the return of the amplified light to the laser. Created in the atmosphere, this backward signal would, for example, enhance remote sensing capabilities, or even the creation of artificial “guide stars” used for the adaptive optics of astronomical telescopes. “I have given a talk about this at a meeting with astronomers, and they are interested,” says Zheltikov.