Possibility of creating a laser in the sky has been a hotly debated topic. Rather than a traditional laser with a cavity, what is generally understood is super-radiant emission, where a long and narrow gain volume produces coherent directional stimulated emission. Some filaments can provide the required geometry for an extended gain medium, necessary condition to create lasing in air. Ideal pulse parameters, including wavelength, duration energy and repetition rate will be discussed. Shadowgraphy is presented as a powerful method to analyze the shock wave created by the filament and its guiding-antiguiding properties. The best filaments to create a waveguide are not necessarily the same that will produce the optical excitation of air molecules leading to optical gain. The solution to ‘lasing in air” may reside in creating two color or “nested” filaments. It is shown that such combination can form a stable stationary waveguide.
The gain produced by 800 nm femtosecond filaments is analyzed in different conditions of pulse duration, energy and pressure. At certain wavelengths, lasing in the sky will be bet achieved at the low pressures found in the upper atmosphere. Time resolved high resolution spectroscopy of nitrogen plasma emission excited with 800 nm filament reveals the contribution of rotational wave packets in the nitrogen ion emission. The ultrashort 800nm pulse launches a wavepacket in all the occupied states of neutral and ionized molecule. As the states in B and X of the ion evolve with different phase and respond with opposite polarity to the applied field, the emission alternates between “P” and “R” branch across the accessible rotational manifold. The rovibrational transition can be retrieved back to understand the process by which optical gain is achieved.
Jean-Claude M. Diels and Ladan Arissian, "Conditions for creating super-radiant emission in the sky (Conference Presentation)," Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105181F (Presented at SPIE LASE: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2295749.5751436421001.
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