Creating Long-Lived Optical Waveguides in Air Using Femtosecond Lasers

Through a dynamic balance of nonlinear self-focusing, diffraction, and ionization, ultrafast laser pulses can propagate over long distances in the atmosphere while maintaining a high peak intensity. This phenomenon, known as "filamentation," is being investigated for a number of applications, including few-cycle pulse generation, high harmonic generation, laser-triggered electrical discharges, and remote sensing. I will discuss new applications based on the nonlinear deposition of energy in the air by ultrashort pulses. By harnessing the acoustic and thermal response of the gas, we have created optical waveguides with lifetime exceeding a millisecond. We have used these waveguides to guide high average power laser beams and to collect and guide fluorescence, potentially useful for many remote sensing schemes. I will describe the physics of filamentation and long timescale gas hydrodynamics, including the important role of molecular rotational states, both in the propagation and in the energy deposition. Using coherent control of these rotational states, we have shown that the absorption of a train of short pulses can be greatly enhanced.