Spatial characterization of sideways lasing

Strong excitation with high electromagnetic fields can generate lasing in gaseous media by resonant excitation. Previous efforts have shown that the lasing signal can be emitted perpendicular to the direction of the pump laser if the shape of the gain volume can be controlled. This paper reports on the characterization of sideways emitted lasing obtained through two-photon pumping in oxygen atoms present in a flame. Intense 35-fs laser pulses of 226-nm wavelength focused into a sheet by a cylindrical mirror are used to form a gain volume geometry that yields an 845-nm lasing beam that is emitted perpendicular to the pump-beam direction. The sideways lasing has been investigated in terms of its pump laser energy dependence, excitation and emission spectra, and spatial characteristics. Single-shot images of the emission exhibit interference fringes, revealing that the emission contains multiple coherent spatial modes. The results indicate that superfluorescence constitutes a significant contribution to the sideways lasing. We believe that this is the first important step for understanding and using sideways lasing as a phenomenon and tool for future applications and inventions.