Direct four-photon excitation of amplified spontaneous emission in a nonlinear organic chromophore

We report four-photon pumped amplified spontaneous emission in an organic chromophore. The phenomenon is observed when intense infrared laser radiation illuminates a nonlinear chromophore solution. As a result of this illumination, a strong and highly directional radiation in the visible wavelength range is generated in both the forward and the backward directions, with an angular divergence similar to that of the pump beam. (c) 2005 Optical Society of America.