Frequency doubling
Straightforward demonstration of frequency doubling existed in the femtosecond laser-induced air plasma. A piece of white paper is used to intercept the forward transmitting light of the little ionized femtosecond laser pulses in air after focus point. And the central blue speckle due to fluorescence in white paper signifies the existence of frequency doubling of near-infrared femtosecond laser pulses. Olympus C5050Z.
—Zhijun Xu Discipline of Optical Engineering, College of Science, Zhejiang University of Technology