We present for the first time in this report is the observation of harmonic emission induced by a single ten-micrometer-scale laser spot in a preformed plasma, instead of the previous observation, in which the laser plasma harmonic emission, induced from a whole laser spot, was accounted as a collected effect. By the employment of mask image formation technique, a single 10-micrometer-radius laser spot was built up in a preformed plasma corona, which resulted in ponderomotive self-focusing. Furthermore, the single spot shrunk with more intense laser irradiance, which brought about the 3 ωj2 and or 2 ω0 harmonic emission. Experimental observation of the spatial and temporal evolution of the harmonic emission is important for the study of the self-focusing dynamics. Meanwhile this research can also provide information on the required small length of laser intensity perturbation, which need to be compensated for laser fusion.

© 1991 Optical Society of America

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