David  Shwa, Eisenmann, Shmuel , Marcus, Gilad , and Zigler, Arie . 2009. ?Using The Self-Filtering Property Of A Femtosecond Filament To Improve Second Harmonic Generation?. Optics Express, 17, Pp. 6451 - 6456. http://www.opticsexpress.org/abstract.cfm?URI=oe-17-8-6451.