A new double laser recrystallization technique utilizing a temporally modulated continuous wave (CW) Ar+ laser in conjunction with a superposed KrF excimer laser has been shown to achieve lateral grain growth on amorphous silicon (a-Si) thin films. In the present investigation, instead of excimer laser, a more adaptable frequency-doubled Q-switched Nd:YLF laser (wavelength lambda= 524 nm) is utilized. Lateral grains of larger than 20 mum are obtained in 50 nm-thick a-Si films. In order to understand the recrystallization mechanism, high-resolution laser flash photography employing a nitrogen laser-pumped dye laser (lambda = 445 nm, full-width-half-maximum pulse duration FWHM = 2 ns) as illumination source enables in situ direct visualization of the resolidification process for the first time. The images reveal lateral solidification velocity of about 10 m/s. The nuclei created by the Nd:YLF laser and the reduction in cooling rate by the Ar+ laser are important for inducing long lateral grains.