HIGH-SPEED ELECTRON-MICROSCOPY OF LASER-INDUCED VAPORIZATION OF THIN-FILMS

Laser-driven explosive vaporization of free germanium films was investigated with a high-speed transmission electron microscope. Double frame and streak imaging with nanoseconds exposure and sweep times were exploited to visualize the fast processes. Using focused 30-ns laser pulses of about 1-mu-J and 10 MW/cm2, the films were shattered within 100-300 ns by three cooperating mechanisms: Marangoni flow, jerky boiling, and capillary forces. These effects are very different from those ablating supported films. They suggest that large temperature gradients of 1000 K/mu-m centered near 4000 K are generated within the laser spot, and the homogeneous nucleation theory of boiling is not applicable.