Paper 6.9: P. Saeta, J.K. Wang, Y. Siegal, E. Mazur and N. Bloembergen, “Ultrafast electronic disordering during femtosecond laser melting of GaAs,” Phys. Rev. Lett. 67, 1023–1026, 1991.

This paper is mainly the work of my colleague Eric Mazur and his graduate students, who introduced femtosecond pulsed lasers at the Gordon McKay laboratory. At the time of this publication, I had become professor emeritus. In the preceding years, Eric Mazur had gradually taken over all laboratory space of my former nonlinear optics group. He removed all obsolete and obsolescent equipment which my associates had used and started several new avenues of research. The work in this note is, however, a natural continuation of the investigation with picosecond pulses reported in the preceding paper. The potential of femtosecond techniques for solid state investigations was already evident to me a decade earlier. On this timescale, the energy absorbed by the electrons from the laser pulse has not yet been shared with lattice phonons. So melting, in the conventional sense, cannot take place in such a short time interval. It was a surprise to find that the second harmonic signal nevertheless diminishes rapidly on this short timescale. The change in electronic structure, originally proposed by J.A. van Vechten, to occur on nanoand picosecond time scales, finally manifests itself.

These femtosecond investigations have been pursued further by Mazur and coworkers to obtain the temporal behavior of the complex dielectric response function, which gives more detailed information about the collapse of the electronic band structure.