Paper 6.1: E. Yablonovitch and N. Bloembergen, “Avalanche ionization and the limiting diameter of filaments induced by light pulses in transparent media,” Phys. Rev. Lett. 29, 907–910, 1972.

This note demonstrates that the considerations governing electric breakdown in d.c. electric fields can be extended through the microwave region to the far infrared, and perhaps even to visible frequencies. Avalanche ionization must start from a free electron which could be supplied by thermal ionization from a shallow impurity level or by multiphoton excitation. This breakdown mechanism is certainly correct for radiation at 10.6μm wavelength. Very recent studies in alkali halide crystals have shown that heating by multiphoton absorption processes may be more important at visible and shorter wavelengths. Avalanche breakdown studies were reported in detail in E. Yablonovitch's Ph.D. thesis, “Nonlinear Optics with the CO₂ Laser.” He stayed on as an assistant and associate professor at Harvard to conduct original studies on collisionless multiphoton dissociation of molecules. During the eighties, he worked at the industrial research laboratories of Exxon and Bellcore. He carried out research on photon band structures in three-dimensional media with periodic variations in the index of refraction, and on other topics in opto-electronics. He is now professor of electrical engineering at the University of California in Los Angeles. My contacts with Eli are always stimulating.