Sparks produced by focusing the beam from a single-mode ruby laser have been investigated, and photographs of radiation scattered at 90° to the incident beam show that breakdown occurs in filaments or points having a diameter of 5 μ or less. Intense pulses of coherent radiation scattered in the forward direction have also been observed, and the measured divergence of this light indicates the presence of self-focused regions having a diameter of ∼1.7 μ. These observations support the hypothesis that self-focusing of the beam may initiate laser-induced breakdown.

• Received 4 October 1968

DOI:https://doi.org/10.1103/PhysRevLett.21.1433