Self-Focusing of Light

the phenomenon of the concentration of the field of a light wave in a nonlinear medium whose refractive index depends on the field intensity. The refractive index η of the medium can increase with increasing field strength as a result of a nonlinear variation in electron polarization of the substance owing to, for example, the high-frequency Kerr effect, electrostriction, or heating (see NONLINEAR OPTICS). This increase in n causes nonlinear refraction in the medium: rays are deflected in the direction of great field intensity. When the power of the light flux exceeds a certain threshold value, nonlinear refraction suppresses the diffraction broadening of the light beam, and focal points arise in the medium. As the power increases, the number of focuses increases and the focuses move at speeds close to the speed of light. The concentration of the field in the self-focusing of light is much stronger than in ordinary focusing by a lens. Self-focusing of light can lead to electric breakdown or can contribute to the development of such nonlinear processes as the stimulated scattering of light.

Under certain conditions the number of focuses can become so large that the light will propagate in an oscillating dielectric wave guide formed in the nonlinear medium by the light beam itself. For a beam of a certain cross section carrying the critical power, the cross section remains constant. Light energy can be transmitted over long distances by means of such wave guides.

In the case of a moving medium, as in convective flows of liquids and gases, or a scanning beam, the beam is deflected from its initial direction. The angle of deflection depends on beam power and on the lateral velocity of the medium.

The inverse phenomenon—the nonlinear broadening of a light beam—is called defocusing. This effect occurs in media whose refractive index decreases with increasing intensity. Thermal defocusing is most widely encountered. It results from a decrease in the refractive index owing to the expansion of the substance when heated by the light. Self-focusing and defocusing are observed in experiments with laser radiation passing through condensed and gaseous media, including air and plasmas.