Optimal Concentration of Electromagnetic Radiation

For complete spherical concentration of light the maximum theoretically possible total energy density for a given power input can be, in principle, achieved by appropriate choice of polarization and angular amplitude variation. Illumination of a focusing system with a plane-polarized wave creates at the focus equal electric and magnetic energy densities. By appropriate choice of radial variation this energy density can be maximized. For hemispherical concentration the electric energy density can be seven sixteenths of the maximum possible for a given power input, and the total energy density can be seven eighths of the maximum possible. Focusing by optical systems satisfying the sine condition amongst others is also considered. For a system satisfying the sine condition, the total energy density can be 64/75 of the maximum possible.