Limit to Concentration by Focusing

Abstract

Abstract If light of power W and wavenumber approximately k is concentrated by a passive device (such as a lens or mirror), the stationary energy density Uv attainable at any point O is shown to satisfy Uv r Wk2/3 ~ c provided (a) O is many wavelengths from the nearest object and (b) after the light has first passed through the neighbourhood of O, any which is subsequently reflected or scattered back to O can be neglected. The right-hand side of the inequality is subject to a fractional error O(ik/k), where ik is the range of wavenumbers present.