Focusing in microlenses close to a wavelength in diameter

Light focused from air into a spherical microlens is affected by diffractio n at the lens surface as its diameter approaches the wavelength of light. T hrough an extension of Mie theory, we show that a converging wave that is i ncident upon a Si microlens with a diameter less than approximately 4 lambd a creates a spot as much as 25% smaller than predicted with vector diffract ion theory. Si microlenses only a wavelength in diameter are shown to be vi rtually insensitive to variations in the maximum illumination angle, and ch anges in index of refraction are not found to cause the proportional change s in spot size that would be expected from vector diffraction theory. (C) 2 001 Optical Society of America.