Characterization of acceptance angles of small circular apertures.

We characterize the acceptance angles of small circular apertures for light collection by simulations and experimental measurements. By examining the full width half maximum acceptance angle as a function of the aperture size, we show that the acceptance angle of a circular aperture reaches a minimum of 67 deg before rebounding around the transition between single mode and multimode transmission (approximately 400 nm). This behavior can be explained by the change of mode-coupling efficiency during the transition from single mode to multimode propagation regime. This work in understanding of the behavior of light transmission through subwavelength apertures will guide the design of better aperture based imaging devices where apertures are used as light collection units.