Abstract

The minimum spot size of a focused homogeneous beam is determined by wavelength and numerical aperture, scanning an object with a spot yield transfer of spatial frequencies up to a definite cutoff frequency which is determined by the same parameters. The frequency range may be enlarged, however, by simply placing a screen with a small hole in the center of the focused spot. In general any local optical disturbance in the spot gives rise to resolution above the conventional cutoff frequency. A disturbance of the spot can be realized with thin layers of materials which behave nonlinearly with respect to transmission or reflection; i.e., the transmissivity depends on the incident power density. For example, in the case of a saturable absorber the top of the spot makes a hole in the layer and the long, tailing slopes disappear; the diffraction-limited spot is sharpened. The nonlinearity is often due to a highly temperature dependent (complex) index of refraction. Experiments with optical disks coated with proper layers, yielded signals at more than twice the conventional cutoff frequency, albeit at a low level.

© 1989 Optical Society of America