Spatial-frequency multiplication via absorbance modulation

The absorbance of a thin film of photochromic material can be reversibly modified by exposure to two different wavelengths, λ1 and λ2. When such a film is illuminated by both wavelengths simultaneously, and the longer wavelength λ2 possesses a node in its intensity distribution, then the absorbance of the layer can be made high except at an arbitrarily small region near the node. By exploiting the large nonlinearity introduced by this mechanism, combined with the reversibility of the absorbance of the photochromic layer, the authors demonstrate that spatial frequencies larger than those present in incident intensity distributions may be generated. They show photoresist exposures to demonstrate this technique. © 2007 American Institute of Physics.