Diffractive optical elements for multi-dimensional subdiffraction-limit spot generation: design, demonstration, and characterization

Abstract

We present diffractive optical elements (DOEs) for generating wavelength-multiplexing and three-dimensional (multi-dimensional) arrays of spots that are smaller than the single diffraction limit spot. Such small spots are referred to as subdiffraction-limit spots. This method is based on the use of propagating light, and its superior features, including spatial parallelism and remote accessibility to matter, can be utilized in applications. In design, special constraints are introduced into the Gerchberg-Saxton algorithm to control the phases of spots and to disperse light energy to a surrounding area. In each iteration, the phase distribution of a wavelength- multiplexing DOE is synthesized from the phase distributions obtained tentatively for the individual wavelengths. As an example, a two-wavelength two-plane DOE was investigated. The design result showed that expected spot arrays were generated for two wavelengths on the individual output planes. We constructed an experimental system with a spatial light modulator and demonstrated that the generated spot arrays were in conformity with the design result. Spot sizes were reduced to about 80% of that of the diffraction limit spot. Characteristics of this type of DOEs were also evaluated by numerical calculation. For example, the result shows that the depth of focus is extended in subdiffraction-limit spot generation. We believe that this study contributes to expand application fields of DOEs.