Double-layer Structures for Multifocal Diffractive Optical Elements: Engineering Wavelength- Dependent Efficiency Through Adjusted Material Selection.

Multifocal diffractive optical elements (MFDOEs) as interleaved sawtooth structures with alternating heights are established as single-material layers. While the distribution of diffraction efficiencies across different orders can be selected for a specific wavelength, it is fixed for other wavelengths due to the dispersion of the material. In this work, we investigate multilayer MFDOEs consisting of two layers of sawtooth structures to enable tailored wavelength selectivity, especially the controlled efficiency distribution into specific diffraction orders for selected wavelengths or wavelength ranges. Using scalar diffraction theory, we classify material combinations into four categories based on their dispersion characteristics. This allows for the design of structures that achieve high diffraction efficiencies in either narrow or broadband spectral ranges, directing light into the zeroth, multiple positive, or negative diffraction orders. Using representative material combinations, we evaluate key quantitative measures, including maximum efficiency, wavelength of maximum efficiency, and the full width at half maximum for narrowband efficiency peaks. Broad spectral efficiency maxima are characterized by their average efficiency and root mean square error as a measure of uniformity. These parameters can be adjusted by varying the structure depths, paving the way for wavelength-selective multifocal imaging in microscopy and medical applications.