Compact Dual-Band Multi-Focal Diffractive Lenses

This paper presents the design, fabrication, and characterization of dual-band multi-focal diffractive microlenses with sub-wavelength thickness and the capability to simultaneously focus visible and near-infrared spectral bands at two different focal positions. This technology utilizes high-index and low-loss sputtered hydrogenated amorphous Si, enabling a sub-wavelength thickness of only 235 nm. Moreover, the proposed flat lens concept is polarization insensitive and can be readily designed to operate across any desired wavelength regime. Imaging under unpolarized broadband illumination with independent focal planes for two targeted spectral bands is experimentally demonstrated, enabling the encoding of the depth information of a sample into different spectral images. In addition, with a small footprint of only 100 mu m and a minimum feature size of 400 nm, the proposed dual-band multi-focal diffractive microlenses can be readily integrated with vertical detector arrays to simultaneously concentrate and spectrally select electromagnetic radiation. This approach provides novel opportunities for spectroscopic and multispectral imaging systems with advanced detector architectures.