  We design, fabricate, and characterize multifunctional and compact
diffractive microlenses with sub-wavelength thickness and the capability to
simultaneously focus visible and near-infrared spectral bands at two different
focal positions with 24% and 15% measured focusing efficiency, respectively.
Our technology utilizes high-index and low-loss sputtered hydrogenated
amorphous Si, enabling a sub-wavelength thickness of only 235nm. Moreover, the
proposed flat lens concept is polarization insensitive and can be readily
designed to operate across any desired wavelength regime. Imaging under
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 400nm, the
proposed multifunctional and compact diffractive microlenses can be readily
integrated with vertical detector arrays to simultaneously concentrate and
spectrally select electromagnetic radiation. This provides novel opportunities
for spectroscopic and multispectral imaging systems with advanced detector
architectures.
