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 µm 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.
Flat lenses with focal length tunability can enable the development of highly integrated imaging systems. This work explores machine learning to inverse design a multifocal multilevel diffractive lens (MMDL) by wavelength multiplexing. The MMDL output is multiplexed in three color channels, red (650 nm), green (550 nm), and blue (450 nm), to achieve varied focal lengths of 4 mm, 20 mm, and 40 mm at these three color channels, respectively. The focal lengths of the MMDL scale significantly with the wavelength in contrast to conventional diffractive lenses. The MMDL consists of concentric rings with equal widths and varied heights. The machine learning method is utilized to optimize the height of each concentric ring to obtain the desired phase distribution so as to achieve varied focal lengths multiplexed by wavelengths. The designed MMDL is fabricated through a direct-write laser lithography system with gray-scale exposure. The demonstrated singlet lens is miniature and polarization insensitive, and thus can potentially be applied in integrated optical imaging systems to achieve zooming functions.
more » « less- PAR ID:
- 10451572
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Applied Optics
- Volume:
- 62
- Issue:
- 26
- ISSN:
- 1559-128X; APOPAI
- Format(s):
- Medium: X Size: Article No. 6931
- Size(s):
- Article No. 6931
- Sponsoring Org:
- National Science Foundation
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