Low-cost, high-volume, manufacturable 0.88 NA multi-wavelength diffractive lens array for optical document security

Area of Science:

• Optics and Photonics
• Materials Science
• Nanotechnology

Background:

• Refractive microlens arrays (MLAs) face limitations in achieving high-numerical-aperture (NA) and multi-wavelength focusing.
• Existing solutions for document security lack the integration of advanced optical features for robust anti-counterfeiting.

Purpose of the Study:

• To design, manufacture, and characterize a high-NA, multi-wavelength diffractive MLA.
• To demonstrate the application of this MLA in color-integral imaging for anti-counterfeiting purposes.
• To highlight the advantages of diffractive MLAs over refractive ones for specific applications.

Main Methods:

• Fabrication of a multilevel diffractive microlens array (MLA) using UV casting on a polymer film.
• Characterization of the MLA's optical performance, including numerical aperture (NA=0.88), multi-wavelength focusing (480 nm, 550 nm, 650 nm), and depth of focus (DoF).
• Integration of the MLA with high-resolution banknote prints for anti-counterfeiting demonstrations.

Main Results:

• Achieved multi-wavelength focusing with a DoF twice the diffraction-limited value.
• Focused light at three design wavelengths into a spot with a full width at half maximum (FWHM) of approximately 310 nm.
• Demonstrated effective color-integral imaging for anti-counterfeiting on banknotes with minimum feature widths of 10-15 µm.

Conclusions:

• The developed diffractive MLA offers superior performance compared to refractive MLAs for high-NA, multi-wavelength applications.
• UV casting enables low-cost, high-volume production of these advanced flat optics.
• The MLA is highly suitable for enhancing security features in banknotes and documents.