Large-scale vivid metasurface color printing using advanced 12-in. immersion photolithography.

Nanostructures exhibiting optical resonances (so-called nanoantennas) have strong potential for applications in color printing and filtering with sub-wavelength resolution. While small scale demonstrations of these systems are interesting as a proof-of-concept, their large scale and volume fabrication requires deeper analysis and further development for industrial adoption. Here, we evaluate the color quality produced by large size nanoantenna arrays fabricated on a 12-in. wafer using deep UV immersion photolithography and dry etching processes. The color reproduction and quality are analyzed in context of the CIE color diagram, showing that a vivid and vibrant color palette, almost fully covering the sRGB color space, can be obtained with this mass-manufacturing-ready fabrication process. The obtained results, thus, provide a solid foundation for the potential industrial adoption of this emerging technology and expose the limits and challenges of the process.

A Metalens with a Near-Unity Numerical Aperture.

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high-NA lenses in an ultraflat fashion. However, so far, these have been limited to numerical apertures on the same order of magnitude as traditional optical components, with experimentally reported NA values of <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (∼λ/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in subdiffractive diamond nanocrystals. This work, based on diffractive elements that can efficiently bend light at angles as large as 82°, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated with the standard, phase mapping approach.

Asymmetric Nanoantennas for Ultrahigh Angle Broadband Visible Light Bending.

Wavefront manipulation in metasurfaces typically relies on phase mapping with a finite number of elements. In particular, a discretized linear phase profile may be used to obtain a beam bending functionality. However, discretization limits the applicability of this approach for high angle bending due to the drastic efficiency drop when the phase is mapped by a small number of elements. In this work, we discuss a novel concept for energy redistribution in diffraction gratings and its application in the visible spectrum range, which helps overcome the constraints of ultrahigh angle (above 80°) beam bending. Arranging asymmetric dielectric nanoantennas into diffractive gratings, we show that one can efficiently redistribute the power between the grating orders at will. This is achieved by precise engineering of the scattering pattern of the nanoantennas. The concept is numerically and experimentally demonstrated at visible frequencies using several designs of TiO2 (titanium dioxide) nanoantennas for medium (∼55°) and high (∼80°) angle light bending. Results show efficient broadband visible-light operation (blue and green range) of transmissive devices, reaching efficiencies of ∼90% and 50%, respectively, at the optimized wavelength. The presented design concept is general and can be applied for both transmission and reflection operation at any desired wavelength and polarization.