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Two approaches to making polarization insensitive metalenses using the Pancharatnam-Berry phase.

The Pancharatnam-Berry or geometric phase is widely used for making metalenses and metasurfaces. It allows for broadband performance and relatively straightforward design. In particular dielectric rectangular structures are widely used.

One disadvantage of these anisotropic structures is that they require a circularly polarized incident light. Recently some papers have shown that a polarization insensitive metalens can be made even with polarization sensitive structures.

A polarization insenstive metalens by interleaving nano-fins for left and right circular polarized light.

A polarization insenstive metalens by interleaving nano-fins for left and right circular polarized light.

By using only 2 orientations (0 and 90 degrees) of nano-fins a polarization insenstive metalens is made.

By using only 2 orientations (0 and 90 degrees) of nano-fins a polarization insenstive metalens is made.

The picture on the left shows the work of Lin et al. who interleaved two circularly polarizing metalens designs (red and green). One for left and one for right circular polarized light, the result is a polarization insensitive lens. The drawback of this approach is that the focussing efficiency decreases since each polarization state effectively “sees“ only half a lens. Read the full paper here:

Lin, D. et al. Polarization-independent metasurface lens employing the Pancharatnam-Berry phase. Opt. Express 26, 24835 (2018).

The rightpicture shows an alternative approach. Chen and colleagues use only two orientations of nano-fins: 0 and 90 degrees. These two orientations cause respecitvely a 0 and 180 degree phase shift for both left and right circular polarized light. By this choice the resulting metalens is automatically polarization insensitive. This elegant approach effectively means that only a binary hologram can be made to reproduce the desired phase profile. The entire paper is in the link below:

Chen, W. T., Zhu, A. Y., Sisler, J., Bharwani, Z. & Capasso, F. A broadband achromatic polarization-insensitive metalens consisting of anisotropic nanostructures. (2018).

The effect of fabrication-imperfections on metalens performance

2018_imperfection_factor.PNG


Planar dielectric cylindrical lens at 800 nm and the role of fabrication imperfections

The full paper is found here.
 

Metalenses are perfect lenses, at least they could be. In reality all kinds of fabrication imperfections and assembly errors reduce their performance. For real-life applications robust designs are needed so that the fabricated lenses perform within tolerance. Producers have to know which imperfections are critical and which deviations are acceptable.

This interesting paper discusses the effect of changes in a nano-pillars on their performance in a cylindrical lens. The authors examine how the intercept factor changes with random imperfections in the radius and flatness of the nano-pillars.
It made me wonder what the effect of all other kinds of imperfections are such as steepness of the sidewalls, warping of the substrate or roughness of the material.

Optical modelling tools are essential for tolerancing and robustness studies.