Reversal Asymmetry of Reciprocal Metasurface Enables Ultra‐Compact Varifocal Reflective Lens

Abstract Reversing a ray through a conventional lens will bring it back to its starting point. Although such reversal symmetry is characteristic of reciprocal systems, such symmetry can be broken without violating reciprocity. This is demonstrated in the case when utilizing circular polarization states and chiral structures: Geometric phase metasurface lenses (metalenses) can be shown to act as positive (converging) or negative (diverging) lenses depending on transmission through the front or rear of the lens, respectively, for a given circular polarization state. Ray‐tracing for such a metalens placed in front of a mirror reveals that reversal symmetry is broken: The reversed ray after the mirror does not end at the starting point. In this paper, such an arrangement is used to realize a varifocal reflective doublet that is equivalent to a transmissive doublet with a consecutive positive and negative lens. Such a reflective arrangement is not possible using a reversal symmetric lens, and allows for realizing an ultra‐compact varifocal lens component. A proof‐of‐concept implementation using a 1550 nm NIR metalens is demonstrated, attaining a diopter change on the order of 1052 for a focal length shift of 240 µm, caused by a 53 µm thin‐film piezoelectric micromirror displacement.