High-efficiency chiral meta-lens

Abstract

We present here a compact metasurface lens element that enables simultaneous and spatially separated imaging of light of opposite circular polarization states. The design overcomes a limitation of previous chiral lenses reliant on the traditional geometric phase approach by allowing for independent focusing of both circular polarizations without a 50% efficiency trade-off. We demonstrate circular polarization-dependent imaging at visible wavelengths with polarization contrast greater than 20dB and efficiencies as high as 70%.

Figure 1

Chiral meta-lens principle. (a) Illuminated with LCP or RCP light, the meta-lens focuses an RCP - LCP image on two different positions. (b) Analogy demonstrating fundamental design differences with respect to previous chiral-lenses. A geometric phase lens designed for RCP has an equal and opposite phase profile for LCP. It focuses RCP to the right and defocus LCP to the left, here in analogy represented with a refractive lens and a wedge. The combined approach (propagation & geometric phase) allows focusing of LCP and RCP independently from each other.

Figure 2

Device and measurement setup. (a) Top-side view of the SEM micrograph picture at the edge of the metasurface lens. Scale bar: 1 (b) Top-view SEM micrograph picture close to the center of the chiral meta-lens. Scale bar: 1 (c) The 1951 USAF resolution test chart illuminated with a supercontinuum laser. The polarization is controlled with a linear polarizer LP and a quarter-wave plate QWP. The sample focuses the LCP and RCP image of the resolution test chart on detector.

Figure 3

Device Performance. (a) RCP and LCP images taken of the 1951 USAF resolution test chart with the chiral meta-lens for RCP and LCP illumination at 500 nm. Scale bar: 180. In group 4 the bars have a spacing and width ranging from 31.25 to 17.54, in group 5 from 15.63 to 8.77. (b) Focusing efficiency for RCP (I_RCP), LCP (I_LCP) and transmission efficiency (I_trans) at different quarter wave plate (QWP) angles for 500 nm illumination. The transmission efficiency through the chiral meta-lens is about 80% from which LCP and RCP have a combined focusing efficiency of 70%. Most of the remaining light couples to the 0th order while higher orders can be neglected. The data for LCP and RCP focusing efficiency is fitted with: $I_{\mathrm{RCP}}=0.3377\sin (-0.0359\theta -\mathrm{0.0065)}+0.3509$ and $I_{\mathrm{LCP}}=0.3451\sin (-0.0334\theta +\mathrm{3.1328)}+0.3463$, here θ is the QWP angle in degrees.

Figure 4

Broadband properties. (a) Focusing efficiency for LCP - RCP and 0th order coupling loss for LCP illumination at different wavelengths. The LCP focusing efficiency is peaked at 500 nm. The intensity in the 0th order increases gradually from to 0% to almost 80% in the wavelength range 470 nm to 650 nm. (b) The polarization contrast for RCP - LCP from 470 nm to 650 nm.