Planar gradient meta-surface to focus EM waves in reflection geometry Xin Li1, Shiyi Xiao1, Qiong He1, Bengeng Cai2, Tiejun Cui2, Lei Zhou1* 1State Key Laboratory of Surface Physics and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Fudan University, Shanghai, 200433, China 2 State Key Laboratory of Millimeter Waves and Institute of Target Characteristics and Identification, Southeast University, Nanjing 210096, China Backgrounds: III. FDTD & Experimental Verification PROBLEMS ! Metamaterials are used to focus plane waves to point image in transmission geometry1,2,3. Energy loss due to reflection at surface caused by impedance mismatch. Using high impedance surface as basic structure Sandwich structure: metallic H-shape + dielectric slab + metal Magnetic resonance Different Ly in a super cell Hyperbolic φ Motivation: Planar lens in reflection geometry Fig. 1 Various structures to focus PWs in transmission geometry I. Concept – Radiation of A Surface Current Fig. 5 (a) φ dependence on Ly (b) φ distribution in a super cell. Fig. 6 Fabricated sample Electric field at different z planes Fig. 2 Scheme of planar lens focusing Surface current distribution Fig. 4 Electric field at (a) z=f (b) z=2f FIG. 7 (a) Scheme for experiment setup. (b) Electric field of FDTD simulation. (c) Corresponding experiment result. (d) Electric field distribution at focal plane of FDTD simulation (green line) and experiment (purple circle). Field enhancement~3 Half peak width~ : focus II. How to Realize Such A Surface Current? added phase term to compensate propagation phase difference Oblique incidence Applying dyadic Green’s function For a homogeneous system: Polarization current for TE wave Radiated electric field Reflected electric field Numerical calculation ( ) whereφis reflection phase FIG. 8 FDTD simulations for oblique incidence (a) 10° (b) 20° (c) 30° (d) 45° or By LDA, the system must be inhomogeneous with position-dependentφ Conclusions: Criterion for focusing: surface current with hyperbolic φ(x) Design samples and employ both FDTD simulation and experiment to verify our idea Ultra-thin device, works for oblique incidence Different focuses based on different parameters Such a hyperbolic profile ofφ(x) can be realized by carefully designed gradient meta-surface Fig. 3 Numerical calculated radiated electric field [1] L. Verslegers, et al., Nano Lett. 9, 235 (2009) [2] L. Lin, et al., Nano Lett. 10, 1936 (2010) [3] O. Paul, et al., Appl. Phys. Lett. 96, 24110 (2010)