An optical funnel, which performs as a passive electromagnetic compressor, can guide electromagnetic waves from a wide inlet to a narrow outlet without reflectance/scattering and squeeze electromagnetic fields uniformly to an air neck. In this study, an optical funnel is designed by precisely filling subwavelength ceramic blocks with a gradient refractive index inside a tapered waveguide. The gradient refractive index is designed by transformation optics, which is isotropic and all above unit, thus exhibiting a broadband feature. Due to the mechanism of impedance matching over the whole funnel, extremely low reflectance/scattering and stable enhancement of fields can be achieved. The field enhancement factor in different regions of the funnel (e.g., in the air neck) can be flexibly designed just by modifying the funnel-width ratios.

文中利用热蒸发以及退火等工艺制备了支持局域表面等离子体激元（LSP）的微纳结构，来增强硫系玻璃Ge₂₈Sb₁₂Se₆₀ (GSS)薄膜的非线性吸收效应；搭建了Z-scan光路，实现了对样品非线性折射与吸收的测量；通过对样品透射光谱的分析，揭示了GSS非线性吸收增强效应的原理。并研究了该微纳结构对不同厚度GSS非线性吸收的增强规律。文中用到的LSP微纳结构制作简单，无需复杂光刻工艺，可为增强材料光学非线性研究提供重要参考。

A novel way to design arbitrarily shaped retro-reflectors by optics surface transformation is proposed. The entire design process consists of filling an optic-null medium between the input and output surfaces of the retro-reflector, on which the points have 180 deg reverse corresponding relations. The retro-reflector can be designed to be very thin (a planar structure) with high efficiency. The effective working angles of our retro-reflector are very large (from ?80 deg to +80 deg), which can, in principle, be further extended. Layered metal plates and zero refractive index materials are designed to realize the proposed retro-reflector for a TM polarized beam.