Optical funnel:broadband and uniform compression of electromagnetic fields to an air neck

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.