The positive refractive index of conventional optical lenses means that they need curved surfaces to form an image, whereas a negative index of refraction allows a flat slab of a material to behave as a lens and focus electromagnetic waves to produce a real image¹. Here we demonstrate this unique feature of imaging by a flat lens, using the phenomenon of negative refraction in a photonic crystalline material. The key advance that enabled us to make this observation lies in the design of a photonic crystal^{2, 3} with suitable dispersion characteristics to achieve negative refraction over a wide range of angles.

1. Department of Physics and Electronic Materials Research Institute, Northeastern University, Boston, Massachusetts 02115, USA

Correspondence to: Srinivas Sridhar¹ Email: s.sridhar@neu.edu