Near-perfect absorption by photonic crystals with a broadband and omnidirectional impedance-matching property.

We propose a method to realize near-perfect absorption of light by using lossy photonic crystals with a broadband and omnidirectional impedance matching property. Different from traditional impedance matching methods that require surface decorations, here we show that the effective impedance of bulk photonic crystals can be designed to be omnidirectionally matched with free space in a broad spectrum. By adding some loss into the system, omnidirectional and broadband absorption can be realized with near 100% efficiency. Based on this principle, photonic crystals composed of a large variety of dielectric materials can become near-perfect absorbers at a broad spectrum range. For example, we have demonstrated a multi-layer photonic crystal composed of titanium dioxide and silicon, which exhibits significant absorption efficiency in the optical spectrum of wavelength between 440nm and 640nm. Our work paves a road towards configurable broadband and omnidirectional near-perfect absorption using bulk photonic crystals without surface decoration, which could be useful in many applications including micro-bolometers, solar cells, selective thermal emitters and structure color.