Light trapping in photonic crystals

We consider light trapping in photonic crystals in the weak material absorption limit. Using a rigorous electromagnetic approach, we show that the upper bound on the angle-integrated absorption enhancement by light trapping is proportional to the photonic density of states. The tight bound can be reached if all the states supported by the structure are coupled to external radiation. Numerical simulations are used to illustrate the theory and the design of both two- and three-dimensional photonic crystals for the purpose of light trapping. Using the van Hove singularities, the angle-integrated absorption enhancement in two-dimensional photonic crystals could surpass the conventional limit over substantial bandwidths.