Optimization by a genetic algorithm of pyramidal structures made of one, two or three stacks of metal/dielectric layers for a quasi-perfect broadband absorption of UV to near-infrared radiations

We use a genetic algorithm to optimize 2-D periodic arrays of truncated square-based pyramids made of successive stacks of metal/dielectric layers. The objective is to achieve a quasi-perfect broadband absorption of normally incident radiations with wavelengths comprised between 420 and 1600 nm. We compare the results one can obtain by considering (i) Ni, Ti, Al or Cr for the metal, and
(ii) poly(methyl methacrylate) (PMMA) or TiO2 for the dielectric. The structures considered consist of only one, two or three stacks of each metal/dielectric combination. The absorption spectrum of these structures is calculated by a Rigorous Coupled Waves Analysis method. A genetic algorithm is then used to determine optimal values for the period of the system, the lateral dimensions of each stack of metal/dielectric and the width of each dielectric. The results show that Ni/PMMA represents the best metal/dielectric combination. With an optimized structure made of only three stacks of Ni/PMMA, it is possible indeed to absorb 99.8% of the considered incident radiations. An integrated absorptance of 99.4% is achieved with three stacks of Ti/PMMA or Cr/PMMA. Aluminium is not to recommend for this application. The solutions obtained with this metal are indeed too sensitive on the geometrical parameters of the system.