Aperiodic stacks of dielectric low- and high-index films can be designed to enhance third-harmonic generation (THG) in reflection of near infrared laser pulses using computer optimization. Numerical and analytical results suggest that the TH energy increases rapidly with increasing number of films and the ration of the high and low index. 

Our optical matrix based THG model that takes into account the full pulse bandwidth predicts conversion efficiencies of about 7 % for transform-limited Gaussian pulse bandwidths of 16 nm for mirrors with 45 layers, which exceed those expected from periodic designs. Stability against film thickness fluctuations expected from the deposition process and information about laser-induced damage are components of the design process.  

Frequency tripling mirrors manufactured from hafnia and silica by our collaboration partners in the Laser Center in Hannover according to our designs showed maximum conversion efficiencies of about 2% for 60 fs pulses at 815 nm. 

The main obstacle to achieving higher conversion efficiencies in practice are undesired material modifications in the currently used dielectric materials, observed during a single pulse and pulse trains in the high-index layer.