Order-disorder duality of high entropy alloys extends non-linear optics

Order versus disorder in the structure of materials plays a key role in the theoretical prediction of their properties. However, this structural description appears to be ineffective for new families of materials such as high entropy alloys (HEAs), which combine crystallographic order with chemical disorder. Here, we demonstrate for five-element HEAs as pure solid solutions that the chemical disorder of the elements decorating their cubic structure underlies the generation of second optical harmonics, overcoming the theoretical limit imposed on centrosymmetric crystals. Moreover, we discover that this disorder, inherent to HEAs, sets a threshold for non-linear light emission from the 4th to the 26th order. As a consequence of the 0.5 eV broadening of the energy levels of the five elements of the HEA, the emission spectrum covers broad visible (400-650 nm) and infrared (800-1600 nm) ranges. In addition to the challenge of theoretically predicting non-linear effects in unconventional materials, the duality of structural order and chemical disorder in HEAs offers the opportunity to design sustainable alternatives to urgently needed optical materials.