Optical responses from high-entropy alloys: experimental results and perspectives

High-entropy alloys (HEAs) combine five or more elements in near-equiatomic ratios, opening an immense compositional space whose optical behaviour is still largely unknown. Phase-modulated ellipsometry on bulk CrMnFeCoNi (Cantor) shows that its intrinsic optical constants, <i>n</i>, <i>k</i>, <i>ε</i>₁ and <i>ε</i>₂, deviate strongly from the arithmetic means of the constituent elements-by up to a factor of two beyond 1 μm-yet the derived functional responses, reflectance <i>R</i> and absorption coefficient <i>α</i>, are reproduced to within ∼20%. Cantor nanoparticles have been produced by nanosecond electric discharges in liquid nitrogen. Dark-field spectroscopy and Mie calculations reveal a dominant scattering mode near 100 nm that red-shifts and broadens with increasing size; the steady-state photothermal rise calculated from the absorption cross-section <i>σ</i>_abs falls between those of the constituent pure metals. Generalising the averaging rule, we compute proxy values of <i>R</i> and <i>α</i> for 10 994 density-functional-theory-predicted HEAs. Successive optical, thermal and resource filters condense the space to 58 candidates at 355 nm and eight refractory alloys at 1064 nm, illustrating a "sustainable-by-design" route for future HEA photonics.