The True Value of Disorder

Abstract Border regions between areas where physical problems involve very few bodies with well‐described interactions or immensely many (amenable to statistical description) are prone to fall in the category usually associated with complexity. Disorder is an important factor to consider when examining complexity. Disordered optical materials per se would not qualify as complex but when disorder is partial or merely incipient or nonlinear interactions are included, an entirely new character is added. Here, attention is paid to three examples of these kinds of optical systems. The propagation of light through diffusive media and image formation behind an opaque screen is proposed as a problem where an exact computable solution can be expected but is impractical. Analysis shows that the problem is akin to condensed matter physics concepts related to narrow leads conductance. Another condensed matter related phenomenon brought to photonics is that of localization: disorder can induce a cessation of transport when interference of scattered fields is destructive in all directions forcing light to be exponentially confined. Finally, random lasers are a problem where a disordered, open, photonic system is governed by immensely many modes interacting nonlinearly.