Nature Mater. 13, 720–725 (2014)

Francesco Riboli and colleagues in Italy and Switzerland have experimentally realized control of the spectral properties of individual photonic modes in a two-dimensional disordered photonic structure. The researchers fabricated 320-nm-thick GaAs waveguides containing three dense layers of InAs quantum dots. Random patterns of holes were etched, and the fill factor was varied between 0.13 and 0.35 and the hole diameter was varied from 180 nm to 250 nm until strongly localized optical modes were obtained. The structures were designed to support localized modes at wavelengths around ∼1,300 nm. The researchers exploited the fact that the resonant frequency of the modes is sensitive to the parameters of the surrounding media, and it can therefore be selectively perturbed by introducing a near-field probe tip. Perturbation can also be induced by local sub-micrometre-scale oxidation. In a sample supporting spatially overlapping modes with different resonant frequencies, local tuning pushed modes into spectral superposition. Frequency crossing and anti-crossing were observed, with the latter indicating mode interaction. The researchers claim optically isolated regions can thus be connected by such modes, which might offer new options for controlling light propagation, such as transmission channels in strongly scattering media.