Anderson localization is an interference effect pivotal to the understanding of waves in disordered media. As with all interference effects, localization is expected to diminish and disappear when the features of the disordered structure are much smaller than the wavelength.

Here we demonstrate theory and experiment of light being localized by a disorder whose features are deeply subwavelength. We consider multilayer structures where the layer thickness is ~15 nm (λ/40), in experiments and up to λ/1000 in theory (we provide an analytical scaling relation and extensive numerical calculations). But surprisingly, localization does not disappear; rather it remains a dominant effect when waves are incident in the vicinity of a certain critical angle. This localization regime is shown to be physically rich and display unusual properties such as disorder enhanced transmission, and node-less strongly localized modes.

To highlight the sensitivity to deep-subwavelength features exhibited in this regime, we experimentally demonstrate that varying the thickness of a single layer by 2 nm changes the reflection appreciably. This sensitivity, almost down to the atomic scale, is unique in any type of photonic structure and holds the promise of extreme subwavelength sensing.