Controlling Anderson localization in disordered photonic crystal waveguides

Quantum optics and quantum information technologies require enhancement of light-matter interaction by, for example, confining light in a small volume. A very recently demonstrated route towards light confinement makes use of multiple scattering of light and wave interference in disordered photonic structures [1,2]. Originally proposed for electrons by P. W. Anderson [3], only completely random systems without any long-range correlation between the scattering sites have been used so far, meaning that the Anderson-localized modes cannot be controlled. In disordered photonic crystals, these modes are predicted to appear at frequencies in or near a band gap [4] providing a possible way to control Anderson-localized modes. We have tested this hypothesis by measuring the light localization length, ξ, in a disordered photonic crystal waveguide (PCW) as a function of the dispersive slowdown factor of light denoted by ng. By coupling light into a PCW with a tapered fiber (Fig. 1a), we have measured the ensemble-averaged exponential decay of the light distribution in the range 885 nm <λ <930 nm, λ being the wavelength of light. The inset of Fig. 1b shows two different exponential fits to the intensity decay at two different wavelengths in the fast- (black) and slow-light (red) regimes, respectively. From these fits we extract a strongly dispersive localization length (Fig. 1b). We attribute this effect to the dispersion in the electromagnetic density of states of the waveguide mode which determines ng of the waveguide. Our measurements demonstrate for the first time the close relation between light localization and density of states [5], which can be used ultimately for controlling the extension and spectral position of Anderson-localized modes.