Ad Lagendijk

Localization of light in a disordered medium

Among the unusual transport properties predicted for disordered materials is the Anderson localization phenomenon. This is a disorder-induced phase transition in the electron-transport behaviour from the classical diffusion regime, in which the well-known Ohms law holds, to a localized state in which the material behaves as an insulator. The effect finds its origin in the interference of electrons that have undergone multiple scattering by defects in the solid. A similar phenomenon is anticipated for multiple scattering of electromagnetic waves, but with one important simplification: unlike electrons, photons do not interact with one another. This makes transport of photons in disordered materials an ideal model system in which to study Anderson localization. Here we report direct experimental evidence for Anderson localization of light in optical experiments performed on very strongly scattering semiconductor powders.

Design of Light Scattering in Nanowire Materials for Photovoltaic Applications

We experimentally investigate the optical properties of layers of InP, Si, and GaP nanowires, relevant for applications in solar cells. The nanowires are strongly photonic, resulting in a significant coupling mismatch with incident light due to multiple scattering. We identify a design principle for the effective suppression of reflective losses, based on the ratio of the nondiffusive absorption and diffusive scattering lengths. Using this principle, we demonstrate successful suppression of the hemispherical diffuse reflectance of InP nanowires to below that of the corresponding transparent effective medium. The design of light scattering in nanowire materials is of large importance for optimization of the external efficiency of nanowire-based photovoltaic devices.

Fifty years of Anderson localization

What began as a prediction about electron diffusion has spawned a rich variety of theories and experiments on the nature of the metal–insulator transition and the behavior of waves—from electromagnetic to seismic—in complex materials.

Resonant multiple scattering of light

Abstract This educational work presents a new approach towards resonant interaction between classical light and matter. The interaction between light and matter is considered from three different points of view: the light picture (where the material degrees of freedom have been integrated out, and leaving one with scattering theory), the matter picture (where the radiative degrees of freedom have been eliminated and providing one essentially with atomic physics). In addition the polariton approach is discussed, in which the degrees of freedom of light and matter are treated on the same footing. Although the first approach will by far be given most of the attention, we will frequently emphasize the equivalence of the three methods. Much of the presented material is selfcontained. We demonstrate that in the dynamical properties of multiple scattering of light the “matter” properties play a dominant role. Several “paradigms of atomic physics” will be discussed from the view point of light scattering theory. We shall introduce the far-reaching analogy between the dielectric “Mie” sphere in classical optics, and the two-level atom in semi-classical atomic physics. This mapping turns out to be much more faithful than the widely used analogy between scattering theory for De Broglie waves and classical waves. In scattering theory the semi-classical two-level atom is equivalent to a point scatterer.

Control of Light Transmission through Opaque Scattering Media in Space and Time

We report the first experimental demonstration of combined spatial and temporal control of light transmission through opaque media. This control is achieved by solely manipulating spatial degrees of freedom of the incident wave front. As an application, we demonstrate that the present approach is capable of forming bandwidth-limited ultrashort pulses from the otherwise randomly transmitted light with a controllable interaction time of the pulses with the medium. Our approach provides a new tool for fundamental studies of light propagation in complex media and has the potential for applications for coherent control, sensing and imaging in nano- and biophotonics.

Demixing light paths inside disordered metamaterials

We experimentally demonstrate the first method to focus light inside disordered photonic metamaterials. In such materials, scattering prevents light from forming a geometric focus. Instead of geometric optics, we used multi-path interference to make the scattering process itself concentrate light on a fluorescent nanoscale probe at the target position. Our method uses the fact that the disorder in a solid material is fixed in time. Therefore, even disordered light scattering is deterministic. Measurements of the probes fluorescence provided the information needed to construct a specific linear combination of hundreds of incident waves, which interfere constructively at the probe.

Spatial Extent of Random Laser Modes

We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.

Optical extinction due to intrinsic structural variations of photonic crystals

Unavoidable variations in size and position of the building blocks of photonic crystals cause light scattering and extinction of coherent beams. We present a model for both two- and three-dimensional photonic crystals that relates the extinction length to the magnitude of the variations. The predicted lengths agree well with our experiments on high-quality opals and inverse opals, and with literature data analyzed by us. As a result, control over photons is limited to distances up to 50 lattice parameters 15 m in state-of-the-art structures, thereby impeding applications that require large photonic crystals, such as proposed optical integrated circuits. Conversely, scattering in photonic crystals may lead to different physics such as Anderson localization and nonclassical diffusion.

Large Photonic Strength of Highly Tunable Resonant Nanowire Materials

We demonstrate that highly tunable nanowire arrays with optimized diameters, volume fractions, and alignment form one of the strongest optical scattering materials to date. Using a new broad-band technique, we explore the scattering strength of the nanowires by varying systematically their diameter and alignment on the substrate. We identify strong Mie-type internal resonances of the nanowires which can be tuned over the entire visible spectrum. The tunability of nanowire materials opens up exciting new prospects for fundamental and applied research ranging from random lasers to solar cells, exploiting the extreme scattering strength, internal resonances, and preferential alignment of the nanowires. Although we have focused our investigation on gallium phosphide nanowires, the results can be universally applied to other types of group III-V, II-VI, or IV nanowires.

INFLUENCE OF INTERNAL REFLECTION ON DIFFUSIVE TRANSPORT IN STRONGLY SCATTERING MEDIA

We treat the effect of internal reflection of the propagation of waves in strongly scattering media. The theory will be applied to the recently discovered phenomenon of coherent backscattering of light and to the time-dependent transport of light intensity in reflection and in transmission geometries. For backscattering and for transmission through relatively thin slabs the influence of internal reflection can be very strong. For transmission through relatively thick slabs the effect can be accounted for by renormalizing the diffusion coefficient with a length-scale dependent reduction factor.