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Disordered dielectric materials with structural correlations show unconventional optical behavior: They can be transparent to long-wavelength radiation, while at the same time have isotropic band gaps in another frequency range. This phenomenon raises fundamental questions concerning photon transport through disordered media. While optical transparency in these materials is robust against recurrent multiple scattering, little is known about other transport regimes like diffusive multiple scattering or Anderson localization. Here, we investigate band gaps, and we report Anderson localization in 2D disordered dielectric structures using numerical simulations of the density of states and optical transport statistics. The disordered structures are designed with different levels of positional correlation encoded by the degree of stealthiness χ. To establish a unified view, we propose a correlation-frequency (χ–ν) transport phase diagram. Our results show that, depending only on χ, a dielectric material can transition from localization behavior to a band gap crossing an intermediate regime dominated by tunneling between weakly coupled states.

During the last 30 years, the search for Anderson localization of light in three-dimensional (3D) disordered samples yielded a number of experimental observations that were first considered successful, then disputed by opponents, and later refuted by their authors. This includes recent results for light in TiO2 powders that Sperling et al now show to be due to fluorescence and not to Anderson localization (2016 New J. Phys. 18 013039). The difficulty of observing Anderson localization of light in 3D may be due to a number of factors: insufficient optical contrast between the components of the disordered material, near-field effects, etc. The way to overcome these difficulties may consist in using partially ordered materials, complex structured scatterers, or clouds of cold atoms in magnetic fields.

When waves scatter multiple times in 3D random media, a disorder driven phase transition from diffusion to localization may occur (Anderson 1958 Phys. Rev. 109 1492-505; Abrahams et al 1979 Phys. Rev. Lett. 42 673-6). In 'The question of classical localization: a theory of white paint?' Anderson suggested the possibility to observe light localization in TiO2 samples (Anderson 1985 Phil. Mag. B 52 505-9). We recently claimed the observation of localization effects measuring photon time of flight (ToF) distributions (Störzer et al 2006 Phys. Rev. Lett. 96 063904) and evaluating transmission profiles (TPs) (Sperling et al 2013 Nat. Photonics 7 48-52) in such TiO2 samples. Here we present a careful study of the long time tail of ToF distributions and the long time behavior of the TP width for very thin samples and different turbidities that questions the localization interpretation. We further show new data that allow an alternative consistent explanation of these previous data by a fluorescence process. An adapted diffusion model including an appropriate exponential fluorescence decay accounts for the shape of the ToF distributions and the TP width. These observations question whether the strong localization regime can be reached with visible light scattering in polydisperse TiO2 samples, since the disorder parameter can hardly be increased any further in such a 'white paint' material.

Lead halide perovskite light-emitting diodes (PeLEDs) have demonstrated remarkable optoelectronic performance 1 – 3 . However, there are potential toxicity issues with lead 4 , 5 and removing lead from the best-performing PeLEDs—without compromising their high external quantum efficiencies—remains a challenge. Here we report a tautomeric-mixture-coordination-induced electron localization strategy to stabilize the lead-free tin perovskite TEA 2 SnI 4 (TEAI is 2-thiopheneethylammonium iodide) by incorporating cyanuric acid. We demonstrate that a crucial function of the coordination is to amplify the electronic effects, even for those Sn atoms that aren’t strongly bonded with cyanuric acid owing to the formation of hydrogen-bonded tautomeric dimer and trimer superstructures on the perovskite surface. This electron localization weakens adverse effects from Anderson localization and improves ordering in the crystal structure of TEA 2 SnI 4 . These factors result in a two-orders-of-magnitude reduction in the non-radiative recombination capture coefficient and an approximately twofold enhancement in the exciton binding energy. Our lead-free PeLED has an external quantum efficiency of up to 20.29%, representing a performance comparable to that of state-of-the-art lead-containing PeLEDs 6 – 12 . We anticipate that these findings will provide insights into the stabilization of Sn(II) perovskites and further the development of lead-free perovskite applications. Lead-free perovskite light-emitting diodes (LEDs) prepared using tautomeric mixture coordination provide improved ordering in the crystal structure, reduced recombination and enhanced exciton binding energy compared with lead-containing perovskite-based LEDs.

Dynamic localization (DL) of photons, i.e., the light-motion cancellation effect arising from lattice’s quasi-energy band collapse under a synthetic ac-electric-field, provides a powerful and alternative mechanism to Anderson localization for coherent light confinement. So far only low-order DLs, corresponding to weak ac-fields, have been demonstrated using curved-waveguide lattices where the waveguide’s bending curvature plays the role of ac-field as required in original Dunlap-Kenkre model of DL. However, the inevitable bending losses pose a severe limitation for the observation of high-order DL. Here, we break the weak-field limitation by transferring lattice concepts from spatial to synthetic time dimensions using fiber-loop circuits and observe up to fifth-order DL. We find that high-order DLs possess superior localization and robustness against random noise over lower-order ones. As an exciting application, by judiciously combining low- and high-order DLs, we demonstrate a temporal cloaking scheme with flexible tunability both for cloak’s window size and opening time. Our work pushes DL towards high-order regimes using synthetic-lattice schemes, which may find potential applications in robust signal transmission, protection, processing, and cloaking. Dynamic localization is a method of confining light. Here the authors demonstrate higher-order dynamic localization of photons in a synthetic temporal mesh lattice and discuss the idea of tunable temporal cloaking by combining different-order localizations.

Transmission eigenchannels are building blocks of coherent wave transport in diffusive media, and selective excitation of individual eigenchannels can lead to diverse transport behaviour. An essential yet poorly understood property is the transverse spatial profile of each eigenchannel, which is relevant for the associated energy density and critical for coupling light into and out of it. Here, we discover that the transmission eigenchannels of a disordered slab possess exponentially localized incident and outgoing profiles, even in the diffusive regime far from Anderson localization. Such transverse localization arises from a combination of reciprocity, local coupling of spatial modes and non-local correlations of scattered waves. Experimentally, we observe signatures of such localization even with finite illumination area. The transverse localization of high-transmission channels enhances optical energy densities inside turbid media, which will be important for light–matter interactions and imaging applications.Transverse localization of transmission eigenchannels is discovered in random optical media even in the diffusive regime of propagation far from Anderson localization. These findings will have significant impact on imaging and the control of light–matter interactions in scattering systems.

Light in biological media is known as freely diffusing because interference is negligible. Here, we show Anderson light localization in quasi-two-dimensional protein nanostructures produced by silkworms ( Bombyx mori ). For transmission channels in native silk, the light flux is governed by a few localized modes. Relative spatial fluctuations in transmission quantities are proximal to the Anderson regime. The sizes of passive cavities (smaller than a single fibre) and the statistics of modes (decomposed from excitation at the gain–loss equilibrium) differentiate silk from other diffusive structures sharing microscopic morphological similarity. Because the strong reflectivity from Anderson localization is combined with the high emissivity of the biomolecules in infra-red radiation, silk radiates heat more than it absorbs for passive cooling. This collective evidence explains how a silkworm designs a nanoarchitectured optical window of resonant tunnelling in the physically closed structures, while suppressing most of transmission in the visible spectrum and emitting thermal radiation. Light in biological media is known as freely diffusing because interference is negligible. Here, the authors demonstrate Anderson localization of light from quasi-two-dimensional nanostructures in silk fibres.

In this letter, we investigate localized states within a fractional optical system featuring self-defocusing Kerr nonlinearity influenced by a disordered lattice. Modulating the disordered lattice’s amplitude reveals that higher values yield narrower, Gaussian-like profiles, while lower values produce broader profiles with lattice-modulated tails. Also, the fractional dispersion substantially influences localization. Variations in the Lévy index induce optical field shifts and fragmented solutions, while the self-defocusing parameter prompts abrupt width transitions. Anderson localization is observed in cases of weak self-repulsion within low-amplitude lattices. During the expansion of a Gaussian optical field under lattice influence, oscillation intensity correlates with the self-defocusing parameter and Lévy index, inversely with lattice amplitude. These findings enhance our understanding of complex wave propagation in fractional systems.

Photo-induced dynamics of electronic processes are driven by the coupling between electronic and nuclear degrees of freedom. Here, we construct one- and two-dimensional organic-inorganic tin halides to investigate how dimensionality controls exciton-phonon coupling and exciton self-trapping. The results show that a one-dimensional system has strong exciton-phonon coupling leading to excitation-independent self-trapped exciton emission, whereas a two-dimensional system exhibits over ten times weaker coupling resulting in free exciton emission. The difference originates from enhanced Anderson localization in a one-dimensional system. Femtosecond transient absorption experiments directly resolve room-temperature vibrational wavepackets in a one-dimensional system, some of which propagate along the self-trapped-exciton potential energy surface. A combination of wagging and asymmetric stretching motions (~106 cm -1 ) in tin iodide is identified as such a mode, inducing exciton self-trapping. While no room-temperature wavepackets are observed in a two-dimensional system. These findings uncover the interplay between dimensionality-dependent exciton-phonon coupling and electronic/nuclear dynamics, offering constructive guidance to develop multifunctional organic-inorganic metal halides. The study shows that tuning organic–inorganic tin halides from 2D to 1D significantly enhances coupling between excitons and lattice vibrations, thereby switching emission from free excitons to self-trapped states. Ultrafast spectroscopy identifies the key electronic and vibrational dynamics.

Strong Anderson Localization manifests itself as an interference wave phenomenon, potentially leading to completely localized states under infinite extension. We propose a framework for characterizing light transmission through two-dimensional high-refractive disordered materials, showcasing both localization and photonic band gaps (PBG). Leveraging advanced numerical techniques and recent advancements in Finite-Difference Time Domain (FDTD) simulations, we explore how light behaves in complex dielectric materials and how these effects interact near the bandgap. Based on FDTD numerical simulations, we studied the transverse spreading of a focused beam through various realizations of 2D Stealthy Hyperuniform (SHU) patterns silicon rods. For frequencies close to the bandgap, the transverse confinement of the transmitted intensity indicates the presence of Anderson localization, while far from the gap we observe rapid spreading of the intensity toward to the edges of the slabs suggesting diffusive behavior. We identified all transport regimes: bandgap, pseudotunneling, Anderson localization, and diffusion, and we present our findings in transport phase diagrams.