Tal Schwartz

Transport and Anderson localization in disordered two-dimensional photonic lattices

One of the most interesting phenomena in solid-state physics is Anderson localization, which predicts that an electron may become immobile when placed in a disordered lattice. The origin of localization is interference between multiple scatterings of the electron by random defects in the potential, altering the eigenmodes from being extended (Bloch waves) to exponentially localized. As a result, the material is transformed from a conductor to an insulator. Anderson’s work dates back to 1958, yet strong localization has never been observed in atomic crystals, because localization occurs only if the potential (the periodic lattice and the fluctuations superimposed on it) is time-independent. However, in atomic crystals important deviations from the Anderson model always occur, because of thermally excited phonons and electron–electron interactions. Realizing that Anderson localization is a wave phenomenon relying on interference, these concepts were extended to optics. Indeed, both weak and strong localization effects were experimentally demonstrated, traditionally by studying the transmission properties of randomly distributed optical scatterers (typically suspensions or powders of dielectric materials). However, in these studies the potential was fully random, rather than being ‘frozen’ fluctuations on a periodic potential, as the Anderson model assumes. Here we report the experimental observation of Anderson localization in a perturbed periodic potential: the transverse localization of light caused by random fluctuations on a two-dimensional photonic lattice. We demonstrate how ballistic transport becomes diffusive in the presence of disorder, and that crossover to Anderson localization occurs at a higher level of disorder. Finally, we study how nonlinearities affect Anderson localization. As Anderson localization is a universal phenomenon, the ideas presented here could also be implemented in other systems (for example, matter waves), thereby making it feasible to explore experimentally long-sought fundamental concepts, and bringing up a variety of intriguing questions related to the interplay between disorder and nonlinearity.

Modifying Chemical Landscapes by Coupling to Vacuum Fields

is typically achieved by placing the material in an optical cavity, such as that formed by two parallel mirrors, which is tuned to be resonant with a transition to an excited state. Theory, discussed below, shows that even in the absence of light, a residual splitting always exists due to coupling to vacuum (electromagnetic) fields in the cavity. While cavity strong coupling and the associated hybrid states have been extensively studied due to the potential they offer in physics such as room temperature Bose–Einstein condensates and thresholdless lasers, the implication for chemistry remains totally unexplored. This is despite the fact that strong coupling with organic molecules lead to exceptionally large vacuum Rabi splittings (hundreds of meV) due to their large transition dipole moments. The molecules plus the cavity must thus be thought of as a single entity with new energy levels and therefore should have its own distinct chemistry. We demonstrate here that one can indeed influence a chemical reaction by strongly coupling the energy landscape governing the reaction pathway to vacuum fields. In the absence of dissipation, the Rabi splitting energy h WR (Figure 1) between the two new hybrid light–matter states is given, for a two-level system at resonance with a cavity mode, by the product of the electric field amplitude E in the cavity and the transition dipole moment d :

Spatial photonics in nonlinear waveguide arrays

The recent proposal of optical induction for producing nonlinear photonic lattices has revolutionized the study of nonlinear waves in waveguide arrays. In particular, it enabled the first observation of (2+1) dimensional lattice solitons, which were the first 2D solitons observed in any nonlinear periodic system in nature. Since then, progress has been rapid, with many fundamental discoveries made within the past two years. Here, we review our theoretical and experimental contributions to this effort.

Tuning the Work‐Function Via Strong Coupling

The tuning of the molecular material work-function via strong coupling with vacuum electromagnetic fields is demonstrated. Kelvin probe microscopy extracts the surface potential (SP) changes of a photochromic molecular film on plasmonic hole arrays and inside Fabry-Perot cavities. Modulating the optical cavity resonance or the photochromic film effectively tunes the work-function, suggesting a new tool for tailoring material properties.

Polariton dynamics under strong light-molecule coupling.

We present a comprehensive experimental study of the photophysical properties of a molecule-cavity system under strong coupling conditions, using steady-state and femtosecond time-resolved emission and absorption techniques to selectively excite the lower and upper polaritons as well as the reservoir of uncoupled molecules. Our results demonstrate the complex decay routes in such hybrid systems and that, contrary to expectations, the lower polariton is intrinsically long-lived.

Absorption-Induced Transparency†

In the past decade, the field of optics has been stimulated by new concepts such as plasmonics and extraordinary optical transmission, which are paving the way for next-generation photonic components. In this context, hybrid materials that combine the properties of structured metals with semiconductor or molecular materials to create novel functionalities offer much potential. While studying molecule– metal interactions, we have found a new phenomenon whereby molecules can induce transparency in optically thick metal films perforated with subwavelength holes. Nonintuitively, transparent windows are opened up at wavelengths at which the molecules absorb strongly, that is, where one would normally expect no transmission. Here we report a detailed study of this phenomenon showing, among other things, that the molecular material must be within the dipole coupling distance (less than ca. 20 nm) from the metal surface, and that the mechanism involves surface plasmons but is independent of the arrangement of the holes. The phenomenon thus provides new flexibility for tailoring extraordinary optical transmission through subwavelength holes and points to new directions for preparing plasmonic hybrid materials for photonics and energy-conversion applications. Absorption-induced transparency (AIT) is best illustrated by the schematic and spectra in Figure 1. Figure 1a shows the transmission spectrum of a square array of 100 nm-diameter holes milled by focused ion beam (FIB) in a 200 nm-thick Ag film with a period of 250 nm (black curve). Only the transmission peak associated with the (1,0) surface plasmon (SP) resonance on the glass/metal interface of the hole array is visible at 518 nm. When an approximately 30 nm layer of a J-aggregate of a cyanine compound (2,2’-dimethyl-8-phenyl5,6,5’,6’-dibenzothiacarbocyanine chloride) is adsorbed on the hole array, its transmission spectrum shows an intense new transmission with a sharp onset at 685 nm (red curve). Figure 1b shows the absorption spectrum of the cyanine Jaggregate layer (measured as 1 reflection) taken on a

Light-emitting self-assembled peptide nucleic acids exhibit both stacking interactions and Watson–Crick base pairing

The two main branches of bionanotechnology involve the self-assembly of either peptides or DNA. Peptide scaffolds offer chemical versatility, architectural flexibility and structural complexity, but they lack the precise base pairing and molecular recognition available with nucleic acid assemblies. Here, inspired by the ability of aromatic dipeptides to form ordered nanostructures with unique physical properties, we explore the assembly of peptide nucleic acids (PNAs), which are short DNA mimics that have an amide backbone. All 16 combinations of the very short di-PNA building blocks were synthesized and assayed for their ability to self-associate. Only three guanine-containing di-PNAs-CG, GC and GG-could form ordered assemblies, as observed by electron microscopy, and these di-PNAs efficiently assembled into discrete architectures within a few minutes. The X-ray crystal structure of the GC di-PNA showed the occurrence of both stacking interactions and Watson-Crick base pairing. The assemblies were also found to exhibit optical properties including voltage-dependent electroluminescence and wide-range excitation-dependent fluorescence in the visible region.

Thermodynamics of Molecules Strongly Coupled to the Vacuum Field

The thermodynamics of strong coupling between molecules and the vacuum field is analyzed and the Gibbs free energy, the enthalpy, and entropy of the coupling process are determined for the first time. The thermodynamic parameters are a function of the Rabi splitting and the microscopic solvation. The results provide a new framework for understanding light-molecule strong coupling.

Polychromatic partially spatially incoherent solitons in a noninstantaneous Kerr nonlinear medium

We analytically and numerically find families of polychromatic partially spatially incoherent solitons in a noninstantaneous Kerr nonlinear medium and analyze their coherence properties. We find that the polychromatic incoherent solitons exist when higher temporal frequency constituents of the light are less spatially coherent than smaller temporal frequency constituents.

Photorefractive solitons and light-induced resonance control in semiconductor CdZnTe.

We demonstrate the formation of (1+1) - and (2+1) -dimensional solitons in photorefractive CdZnTe:V, exploiting the intensity-resonant behavior of the space-charge field. We control the resonance optically, facilitating a 10-mus soliton formation times with very low optical power.