Efficient, designable, and broad-bandwidth optical extinction via aspect-ratio-tailored silver nanodisks

Subwavelength resonators, ranging from single atoms to metallic nanoparticles, typically exhibit a narrow-bandwidth response to optical excitations. We computationally design and experimentally synthesize tailored distributions of silver nanodisks to extinguish light over broad and varied frequency windows. We show that metallic nanodisks are two-to-ten-times more efficient in absorbing and scattering light than common structures, and can approach fundamental limits to broadband scattering for subwavelength particles. We measure broadband extinction per volume that closely approaches theoretical predictions over three representative visible-range wavelength windows, confirming the high efficiency of nanodisks and demonstrating the collective power of computational design and experimental precision for developing new photonics technologies

Fundamental limits to optical response in absorptive systems

At visible and infrared frequencies, metals show tantalizing promise for strong subwavelength resonances, but material loss typically dampens the response. We derive fundamental limits to the optical response of absorptive systems, bounding the largest enhancements possible given intrinsic material losses. Through basic conservation-of-energy principles, we derive geometry-independent limits to per-volume absorption and scattering rates, and to local-density-of-states enhancements that represent the power radiated or expended by a dipole near a material body. We provide examples of structures that approach our absorption and scattering limits at any frequency, by contrast, we find that common "antenna" structures fall far short of our radiative LDOS bounds, suggesting the possibility for significant further improvement. Underlying the limits is a simple metric, $|\chi|^2 / \operatorname{Im} \chi$ for a material with susceptibility $\chi$, that enables broad technological evaluation of lossy materials across optical frequencies.Comment: 21 pages and 6 figures (excluding appendices, references

Coherent Plasmon-Exciton Coupling in Silver Platelet-J-aggregate Nanocomposites

Hybrid nanostructures that couple plasmon and exciton resonances generate hybridized energy states, called plexcitons, which may result in unusual light-matter interactions. We report the formation of a transparency dip in the visible spectra of colloidal suspensions containing silver nanoplatelets and a cyanine dye, 1,1′-diethyl-2,2′-cyanine iodide (PIC). PIC was electrostatically adsorbed onto the surface of silver nanoplatelet core particles, forming an outer J-aggregate shell. This core–shell architecture provided a framework for coupling the plasmon resonance of the silver nanoplatelet core with the exciton resonance of the J-aggregate shell. The sizes and aspect ratios of the silver nanoplatelets were controlled to ensure the overlap of the plasmon and exciton resonances. As a measure of the plasmon-exciton coupling strength in the system, the experimentally observed transparency dips correspond to a Rabi splitting energy of 207 meV, among the highest reported for colloidal nanoparticles. The optical properties of the silver platelet-J-aggregate nanocomposites were supported numerically and analytically by the boundary-element method and temporal coupled-mode theory, respectively. Our theoretical predictions and experimental results confirm the presence of a transparency dip for the silver nanoplatelet core J-aggregate shell structures. Additionally, the numerical and analytical calculations indicate that the observed transparencies are dominated by the coupling of absorptive resonances, as opposed to the coupling of scattering resonances. Hence, we describe the suppressed extinction in this study as an induced transparency rather than a Fano resonance.United States. Army (Basic Research Program)United States. Army Edgewood Chemical Biological CenterUnited States. Army Research Office. Institute for Soldier Nanotechnologies (Contract No. W911NF-13-D-0001

Theoretical Criteria for Scattering Dark States in Nanostructured Particles

Nanostructures with multiple resonances can exhibit a suppressed or even completely eliminated scattering of light, called a scattering dark state. We describe this phenomenon with a general treatment of light scattering from a multiresonant nanostructure that is spherical or nonspherical but subwavelength in size. With multiple resonances in the same channel (i.e., same angular momentum and polarization), coherent interference always leads to scattering dark states in the low-absorption limit, regardless of the system details. The coupling between resonances is inevitable and can be interpreted as arising from far-field or near-field. This is a realization of coupled-resonator-induced transparency in the context of light scattering, which is related to but different from Fano resonances. Explicit examples are given to illustrate these concepts.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0819762

Optimization of broadband optical response of multilayer nanospheres

We propose an optimization-based theoretical approach to tailor the optical response of silver/silica multilayer nanospheres over the visible spectrum. We show that the structure that provides the largest cross-section per volume/mass, averaged over a wide frequency range, is the silver coated silica sphere. We also show how properly chosen mixture of several species of different nanospheres can have an even larger minimal cross-section per volume/mass over the entire visible spectrum.United States. Army (Basic Research Program)United States. Army. Edgewood Chemical Biological CenterUnited States. Army Research Office (contract W911NF-07-D-004

Adsorption of Anionic Thiols on Silver Nanoparticles

The adsorption of negatively charged 3-mercaptopropanesulfonate (MPS) on the surface of citrate-stabilized Ag nanoparticles in water is investigated using colloidal particle surface sensitive techniques. The adsorption of this negatively charged thiol appears to be qualitatively different from that of neutral thiols and highlights the importance of repulsive interactions of electrostatic and steric origins pertaining to charged thiols. For the charged MPS thiol, the adsorption process occurs in two phases. At low surface coverage, where the intermolecular repulsion is negligible and the adsorption is dominated by the formation of the S–Ag bond, MPS molecules need to overcome an activation energy barrier <i>E</i>_<b>a</b> = (7.5 ± 0.9) kcal/mol with an associated free energy change Δ<i>G</i>_ads = −(14.3 ± 0.3) kcal/mol and behave similar to neutral thiols. On the other hand, at high surface coverage where the repulsive interactions among MPS molecules cannot be neglected, the adsorption is characterized by a higher <i>E</i>_a = (12.4 ± 0.5) kcal/mol and lower Δ<i>G</i>_ads = −(7.4 ± 0.1) kcal/mol

Electron Injection from a Carboxylic Anchoring Dye to TiO2 Nanoparticles in Aprotic Solvents

Injection of photoexcited electrons in the para-Ethyl Red dye to TiO2 nanoparticles (Anatase, 40 nm diameter) is characterized by transient absorption on ultrafast time scales. This study focuses on understanding the effect of aprotic solvents on the injection rate. Transient absorption at 1900 cm−1 is probed following a 400 nm pulse which excites the electronic transition of p-ER adsorbed on TiO2 through its carboxylic group. Measurements conducted in three different solvents show that electron injection lifetimes are in the 250–300 fs range but display a trend in correlation with solvent polarity: the electron injection lifetime is the shortest (257 fs) in acetonitrile followed by dichloromethane (271 fs) and chloroform (296 fs). This trend can be understood by using the Marcus theory in which the reorganization energy varies correspondingly in the three different solvents. This study shows that for aprotic solvents the one with the highest polarity facilitates the fastest electron injection

ARTICLE Transparent displays enabled by resonant nanoparticle scattering

The ability to display graphics and texts on a transparent screen can enable many useful applications. Here we create a transparent display by projecting monochromatic images onto a transparent medium embedded with nanoparticles that selectively scatter light at the projected wavelength. We describe the optimal design of such nanoparticles, and experimentally demonstrate this concept with a blue-color transparent display made of silver nanoparticles in a polymer matrix. This approach has attractive features including simplicity, wide viewing angle, scalability to large sizes and low cost

Theoretical Criteria for Scattering Dark States in Nanostructured Particles

Nanostructures with multiple resonances can exhibit a suppressed or even completely eliminated scattering of light, called a scattering dark state. We describe this phenomenon with a general treatment of light scattering from a multiresonant nanostructure that is spherical or nonspherical but subwavelength in size. With multiple resonances in the same channel (i.e., same angular momentum and polarization), coherent interference always leads to scattering dark states in the low-absorption limit, regardless of the system details. The coupling between resonances is inevitable and can be interpreted as arising from far-field or near-field. This is a realization of coupled-resonator-induced transparency in the context of light scattering, which is related to but different from Fano resonances. Explicit examples are given to illustrate these concepts

Nanophotonic particle simulation and inverse design using artificial neural networks

© 2018 SPIE. We propose a method to use artificial neural networks to approximate light scattering by multilayer nanoparticles. We find the network needs to be trained on only a small sampling of the data in order to approximate the simulation to high precision. Once the neural network is trained, it can simulate such optical processes orders of magnitude faster than conventional simulations. Furthermore, the trained neural network can be used solve nanophotonic inverse design problems by using back-propogation - where the gradient is analytical, not numerical