Ludmila J. Prokopeva

Optical Dispersion Models for Time-Domain Modeling of Metal-Dielectric Nanostructures

We discuss second-order complex Padé approximants which give a systematic approach to time-domain modeling of dispersive dielectric functions. These approximants, which also reduce to the classical Drude, Lorentz, Sellmeier, critical points and other models upon appropriate truncation, are used to compare frequency domain (FD) versus time-domain (TD) simulations of local optical responses and the transmission-reflection spectra for a plasmonic nanostructure. A comparison is also made using auxiliary differential equations (ADE), and second order recursive convolution (RC) formulations embedded in finite-difference, finite-volume, and finite-element time-domain solvers.

Fabrication and realistic modeling of three-dimensional metal-dielectric composites

Historically, the methods used to describe the electromagnetic response of random, three-dimensional (3D), metal-dielectric composites (MDCs) have been limited to approximations such as effective-medium theories that employ easily-obtained, macroscopic parameters. Full-wave numerical simulations such as finite-difference time domain (FDTD) calculations are difficult for random MDCs due to the fact that the nanoscale geometry of a random composite is generally difficult to ascertain after fabrication. We have developed a fabrication method for creating semicontinuous metal films with arbitrary thicknesses and a modeling technique for such films using realistic geometries. We extended our two-dimensional simulation method to obtain realistic geometries of 3D MDC samples, and we obtained the detailed near- and far-field electromagnetic responses of such composites using FDTD calculations. Our simulation results agree quantitatively well with the experimentally measured far-field spectra of the real samples.

Plasmon resonance in multilayer graphene nanoribbons

Plasmon resonances in nanopatterned single-layer graphene nanoribbons (SL-GNRs), double-layer graphene nanoribbons (DL-GNRs) and triple-layer graphene nanoribbons (TL-GNRs) are studied experimentally using ‘realistic’ graphene samples. The existence of electrically tunable plasmons in stacked multilayer graphene nanoribbons was first experimentally verified by infrared microscopy. We find that the strength of the plasmonic resonance increases in DL-GNRs when compared to SL-GNRs. However, further increase was not observed in TL-GNRs when compared to DL-GNRs. We carried out systematic full-wave simulations using a finite-element technique to validate and fit experimental results, and extract the carrier-scattering rate as a fitting parameter. The numerical simulations show remarkable agreement with experiments for an unpatterned SLG sheet, and a qualitative agreement for a patterned graphene sheet. We conclude with our perspective of the key bottlenecks in both experiments and theoretical models.

Experimental retrieval of the kinetic parameters of a dye in a solid film

Effects of a solid matrix on the dye kinetic parameters for Rh800 were experimentally studied. Saturation intensity dependencies were measured with a seeding pulse amplification method using a picosecond and a femtosecond white light supercontinuum source. The kinetic parameters were obtained by fitting experimental dependencies with Yees finite-difference time-domain model coupled to the rate equations of the 4-level Rh800-system. The comparison of these parameters (Rh800-solid host) with liquid host parameters revealed a slight change of the radiative lifetime and a strong change of the non-radiative decay rate. This experimentally determined model enables predictive simulations of time-domain responses of active metamaterials.

Time-domain modeling of silver nanowires-graphene transparent conducting electrodes

Transparent conducting electrodes (TCE) consisting of silver nanowires (SNW) with a single-layer graphene (SLG) cover demonstrate higher optical transparency and lower sheet resistance than indium tin oxide (ITO) and are comparable to the best reported results in TCEs. SNW layer is simulated using the spectral averaging of the FDTD transmittance data from indiscriminately selected frames. Simulations are done for a number of frames until a convergent set of averaged spectra is obtained. SLG layer is simulated separately and contributes to the total transmittance as a multiplicative constant.

Tunable Pulse-Shaping with Gated Graphene Nanoribbons

We propose a pulse-shaper made of gated graphene nanoribbons. Simulations demonstrate tunable control over the shapes of transmitted and reflected pulses using the gating bias. Initial fabrication and characterization of graphene elements is also discussed.

Numerical modeling of active plasmonic metamaterials

The paper addresses numerical time-domain methods for modeling of active and passive dispersive media, needed for simulations of plasmonic metamaterials. The proposed algorithms differ from published results, as our models employ more general formalisms and are more computationally efficient. The frequency dispersion of the permittivity is considered as an arbitrary Pade approximant, its numerical implementation is more universal and effective for all known ADE and RC methods. The gain model is implemented for an arbitrary topology of transitions with the ADE method. The proposed dispersion models are in a good fit with spectroscopic data and are included into a database of optical materials at nanohub.org.

Tuning Fano resonances with graphene

We demonstrate strong electrical control of plasmonic Fano resonances in dolmen structures using tunable interband transitions in graphene. Such graphene-plasmonic hybrid devices can have applications in light modulation and sensing.

Optical black hole: Design and performance

We discuss the design of a realistic optical black hole (OBH) using the exact frequency domain solutions for TM and TE waves. The design includes thin separating layers between the absorbing core and the metamaterial shell. The overall objective is to develop an on-line simulation tool, which could help to design experimentally realizable OBH with a continuous radius-dependent index. Example structures working at 1.5 μm are presented and analyzed.

Designing optimal nanofocusing with a gradient hyperlens

Abstract We report the design of a high-throughput gradient hyperbolic lenslet built with real-life materials and capable of focusing a beam into a deep sub-wavelength spot of λ/23. This efficient design is achieved through high-order transformation optics and circular effective-medium theory (CEMT), which are used to engineer the radially varying anisotropic artificial material based on the thin alternating cylindrical metal and dielectric layers. The radial gradient of the effective anisotropic optical constants allows for matching the impedances at the input and output interfaces, drastically improving the throughput of the lenslet. However, it is the use of the zeroth-order CEMT that enables the practical realization of a gradient hyperlens with realistic materials. To illustrate the importance of using the CEMT versus the conventional planar effective-medium theory (PEMT) for cylindrical anisotropic systems, such as our hyperlens, both the CEMT and PEMT are adopted to design gradient hyperlenses with the same materials and order of elemental layers. The CEMT- and PEMT-based designs show similar performance if the number of metal-dielectric binary layers is sufficiently large (9+ pairs) and if the layers are sufficiently thin. However, for the manufacturable lenses with realistic numbers of layers (e.g. five pairs) and thicknesses, the performance of the CEMT design continues to be practical, whereas the PEMT-based design stops working altogether. The accurate design of transformation optics-based layered cylindrical devices enabled by CEMT allow for a new class of robustly manufacturable nanophotonic systems, even with relatively thick layers of real-life materials.