Generalized Poynting vector model to calculate the spatial and spectral profiles of the electric field intensity, optical power flow, and optical absorption for all optical modes of organic light-emitting diodes.

We propose a generalized Poynting vector model (GPVM) that can simultaneously calculate the spatial and spectral distributions of the electric field intensity, optical power flow, and optical absorption as well as the power dissipation spectrum for all optical modes of organic light-emitting diodes (OLEDs). The theoretical formulation of the GPVM with respect to the dipole orientation and light polarization is derived by combining the dipole source term and transfer matrix method as a function of the normalized in-plane wave vector u. In the GPVM, the theoretical expression of the spectral power density, derived from the time-averaged Poynting vector at the emission layer, proves to be identical to that presented by the currently-used point dipole model. In a bottom-emitting OLED, the electric field profiles of the waveguide (WG) and surface plasmon polariton (SPP) modes obtained by the GPVM are nearly same as those calculated by the boundary eigenvalue solver except the slight difference at the position of the dipole emitter, which only occurs in the case that the excitation efficiency of a WG or SPP mode is relatively small. Finally, two-dimensional plots of the internal optical power flow and optical absorption, providing physical and intuitive information on the internal emission process as well as the absorption loss of all the optical modes, are calculated as a function of the longitudinal position and normalized in-plane wave vector. Compared with the currently-used electromagnetic methods of the Green's function, dipole radiation, and point dipole models, the proposed GPVM has the advantage that it can provide all the spatial and spectral calculation results of the electric field intensity, optical power flow, and optical absorption with respect to the dipole orientation and light polarization, which are essential in the optical modeling and analysis of OLEDs.

Theoretical comparison of the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and electromagnetic optical models of organic light-emitting diodes.

We theoretically compare the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and classical electromagnetic optical models of organic light-emitting diodes (OLEDs). A sophisticated optical model combining the two approaches is required to obtain an accurate calculation result and a comprehensive understanding of the micro-cavity effect in OLEDs. In the quantum-mechanical approach based on the Fermi's golden rule, the mode expansion method is used to calculate the excitation efficiency. In the classical electromagnetic approach, the spectral power density calculated by the point dipole model is fitted by the summation of the Lorentzian line shape functions, which provide the excitation probability of each waveguide and surface plasmon modes. The mode coupling efficiencies on the basis of the two approaches are calculated in a bottom-emitting OLED when the position of a dipole emitter is varied. By comparing the calculation results, we confirm the equivalence of two approaches and obtain the better optical interpretation to the calculated excitation efficiency of waveguide and surface plasmon modes. The ratio of mode excitation efficiencies calculated by two approaches agrees well with each other except the contribution of the near-field absorption component.

Optical modeling of the emission zone profile and optimal emitter position based on the internal field profile of the air mode in organic light-emitting diodes.

We propose a theoretical formulation to calculate the internal profile of the air mode in the organic light-emitting diode (OLED) on the combination of the transfer matrix method and source-term method. The spatial distributions of the air mode are calculated in a top-emitting OLED with respect to the light polarization, extraction angle, dipole orientation, and dipole position. Air modes are also calculated on the basis of the previously used external source model, where the input optical wave is injected from the air into the OLED multilayer. Comparison of the calculated air modes between two models checks the validity of the external source model. In addition, we propose an improved formula to determine the optimal emitter positions that maximize the two-beam interference of the micro-cavity effect. In the improved formula, a non-ideal reflection phase shift at a reflective metal anode is treated as the skin depth of the air mode. Finally, the effect of the dipole orientation on the air mode is investigated. Compared with the air mode emitted by the horizontally oriented dipole, the air mode generated by the vertically oriented dipole has relatively small intensity and shows the opposite dependence of the emitter position variation. The calculation results of the internal profile of the air mode within the emission layer are matched with the profile of the emission zone obtained by output radiant flux on the basis of the currently used point dipole model.

Multiobjective optimization for a plasmonic nanoslit array sensor using Kriging models.

We propose an efficient multiobjective optimization approach for a plasmonic nanoslit array sensor using Kriging surrogate models. The universal Kriging models whose regression functions are zeroth-, first-, and second-order polynomials are adopted to estimate objective functions. The multiobjective extension of the genetic algorithm is used for Pareto optimal sensor geometry. The objective functions are the figure of merit defined as a ratio of peak wavelength shift at molecular adsorption and 3 dB bandwidth of transmission spectrum, and peak transmission power, respectively. The optical properties of a plasmonic slit sensor are investigated, such as transmission power, bandwidth, and peak shift, using the finite element method.

Spinning of a submicron sphere by Airy beams.

We show that by employing two incoherent counter-propagating Airy beams, we can manipulate a submicron sphere to spin around a transverse axis. We can control not only the spinning speed, but also the direction of the spinning axis by changing the polarization directions of Airy beams.

Slow-light effect via Rayleigh anomaly and the effect of finite gratings.

In this Letter, we investigate the slow-light effect of subwavelength diffraction gratings via the Rayleigh anomaly using a fully analytical approach without needing to consider specific grating structures. Our results show that the local group velocity of the transmitted light can be significantly reduced due to the optical vortex, which can inspire a new mechanism to enhance light-matter interactions for optical sensing and photodetection. However, the slow-light effect will diminish as the transmitted light propagates farther from the grating surface, and the slowdown factor decreases as the grating size shrinks.

Effect of finite metallic grating size on Rayleigh anomaly-surface plasmon polariton resonances.

Rayleigh anomalies (RAs) and surface plasmon polaritons (SPPs) on subwavelength metallic gratings play pivotal roles in many interesting phenomena such as extraordinary optical transmission. In this work, we present a theoretical analysis of the effect of finite metallic grating size on RA-SPP resonances based on the combination of rigorous coupled wave analysis and finite aperture diffraction. One-dimensional arrays of gold subwavelength gratings with different device sizes were fabricated and the optical transmission spectra were measured. As the grating size shrinks, the broadening of the RA-SPP resonances is predicted by the theoretical model. For the first order RA-SPP resonances, the results from this model are in good agreement with the spectra measured from the fabricated plasmonic gratings.

Spin angular momentum of surface modes from the perspective of optical power flow.

We show that the spin angular momentum (SAM) carried by a surface mode can be linked to the expectation value, with respect to the distribution of optical power flow, of its decay constant by itself or divided by the product of permittivity and permeability of the medium. Rewriting the formulas for the SAM of a surface mode using the relation between the SAM density and the Poynting vector and then normalizing the light field so that the surface mode carries unit power, we derive novel formulas that show the linear relation between the SAM and those expectation values. The effect of propagation loss is also discussed briefly.

Hybrid states of propagating and localized surface plasmons at silver core/silica shell nanocubes on a thin silver layer.

Hybrid characteristics of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs) appear at a combined structure of a thin silver (Ag) layer and silver core/silica shell nanocubes (AgNC@SiO(2)s) in the Kretschmann configuration, because the resonant condition of PSPs on the thin Ag layer is significantly modified by LSPs of the AgNC@SiO(2)s. We investigate theoretically and experimentally that due to the hybrid property, the slope and position of the minimum reflectance band can be controlled on a graph of incident angle versus wavelength of reflected light, by changing structural parameters. The hybrid properties of PSPs and LSPs have a potential to simultaneously detect surface plasmon resonance signals and fluorescence images.

Origin of the Abraham spin angular momentum of surface modes.

By considering the transverse spin angular momentum (SAM) that results from the rotation of the electric-field component of a surface mode as a longitudinal SAM of an elliptically polarized light propagating through a homogeneous medium, an alternate route to deriving the formula of the Abraham SAM carried by the surface mode can be achieved. The findings prove in an explicit manner that it is the Abraham SAM that is directly related to the rotation of the electric field.

Intermediate plasmonic characteristics in a quasi-continuous metallic monolayer.

There has been a significant interest on plasmonics in a metallic structure with very narrow gaps for studies of nanophotonics. However, little attention has been paid to the behavior of surface plasmons (SPs) in quasi-continuous metallic structures. This study observes and analyzes intermediate characteristics between propagating SPs (PSPs) and localized SPs (LSPs) in a quasi-continuous metallic monolayer of core-shell nanocubes. We reveal that, in a very narrow region of few-nanometer gaps among the nanocubes, the intrinsic energy bands of PSPs and LSPs intersect each other to generate two hybrid bands and an anti-crossing. Using a self-assembly method instead of the lithographic techniques which have several limitations as of now, we materialize the quasi-continuous metallic layer with plenty of nano-gaps that exhibit intermediate plasmonic characteristics. The intermediate plasmonic characteristics observed in this study will lead to interesting subjects, such as band engineering and slow SPs, in nanophotonics.

Relation of the angular momentum of surface modes to the position of their power-flow center.

We show that the value of the total angular momentum (AM) carried by a surface mode can be interpreted as representing the transverse position of the center or balance point of the power flow through the mode. Especially in the lossless cases, the value of the Abraham AM per unit power (multiplied by the square of the speed of light in vacuum) is exactly the same as the transverse position of this power-flow center. However, the Minkowski counterpart becomes proportional to that position with a coefficient in the form of 1 + η, where η is determined mainly by the constitutive parameters of media.

Generation of finite power Airy beams via initial field modulation.

We investigate the finite power Airy beams generated by finite extent input beams such as a Gaussian beam, a uniform beam of finite extent, and an inverse Gaussian beam. Each has different propagation behavior: A finite Airy beam generated by a uniform input beam keeps its Airy profile much longer than the conventional finite Airy beam. Also, an inverse Gaussian beam generates a finite Airy beam with a good bent focusing in free space. In this paper, the analysis and experimental results of finite Airy beams are presented.

Role of magnetic induction currents in nanoslit excitation of surface plasmon polaritons.

We present a method for exciting surface plasmon polaritons (SPPs) caused by a magnetic field component perpendicular to the direction of slit. The excitation mechanism is based on the spatially oscillating induced current along the edges of the slit under obliquely incident electromagnetic waves. Our finding distinguishes itself from previous mechanisms based on transverse electric fields and unveils the missing point of the SPP-excitation problem in a nanoslit. The use of a magnetic field for SPP excitation can be highly efficient and even comparable to that with an electric field, so that their composition can lead to selective unidirectional excitation.

A compact light concentrator by the use of plasmonic faced folded nano-rods.

We propose a compact nano-metallic structure for enhancing and concentrating far-field transmission: a faced folded nano-rod (FFR) unit, composed of two folded metallic nano-rods placed facing each other in an aperture. By analyzing local charge, field, and current distributions in the FFR unit using three-dimensional finite difference time domain (FDTD) calculation results, we show that although charge and field configurations become somewhat different depending on the polarization states of the illumination, similar current flows are formed in the FFR unit, which entail similar far-field radiation patterns regardless of the polarization states, making the FFR unit a quasi-polarization-insensitive field concentrator. We demonstrate this functionality of the FFR unit experimentally using the holographic microscopy which provides us a three-dimensional map of the complex wavefronts of optical fields emanating from the FFR unit.

Origin of surface modes in nonconducting metamaterials: from the viewpoint of bound charges.

Conventional optical modes are supported by the total internal reflection occurring due to refractive index difference. Surface modes guided through an interface seem to have a different origin from them because they are supported by the difference in the signs of constitutive parameters of two media comprising the interface. Here, we propose that these surface modes have their origin in the accumulated polarization charges (or magnetization currents) near the interface. To induce such an accumulation, we need to make the normal component of the electric field (or the tangential component of magnetic flux density) on each side of the interface mutually antiphase, which entails different signs of constitutive parameters across the interface.

Bessel-like beam generation by superposing multiple Airy beams.

We propose a way of generating Bessel-like non-diffracting beams based on the superposition of multiple Airy beams. We also demonstrate, through numerical simulations of the propagation dynamics of the Bessel-like beams, that these Bessel-like beams can be modified to show the feature of vortex power flow by controlling the initial positions of each single Airy beam.

Slow non-dispersing wavepackets.

We show that in plasmonic or metamaterial slab waveguides, it is possible to generate slow non-dispersing wavepackets which undergo neither spatial diffraction nor temporal spreading with no nonlinear effects by forming a type of hybrid wavepacket between slow-light waveguide modes and diffraction-free Airy wavepackets. Three mechanisms are involved in their slowness: the slow-light feature of waveguide modes, the initial launching speed of hybrid wavepackets, and their acceleration along the time domain in a moving frame.

Dual Airy beam.

We derive a general form of Airy wave function which satisfies paraxial equation of diffraction. Based on this, we propose a new form of Airy beam, which is composed of two symmetrical Airy beams which accelerate mutually in the opposite directions. This 'dual' Airy beam shows several distinguishing features: it has a symmetric transverse intensity pattern and improved self-regeneration property. In addition, we can easily control the propagation direction. We also propose 'quad' Airy beam, which forms a rectangular shaped optical array of narrow beams that travel along a straight line. We can control its propagation direction without changing transverse intensity patterns. These kinds of superposed optical beams are expected to be useful for various applications with their unique properties.

A non-reflecting metamaterial slab under the finite-embedded coordinate transformation.

Under the restrictions that the mapping functions of transformation are defined in extended two-dimensional (2D) forms and the incident waves are 2D propagating fields, the conditions for non-reflecting boundaries in a finite-embedded coordinate transformation metamaterial slab are derived. By exploring several examples, including some reported in the literatures and some novel ones developed in this study, we show that our approach can be efficiently used to determine the condition for a finite-embedded coordinate transformed metamaterial slab to be non-reflecting.