Surface Waveguide Modes Research Articles

Nanostructures in Organic Light‐Emitting Diodes: Principles and Recent Advances in the Light Extraction Strategy

AbstractOrganic light‐emitting diodes (OLEDs) in recent years have emerged as a leading display technology and the popularity of OLEDs is attributed to their numerous advantages, including the ability to produce natural color, achieve a true black state, consume low consumption, exhibit fast response, and be compatible with flexible devices. However, limitations in the performance persist, e.g., the out‐coupling efficiency, which currently stands at ≈20% due to issues such as trapped modes and plasmon loss. Many researchers, therefore, have actively proposed the integration of various nanostructures to address the challenges and enhance OLED performance. The structures play a crucial role in facilitating strong optical interaction with surface plasmon and waveguide modes, thereby improving the extraction of trapped modes. To mitigate the confinement, layers to modulate the refractive index are introduced to extract the confined light and redirect it into the out‐coupled mode. In this review, a comprehensive overview of the principle and effectiveness of these nanostructures in enhancing OLED performance is provided. Various applications of OLEDs are explored based on nanostructures such as nanoparticles, nanomeshes, metasurface, bioinspired structures, and scattering layers. By implementing and refining these nanostructures, significant advancements in OLED performance are anticipated.

High index dielectric films on metals: An island of emission.

Fluorescent emitters are quenched near the surfaces of metals via rapid energy transfer to the metal, via surface plasmons, waveguide modes, and absorption. Commonly, this quenching is reduced by introducing a polymeric or dielectric spacer but requires large distances, at least a fraction of the wavelength, between the metal and chromophore. Using the classical theory for a dipole above a metal/dielectric substrate, we investigate the fluorescent yield for emitters above a wide range of metals and spacers. For metals with low loss and low plasma frequencies, a high index spacer is shown to be advantageous for obtaining higher fluorescent yield in an "island of emission" at finely tuned spacer thickness just 20-30nm from the metal surface. For such metal-dielectric combinations, fluorophores can be placed surprisingly close to the metal surface while remaining significantly emissive.

Photonic Crystal Waveguides Composed of Hyperbolic Metamaterials for High-FOM Nano-Sensing

This study introduces an innovative integration of hyperbolic metamaterials (HMMs) and photonic crystals (PtCs), each possessing unique dispersion properties that effectively manipulate the propagation of light. We present a PtC waveguide consisting of arrays of HMM nanorods, denoted as HMM PtCs. This waveguide configuration enables the realization of a high figure of merit (FOM) nano-sensor. HMMs and PtCs share the same underlying physics. HMMs can generate surface plasmonics, while PtCs offer a bandgap for the waveguide. This configuration presents a novel sensing solution that directly couples surface plasmonics and waveguide modes. By modifying the refractive indices of the surrounding materials, the PtC waveguide exhibits alterations in absorption and transmission, allowing for the detection of temperature, pressure, and material variations. The refractive indices of the surrounding materials can be adjusted based on the sensor’s intended application. For instance, when the sensor is utilized for temperature sensing, thermal infrared materials can serve as the surrounding medium. As the temperature rises, the refractive index of the surrounding material changes accordingly, impacting the waveguide modes and thereby altering absorption and transmission. We utilized the finite element method to conduct numerical simulations in order to assess the absorption and transmission characteristics of the proposed system. Given that this approach involves a full electromagnetic calculation based on Maxwell’s equations, it closely approximates real-world scenarios. The employed numerical method demonstrates the remarkable performance of this proposed system, achieving a sensitivity of 324.16 nm/RIU (refractive index unit) and an impressive FOM of 469.58 RIU−1. These results signify a substantial improvement over surface plasmonic sensors, which typically exhibit limited FOMs. The direct coupling between surface plasmonics and waveguide modes provides a distinct advantage, allowing the proposed sensor to deliver a superior performance. As a consequence, the HMM PtC waveguide sensor emerges as an exceptionally appealing option for photonic sensing applications. The complexity of the proposed system presents a fabrication challenge. Nevertheless, as fabrication technology continues to advance, we anticipate that this issue can be effectively resolved. The proposed HMM PtC waveguide holds vast potential across diverse fields, including biology, medicine, and clinics, representing an exciting advancement for both industry and scientific research.

Models of waveguides combining gradient and nonlinear optical layers

Objectives. Theoretical studies of the waveguide properties of interfaces between nonlinear optical and graded-index media are important for application in optoelectronics. Waveguides combining layers with different optical properties seem to be the most promising, since they can be matched to optimal characteristics using a wide range of control parameters. The paper aims to develop a theory of composite optically nonlinear gradedindex waveguides with an arbitrary profile, within which it is possible to obtain exact analytical expressions for surface waves and waveguide modes in an explicit form. The main feature of the theory proposed in this paper is its applicability for describing surface waves and waveguide modes, in which the field is concentrated inside the gradient layer and does not exceed its boundary, avoiding contact with the nonlinear layer.Methods. Analytical methods of the theory of optical waveguides and nonlinear optics are used.Results. A theoretical description of the waveguide properties of the interface between two media having significantly different optical characteristics is carried out. The formulated model of a plane waveguide is applicable to media having an arbitrary spatial permittivity profile. An analytical expression describing a surface wave propagating along the interface between a medium having stepwise nonlinearity and a gradient layer with an arbitrary permittivity profile is obtained. Additionally, analytical expressions for surface waves propagating along the interface between a medium with Kerr nonlinearity (both self-focusing and defocusing), as well as graded-index media characterized by exponential and linear permittivity profiles, are obtained.Conclusions. The proposed theory supports a visual description in an explicit analytical form of a narrowly localized light beam within such waveguides. It is shown that by combining different semiconductor crystals in a composite waveguide, it is possible to obtain a nonlinear optical layer on one side of the waveguide interface and a layer with a graded-index dielectric permittivity profile on the other.

Terahertz amplification and lasing by using transverse electric modes in a two-layer-graphene-dielectric waveguide structure with direct current

We study for the first time the interaction between the waveguide modes of graphene structure and freely propagating terahertz (THz) electromagnetic waves (this interaction takes place within the light cone). We revealed a new and rather unexpected physical phenomenon by showing that freely incident THz electromagnetic waves can resonate with the surface transverse electric (TE) modes of the graphene waveguide in virtue of these modes having their dispersions in the vicinity of the light cone. The dispersion and amplification of surface TE modes in a dielectric waveguide covered with two graphene layers biased by direct current (DC), as well the amplification and lasing of incident THz wave by excitation of TE mode resonances, are investigated. The DC flows perpendicular to the direction of the surface wave propagation and creates the capacitive complex conductivity of graphene at THz frequencies, which is necessary for the existence of surface TE modes in graphene. The real part of graphene conductivity can be negative at THz frequencies due to DC in graphene which leads to amplification and lasing of THz radiation. Such structure can be of great practical importance because an external THz wave can be amplified or generated in lasing process without using special coupling elements commonly needed for ensuring the interaction between external THz wave and surface waveguide modes. The use of a two-layer graphene structure makes it possible to reduce the charge–carrier drift velocity required for reaching the lasing threshold at those resonances, as compared to a structure with a single graphene layer.

Fiber-optic surface waveguide resonance in gaseous medium: Tunable generation with all fiber modes

The integration of metal clad with dielectric waveguide offers numerous advantages for generating optical surface waves. Here we perform a comprehensive study on the principle and tunable characteristics of fiber-optic surface waveguide resonances (SWRs) excited with a metal-clad film waveguide deposited on optical fiber in gaseous medium. A versatile approach is proposed for the analysis of fiber-optic SWRs and it is also available for analyzing other kinds of optical surface waves. The essential mechanism for generating surface waveguide modes is obtained. The results show that the film waveguide supports both P- and S-polarized discrete surface waveguide modes including guided modes, cladding modes and leaky modes. It is found that the introduction of a metal clad strengthens the guidance of these modes and enhances the mode coupling with fiber modes. This makes it possible to generate strongly polarization-dependent fiber-optic SWRs with all fiber modes by simply tailoring the thickness of film waveguide.

Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

Fiber-Optic Surface Waveguide Modes Excited by Inter/Intra Mode Transition for Refractometric Sensitivity Enhancement

The generation of surface waveguide modes in telecommunication-grade optical fibers is highly desirable, since it would enable the development of novel all-fiber devices with high performance. Herein, we carry out a comprehensive theoretical investigation on the intrinsic excitation mechanism and sensing performance of fiber-optic surface waveguide modes supported by a tilted fiber grating (TFG) with nanometric-scale gold and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films. The results reveal that both P-and S-polarized surface waveguide modes originate from the mode transition tuned by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, including inter mode transition associated with two adjacent modes and intra mode transition only related to the mode itself. During the mode transition, the power carried by cladding modes is absorbed by the Au-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite, thereby yielding a strong surface-localized evanescent field that is sensitive to external medium. It is found that for both polarizations the surface waveguide modes induced by the inter/intra mode transition present almost the same refractometric sensitivity, but with the P-polarization (605.6 nm/RIU) being more sensitive than the S-counterpart (339.2 nm/RIU), the state-of-the-art surface plasmon resonance (539.7 nm/RIU), and also the approach enhanced by high-refractive-index film (7.4 nm/RIU). The results attained here represent a breakthrough since they would open new pathways toward single molecular detection.

Sensing performance of surface waveguide modes excited in long-period fiber grating with gold-silicon nanocoatings.

We carry out a very early theoretical study on surface waveguide modes excited in a long-period fiber grating (LPFG) coated with gold-silicon thin films for refractive index sensing. The surface waveguide modes originate from the intermode transition of EH cladding modes and present a very strong evanescent field penetrating into the surrounding medium, which makes them ultrasensitive to external changes. By tracking the dual resonances of surface waveguide modes, ultrahigh sensitivity up to 7267.7nm/RIU around 1.315 is obtained, which is 76-/4-fold higher than the case of bare LPFG/LPFG-assisted surface plasmon resonance, together with the capability of self-referenced measurement. This new, to the best of our knowledge, concept is expected to find wide applications based on refractive index sensing.

Classical Analogy of Double Electromagnetically Induced Transparencies from Interactions of Surface Plasmon Resonances and Waveguide Modes

Classical Analogy of Double Electromagnetically Induced Transparencies from Interactions of Surface Plasmon Resonances and Waveguide Modes

Flexibly Tunable Surface Waveguide Resonances in Cylindrical Waveguide-Metal-Waveguide Configuration Assisted by Tilted Fiber Grating

Surface waveguide modes/resonances (SWRs) are a promising and relatively new research route toward advanced photonic applications ranging from spectroscopy and sensing to communications. However, the conventional planar setups for exciting the SWRs suffer from its intrinsic shortcomings. Herein, we propose and demonstrate a new cylindrical waveguide-metal-waveguide (WMW) configuration consisting of a tilted fiber grating (TFG) coated with nanometric-scale gold and silicon films for exciting fiber-optic SWRs. The investigations are performed using the ray-optic approach and rigorous coupled mode theory. It is found that the surface waveguide modes essentially stem from polarization-dependent mode transition induced by the dielectric silicon film, including inter mode transition dependent on two adjacent modes for transverse magnetic (TM) polarization and intra mode transition related to only the mode itself for transverse electric (TE) polarization. This is confirmed by the dispersion characteristics, field localization and reflectivity/absorptivity of the metal-silicon composite. Tuned by the silicon film, different surface waveguide modes from low- to high-order are excited. When these surface waveguide modes are phase-matched with the core mode in the TFG, flexibly tunable TM-/TE-polarized SWRs are attained in a wide wavelength range. This work provides new prospects and general guidelines for developing novel robust fiber-optic SWR-based devices with huge potential applications.

Dimensional confinement and waveguide effect of Dyakonov surface waves in twisted confined media

Abstract We theoretically study Dyakonov surface waveguide modes that propagate along the planar strip interfacial waveguide between two uniaxial dielectrics. We demonstrate that owing to the one-dimensional electromagnetic confinement, Dyakonov surface waveguide modes can propagate in the directions that are forbidden for the classical Dyakonov surface waves at the infinite interface. We show that this situation is similar to a waveguide effect and formulate the resonance conditions at which Dyakonov surface waveguide modes exist. We demonstrate that the propagation of such modes without losses is possible. We also consider a case of two-dimensional confinement, where the interface between two anisotropic dielectrics is bounded in both orthogonal directions. We show that such a structure supports Dyakonov surface cavity modes. Analytical results are confirmed by comparing with full-wave solutions of Maxwell’s equations. We believe that our work paves the way toward new insights in the field of surface waves in anisotropic media.

Dyakonov-like waveguide modes in an interfacial strip waveguide

We study Dyakonov surface waveguide modes in a waveguide represented by an interface of two anisotropic media confined between two air half-spaces. We analyze such modes in terms of perturbation theory in the approximation of weak anisotropy. We show that in contrast to conventional Dyakonov surface waves that decay monotonically with distance from the interface, Dyakonov waveguide modes can have local maxima of the field intensity away from the interface. We confirm our analytical results by comparing them with full-wave electromagnetic simulations. We believe that this work can bring new ideas in the research of Dyakonov surface waves.

Differentiating surface plasmon-polariton waves and waveguide modes guided by interfaces with one-dimensional photonic crystals

One-dimensional photonic crystals (PCs) are made of planar layers of different materials with periodic arrangement. Such PCs can support multiple surface waves at their interfaces with homogeneous materials as well as multiple waveguide modes guided by individual constituent layers. The numerical results comparing multiple surface plasmon-polariton (SPP) waves and waveguide modes guided by a metal and a PC with only two layers in each period indicated that the periodic nonhomogeneity and the waveguide modes guided by the individual layers in the PC engender the multiplicity of SPP waves. For smaller value of the period of the PC, the coupling of the individual layers in the PC near the metal/PC interface gives rise to new SPP waves but the coupling reduces as the period increases and waveguide modes dominate. The differentiation between the waveguide modes and the surface waves is necessary to fully exploit the promise of photonic crystals in integrated optics.

Giant enhancement of reflectance due to the interplay between surface confined wave modes and nonlinear gain in dielectric media.

We study theoretically the interplay between the surface confined wave modes and the linear and nonlinear gain of the dielectric layer in the Otto configuration. The surface confined wave modes, such as surface plasmons or waveguide modes, are excited in the dielectric-metal bilayer by obliquely incident p waves. In the purely linear case, we find that the interplay between linear gain and surface confined wave modes can generate a large reflectance peak with its value much greater than 1. As the linear gain parameter increases, the peak appears at smaller incident angles, and the associated modes also change from surface plasmons to waveguide modes. When the nonlinear gain is turned on, the reflectance shows very strong multistability near the incident angles associated with surface confined wave modes. As the nonlinear gain parameter is varied, the reflectance curve undergoes complicated topological changes and sometimes displays separated closed curves. When the nonlinear gain parameter takes an optimally small value, a giant amplification of the reflectance by three orders of magnitude occurs near the incident angle associated with a waveguide mode. We also find that there exists a range of the incident angle where the wave is dissipated rather than amplified even in the presence of gain. We suggest that this can provide the basis for a possible new technology for thermal control in the subwavelength scale.

High-performance near-field thermophotovoltaics for waste heat recovery

The US industries reject nearly 20–50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~ 500–900K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system consisting of a plasmonic emitter (indium tin oxide) and a narrow-bandgap photovoltaic cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11W/cm2 power density at a 900K emitter temperature, because of the spectral reshaping and enhancement by the thermally excited surface plasmons and waveguide modes. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems. We propose a modified system to further enhance the power density by using a thin metal film on the cell, achieving a counterintuitively “blocking-assisted” heat transfer and power generation in the near-field regime.

High light extraction efficiency into glass substrate in organic light-emitting diodes by patterning the cathode in graded superlattice with dual periodicity and dual basis

The newly discovered graded, superlattice photonic crystals with dual periodicity and dual basis present great opportunity for electromagnetic wave control in photonic devices. These graded superlattices can be holographically fabricated by eight beam interference lithography. We have computed, through electrodynamic simulation, the light extraction efficiency of planar, white organic light-emitting diodes where the Al cathode is patterned with the graded superlattice with dual basis. Two graded super-lattices with four-fold and two-fold symmetries are used to pattern the Al cathode. The decrease in power losses to surface plasmon and waveguide modes is explained by the varying plasmon path length and grating cycle, respectively, in the graded pattern. To the authors' best knowledge, the highest light extraction efficiency of 73.1% into the glass substrate in organic light-emitting diodes has been predicted through simulations.

Inverted organic solar cells enhanced by grating-coupled surface plasmons and waveguide modes.

In this study, we demonstrate improved photovoltaic properties in inverted organic thin-film solar cells by simultaneous excitation of grating-coupled surface plasmons and grating-coupled waveguide modes on gold grating surfaces. The cell consists of a glass-ITO substrate/titanium dioxide/poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/gold structure. The grating structures were fabricated on P3HT:PCBM layers using a nanoimprinting technique with a PDMS stamp. The grating-structured PDMS stamps were fabricated using a DVD-R grating template with a grating pitch, Λ, of 740 nm. Reflectivity measurements made using p-polarized light clearly indicate 2 types of excitation modes, i.e., surface plasmons and waveguide modes, while s-polarized light produces only waveguide modes. Incident photon-to-current efficiency measurements exhibited increased photocurrent wavelengths corresponding to the wavelengths of surface plasmon excitations and waveguide mode excitations. Through the simultaneous excitation of surface plasmons and waveguide modes, short-circuit photocurrents in the grating-structured cells exhibited an improvement of up to 11% in the solar cells, leading to an efficiency increase of 16%.

Intensity and phase sensitivities in metal/dielectric thin film systems exhibiting the coupling of surface plasmon and waveguide modes.

In this paper, we discuss four different configurations of metal/dielectric thin films systems: surface plasmon resonance, coupled plasmon-waveguide resonance, metallic waveguide-coupled surface plasmon resonance, and dielectric waveguide-coupled surface plasmon resonance. For the waveguide-coupled surface plasmon resonance thin film systems, we explore several waveguide thicknesses that produce different resonant line shapes, including plasmon-induced transparency and waveguide-induced transparency. This paper presents a theoretical analysis and comparison of the intensity and phase sensitivities to changes in the index of refraction in a sensing layer external to the thin film system. We discuss the material parameters and the variations that led to the sensitivity variations in each thin film system. We show that when the surface plasmon polariton mode is coupled with dielectric waveguides, there is an enhancement of the sensitivity. The plasmon-induced transparency is used to increase the dynamic range of the system and shows a monotonically increasing intensity in the range of 0.4 refractive index units.

Magneto-optical switching of Bloch surface waves in magnetophotonic crystals

Bloch-surface-wave (BSW) excitation controlled by Faraday rotation in one-dimensional magnetophotonic crystals is presented. Dispersion curves of the Bloch surface wave and waveguide modes of magnetophotonic crystals consisting of silicon dioxide and bismuth-substituted yttrium-iron-garnet (Bi:YIG) quarter-wavelength-thick layers are calculated using Berreman's 4×4 transfer matrix method. Enhanced Faraday rotation observed in the magnetophotonic crystals in the spectral vicinity of the BSW resonance enables the magneto-optical switching of BSWs. The excitation of the BSWs at the magnetophotonic crystal surface for p-polarized incident light is induced by magneto-optical activity in the Bi:YIG layers.