Low Index Medium Research Articles

Coherent all-optical transistor based on frustrated total internal reflection

This study aims to design an all-optical transistor based on tunneling of light through frustrated total internal reflection. Under total internal reflection, the electromagnetic wave penetrates into the lower index medium. If a medium with high refractive index is placed close to the boundary of the first one, a portion of light leaks into the second medium. The penetrated electromagnetic field distribution can be influenced by another coherent light in the low refractive index medium via interference, leading to light amplification. Upon this technique, we introduce coherent all-optical transistors based on photonic crystal structures. Subsequently, we inspect the shortest pulse which is amplified by the designed system and also its terahertz repetition rate. We will show that such a system can operate in a cascade form. Operating in terahertz range and the amplification efficiency of around 20 are of advantages of this system.

Thermodynamics of Light Extraction from Luminescent Materials

Light extraction from luminescent materials, where luminescence generated in a high refractive index medium must be coupled to a lower index medium, is a complex problem with significant ramifications for efficient LED and phosphor converted LED lighting. We derive thermodynamic arguments which show that light transmission for incident Lambertian light through arbitrary structured or non-structured surfaces is always limited by the ratio of incident to output étendues. Numerical simulations of various strongly scattering surfaces in the wave-optic regime are made to confirm the surface extraction results. The results are also extended to arbitrary angular radiance distributions of incident light as in Lenef, et al. [Opt. Lett., 39, 3058 (2014)], but also generalized to cases of restricted angular output emission. Furthermore, we show how cavity effects and scattering are both needed to overcome the thermodynamic surface extraction limits. Examples of both surface scattering and a converted LED with volume scattering are given. Finally, a derivation of the Lambertian light transmission theorem is developed from scalar wave-optics to highlight the physics of the thermodynamic result and to provide limits of applicability of the surface extraction limit.

Beyond the perturbative description of the nonlinear optical response of low-index materials.

We show that standard approximations in nonlinear optics are violated for situations involving a small value of the linear refractive index. Consequently, the conventional equation for the intensity-dependent refractive index, n(I)=n0+n2I, becomes inapplicable in epsilon-near-zero and low-index media, even in the presence of only third-order effects. For the particular case of indium tin oxide, we find that the χ(3), χ(5), and χ(7) contributions to refraction eclipse the linear term; thus, the nonlinear response can no longer be interpreted as a perturbation in these materials. Although the response is non-perturbative, we find no evidence that the power series expansion of the material polarization diverges.

Augmented low index waveguide for confining light in low index media

AbstractGuiding light in the low index region of a high refractive index contrast waveguide can be beneficial for many applications including sensing, nonlinear optics and electro‐optics. Existing methods to achieve this goal suffer from fabrication complexity, large loss, or poor optical confinement. We propose a simple structure to achieve a significant enhancement of light confinement in the low index medium. We explain the guiding principle of this structure using geometrical optics, and suggest a number of applications where this guiding structure can provide significant performance benefits. The combination of simplicity, ease of fabrication, and good confinement makes this waveguide an attractive choice for a wide range of applications. To illustrate this, we consider the integration of a nonlinear polymer with a silicon photonic waveguide, and show that significantly better performance with an easier fabrication process can be achieved using this new scheme. image

Hybrid plasmonic waveguide consisting of two identical semiconductor nanowires and metal film with semi-cylinder ridges

In this article, a hybrid plasmonic waveguide (HPW) consisting of a thin metal film sandwiched between two identical cylindrical semiconductor nanowires is proposed and investigated numerically. With two air grooves carved symmetrically on the upper and lower surfaces of the metal film and two nanoscale semi-cylindrical ridges formed, the structured metal film and the semiconductor nanowires are embedded in a low-index silicon-dioxide medium. Based on the finite element method, our simulation results show that the proposed HPW can achieve a propagation length longer than 1000 μm in all circumstances, as well as a mode area as small as 5.21 × 10−4λ2, and an excellent figure of merit. The high-performance of the novel HPW may provide theoretical guidance for further research of HPW and related applications in photonic integrated circuits.

Model Analysis of Ridge and Rib Types of Silicon Waveguides With Void Compositions

Detailed numerical study on mode characteristics and light confinement properties of ridge and rib waveguides with vertical, horizontal, and cross types of void configurations is presented. The study starts from the mode analysis for rectangular ridge and rib waveguides. Then, the analysis is extended to semicircular ridge and rib waveguides with and without slots. The percentages of the electric and magnetic field components and light confinement capabilities are compared. We observe that polarization type of the mode and strong field confinement in the low refractive index medium can be achieved by the incorporation of void structures into the regular optical waveguide.

Omnidirectional reflector using one-dimensional dispersive photonic heterostructure

We present theoretical design for omnidirectional reflector (ODR) using one-dimensional photonic heterostructure. With the help of transfer matrix approach, we present a simple design to achieve the ODR at a desired wavelength range of interest using a high refractive index material which is dispersive at that wavelength range. Taking material combinations of SiO2 and GaSb as low and high index medium a perfect ODR is designed in the 1480–1680nm wavelength region.

超低折射率特异材料天线

超低折射率特异材料天线

Digital in-line holography in a droplet with cavitation air bubbles

In this publication, the modelisation of an air bubble as inclusion in a droplet is treated from scalar theory point of view (Fresnel’s theory). The elaborated model is compared with Lorenz–Mie scattering theory and with an experimental results. Circle polynomials and scaled pupil function are the background of this work to take into account the critical angle effect that arises at a transition from a higher index to a lower index medium.

A Detailed Study of a Dielectric Waveguide Structure for an Enhanced Evanescent Wave Atom Mirror

A Detailed Study of a Dielectric Waveguide Structure for an Enhanced Evanescent Wave Atom Mirror S. Ghezali, A. Taleb Abstract We are interested in a coherent atom mirror (specular atomic reflections in vacuum). A resonant dielectric waveguide structure is placed upon the prism so that a large energy is confined into the waveguide which is placed between two mediums of low index medium. The incident laser beam is resonantly coupled to a waveguide mode, through the gap, by frustrated total internal reflection. We obtain a large amplitude evanescent wave in the vacuum above the waveguide. We study the influence of the thickness of the gap and the losses inside the waveguide. Full Text: PDF DOI: 10.15640/ijpa.v2n3-4a3

Super-collimation with high frequency sensitivity in 2D photonic crystals induced by saddle-type van Hove singularities

Using detailed numerical simulations, and theoretical modeling, we predict a new super-collimation operation regime which is very sensitive on frequency. This operation regime is predicted to exist in 2D photonic crystals of dielectric rods in low index media. We explain the physical origin of this operation regime, as well as discuss how it could be of interest for implementation of low-power non-linear devices, novel sensors, as well as low-threshold lasers.

Strong confinement of light in low index materials: the Photon Cage

New photonic microstructures are proposed for an efficient light trapping in low index media. Cylindrical hollow cavities formed by bending a photonic crystal membrane are designed. Using numerical simulations, strong confinement of photons is demonstrated for very open resonators. The resulting strong light matter interaction can be exploited in optical devices comprising an active material embedded in a low index matrix like polymer or even gaz.

Gain enhancement in a V-shaped plasmonic slot waveguide for efficient loss compensation at the subwavelength scale

An active plasmonic slot waveguide comprising an inverted triangular metal wedge incorporated inside a V-shaped plasmonic groove with a low-index gain medium embedded between them is presented, and its guiding properties are investigated numerically at the wavelength of 1550nm. The presented waveguide is shown to be capable of supporting two fundamental plasmonic slot modes with high field localization to the V-shaped low-index slot region. Due to such strong optical confinement and significant field enhancement, the introduced gain in the slot could effectively compensate the propagation loss of the supported plasmonic modes. It is revealed that for the studied channel plasmonic slot and wedge plasmonic slot modes, notable gain enhancements are observable within a wide range of geometric parameters. For the considered structure with a 10–40nm-wide slot, the enhancements of gain can be as large as 11%–159% for the CPS mode while 43%–174% for the WPS mode. These values could be further improved by adopting even narrower slots. It is shown that, by introducing a gain medium with coefficients around hundreds of cm−1, the modal loss can be largely or even fully compensated, with a subwavelength mode area achievable simultaneously. These unique features of the studied V-shaped plasmonic slot waveguide might be useful for its potential applications in compact, active plasmonic components.

Broadband cloaking with transmission-line networks and metamaterial

Cloaking objects by transmission line and metamaterial is presented. Simulation results reveal that the bandwidth of this method is increased by embedding the transmission networks in a medium whose refractive index is smaller than unity. The low refractive index medium is realized by embedding thin wires in a host medium. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 22: 663–668, 2012. © 2012 Wiley Periodicals, Inc.

BANDWIDTH ENHANCEMENT OF A CLASS OF CLOAKS INCORPORATING METAMATERIALS

A new method for increasing the bandwidth (BW) of a class of cloaks is presented. Simulation results reveal that the bandwidth of this class of cloaks is increased by embedding the two- dimensional transmission networks in a medium whose refractive index is smaller than unity. The low refractive index medium is realized by embedding several thin wires in a host medium. The overall bandwidth of the proposed cloak for re∞ectance less than i25dB is revealed to be increased by more than 17% compared to its conventional counterpart.

Dielectric particle and void resonators for thin film solar cell textures

Using Mie theory and Rigorous Coupled Wave Analysis (RCWA) we compare the properties of dielectric particle and void resonators. We show that void resonators-low refractive index inclusions within a high index embedding medium-exhibit larger bandwidth resonances, reduced peak scattering intensity, different polarization anisotropies, and enhanced forward scattering when compared to their particle (high index inclusions in a low index medium) counterparts. We evaluate amorphous silicon solar cell textures comprising either arrays of voids or particles. Both designs support substantial absorption enhancements (up to 45%) relative to a flat cell with anti-reflection coating, over a large range of cell thicknesses. By leveraging void-based textures 90% of above-bandgap photons are absorbed in cells with maximal vertical dimension of 100 nm.

Total internal reflection and evanescent gain

Total internal reflection occurs for large angles of incidence, when light is incident from a high-refractive-index medium onto a low-index medium. We consider the situation where the low-index medium is active. By invoking causality in its most fundamental form, we argue that evanescent gain may or may not appear, depending on the analytic and global properties of the permittivity function. For conventional, weak gain media, we show that there is an absolute instability associated with infinite transverse dimensions. This instability can be ignored or eliminated in certain cases, for which evanescent gain prevails.

Integrated Bragg reflectors in low-index media: enabling strategies for wavelength tunability in electro-optic liquid crystals

This paper reports two configurations of Bragg reflectors based on liquid crystals confined between two small glass plates. Both approaches employ the efficient electro-optic effect in liquid crystals, which allows tunability of the reflectors by using low voltages. The molecular reorientation induced by an applied electric field implies a refractive index modulation seen by polarized light propagating into the liquid crystal. We show design criteria and profile optimization of the electrodes to induce a liquid crystal refractive index periodic modulation, providing a wavelength selective propagation of confined light in the liquid crystal. The two proposed device configurations differ for the top-bottom electrode configuration in one case and coplanar electrodes in the other case. Modeling of both configurations has been carried by calculating the applied electric field distribution and its interaction with the liquid crystal elastic properties taking into account the boundary conditions due to the alignment layer on the inner faces of the glass substrates. The calculated performance in terms of high wavelength selectivity and ultrawide spectral tuning range indicate that the two designed structures can be proposed for both optical filtering and to produce novel low power integrated distributed feedback resonators in dense wavelength division multiplexed fiber optic systems.

Gain enhancement in a hybrid plasmonic nano-waveguide with a low-index or high-index gain medium

A theoretical investigation of a nano-scale hybrid plasmonic waveguide with a low-index as well as high-index gain medium is presented. The present hybrid plasmonic waveguide structure consists of a Si substrate, a buffer layer, a high-index dielectric rib, a low-index cladding, a low-index nano-slot, and an inverted metal rib. Due to the field enhancement in the nano-slot region, a gain enhancement is observed, i.e., the ratio ∂G/∂g >1, where g and G are the gains of the gain medium and the TM fundamental mode of the hybrid plasmonic waveguide, respectively. For a hybrid plasmonic waveguide with a core width of w(co)=30nm and a slot height of h(slot)=50nm, the intrinsic loss could be compensated when using a low-index medium with a moderate gain of 176dB/cm. When introducing the high-index gain medium for the hybrid plasmonic waveguide, a higher gain is obtained by choosing a wider core width. For the high-index gain case with h(slot)=50nm and w(co)=500nm, a gain of about 200dB/cm also suffices for the compensation of the intrinsic loss.

Optical surface Bloch modes of complete photonic bandgap materials as a basis of optical sensing

Surface Bloch modes induced on three-dimensional dielectric photonic crystals with a complete photonic bandgap are evanescently decaying states at surfaces and have large field overlap with low-index media, resulting in narrow spectrum linewidth and simultaneously a large resonance shift due to a perturbation of the refractive index in the background media. Surface Bloch resonance states are analyzed for (001), (100), and (110) woodpile surface planes. Low-loss, high-sensitivity surface Bloch modes are also analyzed on a flat-top (001) woodpile planar surface. These analyzed surface Bloch modes are confined in a subwavelength scale and are expected to form a basis set used for optical resonance sensing.