Asymmetric Slab Waveguide Research Articles

Modal properties of an Erbium-doped asymmetric single-mode slab waveguide in the glass-ceramics SnO2-SiO2 system

We investigate the modal properties (effective index and modal gain) of an active, Erbium-doped single-mode slab waveguide made of silica tin-dioxide glass-ceramics on Silica. Completed with a photo-inscribed Bragg grating, such a structure is a good candidate for realizing an optically pumped laser emitter. For total Erbium concentration of 9.84 × 10−4 mol/cm3 and a cross-section (for stimulated transitions) of σ ≈ 2 × 10−25 m2 in a discussed structure, the maximum achievable material gain in DFB with Quarter-Wave phase Shift (QWS-DFB) is σ Ntot ≈ 23 m−1. Taking into account the confinement factor, the maximum achievable modal gain, in complex amplitude, over the length of 10 mm is at about (αmaxL) ≈ 0.09.

Propagation of Electromagnetic Waves in Slab Waveguide Structure Consisting of Chiral Nihility Claddings and Negative-Index Material Core Layer

The dispersion equation of an asymmetric three-layer slab waveguide, in which all layers are chiral materials is presented. Then, the dispersion equation of a symmetric slab waveguide, in which the claddings are chiral materials and the core layer is negative index material, is derived. Normalized cut-off frequencies, field profile, and energies flow of right-handed and left-handed circularly polarized modes are derived and plotted. We consider both odd and even guided modes. Numerical results of guided low-order modes are provided. Some novel features, such as abnormal dispersion curves, are found.

Guided waves in asymmetric hyperbolic slab waveguides: the TM mode case

Nonlinear guided wave modes in an asymmetric slab waveguide formed by an isotropic dielectric layer placed on a linear or nonlinear substrate and covered by a hyperbolic material are investigated. Optical axis is normal to the slab plane. The dispersion relations for TM waves are found. It is shown that there are additional cut-off frequencies for each TM mode. The effects of the nonlinearity on the dispersion relations are investigated and discussed. There are the modes, which are corresponded with situation where the peak of electric field is localized in the nonlinear substrate. These modes are absent in the linear waveguide. To excite these modes the power must exceed certain threshold value.

PassivePT-symmetric couplers without complex optical potentials

In addition to the implementation of parity-time ($\mathcal{PT}$)-symmetric optical systems by carefully and actively controlling the gain and loss, we show that a $2\times 2$ $\mathcal{PT}$-symmetric Hamiltonian has a unitarily equivalent representation without complex optical potentials in the resulting optical coupler. Through the Naimark dilation in operator algebra, passive $\mathcal{PT}$-symmetric couplers can thus be implemented with a refractive index of real values and asymmetric coupling coefficients. This opens up the possibility to implement general $\mathcal{PT}$-symmetric systems with state-of-the-art asymmetric slab waveguides, dissimilar optical fibers, or cavities with chiral mirrors.

Dispersion properties of lossy, dispersive, and anisotropic left-handed material slab waveguide

An asymmetric three-layer slab waveguide with air as a substrate, lossless dielectric medium as a guiding layer, and lossy, dispersive, and anisotropic left-handed material (LHM) as a cladding was investigated. The LHM cladding layer was assumed to exhibit positive permittivity and permeability in the transverse directions and negative ones along the longitudinal direction (z-axis). The dispersion equations were derived and corresponding dispersion curves were plotted and studied for TE and TM polarized light. The dispersion properties were studied with different parameters of the proposed structure. A set of interesting features were observed such as the existence of the fundamental mode in a narrow frequency range, positive group velocity which becomes faster as the frequency increases, and crucial dependence of the dispersion curves on the anisotropy of the LHM cladding.

Oblique incidence of semi-guided waves on rectangular slab waveguide discontinuities: A vectorial QUEP solver

The incidence of thin-film-guided, in-plane unguided waves at oblique angles on straight discontinuities of dielectric slab waveguides, an early problem of integrated optics, is being re-considered. The 3-D frequency domain Maxwell equations reduce to a parametrized inhomogeneous vectorial problem on a 2-D computational domain, with transparent-influx boundary conditions. We propose a rigorous vectorial solver based on simultaneous expansions into polarized local slab eigenmodes along the two orthogonal cross section coordinates (quadridirectional eigenmode propagation QUEP). The quasi-analytical scheme is applicable to configurations with — in principle — arbitrary cross section geometries. Examples for a high-contrast facet of an asymmetric slab waveguide, for the lateral excitation of a channel waveguide, and for a step discontinuity between slab waveguides of different thicknesses are discussed.

Ultra-compact and fabrication-tolerant polarization rotator based on a bend asymmetric-slab waveguide

We propose and analyze a polarization rotator based on a bend asymmetric-slab waveguide on the silicon-on-insulator platform. The device can be fabricated using standard complementary metal-oxide-semiconductor process involving only two dry etching steps. Compared with the formerly reported polarization rotators based on two-step etching, our introduced device demonstrates a significant improvement for fabrication tolerance. Furthermore, an ultra compact structure of ~5 μm conversion length, an insertion loss of only 0.5 dB, and an extinction ratio of >40 dB for both TE to TM polarization conversion and TM to TE polarization conversion are exhibited. Operation wavelength and the influence of environmental temperature on our device are also discussed.

Guided modes in slab waveguides with negative index cladding and substrate

In this work, three normalized parameters are used to study the dispersion characteristics of three different asymmetric slab waveguide structures. Each structure contains at least one layer made of left-handed material (LHM). The first structure comprises a LHM cladding layer, the second contains a LHM substrate layer, and the third has a LHM cladding and substrate. The results show that the dispersion characteristics of these structures differ from conventional guided waves in waveguide structures comprising right-handed materials. A number of unusual properties are observed including the dispersion characteristics of the fundamental mode for the first structure is different for different μc and μs. The fundamental mode exists and can only propagate within a small frequency range in the second structure. In the third structure, the fundamental mode does not exist whereas the first-order mode exists in a narrow frequency range. No propagation modes in some frequency ranges is observed which enables the structure to behave as a bandpass filter.

Nanoscale Optical Dielectric Rod Antenna for On-Chip Interconnecting Networks

A nanoscale on-chip optical dielectric rod antenna is demonstrated in this paper. The antenna is designed and fabricated on a 200-mm silicon-on-insulator platform, using IMEC 193-nm-deep UV lithography. A 500-nm-thick polymer layer is designed and deposited to act as an asymmetric slab waveguide, confining the radiated wave within the layer. Full-wave analysis predicts antenna return loss above 25 dB, and end-fire gain greater than 9 dBi from 172 to 222 THz. Six antenna pairs with 1-, 3-, 5-, 7-, 12-, and 17- μm separations are fabricated. Corresponding transmissions are measured from 190 to 200 THz. Two on-chip optical signal hubs composed of 16 and 32 antennas designed for core-to-core interconnection for the next-generation multicore microprocessors are also demonstrated. Good agreement between the modeling and measurement is obtained.

Optical amplification of two different propagation modes in organic small molecular waveguide structure

We investigate the emission spectra from the edge of optically pumped waveguide. The waveguide is based on vacuum-deposited thin films of small molecular, 2,5,2',5'-tetrakis(2,2-diphenylvinyl)biphenyl (TDPVBi). Narrowed emissions are observed both at high (> 6 KW/cm2) and low (< 1 W/cm2) pump power sources, which are attributed to two different propagation modes in the asymmetric slab waveguides, guided mode and cutoff mode, respectively. The peak wavelengths of the guided mode appear at the maximum of the photoluminescence (PL) spectrum of the TDPVBi film. In contrast, both the peak wavelength and polarization of the cutoff mode are thickness dependent. The optical gains of the two modes are measured by the variable stripe length (VSL) method. The amplification with an exceptional low threshold for the cutoff mode has been demonstrated. Our results suggest that the cutoff mode is a promising route for the reduction of lasing threshold.

Laser emission from mirrorless waveguides based on photosensitized polymers incorporating POSS

Laser emission from leaky waveguides based in dye-doped organic gain media incorporating Polyhedral Oligomeric Silsesquioxanes (POSS) nanoparticles is reported. The samples consist of thin film gain media deposited onto glass substrate defining a planar asymmetric slab waveguide, which does not incorporate any resonant substructure. The presence of POSS results in additional amplified spontaneous emission (ASE) spectral narrowing, and conditions have been found for which directional multimode laser emission is achieved. The spectral narrowing is ascribed to the photon path enlargement caused by a non-resonant feedback mechanism provided by individual scatterers, which enhances incoherently the magnitude of the amplification process. On the contrary, the appearance of multimode lasing is attributed to coherent random lasing from a many scatterers collective effect.

Out-of-plane scattering from vertically asymmetric photonic crystal slab waveguides with in-plane disorder

We characterize optical wave propagation along line defects in two-dimensional arrays of air-holes in free-standing silicon slabs. The fabricated waveguides contain random variations in orientation of the photonic lattice elements which perturb the in-plane translational symmetry. The vertical slab symmetry is also broken by a tilt of the etched sidewalls. We discuss how these lattice imperfections affect out-of-plane scattering losses and introduce a mechanism for high-Q cavity excitation related to polarization mixing.

Transmission properties of asymmetric slab waveguides with left-handed materials

A new dielectric slab waveguide with a left-handed material (LHM) cover and substrate is proposed. The dispersion relations and normalized effective thickness of the asymmetric LHM slab waveguide are investigated, in view of the normalized parameters. A number of unusual properties are found, for example, the fundamental and first-order modes do not exist and higher-order modes have double degeneracy. The propagation modes are absent at the low normalized frequency, and the cutoff frequencies of some LHM slab waveguide modes decrease with increase in the asymmetry measure. Unlike traditional slab waveguides, the V –H curves of the LHM slab waveguides are in one-to-one correspondence. Both TE and TM modes are discussed; in addition, the dispersion relations and normalized effective thicknesses of the TM modes are discussed in detail, when the difference in refractive indices of the film and the substrate is small. The results show that the region of mode coexistence taking place near the cutoff frequency becomes narrower with increase in the difference in refractive indices of the film and the substrate. The influence of this difference on the normalized effective thickness curves is different, and becomes smaller and smaller with increase in the value of the asymmetry measure, if different values of the refractive indices are employed.

Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films

The authors investigated the characteristics of spectrally narrow emissions at the cutoff wavelength from the edges of optically and electrically pumped organic semiconductor films. They estimated the optical properties of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) films using variable angle spectroscopic ellipsometry, revealing that the BSB-Cz films have uniaxial anisotropy and that BSB-Cz molecules lie nearly parallel to substrate surfaces. The authors discuss here the spectral changes in the edge emissions from neat BSB-Cz and tris(8-hydroxyquinoline)aluminum (Alq3) films of 15 different thicknesses under continuous-wave (cw) optical excitation. The spectrally narrow emissions of transverse electric (TE) and transverse magnetic (TM) modes were observed. The authors showed that the peak wavelengths of these emissions varied according to the cutoff wavelengths of asymmetric slab waveguides and that their spectral widths changed depending on the angular dispersion of the Fabry–Pérot interferometer composed of the organic film and its interfaces. The edge emissions from the BSB-Cz films had narrower bands than those from the Alq3 films and increased with a superlinear dependence on the stripe length of the excitation area, indicating the occurrence of light amplification under cw optical excitation. Finally, the authors demonstrate spectrally narrow emissions from the edges of electrically pumped organic light-emitting diodes (OLEDs) having a fine line-shaped waveguide structure with a silver metal cladding layer. Narrow emissions of TE mode were observed, and the peak wavelengths of the emission spectra corresponded well to the cutoff wavelength of the waveguide with the silver cladding layer. In addition to the narrow emissions, the authors observed some intriguing phenomena suggestive of the occurrence of light amplification in OLEDs under electrical pumping.

Enhancing the radiative decay rate of fluorescent organic molecules using micropillar microcavities and optical nanocavities

We discuss the enhancement of spontaneous emission of fluorescent organic dyes when placed in micropillar microcavities and also in an optical nanocavity. We show experimentally that an enhancement in the spontaneous-emission rate of 1.78 ± 0.04 times can be realized by placing a thin-film of the molecular dye Lumogen Red dispersed in a polymer matrix into a 3 μm diameter micropillar having a Q-factor of 680. We then model the optical properties of Noda nanocavities in vertically symmetric and asymmetric slab waveguides using TiO 2 as a high-index dielectric, with the cavity mode wavelength tuned to 713 nm. We estimate that such cavities will have a maximum Q-factor of 4455 and a Purcell Factor (maximum enhancement in the spontaneous-emission rate) of 340.

Fabrication and Properties of an Asymmetric Waveguide Containing Nanoparticles

A wavelength-tunable light-emitting source consisting of a multilayer asymmetric waveguide structure was designed, modeled, and fabricated on a silicon substrate. The structure has two layers: a silica cladding and a zirconia-Glymo (γ-glycidoxypropyltrimetoxysilane) sol–gel-based waveguide layer. CdS and CdSe/ZnS colloidal nanoparticles (NPs) used as light-emission sources were incorporated into the waveguide layer. These NPs are however unstable in a low-pH environment. A zirconia-Glymo sol–gel film using acetylacetone as the zirconium chelating agent was developed, thus eliminating the often used acetic acid from the sol–gel synthesis and preserving the NPs in the waveguide formation process. The acetylacetone chelating agent allowed for a high degree of cross-linking between the Zr and the Glymo as indicated by the Zr-O-Si absorption observed by infrared spectroscopy. The incorporation of the photoemissive CdSe/ZnS core/shell nanoparticles within the zirconia gel asymmetric slab waveguide was demonstrated as a means to create miniature multispectral light sources.

TM modes in asymmetric slab waveguides with left-handed material

Three-layer slab waveguide with left-handed materials (LHM) is discussed using the Maxwell’s equations and their equations of dispersion relation for TM modes are obtained. By seeking the power of the slab waveguide, their equations of effective thickness for TM modes are obtained. Furthermore, the interrelated equations for right-handed materials (RHM) slab waveguide are introduced and their curves plotted. To find new properties for LHM slab waveguide, the equations and their curves for the LHM and RHM slab waveguide are compared respectively.

Effects of metal irises on the guided-mode properties in asymmetrical slab waveguides

The diffraction phenomenon caused by metal transverse irises placed into an asymmetrical slab waveguide is examined by using the integral equation method. We concentrate on the possibility of controlling the radiation characteristics of the structure by changing the irises positions and the slab waveguide asymmetry. The aperture electric-field distribution is expressed in terms of a finite series of Chebyshev polynomials. The dominant TE guided-mode reflection and transmission coefficients, the near-field distribution and the far-field radiation pattern are calculated, while numerical results are presented for several cases of asymmetrical slab waveguides and different irises’ positions.

Wavelength-selective core-to-jacket coupling in optical fiber

Mode coupling between the fundamental core mode of an as-drawn optical fiber operating far from cutoff and higher-order modes in the plastic jacketing has been observed. In one fiber, the transmission spectrum of the core exhibited five distinct minima between 1200 and 1900 nm. By approximating the fiber jacket as an asymmetric slab waveguide of infinite lateral extent, the wavelengths of these transmission features were successfully predicted using coupled-mode theory. Since conditions at the jacket surface influence the transmission spectrum, this coupling effect potentially enables new applications for environmental sensing.

Optical fibers with high-index-contrast dielectric thin films

The authors analyze electromagnetic wave propagation through optical fibers consisting of a glass core surrounded by a high-refractive-index dielectric thin film, in turn surrounded by a glass cladding. The asymmetry of the structure due to different core and cladding refractive indices results in two types of guided modes: they propagate either in the core and film or only in the film. The dispersion equations are derived, and mode classifications are established for transverse and hybrid modes. The cutoff conditions of transverse modes are given. Numerical simulations are performed to obtain the effects of film index and thickness on the cutoff frequencies and dispersion curves. It is shown that a high-index film causes the modes to become evanescent in the core and guided only in the film. These modes form two types of linearly polarized modes, according as they follow the TE or the TM mode propagation curves of asymmetric slab waveguide. Investigation of power flow along the propagation direction shows that the higher the refractive index of the film, the higher the power density in the film.