Optical Waveguide Modes Research Articles

Calculation of bending losses for highly confined modes of optical waveguides with transformation optics

We revisited the fundamental problem of bending loss calculation in optical waveguide theory using transformation optics (TOs). Due to the fact that TOs is based on the form invariance property of Maxwell equations, this new approach provides more accurate calculation of waveguide bending loss compared to the conventional refractive index conformal mapping method, especially for small bending radii typical for plasmonic waveguides or photonic waveguides with high-index contrast. We believe our results provide a simple yet reliable way of bending loss calculation for highly confined optical (including plasmonic) waveguides.

Direct fabrication of PDMS waveguides via low-cost DUV irradiation for optical sensing

We demonstrate the fabrication of single mode optical waveguides by irradiating polydimethylsiloxane (PDMS) with a low cost Hg lamp through a conventional quartz mask. By increasing the refractive index of the irradiated areas, waveguiding is achieved with an attenuation of 0.47 dB/cm at a wavelength of 635 nm. The refractive index change is stable in ambient air and water for time periods of more than 3 months. The excitation of water-dispersed fluorescent nanoparticles in the evanescent field of the waveguide is demonstrated.

Biosensor based on hydrogel optical waveguide spectroscopy for the detection of 17β-estradiol

Rapid detection of hormones at sub-ng/ml concentrations is of tremendous importance for diagnostic purposes, quality control, and environmental monitoring. In this respect, we report a novel label-free biosensor based on hydrogel optical waveguide spectroscopy (HOWS) for the sensitive detection of 17β-estradiol (E2). This approach was implemented by using a thin hydrogel layer of a carboxylated poly(N-isoproprylacrylamide) (PNIPAAm) terpolymer that was attached to a metallic sensor surface in order to simultaneously serve as a binding matrix and an optical waveguide. Refractive index changes that are accompanied with the specific capture of biomolecules from an aqueous sample on the sensor surface were probed by resonantly excited hydrogel optical waveguide modes. To optically excite and interrogate these waves, an optical setup based on Kretschmann configuration of attenuated total reflection (ATR) method that is compatible with surface plasmon resonance (SPR) was used. We demonstrate that HOWS offers a higher binding capacity, good anti-fouling properties, improved figure of merit, and E2 detection limit of 50pg/ml which is seven times better than that obtained by a regular surface plasmon resonance (SPR) biosensor.

Plasmonic Nanostructure for Enhanced Light Absorption in Ultrathin Silicon Solar Cells

The performances of thin film solar cells are considerably limited by the low light absorption. Plasmonic nanostructures have been introduced in the thin film solar cells as a possible solution around this issue in recent years. Here, we propose a solar cell design, in which an ultrathin Si film covered by a periodic array of Ag strips is placed on a metallic nanograting substrate. The simulation results demonstrate that the designed structure gives rise to 170% light absorption enhancement over the full solar spectrum with respect to the bared Si thin film. The excited multiple resonant modes, including optical waveguide modes within the Si layer, localized surface plasmon resonance (LSPR) of Ag stripes, and surface plasmon polaritons (SPP) arising from the bottom grating, and the coupling effect between LSPR and SPP modes through an optimization of the array periods are considered to contribute to the significant absorption enhancement. This plasmonic solar cell design paves a promising way to increase light absorption for thin film solar cell applications.

Optical waveguide mode control by nanoslit-enhanced terahertz field

In this Letter we propose a scheme providing control over an optical waveguide mode by a terahertz (THz) wave. The scheme is based on an optimization of the overlap between the optical waveguide mode and the THz field, with the THz field strength enhanced by the presence of a metallic nanoslit surrounding the waveguide. We find an optimum balance between the optical mode attenuation and Kerr-induced change in the propagation constant. The criterion for a π/2-cumulative phase shift, for instance for application in a Mach-Zehnder interferometer configuration, requires 10 kV/cm THz field, which in turn is estimated to result in a nonlinear change of the refractive index in the waveguide of 0.001. Our simulations prove that it is quite reasonable to observe the effect experimentally.

Analysis of coupling strength apodization among the three co-directional guided modes in optical period dielectric waveguides

We describe and analyze the coupling strength apodization effect among the three co-directional guided modes using the supermodes theory. The propagation condition is related to the two phase matching distribution induced by two long period co-spatial gratings. It is seen that one input mode can be converted into other two modes with one mode having two peaks at its transmission spectrum. Through adopting suitable apodization functions and adjusting the maximum coupling apodization strength from 100 to 1500, the frequency space between the two peaks in one converted mode can be tuned nearly linearly from 336GHz to 5400GHz. Moreover, the other converted mode which has one peak and maximum output power around the central frequency can be used in the transmission operation mode. This kind of design can form the basis for a new generation of optical filtering and mode converting.

A Multidomain Pseudospectral Mode Solver for Optical Waveguide Analysis

We propose a pseudospectral mode solver for optical waveguide mode analysis formulated by the frequency-domain Maxwell equations. Special attention is paid upon identifying the required boundary operator for the formulation and the relationships between the derived operator and the physical boundary conditions. These theoretical results are adopted into a Legendre pseudospectral multidomain computational framework to compute the propagation characteristics of optical waveguides. Numerical experiments are conducted, and the expected spectral convergence of the scheme is observed for smooth problems and for problems having field jumps at material interfaces. For dielectric waveguides with sharp corners, the spectral convergence is lost due to the singular nature of fields at the corner. Nevertheless, compared with other methods, the present formulation remains as an efficient approach to obtain waveguide modes.

Wideband slow-light modes for time delay of ultrashort pulses in symmetrical metal-cladding optical waveguide

A widebandwidth optical delay line is a useful device for various fascinating applications, such as optical buffering and processing of ultrafast signal. Here, we experimentally demonstrated effective slow light of sub-picosecond signal over 10 THz frequency range by employing the wide slow light modes in thick symmetrical metal-cladding optical waveguide (SMCOW). Ultrahigh-order guided modes travelling as slow light in waveguide together with strong confinement provided by metal-cladding makes this scheme nearly material dispersion independent and compatible with wide bandwidth operation.

Numerical analysis of the three co-directional guided modes in optical period dielectric waveguides using inverse Gaussian apodization

We describe and analyze the inverse Gaussian apodization among the three co-directional guided modes using the supermodes theory and the transfer matrix method. The propagation condition is related to the two phase matching distribution induced by two long period co-spatial gratings. It is seen that one input mode can be converted into another two modes with each mode having two peaks at its transmission spectrum. Through adopting the inverse Gaussian apodization, the input mode A turns to having two peaks while the converted mode B has six peaks and mode C has three peaks around the central frequency in the transmission spectrum. Of particular interest is when adjusting the maximum coupling apodization strength from 100 to 1500, the frequency space in mode A changes nearly linearly from 340 GHz to 5100 GHz which is twice of the mode B at each coupling strength, while the mode C can be turned nearly linearly from 350 GHz to 5200 GHz. Additionally, the power at the central frequency of input mode A will almost totally move to the converted mode C. This kind of design can form the basis for a new generation of multi-wavelength-tunable bandpass optical filtering mode converting, add/drop multiplexing and optical switching.

Retracted: Optimization of radiation modes in asymmetric planar slab optical waveguides

In this paper, we try to optimize the substrate-radiation/substrate-cladding (cover) radiation modes in terms of their performance parameters. It is well known that the guided modes can only be normalized. However, the radiation modes can also be normalized using the delta/Dirac function. We try to optimize the waveguide design parameters for the known cases to achieve performance as good as guided modes. The formal electromagnetic theory is applied to study the radiation modes. The normalization condition on radiation mode has been carefully used during analysis. The results are found to be satisfactory. It has been concluded that we can modify the performance of radiation modes according to our requirements. The performance is compared with the guided mode. The paper discusses guidelines to optimize the radiation modes for various constraints.

Thin hydrogel films for optical biosensor applications.

Hydrogel materials consisting of water-swollen polymer networks exhibit a large number of specific properties highly attractive for a variety of optical biosensor applications. This properties profile embraces the aqueous swelling medium as the basis of biocompatibility, non-fouling behavior, and being not cell toxic, while providing high optical quality and transparency. The present review focuses on some of the most interesting aspects of surface-attached hydrogel films as active binding matrices in optical biosensors based on surface plasmon resonance and optical waveguide mode spectroscopy. In particular, the chemical nature, specific properties, and applications of such hydrogel surface architectures for highly sensitive affinity biosensors based on evanescent wave optics are discussed. The specific class of responsive hydrogel systems, which can change their physical state in response to externally applied stimuli, have found large interest as sophisticated materials that provide a complex behavior to hydrogel-based sensing devices.

Analysis of Silicon-On-Insulator (SOI) Buried Waveguide Phase Modulator

The analyses of the simulation of a single mode buried waveguide optical phase modulator based on SOI material are here reported. The structure has been simulated by Athena from Silvaco simulation package. The buried waveguide is created by doping phosphorus with concentration of 10e15 cm-3 into the substrate. The real refractive index and the absorption coefficient of the waveguide are changed using the free carrier dispersion effect via carrier injection of a pn junction. The efficiency, VπLπ is calculated and the performance is compared with that of the rib waveguide optical phase modulator of the same material and dimensions. Simulation shows that the device can be an efficient device for application in intensity modulation.

Analysis of the temperature coefficient of effective refractive index of the TE1 and TM1 modes in optical sol-gel waveguides

The relation between the temperature coefficient of effective refractive index and the density of the wave field in the sol-gel film is determined. Dependences of the temperature coefficient of effective refractive index on the parameters of the sol-gel film are obtained. Features of the temperature dependence of the effective refractive index are revealed. Differences between the temperature characteristics of sol-gel waveguides with the TE and TM modes are found and explained.

Photocrosslinkable dextran hydrogel films as substrates for osteoblast and endothelial cell growth

Functional hydrogel films on solid supports are versatile materials with large potential for cell growth and tissue engineering. Here, we report on a modular approach to generate functional hydrogel composite films for endothelial and osteoblast cell co-culture. The polymer network of the parent hydrogel was formed by a dextran derivative (BP-CMD), which contained carboxymethyl (CM) groups for further chemical functionalization and benzophenone (BP) moieties as a photocrosslinkable unit. BP-CMD could be synthesized by three different routes, first with the benzophenone unit attached via an amide bond, or second by an ether bond, or third as an ion pair between the benzophenone ammonium salt and the carboxylate groups on the dextran backbone. For composite formation, BP-functionalized silica nanoparticles, as well as gelatin particles were mixed with BP-CMD and permanently fixed in the hydrogel matrix by photocrosslinking for mechanical reinforcement and to facilitate cell growth. In the last preparation step the BMP-2 growth factor was covalently coupled to the polymer backbone, which enhanced osteoblast and endothelial cell growth. The swelling behavior and successful BMP-2 immobilization of the hydrogel composite films were investigated with surface plasmon resonance and optical waveguide mode spectroscopy coupled with fluorescence detection. This modular approach allows independent selection and optimization of each component in the hydrogel composite for a wide range of potential applications for targeted cell growth, as successfully shown here with osteoblast–endothelial cell co-culture for bone tissue regeneration.

A search-and-track algorithm for controlling the number of guided modes of planar optical waveguides with arbitrary refractive index profiles

A search-and-track algorithm is proposed for controlling the number of guided modes of planar optical waveguides with arbitrary refractive index profiles. The algorithm starts with an initial guess point in the parameter space that supports a specific number of guided modes. Then, it searches for, and tracks, the boundaries of this space or another space supporting different number of modes. It does so by monitoring the sign of a unified cutoff dispersion function. The algorithm is applied to both symmetric and asymmetric silicon-based parabolic-index waveguides. It shows that unlike asymmetric waveguides, the single-mode condition of symmetric waveguides is controlled by TM-, as opposed to TE-, polarization. This abnormal polarization control is strongest for high index contrast waveguides of sub-micrometer core sizes. The results are verified by full-vectorial beam propagation method.

Magnetic Composite Thin Films of FexOy Nanoparticles and Photocrosslinked Dextran Hydrogels

Magnetic hydrogel composites are promising candidates for a broad field of applications from medicine to mechanical engineering. Here, surface-attached composite films of magnetic nanoparticles (MNP) and a polymeric hydrogel (HG) were prepared from magnetic iron oxide nanoparticles and a carboxymethylated dextran with photoreactive benzophenone substituents. A blend of the MNP and the dextran polymer was prepared by mixing in solution, and after spin-coating and drying the blend film was converted into a stable MNP–HG composite by photocrosslinking through irradiation with UV light. The bulk composite material shows strong mobility in a magnetic field, imparted by the MNPs. By utilizing a surface layer of a photoreactive adhesion promoter on the substrates, the MNP–HG films were covalently immobilized during photocrosslinking. The high stability of the composite was documented by rinsing experiments with UV–Vis spectroscopy, while surface plasmon resonance and optical waveguide mode spectroscopy was employed to investigate the swelling behavior in dependence of the nanoparticle concentration, the particle type, and salt concentration.

Higher-Order Poincaré Sphere, Stokes Parameters, and the Angular Momentum of Light

A higher-order Poincaré sphere and Stokes parameter representation of the higher-order states of polarization of vector vortex beams that includes radial and azimuthal polarized cylindrical vector beams is presented. The higher-order Poincaré sphere is constructed by naturally extending the Jones vector basis of plane wave polarization in terms of optical spin angular momentum to the total optical angular momentum that includes higher dimensional orbital angular momentum. The salient properties of this representation are illustrated by its ability to describe the higher-order modes of optical fiber waveguides, more exotic vector beams, and a higher-order Pancharatnam-Berry geometric phase.

Light Localization in Woodpile Photonic Crystal Built via Two-Directional Etching Method

This paper describes designs, fabrication, and optical properties of subwavelength-scale light localization components in 3-D woodpile photonic crystal materials. Designed microcavities and waveguide geometries can be built with N-directional etching fabrication method (N = 2), which produces structures consisting of a combination of N 2-D air trench geometries. The microcavities and waveguides consist of straight dielectric rods only. In surface reflectance measurements, fabricated woodpile photonic crystals exhibit high reflectivity due to a photonic bandgap in infrared optical wavelength ranging from 1200 nm to about 1550 nm, matching with numerical calculation results. A variety of woodpile photonic crystals are fabricated with 2-D etching method, including (100) and (110) woodpile photonic crystals in silicon and gallium arsenide. This opens up the possibility of designing surface Bloch modes. A high quality factor is expected in the microcavity fabricated by vertical etching on top and side wafer facets. For a dipole mode, the Q factor of 105 requires a woodpile of 9 × 9 × 7 unit cells only. Three types of waveguide designs are studied. In a double-hetero junction woodpile, a self-collimation state is found and analyzed. Single-mode lateral and vertical optical waveguide modes in a complete photonic bandgap are presented, and would become basic components in 3-D integrated optics based on woodpile photonic crystals. Due to the confinement of light in complete photonic bandgap materials, the loss can be suppressed in a large woodpile photonic crystal.

Electro-optic and dielectric characterization of ferroelectric films for high-speed optical waveguide modulators

We investigate the electro-optic properties and high-frequency relative dielectric constants of epitaxially grown lanthanum-modified lead zirconate titanate (PLZT) and polycrystalline barium titanate (BaTiO3) films in a high-frequency range of up to 40 GHz for designing a high-speed ferroelectric thin-film modulator. We also demonstrate a ferroelectric thin-film Mach-Zehnder-type waveguide modulator using the epitaxially grown PLZT film. The use of a composite structure with a low dielectric coefficient substrate and a buffer layer enabled of a ferroelectric thin-film waveguide modulator with 40-Gb/s operation.

Photon management for photovoltaics

Abstract