Low-index Region Research Articles

Refractometer probe based on a reflective carbon nanotube-modified microfiber Bragg grating.

A carbon nanotube (CNT)-modified microfiber Bragg grating (MFBG) is proposed to measure the refractive index with a strong enhancement of the sensitivity in the low refractive index region. The introduction of the CNT layer influences the evanescent field of the MFBG and causes modification of the reflection spectrum. With the increase of the surrounding refractive index (SRI), we observe significant attenuation to the peak of the Bragg resonance, while its wavelength remains almost unchanged. Our detailed experimental results disclose that the CNT-MFBG demonstrates strong sensitivity in the low refractive index range of 1.333-1.435, with peak intensity up to -53.4 dBm/refractive index unit, which is 15-folds higher than that of the uncoated MFBG. Therefore, taking advantage of the CNT-induced evanescent field enhancement, the reflective MFBG probe presents strong sensing capability in biochemical fields.

Hybrid rib-slot-rib plasmonic waveguide with deep-subwavelength mode confinement and long propagation length

We propose a hybrid plasmonic waveguide where a combined rib-slot-rib structure is added on the metal substrate within a low-index gap region. The optical properties of the quasi-TM fundamental mode are numerically calculated using the finite element method. Compared to the traditional hybrid plasmonic waveguide, our designed waveguiding structure can support modes with tighter confinement and longer propagation length, by properly adjusting the geometry of the rib-slot-rib structure and the gap height. In details, it can provide the hybrid mode with mode area λ2/12000 and reasonable propagation distance 23.78 μm, simultaneously. Its excellent optical performance can facilitate potential applications in ultra-compact nanophotonic devices and circuits.

Single-polarization hollow-core square photonic bandgap waveguide

Materials with a periodic structure have photonic bandgaps (PBGs), in which light can not be guided within certain wavelength ranges; thus light can be confined within a low-index region by the bandgap effect. In this paper, rectangular-shaped hollow waveguides having waveguide-walls (claddings) using the PBG have been discussed. The design principle for HE modes of hollow-core rectangular PBG waveguides with a Bragg cladding consisting of alternating high- and low-index layers, based on a 1D periodic multilayer approximation for the Bragg cladding, is established and then a novel single-polarization hollow-core square PBG waveguide using the bandgap difference between two polarized waves is proposed. Our results demonstrated that a single-polarization guiding can be achieved by using the square Bragg cladding structure with different layer thickness ratios in the mutually orthogonal directions and the transmission loss of the guided mode in a designed hollow-core square PBG waveguide is numerically estimated to be 0.04 dB/cm.

Optimization of One Dimensional Photonic Crystal Elliptical-Hole Low-Index Mode Nanobeam Cavities for On-Chip Sensing

We report the design of one dimensional photonic crystal nanobeam cavities with elliptical holes that is fully encapsulated in the water environment and can confine the light in the low index region. The proposed structure, based on the principle of gentle confinement of the electromagnetic field, is designed by tapering the width of the host photonic crystal waveguide away from the center of the cavity while keeping other parameters constant. With elliptical-hole low-index mode nanobeam cavities and tapered waveguide widths, through three dimensional finite-difference time-domain simulations, large band-gap and high reflectivity are achieved to confine the optical mode. Also, the electric field is confined in the elliptical holes, which helps to enhance the sensitivity. The simulation results demonstrate that we achieve the highest quality factor of $\rm{1.35\times10}^{5}$ when 15 taper segments and 15 additional mirror segments are placed on both sides of the host waveguide. A sensitivity of 390 nm/RIU (refractive index unit) and mode volume of 2.23 $\rm{(\lambda}_{\rm{res}}\rm{/n}_{\rm{si}}\rm{)}^{3}$ are achieved in the water environment, thus showing exceptionally good on-chip sensing properties with respect to high sensitivity, small footprints and masses.

Modelling of Photonic Crystal Structure by Opti FDTD

A photonic crystal is a material in which the refractive index (RI) is periodically modulated on a length scale comparable to the desired operation wavelength. Photonic crystal fibres (PCFs) are optical fibres with a periodic arrangement of low index material in a background with higher RI as compared with optical fibre, a solid thread surrounded by another material with a lower RI. The background material in PCF is usually ‘Un-doped Silica’, and the low-index region is typically provided by air holes running along their entire length. In this paper, we have designed a power divider on the basis of photonic crystal structure with the help of OptiFDTD. The numerical technique FDTD (finite difference time domain) is examined considering anisotropic perfectly matched layer for transparent boundary condition. Important properties of optical power divider such as RI, discrete Fourier transform (DFT) Ey, DFT Hx, DFT Hz, observation points, power at the input and output ports and pointing vector are investigated.

Carbon nanotube-deposited tilted fiber Bragg grating for refractive index and temperature sensing

A tilted fiber Bragg grating (TFBG) deposited with carbon nanotube (CNT) is proposed to simultaneously measure the surrounding refractive index (SRI) and temperature. Assisted by the interaction between the CNT deposition layer and the TFBG’s cladding mode, the increase in SRI could significantly attenuate the coupling intensity of cladding modes, while the resonant wavelengths keep almost unchanged. Therefore, the temperature response of the CNT-deposited TFBG could be further obtained by tracking the shifted resonant wavelength. In addition, the CNT deposition greatly enhances the sensing capability in a low refractive index region of $1\sim 1.428$ , presenting great potentials in biochemical sensing.

Single-pass and omniangle light extraction from light-emitting diodes using transformation optics.

We present a light-extraction approach allowing for single-pass and omniangle outcoupling of light from light-emitting diodes (LED). By using transformation optics, we perceive a feasible graded-index structure that is a transition from the LED exit facet to a low refractive index region with expanded space that represents air. Apart from the material dispersion of the constituents, our approach is wavelength independent. The suggested extractor is geometrically compact with size parameters comparable to the width of an LED and therefore well adapted for pixelated LEDs. A beam-expanding functionality is possible while fully preserving the outcoupling efficiency by applying index and geometry truncation.

Modal reduction in single crystal sapphire optical fiber

A new type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high and low index regions in the azimuthal direction. The structure retains a “core” region of pure single crystal (SC) sapphire in the center of the fiber and a “cladding” region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the “windmill” shaped cladding. The simulation results showed that the number of guided modes were significantly reduced in the “windmill” fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the “windmill” SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF.

High-Q and High-Sensitivity Photonic Crystal Cavity Sensor

We present the design, fabrication, and the characterization of a high-qualityfactor (Q) and high-sensitivity photonic crystal (PhC) cavity sensor on the silicon-on-insulator platform. By optimizing the structure of the PhC cavity, most of the light can be distributed in the lower index region; thus, the sensitivity can be dramatically improved. The transmission spectrum of the PhC cavity sensor is measured by immersing the device into a sodium chloride (NaCl) solution of different mass concentrations. A Q factor of 1.3 × 10 4 and a very high sensitivity of 428 nm/RIU have been experimentally demonstrated. Furthermore, the total size for the sensing part of the PhC cavity sensor is only 14.5 × 0.75 μm 2 .

Ultrahigh- <inline-formula> <tex-math notation="LaTeX">$Q$</tex-math></inline-formula> and Low-Mode-Volume Parabolic Radius-Modulated Single Photonic Crystal Slot Nanobeam Cavity for High-Sensitivity Refractive Index Sensing

We present a novel optical sensor based on the design of ultrahigh-Q and low-mode-volume 1-D single photonic crystal (PhC) slot nanobeam cavity (SNC) in which the air-hole radius is parabolically tapered. The performance of the device is investigated theoretically. In order to achieve high Q-factor and high sensitivity simultaneously, the slot geometry is exploited to make the optical field strongly localized inside the low index region and overlaps sufficiently with the analytes. With the three-dimensional finite-difference time-domain (3D-FDTD) method, we demonstrate that the proposed single 1-D PhC-SNC sensor device possess an ultrahigh sensitivity (S) up to ~900 nm/RIU (refractive index unit, RIU) and a high Q-factor in air up to > 10 7 at the telecom wavelength range. The optimized figure of merit is > 10 7 . In addition, an ultrasmall mode volume of V m ~0.01 (λ/η air )3 has been achieved, which is more than three orders of magnitude smaller than our previous works [Appl. Phys. Lett. 105, 063118 (2014)] and, thus, is potentially an ideal platform for realizing ultracompact laboratory-on-a-chip applications with dense arrays of functionalized spots for multiplexed gas sensing.

VCMIPWM Scheme to Reduce the Neutral-Point Voltage Variations in Three-Level NPC Inverter

ABSTRACTThis paper presents variable common mode injection pulse width modulation (VCMIPWM) technique to reduce the neutral-point voltage variations in three-level neutral-point-clamped (NPC) inverter. The neutral-point voltage varies with the modulation index and the variation is high in the low modulation index region. Therefore, proper common mode voltages are calculated for various modulation index regions and are carefully injected with the reference phase voltages. The calculation is based on equalizing the switching times of the zero/redundant vectors in a switching cycle for various modulation index regions. The simulation results show the improvement in the neutral-point voltage balancing of the inverter in the VCMIPWM scheme compared with the sinusoidal pulse width modulation (PWM). The proposed PWM is implemented in field programmable gate array (FPGA) and the experimental results are shown to verify the practicability of the proposed method.

An investigation of transverse localization in a disordered waveguide array containing plasma materials

We investigate wave propagation through a disordered waveguide array composed of plasma materials. We first consider a system in which both the low and high index regions are plasma materials. To introduce disorder through the system, the electron plasma densities of the high index regions are selected to be random numbers. We study the effect of disorder strength on transverse localization. Our numerical results reveal that increasing the disorder level improves the quality of the transverse localization. The dependence of the localization features on the plasma density of the low index media and average of the plasma density of the high-index regions is also studied. Localization degrades with increasing plasma density of the low index media. However, transverse localization improves with increasing average plasma density of the high-index regions. Thus, using plasma materials in the disordered photonic lattices makes it possible to control transverse localization characteristics with plasma parameters, as well as applying an external magnetic field. Second, we consider a disordered waveguide array composed alternately of normal and plasma materials. The influence of the operating wavelength variation on the transverse localization is also discussed in this disordered system. It is demonstrated that the effective width of the injected wave at the output end increases with increasing wavelength. In this case, the increase of the average refractive index of normal materials leads to the improvement of transverse localization.

Ultralow Propagation Loss Slot-Waveguide in High Absorption Active Material

A slot waveguide formed by high absorption active material is proposed to reduce the propagation loss for monolithic integration. The low propagation loss is attained by concentrating the optical field inside the low-index slot region without absorption. The simulation results show that the propagation loss at 1.55 μm for the slot waveguide can be as low as 1.5 dB/cm in active material with an absorption coefficient of 3000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , whereas the optical profile is set to be confined within 650 nm.

Slot Spiral Silicon Photonic Crystal Fiber With Property of Both High Birefringence and High Nonlinearity

A photonic crystal fiber (PCF) made of silicon is proposed and numerically optimized with both ultrahigh birefringence and nonlinearity. By introducing an elliptical low-index slot in the core region, one of the orthogonally polarized fundamental modes can be confined in the low-index region due to the discontinuity of its electric field at the interface of the slot and the surrounding material, while the other fundamental mode is dominated by the modified total internal reflection (MTIR) induced by air holes of the cladding, leading to a birefringence as high as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> order. On the other hand, benefiting from the tight field confinement and highly nonlinear material, nonlinear coefficients up to 1068 W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 162 W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> can be achieved for the slot mode and the MTIR mode, respectively.

Dispersion modified slot optical waveguides

Advent of slot waveguide structures had opened a new era where light can be confined in low index slot guarded by high index slabs. Already in use SOI slot waveguides (contrast ratio is 2.42) have two distinct properties over the conventional waveguides, i.e. high E-field amplitude, optical power, optical intensity in low index materials, and strong E-field confinement localized to nanometer-size low index regions. We hereby propose a low refractive index contrast ratio slot waveguide structure (ratio is 1.18) comprising of commercially available glass material. Novelty lies in showing high E-field amplitude, optical power, optical intensity, and strong E-field confinement in low index slot regions despite of lowest ever reported contrast ratio. A systematic numerical study on the higher order dispersion characteristics of the widely studied SOI-based slot structure and of our proposed low refractive index contrast slot structure is carried out. It has been demonstrated that low refractive index contrast ratio slot optical waveguide GVD properties are quite different than SOI slot optical waveguide. The less normal dispersion existing in this kind of waveguide could have an impact on their applications in various nonlinear or linear applications.

Blue light absorption enhancement based on vertically channelling modes in nano-holes arrays

We investigate the specific optical regime occurring at short wavelengths, in the high absorption regime, in silicon thin-films patterned by periodically arranged nano-holes. Near-field scanning optical microscopy indicates that the incoming light is coupled to vertically channelling modes. Optical modelling and simulations show that the light, travelling inside the low-index regions, is absorbed at the direct vicinity of the nano-holes sidewalls. This channelling regime should be taken into account for light management in optoelectronic devices.

Electrochemical reductive desorption of alkyl self-assembled monolayers studied in situ by spectroscopic ellipsometry: evidence for formation of a low refractive index region after desorption.

Gold-sulphur bonds holding self-assembled monolayers (SAMs) on their gold substrate can be broken by electrochemical reduction, which typically occurs in an electrode potential range where the electrochemical hydrogen evolution reaction (HER) is thermodynamically possible. This work uses an in situ coupling between cyclic voltammetry and spectroscopic ellipsometry to compare the interfacial structure after desorption of the aliphatic thiols 1-Dodecanethiol (DDT) and 1-Octadecanethiol (ODT), and the ω-hydroxythiol 11-Mercapto-1-undecanol (MUD). For MUD and DDT, the data can only be explained by the presence of a substance with a significantly lower refractive index than the aqueous electrolyte in the interfacial region. This substance is likely to be H2. The hypothesis is put forward here that for MUD and DDT, desorbed molecules stabilise "nanobubbles" of H2. The resulting aggregates form as initial stages of the process of complete disintegration of the SAMs, i.e. the loss of the SAM-forming molecules into solution. On the other hand, desorption and readsorption of ODT are fully reversible - the presence of a layer with low refractive index can neither be excluded nor confirmed in this case. The results indicate that different SAM-stabilities are a consequence of solubility of the thiolates.

Enhancing surface plasmon polariton propagation by two-layer dielectric-loaded waveguides on silver surface

We report a study of a two-layer dielectric-loaded surface plasmon polariton waveguide (TDLSPPW) which consisted of two dielectric layers (high-index/low-index) on a silver film. The discontinuity of the electric field at the interfaces resulted in a concentrated field in the low-index region. It efficiently reduced the propagation loss of the surface plasmon polariton mode. The mode fields and corresponding complex propagation constants were calculated by a vector finite-difference method. The propagation properties were measured by a modified near-field optical microscope. It is confirmed that the propagation length of the proposed TDLSPPW was about 1.6 times longer than conventional single-layer SPP waveguides. In addition, a 90∘ waveguide turn with 3 μm radius showed that the bending loss was smaller than 2 dB.

Metallic Nanowire-Loaded Plasmonic Slot Waveguide for Highly Confined Light Transport at Telecom Wavelength

The optical properties of a plasmonic slot waveguide that comprises a metallic nanowire supported by a thin-dielectric-coated metal surface are investigated at the telecom wavelength. The fundamental plasmonic mode sustained by the waveguiding configuration is shown to be strongly confined inside the gap between the nanowire and the metallic substrate, along with moderate propagation loss and deep-sub-wavelength mode size achievable simultaneously. Studies on the effects of key geometric parameters on the modal properties reveal that the tradeoff between confinement and loss could be well controlled through tuning the dimensions of the nanowire and the gap. Through conducting 3-D numerical simulations, we further show that its guided plasmonic mode can be directly excited using similar strategies to those of the plasmonic nanowire mode. The nice optical performance, in conjunction with significant field enhancement inside the nanoscale low-index gap region, could facilitate potential applications in sensing, nonlinear light processing, optical manipulation as well as the implementations of numerous ultra-compact passive, and active nanophotonic devices.

Space vector‐based three‐level discontinuous pulse‐width modulation algorithm

Discontinuous pulse-width modulation (PWM) techniques result in reduced power loss in voltage source inverters. This study presents the space vector approach to the three-level discontinuous PWM technique. The switching sequences for four basic types of discontinuous PWM and the implementation are presented. An improved discontinuous PWM algorithm is presented for low modulation index region. All these algorithms are verified experimentally and the results are presented. The performance of discontinuous PWM algorithms is compared with that of conventional space vector-based PWM algorithms.