Low Index Contrast Research Articles

Estimation of photonic band gap in the hollow core cylindrical multilayer structure

The propagation characteristic of two hollow core cylindrical multilayer structures having high and low refractive index contrast of cladding regions have been studied and compared at two design wavelengths i.e. 1550 nm and 632.8 nm. With the help of transfer matrix method a relation between the incoming light wave and outgoing light wave has been developed using the boundary matching technique. In high refractive index contrast, small numbers of layers are sufficient to provide perfect band gap in both design wavelengths. The spectral position and width of band gap is highly depending on the optical path of incident light in all considered cases. For sensing application, the sensitivity of waveguide can be obtained either by monitoring the width of photonic band gap or by monitoring the spectral shift of photonic band gap. Change in the width of photonic band gap with the core refractive index is larger in high refractive index contrast of cladding materials. However, in the case of monitoring the spectral shift of band gap, the obtained sensitivity is large for low refractive index contrast of cladding materials and further it increases with increase of design wavelength.

High Reflectivity Hybrid AlGaN/Silver Distributed Bragg Reflectors for Use in the UV-Visible Spectrum

Indium-free AlGaN-based distributed Bragg reflectors (DBRs) in the UV spectrum are known to have very low reflectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of ~2.41% for AlN on GaN). Near-bandedge excitonic resonances influence the real part of AlGaN's dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si: 10 20 cm -3 ) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reflectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240 to 370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reflectivity exceeds 99%, but this can be shrunk down to 20-30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit 90% reflectivity at the AlGaN/metal interface between wavelengths ranging from >360 to 180-670 nm, respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain.

Modal properties of a strip-loaded horizontal slot waveguide

BackgroundChannel waveguides have been developed for years to comply with the need for smaller footprint and better integration in photonics circuits. However, they are still suffering from fabrication complexity and optical losses. These two limitations can be addressed by combining two types of channel waveguides, namely strip-loaded and slot waveguide.MethodsWe present a systematic study of the geometrical parameters of a strip-loaded horizontal slot waveguide and their influence on the characteristics of the device. Simulations and experiments are carried out to understand the behavior of the electromagnetic field inside such a waveguide. In particular, the influence of the high and low refractive index layers and the loading-strip on the key characteristics of the structure, i.e., the confinement factor and the effective index, are investigated theoretically and compared with experimental values.ResultsThe main properties of this waveguide platform are a low lateral index contrast, a high vertical field confinement, and low-propagation losses (1.4 dB/cm).ConclusionsOur results show that this platform is highly versatile, easy to fabricate and low-losses. We also show that the geometry of the mode can be tuned to suit the application.

Subwavelength Focusing of Bloch Surface Waves

Microsized spheres can focus light into subwavelength spatial domains, a phenomenon called photonic nanojet. Even though well studied in three-dimensional (3D) configurations, only a few attempts have been reported to observe similar phenomena in two-dimensional (2D) systems. This, however, is important to take advantage of photonic nanojets in planar optical systems. Usually, surface plasmon polaritons are suggested for this purpose, but they suffer notoriously from rather low propagation lengths due to intrinsic absorption. Here, we solve this problem and explore, numerically and experimentally, the use of Bloch surface waves sustained by a suitably structured all-dielectric media to enable subwavelength focusing in a planar optical system. Since only a low-index contrast can be achieved while relying on Bloch surface waves, we perceive a new functional element that allows a tight focusing and the observation of a photonic nanojet on top of the surface. We experimentally demonstrate a spot size of 0.66λ...

Enhanced light extraction of LYSO scintillator by photonic crystal structures from a modified porous anodized aluminum oxide layer

Although porous anodized aluminum oxide layer can be used to extract scintillation light from a LYSO scintillator, the low refractive index contrast of porous AAO layer obtains a moderate enhancement. In this investigation, we have designed and fabricated a modified porous anodized aluminum oxide layer with conformal deposition layer of high refractive index material of TiO2 on the surface of LYSO scintillator, achieving a significant enhancement by 60% with wavelength- and angle-integrated emission intensity. The fabrication method of the present study is simple and low-cost for the large area applications in the field of radiation detection.

Thermally controlled mid-IR band-gap engineering in all-glass chalcogenide microstructured fibers: a numerical study

Presence of photonic band-gap (PBG) in an all-glass low refractive index (RI) contrast chalcogenide (Ch) microstructured optical fibers (MOFs) is investigated numerically. The effect of external temperature on the position of band-gap is explored to realize potential fiber-based wavelength filters/sensors at functional mid-IR spectral range. The cross-sectional geometry of the MOF is formed by considering a Ch glass to form the overall background cross-section as well as the central fiber core. The core region is surrounded by periodically arranged (hexagonal pattern) smaller holes, which are assumed to be filled up with another Ch glass. Thermally compatible and fabrication suitable, two Ch glasses are chosen, one (higher RI) as background material and the other (of lower RI) to fill up the holes. Two sets of such pairs of thermally compatible Ch-glasses are considered as fiber structural materials with relative RI contrast of ∼12% and ∼24%. For both such low RI contrast hexagonal structures, PBG appears only for suitable finite values of longitudinal wave vector. The structures are suitable to realize band-gap at mid-IR wavelengths and specifically optimized for operation around the ∼2 μm region. Then the temperature sensitivity of band-gaps is investigated to design fiber-based mid-IR wavelength filters/sensors.

F/Yb-codoped sol-gel silica glasses: toward tailoring the refractive index for the achievement of high-power fiber lasers.

Accurate control of both the doping distribution inside the fiber core and the low refractive index contrast between the fiber core and cladding materials is essential for the development of high-power fiber lasers based on the use of single-mode large-mode-area (LMA) optical fibers. Herein, sol-gel monolithic F/Yb3+-codoped silica glasses were prepared from porous large silica xerogels doped with ytterbium salt solution, which had been subjected to fluorination with hexafluoroethane gas, before subsequent sintering. The fluorine content inside the doped glass has been varied by adjusting the fluorination duration. The space homogeneity of fluorine and ytterbium concentrations in the cylindrical preforms has been checked by chemical analysis and Raman spectroscopy. Moreover, the glass with the lowest fluorine content has been successfully integrated as a core material in a microstructured optical fiber made using the stack-and-draw method. This fiber was tested in an all-fiber cavity laser architecture to evaluate potential lasing performances of the F/Yb3+-codoped silica glass. It presents a maximum efficiency of 70.4%, achieved at 1031 nm from a 1.16 m length fiber. These results confirm the potentialities of the obtained F/Yb3+-codoped glasses for the fabrication of LMA optical fiber lasers.

Large field enhancement obtained by combining Fabry–Perot resonance and Rayleigh anomaly in photonic crystal slabs

By applying the properties of Fabry–Perot resonance and Rayleigh anomaly, we have shown that a photonic crystal slab can scatter the light from an incident plane wave into a diffracted light with a very large reflection or transmission coefficient. The enhanced field is either a propagating diffracted wave (with a grazing angle of diffraction) or a weakly evanescent diffracted wave, so it can be particularly useful for applications requiring an enhanced propagating field (or an enhanced field with a low attenuation). An efficient effective medium technique is developed for the design of the resonant photonic crystal slabs. Numerical simulations have shown that photonic crystal slabs with low index contrast, such as the ones found in the cell wall of diatoms, can enhance the intensity of the incident light by four orders of magnitude.

Concurrency of anisotropy and spatial dispersion in low refractive index dielectric composites.

The article demonstrates uncommon manifestation of spatial dispersion in low refractive index contrast 3D periodic dielectric composites with periods of about one tenth of the wavelength. First principles simulations by the well established plane wave method reveal that spatial dispersion leads to appearance of additional optical axes and can compensate anisotropy in certain directions.

Manipulating the emission intensity and lifetime of NaYF4:Yb3+,Er3+ simultaneously by embedding it into CdS photonic crystals.

Photonic crystals (PCs) have long been considered effective for tuning upconversion luminescence due to their photonic band gap (PBG) and the redistribution of density of optical states (DOS). Although the emission intensity can be changed obviously by the PC effect, rarely an obvious lifetime change consistent with theory is observed due to the low refractive index of PS or SiO2 spheres in the commonly used PCs. Herein, CdS/NaYF4:Yb3+,Er3+ composite PCs with a high refractive index contrast are fabricated in one step with upconversion nanoparticles filled inside CdS PCs. When the upconversion emission peak lies at the edge of the PBGs of the composite PCs, a dramatic decrease in lifetime by 28% and 41% is observed for the green and red emissions, respectively. At the same time, obvious emission intensity enhancements are also observed. In contrast, PS PCs with a low refractive index contrast show a slight effect on the lifetime of upconversion luminescence with their emission peak at the edge of the PBGs. Our results agree well with theory and prove that a sufficiently large refractive index contrast is necessary for PCs to dramatically tune the luminescence lifetime and intensity simultaneously.

Contrast-Enhanced Tissue Harmonic Imaging versus Phase Inversion Harmonic Sonographic Imaging for the Delineation of Hepatocellular Carcinomas

Objective: To compare contrast tissue harmonic imaging (THI) with low mechanical index (MI) and conventional contrast harmonic imaging (CHI) with respect to lesion visibility of hepatocellular carcinoma (HCC). Methods: One hundred and twenty-five patients (84 men and 41 women, age range 39-94 years, mean age 74 years) with 100 naïve HCCs and 30 lesions after radiofrequency ablation (RFA) for HCC were evaluated. One hundred and four patients had liver cirrhosis of Child-Pugh class A, and the remaining 21 had Child-Pugh class B cirrhosis. The lesion conspicuity and intratumoral echogenicity during the postvascular phase were compared using conventional CHI and contrast THI with low MI. Results: The MI values ranged from 0.20 to 0.30 on conventional CHI and from 0.30 to 0.35 on contrast THI. Regarding HCC lesion conspicuity, contrast THI with low MI was clearer in 79 lesions (60.8%), equal in 34 lesions (26.2%), and less clear in 17 lesions (13.1%) when compared with conventional CHI. The lesion conspicuity with contrast THI was significantly better than that with conventional CHI (p < 0.01). All of the postablative lesions were well delineated in patients who received RFA. Conclusion: Low-MI contrast THI was superior to conventional CHI with respect to lesion visibility of HCCs and might offer good imaging for the guiding of RFA.

Digital Detection of Exosomes by Interferometric Imaging.

Exosomes, which are membranous nanovesicles, are actively released by cells and have been attributed to roles in cell-cell communication, cancer metastasis, and early disease diagnostics. The small size (30–100 nm) along with low refractive index contrast of exosomes makes direct characterization and phenotypical classification very difficult. In this work we present a method based on Single Particle Interferometric Reflectance Imaging Sensor (SP-IRIS) that allows multiplexed phenotyping and digital counting of various populations of individual exosomes (>50 nm) captured on a microarray-based solid phase chip. We demonstrate these characterization concepts using purified exosomes from a HEK 293 cell culture. As a demonstration of clinical utility, we characterize exosomes directly from human cerebrospinal fluid (hCSF). Our interferometric imaging method could capture, from a very small hCSF volume (20 uL), nanoparticles that have a size compatible with exosomes, using antibodies directed against tetraspanins. With this unprecedented capability, we foresee revolutionary implications in the clinical field with improvements in diagnosis and stratification of patients affected by different disorders.

Silicon Photonic Integrated Circuits for Wavelength-Division Multiplexing Applications

Silicon photonics will provide low-cost, high-bandwidth and compact optical components for a wide range of applications in optical communications and interconnects. One of the cited key advantages is the capability of wavelength-division multiplexing (WDM). However, the nature of high-index contrast of silicon photonic devices leads to significant challenges when implementing on-chip WDM filters, which is one of the key components in WDM circuits. In this paper, we review several demonstrated silicon photonic WDM circuits based on monolithically integrated silicon nitride (SiN) arrayed-waveguide gratings (AWGs) and thermally tunable silicon microring filters. The use of SiN waveguides with lower index contrast than silicon waveguides enables the realization of high-performance AWGs. Meanwhile, they can evanescently couple to silicon waveguides with high efficiency. The thermally tunable silicon microrings can be used as modulators and wavelength (de)multiplexing filters to implement versatile WDM circuits. Reconfigurability of channel spacing and central wavelengths is achieved by individual tuning of the rings. In this paper, we review silicon photonic circuits for multiple-channel modulators, polarization-insensitive WDM receiver, and variable optical attenuators with multiplexer.

Adaptive Graded Index Photonic Crystal Lens Design via Nematic Liquid Crystals

An adaptive graded index photonic crystal (GRIN PC) lens system is designed by using liquid crystal (LC) infiltration. LCs have the property to change their refractive indices when an external voltage is applied. This feature allows for the modulation on the effective index profile of the low-index contrast GRIN PC lens. Alongside this property, without changing the length of the designed 2D GRIN PC, the corresponding focal distance can be adapted from infinity to a certain positive or negative focal point by controlling the applied voltage. The effects of random perturbations and intentional line defects on the focal tuning capability of proposed 2D GRIN PC are also studied. Moreover, due to the scalability of PC’s dispersion relations, the sizes of the proposed devices can be adapted so that it may operate in either infrared or microwave regimes. Consequently, the proposed tunable polymeric GRIN PC can be implemented in various optical applications, such as near-field imaging and scanning systems, vision correction, and auto-focusing.

3D Printed Hollow‐Core Terahertz Optical Waveguides with Hyperuniform Disordered Dielectric Reflectors

Novel hollow‐core THz waveguides featuring hyperuniform disordered reflectors are proposed, fabricated, and characterized. The reflector comprises aperiodically positioned dielectric cylinders connected with dielectric bridges. The proposed waveguides are fabricated using a 3D MultiJet printer. Optical properties of the fabricated waveguides are investigated numerically using finite element method, as well as experimentally using terahertz time‐domain spectroscopy. The results confirm that proposed waveguides exhibit sizable photonic bandgaps (21%) even with relatively low refractive index contrast (resin/air). Position of the bandgaps can be easily tuned by varying reflector geometrical parameters.

Linearly combined optical structures based on SiON

We investigate an optical structure that results from a linear combination of two periodic, multilayer dielectric functions of slightly different periodicities. By solving the corresponding Maxwell's equation, we show that such linearly combined optical structure supports a dense number of localized (slow) modes. For a practical implementation of the linear combination of the dielectric functions, we make use a silicon oxynitride (SiON) material platform where the refractive index of SiON can be tuned over a continuous range, by changing the percentage of oxygen. The proposed optical structure works, despite a system of low refractive index contrast, and can be effectively used to create efficient polychromatic slow light systems and devices.

Design of a Polymer-Based Hollow-Core Bandgap Fiber for Low-Loss Terahertz Transmission

We use numerical simulations to design a hollow-core microstructured polymer optical fiber (HC-mPOF) suitable for broadband, terahertz (THz) pulse transmission with relatively low losses and small dispersion. The HC-mPOF consists of a central large air-core surrounded by periodically arranged wavelength-scale circular air holes in a hexagonal pattern, embedded in a uniform Teflon matrix. The THz guidance in this fiber is achieved by exploiting the photonic bandgap (PBG) effect. In our low index contrast Teflon-air (1.44:1) hexagonal periodic lattice, the PBG appears only for a certain range of non-zero values of the longitudinal wavevector. We have achieved PBG over a broad spectral range (bandwidth $\sim 400$ GHz) ranging from 1.65 to 2.05 THz in the proposed HC-mPOF. The achievable loss coefficient in our designed HC-mPOF is $ and the group velocity dispersion parameter is $\cdot $ cm over a 300-GHz bandwidth ( $1.65\sim 1.95$ THz).

Femtosecond laser direct inscription of surface skimming waveguides in bulk glass.

We present a detailed study of waveguide inscription near the surface of bulk glass using a femtosecond laser. Three silicate glasses used extensively as hosts for photo-induced photonic devices were examined. Our results show that near-surface waveguides generally present a low-index contrast, as the pulse energy damage threshold decreases sharply at close proximity to the surface. We devised a novel method to allow the formation of optical waveguides that exhibit a high-index contrast up to the surface of any transparent material. As a proof of concept, the inscription of near-surface single-mode waveguides operating at a wavelength of 405 nm is demonstrated.

Production of Structural Colors with High Contrast and Wide Viewing Angles from Assemblies of Polypyrrole Black Coated Polystyrene Nanoparticles.

Structural color with wide viewing angles has enormous potential applications in pigment, ink formulation, displays, and sensors. However, colors obtained from colloidal assemblies with low refractive index contrast or without black additives typically appear pale. Here, we prepare polypyrrole (PPy) black coated polystyrene (PS) nanoparticles and demonstrate well-defined colors with high color contrast and wide viewing angles under ambient light. Depending on the loading of pyrrole during polymerization, PPy nanogranules of different sizes and coverages are grafted to the surface of PS nanoparticles. The bumpy particles can self-assemble into quasi-amorphous arrays, resulting in low angle dependent structure colors under ambient light. The color can be tuned by the size of the PS nanoparticles, and the presence of the PPy black on PS nanoparticles enhances the color contrast by suppressing incoherent and multiple scattering.

Evanescent field trapping of nanoparticles using nanostructured ultrathin optical fibers

While conventional optical trapping techniques can trap objects with submicron dimensions, the underlying limits imposed by the diffraction of light generally restrict their use to larger or higher refractive index particles. As the index and diameter decrease, the trapping difficulty rapidly increases; hence, the power requirements for stable trapping become so large as to quickly denature the trapped objects in such diffraction-limited systems. Here, we present an evanescent field-based device capable of confining low index nanoscale particles using modest optical powers as low as 1.2 mW, with additional applications in the field of cold atom trapping. Our experiment uses a nanostructured optical micro-nanofiber to trap 200 nm, low index contrast, fluorescent particles within the structured region, thereby overcoming diffraction limitations. We analyze the trapping potential of this device both experimentally and theoretically, and show how strong optical traps are achieved with low input powers.