Low Index Contrast Research Articles

Controlling the propagation of an optical vortex through two-dimensional ordered and disordered waveguide arrays using topological charge.

In this paper, we study the propagation of a radially symmetric optical vortex whose amplitude is independent of topological charge in ordered and disordered 2D arrays of coupled waveguides. It is first demonstrated that the topological charge variation affects the beam spreading in the completely ordered arrays. For a low refractive index contrast between waveguides and their surroundings, the effective width at the output end of the optical lattice versus topological charge shows an oscillatory behavior. However, for a higher refractive index contrast, as the topological charge increases from 0 to 10, the effective width reaches a maximum value and then falls. Then, we investigate the effects of topological charge variation on the wave propagation through the waveguide array in the presence of different disorder strengths. Our results here confirm that the behavior of effective width versus the topological charge in the disordered array significantly depends on the average of the refractive index of the waveguides. Although the intensity of the input radially symmetric vortex beam is independent of the topological charge, for low disorder levels, the effective width and intensity distribution at the output end is strongly sensitive to the topological charge or the polar phase of the vortex beam. It is also demonstrated that, for strongly disordered arrays, the effective width and output beam profile shows no considerable change with variation of the topological charge. These effects are due to the discrete diffraction phenomenon and its dependence on the helical wavefront of the optical vortex whose form is determined by the topological charge. Therefore, it is demonstrated here that angular phase affects the beam broadening in an array of coupled optical waveguides.

Center mode of a doubly resonant optical periodic structure

An optical periodic structure with a single spatial resonance exhibits a stopband. When a second spatial resonance very close to the first one is added, the resulting doubly resonant structure exhibits a Gaussian enveloped, high quality factor transmission state right at the center of the original stopband. Using a slowly varying envelope approximation, we describe the optical characteristics of this transmission state analytically. The transmission state exists despite an optical structure of low refractive index contrast, and has potential applications in nano-optics, and photonics.

Rigorous analysis of numerical methods: a comparative study

For any photonic device simulation, the accuracy of the numerical solution not only depends on the methods being used but also on the discretization parameters used in that numerical method. In this work, Finite Element Method and Finite Difference Time Domain Method based on Maxwell’s equations were used to simulate optical waveguides and directional couplers. As the solution accuracy may also depend on the index contrast used in such photonic devices, the characteristics of low-index contrast Germanium doped Silica and high-index contrast Silicon Nanowire Waveguides were analyzed, evaluated and benchmarked. Numerical results to benchmark Directional Couplers are also reported in this paper.

Performance analysis of polarization transformation waveguide structures for planar light circuits

Polarization management is an important issue in current planar light circuit as there are many photonic devices that only work with one type of polarization and also there is much effort to increase network capacity by polarization division multiplexing. Polarization transformers are the most important and challenging elements to realize polarization diversity systems. In this paper we introduce stepped waveguide approach to polarization rotation as a promising method regarding device compactness and fabrication complexity, then structures reported in literature which are based on this concept are reviewed and analyzed. Furthermore we use this approach to propose a novel structure for polarization rotation in low index contrast substrates of silica-on-silicon, with an index contrast of 0.75, to lower the polarization dependence loss of devices realized in such substrates. This new design promises a conversion efficiency of 99 % over 854 um at the wavelength of 1.55 um within a CMOS compatible fabrication procedure.

Thermal activation of regenerated fiber Bragg grating in few mode fibers

This work demonstrated for the first time, the thermal regeneration of two and four modes graded index fiber Bragg gratings using high temperature tube furnace. During the regeneration process, the seed grating is erased and a new grating with lower index contrast is formed. The thermal calibration shows that the temperature sensitivity of regenerated grating is slightly higher for fiber with larger core. On the other hand, the regeneration temperature is lower for fiber with smaller core. The temperature sustainability up to 900 °C is observed for the produced regenerated gratings in few mode fibers.

Block Copolymer Nanocomposites with High Refractive Index Contrast for One-Step Photonics.

Photonic crystals (PhCs) prepared using the self-assembly of block copolymers (BCPs) offer the potential for simple and rapid device fabrication but typically suffer from low refractive index contrast (Δn ≤ 0.1) between the phase-segregated domains. Here, we report the simple fabrication of BCP-based photonic nanocomposites with large differences in refractive index (Δn > 0.27). Zirconium oxide (ZrO2) nanoparticles coated with gallic acid are used to tune the optical constants of the target domains of self-assembled (polynorbornene-graft-poly(tert-butyl acrylate))-block-(polynorbornene-graft-poly(ethylene oxide)) (PtBA-b-PEO) brush block copolymers (BBCPs). Strong hydrogen-bonding interactions between the ligands on ZrO2 and PEO brushes of the BBCPs enable selective incorporation and high loading of up to 70 wt % (42 vol %) of the ZrO2 nanoparticles within the PEO domain, resulting in a significant increase of refractive index from 1.45 to up to 1.70. Consequently, greatly enhanced reflection at approximately 398 nm (increases of ∼250%) was observed for the photonic nanocomposites (domain spacing = 137 nm) relative to that of the unmodified BBCPs, which is consistent with numeric modeling results using transfer matrix methods. This work provides a simple strategy for a wide range tuning of optical constants of BCP domains, thereby enabling the design and creation of high-performance photonic coatings for various applications. The large refractive index contrast enables high reflectivity while simultaneously reducing the coating thickness necessary, compared to pure BCP systems.

Fabrication and Characterization of Optical Devices on Lithium Niobate on Insulator Chips

The high electro-optic coefficients and nonlinear coefficients of lithium niobate make it a highly promising material for optical modulator design, experiments in cavity quantum electrodynamics, and microwave photonics. The two most common techniques for photonic device fabrication on lithium niobate are titanium diffusion and proton exchange. However, due to very low refractive index contrast (usually less than 0.1), the waveguides formed through these processes have low mode confinement, high bending losses, and often, are not polarization independent, making it difficult to fabricate compact photonic structures on the material. Here, we report our new design: rib waveguide on lithium niobate on insulator chips. Lithium niobate on insulator chips contains a thin film of lithium niobate (thickness: ∼700 nm) adhering on an insulator substrate like SiO2. Such devices provide good vertical and lateral index contrast and mode confinement. Also, because of the smaller waveguide widths attainable through etch based techniques, a greater control of the critical dimensions of devices is possible. We fabricated and characterized optical devices - waveguides, power splitters, and microring resonators on lithium niobate on insulator chips, employing robust fabrication techniques, namely EBL and ion milling. The structures were characterized in terms of propagation loss and quality factor. The fabricated structures showed good performance, and the fabrication techniques have potential for use in the mass manufacture of lithium niobate based optical devices.

Ellipsometric study of molecular orientations of Thermomyces lanuginosus lipase at the air–water interface by simultaneous determination of refractive index and thickness

Ellipsometric studies of very thin organic films suffer from the low refractive index contrast between layer and bulk substrate. We demonstrate that null ellipsometry can not only provide detailed information about the adsorption kinetics and surface excess values, but in addition on layer thicknesses with submonolayer resolution of a lipase from Thermomyces lanuginosus at the air–water interface. While measuring very close to the Brewster angle, refractive indices and layer-thicknesses can both be determined with a precision that is sufficiently high to make conclusions on the density and orientation of the molecules at the interface. The orientation was found to be concentration- and pH value-dependent. At the isoelectric point, the lipase was almost vertically oriented with respect to the surface, while for pure distilled water and low lipase concentration a rather horizontal alignment was found. Further experiments, varying the size of the interfacial area in a Langmuir trough, confirm the different layer structures.

Ultrafast laser inscription of volume phase gratings with low refractive index modulation and self-images of high visibility.

Ultrafast laser inscription of volume phase gratings with low index contrast and self-images with visibility of 0.96 is demonstrated. It is also demonstrated that phase differences of π/2 for visible light are achievable with only one layer of structures induced in bulk borosilicate glass by direct laser writing. The fabrication method avoids the stitching of several layers of structures and significantly reduces the time of process. The increment of visibility with the induced phase difference is proved and results are compared with the expected for planar phase gratings.

A comprehensive comparison of photonic band gap and self-collimation based 2D square array waveguides

In this paper, one of the most recently introduced phenomena in Photonic Crystal structures was investigated to design self-collimation based waveguides. The advantages of using self-collimation property to design waveguides are discussed and the idea is thoroughly compared with conventional band gap engineered waveguides made by defects. The 2D square lattice waveguides presented in this article, guide the 1.55μm wavelength, most commonly used in telecommunication applications. Results indicate the high coupling efficiency enhancement in Self-Collimation based waveguides due to using low index contrast materials. The simulations are done using Plane Wave Expansion (PWE) and Finite Difference Time Domain (FDTD) methods and results confirm the value of using self-collimation phenomena in waveguide design.

Study of all-polymer-based waveguide resonant gratings and their applications for optimization of second-harmonic generation

We investigated theoretically and experimentally the optical properties of all-polymer-based one-dimensional waveguide resonant gratings (WRGs) and their important applications for the optimization of second-harmonic generation (SHG). We first studied the basic theory of the resonant modes of a simple grating-coupled waveguide realized on a material possessing a low refractive index contrast. The optical properties of any WRG were numerically simulated by using the finite-difference time domain method, performed by commercial Lumerical software. The polymer-based surface relief gratings were fabricated on azopolymer Disperse Red 1-Poly-Methyl-Methacrylate (DR1–PMMA) thin films by using the two-beam interference method and mass transport effect. Their experimental reflection spectra measured as a function of incident light wavelength are in good agreement with the theoretical predictions. We then demonstrated a first application of such a polymer-based WRG for nonlinear optics. Thanks to the strong local electrical field in the WRG, due to a guided-mode resonance condition, the SHG signal of an infrared light beam was strongly enhanced by a factor of 25 as compared to the result obtained in a sample without a grating.

(Invited) Si Photonics and Recent Challenges for on-Chip WDM

A trend of high speed computing and communication has demonstrated incredible performance increase, 1000x in every 10yrs. Such enhancements have been realized partly due to Moore’s law based and parallelism based progresses [1]. To keep up such a trend for future, we have to integrate photonics on a chip, i.e., Si photonics, especially on-chip WDM (wavelength division multiplexing) architecture [2]. In the history of the fiber optics, WDM has brought significant increase in bandwidth of optical communication and currently the channel spacing in WDM is as small as 0.8 nm and will soon be 0.4 nm. However, Si photonics has used coarse WDM where the channel spacing is as large as 10nm. The reason for the large spacing is a large TO (thermooptics) coefficient of Si. Due to chip temperature fluctuation on a chip, wavelength is changed widely, malfunctioning WDM. This is the fundamental limit in shrinking the wavelength spacing. In other words, the key challenge to implement on-chip WDM is to lock its channel wavelength within a given channel spacing. In this talk we will focus on silicon nitride based Si photonics, i.e., medium index contrast optics, MiDex Si photonics to lock the channel wavelength within the current dense WDM (DWDM) in optical fiber communication, i.e., 0.8nm spacing [3]. Fig. 1 shows bending radii of various waveguides vs. index difference between the core and cladding of the waveguide. Generally, Si waveguide and SiO2 cladding have been used in Si photonics, where index difference is 2.0. The materials set has a high index contrast, i.e., here we will refer it to as HiDex (high index contrast). The advantage is a sharp bending of the waveguide, which reduces device footfprints and allows high density integration of optical devices on a chip. Althougt the disadvantage is large light propagation loss, Si process technology allows to minimize the roughness of waveguide to reduce the loss to ~1dB/cm. It is known however that Si has a large TO coefficient, i.e., temperature dependence of refractive index. It has been reported that chip temperature has changed about 50˚C during its operation. Thus, on-chip temperature fluctuation changes wavelength, malfunctioning of WDM. To avoid it, a large channel spacing, i.e., 10nm has been chosen. We have been focusing a slightly smaller index contrast system in the following two reasons: When we choose SiNx waveguide and SiO2 cladding, the minimum bending radius is 10 µm which is still small enough and the TO coefficient of SiNx is ~1/10 of that of Si. This indicates this materials system should be robust against temperature fluctuation on a chip. Of course Planar Lightwave Circuit based on silica fiber technology has smaller TO coefficient, here we refer it to as LoDex (low index contrast), but the bending radii should be ~1cm. Thus, we have proposed MiDex Si photonics instead of current HiDex Si photonics. SiNx (0.6-0.8 µm thick) can be deposited on SiO2 (2µm thick) using a physical vapor deposition (PVD) technique. Chemical vapor deposition techniques have widely been used, but CVD SiNx contains N-H bonds that absorb 1520nm, that is in optical communication wavelength range of 1550nm. To dissociate H from SiNx, CVD SiNx should be annealed at high temperature such as 1100˚C. To avoid such high temperature annealing, we have chosen PVD SiNx, which should be free from N-H bonds. We have demonstrated SiNx without such absorption. The present report has verified the advantage of PVD SiNx over CVD SiNx, temperature stability of filter characteristics of ring based resonators, and athermalization using polymer materials.

Optical Characterization and Lasing in Three-Dimensional Opal-Structures

The lasing properties of dye-permeated opal pyramidal structures are compared with the lasing properties of opal films. The opal-structures studied were made by sedimentation of micro-spheres and by sol-gel inversion of the direct-opals. Forced-sedimentation by centrifugation inside wet-etched pyramidal pits on silicon surfaces was used to improve the structural quality of the direct-opal structures. Single crystalline pyramids with the base length of ∼ 100 µm were formed by centrifuged sedimentation. The lasing of dyes in the well-ordered crystalline and poly-crystalline structures showed a distinct multi-modal spectrum. Gain via a distributed feedback was responsible for the lasing since the photonic band gap was negligible in a low refractive index contrast medium; the indices of silica and ethylene glycol are 1.46 and 1.42, respectively. A disordered lasing spectrum was observed from opal films with structural defects and multi-domain regions. The three dimensional structural quality of the structures was assessed by in situ optical diffraction and confocal fluorescence. A correlation between the lasing spectrum and the three-dimensional structural quality was established. Lasing threshold of a sulforhodamine dye in a silica opal was controlled via Forster mechanism by addition of a donor rhodamine 6G dye. The lasing spectrum had a well-ordered modal structure which was spectrally stable at different excitation powers. The sharp lasing threshold characterized by a spontaneous emission coupling ratio β ' 10−2 was obtained.

Spatial Resolution and Refractive Index Contrast of Resonant Photonic Crystal Surfaces for Biosensing.

By depositing a resolution test pattern on top of a Si3N4 photonic crystal resonant surface, we have measured the dependence of spatial resolution on refractive index contrast Δn. Our experimental results and finite-difference time-domain (FDTD) simulations at different refractive index contrasts show that the spatial resolution of our device reduces with reduced contrast, which is an important consideration in biosensing, where the contrast may be of order 10−2. We also compare 1-D and 2-D gratings, taking into account different incidence polarizations, leading to a better understanding of the excitation and propagation of the resonant modes in these structures, as well as how this contributes to the spatial resolution. At Δn = 0.077, we observe resolutions of 2 and 6 μm parallel to and perpendicular to the grooves of a 1-D grating, respectively, and show that for polarized illumination of a 2-D grating, resolution remains asymmetrical. Illumination of a 2-D grating at 45° results in symmetric resolution. At very low index contrast, the resolution worsens dramatically, particularly for Δn < 0.01, where we observe a resolution exceeding 10 μm for our device. In addition, we measure a reduction in the resonance linewidth as the index contrast becomes lower, corresponding to a longer resonant mode propagation length in the structure and contributing to the change in spatial resolution.

Rigorous analysis of acoustic modes in low and high index contrast silica fibers.

The vector acoustic modes in both well-established and emerging designs of optical waveguides have been studied through use of a computer code which has been developed based on the finite element method (FEM). Dispersion curves and the displacement vectors for the transverse and longitudinal acoustic modes and the modal hybridness have been determined and these are shown for both low and high index contrast silica (SiO₂) acoustic waveguides. Stimulated Brillouin scattering (SBS) frequencies are also reported for the subwavelength size SiO₂ optical waveguides.

On-chip optical trapping and Raman spectroscopy using a TripleX dual-waveguide trap.

We present a new approach to the dual-beam geometry for on-chip optical trapping and Raman spectroscopy, using waveguides microfabricated in TripleX technology. Such waveguides are box shaped and consist of SiO2 and Si3N4, so as to provide a low index contrast with respect to the SiO2 claddings and low loss, while retaining the advantages of Si3N4. The waveguides enable both the trapping and Raman functionality with the same dual beams. Polystyrene beads of 1 μm diameter can be easily trapped with the device. In the axial direction discrete trapping positions occur, owing to the intensity pattern of the interfering beams. Trapping events are interpreted on the basis of simulated optical fields and calculated optical forces. The average transverse trap stiffness is 0.8 pN/nm/W, indicating that a strong trap is formed by the beams emitted by the waveguides. Finally, we measure Raman spectra of trapped beads for short integration times (down to 0.25 s), which is very promising for Raman spectroscopy of microbiological cells.

Spontaneous emission modulation of colloidal quantum dots via efficient coupling with hybrid plasmonic photonic crystal.

The spontaneous emission of colloidal CdSe/ZnS quantum dots (CQDs) modified by the hybrid plasmonic-photonic crystal is reported in this paper. By using a spin coater, the spatial overlap between CQDs and the surface resonance modes in this quasi-2D crystal slab is achieved. In this case, the coupling efficiency of them is enhanced greatly and most excited CQDs radiate through the surface modes. Consequently, despite the low refractive index contrast of our hybrid structure, the directionality of spontaneous emission, increased radiative probability and narrowed full width at half maximum of emission peak are all clearly observed by our home-made microscopic angle-resolved spectroscopy and time-resolved photoluminescence system. Our results manifest that the quasi-2D hybrid plasmonic-photonic crystal is an ideal candidate to tailor the radiative properties of CdSe/ZnS CQDs, which might be significant for the applications of light emitting devices.

Contrast-Enhanced Ultrasound Identifies Reduced Overall and Regional Renal Perfusion During Global Hypoxia in Piglets

It is well known from both clinical experience and animal research that renal hypoxia may lead to temporary or permanent renal failure, the severity being dependent largely on the duration and grade of the hypoxia. The medulla is more susceptible to hypoxic injury than the cortex because approximately 90% of the renal blood flow supplies the cortex. Various methods have been applied to evaluate renal perfusion in both experimental and clinical settings, including magnetic resonance imaging, computed tomography, laser Doppler, and contrast-enhanced ultrasound (CEUS). The aim of this study was to evaluate changes in overall and regional renal perfusion with CEUS in response to global hypoxia. Twelve newborn anesthetized piglets were exposed to general hypoxia with a fraction of inspired oxygen of 8% of 30 minutes duration. Resuscitation was performed with either 100% oxygen (n = 6) or air (21% oxygen) (n = 6) for 30 minutes followed by 7 hours of reoxygenation with air. Before, during, and after hypoxia, the left kidney was examined with CEUS using 0.2 mL IV of SonoVue followed by 2 mL saline flush. Five additional piglets served as controls. The kidney was examined using a 9-MHz linear transducer with low mechanical index (0.21) and pulse inversion contrast program. One region of interest was drawn in the renal cortex and 1 in the medulla to obtain the corresponding time intensity curves (TICs). From these curves, the peak intensity (PI), time to peak (TTP), upslope of the curve, area under the curve, and mean transit time (MTT) were recorded. Also, the renal arteriovenous transit time (AVTT) was registered. The resistance index (RI) was repeatedly measured in the renal artery. Contrast-enhanced ultrasound was repeated at regular intervals until the animals were sacrificed 8 hours after the hypoxic period. In the group of 12 piglets subjected to hypoxia, RI increased from 0.69 ± 0.08 at baseline to 0.99 ± 0.09 during hypoxia (P < 0.01), indicating severe general renal vasoconstriction. The AVTT increased from 2.6 ± 0.5 seconds at baseline to 6.7 ± 2.8 seconds during hypoxia (P < 0.001). The PI in the cortex decreased from a mean value of 38.6 ± 6.1 dB at baseline to 30.3 ± 9.7 dB during hypoxia (P < 0.05). In the medulla, only a minor, nonsignificant reduction in PI was observed during hypoxia. In the medulla, TTP and MTT increased from 6.4 ± 1.5 and 9.2 ± 1.7 seconds at baseline to 14.6 ± 8.4 seconds (P < 0.01) and 15.2 ± 5.6 seconds (P < 0.01), respectively, during hypoxia. In the cortex, no statistically significant changes in TTP or MTT were observed during hypoxia. A return to near-baseline values was observed for TTP, PI in both the medulla and cortex, as well as for RI and AVTT within 1 to 3 hours after hypoxia, and they remained relatively constant for the duration of the experiment.Less than 1 hour after the hypoxia, PI both in the cortex and the medulla was significantly higher in the group resuscitated with air than in the group resuscitated with 100% oxygen, 36.0 ± 4.3 versus 27.2 ± 2.2 dB (P < 0.05) and 33.3 ± 8.2 versus 21.1 ± 2.0 dB (P < 0.01), respectively. Global hypoxia induced changes in overall and regional renal perfusion detectable with CEUS. Cortical and medullary flows were affected differently by hypoxia; a strong increase in medullary TTP and MTT was observed, indicating a reduction in medullary blood flow velocity. In the cortex, a significant reduction in PI was found, probably because of a reduction in cortical blood volume. A faster recovery of both medullary and cortical PI in the group resuscitated with air could indicate that air might be more beneficial for renal perfusion than hyperoxia during resuscitation after renal hypoxia.

All-optical on-chip sensor for high refractive index sensing in photonic crystals

In this paper we demonstrate an optical sensor designed to detect material infiltrations with relatively high indices, based on a two-dimensional photonic crystal cavity structure. The locations and sizes of the holes surrounding a L3 cavity were modified to increase the Q factor to a value of 1500 with a high refractive index infilling of . With precise design and simulation, we overcome the difficulty of low index contrast, and observe a very clear wavelength shift of 10.4 nm in simulation and 12.4 nm in experiment between water and oil samples at resonance.

Surface charge effects on optical trapping of nanometer-sized lipid vesicles.

Optical trapping of nanometer-sized lipid vesicles has been challenging due to the low refractive index contrast of the thin lipid bilayer to the aqueous medium. Using an "optical bottle", a recently developed technique to measure interactions of nanoparticles trapped by an infrared laser, we report, for the first time, quantitative measurements of the trapping energy of charged lipid vesicles. We found that the trapping energy increases with the relative amount of anionic lipids (DOPG) to neutral lipids (DOPC) in vesicles. Moreover, as monovalent salt is added into the exterior solution of vesicles, the trapping energy rapidly approaches zero, and this decrease in trapping energy strongly depends on the amount of anionic lipids in vesicles. A simple model with our experimental observations explains that the trapping energy of charged lipid vesicles is highly correlated with the surface charge density and electric double layer. In addition, we demonstrated selective trapping of a binary mixture of vesicles in different mole fractions of charged lipids, a strategy that has potential implications on charge selective vesicle sorting for engineering applications.