Low-index Layer Research Articles

Electrical characterisation of p-doped distributed Bragg reflectors in electrically pumped GaInNAs VCSOAs for 1.3μm operation

► We electrically characterised two p -type doped DBRs for 1.3 μm VCSOA applications. ► Abrupt and graded structures were designed and investigated. ► Longitudinal and vertical transports were measured as function of temperature. ► Low series resistivity achieved using the interface composition grading technique. ► Theoretical model confirmed the obtained results. The high resistivity that is encountered in p -type DBRs is an important problem in vertical cavity surface emitting lasers and optical amplifiers (VCSELs and VCSOAs). This is because the formation of potential barriers at the interfaces between layers of high and low refractive index inhibits the carrier flow, thus increasing the DBR series resistance. In this work, the electrical characteristics of two p -type doped DBR structures grown on undoped and p -type doped GaAs substrates have been investigated. The DBRs are designed for VCSOAs operating at 1.3 μm and consist of 14-periods of alternating GaAs and Al 0.9 Ga 0.1 As in the first sample and 14-periods of GaAs and Al 0.3 Ga 0.7 As/Al 0.9 Ga 0.1 As in the second one. For the longitudinal transport sample, Hall mobility and sheet carrier density were measured in the temperature range from 77 to 300 K. In the vertical transport sample, current–voltage ( I – V ) measurements across the DBR layers were carried out at different temperatures in the range between 15 and 300 K. We achieved resistivity reduction in our samples by using an interface composition grading technique aimed at improving the VCSOA characteristics.

AlN/GaN Distributed Bragg Reflectors Grown via Metal Organic Vapor Phase Epitaxy using GaN Insertion Layers

ABSTRACTDistributed Bragg Reflectors (DBRs) are an important component of various optoelectronic devices for ultra violet and visible wavelengths. In the III-Nitride material system, Aluminum Nitride (AlN) and Gallium Nitride (GaN) offer a large contrast in refractive index and are therefore well suited for fabricating DBRs with high reflectivity and wide bandwidths using relatively few periods. However, the large lattice and thermal mismatch leads to cracking in these heterostructures. In this work short period superlattice layers have been used to fabricate high reflectivity (> 94%) nitride based DBRs via Metal Organic Vapor Phase Epitaxy. Short period AlN/GaN superlattices containing three to four monolayers of GaN have been employed as the low refractive index layer in DBRs to minimize cracking. Using this technique, crack-free DBRs reflecting from 440-475 nm with up to 25 periods have been fabricated. The technique has been proven to be versatile and resulted in large area yield DBRs grown on a variety of different sapphire substrates.

HIGH-Q PHOTONIC CRYSTAL NANOBEAM CAVITY BASED ON A SILICON NITRIDE MEMBRANE INCORPORATING FABRICATION IMPERFECTIONS AND A LOW-INDEX MATERIAL LAYER

We detail the optimization of a nanobeam design and show how the fabrication imperfections can afiect the optical performance of the device. Then we propose the design of a novel conflguration of a photonic crystal nanobeam cavity consisting of a membrane structure obtained by sandwiching a layer of Flowable Oxide (FOx) between two layers of Silicon-Nitride (SiN). Finally, we demonstrate that the presence of a low refractive index layer does not impair the performance of the nanobeam cavity that still exhibits a Q factor and mode volume V of the order of 10 5 and 0.02 (‚=n) 3 , respectively.

High-reflectivity II-VI-based distributed Bragg reflectors for the blue-violet spectral range

We report on the realization of a high quality distributed Bragg reflector for the blue-violet spectral range, with both high and low refractive index layers lattice matched to the GaAs substrate. Our structure is grown by molecular beam epitaxy (MBE). The high refractive index layer is made of ZnMgSSe, while the low index material consists of a short period superlattice containing MgS and ZnCdSe. The refractive index step of Δn = 0.43 results in a stop band width of 40 nm and the normalized reflectivity exceeds 99% for 21 Bragg pairs.

Design and analysis of a phase modulator based on a metal–polymer–silicon hybrid plasmonic waveguide

A plasmonic-hybrid-waveguide-based optical phase modulator is proposed and analyzed. The field enhancement in the low-index high-nonlinear polymer layer provides nanoscale optical confinement and a fast optical modulation speed. At 2.5 V drive voltage, a π phase shift can be obtained for a 13-μm-long plasmonic waveguide. Because of its small capacitance and parasitic resistance, the modulation bandwidth can reach up to 100 GHz with a low power consumption of ∼9 fJ/bit. The plasmonic waveguide is connected to a silicon wire waveguide via an adiabatic taper with a coupling efficiency of ∼91%. The phase modulator can find potential applications in optical telecommunication and interconnects.

Simplified design of thin-film polarizing beam splitter using embedded symmetric trilayer stack

An analytically tractable design procedure is presented for a polarizing beam splitter (PBS) that uses frustrated total internal reflection and optical tunneling by a symmetric LHL trilayer thin-film stack embedded in a high-index prism. Considerable simplification arises when the refractive index of the high-index center layer H matches the refractive index of the prism and its thickness is quarter-wave. This leads to a cube design in which zero reflection for the p polarization is achieved at a 45° angle of incidence independent of the thicknesses of the identical symmetric low-index tunnel layers L and L. Arbitrarily high reflectance for the s polarization is obtained at subwavelength thicknesses of the tunnel layers. This is illustrated by an IR Si-cube PBS that uses an embedded ZnS-Si-ZnS trilayer stack.

Investigation of 1d Photonic Crystal Based on Nano-porous Silicon Multilayer for Optical Filtering

We present the fabrication, simulation, and measurements of 1D photonic crystal based on nano-porous silicon multilayer designed as an optical interference filter. Using electro-chemical etching with timely repeat steps of applied current densities, we fabricated a multilayer structure composed of alternating high- and low-index layer which achieved 90% power reflectivity at wavelength range of 1400-3000 nm. The simulation is relying on the Transfer Matrix Method (TMM) to design and predict the optical properties of nano-porous silicon multilayer as well as the relation between anodization parameters with reflection spectra. The measured reflection and transmission spectra of the nano-porous silicon multilayer show good agreement with simulation. This technique could provide a convenient and economical method to produce filters, cavities, and graded-index dielectric waveguides in the future.

Dielectric shielded nanoscale patch laser resonators

Dielectric shielded nanoscale patch laser resonators are introduced. Low-index dielectric shield layers surrounding a high-index core are shown to significantly reduce both metal and radiation losses. Structures suitable for both optical and electrical pumping and smaller than the vacuum wavelength in all three dimensions are shown to have a low enough threshold gain to lase at room temperature. Shifting the gain medium core provides control over the radiation pattern of the resonator and enables coupling of the laser light into a waveguide, opening opportunities for chipscale integration.

Simulations of the optical properties of broad-area edge-emitting semiconductor lasers at 1060 nm based on the PBC laser concept

The design principles of multi-stripe edge-emitting lasers based on the photonic band crystal (PBC) laser concept are evaluated numerically. The concept is based on a waveguide with a thick quasi-periodic sequence of alternating high- and low-index layers allowing mode expansion across the whole layer sequence. Effective ways to filter out higher-order modes via control of the confinement and leakage are demonstrated as well as enhanced lateral mode selectivity of multi-stripe lasers by changing their etching profile. Thus, for proper epitaxial and lateral design PBC lasers have an enormous potential to provide high-power/brilliance single-mode radiation with broad-area output spot size and nearly circular narrow far-field pattern.

Direct Integration of Nanoscale Conventional and Slot Waveguides

Slot waveguides can provide high optical confinement in a nanoscale low-index layer. While a conventional waveguide has a Gaussian-like Eigenmode profile, the Eigenmode profile of a slot waveguide is quite non-Gaussian type, due to the large discontinuity of refractive indices and thus the transverse electric field component between the high and low index layers of a slot waveguide. Although the field profiles of the two types of waveguides seem different, here we show that direct integration of conventional and slot waveguides yields efficient coupling of light into and out of slot waveguides using the rigorous finite-difference time domain method. The proposed direct coupling method has comparable performance to recently proposed taper based coupling methods, while having advantages in easier integration with conventional waveguide optics and higher integration density. We also show that coupling efficiency is not sensitive to the symmetricity of the slot waveguide, resulting in good manufacturing tolerance. The proposed direct coupler may have a number of applications in lightwave interconnects, sensing and data storage.

Polarization-insensitive subwavelength sharp bends in asymmetric metal/multi-insulator configuration

A new silicon-based sharp waveguide bend in asymmetric metal/multi-insulator configuration is described. TE and TM modes are calculated rigorously electromagnetically from which the general design rules are derived. Numerical simulations show that the respective insertion losses of < 0.085 dB and < 0.229 dB for TE and TM modes can be achieved by introducing a low-index layer between the metal and high-index core. The bending length is determined by the TE mode and has much smaller impacts on the TM that exhibits no resonance-like behavior as does the TE. The combined TE modal and radiation power in the air region is shown to couple back to the Si core through an asymmetric output taper, yielding a high transmission efficiency. Structure-enabled successive photonic-plamsonic mode conversions are shown to increase the TM mode confinement in the high-index core while the plasmonic mode carries up to 42.6% of the input power along the bending section.

赤外線吸収式マルチガスセンサ

We report on a novel MEMS (Micro Electro Mechanical Systems) based Fabry-Perot spectrometer with ultra wide wavelength range (3.2-8.4 μm) compared to the previously reported ones (typically 2.8-5.8 μm). The wavelength range of a Fabry-Perot spectrometer is known to increase by increasing the ratio of the refractive indexes of the multilayer mirrors. Thus, a novel mirror structure was proposed replacing low refractive index layer of SiO2 (nL=1.44) by “air (nL=1)” for wider wavelength range. The proposed device was fabricated by HF sacrificial layer etching of SiO2 between four ultra-thin polysilicon films (ca. 320 nm). The following two ideas were adopted to fabricate this delicate structure properly. (1) Electrostatic force was generated only in the outer region surrounding the air mirror not to deform the mirror. (2) Supercritical drying process was employed after the sacrificial etching to prevent the possible sticking issue.

低损耗大模面积W型布喇格光纤

The influence of Bragg fiber design parameters on photonic bandgap was discussed via the finite element method.The theoretical results show that the bandgap move to long wavelength when the thickness or the refractive index of the Ge-doped layers increases.A novel W-shape-core Bragg fiber was firstly proposed by introducing a F-doped low index layer adjacent of fiber core.The calculation demonstrates the W-shape-core Bragg fiber can reduce the confinement loss and improve the bending-resistant ability effectively.A W-shape-core Bragg fiber was designed with a core diameter of 30 μm,which has a loss of 0.1 dB/km at wavelength of 1.06 μm and is proved single mode propagation theoretically.

High reflectivity monolithic sub-wavelength diffraction grating withGaAs/AlOx stack

This paper presents the fabrication of high reflective sub-wavelength gratings in GaAs on wet oxidizedAlxOy low index layers. This structure can efficiently replace a top Bragg reflector in GaAs-basedvertical cavity lasers (VCSELs), while keeping all the advantages of a monolithic stack.Low refractive index and low loss can be achieved with the oxide sublayer, thus providing ahigh reflectivity over a large wavelength range. Reflectivities above 90% have beenmeasured with stopbands exceeding 130 nm. Further adjustments of the geometricalparameters will enable the reflectivity requirements to be fulfilled for implementation inmicrocavity devices.

A near infrared optical reflector using one-dimensional photonic crystal structure containing chalcogenide glasses

The reflectivity of one-dimensional chalcogenide photonic crystal (CGPC) structure with the first order reflection band in near infrared (NIR) region is theoretically studied. Sb-Se and Ge-S chalcogenide glasses are used as high and low refractive index layers respectively, because these materials have zero absorption in NIR region. The transfer matrix method (TMM) is employed to calculate the reflective spectra of the proposed structure. The theoretical results of reflective spectra of bulk chalcogenide materials with the composition of Sb40Se60 and Ge30S70 for 4, 8, 12 and 15 layers and thicknesses of 117 nm and 183 nm respectively, at normal incidence, are close agreement with the experimental results. Furthermore, by increasing the number of layers of Sb40Se60 and Ge30S70, the reflection bands can be enhanced in the wider range of the NIR region for the polarization at different angles and thus the broadband omnidirectional reflector can be designed.

Mixed metal dielectric gratings for pulse compression.

We report on manufacturing and testing results of high efficiency mixed metal dielectric gratings (MMLD) for high power pulse compression applications. The gratings with 1780 l/mm are etched in the top low index layer of a Au-(SiO2/HfO2)4-SiO2 mirror stack. Various grating profiles manufactured in order to modify the near electric field distribution are damage tested on a facility operating at 1.053 µm, 500 fs pulse duration. We evidence that damage threshold is governed by the value of the maximum electric field intensity inside the grating pillar. Moreover thresholds close to 3 J/cm2 beam normal are obtained with this new MMLD grating being thus an interesting alternative to gold and pure dielectric gratings for pulse compression applications.

Plasma-etched organic layers for antireflection purposes

Organic layers can be used to realize special functions in optical interference coatings. Suitable compounds for such layers were thermally evaporated and characterized. A plasma etching procedure was applied to produce nanostructures on top of the organic layers to reduce their effective refractive indices. Broadband antireflective coatings were obtained by combining these artificial low-index layers with conventionally prepared interference stacks.

Highly emissive and low refractive index layers from doped silica nanospheres for solar cell applications

Confinement of europium (Eu 3+) ions in silica (SiO 2) nanospheres yields efficient red-emitting nanophosphors when excited via charge transfer states (CTS) absorption in UV (393 nm) radiation. This is explained on the basis of modulation of f–f transition due to quantum confinement of rare-earth ion in a nanosize host. It is also evidenced that the short range crystallanity and confinement effects provided by the nanospheres increases the Eu 3+ emission intensity by almost ten times at the expense of CTS. Coating of organically modified SiO 2 nanospheres resulted in low refractive index layers that are highly useful as cover glazing for solar collectors.

Sub-wavelength plasmonic modes in a conductor-gap-dielectric system with a nanoscale gap

We study guided modes in a conductor-gap-dielectric (CGD) system that includes a low-index dielectric gap layer of deep sub-wavelength thickness sandwiched between a conductor and a high-index dielectric cladding. Analysis of the dispersion equation for CGD modes provides an analytical estimation for the cut-off thickness of the gap layer. This guided mode is unusual because it exists when the gap thickness is less than the cutoff thickness. In the direction normal to the interfaces, the modal electric field is tightly confined within the gap. Sub-wavelength lateral mode confinement is readily provided by a spatial variation of the gap-layer thickness: the modal field localizes at the narrowest gap. Various lateral confinement schemes are proposed and verified by numerical simulations. Possible applications of CGD modes include surface-plasmon nano-lasers (SPASERs) and sensors. If these plasmonic waveguides are scaled for operation at far infrared rather than telecomm wavelengths, then the propagation losses are dramatically reduced, thereby enabling the construction of practical chip-scale plasmonic integrated circuits or PLICs.

Thick optical waveguides in lithium niobate induced by swift heavy ions (~10 MeV/amu) at ultralow fluences

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm). They propagate ordinary and extraordinary modes with low losses keeping a high nonlinear optical response (SHG) that makes them useful for many applications. Complementary RBS/C data provide consistent values for the partial amorphization and refractive index change at the surface. The proposed method is based on ion-induced damage caused by electronic excitation and essentially differs from the usual implantation technique using light ions (H and He) of MeV energies. It implies the generation of a buried low-index layer (acting as optical barrier), made up of amorphous nanotracks embedded into the crystalline lithium niobate crystal. An effective dielectric medium approach is developed to describe the index profiles of the waveguides. This first test demonstration could be extended to other crystalline materials and could be of great usefulness for mid-infrared applications.