Low-index Layer Research Articles

Modified multicomponent band pass optical reflection filters at oblique incidence

Reflection and transmission characteristics of a modified long-wavelength cutoff filter using a three-component system with an angular dependence are studied. In this case, a layer of high refractive index is sandwiched between edge layers of equivalent refractive index with the base structure [G(0.5AB0.5A)]N; then, an optimized structure of the form [(0.5AB0.5A)N−X (0.5AB0.5A)X (0.5AB0.5A)N−X] is taken and its spectral properties are studied. In the case of a short-wavelength three-component cutoff filter, a layer of low refractive index is sandwiched between two edge layers of high equivalent refractive index with a base structure [G(0.5BA0.5D)]N and its spectral characteristics are studied. Then, the structure is optimized [(0.5BA0.5B)N− X(0.5BA0.5B)X(0.5BA0.5B)N−tX] and spectral characteristics of this structure are also studied. Spectral properties of band pass optical reflection filters of the type [G(XY)A] whose components are the proposed long-wavelength cutoff [(0.5AB0.5B)N− X(0.5AB0.5A)X(0.5BA0.5B)N−X] and short-wavelength cutoff [(0.5BA0.5B)N−X(0.5BA0.5B)X(0.5BA0.5B)N−X] filter structures are studied with angular dependences. No increase in the bandwidth with the least oscillatory components at the edges can be precisely formed by known classes of a two-component system at normal incidence. It is shown that the use of reflection and transmission filters with oblique bands yields an ideal polarizer. Angular dependences of the filter and the transmission and reflection band shapes on the parameters of a high-reflectance multilayer coating, such as the thickness, refractive indices, and the number of layers, are analyzed in detail.

IR reflection spectra of monolayer films sandwiched between two high refractive index materials.

Monolayer films adsorbed on substrates with high refractive indices such as metals or semiconductors yield strongly enhanced infrared reflection spectra when they are contacted with a transparent, high refractive index ambient medium and are probed with p-polarized light at high incidence angles. The sensitivity increase arises from the enhancement of the perpendicular electric field within a thin, low refractive index layer sandwiched between two high refractive index materials and gives rise to signal intensity gains up to 2 orders of magnitude in combination with an essentially exclusive detection of only perpendicular surface vibrations. Experimental spectra of ordered monolayer films of octadecanethiol on gold and of octadecylsiloxane on silicon in this sandwich configuration yield enhancement factors between 15 (on Si) and 30 (on gold) compared to conventional grazing incidence external reflection spectra and are governed by a common, simple surface selection rule, which allows immediate quantitative evaluation and comparison of the film structures on different substrates.

Analysis of ring-structured Bragg fibres for single TE mode guidance

Ring-structured Bragg fibres that support a single TE-polarisation mode are investigated. The fibre designs consist of a hollow core and rings of holes concentric with the core, which form the low-index layers of the Bragg reflector in the cladding. The effects of varying the air fraction in each ring of holes on the transmission properties of the fibres are analysed and an approximate model based on homogenisation is explored. Surface modes and transitions thereof are also discussed.

Fiber-Optic Multiple-Wavelength Filter Based on One-Dimensional Photonic Bandgap Structures With Defects

In this paper, a theoretical analysis is given of an optical fiber multiple-wavelength tunable filter based on a one-dimensional (1-D) photonic bandgap (PBG) structure with four defects. To understand the positioning of the modes in the bandgap, a previous analysis of structures with one and two defects is performed. By adequate parameterization, it will be possible to control the central wavelengths of the various filters of the device. Parameters responsible for this effect are the contrast of refractive indexes of high- and low-index layers, the optical thickness of the defects, and the number of layers stacked among the defects related to those stacked at the extremes. In addition to this, the finesse of the filters can be controlled by the adequate addition of layers among defects. As a result, a simple 1-D PBG structure with defects will permit designing almost any multiple-wavelength filter, with immediate application in the treatment of wavelength-division-multiplexed (WDM) signals. The possibility of the tunability of this device can be introduced if materials are included whose refractive index changes with some parameter, such as temperature, voltage, or strain. As an example, liquid crystals change their refractive index with an applied voltage, leading to a shift of the central wavelengths of the filters.

GaInN light-emitting diodes with RuO2/SiO2/Ag omni-directional reflector

A GaInN light-emitting diode (LED) employing an omni-directional reflector (ODR) is presented. The ODR consists of a RuO2 ohmic contact to p-type GaN, a quarter-wave thick SiO2 low-index layer perforated by an array of micro-contacts, and an Ag layer. Calculations predict a 98% angle-averaged reflectivity at λ=450 nm for an GaN/SiO2/Ag ODR, much higher than that for a 20 period Al0.25Ga0.75N/GaN distributed Bragg reflector (49%) and an Ag reflector (94%). It is shown that the RuO2/SiO2/Ag ODR has higher reflectivity than Ni/Au and even Ag reflectors, leading to a higher light extraction efficiency of GaInN LEDs with ODR. The electrical properties of the ODR-LED are comparable to those LEDs with a conventional Ni/Au contact.

The ideal vehicle for optical model development: porous silicon multilayers

A proprietary field emission scanning electron microscopy (FESEM) image analysis method is proposed to support the optical modelling of spectroscopic ellipsometry data evaluation of complex porous silicon multilayer stacks (PSM). Sample structure was a 27-layer Fabry–Perot type PSM stack, which consists of nominally λ 0/4 (optical thickness) layers of high and low refractive indices (low and high porosity). Two types of optical models were adopted (derived from FESEM imaging): one considering step-like, abrupt interfaces and another one taking gradual transitions between the layers of high and low index of refraction into account. The results suggest that there is no sense to refine the optical models beyond any limit, when the uncertainties given by the tolerances involved in the interface determination of this random PSM structure do not justify the efforts any more.

Implantation induced wavelength shift in the reflectivity spectra of green and orange Al Ga1−As distributed Bragg reflectors

The effects of ion implantation on the reflectivity spectra of Al0.98Ga0.02As/Al0.7Ga0.3As and Al0.98Ga0.02As/Al0.5Ga0.5As distributed Bragg reflectors (DBRs) are investigated. Three different ions (H+, O+, Si+) are implanted into the DBR for comparison. The reflectivity spectra in the O-implanted and Si-implanted DBRs exhibit large amount of red-shift, but almost the same as unimplanted sample in the H-implanted DBRs is found. The maximum wavelength shift of 20 nm is observed for Si-implanted DBR in our experiment. The peak wavelength shifts toward longer wavelength after ion implantation, which is attributed to the Al–Ga intermixing near interfaces in DBRs. In addition, the wavelength shift of implanted Al0.98Ga0.02As/Al0.5Ga0.5As DBR is more obvious than that of Al0.98Ga0.02As/Al0.7Ga0.3As DBRs. This means the implantation induced intermixing is reduced while decreasing compositional contrast of Al concentration between high- and low-index layers.

Optical waveguides with an aqueous core and a low-index nanoporous cladding

We report measurements on planar optical waveguides having an aqueous core and a low-index nanoporous dielectric cladding. Spin-on deposition of the nanoporous dielectric results in a thin (~0.8 microm) low-index cladding layer on a higher-index fused silica substrate, which produces leaky waveguide modes; however, for the aqueous layer thickness needed in most microfluidic applications, a large number of low-loss modes exist. We demonstrate that such a waveguide can be used for efficient collection and transport of fluorescence generated within the aqueous core and show that the use of these nanoporous materials offers advantages over the principal alternative, Teflon(R) AF.

Condenser-Transducer Configuration for Improving Radiation Efficiency of Near-Field Optical Transducers

ABSTRACTNear-field radiation efficiency of the ridge waveguide transducer is investigated in the vicinity of a recording magnetic medium. Near-field radiation from a ridge waveguide transducer is expressed in terms of power density quantities. This allows us to quantify the near-field radiation efficiency from the near-field transducer with respect to the input optical power. Finite element method (FEM), which is capable of modeling focused beams, is used to simulate various geometries involving ridge waveguides. The incident electric field near the focal region is determined using a Gaussian beam expression and Richards-Wolf vector field equations for low NA and high NA beams, respectively. First, the ridge waveguide transducer is placed at the focal point of an optical lens system. The maximum value of the absorbed optical power in the recording medium is 1.6*10-4 mW/nm3 for a 100 mW input optical power. Finally, the ridge waveguide is placed adjacent to a solid immersion lens but separated by a low-index dielectric layer. For this case, the maximum value of the absorbed optical power in the recording medium is 7.5*10-4 mW/nm3 for a 100 mW input optical power. The improvement in the transmission efficiency is a result of two factors: 1. Increased incident electric field over the transducer surface due to increased NA of the optical system, 2. Surface plasmon enhancement obtained by placing a low-index dielectric material between the solid immersion lens and ridge waveguide.

Digital Design of Broadband Antireflection Coating

A digital search technique based on the flip–flop method was applied to design a broadband antireflection coating using very thin high-, middle- and low-index layers. The optical performance of the average reflectivity over the visible region (from 400 to 750 nm) was 0.19%, which yielded a better result than the design using only very thin high- and low-index layers.

Omnidirectional optical mirror in a cladded-superlattice structure

In recent years, it has been shown that one-dimensional photonic crystals such as superlattices can exhibit an omnidirectional reflection of light for a given frequency range. However, this property requires that the incident wave be launched from vacuum or from a low refractive index material. In this article, we show that the limitation about the choice of the substrate can be removed by adding a clad layer of low refractive index at one boundary of the superlattice. The additional layer acts like a barrier for the propagation of light. An alternative solution based on a combination of two multilayer structures is also mentioned.

Asymmetric anti-resonant reflecting optical waveguides (arrow) as chemical sensors.

Anti-resonant reflecting optical waveguides (ARROW) are described which trap light in a low index layer between a lower, high-index confining layer and an upper total internal reflection boundary. In this configuration, most of the light (greater than 80%) travels in the low index porous polymer layer, the refractive index of which is monitored by examining the angle at which light is coupled out of the waveguide. It is shown that asymmetric ARROW sensors can be constructed using conventional chemical vapour deposition and spin-coating techniques and their sensitivity is as predicted by theoretical modelling.

Preparation and characteristics of porous silica films by a modified base-catalyzed sol-gel process containing PVA: II. Film preparation

Porous SiO2 films were successfully deposited on silicon substrates by a modified base-catalyzed Sol-Gel process (MBCP) containing polyvinyl alcohol (PVA). The process conditions, such as the gelation time, the synthesis temperature, the stabilizing agent of the precursor solution and the spin coating speed, the heat-treatment, the annealing temperature of the film on the microstructure and porosity of porous SiO2 films were systematically investigated by SEM, XRD and ellipsometry techniques. This study provides a novel preparation technique for the porous SiO2 film. Using this process, the resultant film can reach a thickness of 3.6 μm for one layer, a porosity of 25–50%, a low thermal conductivity of 0.11 W/m·K. This film will be used as a low dielectric layer, an thermal-insulating layer and a low refractive index layer.

Photonic band-gap guidance in high-porosity luminescent porous silicon

We demonstrate a nanoporous silicon (nPS) multilayered structure, offering an efficient photonic band-gap (PBG) single-mode guidance in a high-porosity (i.e., low index) layer in the near-infrared. We show the good agreement between the calculated band structure and the measured spectral range of efficient guiding. We discuss the different kinds of guided modes that exist within the structure, and verify that only one PBG mode can be guided efficiently. Quantitative measurements of the losses are performed using the room-temperature photoluminescence of nPS.

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.

Photonic bandgap properties of nanoporous silicon microstructures

We report the recent progress in the structuration of nanoporous silicon at a submicronic scale. Multilayered periodic structures, with low-roughness interfaces, and an index contrast of 0.6 are reported. Their strong photonic bandgap properties are demonstrated by the measurement of a reflectance up to 99.64% ± 0.01%, by mean of ring-down spectroscopy. This property is the base of a novel structure allowing an efficient guiding of light in a low-index layer. The submicronic structuration of the optical index of a planar waveguide along one direction of plane is also described, and a realistic map of the optical index, demonstrating a index contrast of 0.5, is unambiguously deduced from transmission spectra.

Synthesis of thick optical thin-film filters with a layer-peeling inverse-scattering algorithm

We present an efficient and accurate method for synthesis of optical thin-film structures. The method is based on a differential inverse-scattering algorithm and considers therefore both phase and amplitude reflectance data. We apply the algorithm to the synthesis of filters with arbitrary index layers and two-material filters consisting of only high- and low-index layers. The layered structure is approximated by a stack of discrete reflectors with equal distance between all reflectors. This mirror stack is in turn determined from the desired, complex reflection spectrum by a layer-peeling inverse-scattering algorithm. The complexity of the design algorithm is approximately the same as that of the forward problem of computing the spectrum from a known structure.

General electromagnetic density of modes for a one-dimensional photonic crystal

In this paper, we present more general, exact, and concise expressions for calculating the electromagnetic density of modes (EDOM) in one dimension photonic crystal (superlattice) for E and H polarizations. The expression is used for numerical computation of the EDOM in the lower-index (dielectric constant) layer. We discuss the difference between the EDOM in high- and low-index layers as due to the presence of waveguiding modes and evanescent-excited Bloch modes in the higher-index layer. Two methods of computation are presented to compute the EDOM in the lower-index layer. We suggest the possibility of using the EDOM to establish population inversion, which may be useful for higher-frequency lasers (e.g., x rays) and control any radiative processes. We also elaborate on the limitations of the results of Alvarado Rodriguez et al. as due to the approximation used in the evaluation of partial differentialomega/ partial differentialk(y,z) for nabla(k)omega and comment on the limitations of the one-dimensional EDOM expression of Bendickson et al.

Omnidirectional reflection by multilayer dielectric mirrors

Periodic structures reflect strongly at frequencies and angles of incidence corresponding to photonic bandgaps. It has recently been shown that for a suitable choice of high and low refractive indices nh and nl, a high-low dielectric multilayer can reflect strongly at all angles of incidence. An accurate and simple analytic approximation for the location of the band edges for the s and p polarizations is given. The region in the (nl,nh) plane where omnidirectional reflection occurs is determined. This region lies above a curve which has its minimum value of nh(≈2.247n1) at nl≈1.492n1, where n1 is the refractive index of the medium of incidence. The minimum occurs when the optical paths in the high and low refractive index layers are in the ratio 1.362 to 1.

Molecular beam epitaxial growth of vertical cavity surface emitting lasers with digital alloys and digital gradings

Electrical resistance through vertical cavity surface emitting lasers (VCSELs) grown by molecular beam epitaxy is generally higher than in VCSELs grown by metalorganic chemical vapor deposition (MOCVD). The better performance of MOCVD-grown material is attributed to the fact that MOCVD can continuously grade the composition between the high index (narrow band gap) and low index (wide band gap) materials that constitute the Bragg mirror stack. This leads to a lower resistance for the charge to traverse through the mirror stack. In addition, the oxidation of VCSEL apertures dramatically reduces threshold current Ith; however, the oxidation process requires a low index layer with a high aluminum mole fraction for reasonable oxidation rates and for good material stability. The MOCVD process benefits from its ability to tailor the high mole fraction AlGaAs material that is crucial for creating the oxide apertures. The majority of these problems can be alleviated using digital alloys and digital grades.