Low Refractive Index Material Research Articles

Maximum length sequence dielectric multilayer reflector

This paper explores the wavelength-dependent reflectivity of alternating high and low refractive index multilayers with a thickness profile defined by a pseudo-random, maximum length sequence (MLS). An MLS contains all possible combinations of a binary sequence save one; thus, a multilayer with an MLS profile contains a superposition of a broad range of periods. The range of periodicities in an MLS multilayer should make these systems more effective broad wavelength reflectors as compared to purely periodic counterparts. We compute the reflection characteristics of MLS and periodic dielectric sequences at visible wavelengths over a range of incident angles using the transfer matrix method (TMM), a recursive multilayer calculation method. The materials SiO2 and TiO2 are chosen as the low and high refractive index materials, respectively, because these materials are commonly used in optical multilayers and because their wavelength-dependent refractive index is well known. Our results show that it is possible to create an MLS structure with high average reflectivity across the entire visible spectrum (400 nm – 700 nm) at all incident angles and polarizations. Finally, we compare the reflection characteristics of dielectric multilayers with metallic reflectors whose refractive index is based on a Brendel-Bormann (BB) model. The comparison shows that a seventh order MLS aperiodic multilayer exhibits slightly higher average reflectivity over the visible spectum than silver or aluminum metallic reflectors.

Invited Article: Bragg stacks with tailored disorder create brilliant whiteness

The scales of white beetles strongly scatter light within a thin disordered network of chitin filaments. There is no comparable artificial material achieving such a high scattering strength within a thin layer of low refractive index material. Several analyses investigated the scattering but could not explain the underlying concept. Here a model system is described, which has the same optical properties as the white beetles’ scales in the visible wavelength range. With some modification, it also explains the behavior of the structures in the near infrared range. The comparison of the original structure and the model system is done by finite-difference time-domain calculations. The calculations show excellent agreement with the beetles’ scales with respect to the reflectance, the time-of-flight, and the intensity distribution in the far-field.

Experimental demonstration of narrow-band rugate minus filters using rapidly alternating deposition technology**Project supported by the Postdoctoral Science Foundation of China (Grant Nos. 2016M592528 and 2016M602517), the Natural Science Foundation of Guangdong Province, China (Grant No. 2017A030310130), and the National Natural Science Foundation of China (Grant No. 11705259).

The design, fabrication and performance of narrow-band rugate minus filters are investigated in this paper. A method of fabricating graded-index coatings by rapidly alternating deposition of low (SiO2) and high (Al2O3) refractive index materials is presented to fabricate a rugate structure. The narrow-band rugate minus filter design and fabrication approaches are discussed in detail. The experimental results, including transmittance spectrum, surface damage test and damage morphology investigated with a scanning electron microscope, demonstrate the high performance of the as-fabricated spatial filter and confirm the feasibility of the fabrication method for narrow-band rugate minus filters.

Design and optimization of thin film polarizer at the wavelength of 1540 nm using differential evolution algorithm

In this paper, a thin film polarizer at the wavelength of 1540 nm in infrared region was designed and optimized using differential evolution method. It is shown how the algorithm’s parameters can change the output result to obtain the best consequence of optimization. This polarizer consists of a few pairs of high and low refractive index dielectric materials, titanium dioxide and silicon dioxide, respectively, with $$BK_{7}$$ glass substrate and the angle of incident light was supposed 56° that is the Brewster angle for $$BK_{7}$$ glass. Our final optimized polarizer has 91.20 and 0.336% transmittance for P and S polarization, respectively, and a 271 ratio of $$\frac{{T_{P} }}{{T_{S} }}$$ which has high significance for this polarizer. It consists of eight pairs of layers with low and high refractive index materials and 3369.1 nm physical thickness which is used to separate S and P polarized light for Q-switching process.

Bio‐inspired Highly Scattering Networks via Polymer Phase Separation

AbstractA common strategy to optimize whiteness in living organisms consists in using 3D random networks with dense and polydisperse scattering elements constituted by relatively low refractive index materials. Inspired by these natural architectures, a fast and scalable method to produce highly scattering porous polymer films via phase separation is developed. By varying the molecular weight of the polymer, the morphology of the porous films is modified, and therefore their scattering properties are tuned. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low refractive index systems, e.g., standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of ≈75% while only 4 µm thick. In addition, the films are flexible and can be readily index‐matched with water (i.e., they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.

Experimental sol-gel fabrication and theoretical simulation of one-dimensional photonic crystals with a defect state

One-dimensional multilayer structures of the periodically alternating low refractive index (silica) and high refractive index (titania) materials have been deposited by sol–gel spincoating. The possibility of creating an optical filter by introducing a structural defect in the form of a layer of double thickness is shown theoretically and experimentally. All experimental spectra are in agreement with theoretical spectra calculated by transfer matrix method.

Geometric Design of Scalable Forward Scatterers for Optimally Efficient Solar Transformers.

It will be ideal to deliver equal, optimally efficient "doses" of sunlight to all cells in a photobioreactor system, while simultaneously utilizing the entire solar resource. Backed by the numerical scattering simulation and optimization, here, the design, synthesis, and characterization of the synthetic iridocytes that recapitulated the salient forward-scattering behavior of the Tridacnid clam system are reported, which presents the first geometric solution to allow narrow, precise forward redistribution of flux, utilizing the solar resource at the maximum quantum efficiency possible in living cells. The synthetic iridocytes are composed of silica nanoparticles in microspheres embedded in gelatin, both are low refractive index materials and inexpensive. They show wavelength selectivity, have little loss (the back-scattering intensity is reduced to less than ≈0.01% of the forward-scattered intensity), and narrow forward scattering cone similar to giant clams. Moreover, by comparing experiments and theoretical calculation, it is confirmed that the nonuniformity of the scatter sizes is a "feature not a bug" of the design, allowing for efficient, forward redistribution of solar flux in a micrometer-scaled paradigm. This method is environmentally benign, inexpensive, and scalable to produce optical components that will find uses in efficiency-limited solar conversion technologies, heat sinks, and biofuel production.

Ultrahigh Purcell Factor, Improved Sensitivity, and Enhanced Optical Force in Dielectric Bowtie Whispering-Gallery-Mode Resonators

We propose and theoretically investigate an all-dielectric bowtie whispering-gallery-mode (WGM) resonator which consists of two tip-to-tip coupled semiconductor nanorings with triangular cross section separated by low refractive index material gap. Mode splitting of symmetric WGM and antisymmetric WGM is observed and analyzed. Due to extremely field enhancements by the “slot” and “tip” effects, strong localization of light in the gap of symmetric WGMs leads to ultrasmall modal volume of $0.3\,\mu {\rm{m}}^{3}$ . This value is two orders of magnitude smaller than that in toroidal microresonator which is the typical WGM microcavity with tight mode confinement. Importantly, large amount of light confined in the gap in bowtie WGM resonator not only suppresses the radiation loss (radiation-loss-related quality factor is above $10^{8}$ ), but also makes it as an ideal platform for Purcell effect enhancement, ultrasensitive sensing, and optical trapping of nanoparticles. Calculation results show that Purcell factor can reach to $10^{8}$ at room temperature when assuming quantum dots or atoms placed in the gap. Refractive index sensitivity is improved to 700 nm/RIU as compared with conventional slot waveguide with rectangular cross section. The optical gradient force is greatly enhanced and allows efficient trapping of single nanoscale particle with diameter of 5 nm even at a relatively large gap of 100 nm.

Anomalous dispersion engineering of co-sputtering Ag-AZO hybrids for antireflection coatings.

Conventional antireflection coatings (ARCs), including single- and multi-layer, bionic micro/nano, and gradient-index structures, often do not take anomalous dispersive materials into account, but normal dispersion and Rayleigh configuration, known as a step-down index profile, in which low refractive index (RI) materials are adjacent to incident media and the high RI ones, are located on substrates. We found that the anomalous dispersive materials could be good candidates for novel sub-100nm ARCs, considering the ab-initio impedance matching designs. Engineering the anomalous dispersion of co-sputtering Ag-AZO hybrid materials, two types of ARCs were fabricated on silicon substrates: a 70nm tri-layer reverse-Rayleigh and a 140nm bi-layer Rayleigh for comparison. We observed that both exhibited the average reflectance of 5.46% and 1.49% at the wavelength range from 400 to 780nm, respectively, and were much lower than that of bare silicon, 38.02%, where the underlying origin for the reverse-Rayleigh AR was discussed. The studies on the engineering and integration of anomalous dispersive materials into ARCs and optical elements would be of great significance for compact light harvesting, energy conversion, and radar stealth technology.

Vibrational and valence aloof beam EELS: A potential tool for nondestructive characterization of nanoparticle surfaces

In many materials systems, electron beam effects may substantially alter and destroy the structure of interest during observation. This is often true for the surface structures of catalytic nanoparticles where the functionality is associated with thin surface layers which are easily destroyed. The potential application of using aloof beam electron energy-loss spectroscopy as a non-destructive nanoscale surface characterization tool is discussed. Recent developments in monochromators make vibration and valence loss EELS possible in the electron microscope. The delocalization associated with these signals allows spectra to be acquired when the electron beam is position 2nm or more away from the particle surface. This eliminates knock-on damage and significantly reduces ionization damage. Theoretical and experimental results are employed to explore the potential strengths and weaknesses of monochromated aloof beam EELS for surface analysis. The approach is most favored for surface layers on insulators because the bandgap lowers the background for detection of the vibrational signal and bandgap states. Guided light modes and relativistic effects can complicate the interpretation of the spectra. The effects are suppressed at lower accelerating voltages and particle size especially for low refractive index materials.

Study and Fabricate of Reflectance Coating for UV Lithography Machine Arc Lampshade

As one of the key components in lithography, the UV light source has very high demands for wavelength of source and energy. To meet the requirements of UV light source, it demands UV reflectance coating prepared on 78mm Mg-Al alloy paraboloid lampshade . Based on design theory of film, H4 and SiO2 were selected as high and low refractive index materials respectively, and with the help of TFCalc, adopting electron beam vacuum coating method, the reflectance coating was prepared, which satisfied the requirements of reflectivity greater than 98.5% within band of 365-436nm in the case of 0o-45o incident angles. First the reflective band width was broadened through the coating stack; then according to the thickness distribution of film, compensation baffle was designed and modified, which ensured the uniformity of reflectance coating in the lampshade. And last, firmness of coating was strengthened and the reflectivity in the required band was improved by depositing one bonding layer of metal between Mg-Al alloy and dielectric film. By repeated trials, coating absorption problem was solved by changing the process parameters and coating quality of film was improved with the use of ion source assisted deposition, finally the UV film meets the requirements for use.

Gaussian oscillator model of the dielectric constant of SiO2 thin film in infrared range

SiO2 thin film is one of the most important low refractive index materials in the area of optical thin films. It is always used in the design and preparation of many kinds of multilayer films. The dielectric constant of the SiO2 thin film is a key characteristic for design of the multilayer thin film. The composite Gaussian oscillator model is one of the most important dispersion models for the dielectric constant of the amorphous SiO2 in the anomalous dispersion regime in the infrared range. More and more researchers have focused on the number and the physical meaning of the oscillators in the composite oscillator. A method to determine the SiO2 thin film oscillator quantity was proposed. In this method, the quantity of oscillator peaks was equivalent to the oscillator number of chemical composition, based on the factor analysis technology of chemometrics. Concretely, the composite oscillators of the dielectric constant were equivalent to the mixture, and the independent oscillators were equivalent to the compositions of the mixture. The absorbance of the mixture changed with the physical thickness of the thin film. Eight SiO2 film samples with different thickness were prepared on the Si substrate by the ion beam sputtering deposition. The infrared transmittances of the eight samples were used as elements in the spectral matrix. There were nine Gaussian oscillators in the range of 400-4000 cm-1, which was determined by the factor analysis technology. The dielectric constant of the SiO2 thin film in this range was obtained by the inverse calculation from the spectral transmittance. It provides the inverse calculation result for the dielectric constant of the SiO2 thin film with a specific physical meaning. By analyzing the dielectric constant in the range of 400-900 cm-1, the symmetric stretching vibrational frequency and the in-plane rocking frequency of the Si-O-Si bond of the SiO2 thin film can be obtained. Compared with fused silica, the symmetric stretching vibrational frequency increased while the rocking frequency was reduced. In fact, the frequency shifts are caused by the strain of the thin film. By analyzing the dielectric constant in the range of 900-1500 cm-1, four anti-symmetric stretching vibrational frequencies of the Si-O-Si bond in the SiO2 thin film were obtained. They have a certain corresponding relation with the anti-symmetric stretching vibrational frequency of the Si-O-Si bond in the fused silica. What's more, by analyzing the dielectric constant in the range of 3000-4000 cm-1, the water-cut (or hydroxyl) chemical defects in the films were confirmed. The chemical defects can influence the dielectric constant in the whole infrared range.

Easily made antireflective coatings for flexible surfaces

Conventional antiglare coatings for eyeglasses, camera lenses, and car headlights are made of thin films of low refractive index materials such as magnesium fluoride. Although they work well, they can be applied only to flat surfaces and are difficult to make over large areas. Now, a simple method to create durable, antireflective coatings could make it easier to produce large, glare-free windows and flexible electronic displays (ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/acsami.6b10624). Norihiro Mizoshita and Hiromitsu Tanaka of Toyota Central R&D Laboratories made the coatings by spraying plastic substrates with nanosized silica beads dispersed in ethanol. After the ethanol evaporated, the researchers exposed the substrates to chloroform vapor, which softened the plastic and allowed the nanospheres to sink into the surface. Washing away excess nanoparticles left behind a single layer of beads embedded about 50 nm deep in the plastic. Coatings made of 190-nm-wide particles reflected 1.2% of visible

Ray picture and ray-wave correspondence in triangular microlasers

We apply ray-optical methods to dielectric optical microcavities in the shape of triangles made of low refractive index material. We find ray trajectories that maximize the intensity inside the cavity to determine the far-field emission characteristics and to complement the concept of the unstable manifold applicable to chaotic microlasers. As these maximum intensity trajectories need not be periodic, we suggest that they provide a more general explanation for emission patterns of microlasers than short periodic orbits. Further, the geometrical optics description is extended by the inclusion of intensity amplification along the optical path to achieve a better description of active, lasing cavities. Far-field emission patterns of equilateral triangle cavities obtained in this way agree well with our full electromagnetic wave simulations and with previously reported experimental results.

Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices.

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

(Invited) Silicon Oxide Films: Plasma Assisted Formation of Nanostructures from Glass to Organic Polymers

Thin silicon oxide films produced by means of plasma enhanced deposition techniques cover a large area of applications with a long industrial tradition. It is long-recognized that the chemical composition, physical properties and particularly the functionalisation of SiOx films can be adjusted in a wide range by choosing the appropriate plasma conditions for the deposition process. For PECVD, the proper choice of the silicon-organic molecule as thin film precursor is also substantial. Next to the production of amorphous SiO2films by PVD for optical coatings, also SiOx films by PECVD are known that mimic the functionality of a glass surface. Prominent examples include films for corrosion protection, as passivation and insulation layer, and for permeation barrier coatings. Here, dense, pin-hole free films are required that are usually characterized by a stoichiometry close to x=2, with low carbon content (below 5%). With increasing polymeric character and depending on the degree of cross-linking the films become attractive for applications such as adhesion promoting interfaces in multilayer systems, for the integration of catalysts into surfaces or as selective absorber in sensor applications. For these purposes, films with micro- and nano-porous morphologies are preferred. It is generally supposed that films with superior qualities (e.g. spatial morphological homogeneity) are produced by using low pressure methods due their technological maturity in comparison to atmospheric pressure thin film deposition techniques which have found increasing interest over the last decades. In this contribution, several examples of deposition processes for SiOx at atmospheric pressure are presented. The nanostructure of the films is emphasized in particular. An atmospheric pressure dielectric barrier discharge (apDBD) using Ar, O2and OMCTS (D4, octamethylcyclotetrasiloxane) is presented for the deposition of SiOx release films. Deposition using a microwave plasma jet with TTMS (Tetrakis(trimethylsilyloxy)silane) is shown as an example for the deposition of nanostructured dendritic films. A similar experiment with HMDSO (hexamethyldisiloxane) and with an RF plasma jet demonstrates advantages of self-organisation mechanisms of plasma sources under atmospheric pressure for the chemical homogeneity of the film. Compared to other non-thermal plasmas, the geometry of the plasma jet is advantageous for local selective deposition, particularly on 3D shaped substrates. The jet geometry allows better flexibility in terms of distance between active plasma and substrate position and exposure to high electrical fields, as the surfaces to be treated are not necessarily placed between electrodes. Furthermore, the low pressure deposition of amorphous SiO2films using PVD with additional kinetic ions is reported. These films serve as low refractive index material for multiple thin film stacks in applications for high-precision thin film optics, such as laser mirrors or optical filters. The refractive index of the deposited film depends crucially on the plasma conditions. The chemical composition of films is systematically characterized using different surface analysis techniques (XPS, FTIR). The stoichiometry and carbon content are evaluated and robustly classified regarding the chemical structure (polymer-like, crosspolymer-like, inorganic) and allocated to an overview structure diagram proposed as a general classification model for SiOxCy films. The correlation of the chemical properties is thoroughly discussed in relation to the morphologic characterisation (based on SEM and AFM). This research has been supported by the projects R&D center for low-cost plasma and nanotechnology surface modifications CZ.1.05/2.1.00/03.0086, by the project No 7AMB12DE005 funded by the Czech Ministry of Education, by the DAAD project No 54437076 and by the German Research Foundation under grant LO 623/3-1 and TRR 24 ‘Complex Plasmas’.

Sol-gel fabrication of one-dimensional photonic crystals with predicted transmission spectra

One-dimensional multilayer structures of periodically alternating low refractive index (silica) and high refractive index (titania) materials have been deposited by sol-gel spincoating. Experimental spectra of the structures are in agreement with spectra calculated by transfer matrix technique. As an example, theoretical and experimental spectra with a stop band corresponding 600 nm-reflection are shown.

Indium phosphide all air-gap Fabry-Pérot filters for near-infrared spectroscopic applications

Food quality can be characterized by noninvasive techniques such as spectroscopy in the Near Infrared wavelength range. For example, 930 -1450 nm wavelength range can be used to detect diseases and differentiate between meat samples. Miniaturization of such NIR spectrometers is useful for quick and mobile characterization of food samples. Spectrometers can be miniaturized, without compromising the spectral resolution, using Fabry-Pérot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. The most commonly used mirrors in the design of FP filters are Distributed Bragg Reflections (DBRs) consisting of alternating high and low refractive index material pairs, due to their high reflectivity compared to metal mirrors. However, DBRs have high reflectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer detector. Therefore, a bandpass filter is usually required to restrict wavelengths outside the stopband of the FP DBRs. Such bandpass filters are difficult to design and implement. Alternatively, high index contrast materials must be can be used to broaden the stopband width of the FP DBRs. In this work, Indium phosphide all air-gap filters are proposed in conjunction with InGaAs based detectors. The designed filter has a wide stopband covering the entire InGaAs detector sensitivity range. The filter can be tuned in the 950-1450 nm with single mode operation. The designed filter can hence be used for noninvasive meat quality control.

Biomedical bandpass filter for fluorescence microscopy imaging based on TiO2/SiO2 and TiO2/MgF2 dielectric multilayers

We report a design for creating a multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 & 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Reflectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high reflectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer range. Therefore a bandpass filters are required to restrict wavelength outside the stopband of the FP DBRs. The proposed filter shows a high quality with average transmission of 97.4% within the passbands and the transmission outside the passband is around 4%. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptional choice for florescence imaging and Endoscope narrow band imaging.

One-dimensional PMMA–V2O5 photonic crystals used as color indicators of chloroform vapors

The fabrication and characterization of one-dimensional photonic crystals (1D-PC) in the form of quarter-wavelength multilayered stack (Bragg stack) are presented. The 1D-PC comprises alternatively deposited spin-coated polymer PMMA (Poly(methyl methacrylate)) films as low-refractive index material that change thickness upon exposure to chloroform vapors, and both dense and porous V2O5 films providing both high optical contrast. The possibility of using PMMA–V2O5 structure as an omnidirectional reflector in both cases is confirmed experimentally. Besides, the prospect of utilizing the studied structures as optical indicators of chloroform vapors is demonstrated and discussed.