Low Refractive Index Material Research Articles

Structures and Optical Properties of Anodic Aluminum Oxide Thin Films

Low refractive index materials (n<1.3) are not common in nature. However, they are essential for antireflection coatings. In this study porous anodic aluminum oxide (AAO) on glass substrate was fabricated by electrochemical oxidation and subsequent etching. The pore size was modulated from less than 80 nm to more than 250 nm. The pore depth was controlled by electrochemical anodization and/or chemical etching time. It is challenging to effectively quantify the pore structures and the optical properties of such porous materials. Using spectroscopic ellipsometry, the authors showed that the AAO materials had tunable refractive index from 1.25 to 1.40, which is ideal for antireflection coating on glass (n=1.54). In addition, quantitative information on the AAO film porosity, profile structure, film thickness, dielectric constants, and roughness was also derived from the ellipsometry analysis. It was shown that the as-fabricated AAO film included trace amount of residual metal aluminum with an effective thickness ~0.28 nm.

Multilayer dielectric narrow band mangin mirror

The design of multilayer stack of dielectric films for narrow band mirror is developed using thin film coating software. The proposed design is materialized by employing thin film coating (PVD) method and reflectance in narrow band spectrum range is achieved. Thickness of high and low refractive index material is taken precisely up to nanometer level. The curved coated substrate is cemented with another K9 matching substrate that forms a Mangin mirror for wavelength 650nm. Narrow band mirrors with reflectivity more than 90% has been produced by properly stacking of 21 layers and advantage of the use of this type of mirror as an interference filter is discussed.

Non-resonant Mie scattering: Emergent optical properties of core-shell polymer nanowires

We provide the in-depth characterization of light-polymer nanowire interactions in the context of an effective Mie scattering regime associated with low refractive index materials. Properties of this regime sharply contrast with these of resonant Mie scattering, and involve the formation of strictly forward-scattered and coupling-free optical fields in the vicinity of core-shell polymer nanowires. Scattering from these optical fields is shown to be non-resonant in nature and independent from incident polarization. In order to demonstrate the potential utility of this scattering regime in one-dimensional (1D) polymeric nanostructures, we fabricate polycarbonate (PC) - polyvinylidene difluoride (PVDF) core-shell nanowires using a novel iterative thermal drawing process that yields uniform and indefinitely long core-shell nanostructures. These nanowires are successfully engineered for novel nanophotonics applications, including size-dependent structural coloration, efficient light capture on thin-film solar cells, optical nano-sensors with ultrahigh sensitivity and a mask-free photolithography method suitable for the straightforward production of 1D nanopatterns.

Guided-Mode Resonant $\hbox{HfO}_{2}$ Grating at Visible Wavelength Range

Subwavelength HfO2 gratings are realized on a freestanding 200-nm-thick HfO2 membrane with air as the low refractive index materials on top and bottom. Strong coupling between the incident light and HfO2 grating is characterized by angular-resolved reflectivity measurement, and guided-mode resonances are experimentally demonstrated with the sensitivities to the parameters and shapes of grating and the polarization of incident beam. The experimental results are consistent with numerical simulation. This work opens the way to fabricate guided-mode resonant HfO2 photonic devices in the visible wavelength range.

Effects of substrate temperatures on the characterization of magnesium fluoride thin films in deep-ultraviolet region

As a low refractive index material widely used in coatings for deep-ultraviolet optical systems, magnesium fluoride (MgF2) films were prepared by electron beam evaporation at different substrate temperatures. The effects of the substrate temperatures on the optical properties in vacuum and in air, microstructures, and composition were investigated, as were the microstructures, their composition, and the relation between them. In vacuum, the substrate temperature directly affected the microstructures which dominated the packing density and inhomogeneity along the film thickness. When the films were exposed to air, the refractive index increased and a nonmonotonic change trend of the refractive index with substrate temperature was observed due to adsorbed water and magnesium oxide (MgO) formed in the film. While a moderate amount of MgO reduced absorption loss by decreasing vacancy defects, excessive MgO increased the absorption loss because of the high extinction coefficient of the oxide.

Effect of Annealing Temperature on Metal/Dielectric Multilayers for Fabricating Broadband Pulse Compression Gratings

Metal/dielectric multilayers were prepared by physical vapor deposition process using gold as the metal material, HfO2 and SiO2 as high and low refractive index materials, respectively. These stacks were used to fabricate broadband pulse compression gratings. Influences of annealing temperature on the surface root-mean-square roughness, reflectivity and resistance to chemical cleaning damage were investigated. The experimental results indicated that surface root-mean-square roughness of these multilayers changed only slightly after annealing. Their resistance to chemical cleaning damage improved with the annealing temperature increase, whereas their reflectivity decreased. The metal/dielectric multilayers annealed at 250°C for 10 h could not only endure the process of chemical cleaning, but also slightly decrease the reflectivity, suggesting that it is an optimal annealing process for metal/dielectric multilayers.

Reactive ion-assisted deposition of e-beam evaporated titanium for high refractive index TiO2 layers and laser damage resistant, broad bandwidth, high-reflection coatings

High-reflection coatings with broad bandwidth can be achieved by pairing a low refractive index material, such as SiO2, with a high refractive index material, such as TiO2. To achieve high refractive index, low absorption TiO2 films, we optimized the reactive, ion-assisted deposition process (O2 levels, deposition rate, and ion beam settings) using e-beam evaporated Ti. TiO2 high-index layers were then paired with SiO2 low-index layers in a quarter-wave-type coating to achieve a broader high-reflection bandwidth compared to the same coating composed of HfO2/SiO2 layer pairs. However, the improved bandwidth exhibited by the TiO2/SiO2 coating is associated with lower laser damage threshold. To improve the laser damage resistance of the TiO2/SiO2 coating, we also created four coatings where HfO2 replaced some of the outer TiO2 layers. We present the laser damage results of these coatings to understand the trade-offs between good laser damage resistance and high-reflection bandwidth using TiO2 and HfO2.

Interface characteristics of peeling-off damages of laser coatings

Coating stacks of HfO2/SiO2 and Ta2O5/SiO2 were separately prepared by electron beam evaporation and dual ion beam sputtering. Damage characteristics at the interlayer interfaces were analyzed after irradiation of the coatings by a 1064nm laser. The cross-sectional morphologies of damage spots indicated that peeling-off damages always occurred at the interface where the low refractive index material (SiO2) was deposited on the high refractive index material (HfO2 or Ta2O5). The effects of interface microstructure and components on peeling-off damages were also discussed. The microstructure of the interface was not a major factor that influenced peeling-off damages. Incomplete oxides (SiOx) and Na, K, Li ions accumulated near the interface and caused the formation of micro-defects layers with nano-sized thicknesses. Micro-defects layers maybe reduced adhesion of different interfaces and formed plasmas by absorbing laser energy. Finally stripping damages happened from micro-defects layers during irradiation by a 1064nm laser.

Effect of substrate bias and oxygen partial pressure on properties of RF magnetron sputtered HfO2 thin films

Among all dielectric materials, hafnium oxide qualifies to be one of the most significant candidates as high index optical coating material due to its excellent chemical and thermal stability with SiO2, the low refractive index material, apart from its excellent laser radiation resistance. In this article, microstructural, physical, and optical properties of two sets of hafnium oxide thin films deposited by radio frequency (RF) magnetron reactive sputtering under a mixed ambient of argon and oxygen have been investigated: one set at various oxygen partial pressure and with substrate biasing by 50 W pulse direct current (DC) and another set at similar gaseous ambient as above but without any substrate bias. Structure of all the HfO2 thin films have been found to be monoclinic through grazing incidence x-ray diffraction measurements. Mass density of the samples has been estimated by grazing incidence x-ray reflectivity measurements and compared with the atom density of the samples estimated through Rutherford back scattering study. The evolutions of optical properties of the films with respect to the variation of oxygen partial pressure and substrate biasing have been investigated by transmission spectrophotometry and spectroscopic ellipsometry. It has been observed that RF sputtering with pulse DC substrate bias and with 15–30% oxygen partial pressure in the ambient helps in achieving better quality HfO2 films with low void fraction and high refractive index.

Liquids Analysis with Optofluidic Bragg Microcavities

Porous Bragg microcavities formed by stacking a series of porous nanocolumnar layers with alternate low (SiO2) and high (TiO2) refractive index materials have been prepared by physical vapor deposition at glancing angles (GLAD). By strictly controlling the porosity and refractive index of the individual films, as well as the relative orientation of the nanocolumns from one layer to the next, very porous and nondispersive high optical quality microcavities have been manufactured. These photonic structures have been implemented into responsive devices to characterize liquids, mixtures of liquids, or solutions flowing through them. The large displacements observed in the optical spectral features (Bragg reflector gap and resonant peak) of the photonic structures have been quantitatively correlated by optical modeling with the refractive index of the circulating liquids. Experiments carried out with different glucose and NaCl solutions and mixtures of water plus glycerol illustrate the potentialities of these materials to serve as optofluidic devices to determine the concentration of solutions or the proportion of two phases in a liquid mixture.

Sensitivity enhancement in optical micro-tube resonator sensors via mode coupling

A liquid filled, silica micro-tube with a low refractive index material inner-coating has been proposed and theoretically studied as a coupled micro-resonator sensor to greatly enhance biochemical sensor sensitivity. Its unique coupling phenomenon has been analyzed and utilized to boost the device's refractive index sensitivity to 967 nm/Refractive Index Unit (RIU). Through optimization of the coupling strength between the two micro-resonators, further improvement in refractive index sensitivity up to 1100 nm/RIU has been predicted. This mode coupling strategy allows us to design robust, thick-walled micro-tube sensors with ultra-high sensitivity which is useful in practical biochemical sensing applications.

All-solid all-chalcogenide microstructured optical fiber

The realization of an all-solid microstructured optical fiber based on chalcogenide glasses was achieved. The fiber presents As(2)S(3) inclusions selected as low refractive index material (n = 2.4) embedded in a As(38)Se(62) glass matrix (n = 2.8). The single mode regime of the fiber was demonstrated both theoretically by multipole method calculations and experimentally by near field measurements. Optical transmission measurements of the microstructured fiber and single index fibers made of the initial glasses reveal an excess of losses as high as 6-7 dB/m. This excess is not due to the guide geometry but can be explained by the presence of defects in the glass interface regions.

Study and Fabrication of Multi-Band Filter Film on ZnS Substrate

In order to satisfy the special requirements of IR optical instrument, multi-band filter film on the substrate of ZnS is deposited by adopting electron beam vacuum depositing method with the ion assistant deposition technology. At incident angle of 0°～25°, 660nm high reflection and 1064nm and 3～5μm high transmission are realized. ZnS and YbF3 are selected as the high and low refractive index materials, and the film structure prepared easily is getting by optimizing the film system design curve ceaselessly. The precision of thickness control has been improved by adjusting film process parameters and improving film thickness control methods. The adhesion and firmness of film and substrate has been increased by pre-coating and mixed evaporation technology, and the laser induced damage threshold of film has been improved with vacuum annealing. The deposited film can endure the environmental tests such as rain pour, salt fog, and high and low temperature etc, and meet its practical requirement.

Large-area broad band saturable Bragg reflectors using oxidized AlAs in the circular and inverted mesa geometries

A semiconductor Saturable Bragg Reflector (SBR) is a mirror structure comprising alternating layers of high and low refractive index materials with an incorporated saturable absorber. SBRs can be used to initiate and sustain ultra-short pulses in various laser systems. In order to form ultra-short pulses, SBRs with high reflectivity over a broad wavelength range are required. Furthermore, large-area SBRs facilitate easy integration in a laser cavity. One of the key elements for the realization of broad band SBRs is the development of the thermal oxidation process that creates buried low-index AlxOy layers over large areas. The design, fabrication, characterization, and implementation of broad band, high index contrast III-V/AlxOy SBRs in the form of circular mesas, as well as inverted mesa structures, is presented.

Sensitivity enhanced biosensor using graphene-based one-dimensional photonic crystal

In this paper, we propose and analytically demonstrate a biosensor configuration based on the excitation of surface electromagnetic waves in a graphene-based one-dimensional photonic crystal (1D PC). The proposed graphene-based 1D photonic crystal consists of alternating layers of high (graphene) and low (PMMA) refractive index materials, which gives a narrow angular reflectivity resonance and high surface fields due to low loss in PC. A differential phase-sensitive method has been used to calculate the sensitivity of the configuration. Our results show that the sensitivity of the proposed configuration is 14.8 times higher compared to those of conventional surface plasmon resonance (SPR) biosensors using gold thin films. This novel and effective graphene-based 1D PC will serve as a promising replacement of metallic thin film as a sensor head for future biosensing applications.

Aging ef fect of optical properties on low loss antireflection coatings for laser optics

For high precise laser systems like laser gyro, gravitational waves interferometers, the optical components which lie in the cavity or laser path need to control the total loss, scattering, reflection, absorption, and so on. Low loss high-reflection (HR) and antireflection (AR) coating mirrors are their key components and their aging optical properties are the most important performance parameters for long operating lifetime. Low loss AR coatings on quartz substrates for He-Ne laser are designed and manufactured by ion beam sputtering (IBS) technology. Ta2O5 and SiO2 are used as high and low refractive index materials and the obtained total optical loss of AR coating is less than 50 ppm. Aging optical properties of AR coatings are investigated as a function of time placed in the air. When the placed time exceeds 40 days, optical properties tend to be stabilization. The obtained results indicate that the prepared low loss AR coatings have good stability of optical properties and can be applied for high precise laser systems.

Single-mode biological distributed feedback laser

Single-mode second order distributed feedback (DFB) lasers of riboflavin (vitamin B2) doped gelatine films on nanostructured low refractive index material are demonstrated. Manufacturing is based on a simple UV nanoimprint and spin-coating. Emission wavelengths of 543 nm and 562 nm for two different grating periods are reported.

Study on internal short-range order microstructure characteristic of SiO2 thin film

SiO2 is one of important low refractive index materials, and SiO2 films are prepared by both ion-beam sputtering (IBS) and electron-beam evaporating (EB) technology. Dielectric constants of SiO2 films are calculated by infrared spectrum inversion technique in a wavenumber range from 400 cm-1 to 1500 cm-1. Through analyzing dielectric energy loss function, the oscillation frequency and the Si–O–Si angle of two types of SiO2 films are obtained in the transverse optics and longitudinal optics oscillating mode. The research results indicate that the attended modes of SiO4 are main coesite-like structure, three-plane folding ring structure, and keatite-like structure in the range of short-range order for EB-SiO2 films, but the attended modes of SiO4 are main coesite-like structure, three-plane folding ring structure, four-plane folding ring structure, and keatite-like structure in the range of short-range order for IBS-SiO2 films.

One dimensional photonic crystals from As2S3 and PMMA films for photonic and sensor applications

One dimensional photonic crystal (1D-PhC) in form of 19 layered stack was prepared through layer-by-layer deposition of spin coated Poly (methyl methacrylate) (n = 1.49 at 1.55 μm) and vacuum deposited As2S3 (n = 2.27 at 1.55 μm) as low and high refractive index materials, respectively. The optical properties of the single layers and the multilayered stack are studied at normal and oblique incidence of linearly polarized light. The possibility of using the 1D-PhC as an omnidirectional reflector was exploited theoretically and confirmed experimentally. Besides, the prospect of using the studied structure for chemical sensing with optical read-out is demonstrated.

Optical and morphological characterisation of low refractive index materials for coatings on solar collector glazing

Nanostructured coatings based on the elements Si, O, Mg, and F have been deposited as thin films by sol–gel dip-coating in a particle-free atmosphere. The refractive index of the prepared SiO2 and quaternary Mg–F–Si–O thin films has been determined from spectrophotometric and ellipsometric data. The morphology of those thin films has been observed by TEM.Nanoporous SiO2 coatings with a pore size smaller than 3 nm (TEM) and a pore volume fraction of 30% (as inferred from ellipsometric measurements) have been achieved. They are characterised by significantly lower refractive index values (approx. 1.32 at 550 nm) than compact SiO2 (approx. 1.46).Quaternary Mg–F–Si–O thin films are characterised by a surprisingly low refractive index (approx. 1.26 at 550 nm), even lower than that of dense MgF2 coatings (approx. 1.38). Preliminary results of transmission electron microscopy suggest that these films are of nanocomposite nature.In both cases, highly transparent samples have been produced in a single dip-coating step followed by simple thermal annealing in air. Broad spectral transmittance maxima are observed exceeding values of 98.5% (nanoporous SiO2) and 99.5% (quaternary Mg–F–Si–O films).The quaternary films might exhibit a higher ageing stability than porous SiO2 films with respect to pore-filling and could therefore be a promising alternative for single-layered anti-reflection coatings as well as for multi-layered coloured coatings on solar collector glazing.