High Refractive Index Films Research Articles

Azimuthal polarized quasi-bound states in the continuum based on rotational symmetry breaking.

Bound states in the continuum (BICs) allow to obtain an ultrahigh-quality-factor optical cavity. Nevertheless, BICs must be extended in one or more directions, substantially increasing the device footprint. Although super-cavity mode quasi-BICs supported by single nanopillars have been demonstrated recently, their low-quality factor and localized electromagnetic field inside the dielectric nanopillar are insufficient for high-sensitivity refractive index sensing applications. We propose a ring structure rotated by a dielectric sectorial nanostructure, which can achieve a high quality factor by breaking the rotational symmetry of the ring structure with a footprint as small as 3 µm2. As a straightforward application, we demonstrate high performance local refractive index and nanoscale film thickness sensing based on rotational symmetry breaking induced BICs. These BICs reach quality factor and sensitivity of one order of magnitude better than those of conventional super-cavity mode BICs. The proposed method provides insights into the design of compact high quality factor photonic devices, opening up new possibilities for applications in refractive index and nanoscale film thickness sensing.

Effective way to reduce rear-side potential-induced degradation of bifacial PERC solar cells

While bifacial p -type silicon ( p- Si) passivated emitter and rear cells (PERCs) have dominated the current photovoltaic industry, potential-induced degradation (PID), especially in the rear-side, has become a big issue in their practical applications. Here, we have investigated the impact of different rear passivation layers on the anti-PID and electrical property of bifacial p- Si PERCs. Both simulation and characterization showed that the introduction of SiN x films with a high refractive index and additive oxide films were in favor of reducing the rear PID effect. Finally, we proposed a combined structure of stack films as the rear passivation layer, yielding an absolute front-side and rear-side efficiency improvement of 0.09% and 0.18% with 800 pieces of 166 × 166 mm 2 Si wafers on the production line, and the rear PID related power loss dropped from the baseline of 5.46%–1.05% and 5.15%–0.56% by single-cell modules and commercial glass-glass modules, respectively. Together with the well-solved front-side anti-PID technique in industry, the presented novel rear passivation layer establishes a base for the efficient applications of the bifacial p- Si PERCs. • A combined structure of stack films as the rear passivation layer was proposed for bifacial p-Si PERCs, yielding an absolute front-side and rear-side efficiency improvement of 0.09% and 0.18% on the production line. • The rear PID related power loss dropped from the baseline of 5.46% to 1.05% and 5.15% to 0.56% by single-cell modules and commercial glass-glass modules, respectively. • This work has demonstrated that an invaluable improvement in both anti-PID performance and electrical property can be achieved with the introduction of the novel rear passivation layer to the bifacial p-Si PERCs.

Study on Innovative Flexible Design Method for Thin Film Narrow Band-Pass Filters

The thin film narrow band-pass filters are widely used in precision optical systems such as gas detection, fiber optic communication and optical analysis instruments due to their unique optical characteristics. However, as the applications become more widespread, the problems of regulating the half bandwidth and cut-off bandwidth of the thin film narrow band-pass filters based on the Fabry-Perot (F-P) principle are becoming more apparent. In this work, we took the thin film narrow band-pass filter with a center wavelength of 450 nm as the design target, and comprehensively optimized the film system based on standard F-P principle to obtain the film systems with different half bandwidths and their systematic adjustment methods. The half bandwidth can be adjusted effectively by using the conventional half bandwidth control methods such as increasing the interference orders and the number of mirror layers or the cavities, but it is quite difficult to accurately obtain the filters with various half bandwidths since the adjustment range is too large. Based on the principle of equivalent refractive index, the optical thicknesses of the high and low refractive index film layers of the mirror layers were modified in an unconventional way, and thus the half bandwidth can be flexibly adjusted. Moreover, the standard F-P narrow band-pass filter film system was also modified by the principle of induced transmission film system to achieve a significant broadening of the cut-off bandwidth.

Fabrication of TiO2-based broadband single-layer anti-reflection coating by collimated glancing angle deposition technique

Single-sided TiO2 thin films were prepared using a modified glancing angle deposition (GLAD) technique. An additional flux collimation plate was introduced into the GLAD arrangement to enhance the degree of collimation of depositing vapour flux. Enhancement in the ballistic growth of film on the substrate was observed with increasing distance from the vapour source. The substrate position near to the vapour source (i.e. bottom region) showed a high refractive index (RI, ∼1.336 @ 550 nm wavelength) and lower average film transmittance (∼94.5% in 400–900 nm wavelength range) compared to the others. In contrast, the TiO2 coating deposited at a distant position from the source (i.e. top region) showed a remarkably low RI (∼1.190 @ 550 nm wavelength) and excellent anti-reflection over a broad spectral region with a maximum average transmittance (∼95.3% in 400–900 nm wavelength) compared to the other substrate positions. The reduction in film RI was correlated qualitatively with the morphological alterations in the coating for different substrate positions. With a further increase in distance from the vapour source, an ultimate reduction in the RI of TiO2 to ∼1.101 was observed, which was ∼50% lower than the bulk TiO2 value (∼2.221 @ 550 nm wavelength). The present study reports the lowest RI of TiO2 together with fabrication of a TiO2-based broadband single-layer anti-reflection coating.

Enhanced sensitivity of guided mode resonance sensor through super-mode excitation at near cut-off diffraction

This work theoretically and experimentally demonstrates sensitivity enhancement in dielectric guided mode resonance (GMR) sensor via controlling a super-mode resonance at nearly cut-off diffraction (or at Rayleigh wavelength.) A bulk refractive index sensitivity can reach the upper limit at the value linearly proportional to the grating’s periodicity. Super-mode excitation in the multilayer configuration of the 2D GMR sensor (a high refractive index film coated on a low refractive index imprinted grating) is demonstrated. This concept is verified using numerical calculations: Rigorous Coupled Wave Analysis (RCWA) and Homogeneous Waveguide Approximation (HWA). Modal allocation for each resonance in the GMR structure is performed using the approximated four-layer waveguide. The measured super-mode resonance has shown sensitivity enhancement almost three folds of that of the traditional guide mode resonance. Also, this realization occurs in a broad dynamic range through optimization of the structure’s parameters. This guarantees a usage in multiple sensing applications.

Design and Preparation of a 6-Channel Fan-Shaped Integrated Narrow-Band Filter in the Mid-Infrared Band

A six-channel fan-shaped integrated narrow-band filter on a silicon substrate was designed on the basis of the Fabray–Perot (FP) theory and was fabricated using e-beam thermal evaporation. The central wavelength was modulated by modifying the FP cavity thickness using the combination mask method. Germanium and zinc sulfide were selected as the high and low refractive index film materials, respectively. Its average peak transmission reached 83.3%, the cut-off transmittance was less than 1%, and the full width at half maximum (FWHM) changed from 55 and 94 nm, and the central wavelength positioning accuracy error was less than 0.35%. Furthermore, transmittance was also calculated inversely, using the thickness of each layer of films, in order to deduce the reason of the decrease in peak transmittance. Thickness deviation of each layer and interface scattering contributed to the decrease of peak transmittance. Compared with the same type of products in the market, our filter showed better properties than that of some published and commercial filter.

Hydrostatic pressures effect on structure stability, electronic, optical and elastic properties of rutile VO2 doped TiO2 by density functional theory investigation

The hydrostatic pressures impact on structural stability, electronic, optical and elastic properties of Ti1−xVxO2 (x = 0, 0.25, 0.5, 0.75, 1) are studied by using the density functional theory (DFT)+U. The calculated results show that the lattice parameter, cell volume and the bond length of and dV−O are all decreased linearly as the concentration of V increases. As the pressure increases, these results are not changed. It is worth noting that the structure of Ti0.25V0.75O2 has the highest static refractive index n0, and Ti0.75V0.25O2 has high reflectivity at 20 GPa, respectively. The highest static refractive index n0 is 5.677 5, and the reflectivity value is 84.388% at 1.774 eV (698.98 nm). After discussion, it is concluded that the synthetic materials of Ti0.75V0.25O2 and Ti0.25V0.75O2 under the condition of 20 GPa maybe suitable as a high visible light reflective coating material and useful kind of high refractive index film materials for optical coatings, respectively.

Preparation of High Refractive Index Composite Films Based on Titanium Oxide Nanoparticles Hybridized Hydrophilic Polymers.

Optical materials with high refractive index (n) have been rapidly improved because of urgent demands imposed by the development of advanced photonic and electronic devices such as solar cells, light emitting diodes (LED and Organic LED), optical lenses and filters, anti-reflection films, and optical adhesives. One successful method to obtain high refractive index materials is the blending of metal oxide nanoparticles such as TiO2 and ZrO2 with high n values of 2.1–2.7 into conventional polymers. However, these nanoparticles have a tendency to agglomerate by themselves in a conventional polymer matrix, due to the strong attractive forces between them. Therefore, there is a limitation in the blending amount of inorganic nanoparticles. In this paper, various hydrophilic polymers such as poly(N-hydroxyl acrylamide) (pHEAAm), poly(vinyl alcohol), poly(ethylene glycol), and poly(acrylic acid) were examined for preparation of high refractive index film based on titanium oxide nanoparticle (TiNP) dispersed polymer composite. The hydrogen bonding sites in these hydrophilic polymers would improve the dispersibility of inorganic nanoparticles in the polymer matrix. As a result, pHEAAm exhibited higher compatibility with titanium oxide nanoparticles (TiNPs) than other water-soluble polymers. Transparent hybrid films were prepared by mixing pHEAAm with TiNPs and drop casting the mixture onto a glass plate. The refractive indices of the films were in good agreement with calculated values. The compatibility of TiNPs with pHEAAm was dependent on the surface characteristics of TiNPs. TiNPs with the highest observed compatibility could be hybridized with pHEAAm at concentrations of up to 90 wt%, and the refractive index of the corresponding film reached 1.90. The high compatibility of TiNPs with pHEAAm may be related to the hydrophilicity and amide and hydroxyl moieties of pHEAAm, which cause hydrogen bond formation on the TiO2 surface. The obtained thin film was slightly yellow due to the color of the original TiNP dispersion; however, the transmittance of the film was higher than 80% in the wavelength range from 480 to 900 nm.

Fabrication of Al2O3/SiO2 multilayer reflective filters with excellent uniformity for demanding optical interference filters

In this work, we report the fabrication of a high reflective filter with excellent uniformity for demanding optical interference filters. To achieve this, films were deposited in different batches at three different angular positions using electron beam deposition technique. The coatings were carried out using indigenously designed flat jig holder without shapers. Fourteen alternating layers of high and low refractive index films were deposited on BK7 substrates of dimensions 186 × 150 mm by using aluminum dioxide (Al2O3) and silicon dioxide (SiO2) evaporation grade granuals. Individual layers in the stack were deposited at quarter wave optical thickness at a wavelength of 550 nm. To examine the optical properties of multilayer optical films, Gonio-spectrophotometer was used. Roughness and thickness of all the samples were evaluated using Stylus Profilometer and Coherence Correlation interferometer. Surface properties such as wettability, adhesion and hardness were studied. Additionally, to investigate the change in chemical bonding and structural properites of the coating materials used for evaporation before (bulk powdered material) and after deposition of thin films (material left in the crucible after deposition process), FTIR and XRD studies were carried out.

Localized high-Q modes in conical microcavities

We carry out three-dimensional (3D) numerical simulation on a conical microcavity with a half-angle of 11 degree, where there exists high-Q modes by introducing a thin high refractive index film on a conical surface. Our study reveals that, rather than the surface profile of the microcavity, the effective radius plays crucial role in whether the cavity may support localized modes. Specifically, the change of high refractive index film thickness creates an additional angular momentum barrier, so that the conical microcavity may sustain localized high-Q modes. Our study offers a new degree of freedom to control the properties of 3D microcavities, which is useful for microlaser or sensor applications.

Performance of cubic ZrO2 doped CeO2: First-principles investigation on elastic, electronic and optical properties of Ce1−x ZrxO2

The structural, elastic, electronic and optical properties of Ce1−x ZrxO2 (x = 0, 0.25, 0.5, 0.75, 1) fluorite type oxides are studied by researchers using the method of density functional theory (DFT) + U method. The calculated equilibrium lattice parameter, cell volume, elastic and optical properties for CeO2 and ZrO2 are all in good agreement with the available experimental data and other theoretical results. It is found that Ce substituted by Zr leads to the formation of the pseudo-cubic fluorite-type structure. With doping concentration × increasing, the lattice parameter, cell volume and the bond length of dCe−O and dZr−O decrease linearly. It is interesting to find that the hardness of materials increase with Zr concentration increasing. For Ce0.75 Zr0.25 O2, we also find that its ductility is good. Meanwhile, the range of the conduction band energy in the doped system becomes wider than that in the undoped system. The overlapping band phenomenon occur for all substitutions in Ce1−x ZrxO2 (from x = 0.25 to x = 0.75), especially for the structure of Ce0.5 Zr0.5 O2 and Ce0.25 Zr0.75 O2, its second band gaps almost disappear. Based on the dielectric functions obtained, it is shown that the static dielectric constant ε0 and refractive index n0 obviously decrease with Zr concentration increasing. After discussing, we know that CeO2 is suitable as a useful high-refractive index film material in single and multilayered optical coatings, whereas ZrO2 can be used as gate-dielectric materials in metal-oxide semiconductor (MOS) devices, in metallurgy and as a thermal barrier coating in engines.

A DFT study of the structural, electronic and optical properties of transition metal doped fluorite oxides: Ce0.75M0.25O2 (M=Fe, Co, Ni)

Abstract The structural, electronic and optical properties of Ce 1 − x M x O 2 (M=Fe, Co, Ni; x=0, 0.25) fluorite type oxides are studied by using the method of density functional theory (DFT) + U method. The calculated equilibrium lattice parameter, cell volume, bulk modulus and optical properties for CeO2 are in good agreement with the available experimental data and other theoretical results. The lattice parameter, cell volume, bulk modulus and bond length of Ce–O decreased after substituting Ce atom with Fe (or Co, Ni). Meanwhile, the band-gap (Eg) reduction is observed. It is interesting to find that doping of Fe (or Co, Ni) in CeO2 obviously decreases the O 2p-Ce 4f transition intensity and the covalent character of the Ce–O bond. On the contrary, the static dielectric constant e0 and refractive index n0 for the doped system increased. Compared with the undoped CeO2, the doped system has steep absorption peaks ranging from 1.0 eV to 2.0 eV at lower energy, and it can be used for visible light absorption applications. The Ce 0.75 Co 0.25 O 2 has a high refractive index and reflectivity, therefore it is suitable as a high-refractive index film material in single and multilayered optical coatings.

Optimizing HiPIMS pressure for deposition of high-k (k = 18.3) amorphous HfO2

Stoichiometric amorphous HfO2 films have been deposited by reactive High Power Impulse Magnetron Sputtering (HiPIMS) from a Hf target in a 1:1 Ar:O2 atmosphere at pressures 2–4.5mTorr. An optimum pressure was found for depositing smooth, high refractive index and amorphous films. Stress and refractive index reached a maximum as deposition pressure was increased to 3.5mTorr. At 3.5mTorr, HfO2 films were deposited with a refractive index of 2.15 at 500nm, low leakage currents, moderate fixed charge density and a high dielectric constant of ∼18.3. The intensification of energetic ion bombardment upon the film with increase in HiPIMS pressure plays a dominant role in film properties. Increase in pressure above the optimum relieved the stress in the films and degraded the optical and electrical properties. HiPIMS pressure enables to gain indirect control of ion flux and energy in the plasma and can be used to modify the properties of depositing films.

Transparent, high refractive index oxides: Control of the nanostructure of titanium hafnium oxide alloys by variation of the ion energy during reactive magnetron sputtering deposition

ABSTRACTA range of optical and optoelectronic applications would benefit from high refractive index (n), dense and transparent films that guide, concentrate and couple light. However, materials with high n usually have a high optical extinction coefficient (κ) which keeps these materials from being suitable for optical components that require long optical paths. We studied titanium hafnium oxide alloy films to obtain high refractive index (n>2) with minimum optical extinction coefficients (κ < 10−5) over the visible and near IR spectrum (380-930 nm). Titanium hafnium oxide alloys were deposited using pulsed DC reactive magnetron sputtering with and without RF substrate bias on silicon dioxide. For a given deposition condition intended for a specific titanium/hafnium molar fraction ratio, the ion energy of deposition species was explicitly controlled by varying the RF substrate bias. Spectroscopic ellipsometry, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used to characterize the films. It appears that applying RF substrate bias reduces the nanocrystalline size, changes the surface morphology and increases the refractive index while maintaining comparable titanium/hafnium cation molar fraction. Precise control of the nanostructure of ternary metal oxides can alter their macroscopic properties, resulting in improved optical films.

Low loss, long time stable sol–gel derived silica–titania waveguide films

Silica–titania waveguide films were fabricated via the sol–gel method using dip-coating technique. Chemical uniformity, surface morphology, thickness and refractive index as well as optical transmission losses were investigated. The investigations show, that fabricated silica–titania waveguide films of high refractive index (~1.8) are uniform, very smooth (rms~0.298nm) and exhibit ultra low optical transmission losses (below 0.2dB/cm), constancy through over five years.

Study of Edge Emission and its Induced Photo Pumping by Using Microcavity Organic Light Emitting Diodes

The authors have investigated the edge emission of microcavities which are formed by low-molecular organic light emitting diodes sandwiched between dielectric mirrors and the metal cathode. The edge emission relative to the surface emission is found to significantly depend on the layer number of alternating high and low refractive index films in dielectric mirrors. The enhancement of cavity devices can reach a factor of ~3.0 compared with noncavity devices. Spectra of organic film pumped by edge emission of microcavity devices have been observed, a mechanism of achievement of an amplified spontaneous emission has been discussed theoretically.

High refractive index films of ZnS/PVP nanocomposite by in situ thermolysis

A simple and rapid process for deposition of high refractive index films of ZnS/PVP nanocomposite (NC) is described. Precursor films are dip-coated on glass/quartz substrates from methanolic solution of polyvinylpyrrolidone (PVP) containing Zn+2–thiourea (TU) complex. ZnS/PVP nanocomposite films are produced by heating the solid precursor at 200°C for 10 min in air. Heat treatment converts the Zn+2–TU complex to ZnS by thermolysis in situ PVP. The transmission spectra of the films (typically 700 nm thickness) in the wavelength range of 200–1000 nm showed an absorption edge near 300 nm due to ZnS nanoparticles and high transmission of 97% beyond 400 nm. ZnS nanoparticles are uniformly dispersed in PVP matrix having sizes of about 3–4 nm. For ZnS loading of 45% by weight, the refractive index of ZnS/PVP is 1.65 which is in between that of PVP (1.48) and ZnS (2.36). Fourier Transform Infrared (FTIR) spectroscopy of the composite showed that there is a strong interaction between ZnS nanocrystals and PVP. The root mean square (RMS) roughness of the films is about 3 nm as determined by atomic force microscope (AFM).

Preparation and Characterization of the Transparent Hybrids of Silicone Epoxy Resin and Titanium Dioxide Nanoparticles via Sol–Gel Reactions

In order to prepare transparent hybrid films of high refractive index, nanoparticles of TiO2 were prepared and dispersed in a silicone epoxy (SE) resin synthesized from diphenyl silane diol and [2-(3,4-epoxycyclohexyl)ethyl] trimethoxysilane by sol-gel reactions. It was found that amorphous TiO2 nanoparticles of about 5 nm modified with hexahydro-4-methyl phthalic anhydride [HMPA] were dispersed in the SE resin without agglomerations. The refractive index of the hybrids increased linearly with increasing the TiO2 contents. The hybrid containing 30 wt% of the TiO2 particles showed light transmittance of 94% at 450 nm and refractive index of 1.63. The fine dispersion of the TiO2 nanoparticles was attributable to the sol-gel reactions between the SE resin and TiO2 nanoparticles and the modification of the TiO2 particles with HMPA.

Tunable Mesoporous Bragg Reflectors Based on Block‐Copolymer Self‐Assembly

Mesoporous distributed Bragg reflectors (MDBRs) exhibit porosity on the sub-wavelength scale. They are promising device components for biological and chemisal sensing as well as for light management in optoelectronic devices. In this chapter a new route for the fabrication of MDBRs is presented which relies on the structure directing properties of the block copolymer poly(isoprene-block-ethylene oxide) in combination with sol-gel chemistry. The interplay between structure directing organic host and coassembled inorganic guest allows the fine tuning of refractive index in the outcome material. Stacking high and low refractive index films in sequential order enables the fast and reliable construction of MDBRs which exhibit a continuous TiO2 network with large accessible pores and high optical quality.

Nanocomposites of titania and hybrid matrix with high refractive index

A sol–gel route for TiO2 nanocrystals (NCs) synthesis has been developed at low temperature without surfactants. Synthetic and processing parameters have been optimized to maximize particles’ colloidal stability and crystallinity. The obtained TiO2 NCs can be homogeneously dispersed in a sol–gel derived organic–inorganic hybrid material, resulting in homogeneous composite films when prepared by spin coating. High refractive index films were obtained with high TiO2 NCs loading and good transparency. Furthermore, TiO2 colloidal solutions can be used for depositing porous crystalline films, whose structural evolution has been followed under different annealing treatments. Nanocrystals were characterized by UV–Vis absorption, TEM, FT-Raman, and XRD techniques, while nanocomposite and TiO2 films were analyzed by SEM, TEM, and spectroscopic ellipsometry.