Low Refractive Index Layers Research Articles

The role of photonic crystal based architectures on tailoring the laser induced fluorescence from green emitting cesium lead bromide nanocrystals

Researchers have long been fascinated by fluorescence and its potential to be boosted as it unlocks a treasure trove of promising applications. In this context, this experimental study investigates the amplification of laser induced fluorescence (LIF) emission from cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNC) using two distinct all-polymer photonic crystal (PC) architectures made of poly 9-vinyl carbazole (PVK) and cellulose acetate (CA) as high and low refractive index (RI) layers, respectively. First structure is a photonic crystal micro cavity (PCMC) where PNC incorporated PVK (PNC:PVK) polymer matrix forms the defect layer sandwiched between two similar PCs. In the second structure, PNC:PVK composite itself functions as high RI layers within the PC. These two designs exploit different mechanisms to achieve fluorescence enhancement. The findings demonstrate the promise of simple, solution processed polymer PCs on applications demanding intensified fluorescence output and pave way to the development of flexible photonic devices capable of manipulating light emission.

Bragg reflector one-dimensional multi-layer structure sensor for the detection of thyroid cancer cells

In the proposed work, a defect cavity multi-layer Bragg reflector structure is proposed theoretically to find the presence of thyroid cancer cells in the given sample. The modelling, design and analysis of the sensor is performed using characteristic matrix method (CMM). Proposed structure has central defect cavity with 6 pairs of low and high refractive index layers on each side of the defect. To enhances the sensor sensitivity, the incident light in mid-infrared frequency range is used as input light source. The refractive index of normal and thyroid cancer cells is analysed for the performance of the sensor. The obtained Q factor and sensitivity of the sensor design is 3729 and 2828 nm/RIU respectively. The proposed sensor is a best choice of optical sensor for the detection of thyroid cancer cells in the given test sample for accurate analysis in medical applications.

High light extraction performance using evanescent waves for top emission OLED applications with thin film encapsulation.

We report high light extraction from the top emission OLED (TEOLED) device structure by improving mainly the waveguide mode loss in the atomic layer deposition processed thin film encapsulation (TFE) layer. A novel structure incorporating the light extraction concept using evanescent waves and the hermetic encapsulation of a TEOLED device is presented here. When the TEOLED device is fabricated using the TFE layer, a substantial amount of generated light is trapped inside the device due to the difference in refractive index (RI) between the capping layer (CPL) and the aluminum oxide (Al2O3) layer. By inserting a low RI layer at the interface between the CPL and Al2O3, the direction of the internal reflected light is changed by the evanescent waves. The high light extraction with the low RI layer is attributed to the presence of evanescent waves and an electric field in the low RI layer. The novel fabricated TFE structure, CPL/ low RI layer/ Al2O3/ polymer/ Al2O3, is reported here. The current efficiency of the fabricated blue TEOLED device using this low RI layer is improved by about 23% and the blue index value is enhanced by about 26%. This new approach for light extraction will be applicable to future encapsulation technology for flexible optoelectronic devices.

Silicon Oxy-Nitride for the Low Refractive Index Layers in the Mirror Coatings of the Cryogenic Laser Interferometer Gravitational Waves Detector.

The low refractive index layers in the mirror coatings of the room-temperature laser interferometer gravitational waves detectors are silica deposited by the ion beam sputter method. However, the silica film suffers from the cryogenic mechanical loss peak, hindering its application for the next generation detector operated at cryogenics. New low refractive index materials need to be explored. We study amorphous silicon oxy-nitride (SiON) films deposited using the method of plasma-enhanced chemical vapor deposition. By changing the N_{2}O/SiH_{4} flow rate ratio, we can tune the refractive index of the SiON smoothly from nitridelike to silicalike of ∼1.48 at 1064nm, 1550nm, and 1950nm. Thermal anneal reduced the refractive index down to ∼1.46 and effectively reduced the absorption and cryogenic mechanical loss; the reductions correlated with the N─H bond concentration decrease. Extinction coefficients of the SiONs at the three wavelengths are reduced down to 5×10^{-6}∼3×10^{-7} by annealing. Cryogenic mechanical losses at 10K and 20K (for ET and KAGRA) of the annealed SiONs are significantly lower than the annealed ion beam sputter silica. They are comparable at 120K (for LIGO-Voyager). Absorption from the vibrational modes of the N─H terminal-hydride structures dominates over the absorption from other terminal hydrides, the Urbach tail, and the silicon dangling bond states in SiON at the three wavelengths.

P‐81: Enhanced Color‐Conversion Efficiency of Quantum‐Dot Layer Using Low‐Refractive‐Index Layer

In displays that use QD as color conversion materials, the QD layer must re‐absorb the blue light that passes through it to achieve high‐luminance red and green pixels. In this study, a low refractive index layer was introduced between the color filter and the QDs layer to reuse the light that passes through the QDs layer. This result shows that there is an 8% increase in luminescence compared to that in Ref.

Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors

High performance distributed Bragg reflectors (DBRs) are key elements to achieving high finesse MEMS-based Fabry–Pérot interferometers (FPIs). Suitable mechanical parameters combined with high contrast between the refractive indices of the constituent optical materials are the main requirements. In this paper, Germanium (Ge) and barium fluoride (BaF2) optical thin-films have been investigated for mid-wave infrared (MWIR) and long-wave infrared (LWIR) filter applications. Thin-film deposition and fabrication processes were optimised to achieve mechanical and optical properties that provide flat suspended structures with uniform thickness and maximum reflectivity. Ge-BaF2-Ge 3-layer solid-material DBRs have been fabricated that matched the predicted simulation performance, although a degradation in performance was observed for wavelengths beyond 10 μm that is associated with optical absorption in the BaF2 material. Ge-Air-Ge 3-layer air-gap DBRs, in which air rather than BaF2 served as the low refractive index layer, were realized to exhibit layer flatness at the level of 10 to 20 nm across lateral DBR dimensions of several hundred micrometers. Measured DBR reflectance was found to be ≳90 % over the entire wavelength range of the MWIR band and for the LWIR band up to a wavelength of 11 μm. Simulations based on the measured DBR reflectance indicates that MEMS-based FPIs are able to achieve a peak transmission of ≳90 % over the entire MWIR band and up to 10 μm in the LWIR band, with a corresponding spectral passband of ≲50 nm in the MWIR and <80 nm in the LWIR.

Exploring superlattice DBR effect on a micro-LED as an electron blocking layer.

The role of a superlattice distributed Bragg reflector (SL DBR) as the p-type electron blocking layer (EBL) in a GaN micro-light-emitting diode (micro-LED) is numerically investigated to improve wall-plug efficiency (WPE). The DBR consists of AlGaN/GaN superlattice (high refractive index layer) and GaN (low refractive index layer). It is observed that the reflectivity of the p-region and light extraction efficiency (LEE) increase with the number of DBR pairs. The AlGaN/GaN superlattice EBL is well known to reduce the polarization effect and to promote hole injection. Thus, the superlattice DBR structure shows a balanced carrier injection and results in a higher internal quantum efficiency (IQE). In addition, due to the high refractive-index layer replaced by the superlattice, the conductive DBR results in a lower operation voltage. As a result, WPE is improved by 22.9% compared to the identical device with the incorporation of a conventional p-type EBL.

Detection of oral cancerous cells using highly sensitive one-dimensional distributed Bragg’s Reflector Fabry Perot Microcavity

In the proposed work, a multi-layered Distributed Bragg’s Reflector (DBR) Fabry Perot Microcavity resonator is proposed theoretically to sense bio-analyte. One dimensional Photonic-Crystal (PhC) sensor is designed and analysed to sense the presence of oral cancerous cells in the analyte. The Characteristic Matrix Method (CMM) is used to design, model and analyse the proposed sensor. A multi-layer structure with a central defect having 3 pairs of high and low refractive index layers on either side of the defect is analysed for its sensing performance. The incident light having wavelength in the range of mid-infrared frequency is used at input source, which enhances the sensor sensitivity. Five normal (INOK) cells and oral cancerous (YD-10B) cells are considered for the analysis of sensor performance. The effect of variation in the geometrical length of central defect layer and the number DBR layers on resonant wavelength, sensitivity, and Q factor is performed. A highest sensitivity of 3630 nm/RIU with a Q-factor of 11,323 and a very minimum resolution of 9.5 × 10−5 RIU is obtained. The sensor proposed in this work is suitable for label-free, easy fabrication, cost-effective, and highly sensitive sensor designs for biomedical applications.

Broadband Anti-Reflection Coatings Fabricated by Precise Time-Controlled and Oblique-Angle Deposition Methods

Broadband anti-reflection (AR) coatings are essential elements for improving the photocurrent generation of photovoltaic modules and enhancing visibility in optical devices. In this paper, we report a hybrid-structured, anti-reflection coating that combines multi-layer thin films with a single top-oblique deposited layer. By simply introducing this low-refractive index layer, the broadband anti-reflection properties of optical thin films can be improved while simplifying the preparation. Precise time-controlled and oblique-angle deposition (OAD) methods were used to fabricate the broadband AR coating. By accurately measuring and adjusting the design errors for the thin and thick film layers, 22-layer and 36-layer AR coatings on a sapphire substrate with a 400–2000 nm wideband were obtained. This bottom-up preparation process and AR coating design have the potential to significantly enhance the broadband antireflective properties for many optical systems and reduce the manufacturing cost of broadband AR coatings.

Color Glass by Layered Nitride Films for Building Integrated Photovoltaic (BIPV) System

We investigated layered titanium nitride (TiN) and aluminum nitride (AlN) for color glasses in building integrated photovoltaic (BIPV) systems. AlN and TiN are among suitable and cost-effective optical materials to be used as thin multilayer films, owing to the significant difference in their refractive index. To fabricate the structure, we used radio frequency magnetron deposition method to achieve the target thickness uniformly. A simple, fast, and cheap fabrication method is achieved by depositing the multilayer films in a single sputtering chamber. It is demonstrated that a multilayer stack that allows light to be transmitted from a low refractive index layer to a high refractive index layer or vice-versa can effectively create various distinct color reflections for different film thicknesses and multilayer structures. It is investigated from simulation based on wave optics that TiN/AlN multilayer offers better color design freedom and a cheaper fabrication process as compared to AlN/TiN multilayer films. Blue, green, and yellow color glasses with optical transmittance of more than 80% was achieved by indium tin oxide (ITO)-coated glass/TiN/AlN multilayer films. This technology exhibits good potential in commercial BIPV system applications.

Toward Ultra‐Efficient OLEDs: Approaches Based on Low Refractive Index Materials

AbstractOrganic light‐emitting diodes (OLEDs) have successfully established themselves as light sources vital in high‐end mobile displays and television due to various advances made for a wide range of technical aspects from materials, devices, and processes to air‐tight packaging and systems. However, a significant portion of the generated excitons in OLEDs are still lost to various channels. Among them, excitation of surface plasmon polaritons (SPPs) is a critical source of exciton loss, the recovery of which often involves structuring in nanoscale. Herein, a nano‐pattern‐free approach is reviewed to reducing SPP loss that makes use of materials with low refractive index. Because of the limited availability of such low refractive index materials, a method to use a low extraordinary refractive index of birefringent organic semiconductors is explored. With the proposed method combined with common lens‐based outcoupling schemes and high refractive index substrates, it is shown that an ultrahigh external quantum efficiency of ≈72% in a green, single‐junction phosphorescent OLED is within the reach. Transient photoluminescence measurement is used to monitor the change in Purcell factor incurred by the use of the birefringent low refractive index layer and to further verify the validity of the proposed concept.

Deep-UV hexagonal boron nitride (hBN)/BAlN distributed Bragg reflectors fabricated by RF-sputtering

The hexagonal boron nitride (hBN) and BAlN films were prepared by RF-sputtering, which were used as the low and high refractive index layers. A series of hBN/BAlN distributed Bragg reflectors (DBRs) were prepared on sapphire substrate. The reflectivity of 9-pair hBN/BAlN (39 nm/33 nm) DBR reached 90% at 300 nm with a bandwidth of 45 nm, and which of 6-pair hBN/BAlN (35 nm/29 nm) reached 52% at 280 nm. The hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization.

Nanosheet coated dual-shell TiO2 sphere with high solar reflectance for thermal-shield materials

A dual-shell TiO2 sphere, composed of a TiO2 sphere (TS) core and SiO2 layer (SL)/TiO2 nanosheet (TNS) double shell, was prepared as a novel thermal-shield material. The high reflective rutile TiO2 core was easily synthesized via a solvothermal reaction, and then SL/TNS double shell was sequentially coated on the TiO2 core via two-step coating. This multilayer structure enhanced the solar reflectance via constructive interference between the high- and low-refractive index layers, suggesting a positive impact on potential applications as a thermal-shielding material. Moreover, the multi-faceted reflection derived from vertical growth TNS outer layer can lead to improvement in the solar reflectance caused by maximization of light scattering. As a result, it showed outstanding near infrared- and visible-reflectance value of approximately 87% and 91%, respectively, which are higher than those of neat TS and commercial TiO2.

Influence of Anodization Temperature on Geometrical and Optical Properties of Porous Anodic Alumina(PAA)-Based Photonic Structures

In this work, the influence of a wide range anodizing temperature (5–30 °C) on the growth and optical properties of PAA-based distributed Bragg reflector (DBR) was studied. It was demonstrated that above 10 °C both structural and photonic properties of the DBRs strongly deteriorates: the photonic stop bands (PSBs) decay, broaden, and split, which is accompanied by the red shift of the PSBs. However, at 30 °C, new bands in transmission spectra appear including one strong and symmetric peak in the mid-infrared (MIR) spectral region. The PSB in the MIR region is further improved by a small modification of the pulse sequence which smoothen and sharpen the interfaces between consecutive low and high refractive index layers. This is a first report on PAA-based DBR with a good quality PSB in MIR. Moreover, it was shown that in designing good quality DBRs a steady current recovery after subsequent application of high potential (UH) pulses is more important than large contrast between low and high potential pulses (UH-UL contrast). Smaller UH-UL contrast helps to better control the current evolution during pulse anodization. Furthermore, the lower PSB intensity owing to the smaller UH-UL contrast can be partially compensated by the higher anodizing temperature.

Estimation of power loss spectra in slightly flattened distorted Bragg fiber waveguide

Dispersion characteristic and fiber loss spectra of a Bragg fiber waveguide having slightly flattened distortion are estimated for sensing applications. The characteristic and optical power loss equations of waveguide are obtained by matching the fields at various boundaries under quarter wave stack conditions. It is found that the optimum power loss is inversely proportional to the ninth power of distortion coefficient. The obtained results show that optimum fiber loss decreases with decrease of refractive index of cladding layer 'b' (low refractive index layer) and increase of refractive index of cladding layer 'a' (high refractive index layer). Also, fiber losses are estimated with increase of the high and low refractive index cladding layer losses. This loss spectrum of fiber can be considered as sensing signal. Since obtained fiber losses in distorted Bragg fiber are higher than standard Bragg fiber therefore, the sensitivity of proposed structure is higher than the standard Bragg fiber. Also, the sensitivity increases with the increase of refractive index of core medium. These results would be helpful to design a novel highly sensitive sensor based on hollow core distorted Bragg fiber.

Lossy Mode Resonance-Based Refractive Index Sensor for Sucrose Concentration Measurement

A detailed analysis of Prism/LiF/ZnO structure based lossy mode resonance sensor is presented for measurement of sucrose concentration. Theoretical and experimental studies using angular interrogation technique has been carried out to achieve high sensitivity for a range of surrounding medium refractive indices (1.33-1.45). Simulation outcomes reveal that a low refractive index layer of LiF may be introduced between substrate and the absorbing film of ZnO to develop a high sensitivity refractive index sensor. Various sensor configurations at different thicknesses of LiF are investigated for both s- and p- polarizations of light at the wavelength of 632.8 nm. The best obtained sensor configurations have been developed experimentally to measure concentration of sucrose solution. The maximum sensitivities of 61.92 °/RIU for s- polarization and 68.80 °/RIU for p-polarization are obtained experimentally, which are found to be in close agreement with the theoretically simulated results.

Investigation on photonic band gap of a magneto photonic crystal

Magnetic photonic crystal is a subject of extensive research for their remarkable optical and magneto optical responses. In this work a novel one dimensional magneto photonic crystal having a magnetic nanocomposite medium consists of yttrium iron garnet (YIG)and Silver which act as high refractive index layer and air as the low refractive index layer is theoretically designed. Transmission properties of the designed one dimensional photonic crystal is studied using transfer matrix method. The variation of photonic band gap (PBG) have been analysed by varying applied magnetic field, filling factor and number of layers. Appreciable changes in the photonic band gap occurs with porosity.

Ultra-broad band antireflection coating at mid wave infrared for high efficient germanium optics

This work mainly demonstrates the development process of an ultra-broad band antireflection (AR) coating that maximizes the optical performance of germanium optical elements. A multilayer stack exhibiting high efficient AR performance on Ge consists of thin multi-layers of aluminum oxide (Al2O3) and germanium (Ge) named as low and high refractive index layer materials. A three-layer design with a very tight thickness tolerance (3%) was optimized by Optilayer software. Al2O3 and Ge layers were deposited by a plasma assisted e-beam evaporation system. Ultra-high efficient multilayer AR coating on Ge has a base reflectance 0.005% at 3550 nm and an average reflectance of 0.256% over the entire mid wave infrared spectrum. Multilayer AR coating on Ge offers lower base and average reflectance than the works reported before. Moreover, AR coating on Ge offers a cost effective process cycle due to its number of layers and its thickness for providing ultra-broadband and high efficient optical performance for mid wave infrared electro-optical applications.

Optimised Performance of Non-Dispersive Infrared Gas Sensors Using Multilayer Thin Film Bandpass Filters

In this work, performance improvements are described for a low-power consumption non-dispersive infrared (NDIR) methane (CH4) gas sensor using customised optical thin film bandpass filters (BPFs) centered at 3300 nm. BPFs shape the spectral characteristics of the combined mid-infrared III–V based light emitting diode (LED)/photodiode (PD) light source/detector optopair, enhancing the NDIR CH4 sensor performance. The BPFs, deposited using a novel microwave plasma-assisted pulsed DC sputter deposition process, provide room temperature deposition directly onto the temperature-sensitive PD heterostructure. BPFs comprise germanium (Ge) and niobium pentoxide (Nb2O5) alternating high and low refractive index layers, respectively. Two different optical filter designs are progressed with BPF bandwidths (BWs) of 160 and 300 nm. A comparison of the modelled and measured NDIR sensor performance is described, highlighting the maximised signal-to-noise ratio (SNR) and the minimised cross-talk performance benefits. The BPF spectral stability for various environmental temperature and humidity conditions is demonstrated.

Numerical research on whispering-gallery modes in a triple-layer-coated microsphere resonator

We numerically demonstrate that whispering-gallery modes (WGMs) in a microsphere resonator with three layers of high, low and high refractive index (RI) are analyzed by using the finite difference time domain (FDTD) method. To make the light couple in and out of the microsphere, a phase matched waveguide is used to overlap the WGMs evanescent radiation field. By changing the gap between the microsphere and waveguide, the WGMs of two high-RI layers are efficiently excited. The stored energy and the mode volume are optimized for sensing applications. The coupling structure reveals a good sensitivity of 38.29 nm/RIU (RI unit).