High Refractive Index Layers Research Articles

The Influence of a Power Law Distribution of Cluster Size on the Light Transmission of Disordered 1-D Photonic Structures

A better understanding of the optical properties of random photonic structures is beneficial for many applications, such as random lasing, optical imaging and photovoltaics. Here we investigated the light transmission properties of disordered photonic structures in which the high refractive index layers are aggregated in clusters. We sorted the size of the clusters from a power law distribution tuning the exponent a of the distribution function. The sorted high refractive layer clusters are randomly distributed within the low refractive index layers. We studied the total light transmission, within the photonic band gap of the corresponding periodic crystal, as a function of the exponent in the distribution. We observed that, for a within the interval [0,3.5], the trend can be fitted with a sigmoidal function.

Development of a multianalyte optical sol–gel biosensor for medical diagnostic

This work describes the design and fabrication of a novel multianalyte biosensor platform for medical diagnostic applications. The sensor platform consists of a photonic waveguide-based optical circuit used to deliver excitation light to multiple sensor windows on the platform. The platform is fabricated by UV-photopatterning of photocurable hybrid organic–inorganic sol–gel materials. The sensing mechanism is based upon the detection of fluorescently labelled antibodies, in order to determine the concentration of specific analytes in a test solution. Fluorescence is excited by means of the evanescent wave in each sensor window. It is shown that the sensing properties of the platform can be dramatically enhanced by increasing the intensity of the evanescent field of the light propagating in the optical waveguide by a precise design of a high refractive index layer deposited at the waveguide surface. This work proved the concept of employing a waveguide-based photonic platform for the detection of fluorescently labelled antibodies, with μg/ml detection levels, and as such, we believe this system has immense potential for future applications as a medical diagnostic platform.

Control of the average light transmission in one-dimensional photonic structures by tuning the random layer thickness distribution

In this work we have studied the optical properties of disordered photonic structures, in which we have controlled the distribution of the random layer thickness. Such structures are characterized by the usual alternation of high and low refractive index layers, but the layer thickness follows the aforementioned distribution. We have used two types of distributions: a distribution in which each thickness has the same probability to occur and one in which the thickness follows a Gaussian function. We have simulated the average transmission all over the spectrum for photonic structure characterized by a different width of the distribution. We have found that the choice of the distribution of the layer thickness is a control of the average transmission of a random photonic structure. Photonic structures, fabricated following these distributions, can be interesting for the fabrication of bandpass optical filters.

Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region.

A polarization-independent and omnidirectional nearly perfect absorber in the visible region has been proposed. The absorber is two-layer structure consisting of a subwavelength metal grating layer embedded in the high refractive index and lossless dielectric layer on the metal substrate. Extraordinary optical absorption with absorption peaks of over 99% can be achieved over the whole visible region for both TM and TE polarization. This absorption is attributed to cavity mode (CM) resonance caused by the coupled surface plasmon polaritons (SPP). Through adjusting the grating thickness, the absorption peak can be tuned linearly, which is highly advantageous to design various absorbers. Furthermore, the absorbance retains ultra-high over a wide angular range of incidence for both TM and TE polarization. This nearly perfect absorber offers great potential in the refractive index (RI) sensors, integrated photodetectors, solar cells and so on.

Photonic crystal slabs in flexible organic light-emitting diodes

Photonic crystal slabs integrated into organic light-emitting diodes (OLEDs) allow for the extraction of waveguide modes and thus an increase in OLED efficiency. We fabricated linear Bragg gratings with a 460-nm period on flexible polycarbonate substrates using UV nanoimprint lithography. A hybrid organic–inorganic nanoimprint resist is used that serves also as a high refractive index layer. OLEDs composed of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer anode, an organic emission layer [poly(p-phenylene vinylene) (PPV)-derivative “Super Yellow”], and a metal cathode (LiF/Al) are deposited onto the flexible grating substrates. The effects of photonic crystal slab deformation in a flexible OLED are studied in theory and experiment. The substrate deformation is modeled using the finite-element method. The influence of the change in the grating period and the waveguide thickness under bending are investigated. The change in the grating period is found to be the dominant effect. At an emission angle of 20° a change in the resonance wavelength of 1.2% is predicted for a strain of 1.3% perpendicular to the grating grooves. This value is verified experimentally by analyzing electroluminescence and photoluminescence properties of the fabricated grating OLEDs.

One dimensional disordered photonic structures characterized by uniform distributions of clusters

We investigated one dimensional disordered photonic structures by grouping high refractive index layers in clusters, randomly distributed within layers of low refractive index. We control the maximum size of the high refractive layer clusters and the ratio of the high–low refractive index layers, which we call the dilution of the system. By studying the total transmission of the disordered structure within the photonic band gap of the ordered structure as a function of the maximum cluster size, we observe a dip of the total transmission for a specific maximum cluster size. This value increases with increasing dilution. Moreover, within one dilution we observe oscillations of the total transmission with increasing cluster size.

Laser highlighting on a flat panel display coated with a double-layered anti-reflection film containing a europium(iii) complex

We report a photosensitive, double-layered anti-reflection film, which is composed of a UV cured acrylate film containing a photoluminescent europium(III) complex as a high refractive index layer and a film of a colloidal nano-silica as a low refractive index layer. The refractive index of the high refractive index layer can be tuned from 1. 53 to 1.69 as the composition of the europium(III) complex increases. The reflectance of the anti-reflection film on the glass substrate is as low as 0.48% when the refractive indices and thicknesses of the two layers are controlled. The anti-reflection film is applied as the top layer of a liquid crystal display (LCD). Since the absorption and excitation spectra of the europium(III) complex barely overlap with the backlight spectrum of the LCD, the europium(III) complex in the anti-reflection film does not disturb the display images. The images are visible even under irradiation using an indoor fluorescent lamp. It is also possible to highlight a specific point on the display screen using laser light. When a 405 nm laser pointer is aimed at a specific area, the area appeared as a bright red spot.

Development of a novel ozone gas sensor based on sol–gel fabricated photonic crystal

We have developed a novel ozone gas sensor based on one dimensional photonic crystal with two defects. In this platform, the gas is dissolved in a specific neutral buffer kalium iodide reagent to include the Beer Lambert effect. The corresponding photonic crystal which was fabricated by using sol–gel method consists of a high refractive index layer of TiO2 and a low refractive index layer of SiO2. Prior to the fabrication of corresponding photonic crystal, we grew the TiO2 and SiO2 single layer separately in order to ensure that the performance of each layer fulfilled the required characteristics provided by our simulation. After that, the fabrication processed was conducted layer-by-layer and inspected by spectrophotometer. The performance test of the fabricated photonic crystal, including its validation, accuracy and sensitivity, was then conducted through spectroscopic treatment and we used ozonizer as the gas source. Validation test was performed by comparing the results with measurement using NBKI method and showed a good agreement. It was found that the accuracy value is up to 98.75%. Based on a statistical approach, we found that the limit of detection is 1.067μg/m3 ambient air.

Investigating the medium range order in amorphous Ta2O5 coatings

Ion-beam sputtered amorphous heavy metal oxides, such as Ta2O5, are widely used as the high refractive index layer of highly reflective dielectric coatings. Such coatings are used in the ground based Laser Interferometer Gravitational-wave Observatory (LIGO), in which mechanical loss, directly related to Brownian thermal noise, from the coatings forms an important limit to the sensitivity of the LIGO detector. It has previously been shown that heat-treatment and TiO2 doping of amorphous Ta2O5 coatings causes significant changes to the levels of mechanical loss measured and is thought to result from changes in the atomic structure. This work aims to find ways to reduce the levels of mechanical loss in the coatings by understanding the atomic structure properties that are responsible for it, and thus helping to increase the LIGO detector sensitivity. Using a combination of Reduced Density Functions (RDFs) from electron diffraction and Fluctuation Electron Microscopy (FEM), we probe the medium range order (in the 2-3 nm range) of these amorphous coatings.

Properties of reflecting region of periodic-structured thin film with refractive index dispersion

Periodic structure is the basic physical model in optical film design. Universal conditions of the reflecting center wavelength are given. With the refractive index dispersion of the layers, the reflecting center wavelength and the band characteristic of equal-thickness and unequal-thickness periodic structures are studied. According to experimental results, in both the equal-thickness and unequal-thickness periodic structures with the refractive index dispersion of the coating layers, the reflecting center wavelength moves towards the longer wavelengths, and the reflective series as well as the relative wave number will depart from linear relation. For the same optical thickness of the film with unequal-thickness periodic structure, when the optical thickness of the high refractive index layer is bigger than that of the low refractive index layer, the departure from linear relation between the reflective series and the relative wave number is higher. For the low reflective series, the bandwidth of unequal-thickness periodic structure is smaller than that of equal-thickness periodic structure. The influence of the layer dispersion on the bandwidth is found to be weak.

Simulation and fabrication study of porous silicon photonic crystal

The present work reports design and fabrication of porous silicon based one-dimensional (1D) photonic crystal. Distributed Bragg reflector (DBR) is a 1D photonic crystal composed of multilayer stack of high and low refractive index layers. Design of porous silicon DBR is a complex one and requires appropriate control in optical parameters of its constituent layers. In order to design DBR, two porous silicon single layer samples were fabricated using current density of 10 and 50mA/cm2. Optical characterization of single layer samples showed series of interference fringes. Reflective interferometric Fourier transform spectroscopy (RIFTS) method was employed to determine optical constants of porous silicon single layers. DBR simulation was carried out based on transfer matrix method. DBR was then fabricated using optical parameters obtained from RIFTS method. Reflection bandwidth of prepared DBR was found to be 216nm, which is comparable to the simulated value of 203nm.

Vertical-cavity surface-emitting laser in the long-wavelength (700 nm) region in the visible by energy transfer between organic dyes

In this work, organic vertical-cavity surface-emitting lasers (VCSELs) with single-mode laser output in the long-wavelength region (~700 nm) of the visible were reported based on the energy transfer between dye pairs consisting of pyrromethene 597 (PM597) and rhodamine 700 (LD700). By co-doping PM597 into the polymeric hosts, the fluorescence intensity of LD700 was enhanced by 30-fold and the photophysical parameters of the donor–acceptor pairs were investigated, indicating the involvement of non-radiative resonance energy transfer processes between PM597 and LD700. Active distributed Bragg reflectors (DBR) were made by alternately spin-coating dye-doped polyvinylcarbazole and cellulose acetate thin films as the high and low refractive index layers, respectively. By sandwiching the active layer with 2 DBR mirrors, VCSEL emission at 698.9 nm in the biological first window (650–950 nm) was observed under the 532-nm laser pulses. The laser slope efficiency and threshold were also measured.

Fabrication of Artificially Stacked Ultrathin ZnS/MgF2 Multilayer Dielectric Optical Filters

We report a design and fabrication strategy for creating an artificially stacked multilayered optical filters using a thermal evaporation technique. We have selectively chosen a zinc sulphide (ZnS) lattice for the high refractive index (n = 2.35) layer and a magnesium fluoride (MgF2) lattice as the low refractive index (n = 1.38) layer. Furthermore, the microstructures of the ZnS/MgF2 multilayer films are also investigated through TEM and HRTEM imaging. The fabricated filters consist of high and low refractive 7 and 13 alternating layers, which exhibit a reflectance of 89.60% and 99%, respectively. The optical microcavity achieved an average transmittance of 85.13% within the visible range. The obtained results suggest that these filters could be an exceptional choice for next-generation antireflection coatings, high-reflection mirrors, and polarized interference filters.

Design and fabrication of a planar PDMS transmission grating microspectrometer

We describe the monolithic integration of microfluidic channels, optical waveguides, a collimating lens and a curved focusing transmission grating in a single PDMS-based microsystem. All optical and fluidic components of the device were simultaneously formed in a single layer of high refractive index (n~1.43) PDMS by soft lithography. Outer layers of lower-index (n~1.41) PDMS were subsequently added to provide optical and fluidic confinement. Here, we focus on the design and characterization of the microspectrometer part, which employs a novel self-focusing strategy based on cylindrical facets, and exhibits resolution <10 nm in the visible wavelength range. The dispersive behavior of the grating was analyzed both experimentally and using numerical simulations, and the results are in good agreement with simplified analytical predictions.

Fluorescent polymer coated capillaries as optofluidic refractometric sensors

A capillary microresonator platform for refractometric sensing is demonstrated by coating the interior of thick-walled silica capillaries with a sub-wavelength layer of high refractive index, dye-doped polymer. No intermediate processing, such as etching or tapering, of the capillary is required. Side illumination and detection of the polymer layer reveals a fluorescence spectrum that is periodically modulated by whispering gallery mode resonances within the layer. Using a Fourier technique to calculate the spectral resonance shifts, the fabricated capillary resonators exhibited refractometric sensitivities up to approximately 30 nm/RIU upon flowing aqueous glucose through them. These sensors could be readily integrated with existing biological and chemical separation platforms such as capillary electrophoresis and gas chromatography where such thick walled capillaries are routinely used with polymer coatings. A review of the modelling required to calculate whispering gallery eigenmodes of such inverted cylindrical resonators is also presented.

TiO&lt;sub&gt;2&lt;/sub&gt;-Acrylate Nanocomposites Elaborated by UV-Curing with Tunable Properties

Abstract. Homogeneous TiO2–acrylate nanocomposite with low optical scattering were prepared by UV curing. The surface of the nanoparticles was modified by carbonic acids to control the aggregation between the nanoparticles. Effects of TiO2 content on the properties of nanocomposite (transmission, refractive index, mechanical properties, internal structure) were investigated. Depending on the composition of the matrix hard materials with high refractive index or light sensitive layers for holographic recording can be obtained. The first are interesting for medical applications, the last for photonics. For example, TiO2 nanocomposite was used for in situ fabrication of holographic gratings by interference lithography. Formation of the grating samples with high diffraction efficiency (20 %) is based on the light-stimulated mass-transport processes in the polymerized layer during the recording without any additional treatments. The relief formation has been investigated by AFM measurements too. These materials are designed for applications in integrated optics, elements of optics, in nanolithography.

Theoretical realization of robust broadband transparency in ultrathin seamless nanostructures by dual blackbodies for near infrared light

We propose a counter-intuitive mechanism of constructing an ultrathin broadband transparent device with two perfect blackbodies. By introducing hybridization of plasmon modes, resonant modes with different symmetries coexist in this system. A broadband transmission spectrum in the near infrared regime is achieved through controlling their coupling strengths, which is governed by the thickness of high refractive index layer. Meanwhile, the transparency bandwidth is found to be tunable in a large range by varying the geometric dimension. More significantly, from the point view of applications, the proposed method of achieving broadband transparency can perfectly tolerate the misalignment and asymmetry of periodic nanoparticles on the top and bottom, which is empowered by the unique dual of coupling-in and coupling-out processes within the pair of blackbodies. Moreover, roughness has little influence on its transmission performance. According to the coupled mode theory, the distinguished transmittance performance is physically interpreted by the radiative decay rate of the entire system. In addition to the feature of uniquely robust broadband transparency, such a ultrathin seamless nanostructure (in the presence of a uniform silver layer) also provides polarization-independent and angle-independent operations. Therefore, it may power up a wide spectrum of exciting applications in thin film protection, touch screen techniques, absorber-emitter transformation, etc.

Development of a sol–gel photonic sensor platform for the detection of biofilm formation

This paper focuses on the development and characterization of a waveguide-based photonic sensing platform for the detection of biofilm. This integrated photonic platform is based upon the high sensitivity of an optical field distribution formed in optical waveguides and the resulting changes in the refractive index and absorption of this environment. The sensor platform and materials formulations were established from simulation studies conducted with the Olympios software. These simulations demonstrated the importance of correctly specifying the material refractive index to achieve single-mode waveguides. They also highlighted the necessity to deposit a high refractive index layer (HRIL) on top of the optical waveguides in order to increase the intensity of the evanescent field responsible for the sensing performance of the platform. Platform fabrication exploited a low-cost process using photocurable organic–inorganic hybrid sol–gel materials, which were microstructured by UV-photolithography to form channel optical waveguides. A tantalum-based material was synthesized using the sol–gel process with refractive index as high as 1.87. This material was developed, optically characterized and applied as an evanescent field enhancement layer, deposited at the surface of the waveguide to increase the sensitivity of the sensing platform. The sensor characterization was performed by monitoring the output intensity of the optical waveguide while contaminated water was monitored in a quasi-static flow-rate (0.5 ml/s) on the platform. It is shown that our biosensor platform was able to detect the biofilm formation after 10 min of reactions, demonstrating the early stage biofilm formation in quasi-static flow-rate. Furthermore, the sensing performances of our photonic platform were found to be strongly dependent on the thickness of the HRIL, confirming the simulations studies. This work proved the concept of employing a waveguide-based photonic platform for the early detection of biofilm formation, including the induction phase, and as such, we believe this system has immense potential for future applications as a label-free and real-time biosensor platform.

Optimization of slow light one-dimensional Bragg structures forphotocurrent enhancement in solar cells

In 1D photonic crystal Bragg structures, strong localization of the light occurs in the high refractive index layers at wavelengths on the red edge of the photonic bandgap. We exploit this slow light effect for thin film solar cells in order to increase the absorption of light in silicon, as the latter has a high refractive index. Amorphous silicon and a transparent conductive oxide are chosen as high-index and low-index materials, respectively. Reference thin film cells have equivalent total thickness of amorphous silicon, plus antireflection coating and optional metallic back mirror. Through transfer-matrix calculations, we demonstrate that the spectrally integrated photon flux absorbed in active layers, hence the photocurrent, is enhanced by at least 10% with respect to reference using only a few periods. The enhancement is robust with respect to the light incidence angle. The key of such an enhancement is the tuning of the red edge to both the solar irradiance spectrum and the intrinsic material absorption spectrum, which is achieved by suitably selecting the layer thicknesses.

51.4: High‐Efficiency White OLEDs with Built‐up Outcoupling Substrate

AbstractA built‐up outcoupling substrate with a micro‐patterned high refractive index layer was developed in order to obtain both high efficiency and reliability for OLEDs. Light extraction efficiency of this substrate was about 47%, and high efficacy of about 85 lm/W was achieved by the combination with an all phosphorescent white OLED.