Low Refractive Index Material Research Articles

P-158: Enhanced Efficiency of Organic Light-Emitting Diodes by Inserting Nanosized Dot Pattern by Laser Interference Lithography

We fabricate organic light-emitting diodes (OLEDs) including nano-scale low refractive index periodic dot light scattering structure between transparent conducting oxide layer and organic layer. Low index oxide (LIO) is used for composing the structure, which is cost-effective and low-refractive index material. Fabricated devices show constant tendency by handling parameters (pitch, size and height) of structure. To fabricate these structures, we use simple and fast patterning process, laser interference lithography (LIL)

Efficiency enhancement and angle-dependent color change in see-through organic photovoltaics using distributed Bragg reflectors

A distributed Bragg reflector (DBR) is conducted as a bottom reflector in see-through organic photovoltaics (OPVs) with an active layer of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The DBR consists of alternative layers of the high- and low-refractive index materials of Ta2O5 (n = 2.16) and SiO2 (n = 1.46). The DBR selectively reflects the light within a specific wavelength region (490 nm–630 nm) where the absorbance of P3HT:PCBM is maximum. The see-through OPVs fabricated on DBR exhibit efficiency enhancement by 31% compared to the device without DBR. Additionally, the angle-dependent transmittance of DBR is analysed using optical simulation and verified by experimental results. As the incident angle of light increases, peak of reflectance shifts to shorter wavelength and the bandwidth gets narrower. This unique angle-dependent optical properties of DBR allows the facile color change of see-through OPVs.

Metalorganic chemical vapor phase epitaxy of narrow-band distributed Bragg reflectors realized by GaN:Ge modulation doping

We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2–3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020cm−3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418nm and even 95% at 370nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3nm and significantly increased emission intensity.

Photonic bandgap compensation and extension for hybrid quasiperiodic heterostructures

Based on the idea of multiple photonic bandgap (PBG) overlapping for a one-dimensional photonic crystal heterostructure, a novel hybrid quasiperiodic heterostructure is proposed to enlarge the omnidirectional photonic bandgap (OPBG). The heterostructure is formed by combining Fibonacci and Thue-Morse quasiperiodic structure. The results show that the OPBG of the heterostructure is enlarged obviously, which increases about three times compared with that of Fibonacci quasiperiodic structure, and twelve times compared with that of Thue-Morse quasiperiodic structure. The influences of structural parameters, such as period number and generation number, on PBGs of Fibonacci and Thue-Morse quasiperiodic structure are studied respectively. The results show that the parameters have little effects on PBG widths of the two quasiperiodic structures. The influences of the refractive indexes and thickness values of the high and low refractive index materials on OPBG of the heterostructure are also investigated. The results show that the OPBG of the heterostructure can be further broadened by increasing the refractive index ratios and thickness values of the high and low refractive index materials. The reason why the quasiperiodic structure can easily realize the multiple band gap overlapping is analyzed by comparing the bandgap properties of periodic structure. The number of PBGs of the quasiperiodic structure in the same wavelength range is more than that of the periodic structure. Moreover, with the increase of generation number of the quasiperiodic structure, due to the occurrence of PBG split, the number of PBGs increases obviously, and each PBG width is less than that of the periodic structure. Owing to this kind of PBG characteristic of the quasiperiodic structure, the heterostructure formed by cascading the two quasiperiodic structures is more prone to realizing the multiple PBG overlapping than other heterostructures, thus more easily achieving the expansion of OPBG. These results lay the design foundation for the compensation and broadening of the more complex bandgap structure.

Modelling of multilayer dielectric filters based on TiO2/SiO2 and TiO2/MgF2 for flourescence microscopy imaging

We report a design for creating multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 and 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Re-flectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high re-flectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer. Therefore, bandpass filters are required to restrict the wavelength outside the stopband of the FP DBRs. The proposed filter shows high quality with an average transmission of 97 % within the passbands and the transmission outside the passband is around 3 %. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptionally promising for florescence imaging and narrow-band imaging endoscopy.

Binary particle swarm optimization algorithm assisted to design of plasmonic nanospheres sensor

In this study, a coherent perfect absorption (CPA)-type sensor based on plasmonic nanoparticles is proposed. It consists of a plasmonic nanospheres array on top of a quartz substrate. The refractive index changes above the sensor surface, which is due to the appearance of gas or the absorption of biomolecules, can be detected by measuring the resulting spectral shifts of the absorption coefficient. Since the CPA efficiency depends strongly on the number of plasmonic nanoparticles and the locations of nanoparticles, binary particle swarm optimization (BPSO) algorithm is used to design an optimized array of the plasmonic nanospheres. This optimized structure should be maximizing the absorption coefficient only in the one frequency. BPSO algorithm, a swarm of birds including a matrix with binary entries responsible for controlling nanospheres in the array, shows the presence with symbol of (‘1’) and the absence with (‘0’). The sensor can be used for sensing both gas and low refractive index materials in an aqueous environment.

Ultraviolet optical properties of aluminum fluoride thin films deposited by atomic layer deposition

Aluminum fluoride (AlF3) is a low refractive index material with promising optical applications for ultraviolet (UV) wavelengths. An atomic layer deposition process using trimethylaluminum and anhydrous hydrogen fluoride has been developed for the deposition of AlF3 at substrate temperatures between 100 and 200 °C. This low temperature process has resulted in thin films with UV-optical properties that have been characterized by ellipsometric and reflection/transmission measurements at wavelengths down to 200 nm. The optical loss for 93 nm thick films deposited at 100 °C was measured to be less than 0.2% from visible wavelengths down to 200 nm, and additional microstructural characterization demonstrates that the films are amorphous with moderate tensile stress of 42–105 MPa as deposited on silicon substrates. X-ray photoelectron spectroscopy analysis shows no signature of residual aluminum oxide components making these films good candidates for a variety of applications at even shorter UV wavelengths.

Design of tunable transmission filter using one-dimensional defective photonic crystal structure containing electro-optic material

A narrowband tunable transmission filter suitable for wavelength division multiplexing is designed. The basic structure is a one-dimensional Fabry–Perot structure formed by layers of dielectric magnesium fluoride and electro-optic lithium niobate, which act as low and high refractive index material layers, respectively. A narrowband phase shifted transmission peak occurs within the stopband of the reflectance spectra of the structure by introducing the defect of a low-index material at a suitable position in the structure. The bandwidth of the peak depends on the number of bilayers and also on the operating wavelength. The phase shift of the transmission peak is linearly related to the wavelength under consideration. By adjusting the defect layer width, this shift of the transmission peak from the operating wavelength can be avoided. The device dimensions are so chosen that such a structure can be fabricated and used with presently available technology. A linear transmission peak tunability of 4 nm/10 V is achieved for this device by varying the refractive index of the electro-optic lithium niobate layer with externally applied voltage along its z axis. All the simulations have been carried out using the finite difference time domain method in a MATLAB® environment.

PECVD Synthesis of Flexible Optical Coatings for Renewable Energy Applications

The design, fabrication, and evaluation of flexible, multilayer optical coatings deposited by plasma‐enhanced chemical vapor deposition at low temperature are demonstrated using hybrid nanolaminates consisting of TiO2 and silicone (SiOxCyHz) as the high and low refractive index materials, respectively. A broadband anti‐reflection coating was designed and deposited onto a variety of substrates including flexible polyethylene terephthalate (PET) and CdTe solar cells which was shown to increase absolute transmission by an average of 3% over 410–850 nm wavelengths and results in a commensurate increase in short circuit current density. An infrared reflector was designed and applied to PET which was found to provide 70% reflectance in the near‐IR while maintaining >80% transmittance for visible light. The optical performance of these flexible coatings on PET remained unchanged after automated bend testing, and were shown to be robust with respect to humidity and thermal shock tests.

High refractive index hyperbranched polyvinylsulfides for planar one‐dimensional all‐polymer photonic crystals

ABSTRACTWe report on the growth and characterization of one‐dimensional (1D) planar all‐polymer photonic crystals (PhC) with high dielectric contrast (Δn = 0.3) prepared by spin coating using hyperbranched polyvinylsulfide polymers (HB‐PVS) as high refractive index material and cellulose acetate as low refractive index material. Solution processable HB‐PVS show a near ultraviolet absorption inducing an increased refractive index in the visible‐near infrared (n = 1.68, λ = 1000 nm). HBPVS:Cellulose Acetate Distributed Bragg Reflectors show a very clear fingerprint of the photonic band gap possessing the expected polarized dispersion properties as a function of the incidence angle. Moreover, engineered microcavities tuned on the weak fluorescence spectrum of the HB‐PVS show directional fluorescence enhancement effects due to spectral redistribution of the emission oscillator strength. The combination of all these properties testifies the high optical quality of the obtained photonic structures thus indicating HB‐PVS as an interesting material for the preparation of such PhC. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 73–80

Low temperature processing of multilayer dielectrics mirrors by sol–gel method

Bragg mirrors (BM) based on TiOx and SiO2 layers were built on glass and PET substrates at operating wavelength in the visible region. The layers were processed by the sol–gel method at room temperature and dried in air for one hour at 100°C (glass) and 70°C (PET). At these deposition conditions, the refractive indices of the TiOx and SiO2 layers were 2 and 1.49, respectively. The BM were assembled in a multilayer stack of quarter-wave thickness of alternately high and low refractive index materials. We report the experimental and simulated optical reflection spectra of TiOx–SiO2 BM for different number of layers pairs.

Testing multilayer structures for optical filtering in temperature unstable environments

Two multilayer structures are taken, the first multilayer is having a sequence LHLHLHLHLHLHLHLH of low (denoted by L) and high refractive index (denoted by H) material layers in the structure and the second multilayer is having a sequence LLLHHLHHLLLHHLHH. These two multilayer structures were tested for optical filtering in temperature unstable environments. The first multilayer structure shows a large shift in the output spectrum, even for a slightest temperature change. While the second multilayer structure shows a negligible shift in the output spectrum even for large changes in the environmental temperature. The transmission spectrum shift observed per degree rise in temperature in the second multilayer structure was 62.5% less than the shift observed in first multilayer. Second multilayer structure is a superior substitute to conventional LHLHLHLHLHLHLHLH sequence 1D Photonic Band Gap structure.

P‐137: Improved Light Extraction of Organic Light‐Emitting Diodes using Embedded Nanoscale Vacuum Line Layer

In this study, an organic light‐emitting diode with improved light extraction was fabricated by embedding nanoscale vacuum line layer (nVLL). The nVLL increases the optical efficiency both by the perspective of inserting extremely low refractive index materials between the substrate and the anode, and also by the photonic crystal effect from a nano‐periodic structure.

Research of Thin Film for Laser Polarization Beam Splitter

Based on the design theory of film stack,H4 and SiO2 were selected as the high and low refractive index materials respectively. Through optimization by Macleod and TFCalc software, plus considering the internal electric field intensity distribution of film and laser induced damage threshold as well,the beam-splitting film with Tp=65±5% and Ts=30±5% in the 600-700 band in condition of 45 °± 3 ° incident angle has been achieved. Adopting electron beam ion assisted deposition system to development, the deposition parameters of materials has been optimized via orthogonal matrix experiment. The fabrication of thin film for laser polarization beam splitter has been succeeded. Its optical properties, mechanical properties and resistance to environmental test of the film have been approved to meet all using requirements.

Broadening and tuning of omni-directional reflection gap in a 1D photonic crystal consisting semiconducting constituents

The effect of temperature on omni-directional gap (ODG) in a dispersive one-dimensional photonic crystal has been investigated simultaneously considering thermal expansion effect and thermo-optic effect. The proposed structure consists of a periodic array of alternate layers of ZnS and Ge respectively, as the materials of low and high refractive index materials. The refractive index of both layers is taken as a function of temperature and wavelength both. The effect of temperature on ODR band and the central wavelength of this ODR have been studied. This ODR can be tuned by varying the temperature of the geometry. The propagation characteristics of the proposed structure are analyzed by the transfer matrix method.

Stress relaxation in dual ion beam sputtered Nb2O5 and SiO2 thin films: application in a Fabry–Pérot filter array with 3D nanoimprinted cavities

Miniaturized spectrometers can be implemented using Fabry–Pérot (FP) filter arrays. Such filters are defined by two parallel mirrors with a resonance cavity in between. For high optical quality, ion beam sputtered distributed Bragg reflectors (DBRs), with alternating high and low refractive index material pairs, can be used as the FP mirrors; while 3D nanoimprint technology provides an efficient way of implementing multiple organic FP cavities of different heights in a single step. However, the high residual stress in ion beam sputtered films results in poor adhesion between the DBR films and the organic polymer cavities, causing debonding of the DBR. Therefore, the residual stress of the ion beam sputtered films forming the DBRs must be reduced. Niobium pentoxide (Nb2O5) and silicon dioxide (SiO2) are used as the DBR materials in this work due to their high index contrast, resulting in high reflectivity for only a few alternating pairs. Stress relaxation in ion beam sputtered Nb2O5 and SiO2 films is achieved in this work by deposition under simultaneous high energy ion bombardment (oxygen and argon gas mixture) from a second ion source. Using this technique, the film density and hence compressive film stress for both Nb2O5 and SiO2 films is reduced without introducing any additional optical absorption in the films. FP filter arrays fabricated with stress reduced Nb2O5 and SiO2 as DBR films exhibit high optical and mechanical performance, with good adhesion between the films and the polymer cavity.

Spectral splitting strategy and optical model for the development of a concentrating hybrid PV/T collector

The efficient utilization of the solar spectrum has been intensively addressed by solar researchers in the last decades. For instance, many efforts have been made to improve the efficiency of PV cells and to allow their operation at high temperatures and concentration ratios (CR). On the other hand, solar thermal collectors are generally mature technologies which can be selected according to the outlet temperature required. The study of hybrid photovoltaic/thermal (PV/T) collectors using beam splitting has gained particular interest since the photovoltaic cells can be thermally decoupled from the thermal receiver allowing operation at significantly different temperatures; in addition, the PV receiver is illuminated only with the region of the solar spectrum that matches well with its spectral response curve. In this work, a general methodology for the determination of the optimal spectral splitting parameters in hybrid PV/T collectors is presented. The method is applied in the design of a novel PV/T collector which is also modeled in detail using the ray tracing software Zemax 12. The optimal wavelengths for the spectrum division in this collector correspond to 732 and 1067nm, which represents the region of the spectrum that should be directed to the PV cells. Moreover, the method indicates that under this particular partition, 47% more power can be delivered from the collector in relation to a concentrating PV stand-alone system under the same CR. The design of the beam splitting devices was addressed by using SiNx and SiO2 as high and low refractive index materials respectively, in a multilayer thin film filter profile that was included in the ray tracing model. Finally, the ray tracing model suggests that the optical efficiency of the collector is approximately 65% in real operational scenarios and that the concentrated light can be successfully split independently of the light angle of incidence.

Properties of one Dimensional Photonic Crystals with Defects Thickness and Temperature Dependence

The transmission properties of one dimensional photonic crystals with two defects having structures (HL)n(D1LD2L)m(HL)n, where m is the stack number of the Photonic Quantum Well (PQW) have been theoretically investigated by the transfer matrix method. The thickness and temperature dependence of the defect modes have been studied by considering Si and air as the high and low refractive index materials with GaAs and InSb as the two defects. For a defect lattice with m=1 (two defects) the two peaks due to defects are shifted towards the higher wavelength region, when the thickness of the defect layers is changed. Also, the transmittance peaks are shifted by 0.44nm/K and 0.50nm/K due to temperature variation. The above shifts for other cases (m ≠ 1 and n ≠ 1), have also been obtained. Part of these results is in qualitative agreement with the results available in the literature for other photonic crystals.

红外线治疗仪中宽波段带通滤光片的研究

Design and manufacture of a new type of filters which meet the infrared therapy apparatus needs.By selectively filtering for the light of the infrared light emitted by the therapy apparatus,reducing the portion of the band light carrier energy,improving the safety performance of the therapy apparatus.Based on the use requirements of the filters to infrared therapy apparatus,chose Ti3O5 and SiO2 as high and low refractive index materials respectively.Thin films were prepared through electron beam evaporation method and ion-assisted deposition technique,and the thickness of the filter was controlled using crystal-controlled.By repeatedly optimizing technical parameters,the band-pass filter was prepared.The average transmittance of the filter is higher than 92% at the band of 600~1200nm,the average transmittance of the broad band-pass filter is less than 2% at the band of 300~550nm and1270nm-2000 nm.To solve the problem of the single sided film was too thick to prepared.Based on the concept of splitting technical requirements,the double-sided of the color filter were coated.The results show that the difficulty of preparation are reduced,and filters can be easily manufactured in a large scale,the filter meets the actual requirements.

Antireflection coatings designed by the average uniform algorithm for thin film solar cells.

Effective coatings improve the performance of any device designed to emit or collect light over a range of angles and wavelengths. Improved broadband and wide angle antireflection coatings (ARCs) are a simple and direct way to improve solar cell performance. In this paper we demonstrate a multilayered ARC optimized using a new meta-heuristic algorithm called the average uniform algorithm (AUA). Comparison between the well-known genetic algorithm and the AUA showed that both achieved similar results but the AUA converged much faster. The coating optimized by AUA for broadband and wide-angle emission is applied to a copper zinc tin sulfide based thin film solar cell by co-sputtering of high and low refractive index material. A significant improvement in efficiency was observed over wide angle and bandwidth with a typical improvement of 15% over the uncoated solar cell. This AUA methodology is proven to be an efficient method for design of general ARCs.