Low Refractive Index Material Research Articles

P‐54: Development of Heat Free / Low Temperature Process High Refractive Index Materials for Display

The rapid development of display devices has led to the increasing exploration of high performance materials, especially those with high refractive index. Moreover, process condition control is important for device and material development. To meet such demand, we have developed heat free / low temperature process high refractive index materials.

Effect of different stacking orders of Ta2O5 and SiO2 films on the reflective properties of a blue distributed Bragg reflector

The Ta2O5 and SiO2 single-layer films were deposited on the glass substrates to measure their refractive properties under different optical wavelengths. After that, the Ta2O5 and SiO2 films were used as high and low refractive index materials to design the blue distributed Bragg reflectors (DBRs) with different periods. The approximate formula proposed by Sheppard was used to calculate the maximum reflectance ratio of the bi-layer SiO–Ta2O5 DBRs with two, four, six, and eight periods. The COMSOL Multiphysics software was used to simulate the reflective spectra of the SiO2–Ta2O5 DBRs with different stacking orders and periods by incorporating the refractive indexes of the Ta2O5 and SiO2 films at a wavelength of 550 nm. The SiO2–Ta2O5 DBRs with different periods were fabricated by depositing the Ta2O5 and SiO2 films on glass substrates, and their reflective properties were measured. We compared the measured reflective spectra of the fabricated SiO2–Ta2O5 DBRs with the simulated reflective spectra to show the effect of the different stacking orders.

Plasma-Enhanced Atomic Layer Deposition of HfO2 with Substrate Biasing: Thin Films for High-Reflective Mirrors.

Tuning ion energies in plasma-enhanced atomic layer deposition (PEALD) processes enables fine control over the material properties of functional coatings. The growth, structural, mechanical, and optical properties of HfO2 thin films are presented in detail toward photonic applications. The influence of the film thickness and bias value on the properties of HfO2 thin films deposited at 100 °C using tetrakis(dimethylamino)hafnium (TDMAH) and oxygen plasma using substrate biasing is systematically analyzed. The HfO2 films deposited without a substrate bias show an amorphous microstructure with a low density, low refractive index, high incorporation of residual hydroxyl (OH) content, and high residual tensile stress. The material properties of HfO2 films significantly improved at a low bias voltage due to the interaction with oxygen ions accelerated to the film. Such HfO2 films have a higher density, higher refractive index, and lower residual OH incorporation than films without bias. The mechanical stress becomes compressive depending on the bias values. Further increasing the ion energies by applying a larger substrate bias results in a decrease of the film density, refractive index, and a higher residual OH incorporation as well as crystalline inclusions. The comparable material properties of the HfO2 films have been reported using tris(dimethylamino)cyclopentadienyl hafnium (TDMACpH) in a different apparatus, indicating that this approach can be transferred to various systems and is highly versatile. Finally, the substrate biasing technique has been introduced to deposit stress-compensated, crack- and delamination-free high-reflective (HR) mirrors at 355 and 532 nm wavelengths using HfO2 and SiO2 as high and low refractive index materials, respectively. Such mirrors could not be obtained without the substrate biasing during the deposition because of the high tensile stress of HfO2, leading to cracks in thick multilayer systems. An HR mirror for 532 nm wavelength shows a high reflectance of 99.93%, a residual transmittance of ∼530 ppm, and a low absorption of ∼11 ppm, as well as low scattering losses of ∼4 ppm, high laser-induced damage threshold, low mechanical stress, and high environmental stability.

Realisation of optical filters using multi-layered thin film coatings by transfer matrix model simulations

Anti-reflection (AR) coatings, distributed Bragg reflectors (DBRs), Comb filters and dichroic mirrors were designed by multi-layered thin films using transfer matrix model (TMM) simulations. In AR coatings to increase the zero-reflectance window, a tri-layered optical coating with central layer having an optical thickness of λ/2 may be chosen. DBRs and dichroic filters were designed by multilayers of alternating high and low refractive index materials in a periodic pattern. Comb filters were designed by sandwiching a thick transparent layer between two DBRs.

Low-threshold lasing in a plasmonic laser using nanoplate InGaN/GaN

Plasmonic nanolaser as a new type of ultra-small laser, has gain wide interests due to its breaking diffraction limit of light and fast carrier dynamics characters. Normally, the main problem that need to be solved for plasmonic nanolaser is high loss induced by optical and ohmic losses, which leads to the low quality factor. In this work, InGaN/GaN nanoplate plasmonic nanolaser with large interface area were designed and fabricated, where the overlap between SPs and excitons can be enhanced. The lasing threshold is calculated to be ~6.36 kW/cm2, where the full width at half maximum (FWHM) drops from 27 to 4 nm. And the fast decay time at 502 nm (sharp peak of stimulated lasing) is estimated to be 0.42 ns. Enhanced lasing characters are mainly attributed to the strong confinement of electromagnetic wave in the low refractive index material, which improve the near field coupling between SPs and excitons. Such plasmonic laser should be useful in data storage applications, biological application, light communication, especially for optoelectronic devices integrated into a system on a chip.

Application of static masking technique in magnetron sputtering technology for the production of linearly variable filters

Variable filters are key components for compact spectral imagers. In this paper, we present a method for the fabrication of linearly variable filters based on Bühler HELIOS machine (plasma assisted reactive magnetron sputtering). These filters are obtained by producing a variation of the thickness of all the layers of the coating, using adapted masks placed in between the sputtering targets for the low and high refractive index materials and the substrates. Variable band pass filter from 550 up to 1000 nm is demonstrated.

Numerical and experimental demonstration of inverse designed low-index polarization-insensitive wavelength demultiplexer

The computational inverse design has paved the way for the design of highly efficient, compact, and novel nanophotonic structures beyond human intuition and trial-and-error approaches. Apparently, with nanophotonic design power, the exploration and implementation of multi-objective, complex, and functional nanophotonic devices become feasible. Herein, we used a recently emerged inverse design framework to demonstrate the design of a 1 × 2 polarization-insensitive wavelength division multiplexer (PIWDM) made of a low-refractive-index material with an index of 1.55. The inversely designed PIWDM structure successfully steers toward the targeted channels for 1.30 µm and 1.55 µm with TE and TM polarizations. Taking advantage of the design with a low refractive index material, we scaled the structural dimensions corresponding to the microwave region, fabricated the compact device using a 3D printer, and conducted an experiment as a proof of concept. The transmission values of the fabricated PIWDM device were −4.87 and −2.18 dB for TE and −2.19 and −2.23 dB for TM polarization at WG-I and WG-II, respectively.

Manipulation of thermal radiation by using photonic crystals

This paper focuses on manipulating thermal emission and radiation loss of heat energy in a heat waveguide. A One-Dimensional Photonic Crystal is used as a waveguide clad to prohibit the thermal emission from escaping. The model may reduce the radiation loss of heat energy in the waveguide core, and heat energy can be confined to propagate along the waveguide’s longitude axis. The waveguide clad comprises alternative layers of high and low refractive index materials containing sufficient electromagnetic stop bands to trap the thermal emission from escaping out of the waveguide. The numerical simulation of the model shows that the forbidden bandgap of photonic crystal structures with alternative layers of silica and silicon has width enough to make heat energy be confined within the waveguide core so that efficient heat energy transmission can be achieved along the longitude axis of the waveguide.

Investigation of waveguide modes in EUV mask absorbers

Background: Explaining imaging phenomena in EUV lithography requires more than a single point of view. Traditionally, the diffraction characteristics of EUV masks are analyzed in terms of the amplitude and phase of diffraction orders that are generated by the absorber pattern. Aim: We propose a complementary perspective to view the EUV mask absorber openings as waveguides. Approach: Comparisons between RCWA simulations and analytical solutions to waveguide equations are performed to prove that EUV mask absorbers behave as a waveguide. Results: This perspective can explain phenomena left unexplained by conventional analysis of far-field diffraction orders. Conclusions: The waveguiding effect in EUV mask absorbers explains the need for low refractive index and high extinction materials. The waveguide perspective explains why attenuated phase shift masks behave differently for EUV than our traditional understanding would suggest.

Design High performance multilayers cold mirror

The optical design process includes a myriad of tasks that the designer must be performed and consider in the process of optimizing the performance of the optical system. In this article, a design based on four groups of materials for a multilayer dielectric cold mirror: V2O5 \\MgF2, Sic\\MgF2, TiO2\\MgF2, and Al As\\MgF2 where MgF2 represents low refractive index and other material in each group represent high refractive index. A cold mirrors are a special dielectric mirrors that reflects the entire visible wavelength band, while the transmitting infrared wavelength band. These systems of mirrors are constructed for an event angle from 0° to 45° and are modeled with interference filters-like multi thin layer dielectric coatings. The results of our work designed shows the mirror based on Al As/MgF2 is the most promising design for cold mirror with the highest reflection in the visible region and the highest Transition in the IR region. In the spectrum range of 400-800 nanometers, the average transmission is much less than 1 %, while the average transmission is 95 percent in the wave band 800-2500 nanometer.

Multilayer SiC/C Thin Film Coating on Fiber Tip for Raman Probe Application

A novel technique in the fabrication of an optical filter that is made up of multilayer thin film is introduced in this study. Multiple layers of silicon carbide and carbon with high and low refractive index materials respectively will be deposited alternatively with a various thickness on a glass substrate by radio frequency magnetron sputtering of 99% purity of silicon carbide as the target. The variation of structure and optical properties of the films are studied by using X-ray reflectivity (XRR), Raman scattering spectroscopy, and UV–vis-NIR spectroscopy until it reached the desired results and meet the specification as Raman optical filter. The thin film is then placed on a multimode fiber tip as Raman edge filter. Raman fingerprint of specific alcohol (ethanol) obtained is tested using a fabricated filter with the standard spectrum. The preliminary result of SiC thin film fabrication for 30, 60 and 90 minutes by sputtering process showed rate of deposition obtained is 3.17nm/min. The results demonstrate that the optical properties of silicon carbide thin film can be monitored by modifying the condition of silicon carbide and carbon deposition.

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.

Fabrication and Bonding of Refractive Index Matched Microfluidics for Precise Measurements of Cell Mass.

The optical properties of polymer materials used for microfluidic device fabrication can impact device performance when used for optical measurements. In particular, conventional polymer materials used for microfluidic devices have a large difference in refractive index relative to aqueous media generally used for biomedical applications. This can create artifacts when used for microscopy-based assays. Fluorination can reduce polymer refractive index, but at the cost of reduced adhesion, creating issues with device bonding. Here, we present a novel fabrication technique for bonding microfluidic devices made of NOA1348, which is a fluorinated, UV-curable polymer with a refractive index similar to that of water, to a glass substrate. This technique is compatible with soft lithography techniques, making this approach readily integrated into existing microfabrication workflows. We also demonstrate that this material is compatible with quantitative phase imaging, which we used to validate the refractive index of the material post-fabrication. Finally, we demonstrate the use of this material with a novel image processing approach to precisely quantify the mass of cells in the microchannel without the use of cell segmentation or tracking. The novel image processing approach combined with this low refractive index material eliminates an important source of error, allowing for high-precision measurements of cell mass with a coefficient of variance of 1%.

An ultrabroadband 3D achromatic metalens

Abstract We design and fabricate ultra-broadband achromatic metalenses operating from the visible into the short-wave infrared, 450–1700 nm, with diffraction-limited performance. A hybrid 3D architecture, which combines nanoholes with a phase plate, allows realization in low refractive index materials. As a result, two-photon lithography can be used for prototyping while molding can be used for mass production. Experimentally, a 0.27 numerical aperture (NA) metalens exhibits 60% average focusing efficiency and 6% maximum focal length error over the entire bandwidth. In addition, a 200 μm diameter, 0.04 NA metalens was used to demonstrate achromatic imaging over the same broad spectral range. These results show that 3D metalens architectures yield excellent performance even using low-refractive index materials, and that two-photon lithography can produce metalenses operating at visible wavelengths.

Broadband and lossless lithium niobate valley photonic crystal waveguide [Invited

We investigate the influences of structure parameters and interface shapes on the bandwidth of the edge state of lithium niobate valley photonic crystals. By increasing the size difference of two air holes in the same unit cell, we find that the bandwidth of the lossless nontrivial edge state possesses a peak value of 0.0201(a/λ), which can be used to construct broadband valley photonic crystal waveguides. Mode field distributions verify that the waveguide is robust against sharp bends and exhibits chirality. When the unit cell is arranged in a bearded interface with the top and bottom components showing negative and positive valley Chern numbers, respectively, we find that the lithium niobate valley photonic crystal is more likely to exhibit a lossless edge state, which is difficult to be realized in valley waveguides with low refractive index materials. This work can provide guidance on the design of the high-performance topological waveguide.

Perspectives and tradeoffs of absorber materials for high NA EUV lithography

Next-generation extreme ultraviolet (EUV) systems with numerical apertures of 0.55 have the potential to provide sub-8-nm half-pitch resolution. The increased importance of stochastic effects at smaller feature sizes places further demands on scanner and mask to provide high contrast images. We use rigorous mask diffraction and imaging simulation to understand the impact of the EUV mask absorber and to identify the most appropriate optical parameters for high NA EUV imaging. Simulations of various use cases and material options indicate two main types of solutions: high extinction materials, especially for lines spaces, and low refractive index materials that can provide phase shift mask solutions. EUV phase masks behave very different from phase shift masks for DUV. Carefully designed low refractive index materials and masks can open up a new path toward high contrast edge printing.

Fabricating multilayer antireflective coating for near complete transmittance in broadband visible light spectrum

Abstract To extract maximum performance out of the certain optical devices it is highly desirable to use completely transparent window in a broad wavelength range. In search of large-scale applicable antireflective (AR) coating for such window, herein we report the design and fabrication of multilayer coating with the aim to reduce the reflection of the window in the entire visible light range. Systematic selection of material and thickness of each layer was performed based on the criteria fulfilled by refractive index to obtain zero reflection. Low refractive index material like MgF2 is deposited by e-beam technique to obtain single-layer AR coating. While Al2O3 and ZrO2, deposited by RF-magnetron sputtering, with MgF2 are used to develop multilayer AR coating. Scanning electron microscope image along with elemental line scan obtained along the cross-section of the coating verified the thickness and composition of each layer. Deposition of four-layer AR coating (MgF2/Al2O3/ZrO2/MgF2) on both side of quartz window with optimized thickness showed considerable reduction in the reflection to 0.6% at 550 nm. The average transmittance of 99.1% was recorded in the broadband of visible spectrum in the range of 450–750 nm wavelength. With near to complete transmission, the present AR coating holds encouraging prospect for implementing in windows of digital display, photovoltaic module, and other optoelectronic devices.

Prototypical Simulation of an Optical Edge Filter Design for Three Different Rayleigh Wavelengths Using Octave Software

Optical filters used specifically for special functions to block a particular wavelength or range of wavelength and transmit the rest of the spectrum have received research attention for the past few eras. Recently, optical filter based on multilayer coating are more focused due to potential to manipulate filter properties by changing layer thickness in order to apply in various fields. This Quarter wave stack (QWS) model design is one of the approaches to design optical filter such as edge filter. However, to obtain desire minimum specification need an optimization. Therefore, in this study aims to design optical edge filter based QWS model by optical matrix method in Octave software. In this design, MoS2 and Si are being choose for high (H) and low (L) refractive index materials respectively. The optimum twenty-four (24) number of layers are determined by calculating maximum transmittance obtained. The Rayleigh wavelength ( λex ) of 405 nm ,532 nm and 633 nm are selected and ‘glass|12HL|air’ configuration is set for the design simulation. Then, the cut-on wavelength (λcut-on) and cut-off wavelength ((λcut-off) of successful designed optical edge filter are measured. The result shows that the cut-on wavelength of 408.11 nm, 536 nm and 640.25 nm with minimum effective transmission (MET) of 32.3%, 31.3% and 32% respectively are obtained. In conclusion, this present study shows the deviation between the calculated value and simulation of about 0.2 nm,1.7 nm and 1.15 nm for each λex of 405 nm, 532 nm and 633 nm is determined. But for λcut-off, the value obtained are much higher than the calculated value. The MET of each filter is 32.3% ,31.3% and 32% for Rayleigh wavelength respectively.

Magnesium fluoride as low-refractive index material for near-ultraviolet filters applied to optical sensors

This article describes the fabrication of MgF2 and MgO thin-film-based optical filters and compares the optical transmission of the filters over UV. The MgF2 thin-films were deposited by use of an e-beam technique and their optical properties were characterised by ellipsometry. The effect of substrate temperature on the optical properties was studied. The MgF2 optimum refractive indices were obtained with a substrate temperature between 200 °C and 300 °C. Optical simulations were performed to compare the performance of MgF2 and MgO in the fabrication of near-UV narrow bandpass optical filters. While MgO-based optical filters result in a higher transmittance peak intensity, especially at 350 nm, the MgF2 optical filters are narrower, present lower values of FWHM, a mean value of 20 nm. This feature could be especially relevant for specific applications on fluorescent optical sensors. Finally, a Fabry-Perot based on a MgF2/TiO2 optical filter was deposited, using an e-beam technique for the MgF2 thin-films and RF-sputtering technique for the TiO2 thin-films. The MgF2/TiO2 optical filter peak transmittance is approximately 70% close to 400 nm, as expected. The results are discussed with focus on applications in fluorescent optical sensors for peaks at 350, 370, 380 and 400 nm, respectively.

A hybrid achromatic metalens

Metalenses, ultra-thin optical elements that focus light using subwavelength structures, have been the subject of a number of recent investigations. Compared to their refractive counterparts, metalenses offer reduced size and weight, and new functionality such as polarization control. However, metalenses that correct chromatic aberration also suffer from markedly reduced focusing efficiency. Here we introduce a Hybrid Achromatic Metalens (HAML) that overcomes this trade-off and offers improved focusing efficiency over a broad wavelength range from 1000–1800 nm. HAMLs can be designed by combining recursive ray-tracing and simulated phase libraries rather than computationally intensive global search algorithms. Moreover, HAMLs can be fabricated in low-refractive index materials using multi-photon lithography for customization or using molding for mass production. HAMLs demonstrated diffraction limited performance for numerical apertures of 0.27, 0.11, and 0.06, with average focusing efficiencies greater than 60% and maximum efficiencies up to 80%. A more complex design, the air-spaced HAML, introduces a gap between elements to enable even larger diameters and numerical apertures.