High Refractive Index Materials Research Articles

Improved light output of plastic scintillator by a modified self-assembled photonic crystal

In this investigation, we have demonstrated that a modified self-assembled photonic crystal with conformal high refractive index material TiO2 can achieve a great enhancement of light extraction efficiency. A 2.26 fold wavelength- and angle-integrated enhancement ratio can be achieved. The conformal layer increases the number of leaky modes and thus improve the extraction efficiency. The enhancement is attributed to the leaky modes based on the individual microspheres with conformal layer. Their low quality factors with a broadband characteristic are advantageous to the broadband enhancement for the emission spectra of plastic scintillator. Furthermore, the dense conformal layers have excellent combination with the self-assembled microspheres and the whole preparation process cannot destroy the plastic scintillator, which is beneficial to the practical application.

Metallic nanorings for broadband, enhanced extraction of light from solid-state emitters

We report on the increased extraction of light emitted by solid-state sources embedded within high refractive index materials. This is achieved by making use of a local lensing effect by sub-micron metallic rings deposited on the sample surface and centered around single emitters. We show enhancements in the intensity of the light emitted by InAs/GaAs single quantum dot lines into free space as high as a factor of 20. Such a device is intrinsically broadband and therefore compatible with any kind of solid-state light source. We foresee the fabrication of metallic rings via scalable techniques, like nano-imprint, and their implementation to improve the emission of classical and quantum light from solid-state sources. Furthermore, while increasing the brightness of the devices, the metallic rings can also act as top contacts for the local application of electric fields for carrier injection or wavelength tuning.

Selective Dielectric Metasurfaces Based on Directional Conditions of Silicon Nanopillars.

Dielectric metasurfaces based on high refractive index materials have been proposed recently. This type of structure has several advantages over their metallic counterparts. In this work, we demonstrate that dielectric metasurfaces can be theoretically designed satisfying Kerker’s zero-forward condition. This is the first time that a dielectric metasurface based on this principle has been designed. A selective dielectric metasurface of silicon nanopillars is designed to work at 632.8 nm. This structure could work both as a dielectric mirror and a reject band filter. Furthermore, by scaling up the structure, it could be possible to manufacture a terahertz (THz) dielectric mirror.

Bandgap measurement of high refractive index materials by off-axis EELS

In the present work Cs aberration corrected and monochromated scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) has been used to explore experimental set-ups that allow bandgaps of high refractive index materials to be determined. Semi-convergence and –collection angles in the µrad range were combined with off-axis or dark field EELS to avoid relativistic losses and guided light modes in the low loss range to contribute to the acquired EEL spectra. Off-axis EELS further supressed the zero loss peak and the tail of the zero loss peak. The bandgap of several GaAs-based materials were successfully determined by simple regression analyses of the background subtracted EEL spectra. The presented set-up does not require that the acceleration voltage is set to below the Čerenkov limit and can be applied over the entire acceleration voltage range of modern TEMs and for a wide range of specimen thicknesses.

Effects of hot-isostatic pressing and annealing post-treatment on HfO2 and Ta2O5 films prepared by ion beam sputtering

Hafnium oxide (HfO2) films and Tantalum oxide (Ta2O5) films prepared by ion beam sputtering are important high refractive index materials from ultraviolet to near infrared. In this paper, Hot-isostatic pressing (HIP) technique is applied to the post-treatment of HfO2 and Ta2O5 films, and the effects of HIP and annealing treatment on optical and stress properties of the thin films were compared. Optical properties are mainly including thickness, refractive index, extinction coefficient, band gap (Eg), and tail energy (Eu), which were calculated by the inversion of spectral properties. And stress characteristics were calculated by the curvature change of the samples surface. The results show that the two post-treatment methods both can reduce refractive index and extinction coefficient of the thin films, and can effectively improve band gap of thin films. The annealing treatment is better than HIP for reducing the extinction coefficient of thin films. The tail energies of the thin films show a decreasing trend after post-treatment, which indicates the decrease of the defect density of local state and the disorder degree of microstructures. The change of stress after HIP treatment is less than that of annealing because of the homogeneous pressure in HIP post-treatment, but both the post-treatment methods can transform the stress properties of the films.

Enhanced light extraction of LYSO scintillator by photonic crystal structures from a modified porous anodized aluminum oxide layer

Although porous anodized aluminum oxide layer can be used to extract scintillation light from a LYSO scintillator, the low refractive index contrast of porous AAO layer obtains a moderate enhancement. In this investigation, we have designed and fabricated a modified porous anodized aluminum oxide layer with conformal deposition layer of high refractive index material of TiO2 on the surface of LYSO scintillator, achieving a significant enhancement by 60% with wavelength- and angle-integrated emission intensity. The fabrication method of the present study is simple and low-cost for the large area applications in the field of radiation detection.

Controlling ultrashort pulses shape using one-dimensional graded index photonic crystals

In this study, we analyze the characteristics of a one dimensional coupled cavity-waveguide composed of SiO2/TiO2 layers in which three defects are located symmetrically in structure, using finite-time domain-method and transfer matrix method. Then we introduced a new structure based on mentioned one-dimensional structure in which the higher refractive index material is maintained constant, while lower refractive index slightly increases with a constant step value from the beginning to the end of the structure. Simulation results reveal that the graded structure represents unique features in comparison with the conventional structure.

Study and Fabricate of Reflectance Coating for UV Lithography Machine Arc Lampshade

As one of the key components in lithography, the UV light source has very high demands for wavelength of source and energy. To meet the requirements of UV light source, it demands UV reflectance coating prepared on 78mm Mg-Al alloy paraboloid lampshade . Based on design theory of film, H4 and SiO2 were selected as high and low refractive index materials respectively, and with the help of TFCalc, adopting electron beam vacuum coating method, the reflectance coating was prepared, which satisfied the requirements of reflectivity greater than 98.5% within band of 365-436nm in the case of 0o-45o incident angles. First the reflective band width was broadened through the coating stack; then according to the thickness distribution of film, compensation baffle was designed and modified, which ensured the uniformity of reflectance coating in the lampshade. And last, firmness of coating was strengthened and the reflectivity in the required band was improved by depositing one bonding layer of metal between Mg-Al alloy and dielectric film. By repeated trials, coating absorption problem was solved by changing the process parameters and coating quality of film was improved with the use of ion source assisted deposition, finally the UV film meets the requirements for use.

Fundamental limits of ultrathin metasurfaces

We present a set of universal relations which relate the local transmission, reflection, and polarization conversion coefficients of a general class of non-magnetic passive ultrathin metasurfaces. We show that these relations are a result of equal forward and backward scattering by single layer ultrathin metasurfaces, and they lead to confinement of the transmission, reflection, and polarization conversion coefficients to limited regions of the complex plane. Using these relations, we investigate the effect of the presence of a substrate, and show that the maximum polarization conversion efficiency for a transmissive metasurface decreases as the refractive index contrast between the substrate and cladding layer increases. Furthermore, we demonstrate that a single layer reflective metasurface can achieve full 2π phase shift coverage without altering the polarization if it is illuminated from the higher refractive index material. We also discuss two approaches for achieving asymmetric scattering from metasurfaces, and realizing metasurfaces which overcome the performance limitations of single layer ultrathin metasurfaces.

Enhanced nonlinear optical effects in organic frustum-shaped microresonators

We discuss photonic effects in arrays of frustum-shaped organic microstructures prepared from organic orange dye composed on a solid substrate by self-assembling technique. We demonstrate that such structures reveal strong amplification of second-order nonlinear optical response, including optical second harmonic generation (SHG) and two-photon fluorescence (TPF), as compared to that of a continuous film. This is associated with a strong light localization in microstructures composed of high refractive index material. The TPF and SHG power dependencies show that the observed effects are governed by high second-order nonlinearity of the dye damped by a strong absorption. FDTD calculations confirm that the mechanism of the light localization inside organic frustums can be in the form of whispering gallery modes excitation.

Investigation of AWG demultiplexer based SOI for CWDM application

9-channel Arrayed Waveguide Grating (AWG) demultiplexer for conventional and tapered structure were simulated using beam propagation method (BPM) with channel spacing of 20 nm. The AWG demultiplexer was design using high refractive index (n~3.47) material namely silicon-on-insulator (SOI) with rib waveguide structure. The characteristics of insertion loss, adjacent crosstalk and output spectrum response at central wavelength of 1.55 μm for both designs were compared and analyzed. The conventional AWG produced a minimum insertion loss of 6.64 dB whereas the tapered AWG design reduced the insertion loss by 2.66 dB. The lowest adjacent crosstalk value of -16.96 dB was obtained in the conventional AWG design and this was much smaller compared to the tapered AWG design where the lowest crosstalk value is -17.23 dB. Hence, a tapered AWG design significantly reduces the insertion loss but has a slightly higher adjacent crosstalk compared to the conventional AWG design. On the other hand, the output spectrum responses that are obtained from both designs were close to the Coarse Wavelength Division Multiplexing (CWDM) wavelength grid.

Analysis on Transition between Index- and Bandgap-guided Modes in Photonic Crystal Fiber

We calculate optical properties of guided modes of a hybrid-guiding photonic crystal fiber. The design and modeling of such hybrid-guiding PCF is made by replacing air holes with inserts of high refractive index material layer by layer in order. The optical properties such as mode intensity profile, mode dispersion, optical birefringence, confinement loss, and chromatic dispersion during transition of the guiding mechanism are analyzed and discussed. The guided modes in the hybrid-guiding region are also compared with those of reference index-guiding and bandgap-guiding photonic crystal fibers.

Stratospheric solar geoengineering without ozone loss

Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of -1 W⋅m-2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y-1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.

ChemInform Abstract: Polyferrocenylsilanes: Synthesis, Properties, and Applications

AbstractReview: 409 refs.

Enhanced axial confinement in a monolithically integrated self-rolled-up SiNx vertical microring photonic coupler

We report an efficient method to introduce enhanced axial confinement in the self-rolled-up SiNx vertical microtube coupler by depositing a thin layer of high refractive index material strip within the coupling section and effectively forming a vertical microring. Three times wider mode spacing is observed in such a vertical microring coupler monolithically integrated with a silicon nitride ridge waveguide as compared to the one without such axial confinement. More importantly, single mode operation within the telecomm C-band and S-band is achieved.

Indium phosphide all air-gap Fabry-Pérot filters for near-infrared spectroscopic applications

Food quality can be characterized by noninvasive techniques such as spectroscopy in the Near Infrared wavelength range. For example, 930 -1450 nm wavelength range can be used to detect diseases and differentiate between meat samples. Miniaturization of such NIR spectrometers is useful for quick and mobile characterization of food samples. Spectrometers can be miniaturized, without compromising the spectral resolution, using Fabry-Pérot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. The most commonly used mirrors in the design of FP filters are Distributed Bragg Reflections (DBRs) consisting of alternating high and low refractive index material pairs, due to their high reflectivity compared to metal mirrors. However, DBRs have high reflectivity 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 detector. Therefore, a bandpass filter is usually required to restrict wavelengths outside the stopband of the FP DBRs. Such bandpass filters are difficult to design and implement. Alternatively, high index contrast materials must be can be used to broaden the stopband width of the FP DBRs. In this work, Indium phosphide all air-gap filters are proposed in conjunction with InGaAs based detectors. The designed filter has a wide stopband covering the entire InGaAs detector sensitivity range. The filter can be tuned in the 950-1450 nm with single mode operation. The designed filter can hence be used for noninvasive meat quality control.

Biomedical bandpass filter for fluorescence microscopy imaging based on TiO2/SiO2 and TiO2/MgF2 dielectric multilayers

We report a design for creating a 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 & 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 Reflectors (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 reflectivity 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 range. Therefore a bandpass filters are required to restrict wavelength outside the stopband of the FP DBRs. The proposed filter shows a high quality with average transmission of 97.4% within the passbands and the transmission outside the passband is around 4%. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptional choice for florescence imaging and Endoscope narrow band imaging.

Sol-gel fabrication of one-dimensional photonic crystals with predicted transmission spectra

One-dimensional multilayer structures of periodically alternating low refractive index (silica) and high refractive index (titania) materials have been deposited by sol-gel spincoating. Experimental spectra of the structures are in agreement with spectra calculated by transfer matrix technique. As an example, theoretical and experimental spectra with a stop band corresponding 600 nm-reflection are shown.

High Refractive Index Hyperbranched Polymers Prepared by Two Naphthalene-Bearing Monomers via Thiol-Yne Reaction

High refractive index (HRI) materials play an important role in optic-electronic devices. In this study, two compounds with high content of naphthalene groups, 1,5-dithiolnaphthalene and 1,3,5-tris(naphthalyl–ethylnyl) benzene, are selected as “A2” and “B3” monomers, respectively to prepare hyperbranched HRI polymers. Metal-free radical-initiated “A2 + B3” thiol-yne polyaddition is conducted successfully at different monomer molar ratios even for those sterically demanding molecules being able to adjust the molar mass as well as RI. Polymers with refractive index up to 1.79 at 589.7 nm are obtained, which are among the highest RI values so far reported for pure polymer-based materials. The high RI, based on the high molar refraction of naphthalene group, metal-free and easy one-pot synthesis, high transparency in the visible area, good thermal stability, good solubility, and easy processability into thin films, make these polymers excellent candidates for optic-electronic applications.

High refractive index Fresnel lens on a fiber fabricated by nanoimprint lithography for immersion applications.

In this Letter, we present a Fresnel lens fabricated on the end of an optical fiber. The lens is fabricated using nanoimprint lithography of a functional high refractive index material, which is suitable for mass production. The main advantage of the presented Fresnel lens compared to a conventional fiber lens is its high refractive index (n=1.68), which enables efficient light focusing even inside other media, such as water or an adhesive. Measurement of the lens performance in an immersion liquid (n=1.51) shows a near diffraction limited focal spot of 810 nm in diameter at the 1/e2 intensity level for a wavelength of 660 nm. Applications of such fiber lenses include integrated optics, optical trapping, and fiber probes.