Improvement In Optical Performance Research Articles

Effect of distributed Bragg reflectors on optoelectronic characteristics of GaN-based flip-chip light-emitting diodes

Abstract Distributed Bragg reflectors have been widely utilized in GaN-based flip-chip light-emitting diodes (FCLEDs) owing to their excellent reflection performance. Recently, wide reflected angle DBR (WRA-DBR) has been suggested to enhance the optical characteristics of GaN-based FCLEDs by incorporating multiple sub-DBRs with varying central wavelengths. However, the reflectivity of WRA-DBR decreases at large incident angle from 425 nm to 550 nm, which restricts further optical performance improvement of FCLEDs. Here, we demonstrate a quintuple-stack DBR comprised of five sub-DBRs. The quintuple-stack DBR possesses a high reflectivity (>97.5%) for incident angles below 50° within the blue and green light wavelength ranges. Compared to WRA-DBR, quintuple-stack DBR exhibits a higher reflectivity in wavelength range of 425 nm to 550 nm and thinner multilayer thicknesses. Furthermore, stronger electric field intensities exist in the top facet and sidewalls of FCLED with quintuple-stack DBR, revealing that quintuple-stack DBR is beneficial for enhancing the light extraction efficiency. As a result, the light output power of FCLED with quintuple-stack DBR is ~3% higher than that of FCLED with WRA-DBR at 750 mA.

Performance investigation of elevated source EBG TFET based photosensor for near-infrared light sensing applications

In this manuscript, an elevated source TFET with extended back gate (ES-EBG-TFET) based photosensor is designed to offer improvement in optical performance for detecting incident light of narrow-spaced wavelengths (∼100 nm) and low luminous intensity (<0.9 W/cm2) in the near-infrared (IR) range. The proposed photosensor combines the advantages provided by both the elevated top gate and extended back gate in the sense of increasing the line tunneling region at the source-channel (S–C) interface, resulting to improvement in illumination current (ILight), threshold voltage (Vth) and average subthreshold swing (SSavg) under the light state. Furthermore, the inclusion of an L-shaped pocket around the source region helps to reduce the corner effects at S–C interface in the sub-threshold region, thereby improving the signal-to-noise ratio (SNR) of the proposed photosensor for on-chip applications. Due to improved optical generation in the photosensing gate, a high spectral sensitivity (Sn) of ∼54 is achieved for ES-EBG-TFET based photosensor because the variation in wavelength (λ) of incident light from 1050 to 750 nm shows a two order change in ILight (ID at Vgs = 0.2V under light state) from 3.1 × 10−15 A/μm to 1.72 × 10−13 A/μm, respectively. Since the elevated source provides a space to create larger illumination window above the channel region, a high optical generation rate is observed for the proposed photosensor under the incident light with low intensity (<0.9 W/cm2). The maximum value of SNR is observed to be 66.4 dB at a λ of 750 nm, while the quantum efficiency (ƞ) and responsivity (R) are reported to be 74% and 0.59 A/W, respectively, around λ of 1050 nm. Moreover, an optimum length of back-gate underlap with drain (LBGUD) is found to enhance the magnitude of SNR (at λ of 750 nm) and Sn (750–1050 nm) from 25.6 to 63.1 dB and 14 to 52.8, respectively when the photosensor is operated at the ambipolar state (Vgs = −0.6 V and Vds = 0.5 V). Finally, the photosensing parameters such as Sn and SNR are analyzed under the presence of acceptor and donor interface traps and it is found that there is insignificant degradation in the optical performance of the proposed photosensor.

Improvement of high energy X-ray optical performance of W/Si supermirror by optimizing interface compounds using ultra-thin buffer layer

The high energy spectral properties of X-ray supermirrors are governed mainly by the interface diffusion and compound formation at the interfaces. Using ion beam sputtering technique, a set of W/Si periodic multilayers with C buffer layer (BL) and without C BL have been prepared and the effect of BL has been studied by grazing incidence specular X-ray reflectivity, diffused X-ray scattering and X-ray photoelectron spectroscopy. Using C BL at the W-on-Si interfaces, reduction of interface diffusion and minimization of WSix formation at the interfaces has been observed though signature of carbide formation has been noticed. Using the results an interface diffusion model has been proposed explaining the effect of C BL. Block method of X-ray supermirror design has been effectively used for reducing the number of layers of W/Si supermirror targeting the improvement of the performance. Finally, 36-layer W/Si supermirror with C BL and without C BL have been fabricated and the performance evaluated by reflectivity measurement using 25 keV synchrotron X-rays and cross sectional TEM measurements. This study has both technological implication in X-ray optical devices and also offers atomistic insight into the mechanism of improvement of the interface quality by application of ultra-thin buffer layer.

Anti-phase pulsation of counter-propagating dissipative solitons in a bidirectional fiber laser

Due to its unique geometric structure, the bidirectional ultrafast fiber laser is an excellent light source for dual-comb applications. However, sharing the same gain between the counter-propagating solitons also gives rise to complex dynamics. Herein, we report the anti-phase pulsation of counter-propagating dissipative solitons in a bidirectional fiber laser. The in-phase and anti-phase soliton pulsation can be manipulated by adjusting the intracavity birefringence. The periodic modulation of polarization-dependent gain (PDG) caused by polarization hole burning (PHB) in the gain fiber can be responsible for anti-phase pulsation of bidirectional dissipative solitons. These findings offer new, to the best of our knowledge, insights into the complex dynamics of solitons in dissipative optical systems and performance improvement of bidirectional ultrafast fiber lasers.

Research on the influence of the non-stationary effect of the magnetorheological finishing removal function on mid-frequency errors of optical component surfaces.

With the continuous development of modern optical systems, the demand for full spatial frequency errors of optical components in the system is increasing. Although computer-controlled sub-aperture polishing technology can quickly correct low-frequency errors, this technology significantly worsens the mid-frequency errors on the surface of the component, which greatly inhibits the improvement of optical system performance. Therefore, we conducted in-depth research on the non-stationary effect of the removal function caused by the fluctuation in magnetorheological polishing and their influence on the mid-frequency errors of the component surface. We established a non-stationary profile model of the removal function and applied this model to simulate the distribution of mid-frequency errors on the surface of the processed component, considering the non-stationary effect. The simulation results showed that the non-stationary effect of the removal function weaken the mid-frequency ripple errors but increase other mid-frequency errors. Therefore, we first proposed the optimal single-material removal thickness corresponding to the non-stationary effect and experimentally verified the effectiveness of the optimal material removal thickness in suppressing mid-frequency errors. The experimental results showed that when the magnetorheological finishing single-material removal thickness is set to the optimal value, both the mid-frequency ripple errors and the mid-frequency RMS on the surface significantly decrease. Therefore, this work provides a basis for improving the existing magnetorheological finishing process and effectively suppressing the mid-frequency errors on the surface of processed components. It also provides theoretical and technical support for the magnetorheological processing and manufacturing of high-precision optical components. At the same time, the non-stationary effect and the corresponding analytical models has the potential to be extended to other polishing tools.

Characterising the broadband, wide-angle reflectance properties of black silicon surfaces for photovoltaic applications.

Black silicon nanotextures offer significant optical performance improvements when applied to crystalline silicon solar cells. Coupled with conventional pyramidal textures, to create so-called hybrid black silicon, these benefits are shown to be further enhanced. Presented here is a comprehensive analysis of different variations of this texture, coupled with typical anti-reflectance schemes such as coated pyramids, with a view to the significance of this on subsequent, real-world, solar energy generation. The study uses an angle-resolved spectrophometry system to characterise and compare the optical properties of these surface textures in terms of reflectance versus wavelength and incident angle, with and without encapsulant layers. This analysis, coupled with time-resolved, location specific irradiance data, leads to a new figure-of-merit, the weighted reflectivity, with which to compare surface textures for use in solar cells. Weighted reflectivity for an encapsulated solar cell surface, averaged over a year, for a Southampton, UK, location is calculated to be 7.6% for hybrid black silicon, compared to 10.6% for traditional random pyramids with a thin film anti-reflective coating.

Modeling and in-depth analysis of the mid-spatial-frequency error influenced by actual contact pressure distribution in sub-aperture polishing.

In ultra-precision optical processing, the sub-aperture polishing is prone to produce a mid-spatial-frequency (MSF) error. However, the generation mechanism of the MSF error is still not fully clarified, which seriously affects the further improvement of optical component performance. In this paper, it is proved that the actual contact pressure distribution between the workpiece and tool is a crucial source which affects the MSF error characteristics. A rotational periodic convolution (RPC) model is proposed to reveal the quantitative relationship among the contact pressure distribution, speed ratio (spin velocity/feed speed) and MSF error distribution. In-depth analyses show that the MSF error is linearly related to the symmetry level of contact pressure distribution and inversely proportional to the speed ratio, where the symmetry level is effectively evaluated by the proposed method based on Zernike polynomials. In the experiments, according to the actual contact pressure distribution obtained from the pressure-sensitive paper, the error rate of modeling results under different processing conditions is around 15%, which proves the validity of the proposed model. The influence of contact pressure distribution on the MSF error is further clarified through the establishment of RPC model, which can further promote the development of sub-aperture polishing.

Large field-of-view short-wave infrared metalens for scanning fiber endoscopy.

The scanning fiber endoscope (SFE), an ultrasmall optical imaging device with a large field-of-view (FOV) for having a clear forward view into the interior of blood vessels, has great potential in the cardiovascular disease diagnosis and surgery assistance, which is one of the key applications for short-wave infrared biomedical imaging. The state-of-the-art SFE system uses a miniaturized refractive spherical lens doublet for beam projection. A metalens is a promising alternative that can be made much thinner and has fewer off-axis aberrations than its refractive counterpart. We demonstrate a transmissive metalens working at 1310nm for a forward viewing endoscope to achieve a shorter device length and better resolution at large field angles. We optimize the metalens of the SFE system using Zemax, fabricate it using e-beam lithography, characterize its optical performances, and compare them with the simulations. The SFE system has a resolution of at the center of field (imaging distance 15mm), an FOV of , and a depth-of-focus of , which are comparable with a state-of-the-art refractive lens SFE. The use of the metalens reduces the length of the optical track from 1.2 to 0.86mm. The resolution of our metalens-based SFE drops by less than a factor of 2 at the edge of the FOV, whereas the refractive lens counterpart has a times resolution degradation. These results show the promise of integrating a metalens into an endoscope for device minimization and optical performance improvement.

Treatment of Extended Kalman Filter Implementations for the Gyroless Star Tracker.

The literature since Apollo contains exhaustive material on attitude filtering, usually treating the problem of two sensors, a combination of state measuring and inertial devices. More recently, it has become popular for a sole attitude determination device to be considered. This is especially the case for a star tracker given its unbiased stellar measurement and recent improvements in optical sensor performance. The state device indirectly estimates the attitude rate using a known dynamic model. In estimation theory, two main attitude filtering approaches are classified, the additive and the multiplicative. Each refers to the nature of the quaternion update in the filter. In this article, these two techniques are implemented for the case of a sole star tracker, using simulated and real night sky image data. Both sets of results are presented and compared with each other, with a baseline established through a basic linear least square estimate. The state approach is more accurate and precise for measuring angular velocity than using the error-based filter. However, no discernible difference is observed between each technique for determining pointing. These results are important not only for sole device attitude determination systems, but also for space situational awareness object localisation, where attitude and rate estimate accuracy are highly important.

Microstructures and optical properties of porous PbSe film prepared by ion exchange process

PbSe films can be widely applied in many fields involving industrial temperature measurement, military guidance, agriculture detection and celestial observation due to their high reliability and sensitivity. The porous PbSe film with a grain size of ∼0.5 μm was firstly transformed from flaky pastry-like plumbonacrite (Pb10O(OH)6(CO3)6) precursor film by ion exchange process, where the precursor film was prepared by a low-toxicity chemical bath deposition (CBD) process. The variations of phase composition, surface morphology, and binding state of main surface elements, as well as the optical properties of films with immersion time were mainly investigated. The experimental results show that the flaky pastry-like Pb10O(OH)6(CO3)6 film can be fully converted to cubic PbSe film as the immersion time is over 1.5 h, and the grain size of PbSe is further increased with the immersion time prolonging from 1.5 to 2.5 h. Moreover, the presence of a large number of pores in PbSe film can produce low reflectance, leading to the improvement of optical performance of PbSe film in the infrared wavelength range. The XPS, Raman and TEM analyses demonstrate the presence of trace impurities and lattice defects, which can be related to the oxidation, incomplete transformation and mutual extrusion between grains. The calculated bandgap is decreased from 2.68 eV for Pb10O(OH)6(CO3)6 film to 0.39 eV for PbSe film.

Synergistic Effect of Halogen Ions and Shelling Temperature on Anion Exchange Induced Interfacial Restructuring for Highly Efficient Blue Emissive InP/ZnS Quantum Dots.

InP quantum dots (QDs) have attracted much attention owing to their nontoxic properties and shown great potential in optoelectronic applications. Due to the surface defects and lattice mismatch, the interfacial structure of InP/ZnS QDs plays a significant role in their performance. Herein, the formation of In-S and Sx -In-P1-x interlayers through anion exchange at the shell-growth stage is revealed. More importantly, it is proposed that the composition of interface is dependent on the synergistic effect of halogen ions and shelling temperature. High shelling temperature contributes to the optical performance improvement resulting from the formation of interlayers, besides the thicker ZnS shell. Moreover, the effect relates to the halogen ions where I- presents more obvious enhancement than Br- and Cl- , owing to their different ability to coordinate with In dangling bonds, which are inclined to form In-S and Sx -In-P1-x bonds. Further, the anion exchange under I- -rich environment causes a blue-shift of emission wavelength with shelling temperature increasing, unobserved in a Cl- - or Br- -rich environment. It contributes to the preparation of highly efficient blue emissive InP/ZnS QDs with emission wavelength of 473nm, photoluminescence quantum yield of ≈50% and full width at half maximum of 47nm.

Full-visible-spectrum perovskite quantum dots by anion exchange resin assisted synthesis

Abstract Photoelectric properties of all-inorganic perovskite quantum dots (IPQDs) highly depend on their synthetic route. However, current synthetic processes of IPQDs are widely facing potential unsustainable issues of containing nonreusable and high-cost auxiliary materials. In this work, full-visible-spectrum IPQDs were successfully synthesized by an environmentally friendly ion-exchange approach with a renewable and low-cost anion exchange resin. Introducing anion exchange resin brings the improvement of both optical performance and surface morphology of the prepared IPQDs. The emission wavelength of IPQDs can be precisely controlled without changing their inherent crystal phase, and those IPQD’s single crystals with poor morphology and unstable structure are selectively removed. The photoluminescence quantum yield (PLQY) and the fluorescence lifetime of the three-primary-color IPQDs can be dramatically improved to 93.69, 89.99, and 65.03% and 71.3 ns, 22.2 ns, and 13.2 ns, respectively. Notably, the red-emitting PQDs at 622 nm exhibit a record high PLQY. By using the prepared IPQDs for photoluminescent color conversion, the three-primary-color light-emitting diodes (LEDs) provided high brightness and wide color gamut simultaneously. This study provides new ideas for the environmentally friendly and sustainable synthesis route of IPQDs, and it is expected to show great ambitions in the display field.

Polystyrene-Fiber-Rod Hybrid Composite Structure for Optical Enhancement in Quantum-Dot-Converted Light-Emitting Diodes

High-haze optical polymer diffusers have attracted great attention in terms of application in surface color-conversion films for lighting and displays. An optical diffuser based on polystyrene-fiber rods (PFRs) dispersed in polydimethylsiloxane (PDMS) resin is presented in this study. Using a simple electrospinning and emulsion shear fabrication process, composite films having optical diffusion properties were fabricated owing to index-mismatching between the PFRs and PDMS. During mechanical shearing, it is demonstrated that the rotation speed is the important parameter affecting the length of the PFRs. PFRs are uniformly dispersed in the PDMS matrix as an excellent filler to provide excellent light-scattering performance. The obtained composite films having low concentrations of PFRs (less than 1 wt %) exhibit highly efficient light diffusion in the visible-light region. At a 0.5 wt % PFR concentration, the composite film realizes a high haze of 87.4% while maintaining a high diffuse transmittance of 81.8% at 550 nm simultaneously. The PFRs were then filled in the quantum dot (QD) films for enhancing the photoluminescence (PL) intensity, thus demonstrating their light management ability. Compared to the reference, the PFRs/PDMS film with a 1 wt % concentration provides a 1.98 times PL intensity enhancement. While applying to the QD-based LEDs, the luminous flux of the QD-based LEDs exhibited the greatest improvement, demonstrating an enhancement of 11.37% at a 0.4 wt % PFR concentration. It exhibits potential for optical performance improvement in QD-based light-emitting diodes.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation.

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient dij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs4 V8 O22 (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO4 ] and [VO5 ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V4 O11 ]∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

Improvement in optical and electrical performance of hydrophobic and antireflective silica nanoparticles coating on PMMA for lightweight PV module

Contamination and weather-resistant property of anti-reflection coating obtained by the sol-gel process is crucial for practical applications. In this work, we used the stӧber method to make silica nanoparticles and used the spin coating approach to apply them to a polymethylmethacrylate (PMMA) substrate. We adopted a low-temperature surface treatment approach by placing silica-coated PMMA in chloroform, aqueous ammonia, and hexamethyldisilazane vapor-phase environment. The treated PMMA exhibits a maximum transmittance of 95.68% in the wavelength range of 500–1100 nm. The treated PMMA showed a decrease of 0.21% transmittance after checking it with contamination and weather-resistant test. The treated PMMA exhibits a water contact angle of 115.96°, making it hydrophobic which is almost ten times that of untreated PMMA. An increase in the current density from 33.23 to 34.37 mA/cm 2 and an improvement in the efficiency of 3.54% was observed when the treated PMMA was placed onto a solar cell instead of uncoated PMMA for lightweight photovoltaic module applications. • Improvement in the optical, structural, and electrical properties. • Low temperature vapor phase surface treatment using chloroform, aqueous ammonia, and hexamethyldisilazane. • An only 0.21% decrease in transmittance was observed after contamination and weather-resistant test. • The treated PMMA exhibits a hydrophobicity of 115.96°. • J sc increased from 33.23 to 34.37 mA/cm 2 and η improvement of 3.54%.

Mn2+ ions substitution inducing improvement of optical performances in ZnAl2O4: Cr3+ phosphors: Energy transfer and ratiometric optical thermometry

Zn1-xMnxAl2O4:0.1 mol% Cr3+ (0.04≤x≤0.16) phosphors with single spinel phase were synthesized by using sol–gel method and the structure, optical and temperature sensing performances were reported herein. The results of X-ray photoelectron spectra indicate that the inversion defects related to octahedral Zn are reduced and the crystal field surrounding Al changes with Mn2+ doping in ZnAl2O4 lattices. Mn2+/Cr3+ co-doped ZnAl2O4 nanophosphors reveal a green emission band assigned to Mn2+ and a series of red emission peaks assigned to Cr3+, respectively. With the concentration of Mn2+ increasing, the intensity of zero phonon line (R line) assigned to Cr3+ increases, reaching the maximum at the optimal Mn2+ concentration of x=0.14. The energy transfer from Mn2+ to Cr3+ is confirmed with the energy transfer efficiency of 83%. The separation between 2E(eg) and 2E(tg) of Cr3+ is enlarged due to Mn2+ dopants giving rise to a change of crystal field. The luminous intensity ratio between two separated emission peaks at 685 nm (R3) and 689 nm (R2) reveals an obvious temperature dependence. The relative sensitivity changes from 3.7 %K−1 to 0.25 %K−1 with the temperature increasing from 80 K to 310 K, which is much larger than that of ZnAl2O4:Cr3+ nanophosphors without Mn2+, indicating its good application prospect in optical thermometry.

P‐4.2: Study on Optical Performances of Transparent LCD with Dichroic Dye‐doped Liquid Crystal

With the development of the Internet of Things, transparent display is playing an increasingly important role in people's lives. The most important optical index of transparent display is transmittance. Due to no need for polarizer, transparent liquid crystal display (TLCD) with dichroic dye‐doped can achieve very high transmittance. In this paper, optical performance and image quality improvement of TLCD with dye‐doped liquid crystal were studied. A new TN pixel design with common electrode of ITO instead of metal to form stored capacitance was used to achieve high aperture ratio (AR). And it was found that bright transmittance of NWSC is higher than that of NBSC due to high anchoring effect of PI to positive liquid crystal. In addition, the transmittance decreases gradually with increasing cell gap due to the increased light absorption by the long axis of dye molecules. The results showed that the TLCD with Dichroic Dye‐doped Liquid Crystal (DDLC) has a good display effect.

Spectroscopic and luminescent properties of Nd3+-Al3+ co-doped silica glass prepared by spark plasma sintering

The Nd3+-Al3+ co-doped silica glass has drawn much attention due to the stability and efficient lasing emissions, while the high preparation temperature and residual hydroxyl groups restrict the further improvement of optical performance. In this work, transparent Nd3+-Al3+ co-doped glasses have been synthesized by spark plasma sintering at 980°C within 10 mins. Besides, adopting mesoporous powders as raw materials is beneficial to the homogeneous dispersion of ions in the glass. The transmittance, structure and luminescent properties were systematically investigated. The Judd-Ofelt intensity parameters were obtained to analyze the Al3+ co-doping effects. The PL intensity strongly depends on the ions concentration and the best value is obtained when the Nd3+ concentration is 1.0 mol% and ratio of Nd:Al is 1:7. In addition, the calculated emission cross section is 2.12, which is larger than the majority of previously reported values, suggesting that as-prepared silica glasses are candidates for laser host materials.

ROF systems performance efficiency based on continuous phase frequency shift keying phase modulation scheme

Abstract Radio over fiber (ROF) technique has been presented with hybrid ordinary modulation technology to achieve improvement in optical communication network performance. Fiber Bragg grating (FBG) is employed in single mode (SM) optical channel in order to enhance the transmission environment with dispersion compensation. The continuous phase frequency shift keying phase modulation (CPFSKPM) technique is included in this study. Both previous and proposed models have been inspected and investigated. Min. bit error rate (BER), max. Q factor, signal/noise ratio (SNR), received power (RP), and noise figure (NF) are the most essential parameters able to explain the behavior performance of optical communication network model.

38‐2: Invited Paper: 12.3‐in Highly Transparent LCD by Scattering Mode with Direct Edge Light and Field‐Sequential Color‐Driving Method

We have newly developed 12.3‐inch highly transparent color liquid crystal display (LCD) which has no color filter, no polarizer and no back light. For the improvement of optical performance and lager sized panel, the process of scattering liquid crystal, panel structure and driving scheme were optimized. We have successfully achieved high transmittance as 87% and high contrast ratio as 32:1 which is 2 times larger than previous 4‐inch model.