Reflectivity Of Reflectors Research Articles

Optical and thermal investigation of hyperbolic cavity receiver with secondary reflector for solar parabolic dish collector

A hyperbolic-shaped cavity-type receiver with a second stage reflection for a 40 m2 solar parabolic dish concentrator (PDC) is presented in this paper,along with optical and thermal modeling. The receiver geometry and the secondary reflector complete the hyperbolic geometrical shape of the present receiver. This study aims to estimate heat losses owing to natural convection and radiation under influential parameters associated with solar parabolic dish system. Optical modeling has been performed with verified model using ASAP® 2013 V1R1. The optical simulations have been carried out for varying receiver height (h1), height of the secondary (h2) and diameter-to-total height ratios (d/h) of 0.5, 1, and 1.5 while keeping the diameter of the plane aperture of the receiver (d) constant. The optimum receiver geometry has been selected based on the performance in terms of optical efficiency. The maximum efficiency found is 91.72% for h1 = 0.75 m and d/h = 0.5 for the mounting distance of 4.21 m. The optimized shape is further studied to determine how the receiver absorptivity and the secondary reflector's reflectivity affect the optical performance. The absorptivity and reflectivity range from 0.86 to 0.94. Obtained optimum receiver geometry from the optical investigation is further analyzed to estimate the heat transfer rate. The thermal modeling is performed for 0°, 30°, 60°, and 90° orientations of the receiver in ANSYS Fluent 2020 R1. A comprehensive comparison among the available models has been reported and validated with well-established experimental data. This work has developed free convective Nusselt number correlation. It can be a valuable addition to the existing literature.

An Efficient Photon Utilization Radioisotope Thermophotovoltaic Based on Curled Reflectors

For radioisotope thermophotovoltaic (RTPV) to produce higher output power, it is often required that the irradiance incident on the thermophotovoltaic (TPV) cell array be more uniform. However, the irradiance received by each cell of the TPV array is relatively different under the conventional cell array surround structure of RTPV, resulting in a serious overall electrical output mismatch loss and a lower output power utilization rate. Herein, an RTPV with curled reflectors optimized by finite‐element method is proposed. It is shown in the simulation results that the irradiation uniformity of the cell array can reach 87.5%, the total output power reaches 88.12 W, which is 39.8% higher than that prior to optimization, and the mismatch loss is reduced by 69.2%. In addition, the impact of the reflectivity of reflectors is also targeted from the perspective of practical applications, and it can still produce higher output power than that of the conventional structure even at reflectivity as low as 70%. Ultimately, the RTPV prototype with reflectors is developed, and the electrical performance is tested to verify the effect of irradiance improvement. Herein, reflectors with positive gain for RTPV are proposed, which provides a new idea for the development of high‐efficiency power supplies.

Ultra-compact lithium niobate microcavity electro-optic modulator beyond 110 GHz

A lithium-niobate-on-insulator (LNOI) electro-optic (EO) modulator based on a 2 × 2 FP-cavity was designed and realized with an ultra-compact footprint and an ultra-high bandwidth. A comprehensive analysis on the present LNOI FP-cavity modulator was conducted to reveal the dependence of modulation bandwidth and modulation efficiency on the cavity Q-factor and the operation wavelength detuning to the resonance. In particular, the 2 × 2 FP cavity was designed to achieve an optimal Q factor by reducing the reflectivity of reflectors and the cavity length, thus reducing the photon lifetime in the cavity . An ultra-short effective cavity length of only∼ 50 µm was achieved for the designed LNOI FP-cavity modulator, with itsfootprint being as compact as ∼ 4 × 500 µm2. It was demonstrated theoretically that the modulation bandwidth could be improved significantly to be over 200 GHz by utilizing the peaking enhancement effect. The fabricated device exhibited an excess loss of ∼ 1 dB and an extinction ratio of ∼ 20 dB in experiments, while the measured 3-dB bandwidth was higher than 110 GHz (beyond the maximal range of the facilities in experiments). Up till now, to our best knowledge, this has been the first LNOI microcavity modulator with a bandwidth higher than 110 GHz. Finally, high-quality eye-diagrams of 100 Gbps on-off keying (OOK) and 140 Gbps 4-pulse amplitude modulation (PAM4) signals were demonstrated experimentally, and the energy consumption for the OOK signals was as low as 4.5 fJ/bit.

A Suspended Metamaterial Mirror for Hyperspectral Shortwave Infrared Fabry-Perot Filters

This paper reports the development of a broadband highly reflective and structurally flat large-area 2-D subwavelength grating (SWG) reflector, based on a tensioned single-layer silicon metamaterial membrane. Rigorous coupled-wave analysis is adopted to design the metamaterial reflector, resulting in a wavelength range of 600 nm (1.9- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5~\mu \text{m}$ </tex-math></inline-formula> ) with >99% reflectivity, which represents the largest reported fractional bandwidth <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \lambda /\lambda _{\mathrm {c}}$ </tex-math></inline-formula> of 27%. Effects of design parameter deviations on the SWG reflector reflectivity are studied, showing that the reflector design has good fabrication tolerances. A freestanding 2-mm dimension 2-D SWG reflector has been fabricated with high precision, and optical measurements indicate polarization-independent average reflectivity of 99% over the wavelength range of 1.91- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.49~\mu \text{m}$ </tex-math></inline-formula> , which agrees well with the modeled result. This paper also, for the first time, examines the surface flatness characteristics of a suspended single-layer 2-D SWG reflector. With a tensile stress of 10 ± 5 MPa in the silicon grating membrane, the freestanding reflector achieves nanometer-scale surface flatness, which provides a significant advantage over multilayer DBRs requiring stress-balancing. Furthermore, a hyperspectral Fabry-Perot shortwave infrared filter based on a top suspended 2-D SWG reflector and a bottom 4-pair Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> distributed Bragg reflector (DBR) has been experimentally demonstrated for the first time. The filter is measured to have a peak transmission of 80% with a narrow full-width at half-maximum (FWHM) of about 4 nm. This spectral resolution is one order of magnitude higher than that of other reported MEMS/DBR-based Fabry-Perot filters and is well-suited to highly demanding hyperspectral shortwave infrared imaging applications. [2022-0060]

Performance enhancement of microstrip antenna using artificial magnetic conductor reflection phase characteristics

This paper investigates the behavior of a dipole antenna near the perfect electric conductor, perfect magnetic conductor, and artificial magnetic conductor (AMC). The phase variations due to reflector reflection coefficient and the distance between antenna and reflector are analyzed, and their effect on the antenna radiation patterns are studied for different frequencies. The cause of a null occurrence in the broadside direction of radiation pattern when using the AMC (or HIS) as a reflector is explored. It is shown that this degradation is due to the phase of the reflected waves at different angles from the reflector surface and this phenomenon may occur for any type of reflector. In the next step, a new broadband unit cell is presented. By changing the AMC reflection feature and the distance between the antenna and the ground plane for three different states, the performance of the printed dipole antenna is improved. The maximum gain of the antenna is enhanced by approximately 7 dB, 9 dB, and 9 dB for three different cases of AMC reflectors. Moreover, the 3 dB max-gain bandwidth of the structures are 33.81%, 29.17% and 20.84%, with profile size of 0.16λ, 0.4λ, and 0.4λ, respectively. All cases of the antenna with AMC structures have been fabricated and the measurement results are presented.

High-power narrow spectrum GaSb-based DBR lasers emitting near 2.1 µm

Stable high-power narrow-linewidth operation of the 2.05-2.1 µm GaSb-based diode lasers was achieved by utilizing the sixth-order surface-etched distributed Bragg reflector (DBR) mirrors. The DBR multimode devices with 100 µm wide ridge waveguides generated ∼850mW in the continuous wave (CW) regime at 20°C. The device CW output power was limited by thermal rollover. The laser emission spectrum was defined by Bragg reflector reflectivity at all operating currents in a wide temperature range. The devices operated at DBR line with detuning from gain peak exceeding 10 meV.

Effect of the ember element in increasing the efficiency of liquefied petroleum gas stoves

The purpose of this study was to determine the effect of use ember element from woven nickel wire to increasing the efficiency of the LPG stoves. It is supposed that high-temperature embers can burn more fuel around the wire, thereby increasing the area of complete combustion. The test was conducted by means of a Water Boiling Test (WBT) and the number of ember layers varied from one to four. It was found that the use of elements of fire without reflectors could increase efficiency by 8.32%, with the highest efficiency being with the use of a single layer ember element of the fire, of 61.71%. However, the use of elements of fire in the finned heat reflectors causes efficiency to decrease, as the pattern to put elements of fire interfere with the reflectivity. This means the heat reflection is blocked by the pattern and trapped between the reflector and pattern elements. As a result, the heat energy from the reflector reflection cannot be forwarded to the combustion zone. The test results also show that the temperature distribution from ember element use can increase the area of complete combustion.

Design and performance simulation of microcavity enhanced bilayer graphene infrared photodetector

Photodetectors based on bilayer graphene have raised great interest for its application in infrared and terahertz detection, because bilayer graphene performs well in optical and electrical area. However, weak light absorption of bilayer graphene limits the property of bilayer graphene photodetector. In this paper a design of microcavity enhanced infrared photodetector based on Bernal stacking bilayer graphene is presented. Quantum efficiency and responsivity of detector can be valid improved by confining light in the cavity. Theoretical analysis shows that microcavity length and reflectivity of reflectors are the key factors to influence the performance. When the microcavity length matches the operating wavelength, under the standing wave effect, the quantum efficiency can reach the maximum, while reflectivity becomes the key factor. To obtain a higher and more detailed reflectivity of reflectors, distributed bragger reflectors (DBRs) are chosen as reflectors of microcavity. Reflectivity is determined through the analysis of electromagnetic field distribution of membrane. The reflectivity of reflectors is calculated by transfer matrix. AlAs/Al0.1Ga0.9As and TiO2/SiO2 membranes are chosen as constituents of reflectors. The structure is confirmed through optimization, and the result is presented below: The microcavity length is 467.4 nm, 13/10 pairs of membranes constructs the top/bottom reflector and the bilayer graphene is located in the middle of microcavity. At a nominal operating wavelength of 1.55 μm, a widely used wavelength in laser range finder, the quantum efficiency can reach 99.33% and the responsivity can reach 1.24 A/W. The method provides a possible approach to fabricate high quantum efficiency bilayer graphene photodetector.

Comparison of line focusing solar concentrator fields considering shading and blocking

The performance of line focusing solar collectors [Parabolic Trough Concentrator (PTC), Linear Fresnel Reflector (LFR) and Compact Linear Fresnel Reflector (CLFR)] is affected by many factors. Due to end effect, inter-row shading and blocking (for LFR and CLFR), the complete aperture of the collector cannot be utilized. Besides, the cosine effect, reflectivity of reflectors, intercept factor, transmissivity of receiver cover, absorptivity of absorber tube and thermal losses are the other major contributors to the energy losses. In the current work, PTC, LFR and CLFR fields are compared in terms of energy losses, net energy collection by fluid, electricity generation and cost of electricity. The ratio of collector aperture area to land area is named as land coverage ratio. The appropriate values of land coverage ratio are found out corresponding to minimum cost of electricity for different technologies. The corresponding annual energy collection by fluid and the annual electricity generation have also been calculated. It is seen that there is no significant difference in the performance of LFR and CLFR fields. For low values of receiver height to collector width ratio, the LFR field results in the largest levelised cost of electricity and the PTC field results in the lowest.

Diffuse reflections within CPCs and its effect on energy collection

In practical applications, part of solar rays incidence on reflectors of compound parabolic concentrators (CPC, in short) undergoes diffuse reflections instead of specular reflections due to imperfect reflector profile, rough surface and dust deposition, leading a considerable fraction of solar rays escape from the cavity of CPCs and making their optical performance decrease. In this article, a detailed mathematical procedure to estimate the optical efficiency factor of CPCs with semi-diffuse reflectors and one-sided flat absorber is developed based on the knowledge of two-dimensional radiation transfer, and the effect of diffuse reflections within the CPC on its energy collection is theoretically investigated. Ray-tracing analysis shows that the use of semi-diffuse reflectors in CPCs can not make solar flux on the absorber more uniform except using perfect diffuse reflectors, in turn leads the optical efficiency decrease. Comparisons of the optical efficiency factors between theoretically expected and those obtained by ray-tracing simulation indicate that the mathematical model proposed in this work can accurately estimate the optical performance of such CPC. Theoretical calculations show that the contribution of diffuse reflections within CPCs to the radiation collection of CPCs is limited as a result of the fact that the diffuse radiation transfer shape factor from the reflectors to the absorber is very small, and the annual average optical concentration decreases with the increase of diffuse reflectivity of reflectors for CPCs with a given total reflectivity.

Effects of shading and blocking in linear Fresnel reflector field

In a linear Fresnel reflector field, parallel rows of reflectors in a collector direct the incident sun rays towards a common linear receiver. Some portion of reflector aperture remains unused due to end effect, inter-row shading and blocking. In addition to these factors, cosine effect, cleanliness factor, reflectivity of reflectors, intercept factor, transmissivity of receiver cover, reflectivity of secondary reflector, absorptivity of absorber tube and thermal losses are other factors that contribute to energy losses and thus affect net energy collection by the heat transfer fluid in the absorber, electricity generation and cost of electricity. Conventionally, the collectors are oriented either along North–South or East–West directions in most cases. However the energy collection, electricity generation and cost of electricity need to be found out for all possible collector-orientations lying between North–South and East–West. This can be used in designing collectors for places where the available land strip does not align with any of these two directions. In this work, explicit analytical expressions for energy losses due to cosine effect, end effect, shading and blocking are derived for any desired time interval as functions of length (L) and width (w) of aperture of reflector-row, spacing between adjacent reflector-rows (p), number of reflector-rows in a collector (n), height of receiver (H), collector-orientation angle (Ω) and location. The expressions for the net energy collected by the working fluid, electricity generated by a collector and the cost of electricity are presented. The effects of L, w, p, n, H, Ω and location on energy losses, net energy collection by fluid, electricity generation and cost of electricity are studied. The minimum cost of electricity is found out for different collector-orientations at various locations and relative comparisons have been made. The corresponding collector parameters, annual energy collection by fluid and the annual electricity generation are also found out.

Role of the mirror's reflectivity in forward-pumped random fiber laser.

In this paper, we thoroughly analyze the role of the point reflector's reflectivity in the performance of forward-pumped random fiber laser, in both the long- and short-cavity cases. The results show that the power performance is sensitive to the small reflection added on the pump side of the fiber end, whereas both the power distribution and threshold tend to be stable when the reflectivity reaches a relatively high level (>0.4). Moreover, for the short cavity case (e.g. 500m), the maximum achievable 1st-oder random lasing output can even increase when the reflectivity decreases from 0.9 to 0.01, due to the different lasing power distributions with different reflectivity values. This work reveals a new and unique property of random fiber lasers and provides insights into their design for the applications such as distributed amplification and high power sources.

Uniform sunlight concentration reflectors for photovoltaic cells

Sunlight concentration is essential to reach high temperatures of a working fluid in solar-thermal applications and to reduce the cost of photovoltaic (PV) electricity generation systems. Commonly, sunlight concentration is realized by parabolic or cylindrical reflectors, which do not provide uniform concentration on the receiver finite surface. Uniform concentration of sunlight is favored especially for the PV conversion applications since it not only enhances the conversion efficiency of sunlight but also reduces the thermal variations along the receiving PV cell, which can be a performance and life-span limiting factor. In this paper a reflector profile that uniformly infiltrates the concentrated sunlight into the receiving unit is attempted. The new design accounts for all factors that contribute to the nonuniform concentration, like the reflector curvature, which spatially reflects the sunlight nonuniformly, and the angular dependency of both the reflector reflectivity and the sunlight transmission through the PV cell.

Vertical-Cavity Surface-Emitting Laser With Cholesteric Liquid Crystal Overlay

We study theoretically the spectral and polarization threshold characteristics of vertical-cavity surface-emitting lasers with an overlay of cholesteric liquid crystal (CLC-VCSELs). Due to the existence of CLC band gap for circularly polarized (CP) light the polarization state of the resonant modes in the compound system evolves from linear (LP) inside the VCSEL to elliptical and circular (CP) inside the CLC. As a result, the output mirror reflectivity is increased and CLC-VCSEL threshold gain is decreased. We elucidate the transition from LP to CP state of the generated light by studying the role of the CLC thickness and the top distributed Bragg reflector reflectivity. This transition takes place for 5-6 μm thick CLC and is not gradual but happens in an oscillatory fashion. When the CLC thickness is fixed and the VCSEL top mirror reflectivity is decreased a profound gradual red shift of the resonant mode wavelength and increase of threshold gain are observed.

In-situ spectral reflectance measurement of off-axis paraboloidal reflector with a large aperture for radiometric calibration

Off-axis paraboloidal reflector with a large aperture is often used as a collimator in radiometric calibration system in the space environment or a simulator for space carrier. Spectral reflectance measurement of the reflector is an important issue that determines the accuracy of radiometric calibration, especially, for imaging spectrometer. Currently, the reflectance of the reflector is generally measured by the sample contrast method. So, only a small sample of the reflector rather than the whole can be measured. In order to overcome this limitation, a method for in-situ measuring spectral reflectance of off-axis paraboloidal reflector, especially, with a large aperture, is proposed in this paper. First of all, the proposed in-situ measurement approach is designed according to in-situ contrast measurement. Then, the proposed measurement is realized and evaluated in experiment for measuring spectral reflectance of an off-axis paraboloidal reflector with an aperture of 110mm. Finally, the influence of prodigious temperature fluctuation on the measurement is experimentally investigated and the impact of optical devices and environment on the uncertainty is analyzed. The total experimental uncertainty is obtained as 4.2%. The experimental results proved that the spectral reflectance of the reflector will decrease when the experimental temperature falls from 20°C to −100°C and the decrease degree is different for different wavelength bands.

High-Efficiency White LED Packaging With Reduced Phosphor Concentration

In this letter, important factors for white light emitting diodes (LED) packaging, including lens encapsulation, reflectivity of cavity reflectors, and phosphor concentration are studied. Simulation as well as the corresponding experiment shows that reflectivity of the reflectors in packaging cavity is more effective than lens encapsulation to reach high efficiency. Besides, a white LED packaging without lens encapsulation but with high reflectivity of cavity reflectors may increase interaction possibility between blue photons and the phosphor so that the phosphor concentration can be reduced to meet the requested correlated color temperature.

The Luminescence of a CuI Film Scintillator Controlled by a Distributed Bragg Reflector

We investigate the emission properties of CuI film scintillators and the effect of distributed Bragg reflectors. The free-exciton emission and the donor-acceptor pair emission from the CuI thin film with the peak wavelengths of 410 nm and 420 nm are observed. However, for the two emission bands, the distributed Bragg reflector reflection results in different enhancements, which is interpreted by the varying transmittance with wavelength. Angle-dependence of emission profile is decided by the transmittance and DBR reflection, which may be useful in scintillation detection applications.

Absorption of photons in the thin film AlGaInP light emitting diode

The path of photons in the thin film (TF) light emitting diode (LED) was analyzed. The reflectivity of reflector in AlGaInP TF LED with and without the AlGaInP layer was contrasted. The absorption of the AlGaInP layer was analyzed and then the light extraction was calculated and shown in figure. The TF AlGaInP LED with 8 μm and 0.6 μm GaP was fabricated. At the driving current of 20 mA, the light output power of the latter is 33% higher. For the 0.6 μm GaP LED, the etching of heavily doped GaP except the ohmic contact dot area is advised. The design and optimizing of current spreading between the n-type electrode and the p-type ohmic contact dot need further research.

Effects of Reflective Surfaces on Silicon Emissivity and Temperature Measurements

AbstractThe effects of front-side reflective surfaces on the emissivity and temperature measurement of silicon wafers in a Rapid Thermal Processing chamber with backside heating are examined through optics modeling. Two schemes are contrasted. In (1), the pyrometry detector looks at the back-side of the wafer; in (2), it looks at the front-side of the wafer. In both cases, the temperature errors occur when the reflector's reflectivity deviates from unity. However, the error dependence on the reflector reflectivity exhibits interesting differences in these two schemes. Under the working conditions proposed in this paper, a non-ideal reflector introduces a bigger temperature error in the “backside-detector” scheme than in the “front-detector” scheme. Practical implications of these results are also discussed.

Optimization of Quartz SAW Resonator Structure with Groove Gratings

Abstruct-A new surface acoustic wave (SAW) resonator optimum design method is proposed to reduce resonator sue. The fundamental design feature is optimization of the interdigital transducer (IDT) electrode period to reflector electrode period ratio, in order to adjust the frequency for the maximum IDT admittance to the maximum reflector reflection point. As a result of the optimized electrode period ratio, both reflector and IDT frequency responses are utilized to advantage, and good resonant Characteristics are obtained with a reduced number of reflector electrode fingers. This design method can be effectively applied for SAW resonators with groove gratings, both on reflectors and transducer, that are formed by a reactive sputter etching process. 145 MHz SAW resonators with groove gratings fabricated on a small 6 mm X 3 mm ST-cut quartz chip have provided good electrical performances, such as high Q values above 2 X lo4 and low resonance resistance in the 20-30 range.