Broadband Reflection Research Articles

Broadband and Omnidirectional Antireflection Surfaces Based on Deep Subwavelength Features for Harvesting of the Solar Energy

Broadband and omnidirectional antireflective surfaces are important to various solar energy technologies. The utilization of deep subwavelength features is reported for decrement in the broadband and omnidirectional reflection of the solar spectrum. The study commences with nanopillar (NP) arrays that are well known for their antireflection properties. Deep subwavelength features to the nanopillar arrays are introduced, which yield further decrease in reflection: quasinanolens (qNL), deep subwavelength sidewall scalloping (DSSS), and decreasing the nanopillar bottom diameter to reshape it into a light funnel (LF). Accordingly, surface silicon arrays are realized in a top−down fabrication process: nanopillar arrays, nanopillar arrays incorporated with DSSS, nanopillar arrays incorporated with qNL, nanopillar arrays incorporated with qNL and DSSS, and nanopillar arrays that are transformed into LF arrays and are complemented with qNL and DSSS. It is experimentally shown that utilization of deep subwavelength features produces a broadband reflection decrement with an almost 80% decrement for normal incidence and 60% decrement for an angle of incidence of 80°, for unpolarized illumination. The introduction of deep subwavelength features concludes a 20% decrement in the broadband diffused reflection compared with undecorated NP arrays. The effect of deep subwavelength features is also explored numerically, and the possible underlying light‐trapping mechanisms are discussed.

A Broadband Switchable Metamaterial Absorber/Reflector Based On Multi-Laps Graphene Sheets in the Terahertz Band

Switchable metamaterial absorbers/reflectors (MAs/MRs) are important bifunctional electromagnetic devices and have been the subject of numerous scientific studies. However, there is a lack of bifunctional devices that operate in the terahertz band. Here, we theoretically propose a broadband switchable MA with many excellent properties, such as good thermal stability, high insensitivity to inferior film quality of the graphene, excitation polarization and wide incident angles, and outstanding structural parameter tolerance. The bandwidth of the proposed broadband MA is 3.4 THz with an absorptivity over 90% in the frequency band of 1.6-5 THz. The proposed absorber can switch to a reflector with a reflectivity over 93% by tuning the chemical potential of the graphene and reducing the temperature. Therefore, the switching intensity of the proposed MA exceeds 83%. The physical mechanisms of the broadband absorption of the proposed structure are investigated using the impedance matching theory and the multiple reflection interference theory. The reflection mechanism of the proposed broadband reflector is discussed by analyzing the effective parameters. The absorption and switching mechanism are theoretically investigated by performing detailed numerical calculations to analyze the surface loss intensity, electric field, and magnetic field. These findings can accelerate the development of terahertz broadband switchable devices.

Broadband microwave reflectometry plasma diagnostic based on invariant point of reflection data

Reentry plasma sheath diagnostic plays extremely important roles in the research of the blackout problem. In this paper, a broadband microwave reflectometer based on the invariant point of reflection data has been designed and developed for the reentry plasma sheath diagnostic. The invariant point Y(ωp, 1) is derivable from the broadband reflection data. The electron density and collision frequency are measured simultaneously. The diagnostic algorithm was verified by the simulation analysis and diagnostic experiment. The algorithm has low computational complexity and relatively high precision. This algorithm offers a certain reference value for the plasma sheath diagnostic of reentry vehicles or hypersonic vehicles in the future. Especially, the design of broadband VVD antenna with heat-resistant wave-transparent composites has certain practical values.

Evaluation of Event-Based Sensors for Satellite Material Characterization

Broadband photometry has been used for many years to infer basic information about satellites; however, there has been limited success at remotely determining satellite surface materials, and the variability of brightness and spectral energy distribution of satellite reflections complicates satellite identification. This paper demonstrates the potential utility of event-based sensors for remotely characterizing satellites based on the reflectance properties of their surface materials. Event-based sensors offer three important advantages over traditional frame-based sensors, such as charge-coupled devices (CCDs) for characterizing satellite materials on orbit: very high temporal resolution, high dynamic range, and low data rates. This allows rapid, fine-resolution measurements over a broad range of intensities. An event-based camera was used to characterize the broadband reflectance properties of five common satellite materials over a range of illumination and observation angles in the laboratory. Some of the results are very distinctive, and have not previously been reported, demonstrating that event-based sensors might perform better than CCDs at satellite identification and material characterization. The results also show that different materials can exhibit quite different, and sometimes very distinctive, illumination/observation geometry-dependent reflectance characteristics. Using high angular resolution event-based data to assess broadband reflectance changes with changing geometry could be an effective means of unambiguously identifying satellites or determining the presence of specific satellite surface materials.

Self-diffusion method for broadband reflection in polymer-stabilized cholesteric liquid crystal films

ABSTRACT Chiral nematic liquid crystal (N*-LCs) films with wide pitch gradients covering the near infrared (NIR) region are facilely obtained by a simple controlling diffusion method. The polymer network can not only fix the orientation of N*-LCs, but also serves as a diffusion channel for substances, and has excellent compatibility. First, in the design of the pre-polymerisation (Incomplete polymerisation) layer, mainly by controlling the content of polymerisable monomer and UV-absorbing dyes in N*-LCs, a polymer network with density gradient distribution and suitable for material diffusion is formed, and the substrate of the oil repellent layer on the side of the low-density polymer network is easier to be peeled off. Then, nematic liquid crystals are injected between the pre-polymerisation layer and another oriented glass substrate. Due to the large concentration difference of chiral compounds and UV-327 between the two liquid crystals, the gradient distribution of spacing is formed by diffusion. Because of the gradient distribution of the UV-327, the gradient distribution of the pitch is further increased after polymerisation again. Finally, the effect of the UV-327 on the polymer network of the pre-polymerised layer is observed using SEM. Thus, a new method for reflective broadband in the near-infrared region is proposed.

Reduced ASE threshold from aperiodic photonic structures in rhodamine B-doped king salmon (oncorhynchus tshawytscha) iridophores

The broadband reflectance of iridescent silvery fish is caused by aperiodic photonic structures formed by complex distributions of guanine platelets in their iridophores. Photons can be reflected multiple times inside the iridophores found in many pelagic teleosts allowing for advanced photonic methods of characterization. The one-dimensional aperiodic structures reflect light over a broad spectrum. Enhanced amplified spontaneous emission (ASE) and suppressed laser light in stained skin tissue is attributed to the high group velocities and negligible localization of light over the gain envelope of rhodamine B (RhB). The enhancement to the ASE, reported herein, is shown to be caused by an increase in the effective ASE gain path length of photons generated in the iridophores of skin tissue stained with RhB. We show that the effective ASE gain path length can be used as a figure-of-merit to quantify aperiodic photonic structures, including those found in many underwater animals, by determining the increase of a photon's average time spent in broadband reflective tissue.

Direct method of extracting broadband complex refractive index from spectrophotometric measurements: an application to polydimethylsiloxane for passive radiative cooling.

We describe an algorithm to extract the complex refractive index of a material from broadband reflectance and transmittance measurements taken by spectrophotometers. The algorithm combines Kramers-Kronig analysis with an inversion of Fresnel's equations to provide a direct method of solving for the refractive index which is accurate, even for weakly absorbing materials, and easily applicable to radiative heat transfer calculations. The algorithm is validated by extracting the complex refractive index of polydimethylsiloxane between 0.25 μm and 100 μm and comparing against existing literature. We also discuss the importance of broadband optical properties to passive radiative cooling and details of the uncertainty analysis of the algorithm.

Retrieval of Daily Mean Top-of-Atmosphere Reflected Solar Flux Using the Advanced Very High Resolution Radiometer (AVHRR) Instruments

The records of the Advanced Very High Resolution Radiometer (AVHRR) instrument observations can resolve the current lack of a long global climate data record of Reflected Solar Flux (RSF), by transforming these measurements into broadband flux at the top-of-atmosphere. This paper presents a methodology for obtaining daily mean RSF (Wm−2) from AVHRR. First, the narrowband reflectances are converted to broadband reflectance using empirical regressions with the Clouds and the Earth’s Radiant Energy System (CERES) observations. Second, the anisotropy is corrected by applying Angular Distribution Models (ADMs), which convert directional reflectance into a hemispherical albedo. Third, the instantaneous albedos are temporally interpolated by a flexible diurnal cycle model, capable of ingesting any number of observations at any time of day, making it suitable for any orbital configuration of NOAA and MetOp satellites. Finally, the twilight conditions prevailing near sunrise and sunset are simulated with an empirical model. The entire day is then integrated into a single daily mean RSF. This paper furthermore demonstrates the methodology by validating a full year (2008) of RSF daily means with the CERES SYN1deg data record, both on daily and subdaily scale. Several configurations are tested, each excluding particular satellites from the constellation in order to mimic orbital changes (e.g., orbital drift), and to assess their relative importance to the daily mean RSF. The best performance is obtained by the combination of at least one mid-morning (NOAA-17 or MetOp-A) and one early afternoon (NOAA-18) orbit. In this case, the RMS difference with CERES is about 7 Wm−2. Removing NOAA-18 degrades the performance to an RMS difference of 12 Wm−2, thereby providing an estimate of the impact of NOAA-19’s orbital drift between 2016 and 2020. Very early or late observations (NOAA-15, NOAA-16) provide little added value, and both mid-morning orbits turn out to be almost interchangeable given their close temporal proximity.

Ultra-broadband Bragg concave diffraction grating designs on 220-nm SOI for wavelength demultiplexing.

The appropriate broadband design of a de/multiplexer can significantly increase the channel number and consequently the transmission capacity of a wavelength division multiplexing system. Herein, we present the first ultra-broadband Bragg concave diffraction grating (CDG) on a 220-nm silicon-on-insulator, covering most of the E, S, C, L, and U telecommunication wavebands spanning from 1.425 to 1.675 μm. A wide-band-gap Bragg mirror is employed to facilitate broadband reflection, with a low diffraction order of grating for a sufficient free spectral range. Numerical simulations show that the proposed approaching blazed concave diffraction grating (AB-CDG) for the two-material case exhibits a high integration, simple fabrication process, and promising spectral performance. We fabricate the grating for design verification with a low transmission loss of -0.6 dB and a crosstalk below -33.7 dB for the eight measured wavelength channels covering the spectral range from 1.5 to 1.61 μm that is limited by the bandwidth of the grating coupler. This design can be used for broadband wavelength demultiplexing, frontier astronomical observation, and spectroscopic imaging.

Impedance mismatch-based enhancement of broadband reflectance of tungsten with bio-inspired multilayers

Optical coatings composed of thermomechanically superior materials with high reflectance in the broadband spectrum have recently attracted attention. Tungsten (W), among other VIB group materials, suits well for applications that require high thermomechanical stability but suffer from low reflectivity below 1 µm. This paper proposes a bio-inspired multilayer structure that lowers Tungsten's absorption over a broadband spectrum. The proposed design mimics the patterns observed on Morpho butterfly wings and reduces the absorption of W over a broad spectrum. At the initial stage, the impedance mismatch method is implemented to optimize the number and thickness of the homogeneous multilayers over the broadband spectrum. Then, the proposed pattern is incorporated into the homogenous structure. The effect of inhomogeneities on the transverse directions is investigated. The resulting spectral characteristics of both homogeneous and inhomogeneous structures are analyzed by wave impedance analysis. The total optical path was extracted with this analysis. Using this analysis, the mechanisms that trigger sharp reflection dips in homogenous layers are uncovered. S-parameters of the inhomogeneous structures are extracted and evaluated. Our findings indicate that the increased rate of change of the S-parameters phase in inhomogeneous structures is responsible for increased dips over the spectrum.

Wideband Reflector and Analogue Electromagnetically Induced Reflection in Metamaterials

Metamaterials are highly demanded for advanced applications in absorbers, sensors, and filters. However, metamaterial reflectors, especially broadband reflectors, remain challenging. In this paper, we theoretically investigate a wideband metamaterial reflector which consists of cross shaped graphene strips and a silica (SiO2) substrate. The cross shaped graphene strips are coated on the top of the structure, and the cross shape rotated 45° graphene strips are spun on the bottom of it. The calculated reflection can be tuned through length and width of the graphene strips. By comparison, not only broadband reflection but also analogue electromagnetically induced reflection (EIR) can be realized. Moreover, the structure can generate a bi-directional broadband reflection of insensitive polarization. This kind of bi-directional reflector at microwave frequencies is obtained because the top and bottom graphene strip structures are similar. We employ the electric field distribution of the designed structure to elucidate the mechanism of the analogue EIR effect. We further discuss the influence of incident angle on the analogue EIR effect. Such a bi-directional reflector can be potentially applied to a wideband reflector, antenna, and sensor.

Broadband near-infrared reflector based on double-layer subwavelength gratings

Broadband near-infrared reflector based on double-layer subwavelength gratings

Design and analysis of broadband guided-mode resonant reflectors with coated triangular and trapezoidal profiles in TE polarization.

A low-refractive-index grating layer with symmetrical triangular/trapezoidal grooves covered with a high-refractive-index Si layer is used to design a broadband guided-mode resonant reflector. Software Rsoft is used to simulate the reflection and transmission spectra as well as the internal electric field distribution at the resonant wavelength. It is discovered that the interaction between resonant modes promotes the formation of a wideband spectrum. The reflector has been proven to provide wideband (Δλ > 450 nm) and high reflectivity (R > 98.4%) spectra over a wide range of base angles from 44° to 72°, and the maximum high reflectivity (R > 99%) spectral range in transverse electric polarization is 458 nm, spanning 1422 to 1880nm. The results not only demonstrate excellent tolerance to the base angle and grating depth but also provide more possibilities for the design of broadband reflectors.

A novel frequency selective structure with steady pass-band and broadband absorption properties

Design on integrated thin frequency selective surface (FSS) with broadband absorptive and transmissive characteristics is presented. The structure consists of two FSS layers (FSS I, FSS II), two dielectric spacer layers and a resistor FSS. The structure has the characteristics of broadband absorption, and low transmissive loss. In the case of normal incidence, the overall structure with 5.1 mm thickness exhibits a broad absorption band (reflection coefficient below −20 dB) in the frequency range 8.6–15.1 GHz, and the transmission band appears at 3.0 GHz with an insertion loss 0.71 dB. In the case of oblique incidence, it has a stable response till 45° angle, and the absorption rate is more than 90%. The proposed structure is considered as a good absorptive and transmissive material.

An Effective Acoustic Impedance Imaging Based on a Broadband Gaussian Beam Migration

The estimation of the subsurface acoustic impedance (AI) model is an important step of seismic data processing for oil and gas exploration. The full waveform inversion (FWI) is a powerful way to invert the subsurface parameters with surface acquired seismic data. Nevertheless, the strong nonlinear relationship between the seismic data and the subsurface model will cause nonconvergence and unstable problems in practice. To divide the nonlinear inversion into some more linear steps, a 2D AI inversion imaging method is proposed to estimate the broadband AI model based on a broadband reflectivity. Firstly, a novel scheme based on Gaussian beam migration (GBM) is proposed to produce the point spread function (PSF) and conventional image of the subsurface. Then, the broadband reflectivity can be obtained by implementing deconvolution on the image with respect to the calculated PSF. Assuming that the low-wavenumber part of the AI model can be deduced by the background velocity, we implemented the AI inversion imaging scheme by merging the obtained broadband reflectivity as the high-wavenumber part of the AI model and produced a broadband AI result. The developed broadband migration based on GBM as the computational hotspot of the proposed 2D AI inversion imaging includes only two GBM and one Gaussian beam demigraton (Born modeling) processes. Hence, the developed broadband GBM is more efficient than the broadband imaging using the least-squares migrations (LSMs) that require multiple iterations (every iteration includes one Born modeling and one migration process) to minimize the objective function of data residuals. Numerical examples of both synthetic data and field data have demonstrated the validity and application potential of the proposed method.

Preparation of cholesteric polymer networks with broadband reflection memory effect

ABSTRACT In this study, polymer network is prepared by broadband reflective cholesteric liquid crystal (CLC) film with washing out method. By the method of photoinduced molecular diffusion, the CLC film is prepared by adding different concentrations of ultraviolet (UV) absorption dyes into the LC system and irradiated with a series of UV light intensities for different times. The selective reflection property and broadband reflection memory effect of the polymer network are proved, and the response mechanism is given. By refilling the polymer network with different LCs, reflective bands with different central wavelengths and bandwidths can be obtained. It is confirmed that different fillers can recurrence the initial reflection bandwidth of CLCs in different degrees. These results may provide a new idea for total reflection CLC films.

Broadband, highly reflective thermal protection systems, exploiting photonic additives

Photonic additives have been investigated as a means to enhance the efficiency of thermal protection systems (TPS) against the adverse effects of thermal radiation. State-of-the-art TPS consist of carbon fibers embedded in a phenolic resin matrix. During operation, the TPS is consumed because it is exposed to an excess heat flux, a large fraction of which is due to thermal radiation. Here, we show that a properly modeled and designed additive-impregnated TPS can block a considerable part of this heat influx and quantify how different control parameters, in particular the additives’ amount, placement and alignment, influence the achieved photonic enhancement. More specifically, the intrinsic reflectivity of 8.5% of a conventional TPS can been improved to values exceeding 85% by controllably inserting additives, consisting of a Ta/[SiO2/TiO2]6 heterostructure, here referred to as Type 1, an ideal, optimized, high and broadband reflector. Nevertheless, even simple, commercially available additives composed of TiO2/Al2O3/TiO2, here referred to as Type 2, provide a high reflectivity enhancement with values of up to 76%, when used in larger quantities. The simulations of this work are based on the Monte Carlo Ray Tracing (MCRT) method. The MCRT simulation method has been validated against experiment, using the structure and experiments from a literature reference. Our analysis method allows one to design and model the performance of photonically enhanced TPS that operate in high-flux, radiative conditions, like those expected in future aerospace re-entry missions or next-generation, gas turbines and thermophotovoltaic plants and provides a viable option for efficiently enhancing a TPS.

Tunable narrowband cascaded random Raman fiber laser

Random Raman fiber lasers (RRFLs) with half-opened cavity have been used as a new platform for designing high performance, wavelength-agile laser sources in the infrared region due to their intrinsic modeless property and structural simplicity. To provide the point feedbacks for cascaded random Raman lasing at different wavelengths, wavelength-insensitive broadband reflectors are commonly used in cascaded RRFLs, resulting in the rather broad high-order random Raman lasing with several nanometers of typical spectral width. Here, we experimentally demonstrate a tunable narrowband cascaded RRFL with an air-spaced etalon assisted point reflector. To realize narrowband, single- or dual-wavelength emission for each order of random lasing, the etalon is specially designed to have broad operation wavelength range, narrowband transmission lines and large free spectral range (FSR) associated with the Raman frequency shift. As a result, 1st- to 3rd-order random Raman lasing with single-wavelength emission in 1.1-1.27 μm region are generated in a 15 km single mode fiber (SMF) with -3 dB bandwidths below 0.4 nm, which are approximately four times less than those of cascaded RRFL without etalon. The maximum output power of the 3rd-order random Raman lasing is 615 mW, with 10% of optical conversion efficiency. Moreover, a tunable cascaded RRFL is performed by tuning the wavelength of pump laser or tilting the etalon. Dual-wavelength emission for each order of random lasing can also be realized at specific pump wavelengths. We also verified, by employing shorter fiber (10 km), more than 1.5 W output power of high-order RRFL can be achieved with -3 dB bandwidths less than 0.6 nm. To the best of our knowledge, this is the first demonstration of tunable sub-1 nm narrowband cascaded RRFL with single- or dual-wavelength emission for each order of random lasing.

Miniaturized dual stop band frequency selective surface with broadband linear co to cross polarization conversion ability

This work presents a miniaturized angularly stable frequency selective surface (FSS) on a single layer substrate. Unit cell comprises of convoluted circular rings connected cross dipole and linear arrow headed dipole printed on both sides of the substrate. A closely spaced dual stop band resonances are obtained at 10 and 14.3 GHz with frequency ratio of 1.43. FSS shows a − 10 dB stopband bandwidth (BW) from 7.32 to 11.05 GHz (40.61%) and 12.68 to 15.97 GHz (22.97%) with >30 dB stop band rejection at center frequencies. FSS poses the features of stable performance under change in polarization and angularly stable response up to 60° incident angle. Later the proposed FSS is printed on grounded thick dielectric substrate and it shows broadband reflection type co to cross polarization conversion from 7.2 to 17.32 GHz (82.54%) with polarization conversion ratio >80% and oblique angle stability up to 35°. To the best of authors' knowledge, this is the first time in literature a single unit cell geometry with two different properties, spatial filtering on transmission and polarization conversion on reflection have been designed and analyzed in details. A finite array with size 25 × 25 of the proposed FSS is fabricated and the measured filter response shows a good agreement with simulated performance at normal and oblique incidences. The proposed FSS is used as a reflector of a dual band monopole antenna and the simulation results shows gain improvement of 5.51 and 3.8 dB at the two stopband frequencies.

Broadband visible reflectors using Au and Ag elliptic cylinder pattern

Broadband terahertz reflector is suggested based on the shape of circular- and elliptic-cylinder metamaterials. We periodically arrange the structural pattern with gold and silver thin films on the FR4 substrate and optimize to operate in the wavelength range of 300–750 THz. Comparing the spectral responses of metamaterials with changes in shape and material, the ellipse shape greatly increased the reflection, and the silver pattern led to induce a broadening of the reflection band. Also, we identify the underlying physics necessary to design broadband reflector at any wavelength band of interest. Our results show that we can obtain broad reflection with 126 THz bandwidth at normal incidence, these results have high potential in large area photonic applications and can serve as reflectors in thin active film.