Omnidirectional Reflectance Research Articles

Ultrawide Spectra Camouflage Coatings from Metallic Flake Powder.

Ultrawide-spectra-compatible camouflage materials are imperative for military science and national security due to the continuous advancement of various sophisticated multispectral detectors. However, ultrawide spectra camouflage still has challenges, as the spectral requirements for different bands are disparate and even conflicting. This work demonstrates an ultrawide spectra camouflage material compatible with visible (VIS, 400-800 nm), infrared (IR, 3-5 and 8-14 μm), and microwave (S-Ku bands, 2-12 GHz). The carbon nanotubes adsorbed on porous anodic alumina/aluminum flake powder (CNTs@PAA/AFP) material for ultrawide spectra camouflage is composed of bioinspired porous alumina surface layers for low visible reflection and aluminum flake powder substrate for low infrared emissivity, while the surface of the porous alumina layers is loaded with carbon nanotubes for microwave absorption. Compared with previous low-emissivity materials, CNTs@PAA/AFP has omnidirectional low reflectance (Ravg = 0.29) and high gray scale (72%) in the visible band. Further, it exhibits low emissivity (ε3-5μm = 0.15 and ε8-14μm = 0.18) in the dual infrared atmospheric window, which reduces the infrared lock-on range by 59.6%/49.8% in the mid/far-infrared band at high temperatures (573 K). The infrared camouflage performance calculated from the radiation temperature of CNTs@PAA/AFP coatings is enhanced to over 65%, which is at least 4 times greater than that of its substrate. In addition, the CNTs@PAA/AFP coating achieves high microwave absorption (RLmin = -42.46 dB) and an effective absorption bandwidth (EAB = 7.43 GHz) in the microwave band (S-Ku bands) due to the enhancement of interfacial polarization and conductive losses. This study may introduce new insight and feasible methods for multispectral manipulation, electromagnetic signal processing, and thermal management via bioinspired structural design and fabrication.

Broadband and perfect absorption in monolayer MoS2-based multilayered structure arranged in Kolakoski sequence

Monolayer MoS2 has excellent optical properties, but its low optical absorption hinders its application in optical absorbers. To improve its absorption performance, we design an aperiodic multilayer structure with monolayer MoS2, the dielectric materials Si and SiO2 arranged by Kolakoski sequence, and achieve the perfect absorption in the visible wavelength range because of its omnidirectional reflectivity. By optimizing the thicknesses of SiO2 (db), Si (dc) and the repetition number (N) of the unit cell, the absorption of the aperiodic multilayer structure can be greatly improved at normal incidence. When N is 3, this aperiodic multilayer structure achieves high absorption of up to 98% in the wavelength range from 350 to 450 nm. As N increases to 5 or 7, the average absorption of this aperiodic multilayer structure exceeds 90% in the 350–600 nm wavelength range. In addition, we verify that the broadband and high absorption obtained in this aperiodic multilayer structure is robust against the angle of incidence and polarization of the incident wave. Our results suggest an alternative configuration with monolayer MoS2 to realize the broadband and perfect absorption, which is useful for future applications in solar cell or photodetectors.

Nano-and micro-engineered vertical graphene/Ni for superior optical absorption

Black coatings benefit from low reflectance or high light absorption are utilized in multifarious optical applications. Due to their unique three-dimensional nanostructure and π-band optical transitions, vertical graphene (VG) has drawn great attention to the field of black coatings. However, how the morphology of VGs affects reflectance has not been comprehensively disclosed yet. In addition, the morphology of the substrate for VG growth also affects the absorption properties. Here, we report a nano-and micro-engineered VG/Ni structure based on VG and laser-modified nickel foil, which possesses omnidirectional low reflectance of ∼0.25% over the visible range. A larger spacing of graphene nanowalls is more conducive to the formation of light traps and improves the absorption property. Besides, the surface of nickel substrate by laser irradiation presents a “cauliflower-like” microstructure, which further enhances light-trapping ability.

A universal robust bottom-up approach to engineer Greta-oto-inspired anti-reflective structure

The Greta oto butterfly has transparent wings with extraordinary omnidirectional anti-reflection behavior, owing to unusual nanostructures with random height and pitch/space distribution on its wing surface. The beauty and efficacy of such nanostructures are proven designs but difficult to reproduce en masse. Here, we establish a low-cost bottom-up approach by simply stacking monolayer-aligned agitation-assisted deposition of silver nanowire (AgNW) meshes followed by deposition of various overcoats. Such AgNW meshes provide the template to grow nanostructures, imitating those on the Greta oto’s wings that consist of randomly situated nanocones with controllable mean pitches and heights. The resulting nanostructure enhances anti-reflections in both SiO2/AgNW and VO2/AgNW systems, which showed reduced omnidirectional reflectance up to 33% and 70%, respectively. Furthermore, the VO2/AgNW system exhibits enhanced luminous and infrared transmittance without sacrificing solar modulation. Our robust bottom-up approach provides a simpler alternate non-lithographic way to economically produce controlled biomimetic anti-reflective nanostructured coatings.

Omnidirectional high reflector at 650, 850, 1300 and 1550 nm for optical fiber communication by fused silica/YBCO-superconductor Octonacci photonic crystal

The transfer matrix method (TMM) is deployed to determine the reflectance spectra of Octonacci photonic structures. First, the structures are designed using the TiO2 and SiO2 slabs materials. This design allows the obtainment of limited high reflectors for some optical communication wavelengths and for both TE and TM polarizations. After that, the superconductor YBa2Cu3O7 (YBCO) and the silica slabs replace the previous materials in Octonacci structures. This modification permits to extend the omni-directional reflectance bands for both polarizations. The iteration number of the Octonacci sequence has an important effect on these bands. Later the geometric thickness of the photonic structure is optimized by changing the reference wavelength λ0 and omnidirectional high reflectors that cover all optical communication wavelengths (650, 850, 1300 and 1550 nm) are obtained. In addition, for these wavelengths, a tiny effect of the ambient temperature is noticed. These omnidirectional high reflectors can find application in glass (GFO) and plastic (PFO) fiber optics and may be used in sensing applications.

A wide band porous silicon omnidirectional mirror for the near infrared range

We report the design, fabrication, and characterization of a porous silicon-based omnidirectional mirror for the near infrared range. The structure consists of 300 porous silicon chirped dielectric layers, optimized to have omnidirectional reflectivity response from 1000 to 2000 nm wavelength range. Measurements of reflectivity spectra are presented for non-polarized light at several incident angles (range 8°–65°) with a reflectivity >95% covering a 1μm band-width. Transfer matrix method calculations were carried out to show the complete angular range for both TM and TE polarizations including a simple model to illustrate the interface scattering effects.

Low absorption semi-transparent cathode for micro-cavity top-emitting organic light emitting diodes

Abstract We present a low absorption semi-transparent cathode for top-emitting organic lighting diodes (TEOLEDs). To reduce the light loss by electrode absorption, transparent cathode materials with a high refractive index and low extinction coefficient characteristics at the visible light range are investigated for the highly efficient TEOLEDs. Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) are considered as the transparent materials. Due to the low conductivity of MoO3 and WO3, a small amount of silver (Ag) is blended in order to enhance the electrical property as electrodes. Optimized MoO3:Ag mixed film shows the reduced extinction coefficient by half compared to Mg:Ag. With a general organic material (refractive index∼1.8) as a capping layer, the average absorption, transmittance and omnidirectional reflectance of 18.2, 65.9, and 25.6% are observed, respectively, with a sheet resistance of 12.7 Ω/□. The current efficiencies of fabricated red, green and blue TEOLEDs with optimized MoO3:Ag cathode are 32.4, 76.8, and 2.8 cd/A, respectively. The performance enhancement ratio is 9.8, 13.6, and 86.7% for red, green and blue devices compared with Mg:Ag cathode devices.

Tunable Filter Based upon Thue–Morse Photonic Crystal Structures

Thue-Morse structures are quasicrystals that can provide omni-directional reflectance and light emission enhancement. Starting from a Thue–Morse structure made of alternating layers of air and InP, we analyze the effects of adding a defect to the structure and changing the incidence angle on the response of the device. In fact, we show that the resonance wavelength can be tuned by adequate control of the angle of incidence of the incoming electromagnetic wave. The modified device has nearly 100% transmissivity and quality factor above 3,000, with a total length of less than 3.449 μm. The device may find applications in optical switching and filtering.

Enhanced performance of dye-sensitized solar cells by omnidirectional antireflective coatings

Organically modified silica (ORMOSIL)-coated dye-sensitized solar cells (DSSCs) with improved energy conversion efficiency are demonstrated. ORMOSIL-coated DSSC surfaces exhibit omnidirectional low reflectivity over a broad range of wavelengths (400–800 nm). The short-circuit current density (JSC) is enhanced up to 23% at normal incidence (θ=0 deg) as a result of ORMOSIL coating. In addition, JSC enhancement is even higher at larger angles of incidence; 84% enhancement was observed at θ=30 deg. Moreover, ORMOSIL coating is superhydrophobic with a contact angle of 155 deg.

Both antireflection and superhydrophobicity structures achieved by direct laser interference nanomanufacturing

Inspired by nature, a number of techniques have been developed to fabricate the bionic structures of lotus leaves and moth eyes in order to realize the extraordinary functions of self-cleaning and antireflection. Compared with the existing technologies, we present a straightforward method to fabricate well-defined micro and nano artificial bio-structures in this work. The proposed method of direct laser interference nanomanufacturing (DLIN) takes a significant advantage of high efficiency as only a single technological procedure is needed without pretreatment, mask, and pattern transfer processes. Meanwhile, the corresponding structures show both antireflection and superhydrophobicity properties simultaneously. The developed four-beam nanosecond laser interference system configuring the TE-TE-TE-TE and TE-TE-TE-TM polarization modes was set up to generate periodic micro cone and hole structures with a huge number of nano features on the surface. The theoretical and experimental results have shown that the periodic microcone structure exhibits excellent properties with both a high contact angle (CA = 156.3°) and low omnidirectional reflectance (5.9–15.4%). Thus, DLIN is a novel and promising method suitable for mass production of self-cleaning and antireflection surface structures.

Aperiodic birefringent photonic structures based on Kolakoski sequence

A novel type of aperiodic porous silicon-based multilayered structures which can be used for the electromagnetic radiation manipulation in the visible and infrared parts of spectrum are proposed in this paper. The one-dimensional aperiodic structures under consideration are arranged by stacking together two anisotropic layers according to the Kolakoski self-generation scheme. The spectral properties of the structures are theoretically investigated by using the electromagnetic field scalarization method in coupling with the finite-difference method. It is shown that the Kolakoski multilayers have both completely transparent states in the transmission spectra and pronounced omnidirectional reflectance which can used in integrated optics, optical fiber telecommunications, vertical cavity surface emitting lasers, laser cavities, etc. Also, it is shown numerically that the anisotropic nature of layered media significantly affects on optical properties of the structures.

Experimental Demonstration of 4n2 Classical Absorption Limit in Nanotextured Ultrathin Solar Cells with Dielectric Omnidirectional Back Reflector

The experimental demonstration of the 4n2 classical absorption limit in solar cells has been elusive for the last 30 years. Especially the assumptions on front and internal rear reflectance in a slab of absorbing material are not easily fulfilled unless an appropriate light-trapping scheme is applied. We propose an advanced metal-free light-trapping scheme for crystalline silicon wafers. For different bulk thicknesses, at the front side of the wafers we applied a nanotexture known as black-silicon. At the rear side, we implemented a random pyramidal texture coated with a distributed Bragg reflector. Such a dielectric back reflector was designed to exhibit a maximized omnidirectional internal rear reflectance in the region of weak absorption of crystalline silicon. Integrating the measured absorptance spectra of our wafers with the reference solar photon flux between 400 and 1200 nm, we could calculate the so-called implied photogenerated current densities. For wafers thinner than 35 μm, we achieved more t...

Reflectance properties of silicon moth-eyes in response to variations in angle of incidence, polarisation and azimuth orientation.

We report a study of the optical properties of silicon moth-eye structures using a custom-made fully automated broadband spectroscopic reflectometry system (goniometer). This measurement system is able to measure specular reflectance as a function of wavelength, polar incidence angle and azimuth orientation angle, from normal to near-parallel polar incidence angle. The system uses a linear polarized broadband super-continuum laser light source. It is shown that a moth-eye structure composed of a regular array of protruding silicon rods, with finite sidewall angle reduces reflectance and sensitivity to incident wavelength in comparison to truly cylindrical rods with perpendicular sidewalls. It is also shown that moth-eye structures have omnidirectional reflectance properties in response to azimuth orientation of the sample. The importance of applying the reflectometer setup to study the optical properties of solar cell antireflective structures is highlighted.

Durable, superhydrophobic, antireflection, and low haze glass surfaces using scalable metal dewetting nanostructuring

In this paper we report a multifunctional nanostructured surface on glass that, for the first time, combines a wide range of optical, wetting and durability properties, including low omnidirectional reflectivity, low haze, high transmission, superhydrophobicity, oleophobicity, and high mechanical resistance. Nanostructures have been fabricated on a glass surface by reactive ion etching through a nanomask, which is formed by dewetting ultrathin metal films (< 10 nm thickness) subjected to rapid thermal annealing (RTA). The nanostructures strongly reduce the initial surface reflectivity (∼4%), to less than 0.4% in the 390–800 nm wavelength range while keeping the haze at low values (< 0.9%). The corresponding water contact angle (θc) is ∼24.5°, while that on a flat surface is ∼43.5°. The hydrophilic wetting nanostructure can be changed into a superhydrophobic and oleophobic surface by applying a fluorosilane coating, which achieves contact angles for water and oil of ∼156.3° and ∼116.2°, respectively. The multicomponent composition of the substrate (Corning® glass) enables ion exchange through the surface, so that the nanopillars’ mechanical robustness increases, as is demonstrated by the negligible changes in surface morphology and optical performance after 5,000-run wipe test. The geometry of the nanoparticles forming the nanomask depends on the metal material, initial metal thickness and RTA parameters. In particular we show that by simply changing the initial thickness of continuous Cu films we can tailor the metal nanoparticles’ surface density and size. The developed surface nanostructuring does not require expensive lithography, thus it can be controlled and implemented on an industrial scale, which is crucial for applications. Open image in new window

Effective Bragg conditions in a one-dimensional quasicrystal

We present occurrence of the effective Bragg conditions with wide gapwidth and high reflectance in a Fibonacci superlattice, which is a typical one-dimensional quasicrystal. In the Fibonacci material, the number of effective Bragg conditions is two rather than one which appears in traditional periodic structures. Based on the effective Bragg conditions, this study proposes existence of omnidirectional, wideband and high reflectance in the quasiperiodic materials analogous to that in traditional materials.

Vacuum deposited GeSbSe thin films for photonic applications

The potentialities are studied of Ge28Sb12Se60 thin films deposited by thermal evaporation in vacuum for preparing 1D photonic crystal arranged as a multilayered stack and operating in the NIR. Thin films of PMMA (poly (methyl methacrylate)) deposited by spin-coating are chosen as a second counterpart of the stacks. The morphological and optical characterization results show that both kinds of single layers have a smooth surface and exhibit a large difference in their refractive indices, which presupposes a high optical contrast of the system. Further, a 13-layer stack built by alternating deposition of the two materials is prepared. The formation of an omnidirectional reflectance band (ODR) centered at λ = 1.5 μm with a maximum value of R = 96.3% is demonstrated. The ODR's width at half maximum is 320 nm. On the basis of the results obtained, the possible ways of optimizing further the system studied are outlined. Finally, the prospects of practical applications of the multilayered reflector developed are discussed.

Omnidirectional reflectance gaps and resonant tunneling effect in a one-dimensional photonic crystal consisting of two metamaterials

We investigate the properties of electromagnetic wave propagating in a one-dimensional photonic crystal (PC) consisting of two metamaterials with different dispersive model. The reflection gaps of metamaterials multilayer system are independent of the incident angle. Not only TE wave but also TM wave, the omnidirectional reflection gaps exhibit the same behavior with different incident angle for metamaterials as double negative material. We also observed that the frequency regimes of zero-transmission bands are different for TE and TM wave with the same incident angle, when one of metamaterials is the permittivity negative (e < 0) and the other is the double negative. Correspondingly, we show that the result can be act as an efficient polarization splitter. At last, we discuss the resonant tunneling effect. If the total reflection condition is satisfied, the resonant tunneling effect is enhanced as the incident angle increases, even though the propagation wave is evanescent wave in the single layer medium.

A multilayer mechanism of omnidirectional reflection in silvery pelagic fish

Event Abstract Back to Event A multilayer mechanism of omnidirectional reflection in silvery pelagic fish Tom Jordan1, 2*, Nicholas Roberts1 and Julian Partridge1 1 University of Bristol, School of Biological Sciences, United Kingdom 2 Bristol Centre for Complexity Sciences, United Kingdom Silvery reflections from pelagic fish are produced by a multilayer “stack” of birefringent guanine crystals with cytoplasm gaps. These reflecting structures provide camouflage in the near-axially symmetric underwater pelagic light field, with reflections from the sides of the fishes matching the rays from behind [1]. For optimal concealment, the reflecting structures must be able to produce spectrally broadband and omnidirectional (non-polarizing) reflectivity. Mechanisms capable of producing spectrally broadband reflectivity are well understood [2,3]. However the low polarization, near-omnidirectional reflections reported in previous experimental studies lack an optical explanation [4]. Here we report a novel multilayer mechanism of omnidirectional reflection in silvery pelagic fish. Using reflectance spectrophotometry, we detail the reflectance spectra from two species of fish, Clupea harengus and Sardina pilchardus, and we characterize the polarization dependence of those spectra as a function of angle of incidence. We further measure the birefringent properties of individual guanine crystals from both species and discover that in the reflecting argenteum there are two distinct populations of crystal morphology. From these experimental results we develop an optical model based upon anisotropic reflectivity coefficients [5], that correctly incorporates the birefringence of the guanine crystals in the multilayer structure. We establish that the neutralization of the polarization over the whole visible range and at all angles of incidence is due to both a variation in optical thickness of the layers and a mixture of orientations of the optic axis of the birefringent guanine layers. This polarization behavior uncovered in our study is very different to existing isotropic optical models of guanine-cytoplasm multilayers, which predict full polarization at Brewster’s angle. Our study demonstrates that polarization properties can be controlled by the orientation of the crystals in the multilayer, with both a fully polarizing Brewster’s angle and an entirely polarization-neutral omnidirectional reflection possible from different orientation arrangements. These findings have important consequences for our understanding of sensory adaptations related to how animals control the reflection of polarized light, and there is wide potential for the ideas to influence current debates in vision ecology upon polarization camouflage and polarization signaling. Acknowledgements The authors acknowledge support from the BBSRC (grant no. BB/G022917/1), the EPSRC 334 (grant no. EP/E501214/1) and the Air Force Office of Scientific Research (grant no. FA- 335 9550-09-1-0149).

Enlargement of omni-directional reflection by cascading chalcogenide based photonic crystals

We study 1-D photonic crystal in which alternate layers are of materials like chalcogenide As 40Se 60 − x S x and silica. For our three different structures, namely AB, CD and EF in which the refractive index contrast as well as the filling factor is different. Another structure (AB)/(CD)/(EF) which is a combination of earlier multilayer systems has also been considered to study the effect of cascading. Transfer matrix method is used to calculate the omni-directional reflectivity of each multilayer system and omni-directional photonic band gaps (PBGs). We found that large range of omni-directional PBG can be obtained by cascading chalcogenide As 40S 60 − x Se x/silica system.

Omnidirectional reflectance and vanishing Brewster angle of disordered semiconductor nanopillar surface with gradually changed refractive index

The geometrical-factor dependent Brewster angle shift in angular reflection spectra of the subwavelength nanopillar roughened semiconductor antireflective surface is investigated. Modeling of the nanopillar roughened surface evidently accounts for the shift and vanishing of the high-contrast transverse magnetic (TM) TM-mode angular reflectance spectrum with enlarging nanopillar height. The Brewster angle down-shifts by 20° with corresponding reflectance minimum increasing from &amp;lt;0.01% to 5.3% as the height of nanopillar increases from 0 to 240 nm, and the Brewster angle vanishing phenomenon is persistently observed until its disappearance with the nanopillar height enlarging up to 240 nm, leaving a slightly increased reflectance at larger incident angles.