Specular Reflection Spectra Research Articles

Color Change of Indole Squaraine between Blue and Yellow with Golden Luster.

Color change of materials by external stimulus has attracted much attention for various applications. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ), which was a yellow powder with golden luster (ISQ(gold)) and whose color changed to blue (ISQ(blue)) by being ground using mortar and pestle. Scanning electron microscopy indicated that ISQ(gold) consisted of laminated plate-like crystals with a large flat surface, which was attributable to (2 0-2) facets of monoclinic crystals. Measurements of the diffuse reflectance spectra revealed that J- and H-type aggregates of ISQ were formed by increasing the weight percent of ISQ(blue) in the mixture with BaSO4. A thin film, which was prepared by a spin-coating of ethanol with dissolved ISQ(blue) on glass substrates, looked blue on a white paper, whereas it looked yellowish on a black paper. By repeating the coating process, the film thickness increased, and the thick film exhibited yellow with golden luster regardless of the background color. These results indicate that the color depends on whether the transmitted light through ISQ or the reflected light on the ISQ surface mainly contributes to color vision. To neglect the effect of the transmitted light and intensify the reflection on the surface, a pellet with a smooth surface was prepared by pressing ISQ(gold) or ISQ(blue) at 20 kg/cm2. The ISQ(blue) pellet consisted of the laminated sheets and was dark blue-green. However, under light irradiation, the ISQ(blue) pellet exhibited a yellow color with golden luster, whose specular reflection spectrum was similar to that of the ISQ(gold) pellet. Our results indicate that the color of ISQ is tunable by controlling the transmission and the reflection of the light, including the specular and diffuse reflections.

Empirical BRDF model for goniochromatic materials and soft proofing with reflective inks.

The commonly used analytic bidirectional reflectance distribution functions (BRDFs) do not model goniochromatism, that is, angle-dependent material color. The material color is usually defined by a diffuse reflectance spectrum or RGB vector and a specular part based on a spectral complex index of refraction. Extension of the commonly used BRDFs based on wave theory can help model goniochromatism, but this comes at the cost of significant added model complexity. We measured the goniochromatism of structual color pigments used for additive color printing and found that we can fit the observed spectral angular dependence of the bidirectional reflectance using a simple modification of the standard microfacet BRDF model. All we need to describe the goniochromatism is an empirically-based spectral parameter, which we use in our model together with a specular reflectance spectrum instead of the spectral complex index of refraction. We demonstrate the ability of our model to fit the measured reflectance of red, green, and blue commercial structural color pigments. Our BRDF model enables straightforward implementation of a shader for interactive preview of 3D objects with printed spatially and angularly varying texture.

Anodizing aluminum in a viscous electrolyte to produce one-dimensional photonic crystals

In the paper, the possibility to produce anodic aluminum oxide (AAO) featuring one-dimensional photonic crystal along the normal to the surface is shown. The AAO structure is represented by alternating layers of different porosity and is formed in a viscous electrolyte based on sulfuric acid and ethylene glycol at the periodically varying from high (1.8 mA/cm2) to low (0.4 mA/cm2) current density with a rectangular pulse shape. The pore sizes and interpore distance, pore density and porosity, thickness and period of the AAO structure have been determined. The specular reflection spectra features for single layers that make up the AAO structure and for one-dimensional photonic crystals structures consisting of 165 periods have been studied. An increase in the porosity of the upper layers of the structure due to chemical etching of the pores during the oxide growth is noted. It is shown that the invariance of the spectral position of the photonic band gap for AAO structures is achieved by a 0.1 % decrease in charge at each subsequent anodizing cycle during their formation, which leads to a decrease in the period of the structure in the lower layers, compensating for the increase in the upper layers porosity. The reflection spectra have been analyzed for the incidence angles of 10° and 30° and used to calculate the period of the structure and the effective refractive index. The effective refractive index of the single layers that make up the AAO structure is calculated using the optical Fabry–Perot oscillations. For AAO with the properties of one-dimensional photonic crystal, a green color is observed at normal light incidence, and an iridescent color is observed when the angle changes. AAO can be used as a decorative coating on the housings of electronic devices (tablets, laptops, phones, etc.) and when creating design objects made of aluminum and its alloys.

Preparation of Structurally Colored Fiber with Cellulose Derivative Exhibiting Cholesteric Liquid Crystal at Room Condition

To prepare structural colored fibers by a cholesteric liquid crystal (CLC), cellulose derivatives forming a CLC structure under a room condition were synthesized through the esterification of hydroxypropyl cellulose (HPC) with propionic an hydride. Obtained cellulose derivatives formed CLC under room conditions within the degree of esterification by propionic acid (PA) from 0.83 to 1.1 per glucose unit. It was also possible to mold a film shape or a fiber shape. The derivative was possible to exhibit structural color under a room condition when it contained moisture more than 13.76 wt% after the preparation and 1.85 wt% at the time of exhibiting color. The glass transition temperature of the HPC derivative was between 5-13 °C. Its CLC structure was reconstructed under a room condition after destroyed the CLC structure by straining. The structural colored fiber was wet spun with the HPC derivative acetone solution to water as a coagulation bath. The specular reflection spectrum of the fiber showed the angle-dependence according to Bragg’s law. The pitch of CLC structure in the fiber was 423 nm, and the size of the CLC domain was 5.13 μm. In the scattering spectrum of the fiber, a reflection peak was observed in the fiber diameter direction regardless of the scattering angle, which suggests that the CLC domains have anisotropy and is radially oriented in the radial direction from the center of the fiber.

Photothermal Atomic Force Microscopy Coupled with Infrared Spectroscopy (AFM-IR) Analysis of High Extinction Coefficient Materials: A Case Study with Silica and Silicate Glasses.

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. This calculation explains the differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.

Composition, Structural and Optical Properties of Golden Grass

Golden grass is a vegetal fiber used in arts and crafts for its peculiar optical properties, namely, its shiny golden-like reflectivity. Since vegetal fiber structure is usually heterogeneous, complementary techniques have been used to understand the electronic properties of the external and core regions of golden grass fibers. The composition and dielectric function of the few nanometer-thick external surfaces has been investigated using X-ray photoelectron spectroscopy and energy loss spectroscopy (XPS-PEELS), while a comparison of vibrational and optical properties of the few-micron-thick external and fiber core regions was derived from Raman, optical, photoluminescence (PL) and photoacoustic (PAS) spectroscopies. Raman signature and valence-band distribution indicate that the internal and external parts of the fibers are made of some oxygen-depleted lignin-like carbonaceous material. Besides a plasmon peak at 21 eV, two main absorption bands at 3.7 eV and 8.2 eV have been identified as π−π* and σ−σ* electronic transitions at aromatic moieties. PL excitation and emission behavior confirm heterogeneous molecular structures at the core and external regions. Specular reflectance spectra calculated from the complex refractive index (derived from XPS-PEELS) do not explain the shiny golden-like aspect of golden grass, which may arise from diffuse reflectance within a collection of heterogeneous hollow fibers.

Synthesis and Study of the Optical Properties of PMMA Microspheres and Opals.

Polymethylmethacrylate (PMMA) microspheres were synthesized by surfactant-free emulsion polymerization. These microspheres were used to obtain opals by the self-assembly method. Monomer and initiator quantities were varied systematically to monitor the size of PMMA microspheres. From SEM and DLS measurements, a trend was observed showing as the monomer and initiator amounts increased the average diameter of PMMA microspheres increased except when a minimum monomer amount was reached, for which the size of the microspheres remained practically constant. Diffuse reflectance spectra were processed by the Kubelka–Munk treatment to estimate the energy band gap (Eg) of the PMMA microspheres. It was found that PMMA microspheres present an indirect transition. From SEM micrographs, it is seen that PMMA opals photonic crystals are formed by microspheres in a uniform periodic face-centered cubic (fcc) array. Variable-angle specular reflectance spectra show that the opals possess a pseudo photonic band gap (PBG) in the visible and near-IR regions. Furthermore, it was found that PBGs shift towards larger wavelengths as the average diameter of the PMMA microspheres increases.

Incorporation of Fluorescent Dyes in Electrospun Chitosan/Poly(ethylene oxide)

Polymeric films have been increasingly investigated due to the ease of miniaturization and integration in several sensor devices. Films obtained from the electrospinning technique have a controlled diameter and homogeneity, and substances can be incorporated into the polymeric network. Electrospinning fiber of chitosan (Ch) and poly(ethylene oxide) (PEO) was obtained from solutions prepared at different concentrations in acetic acid, and varying the distance and the voltage applied. The obtained films were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and UV-Visible specular reflectance spectra (UV-SRS). The best conditions for electrospinning were obtained for a 2% m/v (Ch + PEO) solution in the ratio 90:10% m/m (Ch:PEO), applied voltage of 18 kV, and 18 cm distance between the capillary tube and collector. Acridine orange, sodium fluorescein, and erythrosine fluorescence dyes were successfully incorporated into Ch:PEO films. The spectrofluorometric spectra of the films showed excitation and emission processes and the acridine orange film showed evidence of excimer formation by the presence of an excitation peak at 569 nm. Ch-PEO films with the incorporation of fluorescent dyes may well be used as flexible probes or sensors in colorimetric devices in biochemical applications.

Influence of the growth temperature on the spectral dependence of the optical functions associated with thin silicon films grown by ultra-high-vacuum evaporation on optical quality fused quartz substrates

Following up on some recent work that has been presented (Orapunt et al., J Appl Phys 119:065702-1-12, 2016), we report on the optical properties associated with a unique form of thin-film silicon that has been deposited onto optical quality fused quartz substrates through ultra-high-vacuum evaporation. For the purposes of this particular analysis, we focus on how the growth temperature influences the spectral dependence of the optical functions associated with these thin silicon films, for growth temperatures ranging from 98 to 572 °C. Through measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence, we determine the spectral dependence of the refractive index, the extinction coefficient, the real and imaginary parts of the dielectric function, and the optical absorption coefficient for the 11 thin silicon films considered in this analysis. We find that generally the refractive index increases in response to increases in the growth temperature. The optical absorption spectral dependence is also observed to exhibit a fundamental transition in its functional behavior accompanying increases in the growth temperature. Some details, related to recently developed methods employed for the determination of the optical functions from measurements of the reflectance and transmittance spectra, are provided as a complement to this analysis.

Optical properties of silver nanoparticles deposited onto silicon substrates by different soft-solution processing techniques

Silver nanoparticles have been grown onto crystalline silicon substrates. Three soft-solution one-step processing techniques have been assayed: electroless metal deposition (EMD), electrodeposition (ED) and spray pyrolysis (SP). The shapes exhibited by the silver nanoparticles prepared by EMD and ED are hemispherical, while the shapes of those prepared by SP (for long processing times) are similar to disks of aspect ratio 0.5 approximately. Integrated total reflectance spectra and integrated diffuse reflectance spectra shows distinctive features due to the electronic structure of silicon. Specular reflectance spectra of samples (except those grown by SP with long processing times) show a dip, corresponding to a surface plasmon resonance. Simulations, considering granular composite media were made. The simulated spectra confirm the presence of the spectral dip. Surface plasmon resonance show shifts with the angle of incidence. For samples prepared by ED, the tendency is the same as the one of the model, indicating that there is not much coupling between nanoparticles.

Strain-magneto-optics in ferrite-spinel – new magneto-optical phenomena associated with magnetoelastic interactions

The correlation between magnetoreflection, magnetotransmission of unpolarized light in the infrared range and the magnetoelastic properties of magnetics is reported for the ferrimagnetic spinel CoFe2O4 single crystal. The new physical mechanism responsible for the spectral and field-dependent peculiarities of magnetoreflection and magnetotransmission in the ferrite-spinel possessing strong magnetostriction is discussed. The influence of a magnetic field on specular reflection and transmission spectrum is likely to be indirect: magnetic-field-induced strong strains and deformations of the crystal lattice lead to the change in the electron energy structure and, hence, optical properties of CoFe2O4. The revealed mechanism of new infrared magnetooptical effect in magnetostrictive magnetics paves the way towards new research area called as a strain-magneto-optics.

Color generation from self-organized metalo-dielectric nanopillar arrays

Abstract Nanostructures composed of dielectric, metallic or metalo-dielectric structures are receiving significant attention due to their unique capabilities to manipulate light for a wide range of functions such as spectral colors, anti-reflection and enhanced light-matter interaction. The optical properties of such nanostructures are determined not only by the shape and dimensions of the structures but also by their spatial arrangement. Here, we demonstrate the generation of vivid colors from nanostructures composed of spatially disordered metalo-dielectric (In/InP) nanopillar arrays. The nanopillars are formed by a single-step, ion-sputtering-assisted, self-assembly process that is inherently scalable and avoids complex patterning and deposition procedures. The In/InP nanopillar dimensions can be changed in a controlled manner by varying the sputter duration, resulting in reflective colors from pale blue to dark red. The fast Fourier transform (FFT) analysis of the distribution of the formed nanopillars shows that they are spatially disordered. The electromagnetic simulations combined with the optical measurements show that the reflectance spectra are strongly influenced by the pillar dimensions. While the specular and diffuse reflectance components are appreciable in all the nanopillar samples, the specular part dominates for the shorter nanopillars, thereby leading to a glossy effect. The simulation results show that the characteristic features in the observed specular and diffused reflectance spectra are determined by the modal and light-scattering properties of single pillars. While the work focuses on the In/InP system, the findings are relevant in a wider context of structural color generation from other types of metalo-dielectric nanopillar arrays.

Identification of Chemical Compounds by the Reflected Spectra in the Range of 5.3–12.8 μm Using a Tunable Quantum Cascade Laser

We have considered the technique and experimental laboratory setup that allow the recording of the specular and diffuse reflectance spectra of the infrared radiation of substances in solid and liquid aggregate states on various surfaces. A quantum cascade laser, tunable in the wavelength range of 5.3–12.8 μm with a spectral resolution of 2 cm–1 and an average power of 15 mW, was used as a probing radiation source. The experimental setup made it possible to record the spectra of the diffusely reflected radiation of the substances. Methods for the identification of substances based on the scattered radiation spectra are proposed. It is shown that the use of the Kramers–Kronig integral transform increases the degree of selectivity of the spectra and, as a consequence, the reliability of identification.

Optical and thermal properties of sol–gel Al(OH)3–Fe(OH)3–PVA composite films

In this paper, the evaluation of the interaction forces of Poly(vinyl alcohol) (PVA)-coated nanoparticles of aluminum and iron hydroxides was carried out using the classical and extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory on the basis of experimentally determined sizes and electrokinetic potentials of sol particles. Calculations of the interaction energy between nanoparticles showed that the steric stability factor has a decisive contribution to the aggregative stability of these sols. Organic-inorganic Al(OH)3–Fe(OH)3–PVA composite films with different ratio of components were obtained by mixing polymer-containing sols of aluminum and iron (III) hydroxides. UV–vis absorbance and specular reflectance spectra of prepared films were recorded, and the optical constants such as absorption coefficient, absorption edge, optical band gap (both direct and indirect), refractive index, CIE coordinates, as well as film thickness were determined. The possibility of varying the color and optical parameters of the films by changing the ratio of aluminum and iron hydroxides is shown. The presence of hydroxide nanoparticles in polymer films leads to a change in the mechanism of thermal-oxidative destruction of PVA, which is manifested in the acceleration of weight loss with the simultaneous appearance of an endothermic effect at a temperature of about 280 °C, as well as in the displacement of thermal effects.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants.

Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.

Interfacial characterization and supercapacitive behavior of PEDOT nanotubes modified electrodes

PEDOT nanotubes were chemically synthesized using a hexane/water reverse microemulsion polymerization and deposited onto ITO substrate. FTIR polarized specular reflectance spectra are consistent with AOT molecules organized inside the PEDOT nanotubes. The electrochemical properties of PEDOT nanotubes were evaluated by cyclic voltammetry and electrochemical impedance spectroscopy in comparison with PEDOT:PSS modified electrodes. The results indicates an increase in the capacitance and a decrease in the charge transfer resistance for the PEDOT nanotubes in comparison with PEDOT:PSS films due to the higher exposure of the electroactive sites in the nanostructured material. Galvanostatic charge/discharge curves were performed and a maximum specific capacitance of 160 F·g−1 was revealed, indicating that this material has potential to be used in supercapacitors development.

Acoustic Wood anomaly in transmitted diffraction field

In acoustics, the term Wood anomaly, in analogy to the Wood anomaly in optics, has so far referred to the anomalies observed in the specular reflection spectra of acoustic waves perpendicularly incident on periodic surfaces. Inspired by the pioneering work of Jungman et al. on the study of the transmission field of a solid-fluid periodic interface, this work attempts to provide a complete experimental investigation of the transmission fields of a broadband sound pulse transmitted through a periodic liquid-solid interface as well as a periodic solid-liquid interface. At different frequencies, two types of anomalies are observed: a spectral tip and a spectral dip, which correspond, respectively, to the brighter band and the darker band in optical Wood anomalies. The search for their physical origin suggests that the type and location of the observed spectral anomalies are strongly related to the generation and the diffraction of pseudosurface waves on the interface having superimposed periodic corrugations and time-domain windowing in spectral analysis. To compare with the surface waves on a plane surface, the properties of the pseudosurface waves are also investigated through examining their phase and by comparing their amplitudes.

Reflectance spectra of organic matrices on the basis of photonic crystals formed of polystyrene microspheres with a particle size of 230 nm

The specular and diffuse reflectance spectra of organic matrices on the basis of photonic crystals formed of polystyrene microspheres with a particle size of 230 nm and polydimethylsiloxane layers, which are both applied on substrates made of quartz glass and acrylic plates, are recorded. Part of the organic matrix changes color from green to red-yellow 2 to 5 min after contact with a nonpolar solvent. The kinetics of structural changes occurring in the organic matrix under the influence of non-polar organic solvents, such as toluene, benzene, and n-hexane, is studied by diffusion reflectance spectroscopy. The equilibria in the organic matrix are achieved in 15–20 min.

IR-ATR investigation of surface anisotropy in silicate glasses

Several samples of flat soda-lime silicate glass were investigated by the Infrared Attenuated Total Reflection (IR-ATR) spectroscopy technique. The specular reflectance spectra together with the results of the performed dispersion analysis and the generated reflectance spectra, using Fresnel equations, suggest that the samples are isotropic. In contrast, spectra recorded by the ATR technique suggest an anisotropic structure on the surface of the specimen different from that in the bulk. This is established through differences in the s- and p-polarized IR-ATR spectra, which cannot be simply transformed into one another employing Fresnel formula for an isotropic case. It appears that this thin film having a structure different from the bulk is larger than the ATR effective penetration depth of the evanescent ray for each incidence angle above the critical one. The investigation suggests C2 symmetry of the SiO4 unit.

Gyroid Optical Metamaterials: Calculating the Effective Permittivity of Multidomain Samples

Gold gyroid optical metamaterials are known to possess a reduced plasma frequency and linear dichroism imparted by their intricate subwavelength single gyroid morphology. The anisotropic optical properties are, however, only evident when a large individual gyroid domain is investigated. Multidomain gyroid metamaterials, fabricated using a polyisoprene-b-polystyrene-b-poly(ethylene oxide) triblock terpolymer and consisting of multiple small gyroid domains with random orientation and handedness, instead exhibit isotropic optical properties. Comparing three effective medium models, we here show that the specular reflectance spectra of such multidomain gyroid optical metamaterials can be accurately modeled over a broad range of incident angles by a Bruggeman effective medium consisting of a random wire array. This model accurately reproduces previously published results tracking the variation in normal incidence reflectance spectra of gold gyroid optical metamaterials as a function of host refractive index and volume fill fraction of gold. The effective permittivity derived from this theory confirms the change in sign of the real part of the permittivity in the visible spectral region (so, that gold gyroid metamaterials exhibit both dielectric and metallic behavior at optical wavelengths). That a Bruggeman effective medium can accurately model the experimental reflectance spectra implies that small multidomain gold gyroid optical metamaterials behave both qualitatively and quantitatively as an amorphous composite of gold and air (i.e., nanoporous gold) and that coherent electromagnetic contributions arising from the subwavelength gyroid symmetry are not dominant.