Entire Visible Range Research Articles

Structural and optical studies of 7-methoxy-8-(3-metrhylbut-2-enyl)-2-chromenone (osthol), a plant based coumarin

Abstract Osthol, [7-Methoxy-8-(3-metrhylbut-2-enyl)-2-chromenone], a simple Coumarin (Mol. Formula = C 15 H 16 O 3 ), was isolated from petroleum ether extract of the root parts of the plant Prangos pabularia . The compound was characterized by MS, 1 H NMR, 13 C NMR, IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectroscopy and photoluminescence (PL) measurements. Structural analysis confirmed the monoclinic structure for the system. The exploration of SEM predicted the compound to be comprising of micro crystals with irregular morphology. From UV–vis spectroscopy, high transmission was displayed by these crystals in the entire visible range. The optical band gap ( E g ) was found to be around 3.39 eV and the compound exhibited indirect allowed transitions. PL data showed good emission at certain wavelengths in visible region. The present properties of the compound can be explored for optoelectronics.

Investigations on growth and characterization of glycine admixture sodium molybdate crystals for nonlinear optical applications

Single crystals of semi-organic nonlinear optical material tris-glycine sodium molybdate were grown by slow evaporation technique from aqueous solution at room temperature. Good optical quality crystals with dimensions of 2.3×1×0.5cm3 were obtained. Single crystal X-ray diffraction was used to measure the unit cell parameters and affirmed the crystal structure. Powder X-ray diffraction revealed that the crystallinity of the crystals grown. Fourier transform infrared spectra confirmed the complex formation of glycine and sodium molybdate. Ultra violet absorption studies indicate that the crystals were transparent in the entire visible range. Direct optical band gap and indirect optical band gap values of the grown crystals were calculated from the Tauc plot. Vickers microhardness test was performed to understand the mechanical strength of the crystals. Thermal stability and melting point of the grown crystal was identified from thermo gravimetric analysis and differential thermal analysis. The surface morphology of the grown crystals was studied using scanning electron microscope analysis. Chemical composition and purity of the crystal was corroborated by energy dispersive X-ray analysis. The nonlinear optical property of the crystal was measured by Kurtz and Perry powder technique using Nd:YAG laser operating at 1064nm.

Visible Surface Plasmon Modes in Single Bi2Te3 Nanoplate

Searching for new plasmonic building blocks which offer tunability and design flexibility beyond noble metals is crucial for advancing the field of plasmonics. Herein, we report that solution-synthesized hexagonal Bi2Te3 nanoplates, in the absence of grating configurations, can exhibit multiple plasmon modes covering the entire visible range, as observed by transmission electron microscopy (TEM)-based electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Moreover, different plasmon modes are observed in the center and edge of the single Bi2Te3 nanoplate and a breathing mode is discovered for the first time in a non-noble metal. Theoretical calculations show that the plasmons observed in the visible range are mainly due to strong spin-orbit coupling induced metallic surface states of Bi2Te3. The versatility of shape- and size-engineered Bi2Te3 nanocrystals suggests exciting possibilities in plasmonics-enabled technology.

A single-source precursor route to anisotropic halogen-doped zinc oxide particles as a promising candidate for new transparent conducting oxide materials.

Numerous applications in optoelectronics require electrically conducting materials with high optical transparency over the entire visible light range. A solid solution of indium oxide and substantial amounts of tin oxide for electronic doping (ITO) is currently the most prominent example for the class of so-called TCOs (transparent conducting oxides). Due to the limited, natural occurrence of indium and its steadily increasing price, it is highly desired to identify materials alternatives containing highly abundant chemical elements. The doping of other metal oxides (e.g., zinc oxide, ZnO) is a promising approach, but two problems can be identified. Phase separation might occur at the required high concentration of the doping element, and for successful electronic modification it is mandatory that the introduced heteroelement occupies a defined position in the lattice of the host material. In the case of ZnO, most attention has been attributed so far to n-doping via substitution of Zn2+ by other metals (e.g., Al3+). Here, we present first steps towards n-doped ZnO-based TCO materials via substitution in the anion lattice (O2− versus halogenides). A special approach is presented, using novel single-source precursors containing a potential excerpt of the target lattice 'HalZn·Zn3O3' preorganized on the molecular scale (Hal = I, Br, Cl). We report about the synthesis of the precursors, their transformation into halogene-containing ZnO materials, and finally structural, optical and electronic properties are investigated using a combination of techniques including FT-Raman, low-T photoluminescence, impedance and THz spectroscopies.

Sol–Gel Assisted Inkjet Hologram Patterning

Hereby, it is presented for the first time a method of producing holographic text and images using inkjet printing. For this purpose, colorless TiO2 ink with a high refractive index is used and deposited on top of exposed poly(ethylene terephthalate)‐based microembossed paper by an inkjet printer. Nanoscale coating the paper containing printed text or graphics with transparent polymers or lacquers provides an optical effect of selective preservation of the holographic pattern. The resulting image is preserved only at the site of the colorless ink with a high refractive index from an inkjet printer, allowing to quickly generate any image with a holographic effect. Achieving these results has succeeded through the use of a colloidal dispersion of nanocrystalline titania with a refractive index of 1.75 ± 0.08 in the entire visible range, which meets inkjet rheological requirements. It is shown that the diffraction effect and optical transparency in the visible region are fully preserved. For the first time, it is demonstrated the importance of chemically prepared nanomaterials and nanostructures for an application in the field of holography.

Biometamaterials: Black Ultrathin Gold Film Fabricated on Lotus Leaf.

We report on a black metamaterial of gold fabricated on a lotus leaf that was used as a template. In spite of the extremely thin gold coating (10-nm thick) on the lotus leaf, the surface shows reflectivity below 0.01 over the entire visible spectral range. Finite-difference time-domain (FDTD) calculations suggest that the low reflectivity stems from the secondary structures on the lotus leaf, where randomly oriented nanorods are distributed.

First-Principles Screening and Design of Novel Triphenylamine-Based D−π–A Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells

We screen a series of π-conjugated bridge groups and design a range of metal-free organic donor−π–acceptor (D−π–A) SPL101–SPL108 dyes based on the experimentally synthesized C217 dye for highly efficient dye-sensitized solar cells (DSSC) using density functional theory (DFT) and time-dependent DFT (TDDFT), and further calculate their physical and electronic properties, including geometrical structures, electronic cloud distribution, molecular orbital energy levels, absorption spectra, light harvesting efficiency (LHE), driving force of injection (ΔGinj) and regeneration (ΔGreg), and electron dipole moment (μnormal). Results reveal that the π-conjugated bridge groups in SPL103 and SPL104 are promising functional groups for D−π–A organic dyes. In particular, SPL106 and SPL108 have not only smaller energy gaps, higher molar extinction coefficients, and 128 and 143 nm redshifts, but also a broader absorption spectrum covering the entire visible range up to the near-IR region of 1200 nm compared to C217 dye.

Experiments on cloaking in optics, thermodynamics and mechanics.

Spatial coordinate transformations can be used to transform boundaries, material parameters or discrete lattices. We discuss fundamental constraints in regard to cloaking and review our corresponding experiments in optics, thermodynamics and mechanics. For example, we emphasize three-dimensional broadband visible-frequency carpet cloaking, transient thermal cloaking, three-dimensional omnidirectional macroscopic broadband cloaking for diffuse light throughout the entire visible range, cloaking for flexural waves in thin plates and three-dimensional elasto-static core-shell cloaking using pentamode mechanical metamaterials.

Spectral properties of a lateral p–n homojunction-structured visible silicon light-emitting diode fabricated by dressed-photon–phonon-assisted annealing

We developed a visible silicon light-emitting diode (Si-LED) with a lateral p–n homojunction using dressed-photon–phonon (DPP)-assisted annealing. The lateral p–n homojunction was fabricated in order to decrease the absorption loss inherent in light emission with a photon energy higher than the band-gap energy of the material. The fabricated Si-LED emitted light in the entire visible range, including the three primary colors. The light extraction efficiency of the Si-LED was estimated to be 7.8 times higher than that of a conventional LED structure with a vertical p–n homojunction. Owing to the efficient light extraction, we clearly observed two novel features in the electroluminescence (EL) spectrum: a nonlinear increase in the EL intensity with the injected forward current, and an emission peak at 2.7 eV, at which there is no singular point in the electronic structure. From these features, we concluded that the EL from the Si-LED originated from the phonon-assisted radiative recombination of carriers with much higher energy than that of the bottom of the conduction band, via DPPs.

Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites.

Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from ∼10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX3 (X=Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440–700 nm) with low pump thresholds down to 5±1 μJ cm−2 and high values of modal net gain of at least 450±30 cm−1. Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 nanocrystals and random lasing in films of CsPbX3 nanocrystals.

Experiment evaluation of speckle suppression efficiency of 2D quasi-spiral M-sequence-based diffractive optical element.

The quasi-spiral 2D diffractive optical element (DOE) based on M-sequence of length N=15 is designed and manufactured. The speckle suppression efficiency by the DOE rotation is measured. The speckle suppression coefficients of 10.5, 6, and 4 are obtained for green, violet, and red laser beams, respectively. The results of numerical simulation and experimental data show that the quasi-spiral binary DOE structure can be as effective in speckle reduction as a periodic 2D DOE structure. The numerical simulation and experimental results show that the speckle suppression efficiency of the 2D DOE structure decreases approximately twice at the boundaries of the visible range. It is shown that a replacement of this structure with the bilateral 1D DOE allows obtaining the maximum speckle suppression efficiency in the entire visible range of light.

Plasmonic photo-thermoelectric energy converter with black-Si absorber

Thermal to electrical energy conversation by Seebeck element was enhanced by up to ∼ 50% by dispersing 50nm diameter Au nanoparticles over the black-Si light harvesting surface at an optimized concentration. Size of Au nanoparticles is the defining factor for the spectral position of the extinction maximum at which the cumulative absorption of black-Si (without nanoparticles) can be augmented resulting in an increase of voltage produced by a Seebeck element. Black-Si with reflectivity of 1–2% over the entire visible spectral range is a promising material for extending the operational range of solar and thermal energy converters into longer wavelength regions. Numerical simulations reveal efficient localization of light energy absorption inside black-Si.

Growth and characterization of Piperazinium adipate: A third order NLO single crystal

Good quality single crystals of Piperazinium adipate were grown from aqueous solution at room temperature by slow solvent evaporation method. The crystal belongs to triclinic crystal system with centrosymmetric space group P1¯. Various functional groups present in the title compound were identified through FTIR, FTRaman and FTNMR spectral analyses. The UV–vis absorption studies of the crystal showed that the crystal is highly transparent in the entire visible range above 210nm. The PL emission spectrum recorded at an excitation wavelength of 245nm depicted a broad blue emission centered at 475nm. The crystal is thermally stable upto 215°C as found by TG–DTA and DSC analyses. The third-order nonlinear optical behavior of the title crystal was studied using Z-scan measurements. The negative nonlinear refractive index (n2) of the crystal suggested self-defocusing effect. The nonlinear absorption coefficient (β) and third order nonlinear optical susceptibility (χ(3)) were also calculated from the measurements.

The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy

Integral field unit (IFU) data of the iconic Pillars of Creation in M16 are presented. The ionisation structure of the pillars was studied in great detail over almost the entire visible wavelength range, and maps of the relevant physical parameters, e.g. extinction, electron density, electron temperature, line-of-sight velocity of the ionised and neutral gas are shown. In agreement with previous authors, we find that the pillar tips are being ionised and photo-evaporated by the massive members of the nearby cluster NGC 6611. They display a stratified ionisation structure where the emission lines peak in a descending order according to their ionisation energies. The IFU data allowed us to analyse the kinematics of the photo-evaporative flow in terms of the stratified ionisation structure, and we find that, in agreement with simulations, the photo-evaporative flow is traced by a blueshift in the position-velocity profile. The gas kinematics and ionisation structure have allowed us to produce a sketch of the 3D geometry of the Pillars, positioning the pillars with respect to the ionising cluster stars. We use a novel method to detect a previously unknown bipolar outflow at the tip of the middle pillar and suggest that it has an embedded protostar as its driving source. Furthermore we identify a candidate outflow in the leftmost pillar. With the derived physical parameters and ionic abundances, we estimate a mass loss rate due to the photo-evaporative flow of 70 M$_{\odot}$ Myr$^{-1}$ which yields an expected lifetime of approximately 3 Myr.

Reflective liquid crystal display for better productivity.

Most reflective LCDs so far proposed require a very thin cell gap of approximately 1.5μm to satisfy the quarter-wave retardation condition when used with a half-wave retardation film to obtain an acceptable broadband performance over the entire visible range. On the other hand, the inevitable difficulty associated with precise manufacturing of thin cell gap devices is likely to deteriorate the yield, thereby increasing the production cost. This paper proposes a reflective LCD with a larger cell thickness to achieve better productivity. The proposed reflective LCD consists of a tactically arranged stack of a half-wave retardation film, a quarter-wave retardation film, and a liquid-crystal (LC) layer whose optical performance has been confirmed both by simulation and experiment. The optimal optical configuration to obtain an excellent dark state in the visible range was determined by the Mueller matrices calculus as applied to each optical component. The simulated and experimental results showed that the proposed reflective LC structure has excellent electro-optical properties and is expected to be useful for the next generation LCD industry.

A cotton fabric modified with a hydrogel containing ZnO nanoparticles. Preparation and properties study

Two different methods were used to obtain composite materials based on a ZnO nanoparticles–hydrogel–cotton fabric. The hydrogels, synthesized by photopolymerization, were utilized to provide uniform distribution and binding of the nanoparticles to the fiber surface and to prevent their agglomeration. N-methyldiethanolamine (MDEA) was used as a co-initiator in hydrogel photopolymerization and as an alkaline agent in the synthesis of ZnO nanoparticles. Due to the difference in size, shape and morphology of the nanoparticles, examined by a TEM and SEM, it was found that the materials have distinct photoluminescence properties, e.g. in the entire visible or UV range. The composite materials with small size nanoparticles and photoluminescence in near UV range were investigated for antibiotic activity against the bacterial strains Pseudomonas aeruginosa and Acinetobacter johnsonii known as important pathogens in clinical infections. Significantly high antibacterial effect on the bacteria tested was achieved, especially on A. johnsonii. This suggests potential application of the new formulations as effective wound dressings to control the spread of infections.

Quantum efficiency and dark current evaluation of a backside illuminated CMOS image sensor

We report on the development and characterization of monolithic backside illuminated (BSI) imagers at imec. Different surface passivation, anti-reflective coatings (ARCs), and anneal conditions were implemented and their effect on dark current (DC) and quantum efficiency (QE) are analyzed. Two different single layer ARC materials were developed for visible light and near UV applications, respectively. QE above 75% over the entire visible spectrum range from 400 to 700 nm is measured. In the spectral range from 260 to 400 nm wavelength, QE values above 50% over the entire range are achieved. A new technique, high pressure hydrogen anneal at 20 atm, was applied on photodiodes and improvement in DC of 30% for the BSI imager with HfO2 as ARC as well as for the front side imager was observed. The entire BSI process was developed 200 mm wafers and evaluated on test diode structures. The knowhow is then transferred to real imager sensors arrays.

Optical properties of porous anodic alumina embedded Cu nanocomposite films

Porous anodic alumina embedded Cu with iridescent colors were fabricated in copper sulfate electrolyte. The films display highly saturated colors after being synthesized by an ac electrodeposition method. Tunable color in the films is obtained by adjusting anodization time, and can be adjusted across the entire visible range. Theoretical results of the changes in the structural color according to the Bragg–Snell formula are consistent with the experimental results. The films could be used in many areas including decoration, display and multifunctional anti-counterfeiting applications.

Simple approach for the fabrication of PEDOT-coated Si nanowires.

The synthesis of a conformal poly(3,4-ethylenedioxythiophene) (PEDOT) layer on Si nanowires was demonstrated using a pulsed electrodeposition technique. N-type Si nanowire (SiNWs) arrays were synthesized using an electroless metal-assisted chemical etching technique. The dependence of the SiNW reflection on the concentration of the AgNO3 solution was identified. A reflection of less than 2% over the entire visible spectral range was obtained for these structures, evidencing their excellent antireflective properties. The etched SiNWs nanostructures can be further modified by using a tapering technique, which further preserves the strong light trapping effect. P-type PEDOT was grown on these SiNWs using electrochemical methods. Since the polymerization reaction is a very fast process with regards to monomer diffusion along the SiNW, the conformal deposition by classical, fixed potential deposition was not favored. Instead, the core–shell heterojunction structure was finally achieved by a pulsed deposition method. An extremely large shunt resistance was exhibited and determined to be related to the diffusion conditions occurring during polymerization.

Shape-dependent light scattering properties of subwavelength silicon nanoblocks.

We explore the shape-dependent light scattering properties of silicon (Si) nanoblocks and their physical origin. These high-refractive-index nanostructures are easily fabricated using planar fabrication technologies and support strong, leaky-mode resonances that enable light manipulation beyond the optical diffraction limit. Dark-field microscopy and a numerical modal analysis show that the nanoblocks can be viewed as truncated Si waveguides, and the waveguide dispersion strongly controls the resonant properties. This explains why the lowest-order transverse magnetic (TM01) mode resonance can be widely tuned over the entire visible wavelength range depending on the nanoblock length, whereas the wavelength-scale TM11 mode resonance does not change greatly. For sufficiently short lengths, the TM01 and TM11 modes can be made to spectrally overlap, and a substantial scattering efficiency, which is defined as the ratio of the scattering cross section to the physical cross section of the nanoblock, of ∼9.95, approaching the theoretical lowest-order single-channel scattering limit, is achievable. Control over the subwavelength-scale leaky-mode resonance allows Si nanoblocks to generate vivid structural color, manipulate forward and backward scattering, and act as excellent photonic artificial atoms for metasurfaces.