Wide Spectral Window Research Articles

A Polarization Insensitive Wide‐Band Perfect Absorber

In this article, a wide‐band and polarization‐insensitive perfect absorber composed of 4 sandwiched layers of dielectric and metal disks is introduced. Compared to classical perfect absorbers, the system supports near‐unity absorption within a wider spectral window through multiple perfect absorption mechanisms that exist due to a constituting inter‐metal disk, functioning either as a dipolar antenna or a conducting ground for different perfect absorption mechanisms. Circular shape of the antenna makes the working mechanism of the system polarization insensitive. The working principle of the system is investigated through near‐ and far‐field calculations by finite difference time domain (FDTD) simulations. A fine‐tuning mechanism of the wide‐range perfect absorption window is introduced through geometrical device parameters. The multilayer perfect absorber system is fabricated through a high‐quality fabrication method based on electron beam lithography, lift‐off method, and multi‐step deposition of metal and dielectric layers. The spectral behavior of the perfect absorber system is finally experimentally investigated through Fourier transform infrared (FTIR) spectroscopy.

Synergistic effect of 9-(pyrrolidin-1-yl)perylene-3,4-dicarboximide functionalization of amino graphene on photocatalytic hydrogen generation

A novel hybrid material, AG-PYPMI, with push-pull 9-(pyrrolidin-1-yl)perylene-3,4-dicarboximide (PYPMI) attached to amine functionalized graphene (AG) is reported. The wide spectral window of attached push-pull perylene dye enhances the light harvesting ability of AG-PYPMI. The catalytic composite of the AG-PYPMI with platinum (AG-PYPMI/Pt) is proved to be a proficient material for the generation of hydrogen from water by utilizing solar energy. The synergistic effect of PYPMI in the synthesized composite material, AG-PYPMI, has enhanced 4.3 fold of its hydrogen generation (1009 µmol g−1) efficiency with three hours of visible light irradiation. The thermal and photochemical stability of AG-PYPMI make it as a promising photocatalyst for solar energy conversion.

Broadband 19F TOCSY using BURBOP-based spin lock

A train of BURBOP universal rotation pulses has been used to generate a spin lock sufficient to observe TOCSY correlations over a 46 kHz 19F spectral window (i.e. 122 ppm on a 9.4 T spectrometer). This spin lock requires lower RF field (γB1 = 15 kHz), and was employed over a wider spectral window, than previously reported DIPSI-2 spin locks. The BURBOP-based spin lock was effected for 80–160 ms periods with a 2% duty cycle without evidence of harm to the RF coil of the probehead. Spectral separation and full set of correlations were obtained for a mixture of perfluorocarbons.

Focusing subwavelength grating coupler for mid-infrared suspended membrane germanium waveguides

We present a focusing subwavelength grating (SWG) for efficient coupling of mid-infrared (mid-IR) light into suspended membrane Ge photonic integrated circuits (PICs) that enable mid-IR applications in the entire fingerprint region. By virtue of their wide spectral transparency window and air-cladding device configuration, the suspended membrane Ge PICs are expected to be effective for mid-IR applications over the spectral region covering from 2 to 15μm. Specifically, we demonstrate the maximum coupling efficiency of -11 dB with a 1-dB bandwidth of ∼58 nm at the SWG's center wavelength of 2.37μm. Our focusing SWG is expected to advance the development of on-chip long-wavelength mid-IR applications such as biochemical sensing, thermal imaging, and nonlinear optics in the fingerprint region.

Low-Loss Highly Tolerant Flip-Chip Couplers for Hybrid Integration of Si3N4 and Polymer Waveguides

In this letter, low-loss and highly fabrication-tolerant flip-chip bonded vertical couplers under single-mode condition are demonstrated for the integration of a polymer waveguide chip onto the Si3N4/SiO2 passive platform. The passively aligned vertical couplers have a lateral misalignment between polymer and Si3N4 waveguide cores of $\pm 1.25~\mu \text{m}$ . Low-loss operation has been experimentally demonstrated over a wide spectral window of 1480–1560 nm, with measured coupler losses below 0.8 dB for Si3N4 taper angles below 1.2°, in good agreement with the calculated values. Furthermore, thermal shock test results show less than 0.1 dB degradation, indicating a robust coupling performance.

Assessment of the most effective part of echelle laser-induced plasma spectra for further classification using Czerny-Turner spectrometer

The objective of this work was to assess a part of echelle Laser-Induced Plasma spectra (ranging from 200 to 1000nm) that could be most effectively employed for rocks classification. Therefore, a 60nm wide spectral window mask was iteratively moved over the broadband echelle spectra. Each created narrow artificial spectral windows (60nm) was used for the classification of rock samples using various Multivariate Data Analysis (MVDA) algorithms, reaching more than 99% of the overall accuracy in certain cases. Afterwards, the Czerny-Turner spectrometer (having higher sensitivity compared to the echelle spectrometer) was aligned to the a priori selected and the most effective spectral regions and rocks samples were re-measured. Consequently the MVDA analyses were utilized again, providing also satisfying classification results yielding more than 99% of the overall accuracy. Measurements of 28 sedimentary ores (certified reference materials) were done utilizing commercially available X-Trace device (AtomTrace), where spectrometers in both configurations (echelle and Czerny-Turner) were exploited.

TESTING THE WAVELENGTH DEPENDENCE OF COSMOLOGICAL REDSHIFT DOWN TO Δz ∼ 10−6

ABSTRACT At the core of the standard cosmological model lies the assumption that the redshift of distant galaxies is independent of photon wavelength. This invariance of cosmological redshift with wavelength is routinely found in all galaxy spectra with a precision of Δz ∼ 10−4. The combined use of approximately half a million high-quality galaxy spectra from the Sloan Digital Sky Survey (SDSS) allows us to explore this invariance down to a nominal precision in redshift of 10−6 (statistical). Our analysis is performed over the redshift interval 0.02 < z < 0.25. We use the centroids of spectral lines over the 3700–6800 Å rest-frame optical window. We do not find any difference in redshift between the blue and red sides down to a precision of 10−6 at z ≲ 0.1 and 10−5 at 0.1 ≲ z ≲ 0.25 (i.e., at least an order of magnitude better than with single galaxy spectra). This is the first time the wavelength-independence of the (1 + z) redshift law is confirmed over a wide spectral window at this precision level. This result holds independently of the stellar population of the galaxies and their kinematical properties. This result is also robust against wavelength calibration issues. The limited spectral resolution (R ∼ 2000) of the SDSS data, combined with the asymmetric wavelength sampling of the spectral features in the observed restframe due to the (1 + z) stretching of the lines, prevent our methodology from achieving a precision higher than 10−5, at z > 0.1. Future attempts to constrain this law will require high quality galaxy spectra at higher resolution (R ≳ 10,000).

Metal-dielectric-metal plasmonic resonators for active beam steering in the infrared

Active beam-steering devices near the optical frequencies have long been sought after due to their applications in communication, defense, and display technologies; however, the challenge lies in achieving actively tunable structures near these frequencies. An array of metal-dielectric-metal plasmonic resonators is demonstrated as a dynamic beam-steering device to operate at midinfrared wavelengths. We numerically demonstrate continuous-angle beam steering of 8.75° by making use of tunable properties of silicon as the active dielectric. The proposed device achieves a refractive index insensitive divergence angle and it operates in a 650 nm wide spectral window around 10 μm wavelength. The results of this Letter pave the way to exploiting active beam steering in various applications at midinfrared wavelengths.

Sensitive detection of acrolein and acrylonitrile with a pulsed quantum-cascade laser

We report on spectroscopic measurements of acrolein and acrylonitrile at atmospheric pressure using a pulsed distributed feedback quantum-cascade laser in combination with intra- and inter-pulse techniques and compare the results. The measurements were done in the frequency region around 957 cm−1. In the inter-pulse technique, the laser is excited with short current pulses (5–10 ns), and the pulse amplitude is modulated with an external current ramp resulting in a ∼2.3 cm−1 frequency scan. In the intra-pulse technique, a linear frequency down-chirp during the pulse is used for sweeping across the absorption line. Long current pulses up to 500 ns were used for these measurements which resulted in a spectral window of ∼2.2 cm−1 during the down-chirp. These comparatively wide spectral windows facilitated the measurements of the relatively broad absorption lines (∼1 cm−1) of acrolein and acrylonitrile. The use of a room-temperature mercury-cadmium-telluride detector resulted in a completely cryogen-free spectrometer. We demonstrate ppb level detection limits within a data acquisition time of ∼10 s with these methodologies.

Photophysics of pentacene thin films: The role of exciton fission and heating effects

There is evidence that the photo-generated singlet exciton in polycrystalline pentacene films undergoes rapid and efficient fission to form two triplet excitons. However, the role of exciton fission in pentacene has been controversial, with previous studies putting forward alternate relaxation pathways for the singlet exciton, such as excimer or charge formation, or internal conversion to a doubly excited exciton. We report temperature- and angular-dependent ultrafast transient optical absorption measurements in a wide spectral and temporal window. Angular-dependent transient spectra identify a common origin to photo-induced absorptions at 530 and 860 nm, which we associate with triplet excitons. These constitute the dominant relaxation channel for singlet excitons. Other features, particularly near 620 nm, previously assigned to excimers or electronic charges, are shown to be caused by thermal modulation from the optical pump.

Doping Cu in Semiconductor Nanocrystals: Some Old and Some New Physical Insights

Cu-doped inorganic semiconductors with concomitant optical properties have garnered enormous research interest in the last two decades. However, uncertainties over the origin of Cu emission, its oxidation state, resemblance with trap state emission, position of Cu d-state, emission spectral width, and moreover understanding of the doping mechanism restricted the wide development of the synthetic methodology for high-quality Cu-doped nanocrystals. It has been shown recently that the emission from Cu-doped semiconductor nanocrystals can span over a wide spectral window and could be a potential color tunable dispersed nanocrystal emitter. Herein, we report the size and composition of variable Cu-doped ZnS/Zn(1−x)Cd(x)S zinc-blende (ZB) surface alloyed nanocrystals with intense, stable, and tunable emission covering the blue to red end of the visible spectrum. Further, the Cu dopant emission is distinguished from trap state emission, and the composition variable spectral broadening has been justified on the account of a different environment around the Cu ions in the host lattice. Whereas some findings are in agreement with past reports, several new physical insights presented here would help the community for an in-depth mechanistic study on Cu doping. Moreover, these doped nanocrystal emitters can be a promising candidate for application ranging from optoelectronics to bio-labeling.

Fluorescent Quantum Dots as Artificial Antennas for Enhanced Light Harvesting and Energy Transfer to Photosynthetic Reaction Centers

The development of artificial photosynthetic systems that utilize solar energy is one of the most challenging goals of chemistry and material sciences. The straightforward way to construct an artificial photosynthetic device for practical solar fuel production for the practical use of solar energy is to mimic the structural and functional organization of the natural photosynthetic machinery. In photosynthetic organisms, light is initially absorbed by antenna protein–pigment complexes in which it induces an excited electronic state (exciton), and then excitons (or electron–hole pairs) are transferred by means of F rster resonance energy transfer (FRET) to specialist chlorophyll cofactors in specialized reaction centers (RCs); here, excitons dissociate into their constituent carriers which are used in chemical transformations for the synthesis of high-energy molecules that fuel the organism. An artificial device that mimics this process for solar energy conversion should include, among other components, an efficient light-harvesting antenna capable of transferring the excitation energy to the RC. Based on the principle of photosynthesis, a variety of artificial antenna systems have been developed using supramolecular chemistry in which dendrimers incorporate porphyrins or other organic fluorophores or organometallic complexes. Although efficient excitation-energy transfer was obtained in such systems, the use of organic fluorophores in light-harvesting systems is rather limited because of their narrow spectral windows for light-collecting and lack of photostability. Recently it was suggested that inorganic nanocrystals, which are able to collect light over a wide spectral window, may achieve significantly greater absorption than natural photosystems, thus enhancing and could thus be used to enhance the light-harvesting process. Simultaneously, these nanocrystals may also be very efficient in excitationenergy transfer. This has led us to contemplate the development of hybrid materials in which light energy harvested by the nanocrystals in the optical region may be transferred to the RC in order to enhance the efficiency of the photosynthetic process. The simplest and best understood photosynthetic RC is that found in purple bacteria (Rhodobacter sphaeroides, for example). Although RCs from different photosynthetic organisms vary in their structure and composition, they are always composed of complexes of pigments and proteins, and RC fromRb. sphaeroides is known to be a good model of all the photosynthetic RCs. Here, we demonstrate that photoluminescent quantum dots (QDs) of these selected photoluminescence (PL) wavelengths may be tagged with the RC of Rh. sphaeroides in such a way that FRET from the QD to the RC is realized (Figure 1). A nearly threefold increase in the rate of generation of excitons in the RC is demonstrated, and theoretical estimates predict even stronger enhancements, thus indicating that further optimization is possible. Advances in inorganic synthesis have resulted in the production of monodispersed QDs such as highly photoluminescent CdSe/ZnS core/shell and CdTe nanocrystals. The light absorption by these QDs appears as a quasicontinuous superposition of peaks with extinction coefficients orders of magnitude higher than those of organic molecules. QDs are ultrastable against photobleaching, and the quantum [*] Prof. I. Nabiev CIC NanoGUNE Consolider, 20018 San Sebastian (Spain) and EA n83798, Universit de Reims Champagne-Ardenne 51100 Reims (France) and Ikerbasque, Basque Foundation for Science 48011 Bilbao (Spain) Fax: (+34)943-574-001 E-mail: i.nabiev@nanogune.eu

The macromolecular neutron diffractometer (MaNDi) at the Spallation Neutron Source, Oak Ridge: enhanced optics design, high-resolution neutron detectors and simulated diffraction

The macromolecular neutron diffractometer MaNDi is currently under construction at the first target station of the Spallation Neutron Source at Oak Ridge National Laboratory. This instrument will collect neutron diffraction data from small single crystals (0.1–1 mm3) with lattice constants between 100 and 300 Å, as well as data from less well ordered systems such as fibers. A focusing neutron guide has been designed to filter the high-energy neutron component of the spectrum and to provide a narrow beam with a wide spectral window and angular divergence almost insensitive to neutron wavelength. The system includes a final interchangeable section of neutron guide and two slits, which enable tuning of the horizontal and vertical beam divergence between 0.12 and 0.80° (full width at half-maximum) at the sample position. This allows the trading of intensity for resolution, depending on the scientific requirements. Efforts to enhance and develop suitable high-resolution neutron detectors at an affordable price are also discussed. Finally, the parameters of the neutron guide and detectors were used to simulate diffraction from a large unit cell.

Sun-induced leaf fluorescence retrieval in the O_2-B atmospheric absorption band

Sun-induced leaf fluorescence was inferred by using high resolution (0.5 cm(-1)) radiance measurements and simulated spectra of the solar irradiance at the ground level, in the region of the O(2)-B absorption band. The minimization of a cost function was performed in the Fourier transform domain in order to make an accurate fit of the Instrumental Line- Shape that convoluted the simulated spectrum. Second- order polynomials were used to fit the leaf fluorescence and reflectance in the 100-cm(-1)-wide spectral window. The scale and the instrumental conversion factor were also fitted in order to obtain an accuracy that could not be attained by using the radiance measurements alone.

Synthesis and Characterization of Highly Transparent Sol-Gel Glass for Photovoltaic Applications

ABSTRACTRoom temperature sol-gel synthesis and optical characterization of highly transparent silica glass for photovoltaic (PV) applications is presented in this paper. Tetraethyl orthosilicate (TEOS), Ethanol, Hydrochloric acid (HCl) and deionized water were used as precursors in the volumetric ratio of 4:4.3:0.1:3.2 ml. Silica glass of thickness in the range of 0.5-1 cm were obtained with an average transmittance of 93% and absorption coefficient (α) of 0.08 cm−1 in a wide wavelength window of 350-1100 nm. Application of the developed sol-gel silica glass on solar ray concentration, anti-reflective coating (ARC) and effect of surface passivation on silicon wafers were examined. Carrier lifetime of the sol-gel silica passivated silicon substrate was 16 s and the calculated surface recombination velocity of the was 2200 cm/sec. Very low value of α, high transparency in a wide spectral window and effective surface passivation on silicon suggest that sol-gel processed silica glass can be a potential cost effective candidate for different PV applications.

Integrated Mach-Zehnder micro-interferometer on LiNbO 3

This work presents a scanning micro-interferometer, without moving parts, based on the well-known Mach-Zehnder geometry. The micro-system was obtained by using non-standard processes of planar technology on lithium niobate crystals, in particular the waveguide fabrication was obtained by using high energy ion implantation of medium light mass elements. The scanning effect without moving parts has been obtained by changing the refractive index of the optical waveguides by using electric field. The whole device is 60 mm long with a 0.5×1 mm 2 cross section, weights only few grams and its power consumption lies in the milliwatt range. The performances were preliminary tested in the spectral window ranging from 0.4 to 1.0 μm. By using standard radiation sources, this micro-system demonstrated a spectral resolution suitable for detecting the characteristic spectral lines of a Xe-arc lamp on a 400 nm wide spectral window. In a further experiment we tested the performances of the microinterferometer for gas trace detection by using a calibrated NO 2 optical gas cell. A sensitivity of about 10 ppb for NO 2 detection, when suitable optical paths are used, was evaluated.

Spectral interference in the determination of Ho, Tm and Ga as dopants in single crystals of potassium titanylphosphate by inductively coupled plasma atomic emission spectrometry

The present article discusses the spectral interferences affecting the determination of Ga, Ho and Tm as dopants in single crystals of potassium titanylphosphate (KTiOPO 4) by inductively coupled plasma atomic emission spectrometry (ICP-AES). The Q concept, as proposed by Boumans and Vrakking [Spectrochim. Acta Part B 43 (1988) 69] was used for this study, which covers: (a) spectral data for potassium, titanium and phosphorous as interferents (at a concentration of 4 mg/ml) in 400 pm wide spectral windows around the prominent lines of the analytes; (b) a database of Q values for line interferences ( Q I) and for wing interferences ( Q W); and (c) the interelement interferences in doubly doped crystals of KTP.

Dispersion of the optical constants of quartz and polymethyl methacrylate glasses in a wide spectral range: 0.2–3 μm

The optical constants, the absorption coefficients, and the Fermi energies of the quartz and the polymethyl methacrylate glasses are determined from the reflectance and the transmittance spectrograms of their slabs. These parameters are related to the Fresnel interface reflectance and transmittance by means of rigorous formulae taking into account the incoherent interference and the attenuation of light inside the slab. An iteration procedure is adopted in order to determine the Fresnel and the attenuation coefficients, which lead directly to the determination of the refractive index and the extinction coefficient. The dispersions of these two parameters are compared for the two given glasses inside two wide spectral windows bounded by two strong absorption bands at the extreme ultraviolet and infrared ends of the spectral range: 0.2–3 μm.

Compact Raman Spectrometers: Data Handling Methods

Measurements have been made on a compact dispersive Raman spectrometer of the type often proposed for on-line measurements. Quantitative use of the details of spectra that are produced by this system require different data treatments than those which have been described for longer-focal-length instruments. The data sets are sparsely sampled, and they cover a wide spectral window. Owing to sparse sampling, spectra as recorded have the appearance of piecewise linear approximations. It is necessary to interpolate additional data points in order to observe details of peaks, such as exact positions of maxima. With wider spectral windows, procedures for calibration of the wavenumber axis which have been previously described do not meet our performance criterion: an accuracy of 0.1 cm−1 or better. A model that involves a fifth-order polynomial is needed. On the basis of the performance of this model, atomic lines which give satisfactory results when used as wavenumber references have been identified. Details of a self-modeling procedure for removal of broad-band spectral interferences are described, and its performance is demonstrated.

Simulation of atomic spectra—II. Mutual spectral interferences of rare earth elements in the inductively coupled plasma

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta B (SAB). The hardcopy text, comprising the main article and an appendix, is accompanied by a diskette with a program, data files, and a manual. The text details the purpose of the work, with emphasis on the spectroscopic aspects, and the appendix provides the essential information for accessing the diskette and using program and data. Additional tutorial guidance is provided by the manual. The program primarily concerns the simulation of the spectra of rare earth elements (REE) as interferents in 80-pm wide spectral windows centred about the wavelengths of 26 prominent lines of Ce, La, Nd, Pr, and Sm. The program essentially covers the model described in Spectrochim. Acta 43B, 1365 (1988) and the database is identical to the experimental database published in Spectrochim. Acta 44B, 31 (1989). Accordingly, the data are for an inductively coupled plasma (ICP). The program enables the user to simulate spectra with the spectral bandwidth as an optional variable. The user may then generate spectra of both single REEs, without or with analyte, and mixtures of REEs of any composition, also without or with analyte, whereby for spectra with analyte the display includes the blank spectrum. Displays are accompanied by legends providing a set of essential numerical data. In addition, particular program options allow the generation of numerical data only, one of them being the calculation of true detection limits. This option permits it to perform, within the limited scope of the database, line selection for complex REE samples in a rational way. The program operates, in principle, with the default values of the ICP Doppler temperature (6800 K) and “average” ICP a-parameter of the Voigt profile (0.5), but these values may be optionally modified. Similarly, the user may add new data files to the base and thus also apply the program for other purposes and outside the REE environment, such as model studies in general, and for learning or teaching, as explained in the manual on the diskette. The tutorial part of this manual provides the user not only with elementary instructions for using the program as a practical tool, but also incorporates a variety of instructive examples, including a brief “course” on spectral line profiles and spectral interferences.