Ultraviolet Domain Research Articles

Extreme self-compression along with superbroad spectrum up-conversion of few-cycle optical solitons in the ionization regime

A regime of extreme self-compression of optical solitons to single-cycle duration with further shortening along with superbroad spectrum up-shifting is revealed when the Kerr nonlinearity and ionization process are independently controlled. This results in efficient optical-pulse compression as a whole towards extremely short single-cycle pulses at essentially shorter wavelengths, which may open a new way to generate optical pulses with durations of hundreds of attoseconds in the ultraviolet domain.

Enhanced 355 nm Lasers Obtained from BBO and KDP by Sum Frequency Mixing

A key challenge to researchers in optics and photonics is the lack of suitable broadband lasing materials for the ultraviolet (UV) domain. Hence, the use of nonlinear optics for generation of harmonics for the UV domain, from either infrared (IR) or visible (Vis) lasers. In this work, we made use of nonlinear optical conversion to develop a compact 355 nm pulsed UV laser from a passive Q-switched pulsed infrared laser (1,064 nm). Two nonlinear crystals, barium borate (BBO) and potassium dihydrogen phosphate (KDP), are, in turn, used to generate the third harmonic of the IR laser by frequency mixing, after initial frequency doubling through a potassium titanyl phosphate (KTP) crystal. We make the laser user friendly by constructing crystal holders for the nonlinear crystals so that it becomes easier to angle tune the crystals for optimum generation of the third harmonic. The UV laser is stable and has output pulse energy up to 148 and 110 mJ, with conversion ratio from IR to UV of about 0.29 and 0.21 for BBO and KDP, respectively.

Effective compression and reconstruction of human skin hyperspectral reflectance databases.

In this paper, we investigate techniques for reducing the dimensionality of skin hyperspectral reflectance databases and maintaining a high degree of fidelity during data reconstruction. We compare results obtained using principal components analysis (PCA) with results provided by a piecewise PCA approach that explores the different roles performed by the main light attenuation agents acting within the cutaneous tissues in the ultraviolet (UV), visible and near-infrared (NIR) domains. Our investigation encapsulates not only skin spectral responses obtained by varying the contents of these agents, but also responses resulting from the absence of melanin pigmentation associated with the vitiligo condition.

Visual and ultraviolet flux variability of the bright CP starθAurigae

Chemically peculiar stars of the upper part of the main sequence show periodical variability in line intensities and continua, modulated by the stellar rotation, which is attributed to the existence of chemical spots on the surface of these stars. The flux variability is caused by the changing redistribution rate of the radiative flux predominantly from the short-wavelength part of the spectra to the long-wavelength part, which is a result of abundance anomalies. We study the nature of the multi-spectral variability of one of the brightest chemically peculiar stars, $\theta$ Aur. We predict the flux variability of $\theta$ Aur from the emerging intensities calculated for individual surface elements of the star taking into account horizontal variation of chemical composition derived from Doppler abundance maps. The simulated optical variability in the Str\"omgren photometric system and the ultraviolet flux variability agree well with observations. The IUE flux distribution is reproduced in great detail by our models. The resonance lines of magnesium and possibly also some lines of silicon are relatively weak in the ultraviolet domain, which indicates non-negligible vertical abundance gradients in the atmosphere. We also derive a new period of the star, $P=3.618\,664(10)$ d, from all available photometric and magnetic measurements and show that the observed rotational period is constant over decades. The ultraviolet and visual variability of $\theta$ Aur is mostly caused by silicon bound-free absorption and chromium and iron line absorption. These elements redistribute the flux mainly from the far-ultraviolet region to the near-ultraviolet and optical regions in the surface abundance spots. The light variability is modulated by the stellar rotation. The ultraviolet domain is key for understanding the properties of chemically peculiar stars. (abridged)

Spectropolarimetric Properties of a Gallium Nanoparticle Layer on a Sapphire Substrate

Gallium nanoparticles (Ga NPs) are currently the subject of vigorous research as possible substrates in surface-enhanced Raman scattering (SERS) experiments in the ultraviolet spectral domain. Verification of any comprehensive model of the interaction of electromagnetic radiation with Ga NPs requires that complete polarimetric measurements be made. These spectropolarimetric properties can be obtained using a Mueller matrix spectropolarimeter (MMSP). The position of localized surface plasmon resonances (LSPRs) and spectral depolarization data of Ga NPs in the 300 to 1100 nm spectral region are presented. Spectral depolarization data may be of value in creating a better understanding of how light couples to individual nanoparticles, as well as the role played by interparticle coupling and the connection to phenomena such as SERS.

Optical layouts of the WSO-UV spectrographs

World Space Observatory-Ultraviolet project is an international space observatory for spectroscopy and imaging in 115–310 nm spectral range. Significant progress in the CCD development allows to use the back illuminated CCD detectors with anti-reflection coating for observations in this ultraviolet domain instead of MCP detectors. This detector change was one of the main reason for significant correction of layouts of all three WSO-UV spectrographs. In this paper we present the final optical layouts that are now being realized in the WUVS instrument. The using of implemented technological changes in optical layout allows to keep main requirements to WSO-UV spectrographs.

Asymptotic description of finite lifetime effects on the photon emission from a quark-gluon plasma

Direct photons play an important role as electromagnetic probes from the quark-gluon plasma (QGP) which occurs during ultrarelativistic heavy-ion collisions. In this context, it is of particular interest how the finite lifetime of the QGP affects the resulting photon production. Earlier investigations on this question were accompanied by a divergent contribution from the vacuum polarization and by the remaining contributions not being integrable in the ultraviolet (UV) domain. In this work, we provide a different approach in which we do not consider the photon number density at finite times, but for free asymptotic states obtained by switching the electromagnetic interaction according to the Gell-Mann and Low theorem. This procedure eliminates a possible unphysical contribution from the vacuum polarization and, moreover, renders the photon number density UV integrable. It is emphasized that the consideration of free asymptotic states is, indeed, crucial to obtain such physically reasonable results.

Light absorption and partitioning in Arctic Ocean surface waters: impact of multiyear ice melting

Abstract. Ice melting in the Arctic Ocean exposes the surface water to more radiative energy with poorly understood effects on photo-biogeochemical processes and heat deposition in the upper ocean. In August 2009, we documented the vertical variability of light absorbing components at 37 stations located in the southeastern Beaufort Sea including both Mackenzie River-influenced waters and polar mixed layer waters. We found that melting multiyear ice released significant amount of non-algal particulates (NAP) near the sea surface relative to subsurface waters. NAP absorption coefficients at 440 nm (aNAP(440)) immediately below the sea surface were on average 3-fold (up to 10-fold) higher compared to subsurface values measured at 2–3 m depth. The impact of this unusual feature on the light transmission and remote sensing reflectance (Rrs) was further examined using a radiative transfer model. A 10-fold particle enrichment homogeneously distributed in the first meter of the water column slightly reduced photosynthetically available and usable radiation (PAR and PUR) by ∼6 and ∼8%, respectively, relative to a fully homogenous water column with low particle concentration. In terms of Rrs, the particle enrichment significantly flattered the spectrum by reducing the Rrs by up to 20% in the blue-green spectral region (400–550 nm). These results highlight the impact of meltwater on the concentration of particles at sea surface, and the need for considering non-uniform vertical distribution of particles in such systems when interpreting remotely sensed ocean color. Spectral slope of aNAP spectra calculated in the UV (ultraviolet) domain decreased with depth suggesting that this parameter is sensitive to detritus composition and/or diagenesis state (e.g., POM (particulate organic matter) photobleaching).

Off-equilibrium photon production during the chiral phase transition

In the early stage of ultrarelativistic heavy-ion collisions chiral symmetry is restored temporarily. During this so-called chiral phase transition, the quark masses change from their constituent to their bare values. This mass shift leads to the spontaneous non-perturbative creation of quark–antiquark pairs, which effectively contributes to the formation of the quark–gluon plasma. We investigate the photon production induced by this creation process. We provide an approach that eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable in the ultraviolet domain. The off-equilibrium photon numbers are of quadratic order in the perturbative coupling constants while a thermal production is only of quartic order. Quantitatively, we find, however, that for the most physical mass-shift scenarios and for photon momenta larger than 1 GeV the off-equilibrium processes contribute less photons than the thermal processes.

Enhanced HER and ORR behavior on photodeposited Pt nanoparticles onto oxide–carbon composite

The photodeposition process under ultraviolet domain for platinum nanoparticles was explored. The concomitant presence of different mechanisms during the photodeposition of Pt nanoparticles onto TiO2 in the presence of water and alcohol is evidenced. According to the process, one can devise various complex mechanisms. The presence of nanoparticulated oxide anatase phase enhances the photodeposition process of metal nanoparticles via the so-called heterogeneous photocatalysis. A description and the effect of mixing of various chemicals in the reactor reveal interesting information, which allows controlling the size of nanoparticles by the photodeposition process. This study also paves the way to decrease the amount of precious metals used in material composition used as catalysts towards hydrogen evolution reaction and oxygen reduction reaction for fuel cell technologies.

Fingerprints of level depletion in the photoelectron spectra of small Na clusters in the ultraviolet domain

In the framework of time-dependent density functional theory (TDDFT), we study electronic emission from small Na clusters after irradiation by an intense femtosecond laser pulse. Photoelectron spectra (PES) are compared with level depletion, i.e., the electron loss at each single-electron level. Laser frequencies in the UV range (from 9 to 19 eV) are explored. We find a clear correlation of the peak areas in the photoelectron spectra with the state depletion. This thus allows experimental access to the ionization mechanism. This correlation, moreover, demonstrates that one can find a physical interpretation of the Kohn–Sham orbitals in PES, as in real-time TDDFT.

The PROCESS Experiment: Exposure of Amino Acids in the EXPOSE-E Experiment on the International Space Station and in Laboratory Simulations

To understand the chemical behavior of organic molecules in the space environment, amino acids and a dipeptide in pure form and embedded in meteorite powder were exposed in the PROCESS experiment in the EXPOSE-E facility mounted on the European Technology Exposure Facility (EuTEF) platform on board the International Space Station (ISS). After exposure to space conditions for 18 months, the samples were returned to Earth and analyzed in the laboratory for reactions caused by solar UV and cosmic radiation. Chemical degradation and possible racemization and oligomerization, the main reactions caused by photochemistry in the vacuum ultraviolet domain (VUV, wavelength range 100-200 nm for photon energy from 6.2 to 12.4 eV) were examined in particular. The molecules were extracted and derivatized by silylation and analyzed by gas chromatograph coupled to a mass spectrometer (GC-MS) to quantify the rate of the degradation of the compounds. Laboratory exposure in several wavelength ranges from UV to VUV was carried out in parallel in the Cologne Deutsches Zentrum für Luft- und Raumfahrt (DLR) Center and Centre de biophysique moléculaire (CBM) laboratories. The results show that resistance to irradiation is a function of the chemical nature of the exposed molecules and the wavelengths of the UV light. The most altered compounds were the dipeptide, aspartic acid, and aminobutyric acid. The most resistant were alanine, valine, glycine, and aminoisobutyric acid. Our results also demonstrate the protective effect of meteorite powder, which reemphasizes the importance of exogenic contribution to the inventory of prebiotic organics on early Earth.

Veneziano-like amplitude as a test for AdS/QCD models

The high energy asymptotics of QCD correlation functions is often used as a test for bottom-up holographic models. Since QCD is not strongly coupled in the ultraviolet domain, such a test may look questionable. We propose that the sum over resonance poles emerging in correlators of a bottom-up model should reproduce the structure of a Veneziano like amplitude at zero momentum transfer assuming equivalence of spin and radial states in the latter. This requires a five-dimensional background that suppresses the ultraviolet part in the effective action of a model. We give examples of emerging low-energy holographic models.

DUVEX: An X-ray counting system based on YAG:Ce scintillator

A detector system, called DUVEX, has been developed for the soft-x-ray and extreme ultraviolet domain. It consists of a YAG :Ce scintillator coupled to a photomultiplier module working under vacuum in counting mode. The design and the performances of this detector in terms of yield, absolute efficiency, response and noise are reported. Spectra in the soft x-ray range of different elements (W, Ag, Al, Mg, Cu, N, C and B) obtained in WDS mode using this detector are presented. DUVEX appears as a competitive detection tool in terms of cost and easiness of implementation.

Optical Emission Spectroscopic Measurement of Hydroxyl Radicals in Air Discharge with Atomized Water

Effects of discharge mode, voltage applied, size of the nozzle discharge electrode and flow rate of water on the generation of hydroxyl radical were investigated in air discharge with atomized water, by using optical emission spectroscopy (OES). Water was injected into the discharge region through the discharge nozzle electrode, and a large amount of fine water drops, formed and distributed in the discharge region, were observed. It was found that negative DC corona discharge was more effective to generate the hydroxyl radicals in comparison to positive DC corona discharge or negative pulsed discharge. A larger outer diameter of the nozzle electrode or a stronger electric field is beneficial for hydroxyl-radical generation. Moreover, there is a critical value in the flow rate of atomized water against the discharge voltage. Below this critical value, hydroxyl-radical generation increases with the increase in flow rate of the water, while above this value, it decreases. In addition, it is observed that OES from the discharge is mainly in the ultraviolet domain. The results are helpful in the study of the mechanism and application of plasma in pollution-control in either air or water.

New advances in the field of planetary nebulae from ultraviolet observations

AbstractThe ultraviolet (UV) domain, in particular shortwards of Lyα, provides unique information to unravel the physical parameters of Central Stars of Planetary Nebulae (CSPNe) and the paths for this elusive final stage of stellar evolution, thanks to a wealth of diagnostic transitions from ionic species not observable at other wavelengths. Intermediate mass stars are the major providers of important elements like C and N. Understanding how they shed most of their initial mass is critical for understanding the chemical enrichment of the ISM. Mass-loss diagnostic lines abound at UV wavelengths, and when the CSPN reaches the hottestTeffbefore turning on the WD-cooling sequence, and the wind fades, the last wind lines to disappear are found in the far-UV, as well as diagnostic lines for elements such as Ne. This domain also offers a host of H2transitions, tracing the circum-stellar material expelled in previous phases. UV images and spectra of PNe add critical constraints to their ionization structure and to some abundances. Finally, the recent GALEX sky surveys in two UV bands afforded the first unbiased census of hot white dwarfs (WD) and post-AGB objects in the Milky Way, significantly expanding known catalogs and providing statistical constraints to the initial-final mass relation.

FTIR and UV–VIS spectroscopy investigations on the structure of the europium–lead–tellurate glasses

Glasses in the xEu 2O 3·(100-x)[4TeO 2·PbO 2] system where 0 ≤ x ≤ 50 mol% have been prepared using the melt quenching method. The influence of europium ions on the structure of lead–tellurate glasses has been investigated using density measurements, FTIR and UV–VIS spectroscopy. Structural changes produced by increasing the rare earth concentration were followed. The europium and lead ions show a preference towards [TeO 3] structural units causing a deformation of the Te O Te linkages. Structural changes inferred by analyzing the band shapes of IR spectra revealed that the increase of the Eu + 3 content causes the intercalation of [EuO n] entities in the [TeO 4] chain network. The excess of oxygen can be supported into the glass network by the formation of [PbO n] and [EuO n] structural units. The UV–VIS spectroscopy data show that europium ions enter the glass matrix in the Eu 2+ and Eu 3+ valence states, the last being predominant in the studied glasses. The Pb + 2 ions produce strong absorption in the ultraviolet domain.

A space Fresnel Imager for ultra-violet astrophysics: example on accretion disks

The Fresnel Diffractive Imager concept is proposed for space borne astronomical imaging at Ultra-Violet wavelengths, using diffractive focalization. The high angular resolution and high dynamic range provided by this new concept makes it an ideal tool to resolve circumstellar structures such as disks or jets around bright sources, among them, pre-main sequence stars and young planetary disks. The study presented in this paper addresses the following configuration of Fresnel diffractive imager: a diffractive array 4 m large, with 696 Fresnel zones operating in the ultra-violet domain. The diffractive arrays are opaque foils punched with a large number of void subapertures with carefully designed shapes and positions. In the proposed space missions, these punched foils would be deployed in space. Depending on the size of the array and on the working spectral band, the focal length of such imagers will range from a few kilometers to a few tens of kilometers. Thus, such space mission requires a formation flying configuration for two satellites around the L2 Sun-Earth Lagragian point. In this article, we investigate numerically the potential of Fresnel arrays for imaging circumstellar dust environments. These simulations are based upon simple protostellar disk models, and on the computed optical characteristics of the instrument. The results show that protoplanetary disks at distances up to a few thousand parsecs can be successfully studied with a 4 m aperture Fresnel imager in the UV.

Evidence of an irradiated accretion disc in XTE J1818−245★

The X-ray transient source XTE J1818−245 went through an outburst in 2005 that was observed during a multiwavelength campaign from radio to soft γ-rays. Observations in V and R optical bands with the 1-m Swope telescope allowed the discovery of a new bright source. As we aimed to reveal the nature of the companion star, we performed new optical observations with the ESO/NTT telescope at La Silla, both in photometry and spectroscopy. We confirm the optical counterpart found by the Swope telescope, but the spectral type of the secondary star could not be identified. The spectrum showed a blue-dominated shape and an Hα emission line was detected, indicating that the optical emission was dominated by the presence of an accretion disc. The broad-band spectral energy distribution revealed that the outer parts of the accretion disc had to be irradiated by its inner parts to explain the optical emission. New observations of XTE J1818−245 in quiescence are needed to find the nature of the companion star. Moreover, radio-to-X-ray strictly simultaneous observations of transient black holes are needed to disentangle the importance of jets and irradiated accretion discs in the infrared–optical–ultraviolet domain.

Fourier-transform spectroscopy of HD in the vacuum ultraviolet at λ = 87–112 nm

Absorption spectroscopy in the vacuum ultraviolet (VUV) domain was performed on the hydrogen-deuteride molecule with a novel Fourier-transform spectrometer based upon wavefront division interferometry. This unique instrument, which is a permanent endstation of the undulator-based beamline DESIRS on the synchrotron SOLEIL facility, opens the way to Fourier-transform spectroscopy in the VUV range. The HD spectral lines in the Lyman and Werner bands were recorded in the 87–112 nm range from a quasi-static gas sample in a windowless configuration and with a Doppler-limited resolution. Line positions of some 268 transitions in the Lyman bands and 141 transitions in the Werner bands were deduced with uncertainties of 0.04 cm−1 (1σ) which correspond to Δλ/λ ∼ 4 × 10−7. This extensive laboratory database is of relevance for comparison with astronomical observations of H2 and HD spectra from highly redshifted objects, with the goal of extracting a possible variation of the proton-to-electron mass ratio (μ = m p /m e ) on a cosmological time scale. For this reason also calculations of the so-called sensitivity coefficients K i were performed in order to allow for deducing constraints on Δμ/μ. The K i coefficients, associated with the line shift that each spectral line undergoes as a result of a varying value for μ, were derived from calculations as a function of μ solving the Schrödinger equation using ab initio potentials.