Ultraviolet Laser Excitation Research Articles

Remote Raman spectroscopy of explosive precursors

Deep ultraviolet Raman spectroscopy measurements have been performed at the German Aerospace Center (DLR) with the aim of detecting traces (μg range) of explosive precursors. In this study, a backscattering Raman system was setup and optimized to detect urea, sodium perchlorate, ammonium nitrate, and sodium nitrate at a 60-cm short-range remote detection. The sample was tested at 264-nm ultraviolet laser excitation wavelength to experimentally observe any possible trace over textiles samples. For each colored sample textile, Raman spectra were acquired and no background fluorescence interference was observed at this laser excitation wavelength. Detection limits and system sensitivity with an acquisition time up to 3 s for microgram traces are presented.

The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films

AbstractWe utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h‐BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h‐BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h‐BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal‐to‐noise ratio under ambient conditions, we propose a line‐scan method for the acquisition of UV Raman spectra in atomically thin h‐BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures.

Monitoring the Restoration of a Seventeenth-Century Wooden Artwork Using Laser-Induced Fluorescence and Digital Image Analysis.

The present paper is aimed at demonstrating the capabilities of digital image analysis (DIA) to support conservation of painted artwork. Laser-induced fluorescence (LIF) imaging has been usually used in the diagnosis of wall paintings. In this case, LIF is applied to the case study of a painted wooden canopy, and most successful data processing techniques are presented in the discussion of results. The Jesús del Gran Poder canopy, completed by Francisco Ruiz Gijón in 1692, is an oil panel painting on wood. Eight panels of the canopy have been study by LIF. This technique is capable of remotely acquiring hyperspectral images operating in fluorescence mode following ultraviolet laser excitation. LIF spectra combined with principal component analysis, spectral angle mapper, and DIA provide a chemical mapping of the treated wooden surface of the panels. Besides, LIF spectrum is as a fingerprint of the panels that allows stablishing differences between them. LIF imaging analysis has proven to be a very useful tool for mapping retouching work, tracking previous restorations, and detecting chemicals on the wood in order to monitor restorations.

Dual-channel mobile fluorescence lidar system for detection of tryptophan.

We present a dual-channel mobile lidar system based on laser-induced fluorescence (LIF) for real-time standoff detection and concentration distribution analysis of tryptophan. The system employs an ultraviolet laser excitation source and signal detectors for receiving fluorescence signals within two different wavelength bands. The performed experiments measured tryptophan aerosols at two different standoff distances. Moreover, distilled water and ethanol solutions were also detected for comparison. The results show that the system can detect LIF signals of tryptophan, give early warnings, locate the diffusion sources, and monitor the variation of the aerosol concentration distribution in real time.

The effect of stabilized ZnO nanostructures green luminescence towards LPG sensing capabilities

ZnO nano-rods and flower-like structures have been successfully synthesized by the hydrothermal process. These nanostructures were annealed at various temperatures (210, 310, 510, and 710 °C) both in air and vacuum (~10−4 mbar) for 1h and subsequently allowed to cool down slowly or rapidly. These structures were subsequently characterised by various techniques for structure and morphology. Laser photoluminescence spectroscopy performed with a continuous He–Cd laser chopped at 120 Hz and spectra were collected three times to study the emission stability over time. The un-annealed ZnO exhibited a very broad-band of defect related emission due to oxygen vacancies with the exciton recombination completely suppressed. These defect emissions were unstable and degraded under ultraviolet laser excitation, reducing in intensity after each emission measurement. Interestingly, annealing these ZnO samples in air together with thermal quenching significantly improved both the exciton emission and these defect-related emissions, thereby improving the overall emission stability including the green luminescence. The liquefied petroleum gas (LPG) sensing performance has been examined. The ZnO nanostructure with stable green luminescence has been found to exhibit the highest sensing response of about 80 at 200 °C low operating temperature compared to the other nanostructures. This showed a clear link between the green luminescence stability and LPG sensing properties. Therefore, the main innovation of this work is to link the stability of oxygen vacancies to the LPG detection and response and mechanical properties of these ZnO nanostructures, contrary to previous studies which this part is sought. The ZnO sensor further showed an excellent reproducibility for long-term stability, linear dependence, low detection limit, and higher selectivity towards LPG as compared to other interference gases (i.e. NH3, C2H2, H2, and CO2). The response and recovery times were as 400s and 100s, respectively. The sensitivity and stability of the samples towards LPG were also tested. In addition, the mechanical-stress improved sensor proved to be resilient and reliable even after subjected to harsh humid environment of ultra-violet light irradiation of 8 h in air.

Direct observation of magnetic contrast obtained by photoemission electron microscopy with deep ultra-violet laser excitation

Magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) have been investigated in a photoemission electron microscopy (PEEM) system excited by a deep ultra-violet (DUV) laser (with λ = 177.3 nm and hυ = 7.0 eV) for the first time. High resolution PEEM magnetic images (down to 43.2 nm) were directly obtained on a (001)-oriented magnetic FePt film surface with a circularly-polarized light under normal incidence. Furthermore, a stepped Cr seeding layer was applied to induce the formation of large-area epitaxial FePt films with (001) and (111) two orientations, where MLD with large asymmetry was observed in the transition area of two phases. It demonstrates that DUV laser can be a powerful source for high resolution magnetic imaging in the laboratory in absence of synchrotron facilities.

Comparison of directionally outcoupled photoluminescences from luminous layers on Si and Al nanocylinder arrays

An array of nanocylinders can modulate photoluminescence (PL) of emitters via simultaneous excitation of local resonances and light diffractions. Regarding the local resonance, localized surface plasmon polaritons (LSPP) of metals has been commonly applied, while the Mie resonance of dielectrics can also be a candidate. Since these two resonances are excited in different classes of materials, experimental comparisons between their enhancement efficiencies are rarely made. In this study, we fabricated periodic arrays of Si and Al nanocylinders with the identical design and compared their PL outcoupling abilities. We deposited ultraviolet (UV)-absorbing and highly luminous dielectric films on the arrays and observed enhanced PLs from the films on the nanocylinder arrays upon a UV laser excitation. The optical transmission with varied incident angles revealed that the dispersion of the hybrid mode of diffraction with the Mie resonance in the Si nanocylinder array was sharper than that of the diffraction-LSPP hybrid in the Al nanocylinder array. The PL enhancement by the Si nanocylinder array was more selective both spectrally and spatially than that by the Al nanocylinders, which could be attributed to the difference between the mode profiles. Our findings experimentally verified the advantage of choosing Si as a constituent of the cylinder array to provide highly directional light sources.

Storage and transfer of optical excitation energy in GaInP epilayer: Photoluminescence signatures

GaInP alloy could be the most trusted key material for fabricating super-high-efficiency single- and multi-junction solar cells, especially for space applications. The storage and transfer of optical excitation energy in this key alloy is thus a key subject of the energy conversion from optical to electrical. In this article we present a study of the subject through investigating photoluminescence (PL) degradation in the GaInP epilayer at 4 K under the continuous optical excitations of ultraviolet (UV) 325 nm, visible 488.0 and 514.5 nm lasers. It is found that the decline of PL intensity with the irradiation time may be represented by I(t)/I0=1+tτ−1−1+C, where I0 is the luminescence intensity at the beginning of irradiation, τ a time constant, and C a background. Moreover, the PL degradation degree reduces with increasing the excitation wavelength. In addition, some red shift of the PL peak is observed accompanying with the intensity decline under the UV laser excitation. These PL signatures indicate that the localized carriers within the local atomic ordering domains play a major role in the storage and transfer of the excitation energy via photon recycling processes.

Raman evidence for surface oxidation of amorphous As2S3 thin films under ultraviolet irradiation

Amorphous As2S3 films with surface roughness near 0.5 nm were prepared by thermal evaporation. Raman measurements performed under ultraviolet (UV) laser excitation (325 nm) revealed a series of sharp intense peaks appearing at laser power density Pexc above 150 kW/cm2. No such effects were observed at Raman measurements under visible (514.7 or 632.8 nm) light excitation, even for much higher Pexc values. The new Raman peaks are identified as arsenolite (As2O3) features. UV light-assisted oxidation of the As2S3 surface is discussed.

Non-resonant vibrational excitation of HOD and selective bond breaking.

This paper reports a time-dependent quantum mechanical wave packet study for bond-selective excitation and dissociation of HOD into the H + OD and D + OH channels in the first absorption band. Prior to excitation, the HOD molecule is randomly oriented with respect to a linearly polarized laser field and accurate static dipole moment and polarizability surfaces are included in the interaction potential. Vibrational excitation is obtained with intense, non-resonant 800 nm few-cycle excitation using dynamic Stark effect/impulsive Raman scattering. Dissociation is accomplished by another ultrashort vacuum ultraviolet-laser excitation. A laser control scheme is designed with a train of simple, non-resonant laser pulses in order to enhance the selectivity between the fragmentation channels. The effect of the carrier-envelope-phase of the ultrashort laser pulses is also investigated.

Laser‐Mediated Photo‐High‐p,T Intensification of Vitamin D3 Synthesis in Continuous Flow

AbstractA new reactor design for photosynthesis in microflow is presented based on spatial overlap of a laser beam through a capillary tube. A conical Archimedean spiral facilitates exposure of the entire volume of the reaction solution to the irradiation beam. In this fashion, photo‐high‐p,T intensification of the photolysis of 7‐dehydrocholesterol is achieved using either a pulsed ultraviolet (UV) laser excitation (in the order of femtoseconds) alone or in tandem with a conventional UV lamp. The goal is to control the kinetics of the side photoreactions under pulsed photolysis, as well as to reduce the reaction time and enhance the vitamin D3 yield. Three kinds of effects are targeted: a) Utilizing a very large and spatially homogeneous photon flux inside the flowing solution; b) the novel process window of combining photo‐ with thermal intensification to promote the final, irreversible thermal step to vitamin D3; and c) the use of sub‐ps laser pulses to supply excitation energy on a timescale shorter than the formation of side photoproducts. Finally, photo‐processing with the laser alone is compared to the tandem irradiation by the laser‐UV lamp.

Collective plasmonic modes excited in Al nanocylinder arrays in the UV spectral region.

A periodic array of plasmonic nanocylinders can sustain both surface plasmon polaritons (SPPs) and optical diffraction in the plane of the array. Thus the optical energy can be efficiently trapped in the plane of the array, providing a good platform for controlling light. Plasmonic arrays have been investigated in the visible range, while studies in the ultraviolet (UV) range have been limited due to material-related restrictions and higher precision required for optical diffraction in the UV compared to that in the visible range. In this study, we fabricated periodic arrays of Al nanocylinders with periods comparable to optical wavelengths in the UV for simultaneous excitation of both SPPs and optical diffraction in the UV spectral region. We deposited UV-absorbing and highly luminous dielectric films on the arrays, observed enhanced photoluminescence of the film under UV laser excitation, and demonstrated that such periodic arrays can trap the UV light energy. Our findings show that periodic arrays of Al nanocylinders are useful for controlling UV light.

From Futile to Fruitful: Diesel Soot as White Light Emitter.

The present work describes a solution for the effective use of the hazardous particulate matter (diesel soot) from the internal combustion engines (ICEs) as a potential material emitting white light for white light emitting diodes (WLEDs). The washed soot samples are subjected to Field Emission Scanning Electron Microscopy (FESEM), High- Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive Spectroscopy (EDS), UV-Visible, Photoluminescent (PL) Spectroscopy and quantum yield measurements. The CIE plot and Correlated Color Temperature (CCT) reveals the white fluorescence on photoexcitation. The sample on ultraviolet (UV) laser excitation, provides a visual confirmation of white light emission from the sample. The diesel soot collected from public transport buses of different years of manufacture invariably exhibit white fluorescence at an excitation of 350nm. The sample show a quantum yield of 47.09%. The study is significant in the context of pollution and search for low-cost, rare-earth phosphor free material for white light emission and thereby turning the hazardous, futile material into a fruitful material that can be used for potential applications in photonics and electronics.

Spin-phonon coupling in antiferromagnetic nickel oxide

We report the results of ultraviolet Raman spectroscopy of NiO, which allowed us to determine the spin-phonon coupling coefficients in this important antiferromagnetic material. The use of the second-order phonon scattering and ultraviolet laser excitation (λ = 325 nm) was essential for overcoming the problem of the optical selection rules and dominance of the two-magnon band in the visible Raman spectrum of NiO. We established that the spins of Ni atoms interact more strongly with the longitudinal than transverse optical phonons and produce opposite effects on the phonon energies. The peculiarities of the spin-phonon coupling are consistent with the trends given by density functional theory. The obtained results shed light on the nature of the spin-phonon coupling in antiferromagnetic insulators and can help in developing spintronic devices.

Unusual application of aluminium-doped ZnO thin film developed by metalorganic chemical vapour deposition for surface temperature sensor

A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this purpose, the temperature-dependent photoluminescence (PL) properties of aluminium-doped ZnO thin films were studied. The films have been successfully deposited on substrate of Si(100)-orientation by metalorganic chemical vapour deposition (MOCVD) method. For the use of the films as temperature sensors, the Photoluminescence (PL) properties are most important. Consequently, the emission peaks are observed in the undoped and Al-doped films deposited at 550°C and annealed at 900°C for 2h after ultraviolet laser excitation (355nm). The results show that with increasing temperature the PL intensity is quenched for the Al-doped ZnO film (n(Al)/n(Zn)=0.051). As a result, the area under the spectrum changes significantly with temperature, making it useful for temperature evaluation. Al-doped ZnO films can be used as temperature sensors within the range of room temperature to 300°C. Beyond this range the spectrum is no longer sensitive to temperature change.

Inherent Fluorescence Detection of Latent Fingermarks by Homemade Shortwave Ultraviolet Laser.

Detection of latent fingermarks on various substrates is critical in crime investigations. Conventional chemical methods using reagents could contaminate or even destruct biological information of samples. Here, an optical method and successful case application of detecting latent fingermarks through long-wave ultraviolet (UV) fluorescence (300-400 nm) by shortwave UV laser excitation is reported. Experimental results indicate that the recovery rate of the latent fingermarks on various paper items is in the range of 70-80% without chemical treatments. Moreover, the optical method allows for the preservation of samples for further examination, such as polymerase chain reaction (PCR) testing. The technique has also been successfully applied to a criminal case in identifying the suspect, which, to the best of our knowledge, has never been reported in real crime investigations. Therefore, such a method as UV-excited UV fluorescence in detecting latent fingermarks may be better for examination in cases where biological information of samples is needed for consequent testing.

Modulation of voltage-gated conductances of retinal horizontal cells by UV-excited TiO2 nanoparticles

This study examines the ability of optically-excited titanium dioxide nanoparticles to influence voltage-gated ion channels in retinal horizontal cells. Voltage clamp recordings were obtained in the presence and absence of TiO2 and ultraviolet laser excitation. Significant current changes were observed in response to UV light, particularly in the −40 mV to +40 mV region where voltage-gated Na+ and K+ channels have the highest conductance. Cells in proximity to UV-excited TiO2 exhibited a left-shift in the current–voltage relation of around 10 mV in the activation of Na+ currents. These trends were not observed in control experiments where cells were excited with UV light without being exposed to TiO2. Electrostatic force microscopy confirmed that electric fields can be induced in TiO2 with UV light. Simulations using the Hodgkin–Huxley model yielded results which agreed with the experimental data and showed the I-V characteristics of individual ion channels in the presence of UV-excited TiO2.

Synthesis of Spherical ZnO Microcrystals by Laser Ablation in Air

SUMMARYWe have succeeded in synthesizing the zinc oxide (ZnO) microspheres by laser ablation of a ZnO sintered target in air, and have demonstrated ultraviolet whispering‐gallery‐mode lasting from the sphere under ultraviolet pulsed laser excitation. In this study, we investigated the dependence of the laser fluence and the spot size on the growth of the ZnO microspheres. Large sizes of more than 30 μm microspheres were synthesized with increasing the laser fluence up to 440 J/cm2. Although undesired nanoparticles and fragments were also generated at high fluence and large ablation spot size, selective collection of the microspheres can be expected because of different emitting distributions of the microspheres and other fragments.

Alteration of time-resolved autofluorescence properties of rat aorta, induced by diabetes mellitus

Changes in autofluorescence properties of isolated rat aorta, induced by diabetes mellitus, were detected using time-resolved fluorescence spectroscopy with pulsed ultraviolet (UV) laser excitation. We demonstrated that time-resolved spectroscopy was able to detect changes in aorta tissues related to diabetes and unambiguously discriminate diabetic (τ1 0.63 ± 0.05 ns, τ2 3.66 ± 0.10 ns) samples from the control (τ1 0.76 ± 0.03 ns, τ2 4.48 ± 0.15 ns) group. We also report changes in the ratio of relative amplitudes of the two lifetime component in aorta tissue during diabetes, most likely related to the pseudohypoxic state with altered NADH homeostasis.

Resonance Raman of oxygen dangling bonds in amorphous silicon dioxide

We investigate the origin of a resonance Raman band induced by ionizing radiation in amorphous silicon dioxide (silica glass), which can be detected under ultraviolet laser excitation. A silica sample, rich of oxygen-excess related defects, was prepared by treating some length of a pure-silica-core multimode fiber in an O2 atmosphere (at high temperature and pressure) and by irradiating it with X-rays at 10 MGy(SiO2) dose. A micro-Raman study revealed a gaussian band peaking at 896 cm−1 with a full width at half maximum of 32 cm−1, which could be detected by exciting the sample with the 325-nm line of a HeCd laser. This spectral feature is absent in the Raman spectra performed with the 442-nm line of the same laser. On the basis of several experimental evidences and some complementary literature data, the 896 cm−1 band is assigned to a resonance Raman scattering of oxygen dangling bonds, often called non-bridging oxygen hole centers. Copyright © 2016 John Wiley & Sons, Ltd.