Femtosecond UV Research Articles

Gamma-Radiation Effects on Bragg Gratings Written by Femtosecond UV Laser in Ge-Doped Fibers

We studied the effect of Co60 gamma-radiation on the position and the amplitude of fiber Bragg gratings. These gratings were written with a femtosecond UV laser in three different fibers, both intrinsically photosensitive and hydrogen sensitized. We have found that in contrast with the results obtained on gratings written with low-intensity UV lasers, the Bragg peak shift under irradiation is almost independent on fiber type, fiber sensitization or grating amplitude. The grating strength behavior depends on the fiber type.

Wavelength Dependence of Electronic Relaxation in Isolated Adenine Using UV Femtosecond Time-Resolved Photoelectron Spectroscopy

Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ∼5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV.

Ablation of microstructures applying diffractive elements and UV femtosecond laser pulses

A new method for simple and economic fabrication of diffractive optical elements (DOEs) with three and four phase levels, by UV nanosecond (ns) laser ablation is presented. The technique is based on the combination of two sequentially generated complementary 2-level phase elements. During the fabrication, complete ablative removal of a highly absorbing silicon suboxide layer by pixelated backside illumination ensures the necessary high precision and optical quality. Full functionality of the new DOEs is demonstrated by fabricating micro-structures using UV femtosecond pulses.

Point-by-point inscription of 250-nm-period structure in bulk fused silica by tightly-focused femtosecond UV pulses: experiment and numerical modeling

Abstract By conducting point-by-point inscription in a continuously moving slab of a pure fused silica at the optimal depth (170 μm depth below the surface), we have fabricated a 250-nm-period nanostructure with 30 nJ, 300 fs, 1 kHz pulses from frequency-tripled Ti:sapphire laser. This is the smallest value for the inscribed period yet reported, and has been achieved with radical improvement in the quality of the inscribed nanostructures in comparison with previous reports. The performed numerical modeling confirms the obtained experimental results.

Advances in atomic spectrometry and related techniques

This is the second iteration of this review covering developments in ‘Atomic Spectrometry’. It covers atomic emission, absorption, fluorescence and mass spectrometry, but excludes material on speciation and coupled techniques which is included in a separate review.1 It should be read in conjunction with the other related reviews in the series.2–5 A critical approach to the selection of material has been adopted, with only novel developments in instrumentation, techniques and methodology being included. Most techniques have reached a level of maturity which precludes the emergence of ‘stand-out’ new developments. It is noteworthy that there are very few novel publications in sections on chemometrics, fundamental studies, or direct solids analysis. Sample introduction continues to generate a steady stream of research outputs, but these are mainly derivative and focused on applications. The advent of SF-ICP-MS is becoming widely adopted as a reliable technique for IR measurements, so this had been the main growth area. Likewise, the use of femtosecond UV lasers has now entered the mainstream for geological applications.

Photodissociation of group-6 hexacarbonyls: observation of coherent oscillations in an antisymmetric (pseudorotation) vibration in Mo(CO)5 and W(CO)5

On dissociation of M(CO)(6), M = Cr, Mo and W, by a femtosecond UV laser (<270 to 360 nm), pronounced coherent oscillations are observed in the pentacarbonyl products on probing by long-wavelength (810 nm) ionization in the gas phase. They are vibrations in the ground state, driven by the slope from a conical intersection on relaxation from the initially formed excited state (S(1)). Surprisingly, with M = Mo and W we also find a fundamental of an antisymmetric (b(2) in C(4v)) vibration. From positive and negative displacements along such a coordinate one would expect the same signal, so that there should be only overtones. Vibrational selection rules are therefore considered for time-resolved spectroscopy. The reason for the symmetry breaking is suggested to result from the fact that the phase in superposition of wave functions is established by the pump process and this phase is conserved in probing, independently of the probe delay. An antisymmetric fundamental can be observed, if there is a small tunneling splitting in a state involved in the probe process. The observations also imply some conclusions on the dissociation and relaxation processes and the potentials: with longer wavelengths, the wave packet enters on the same surface but from a different direction to S(1). Only a very minor fraction of the available energy appears as coherent oscillation. There is no equipartition at the end, and a second CO is cleaved off in few picoseconds, even if there is only very little excess energy. Triplets do not contribute, even in the tungsten system and at longest wavelengths. The dissociation mechanism involves passage of the wave packet from all initial states over an avoided crossing to a repulsive ligand-field surface. It predicts that in some other molecules, the barrier caused thereby is larger and for long photolysis wavelength the lifetime is long enough for intersystem crossing to take place; it also predicts wavelength dependences in these cases. It is again emphasized that there is no vertical internal conversion; instead, the molecule is controlled by slopes and intersections of potentials. Also lifetimes can be considered as a control parameter in photochemistry.

On the structure and excited electronic state lifetimes of cytidine self-assemblies with extended hydrogen-bonding networks

The nucleoside cytidine (C) assembles in extended hydrogen-bonded aggregates in neat apolar solvents like n-hexane, when carrying inert hydrophobic groups at the 2′,3′,5′-oxygen atoms of the ribose moiety. The ensuing structures, which constitute model systems for DNA super-structures such as triplexes or quadruplexes, were elucidated by FTIR spectroscopy and further characterized by UV spectroscopy. The lifetimes of the optically excited electronic states of the aggregates were investigated using femtosecond UV fluorescence up-conversion spectroscopy. Time profiles were measured after excitation at a number of pump wavelengths between 296 nm ≥ λ pump ≥ 262 nm. The bi-exponential decay curves were characterized by time constants (with 2 σ error limits) of τ 1,C = 0.58(1) ps (88–95% fractional amplitudes) and τ 2,C = 19.4(13) ps (12–5%) independent of the pump wavelength. The results indicate that the C multimers compared to the monomer do not gain photostability by coupled excited-state electron–proton transfer in the H-bonded networks, in contrast with recent findings for G·G and the G·C Watson–Crick dimer.

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight ( M w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.

Thermal sensitivity studies of fibre Bragg gratings inscribed by high-intensity femtosecond UV laser pulses

We studied the thermal sensitivity of fibre Bragg gratings (FBGs), fabricated in single-mode standard telecom (SMF-28) and photosensitive Ge/B codoped (Fibercore PS 1250/1500) fibres with high-intensity (140–210 GW/cm 2) 264 nm 220 fs laser pulses. The accurate measurement of Bragg wavelength thermal shift in the mentioned fibres brought about the values of 9.62 ± 0.04 and 8.03 ± 0.12 pm/°C, respectively. The comparison of the thermal sensitivity values for FBGs made in the same fibres by low-intensity UV irradiation or by another inscription technique (scanning phase mask instead of static one) did not reveal any significant differences. This confirms that the thermal sensitivity of Bragg gratings is determined only by the material parameters of the fibres and does not depend on the inscription mechanism and/or fabrication technique.

Processing of Dielectric Optical Coatings by Nanosecond and Femtosecond UV Laser Ablation

Microprocessing of dielectric optical coatings by UV laser ablation is demonstrated. Excimer laser ablation at deep UV wavelengths (248 nm, 193 nm) is used for the patterning of thin oxide films or layer stacks. The layer removal over extended areas as well as sub-μm-structuring is possible. The ablation of SiO2, Al2O3, HfO2, and Ta2O5 layers and layer systems has been investigated. Due to their optical, chemical, and thermal stability, these inorganic film materials are well suited for optical applications, even if UV-transparency is required. Transparent patterned films of SiO2 are produced by patterning a UV-absorbing precursor SiOx suboxide layer and oxidizing it afterwards to SiO2. In contrast to laser ablation of bulk material, in the case of thin films, the layer-layer or layer-substrate boundaries act as predetermined end points, so that precise depth control and a very smooth surface can be achieved. For large area ablation, nanosecond lasers are well suited; for patterning with submicron resolution, femtosecond excimer lasers are applied. Thus the fabrication of optical elements like dielectric masks, pixelated diffractive elements, and gratings can be accomplished.

Photoinduced ultrafast ring-opening reaction in trithianes in solution

The ring-opening reaction of trithianes possessing aromatic substituents has been studied using femtosecond broadband UV–Vis transient absorption spectroscopy. Photocleavage of the C S bond occurs during the short-lived (1.5 ps) singlet excited state of trithiane. The products are formed with excess of vibrational energy and cooling of the hot molecules occurs with a 12.1 ps time constant in acetonitrile.

Anisotropic Fiber Bragg Gratings Inscribed by High-Intensity Femtosecond-UV Pulses: Manufacturing Technology and Strain Characterization for Sensing Applications

We report the small-range strain characterization of anisotropic fiber Bragg gratings (FBGs) written by high-intensity femtosecond UV pulses. The FBGs are characterized with applied strain of 0-300 muepsiv using a high-speed Czerny-Turner spectrometer. The strain response slope is comparable to typical FBGs.

Quantification of the activity of biomolecules in microarrays obtained by direct laser transfer

The direct-writing technique laser-induced forward transfer has been employed for the micro-array printing of liquid solutions of the enzyme horseradish peroxidase and the protein Titin on nitrocellulose solid surfaces. The effect of two UV laser pulse lengths, femtosecond and nanosecond has been studied in relation with maintaining the activity of the transferred biomolecules. The quantification of the active biomolecules after transfer has been carried out using Bradford assay, quantitative colorimetric enzymatic assay and fluorescence techniques. Spectrophotometric measurements of the HRP and the Titin activity as well as chromatogenic and fluorescence assay studies have revealed a connection between the properties of the deposited, biologically active biomolecules, the experimental conditions and the target composition. The bioassays have shown that up to 78% of the biomolecules remained active after femtosecond laser transfer, while this value reduced to 54% after nanosecond laser transfer. The addition of glycerol in a percentage up to 70% in the solution to be transferred has contributed to the stabilization of the micro-array patterns and the increase of their resolution.

Assessing solvent effects on the singlet excited state lifetime of uracil derivatives: A femtosecond fluorescence upconversion study in alcohols and D 2O

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ ∗ and the dark nπ ∗ states. This relative energy gap controls the non-radiative relaxation of the ππ ∗ state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D 2O where the decays are faster than in H 2O.

Patterning parameters for biomolecules microarrays constructed with nanosecond and femtosecond UV lasers

The development of laser techniques for transferring biomolecules on solid surfaces in a controlled manner has attracted much attention during the last few years. Laser-induced forward transfer (LIFT) is a direct-write laser technique employed in printing micron-sized patterns or features of a wide range of materials in solid or liquid state, in ambient atmosphere and temperature or in vacuum. In this work, we use LIFT with nanosecond and femtosecond UV lasers for patterning the proteins Biotin, Avidin and Titin. In addition to the laser parameters, we also investigate the influence of the solution viscosity and the receiving surface wettability on the formed patterns. To confirm pattern functionality after transfer, we employ the strategy of patterning a coupling molecule with LIFT, immobilizing an active complementary protein on it and performing a fluorescence microscopy assay afterwards. Our results show that both nanosecond and femtosecond lasers are suitable tools for obtaining micron size protein patterns, with femtosecond laser pulses requiring a lower energy for transfer. We also show that the smallest patterns are formed just above the threshold energy for the thinnest target and that the pattern size increases with increasing surface wettability and decreases with solution viscosity.

Study of Concerted and Sequential Photochemical Wolff Rearrangement by Femtosecond UV−vis and IR Spectroscopy

Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-vis and IR transient absorption spectroscopy. For BpCN2COCH3 in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either 1BpCN2COCH3*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene. The slow time constant of the ketene formation dynamics obtained by ultrafast IR (700 ps) spectroscopy agrees with the relaxed carbene decay of 800 +/- 100 ps obtained by UV-vis absorption spectroscopy.

Ultrafast excited-state dynamics of RNA and DNA C tracts

The excited-state dynamics of the RNA homopolymer of cytosine and of the 18-mer (dC) 18 were studied by steady-state and time-resolved absorption and emission spectroscopy. At pH 6.8, excitation of poly(rC) by a femtosecond UV pump pulse produces excited states that decay up to one order of magnitude more slowly than the excited states formed in the mononucleotide cytidine 5′-monophosphate under the same conditions. Even slower relaxation is observed for the hemiprotonated, self-associated form of poly(rC), which is stable at acidic pH. Transient absorption and time-resolved fluorescence signals for (dC) 18 at pH 6.8 are similar to ones observed for poly(rC) near pH 4, indicating that hemiprotonated structures are found in DNA C tracts at neutral pH. In both systems, there is evidence for two kinds of emitting states with lifetimes of ∼100 ps and slightly more than 1 ns. The former states are responsible for the bulk of emission from the hemiprotonated structures. Evidence suggests that slow electronic relaxation in these self-complexes is the result of vertical base stacking. The similar signals from RNA and DNA C tracts suggest a common base-stacked structure, which may be identical with that of i-motif DNA.

Point-by-point inscription of 250 nm period structure in bulk fused silica by tightly focused femtosecond UV pulses

By conducting point-by-point inscription in a continuously moving slab of pure fused silica at the optimal depth (170 μm depth below the surface), we have fabricated a 250 nm period nanostructure with 30 nJ, 300 fs, 1 kHz pulses from a frequency-tripled Ti:sapphire laser. This is the smallest value for the inscribed period yet reported, and has been achieved with radical improvement in the quality of the inscribed nanostructures in comparison with previous reports.

Separation of Different Hydrogen-Bonded Clusters by Femtosecond UV-Ionization-Detected Infrared Spectroscopy: 1H-Pyrrolo[3,2-h]quinoline·(H2O)n=1,2 Complexes

Experimental and theoretical studies are presented for complexes of water with 1H-pyrrolo[3,2-h]quinoline (PQ), a bifunctional compound acting simultaneously as a hydrogen-bond donor and acceptor. A 1:1 complex, which is not fluorescent and only very short-lived in the electronically excited state, was analyzed by isolating the complex under supersonic jet conditions and characterizing its structure by infrared-induced ion depletion spectroscopy utilizing multiphoton ionization by femtosecond UV pulses (IR/fsMPI spectroscopy). On the other hand, a long-lived 1:2 complex was identified as the smallest microhydrate of PQ contributing to the laser-induced fluorescence excitation spectrum. Its structure was assigned by fluorescence-detected IR spectra and analyzed using density functional theory. The structures of the 1:1 and 1:2 clusters are assigned to species in which the water molecule(s) form a hydrogen-bonded solvent bridge between the two functional groups. In accord with calculations, both 1:1 and 1:2 PQ/water complexes reveal weaker hydrogen bonding than the analogous clusters of PQ with methanol.

Unravelling the ultrafast photodecomposition mechanism of dibenzoyl peroxide in solution by time-resolved IR spectroscopy

The ultrafast photo-fragmentation of dibenzoyl peroxide (DBPO) is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidate the dissociation mechanism. With the help of (13)C-labeled DBPO it was possible to unambiguously assign transient IR bands in the fingerprint region to the benzoyloxy radical. Our experiments show that the fragmentation is controlled by the S(1)-lifetime of DBPO and within 0.4 +/- 0.2 ps leads to a benzoyloxy/phenyl radical pair plus CO(2)via concerted bond breakage of the O-O and the phenyl-C(carbonyl) bond. 20% of the radical pairs geminately recombine to phenyl benzoate on a timescale of 70 ps.