UV Pulses Research Articles

Nonlinear Interaction of Electronic Excitations Created at High-Densities in Luminescent Materials and Scintillators

Nonlinear interaction of electronic excitations has been shown to play a remarkable role in luminescent material performance. Recently, it has been intensely studied in semiconductors and wide-gap scintillators. The nonlinear interaction is a cause of the nonproportionality of scintillator response to the energy of the absorbed ionizing radiation and can limit its energy resolution, which results in crucial deterioration of the scintillator functionality. The type and degree of nonproportionality are material specific and depend little on impurity content or sample treatment. Theoretical models have been developed, which explain the effect of scintillator response nonproportionality by the interplay between the density and mobility of charge carriers created in a track of ionizing radiation [1-3]. Energy losses in relaxation processes and the final distribution of excited luminescence centres in a material are determined by a number of processes including thermalisation of hot carriers, their recombination, interaction, trapping at defects and impurities and exciton formation. In the present contribution, we demonstrate how the mutual interaction of small-radius Frenkel excitons created at high densities by ionizing radiation or intense laser pulses can reveal itself in the luminescence characteristics of a luminescent material. It is shown that the related phenomena observed in various materials under X or γ rays are similar to those detected under UV-XUV excitation, which allows their theoretical description proceeding from the same principles and their experimental study in substantially better controlled experimental conditions of excitation by ultrashort laser pulses with well-defined timing and spatial parameters. A remarkable effect of saturation of excitonic absorption was discovered under excitation by ultrashort UV pulses from the energy region of phonon-assisted electronic transitions. The changes in the luminescence decay kinetics and various saturation effects in excitonic and impurity luminescence induced by high-density or high-energy excitation are demonstrated and theoretically modelled for several wide-gap materials. The experimental results of this contribution originate mainly from the study of time-resolved luminescence in tungstate crystals of different structures, wolframites CdWO4 and ZnWO4, and homologous scheelites CaWO4, SrWO4, BaWO4, by using femtosecond UV-XUV sources such as free-electron lasers, tuneable femtosecond lasers, high-order harmonic generation systems. It is shown that the emission decay and saturation can be described within the frame of Förster’s dipole-dipole interaction model on the basis of theoretical statements developed in [1]. The importance of studying exciton-exciton interactions is demonstrated by revealing a direct link between the Förster interaction radius and nonproportionality of the material light response. A good correlation between the radius of cation, radius of exciton and radius of Förster interaction is shown for the sequence of homologous scheelites CaWO4, SrWO4, BaWO4.

50-kHz, 50-ns UV pulse generation by diode-pumped frequency doubling Pr3+:YLF Q-switch laser with a Cr4+:YAG saturable absorber.

We demonstrate intracavity second-harmonic generation at 320 nm of a diode-pumped praseodymium-doped YLF laser Q-switched by a Cr4+:YAG crystal. By employing two 3.5-W high-power blue InGaN diode lasers as the pump source, we obtained 50-ns Q-switched pulses with a pulse energy of 1.54 μJ at a repetition rate of 50 kHz. A rate equation analysis shows good agreement with the experimental results.

Laser-Induced Periodical Structures Fabrication for Third Harmonic Generation

We propose a novel strategy for self-adjusted fabrication of large-scale array of resonant silicon nanoparticles (metasurface) on a thin silicon film. The self-adjusting mechanism is based on the effect of resonant nanogratings formation under intense multishot femtosecond irradiation of a thin silicon film. The resulting metasurfaces allow for generation of ultraviolet laser pulses at a wavelength of 270 nm with conversion efficiency up to 10-6 and high peak (≈100 kW/sm2) and average power (≈1.5 μW). Such high peak power from ultrathin metasurface makes the generated UV pulses applicable in a wide range of applications: precise nanolithography, ultrafast photoexcitation etc.

Full observation of ultrafast cascaded radiationless transitions from S2(ππ(∗)) state of pyrazine using vacuum ultraviolet photoelectron imaging.

A photoexcited molecule undergoes multiple deactivation and reaction processes simultaneously or sequentially, which have been observed by combinations of various experimental methods. However, a single experimental method that enables complete observation of the photo-induced dynamics would be of great assistance for such studies. Here we report a full observation of cascaded electronic dephasing from S2(ππ(*)) in pyrazine (C4N2H4) by time-resolved photoelectron imaging (TRPEI) using 9.3-eV vacuum ultraviolet pulses with a sub-20 fs time duration. While we previously demonstrated a real-time observation of the ultrafast S2(ππ(*)) → S1(nπ(*)) internal conversion in pyrazine using TRPEI with UV pulses, this study presents a complete observation of the dynamics including radiationless transitions from S1 to S0 (internal conversion) and T1(nπ(*)) (intersystem crossing). Also discussed are the role of (1)Au(nπ(*)) in the internal conversion and the configuration interaction of the S2(ππ(*)) electronic wave function.

A high-sensitivity, fast-response, rapid-recovery UV photodetector fabricated based on catalyst-free growth of ZnO nanowire networks on glass substrate

Here, we report for the first time the fabrication of metal–semiconductor–metal ultraviolet photodetector based on catalyst-free growth of ZnO nanowire networks on ITO seeds/glass substrates by thermal evaporation method. The morphological, structural, and optical properties of the sample were studied by using field emission scanning electron microscopy, X-ray diffraction, photoluminescence, and UV–Vis spectrophotometer. Upon exposure to 365 nm light (1.5 mW/cm2) at five-bias voltage, the device showed 2.32 × 103 sensitivity. In addition, the photocurrent was 1.79 × 10−4 A, and the internal gain of the photodetector was 24.2. The response and the recovery times were calculated to be 3.9 and 2.6 s, respectively, upon illumination to a pulse UV light (365 nm, 1.5 mW/cm2) at five-bias voltage. All of these results demonstrate that this high-quality detector can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications.

Detection of thin oil films on the water surface with the help of UV filaments

The fluorescence of thin oil films on the water surface induced by femtosecond UV laser pulses was studied experimentally. The laser pulses were considered in a wide range of intensities, including the radiation filamentation range. The applicability of the method to femtosecond UV pulses with two central wavelengths (248 and 372 nm) was compared. It was shown that the spatial resolution of the fluorescence localization was no worse than 30 cm.

Development and characterization of hybrid films based on agar and alizarin red S for applications as non-enzymatic sensors for hydrogen peroxide

For the first time, the dye alizarin red S (ARS) was immobilized on indium tin oxide (ITO) electrodes via a layer-by-layer technique (LbL). This was achieved only when ARS was interspersed with the polymers agar (extracted from seaweed Gracilaria birdiae) and PAH [poly(allylamine hydrochloride)]. ARS alone did not show electroactivity when adsorbed onto ITO. Single-walled carbon nanotubes (functionalized with COOH, denoted CNTs) were used to increase the electrochemical signal of the LbL system. Interactions at the molecular level between the CNTs and other materials used in the construction of the films accounted for a threefold increase in the current signal of ARS. The films were developed as trilayer films of agar/PAH/ARS or agar(CNT)/PAH(CNT)/ARS and characterized by differential pulse voltammetry (DPV) and UV–visible spectroscopy and scanning electron microscopy. From the results, it was also possible to calculate the energy diagram for both films. The results showed that the films are promising for applications as electrochemical sensors. Accordingly, the agar(CNT)/PAH(CNT)/ARS film was tested for the reduction of hydrogen peroxide (H2O2). Under a constant potential of −0.5 V versus SCE (saturated calomel electrode), the film exhibited a rapid response for the reduction of peroxide (less than 5 s), and the current stabilized approximately at 30 s. The limit of detection for the amperometric sensor was approximately 0.15 µmol L−1.

High efficiency single frequency 355 nm all-solid-state UV laser

A novel conductively cooled high energy single-frequency 355 nm all-solid-state UV laser is presented based on sum-frequency mixing technique. In this system, a pulsed seeder laser at 1064 nm wavelength, modulated by an AOM, is directly amplified by the cascaded multi-stage hybrid laser amplifiers, and two LBO crystals are used for the SHG and SFG, finally a maximum UV pulse energy of 226 mJ at 355 nm wavelength is achieved with frequency-tripled conversion efficiency as high as 55%, the pulse width is around 12.2 ns at the repetition frequency of 30 Hz. The beam quality factor M2 of the output UV laser is measured to be 2.54 and 2.98 respectively in two orthogonal directions.

Coherence and quasistable states in a strong infrared field

We study the quasistability of UV-pulse-train-excited H atoms in a strong infrared (IR) laser as a function of the phase delay of the UV pulse train relative to the IR laser. The UV pulse train contains two frequency components. When the two components have frequencies separated by two IR photons, the population of surviving electrons is modulated by up to ten percent. When electrons are excited to right above or below the threshold, the survival probabilities have inverted phase delay dependence, which can be explained classically. When the two frequencies are one IR photon apart, the angular symmetry of the quasistable electrons is broken, and the asymmetry is also controlled by the phase delay. The asymmetrical distribution can be observed while the IR is on and smoothly evolves to a nonzero asymmetry that only weakly depends on the duration of the IR field.

Drive laser system for the DC-SRF photoinjector at Peking University**Supported by National Basic Research Project (973) (2011CB808302, 2011CB808304)

Photoinjectors are widely used for linear accelerators as electron sources to generate high-brightness electron beams. The drive laser, which determines the timing structure and quality of the electron beam, is a crucial component of a photoinjector. A new drive laser system has been designed and constructed for the upgraded 3.5-cell DC-SRF photoinjector at Peking University. The drive laser system consists of a 1064 nm laser oscillator, a four-stage amplifier, second and fourth harmonic generators, an optical system to transfer the UV pulses to the photocathode, and a synchronization system. The drive laser system has been successfully applied during stable operation of the DC-SRF photoinjector and its performance meets requirements. A 266 nm laser with an average power close to 1 W can be delivered to illuminate the Cs2Te photocathode and the instability is less than 5% for long time operation. The design considerations for improving the UV laser quality, a detailed description of the laser system, and its performance are presented in this paper.

Research progress of the control and measurement of the atomic and molecular ultrafast electron dynamics using two-color field

The advent of the ultrafast laser pulse provides the powerful and efficient tool for probing the ultrafast electron dynamics in atoms and molecules. The various nonlinear process induced by the laser-matter interaction allows one to obtain the electron motion information on the sub-femtosecond time scale. A series of the ultrafast spectroscopic technique, such as attosecond streak camera, attosecond transient absorption spectrum, and etc., have been successfully applied to the probe of electron dynamics in atoms, molecules, and solids. Using two-color field is one of the significant methods to achieve the coherent control and exploring of the electron motion. This paper summarizes recent research activities in the field of the atomic and molecular ultrafast process investigated in State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, including the detection of the electron dynamics of the multi-bound states, measurement of the carrier envelope phase (CEP) and the phase of the attosecond pulse, and the ultrafast electron control with the THz/UV and MIR/IR field. To measure the dynamics of the multi-bound states, a broadband attosecond pulse can be used to ionize the electrons after it is excited by the pump laser. By changing the delay between the pump laser and the attosecond pulse, the measured electrons ionized by the broadband xuv attosecond pulse can present the multi-bound states dynamics simultaneously. The XUV/IR scheme is popularly used in attosecond dynamics measurement. But usually, the IR field is not very strong. We find that, if the IR field is strong enough to induce the above threshold ionization (ATI), the interference between the ATI electron and the electron from XUV pulse can be used to measure the CEP of the attosecond XUV pulse. Besides, if the electron ionized by attosecond pulse can be pushed back to the nuclei, the emission from the recombination can be used to determine the spectral phase of the attosecond pulse, which is an all-optical measurement. We also investigate the two color scheme of THz/UV and MIR/IR fields. With THz/UV two color scheme, very high electron localization can be achieved duration molecular dissociation when we use the UV pulse to excite the electron and the THz pulse to control the following electron movement. When we use the MIR/IR field to control the electron motion during the high harmonic generation, the recollision can be greatly decreased and the single attosecond pulse can be produced with multi-cycle MIR laser field.

Ultrafast carrier dynamics in a p-type GaN wafer under different carrier distributions

The dependence of the carrier distribution on photoexcited carrier dynamics in a p-type Mg-doped GaN (GaN:Mg) wafer were systematically measured by femtosecond transient absorption (TA) spectroscopy. The homogeneity of the carrier distribution was modified by tuning the wavelength of the UV pulse excitation around the band gap of GaN:Mg. The TA kinetics appeared to be biexponential for all carrier distributions, and only the slower component decayed faster as the inhomogeneity of the carrier distribution increased. It was concluded that the faster component (50–70 ps) corresponded to the trap process of holes by the Mg acceptors, and the slower component (150–600 ps) corresponded to the combination of non-radiative surface recombination and intrinsic carrier recombination via dislocations. Moreover, the slower component increased gradually with the incident fluence due to the saturation of surface states.

Selected problems in IR and UV laser micromachining of Si and GaAs in submillimeter scale

Results of laser micromachining of silicon and gallium arsenide in submillimeter scale were compared. Studies were performed using a laser beam with IR and UV wavelengths and pulses of femto- and nano-second duration. Investigation of effectiveness of micromachining with different parameters of laser beams was presented. Quality of surface and edges was estimated by means of an optical microscope and an electron microscope. Studies of surface profiles were performed using a confocal microscope. Results of micromachining were with good accordance with computer modeling.

Development of human hand phantom containing radiophotoluminescence material

A radiophotoluminescence (RPL) material with high radiation sensitivity was made of polyurethane resin, silver-activated metaphosphate glass particles, and hollow glass microspheres. The density was adjusted to be 1.1 g/cm3 by controlling the amount of hollow glass microspheres. The response to high-energy photons over 100 keV was similar to that of the tissue-equivalent material (polymethylmethacrylate) because the two electron densities were similar. The RPL response had satisfactory linearity in the dose range from 10 to 6 × 104 mGy.An RPL scanner for three-dimensional (3-D) dose measurement was composed of an XYZθ motorized stage, a UV pulse laser, a gated photomultiplier tube (PMT), a red-laser displacement sensor, and an integrating ammeter. The surface profile was measured by the red-laser-displacement sensor. The UV laser was used as an excitation source, and the RPL responses were effectively detected with the gated PMT.An RPL material hand phantom was fabricated to understand the extremity dosimetry of a radiation worker's hand. The hand phantom was exposed to X-rays, and its surface dose profile was obtained by the RPL scanner. Subsequently, the hand phantom was sliced into dozens of square plates using a diamond wire saw. Each inner dose profile was obtained with the RPL scanner. The inner dose profiles were roughly consistent with the computational simulation results. These results indicated that RPL imaging of the hand phantom was useful to understand extremity dosimetry.

DNA Damage in Bone Marrow Cells Induced by Femtosecond and Nanosecond Ultraviolet Laser Pulses.

The purpose of this study was to investigate the possible genotoxic impact of new generation 205 nm femtosecond solid-state laser irradiation on the DNA of murine bone marrow cells in vitro, and to compare the DNA damage caused by both femtosecond and nanosecond UV laser pulses. Recent experiments of corneal stromal ablation in vitro and in vivo applying femtosecond UV pulses showed results comparable with or superior to those obtained using nanosecond UV lasers. However, the possible genotoxic effect of ultrashort laser pulses was not investigated. Mouse bone marrow cells were exposed to different doses of 205 nm femtosecond, 213 and 266 nm nanosecond lasers, and 254 nm UV lamp irradiation. The comet assay was used for the evaluation of DNA damage. All types of irradiation demonstrated intensity-dependent genotoxic impact. The DNA damage induced depended mainly upon wavelength rather than on other parameters such as pulse duration, repetition rate, or beam delivery to a target. Both 205 nm femtosecond and clinically applied 213 nm nanosecond lasers' pulses induced a comparable amount of DNA breakage in cells exposed to the same irradiation dose. To further evaluate the suitability of femtosecond UV laser sources for microsurgery, a separate investigation of the genotoxic and mutagenic effects on corneal cells in vitro and, particularly, in vivo is needed.

ChemInform Abstract: From Ultrafast Structure Determination to Steering Reactions: Mixed IR/Non‐IR Multidimensional Vibrational Spectroscopies

AbstractReview: 146 refs.

Enhancing the branching ratios in the dissociation channels for O(16)O(16)O(18) molecule by designing optimum laser pulses: A study using stochastic optimization.

We propose a strategy of using a stochastic optimization technique, namely, simulated annealing to design optimum laser pulses (both IR and UV) to achieve greater fluxes along the two dissociating channels (O(18) + O(16)O(16) and O(16) + O(16)O(18)) in O(16)O(16)O(18) molecule. We show that the integrated fluxes obtained along the targeted dissociating channel is larger with the optimized pulse than with the unoptimized one. The flux ratios are also more impressive with the optimized pulse than with the unoptimized one. We also look at the evolution contours of the wavefunctions along the two channels with time after the actions of both the IR and UV pulses and compare the profiles for unoptimized (initial) and optimized fields for better understanding the results that we achieve. We also report the pulse parameters obtained as well as the final shapes they take.

From Ultrafast Structure Determination to Steering Reactions: Mixed IR/Non-IR Multidimensional Vibrational Spectroscopies.

Ultrafast multidimensional infrared spectroscopy is a powerful method for resolving features of molecular structure and dynamics that are difficult or impossible to address with linear spectroscopy. Augmenting the IR pulse sequences by resonant or nonresonant UV, Vis, or NIR pulses considerably extends the range of application and creates techniques with possibilities far beyond a pure multidimensional IR experiment. These include surface-specific 2D-IR spectroscopy with sub-monolayer sensitivity, ultrafast structure determination in non-equilibrium systems, triggered exchange spectroscopy to correlate reactant and product bands, exploring the interplay of electronic and nuclear degrees of freedom, investigation of interactions between Raman- and IR-active modes, imaging with chemical contrast, sub-ensemble-selective photochemistry, and even steering a reaction by selective IR excitation. We give an overview of useful mixed IR/non-IR pulse sequences, discuss their differences, and illustrate their application potential.

Fabrication and characterization of metal–semiconductor–metal ultraviolet photodetector based on rutile TiO2 nanorod

The fabrication and characterization of a metal–semiconductor–metal ultraviolet photodetector are studied. The photodetector is based on TiO2 nanorods (NRs) grown on p-type (111)-oriented silicon substrate seeded with a TiO2 layer is synthesized by radio frequency reactive magnetron sputtering. A chemical bath deposition is used to grow TiO2 NRs on Si substrate. The structural and optical properties of the obtained sample are analyzed by using X-ray diffraction and field emission-scanning electron microscopy. Results show a tetragonal rutile structure of the synthesized TiO2 NRs. Optical properties are further examined using photoluminescence spectroscopy. A sharp and high-intensity UV peak at 367nm is observed in comparison with visible defect peaks centered at 432 and 718nm. Upon exposure to 365nm light (2.3mW/cm) at 5V bias, the device displays 76.06×102 sensitivity, internal photodetector gain of 77.06, photocurrent of the device is 2.62×10−5A and photoresponse peak of 69.7mA/W. The response and recovery times are calculated as 18.5 and 19.1ms upon illumination to a pulse UV light (365nm, 2.3mW/cm2) at 5V applied bias. These results demonstrate that the fabricated high-quality photodiode is a promising candidate as a low-cost UV photodetector for commercially-integrated photoelectronic applications.

A novel gadolinium tetra(4-sulfonatophenyl)porphyrin–amino acid complex with interesting photophysical and electrochemical properties

A solvothermal reaction of GdCl3·6H2O, H6TPPS, and histidine led to the formation of a novel gadolinium tetra(4-sulfonatophenyl)porphyrin–amino acid hybrid, namely, {[Gd(His)(H2O)][Gd(H3O)3](H2TPPS)2} n ·3nH2O (1) [His = histidine; H6TPPS = tetra(4-sulfonatophenyl)porphyrin]. The molecular structure of complex 1, which was determined by single-crystal X-ray diffraction technique, shows that the porphyrin macrocycle is saddle-distorted and its center is non-metalated. The gadolinium ions show two kinds of coordination geometry, i.e., a distorted nine-coordinated monocapped square antiprism and a contorted square antiprism. Complex 1 features a three-dimensional structure which is constructed from the gadolinium ions connecting the porphyrin moieties together. The photoluminescence study reveals that it displays an emission in the red region. The cyclic voltammetry/differential pulse voltammetry, FT-IR, and UV–Vis of complex 1 were also investigated in detail. A gadolinium tetra(4-sulfonatophenyl)porphyrin–amino acid hybrid was prepared. It displays an emission in the red region