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The surface morphology of silicon solar cells irradiated with KrF excimer laser pulses (λ = 248 nm, τ = 20 ns) was investigated below the experimentally observed melting threshold fluence (F th) of 0.47 J cm−2 (±20%). At laser fluences of 0.23–0.48 J cm−2 (equivalent to 0.49 F th to 1.0 F th), nanodot structures with a height and width of approximately 60–120 nm were periodically formed with an interdot spacing similar to the laser wavelength. The observed nanodot density (29 dots μ m2) was higher than that previously obtained at longer wavelengths. Furthermore, crystallinity analysis by micro-Raman spectroscopy revealed a Raman shift of 519.56 cm−1 after irradiation (N = 1500 pulses), compared with 518.27 cm−1 prior to irradiation. A laser fluence of 0.41 J cm−2 (= 0.87 F th) was found to induce compressive stress on the silicon solar cell surface. [ABSTRACT FROM AUTHOR]

We propose an original technique for the grating metasurfaces fabrication by low-power ultraviolet laser treatment of fluorinated graphene (FG) films with the focus on terahertz applications. The laser treatment reduces dielectric FG to its conductive counterparts, increasing DC conductivity to 170 S·m−1 for treated areas. The electromagnetic response of the grating metasurfaces studied by THz time-domain spectroscopy in the 100 GHz–1 THz frequency range demonstrates enhanced resonant transmittance through metasurfaces. The intensity and position of transmittance peak could be tuned by changing the metasurface geometry, i.e. the period of the structure and width of the reduced and unreduced areas. In particular, the decrease of the reduced FG area width from 400 to 170 μ m leads to the shift of the resonance peak from 0.45 THz to the higher frequencies, 0.85 THz. Theoretical description based on the multipole theory supported by finite element numerical calculations confirms the excitation of the dark state in the metasurface unit cells comprising reduced and unreduced FG areas at resonance frequency determined by the structure geometrical features. Fabricated metasurfaces have been proved to be efficient narrowband polarizers being rotated by 50° about the incident THz field vector. [ABSTRACT FROM AUTHOR]

Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually, thereby greatly enhancing the scope of Stark decelerated molecules. For comparison, we detect decelerated and trapped ammonia molecules using two different schemes: (i) nonresonant multiphoton ionization using intense femtosecond (fs) pulses in the near infrared, and (ii) resonance-enhanced multiphoton ionization using nanosecond (ns) pulses from a tunable UV laser. The observed number of ions per shot for both schemes is similar. The fs laser detection scheme suffers from an increased background, which can be effectively eliminated by subsequent mass and velocity selection. To determine the detection volume of the ns laser detection scheme, we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below ∼100 ÎĽm. It is concluded that the detection volume for the ns laser detection scheme is 1.5â€"2 times larger than that of the fs laser detection scheme. [ABSTRACT FROM AUTHOR]

In this paper, a facile method was introduced to fabricate fine conductive patterns by low-temperature selective laser sintering of Cu nanoparticles. By virtue of a nanosecond-pulsed ultraviolet laser source, fine circuits with a thickness of ∼8 μm and a conductivity of 3–6.5 × 106 S m−1 was successfully fabricated from Cu nanoparticle paste with a high metal content >80 wt.% on a flexible substrate at a wide scan rate range of 5–500 mm s−1. The sintered circuits exhibited a special sandwich morphology, with fully melted features in the centre and thermally sintered neck-connected features on the edges. A twofold linear relationship between the width of thermally sintered region and the reciprocal of the laser scan rate was revealed, indicating a heat dissipation mode transition from metal layer dominated dissipation to substrate dominated dissipation with the decrease of the laser scan rate. Finite element simulations were carried out to study the evolutions of temperature field and heat dissipation with changing laser scan rate and power, and the results fitted well with experimental results. On this basis, a relational expression was further proposed to determine the optimal processing window for laser sintering of metal nanoparticle layers. Our method extends the producible thickness and improves the fabrication efficiency of laser-printed circuits with desirable conductivity. The findings of this study can provide a guidance for understanding and controlling the sintering and heat transfer process of printing methods with similar materials and techniques. [ABSTRACT FROM AUTHOR]

The paper presents simulation results of hydrogen-air mixture ignition by an ultraviolet followed by a near-infrared laser pulse. We used the three-temperature plasma model in a combination with the beam propagation method. Effect of plasma defocusing on the ignition kernel development is reported. It is shown that the plasma generated by the pre-ionizing pulse affects the position of the focal region of the second heating pulse with resulting changes in the distribution of the laser energy absorption, electron and radical production and ignition kernel dynamics. For the case with the defocusing effect we observed a shift of the peaks of translational and vibrational temperatures towards the direction of the laser pulse and two peaks on the axial profile of the electron temperature, reflecting local changes both in Joule heating and electron energy losses. [ABSTRACT FROM AUTHOR]

Aluminium (Al) composition is a critical parameter of performance for deep ultraviolet (DUV) AlGaN devices. In multiple-quantum-wells (MQW) nanowire laser diode, quantum barriers play an important role in carriers' flow (electrons, holes). In this work, the impact of composition, and the thickness of AlGaN-based quantum barriers is being studied. The study suggests that by properly increasing Al composition of quantum barriers, the height of the conduction band barrier decreases, which enhances electron leakage from MQW, while increasing the height of the valance band barrier, which, in turn, suppresses hole injection efficiency toward MQW. Such variations in band height greatly affect opto-electronic characteristics and performance of AlGaN nanowire. Furthermore, the study also suggests that quantum barrier thickness has much influence on DUV nanowire performance. The decrease in thickness of quantum barriers has little effect on confinement of the optical field, but significant effect on optical emitted power. It is found that by decreasing the thickness of quantum barriers, optical emitted power along with stimulated recombination rate is reduced. Contrastingly, as the thickness of quantum barriers increases, hole injection efficiency gets enhanced while electron leakage from MQW gets reduced. The above observations suggest that the higher the thickness of quantum barriers, the better the carriers' flow toward MQW and hence, the performance is improved. [ABSTRACT FROM AUTHOR]

High-intensity extreme ultraviolet (XUV) pulses from a free-electron laser can be used to create a nanoplasma in clusters. In reference Michiels et al (2020 Phys. Chem. Chem. Phys. 22 7828–34) we investigated the formation of excited states in an XUV-induced nanoplasma in ammonia clusters. In the present article we expand our previous study with a detailed analysis of the nanoplasma evolution and ion kinetics. We use a time-delayed UV laser as probe to ionize excited states of H and in the XUV-induced plasma. Employing covariance mapping techniques, we show that the correlated emission of protons plays an important role in the plasma dynamics. The time-dependent kinetic energy of the ions created by the probe laser is measured, revealing the charge neutralization of the cluster happens on a sub-picosecond timescale. Furthermore, we observe ro-vibrationally excited molecular hydrogen ions being ejected from the clusters. We rationalize our data through a qualitative model of a finite-size non-thermal plasma. [ABSTRACT FROM AUTHOR]

Flexible photodetectors based on two-dimensional (2D) materials hold the promise to open up an unprecedented era for wearable optoelectronic systems. However, there is a major challenge of the inability to directly deposit 2D materials on polymeric substrates due to their high growth temperature. Here, we demonstrate a novel low-cost method to directly fabricate SnS nanoflakes on the poly-ethylene terephthalate substrates. Moreover, the flexible photodetectors based on SnS nanoflakes feature broadband spectral responses from 355 nm to 1550 nm. The responsivity and detectivity can approach up to 1.28 × 103 A W−1 and 3.02 × 1011 Jones, respectively, under the ultraviolet laser of 355 nm. The devices also exhibit excellent NIR photodetection with a specific responsivity of 69 A W−1 towards the 1550 nm regime. Our research represents an important step toward scalable fabrication of the SnS nanoflakes and this method may pave a path for the future commercialization of flexible broadband optoelectronic devices. [ABSTRACT FROM AUTHOR]

Michel Barat passed away in November 2018 at the age of 80 after a rich career in atomic and molecular collisions. He had participated actively in formalizing the electron promotion model, contribuing to low energy reactive collisions at the frontier of chemistry. He investigated electron capture mechanisms by highly charged ions (HCI), switched to collision induced cluster dissociation and finally to UV laser induced fragmentation mechanisms of biological molecules. During this highly active time he created a laboratory, organized ICPEAC and participated actively in the administration of research. This paper covers the 10 years when he mentored my scientific activity in the blossoming field of electron capture by HCI. In spite of an impressive number of open channels, Michel found a way to capture the important parameters and to simplify the description of several electron capture processes; orientation propensity, electron promotion, true double electron capture, transfer ionisation, transfer excitation, formation of Rydberg states, and electron capture by metastable states. Each time Michel established fruitful collaborations with other groups. [ABSTRACT FROM AUTHOR]

The 3p43PJ–3p34p3PJ transition in the sulphur atom is investigated in a precision two-photon excitation scheme under Doppler-free and collision-free circumstances yielding an absolute accuracy of 0.0009 cm−1, using a narrowband pulsed laser. This verifies and improves the level separations between amply studied odd parity levels with even parity levels in S I. An improved value for the 3P2–3P1 ground state fine structure splitting is determined at 396.0564 (7) cm−1. A 34S–32S atomic isotope shift was measured from combining time-of-flight mass spectrometry with laser spectroscopy. [ABSTRACT FROM AUTHOR]

Direct laser writing is a lithography technique for fabricating features in sub-micron dimensions. Major advantages of this technique are the elimination of mask, cost-effectiveness and design scalability. Patterns with different dimensions and geometries can be fabricated using this technique, which are greatly influenced by several laser parameters such as the intensity of the laser, speed of the stage (substrate), and focus level of the laser. To achieve high accuracy, the effects of the operative parameters on width are required to be mapped. In this work, the effect of various parameters, such as laser intensity, stage speed, and focus level (Z-axis movement), on the features of the micropatterns has been investigated. The laser intensity was varied from 25 000 W m−2 to 65 000 W m−2, and the stage speed was varied from 20 steps per minute to 200 steps per minute. The focus level was altered from the far limit of the Z-axis and incremented by a value of 0.1 mm. The patterns were created on glass substrates coated with negative dry film photoresist. The geometry of the obtained patterns was analyzed using optical microscopy and scanning electron microscopy. Subsequently, the statistical tools were employed to frame empirical relationships of the parameters with feature size as a response function. The construction of the efficient experimental combinations has been performed using design of experiments and the response values were analyzed using tools such as analysis of means and analysis of variance. Except knowing the significance of all the individual parameters, it was observed that the focus level highly influenced the feature size compared to the intensity of laser and stage speed. The applications of these devices in the fields of droplet based microfluidics and interdigitated electrodes for electrochemical sensing have also been discussed. [ABSTRACT FROM AUTHOR]

We develop a high-performance ultraviolet (UV) frequency stabilization technique implemented directly on UV diode lasers by combining the dichroic atomic vapor laser lock and the resonant transfer cavity lock. As an example, we demonstrate a stable locking with measured frequency standard deviations of approximately 200 kHz and 300 kHz for 399 nm and 370 nm diode lasers in 20 min. We achieve a long-term frequency drift of no more than 1 MHz for the target 370 nm laser within an hour, which is further verified with fluorescence count rates of a single trapped 171Yb+ ion. We also find strong linear correlations between lock points and environmental factors such as temperature and atmospheric pressure. Our approach provides a simple and stable solution at a relatively low cost, and features flexible control, high feedback bandwidth and minimal power consumption of the target UV laser. [ABSTRACT FROM AUTHOR]

The characteristics of terahertz (THz) radiation generated from large-aperture photoconductive antennas (LAPCAs) were investigated. The antennas were fabricated using different wide-bandgap semiconductor crystals (ZnSe, GaN, 6H–SiC, 4H–SiC and β–Ga2O3) as the substrate. We used an amplified sub-picosecond KrF excimer laser for illumination of the LAPCAs. THz emission scaling was studied as a function of the bias field and the pump laser energy. It was found that the radiated THz energy scales quadratically as a function of the bias field and sub-linearly as a function of the optical fluence for most of the substrates. Further, we demonstrate that SiC, and especially 4H–SiC LAPCAs offer the best THz generation performances. In order to generate intense THz radiation, we fabricated both 6H- and 4H–SiC LAPCAs with an interdigitated structure. From the field autocorrelation trace, it was found that the spectra lie in the sub-THz regime, extending up to 400 GHz, with a peak frequency at 50 GHz, making the bridge between the microwaves band and the THz band. The maximum generated THz energy was 11 μJ, which is to date the highest THz energy measured from LAPCA sources, with a corresponding peak electric field of 115 kV cm−1 and a corresponding ponderomotive potential of 60 eV. Nonlinear THz experiments were performed using these energetic THz pulses, and open aperture Z-scan experiments in an n-doped InGaAs layer revealed a transmission enhancement of 1.7. It was also shown that in order to have efficient THz generation, the energy contrast of the laser must be kept high. [ABSTRACT FROM AUTHOR]

The evolution of the electron energy distribution function in the plasma channel created in air by a UV-laser pulse of femtosecond duration is studied. It is shown that such a channel can be used to amplify few-cycle electromagnetic pulses in the subterahertz frequency range at the time of relaxation of the energy spectrum in air determined by the vibrational excitation of the nitrogen molecules. The gain factor as a function of time, electron concentration and frequency of the amplifying radiation is obtained. The propagation of few-cycle radio-frequency pulses through the amplifying medium is analysed. [ABSTRACT FROM AUTHOR]

State-of-the art pulsed UV excimer laser technology has a proven track record of stable performance under three-shift operation conditions in various industrial thin-film applications at stabilized pulse energy levels ranging from 100 to 1000 mJ per pulse. For the last 25 years excimer laser technology has formed the backbone of pulsed laser deposition advances. The available UV average power level nowadays exceeds 1 kW and novel excimer laser beam utilization schemes in combination with sophisticated substrate–target geometries are ready to utilize the full laser upscaling potential and thereby reduce unit costs. [ABSTRACT FROM AUTHOR]

Optical triggering of high-voltage (18 kV-class) 4H-SiC thyristors with a single amplification step (pilot thyristor) is reported. It is demonstrated that the switch-on processes in such high-voltage structures are quite different from the switch-on processes in 12 kV 4H-SiC thyristors. In particular, the switch-on process occurs in two stages even at maximum light pulse energy and rather high anode biases. [ABSTRACT FROM AUTHOR]

Xenon atoms are double-ionized by sequential two-photon absorption by ultrashort extreme ultraviolet free-electron laser pulses with a photon energy of 23.0 and 24.3 eV, produced by the SPring-8 Compact SASE Source test accelerator. The angular distributions of photoelectrons generated by two-photon double ionization are obtained using velocity map imaging. The results are reproduced reasonably well by the present theoretical calculations within the multi-configurational Dirac-Fock approach. [ABSTRACT FROM AUTHOR]

We report on the laser power dependence of photoabsorptions of Ar clusters irradiated by extreme ultraviolet-free electron laser (EUV-FEL) at the wavelength of 62 nm. We measured kinetic energy distributions of fragment ions with our ion momentum spectrometer. From the measured kinetic energy distributions, we estimated charge states of irradiated Ar clusters using the classical uniformly charged sphere model. Our results show that the EUV-FEL irradiation of Ar clusters results in the frustration of direct photoionization leading to nanoplasma formation and subsequent strong electron-ion charge recombination. [ABSTRACT FROM AUTHOR]

Time-resolved extreme-ultraviolet (EUV) fluorescence spectroscopy has been applied to study the multi-photon, single ionization of Ne irradiated by intense EUV-free-electron laser (FEL) pulses at a wavelength of 51 nm. A broad, intense peak at a wavelength of around 46 nm is observed, which is shorter than the incident FEL wavelength. The time dependence of the fluorescence reveals that the peak has two unresolved components, which we attribute to the decay of the excited ion states Ne+ 2s-1(²S) and 2p-2(³Pe)3s(²Pe) at 46.0 and 44.6 nm. From the observed intensity ratios and fluorescence lifetimes we conclude that the Ne+ 2p-2(³Pe)3s(²Pe) state is populated by two-photon resonance enhancement, via a 2p43p² doubly excited state of Ne. [ABSTRACT FROM AUTHOR]

The two-photon ionization of helium atoms by ultrashort extreme-ultraviolet free-electron laser pulses, produced by the SPring-8 Compact SASE Source test accelerator, was investigated at photon energies of 20.3, 21.3, 23.0 and 24.3 eV. The angular distribution of photoelectrons generated by two-photon ionization is obtained using a velocity map imaging spectrometer. The phase-shift differences and amplitude ratios of the outgoing s and d continuum wave packets are extracted from the photoelectron angular distributions. The obtained values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to a resonance with an excited level or Rydberg manifold. The difference stems from the co-presence of resonant and non-resonant path contributions in the two-photon ionization by femtosecond pulses. Since the relative contribution of both paths can be controlled in principle by the pulse shape, these results illustrate a new way to tailor the continuum wave packet. [ABSTRACT FROM AUTHOR]

Herein we describe how UV excitation of localized electronic states in phosphate glasses can activate structural rearrangements that influence the kinetics of Au nanoparticle (NP) thermal growth in Au-doped glass. The results suggest a novel strategy to address the problem of controlling nano-assembly processes of metal NP patterns in fully inorganic and chemically stable hard materials, such as laser-quality glasses. We show that the mechanism is promoted by opening and subsequent cross-linkage of phosphate chains under UV excitation of non-bridging groups in the amorphous network of the glass, with a consequent modification of Au diffusion and metal NP growth. Importantly, the micro-Raman mapping of the UV-induced modifications demonstrates that the process is restricted within the beam waist region of the focused UV laser beam. This fact is consistent with the need for more than one excitation event, close in time and in space, in order to promote structural cross-linkage and Au diffusion confinement. The stability of the photo-induced modifications makes it possible to design new metal patterning approaches for the fabrication of three-dimensional metal structures in laser-quality materials for high-power nonlinear applications. [ABSTRACT FROM AUTHOR]

Solar flares typically have an impulsive phase that is followed by a gradual phase as best seen in soft X-ray emissions. A recent discovery based on the EUV Variability Experiment observations on board the Solar Dynamics Observatory (SDO) reveals that some flares exhibit a second large peak separated from the first main phase peak by tens of minutes to hours, which is coined as the flare's EUV late phase. In this paper, we address the origin of the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband imaging observations from the Atmospheric Imaging Assembly on board SDO. We find that (1) the late phase emission originates from a different magnetic loop system, which is much larger and higher than the main phase loop system. (2) The two loop systems have different thermal evolution. While the late phase loop arcade reaches its peak brightness progressively at a later time spanning for more than one hour from high to low temperatures, the main phase loop arcade reaches its peak brightness at almost the same time (within several minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be connected magnetically, forming an asymmetric magnetic quadruple configuration. (4) Further, the footpoint brightenings in UV wavelengths show a systematic delay of about one minute from the main flare region to the remote footpoint of the late phase arcade system. We argue that the EUV late phase is the result of a long-lasting cooling process in the larger magnetic arcade system. [ABSTRACT FROM AUTHOR]

Doppler-free dichroic locking (DFDL) spectroscopy on the ¹S0-³P1 transition of neutral mercury (Hg) atoms is observed in a 5 mm long vapour cell. The performance of the DFDL signal, such as its slope and amplitude, is investigated in detail. The optimal axial magnetic field and cell temperature (optical depth) for the observed seven transitions are also highlighted. We also demonstrate an important application of DFDL spectroscopy in the frequency stabilization of an ultraviolet laser on the transition of 200Hg. [ABSTRACT FROM AUTHOR]

In inertial fusion shock ignition, separation of the stages of fuel compression and hot spot creation introduces some degree of design flexibility. A lower implosion velocity can be compensated for by a more intense ignition pulse. Flexibility increases with target (and driver) size and allows for a compromise between energy gain and risk reduction. Having designed a reference ignition target, we have developed an analytical model for (up)-scaling targets as a function of laser energy, while keeping under control parameters related to hydro- and plasma instabilities. Detailed one-dimensional simulations confirm the model and generate gain curves. Options for increasing target robustness are also discussed. The previous results apply to UV laser light (with wavelength λ = 0.35μm). We also show that our scaling model can be used in the design of targets driven by green laser light (λ = 0.53μm). [ABSTRACT FROM AUTHOR]

The aim of this work is to produce 2D plasmonic and diffractive structures in Ag films with sharp features for which both a deeper understanding of laser induced transformation upon modulated laser intensity and a correlation between structural and optical properties are required. We compare results obtained by exposing silver films to an excimer laser operating at 193 nm whose intensity is either modulated or homogeneous. In all cases, one laser exposure is enough to break the film into nanoparticles (NPs). The use of the modulated beam intensity leads to diffractive 2D patterns that are formed by rectangular regions of untransformed material surrounded by transformed regions covered by NPs. The former have sharp edges that are consistent with the absence of significant mass transport that is discussed in terms of the thermal gradient induced. The latter contain NPs whose diameter increases as the initial film effective thickness increases. The surface plasmons associated with the NPs in the transformed regions dominate the reflectivity spectrum and the 2D array formed by the untransformed regions is responsible for the diffractive properties. Evidence for spinodal dewetting is only observed in our case for the steep gradient conditions achieved at the border of the homogeneously irradiated regions. [ABSTRACT FROM AUTHOR]

We present photoionization angular distributions of oriented symmetric (H2+) and asymmetric (HHe2+) molecular ions from numerical solutions of two dimensionally reduced time-dependent Schrödinger equations as a function of ionized electron kinetic energies with intense circularly polarized attosecond extreme ultraviolet laser pulses. Simulations for the equilibrium H2+ show that for single-photon ionization, relative intensities of parallel and perpendicular (to the molecular axis) photoionizations are critically sensitive to laser and photoelectron wavelengths due to ionization rates strongly depending on molecular geometry. Photoionization angular distributions are rotated to particular angles indicating high-order angular momenta and strong molecular potential effects. High-energy Ee spectra show effects of laser-induced electron diffraction (LIED) as the ionized electron wavelengths approach the internuclear distance. For HHe2+, such laser parameter sensitivity is reduced due to strong localization of the electron on one atom, but strong asymmetric LIED patterns are obtained. Angular distributions for both H2+ and HHe2+ are also shown to be dependent on laser-pulse ellipticity and photoelectron energies. [ABSTRACT FROM AUTHOR]

We study time-dependent populations in the excited states of many-electron atoms and molecules with the multiconfiguration time-dependent Hartree–Fock method and reveal the non-stationary behavior of the excited-state populations during field-free propagation originating from the nonlinear character of the equations of motion. We calculate the time-dependent populations of the three lowest singlet S states in a helium atom for two different cases of intense laser-atom interaction. In the first case, He is excited by a VUV pulse with the central wavelength of 108 nm, which is two-photon resonant with the transition. In the second case, He is excited by an off-resonant UV laser pulse with the central wavelength of 200 nm. We obtain the stationary wave functions of the excited states by imaginary time propagation and show that the populations oscillate as a function of time even after the laser pulse has vanished. The time-dependent variations in the populations in the respective eigenstates can be sufficiently suppressed when the number of orbitals adopted in the expansion of the wave function exceeds eight. [ABSTRACT FROM AUTHOR]

To improve the optical and electrical properties of AlGaN-based deep ultraviolet lasers, an inverse-trapezoidal electron blocking layer is designed. Lasers with three different structural electron blocking layers of rectangular, trapezoidal and inverse-trapezoidal structures are established. The energy band, electron concentration, electron current density, P–I and V–I characteristics, and the photoelectric conversion efficiency of different structural devices are investigated by simulation. The results show that the optical and electrical properties of the inverse-trapezoidal electron blocking layer laser are better than those of rectangular and trapezoidal structures, owing to the effectively suppressed electron leakage. [ABSTRACT FROM AUTHOR]

The design of the active region structures, including the modifications of structures of the quantum barrier (QB) and electron blocking layer (EBL), in the deep ultraviolet (DUV) AlGaN laser diode (LD) is investigated numerically with the Crosslight software. The analyses focus on electron and hole injection efficiency, electron leakage, hole diffusion, and radiative recombination rate. Compared with the reference QB structure, the step-like QB structure provides high radiative recombination and maximum output power. Subsequently, a comparative study is conducted on the performance characteristics with four different EBLs. For the EBL with different Al mole fraction layers, the higher Al-content AlGaN EBL layer is located closely to the active region, leading the electron current leakage to lower, the carrier injection efficiency to increase, and the radiative recombination rate to improve. [ABSTRACT FROM AUTHOR]

We demonstrate a 3.65 W, 40 kHz, 17.5 ns end-pumped YVO4/Nd:YVO4 ultraviolet (UV) laser at 355 nm with a beam propagation factor of 1.32 and 1.14. The frequency doubling and tripling are obtained by employing a parallel surfaces LBO crystal and a Brewster angle-cut uncoated LBO crystal in a convex–concave cavity. The arrangement allows the use of optically-uncoated third harmonic generation LBO crystal surfaces while providing wavelength separation. It is proved that the optically-uncoated THG-LBO crystal has a higher damage threshold. Pump-UV conversion efficiency of 15.5% is obtained and it is higher compared with other intra-cavity-wavelength-separation UV nanosecond lasers. By designing a convex–concave cavity structure, four advantages are showed, (1) to compensate for astigmatism caused by the Brewster angle-cut uncoated LBO crystal; (2) to offset the thermal lens effect of the gain medium; (3) to obtain a fundamental light spot size which matches with pump light spot size in the center of gain medium; (4) to obtain a small spot size in the center of the frequency conversion crystals. By employing these designs, UV beam quality Mx2 = 1.32 and My2 = 1.14 at 3.65 W are obtained. [ABSTRACT FROM AUTHOR]

Thermal transport properties of GaN heteroepitaxial structures are of critical importance for the thermal management of high-power GaN electronic and optoelectronic devices. Ultraviolet (UV) lasers are employed to directly heat and sense the GaN epilayers in the transient thermoreflectance (TTR) measurement, obtaining important thermal transport properties in different GaN heterostructures, which include a diamond thin film heat spreader grown on GaN. The UV TTR technique enables rapid and non-contact thermal characterization for GaN wafers. [ABSTRACT FROM AUTHOR]

A double-tapered AlGaN electron blocking layer (EBL) is proposed to apply in a deep ultraviolet semiconductor laser diode. Compared with the inverse double-tapered EBL, the laser with the double-tapered EBL shows a higher slope efficiency, which indicates that effective enhancement in the transportation of electrons and holes is achieved. Particularly, comparisons among the double-tapered EBL, the inverse double-tapered EBL, the single-tapered EBL and the inverse single-tapered EBL show that the double-tapered EBL has the best performance in terms of current leakage. [ABSTRACT FROM AUTHOR]

We report on the precision measurement of aluminum atoms transition at 394 nm and transition at 396 nm in a hollow-cathode lamp. Both absorption and saturated absorption spectra are measured. From the absorption spectra, the Doppler linewidth is estimated to be 2.65 GHz. The saturated absorption spectra are analyzed based on a velocity-changing collisions model. With a frequency comb calibrated wavemeter, the frequencies of transition and transition are measured to be 759.905 204(1) THz and 756.547 133(3) THz, respectively. The hyperfine coupling constants of aluminum atoms are determined, and are compared with previously reported measurement results and theoretical calculation results. Reasonable agreement is found for the magnetic dipole constant (A constant), while the electric quadrupole constant (B constant) has a large deviation. [ABSTRACT FROM AUTHOR]

The waveguide strain and the surface morphologies of AlGaN-based laser heterostructures emitting in the deep UV spectral range have been investigated. In particular, the impact of the AlGaN heterostructure design on the strain relaxation as well as the effect of the growth temperature on the surface morphology were explored. We found strain-induced plastic relaxation for laser heterostructures with 130 nm thick Al0.45Ga0.55N waveguide layers, whereas pseudomorphic growth was obtained for laser heterostructures with Al0.70Ga0.30N waveguide layers. Optically pumped lasing near 280 nm was demonstrated for the coherently grown laser heterostructures. A strong correlation of the surface morphology with the waveguide growth conditions was also observed. Low growth temperatures of 900 °C lead to a high density of V-pits originating from dislocations. By increasing the growth temperatures to 1070 °C the V-pit density significantly decreases, resulting in a more than two-fold reduction of the threshold power density of optically pumped lasers. [ABSTRACT FROM AUTHOR]

By combining the vacuum ultraviolet laser pulsed field ionization-photoion (VUV-PFI-PI) ion source with the double quadruple-double octopole (DQDO) ion-guided mass spectrometer, we have investigated the center-of-mass collision energy (Ecm) and vibrational-state dependences of the ion-molecule reactions of and and 100) + N2 covering the Ecm range of 0.04–10.00 eV. The absolute integral cross sections (σ’s) for the charge transfer (CT) [σCT(v+)] channel to form H2O+ and the H-atom transfer (HT) [σHT(v+)] channel to form N2H+ from the reactions have been determined, revealing the dominance of σCT(v+) over σHT(v+) at Ecm = 0.04–8.00 eV. The Ecm dependence of σCT(v+) at low Ecm < 1.00 eV is consistent with the long-range ion-dipole and ion-induced dipole CT mechanism. Minor vibrational inhibition is observed for the σCT(v+) at low Ecm ≤ 0.30 eV, which can be rationalized by the near-resonance CT mechanism. While the σHT(v+) values are consistent with previous measurements, the σCT(v+) obtained here resolve a hump at Ecm = 1.0–5.0 eV, which is not observed previously. This feature is attributed to the formation of excited H2O+(B2B2) ions via the collision-assisted CT mechanism. The branching ratio for product H2O+[BR(H2O+)] is found to be constant (0.82 ± 0.05) at Ecm = 0.04–1.00 eV, and is independent of v+ vibrational state. As Ecm is increased from 1.0 eV, the BR(H2O+) reaches a maximum of 0.93 at Ecm ≈ 3.00 eV, followed by the decline to 0.20 at Ecm ≥ 9.0 eV, where σHT(v+) becomes dominant compared to σCT(v+). The ) for the formation of N2H+ via the proton transfer (PT) channel of the H2O+(X2B1: 000 and 100) + N2 reaction has also been measured. The comparison of the σPT(000 and 100) values reveals significant (100) vibrational enhancement. Furthermore, the Ecm thresholds determined here for σPT(000 and 100) are in agreement with their thermochemical thresholds. The BR and σ values determined here are valuable for modeling the ion chemistry occurring in planetary atmospheres, in addition to serving as benchmarks for state-of-the-art quantum dynamics calculations. [ABSTRACT FROM AUTHOR]

We report on the experimental investigation and theoretical analysis of a nanosecond pulse high power ultraviolet (UV) 278 nm laser by fourth-harmonic generation (FHG) of a 1112-nm Nd:YAG amplifier in LiB3O5 (LBO) and CsB3O5 (CBO) crystals. The UV laser delivers a maximum average power of 10.3 W at 278 nm with peak power of 36.8 kW under input pump power of 41 W at 556 nm. This is, to the best of our knowledge, the highest output power at the specific UV wavelength of 278 nm. We also performed the theoretical investigation on the FHG with a model in the Gaussian approximation of both spatial and temporal profiles, especially accounting for the two-photon absorption effect in CBO crystal for the first time. The average output power, pulse width, and beam spatial distribution of the UV laser were simulated. The theoretical calculations are in close agreement with the experimental results. [ABSTRACT FROM AUTHOR]

In order to suppress the electron leakage to p-type region of near-ultraviolet GaN/InxGa1–xN/GaN multiple-quantumwell (MQW) laser diode (LD), the Al composition of inserted p-type AlxGa1–xN electron blocking layer (EBL) is optimized in an effective way, but which could only partially enhance the performance of LD. Here, due to the relatively shallow GaN/In0.04Ga0.96N/GaN quantum well, the hole leakage to n-type region is considered in the ultraviolet LD. To reduce the hole leakage, a 10-nm n-type AlxGa1–xN hole blocking layer (HBL) is inserted between n-type waveguide and the first quantum barrier, and the effect of Al composition of AlxGa1–xN HBL on LD performance is studied. Numerical simulations by the LASTIP reveal that when an appropriate Al composition of AlxGa1–xN HBL is chosen, both electron leakage and hole leakage can be reduced dramatically, leading to a lower threshold current and higher output power of LD. [ABSTRACT FROM AUTHOR]

The anisotropy of shales plays a significant role in oil and gas exploration and engineering. Owing to various problems and limitations, anisotropic properties were seldom investigated by direct current resistivity methods. Here in this work, a 248 nm ultraviolet laser was employed to assess the anisotropic electrical response of a dielectric shale. Angular dependence of laser-induced voltages (Vp) were obtained, with a data symmetry at the location of 180° and a ~62.2% Vp anisotropy of the sample. The double-exponential functions have provided an explanation for the electrical field controlled carrier transportation process in horizontal and vertical directions. The results demonstrate that the combination of optics and electrical logging analysis (Opti-electrical Logging) is a promising technology for the investigation of unconventional reservoirs. [ABSTRACT FROM AUTHOR]

A hundred picosecond ultraviolet (UV) laser is demonstrated with a pulse duration of less than 200 ps and peak power of 0.6 GW. With a hundred picosecond stimulated Brillouin scattering compressed pulse as the fundamental light, the UV output at 355 nm is obtained by extra-cavity sum-frequency-mixing in two LiB3O5 crystals. Maximum UV energy was 100 mJ when the incident energy was 280 mJ, yielding an optical-to-optical efficiency of 35.7%. This result is of interest for the generation of high energy sub-nanosecond UV lasers which finds applications in shock ignition and industrial processing. [ABSTRACT FROM AUTHOR]

We demonstrate frequency stabilization of a tunable 318.6 nm ultraviolet (UV) laser system using electronic sideband locking. By indirectly changing the frequency of a broadband electro-optic phase modulator, the laser can be continuously tuned over 4 GHz, while a 637.2 nm laser is directly stabilized to a high-finesse ultra-stable optical cavity. The doubling cavity also remains locked to the 637.2 nm light. We show that the tuning range depends mainly on the gain-flattening region of the modulator and the piezo-tunable range of the seed laser. The frequency-stabilized tunable UV laser system is able to compensate for the offset between reference and target frequencies, and has potential applications in precision spectroscopy of cold atoms. [ABSTRACT FROM AUTHOR]

Electron localization in the dissociation of the symmetric linear molecular ion is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2p by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3p state. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current (DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton. [ABSTRACT FROM AUTHOR]

A dc electric field is utilized to steer the electron motion after the molecular ion is excited by an ultrashort ultraviolet laser pulse. The numerical simulation shows that the electron localization distribution and the dissociation control ratio are dependent on the polarization direction and amplitude of the dc electric field. Most electrons of the dissociation state move opposite to the dc electric field and stabilize at the dressed-up potential well, for the dressed-down well is occupied by the electrons of the 1s state. [ABSTRACT FROM AUTHOR]

We report our numerical simulation on the dynamic interference photoelectron spectra for a one-dimensional (1D) He model exposed to intense ultrashort extreme ultraviolet (XUV) laser pulses. The results demonstrate an unambiguous interference feature in the photoelectron spectra, and the interference is unveiled to originate from the dynamic Stark effect. The interference photoelectron spectra are prompted for intense sub-femtosecond XUV laser pulses in double ionization. The stationary phase picture is corroborated qualitatively in the two-electron system. The ability of probing the dynamic Stark effect by the photoelectron spectra in a pragmatic experiment of single-photon double ionization of He may shed light on further investigation on multi-electron atoms and molecules. [ABSTRACT FROM AUTHOR]

The adaptive module of the 4.1 m SOAR telescope, SOAR Adaptive Module (SAM), corrects ground-layer turbulence using an ultraviolet laser guide star. It has been commissioned in 2013 and it is in regular science operation since 2014. SAM works with the CCD imager covering a 3′ field or with the speckle camera. It operates routinely and stably, delivering resolution in the I band equal to the free-atmosphere seeing. This paper describes the SAM system as a whole, providing essential reference for its users and technical information of interest to instrumentalists. Operation of the instrument, its performance, and science projects done with SAM so far are reviewed. [ABSTRACT FROM AUTHOR]

We report on the detailed molecular beam epitaxial growth and characterization of Al(Ga)N nanowire heterostructures on Si and their applications for deep ultraviolet light emitting diodes and lasers. The nanowires are formed under nitrogen-rich conditions without using any metal catalyst. Compared to conventional epilayers, Mg-dopant incorporation is significantly enhanced in nearly strain- and defect-free Al(Ga)N nanowire structures, leading to efficient p-type conduction. The resulting Al(Ga)N nanowire LEDs exhibit excellent performance, including a turn-on voltage of ∼5.5 V for an AlN nanowire LED operating at 207 nm. The design, fabrication, and performance of an electrically injected AlGaN nanowire laser operating in the UV-B band is also presented. [ABSTRACT FROM AUTHOR]

The trajectory of discovery associated with the study of high-intensity nonlinear radiative interactions with matter and corresponding nonlinear modes of electromagnetic propagation through material that have been conducted over the last 50 years can be presented as a landscape in the intensity/quantum energy [I-ħω] plane. Based on an extensive series of experimental and theoretical findings, a universal zone of anomalous enhanced electromagnetic coupling, designated as the fundamental nonlinear domain, can be defined. Since the lower boundaries of this region for all atomic matter correspond to ħω ~ 103 eV and I  ≈  1016 W cm−2, it heralds a future dominated by x-ray and γ-ray studies of all phases of matter including nuclear states. The augmented strength of the interaction with materials can be generally expressed as an increase in the basic electromagnetic coupling constant in which the fine structure constant α  →  Z2α, where Z denotes the number of electrons participating in an ordered response to the driving field. Since radiative conditions strongly favoring the development of this enhanced electromagnetic coupling are readily produced in self-trapped plasma channels, the processes associated with the generation of nonlinear interactions with materials stand in natural alliance with the nonlinear mechanisms that induce confined propagation. An experimental example involving the Xe (4d105s25p6) supershell for which Z  ≅  18 that falls in the specified anomalous nonlinear domain is described. This yields an effective coupling constant of Z2α  ≅  2.4  >  1, a magnitude comparable to the strong interaction and a value rendering as useless conventional perturbative analyses founded on an expansion in powers of α. This enhancement can be quantitatively understood as a direct consequence of the dominant role played by coherently driven multiply-excited states in the dynamics of the coupling. It is also conclusively demonstrated by an abundance of data that the utterly peerless champion of the experimental campaign leading to the definition of the fundamental nonlinear domain was excimer laser technology. The basis of this unique role was the ability to satisfy simultaneously a triplet (ω, I, P) of conditions stating the minimal values of the frequency ω, intensity I, and the power P necessary to enable the key physical processes to be experimentally observed and controllably combined. The historical confluence of these developments creates a solid foundation for the prediction of future advances in the fundamental understanding of ultra-high power density states of matter. The atomic findings graciously generalize to the composition of a nuclear stanza expressing the accessibility of the nuclear domain. With this basis serving as the launch platform, a cadenza of three grand challenge problems representing both new materials and new interactions is presented for future solution; they are (1) the performance of an experimental probe of the properties of the vacuum state associated with the dark energy at an intensity approaching the Schwinger/Heisenberg limit, (2) the attainment of amplification in the γ-ray region (~1 MeV) and the discovery of a nuclear excimer, and (3) the determination of a path to the projected super-heavy nuclear island of stability. [ABSTRACT FROM AUTHOR]

We theoretically investigate the laser-assisted photoionization of He by an extreme ultra violet (XUV) pulse in the presence of a linearly chirped intense laser pulse by solving the time-dependent Schrödinger equation within the single-active-electron approximation. Analysis based on the time-dependent perturbation theory is also carried out to provide more physical insights. A new scheme is shown to be capable of extracting the arrival time of an XUV free-electron laser (FEL) pulse relative to an external laser pulse as well as the XUV pulse duration from the photoelectron sidebands resulting from XUV ionization in the presence of a chirped laser pulse. This scheme is independent of the energy fluctuation and the timing jittering of the FEL pulse. Therefore it can be implemented in a non-invasive way to characterize FEL pulses on a shot-by-shot basis in time-resolved spectroscopy. [ABSTRACT FROM AUTHOR]

A laser scattering system utilizing an ultraviolet laser with a triple grating spectrometer has been assembled in order to measure gas temperature in atmospheric plasma sources. Such laser scattering interactions offer a non-invasive technique for investigating atmospheric microplasma sources, which have potential applications in remote optical sensing, materials processing, and environmental decontamination. This particular system is unique in that it utilizes a ultraviolet laser line (266 nm), which increases the cross section for Rayleigh and Raman scattering by a factor of 16 in comparison to the more common 532 nm laser operating in the visible range. In this work, the laser scattering system is used to directly compare the rotational gas temperature (Tr) and gas kinetic temperature (Tg) in two different atmospheric plasma sources [1]: a direct current plasma jet operating on nitrogen and [2] a conventional pin–pin glow microdischarge in air. Results show agreement between Tr and Tg both in the low temperature afterglow of the plasma jet (300–700 K) and the hot center of the atmospheric glow (1500–2000 K). These observations lend credence to the common assumption of rotational relaxation in atmospheric plasmas and validate the ultraviolet laser diagnostic for future application in atmospheric microplasma sources. [ABSTRACT FROM AUTHOR]

Neutralization and ash products due to electron capture processes in plasmas reduce the efficiency of energy generation in fusion Tokamak reactors. Therefore, lithium ions have been used to improve the efficiency of energy generation where good control of the electron capture process is required. Here, we show that an intense ( W cm−2), ultra-short (1 fs at full width half-maximum) Gaussian laser pulse in the UV region can enhance the electron capture process on in the low collision keV energy region. We find a factor of 10 enhancement in electron capture cross-section at impact energies lower than 10 keV amu−1 for an 80 nm wavelength laser and a factor of 2 for the excitation process in the hydrogen atom as compared to the laser-free case. In contrast, for a 200 nm wavelength laser the increase of the electron capture cross-sections takes place around 1 keV amu−1 by a factor of 3 and no enhancement for the excitation process. Our results show that the UV assisted production of Li can be controlled, particularly for short UV wave-length for a specific collision energy range. We anticipate that our findings will facilitate UV laser control of the Li production in Tokamak reactors and encourage further experimental work in this system. [ABSTRACT FROM AUTHOR]

A concave microchannel surface was formed by nanosecond pulse laser ablation to allow antibody immobilization on a capillary flow immunoassay chip. Microscopic analysis showed that UV laser ablation of poly(methyl methacrylate) at 193 nm and 1.76 J · cm−2 allowed excellent immobilization of Cy5 conjugated antibody. The concave structure was 10 µm deep and 260 µm wide and supported uniform antibody printing on the microchannel surface. The characteristics of immobilized antibodies on this surface and on a commercially available polymer coating were comparable. Quantitative analysis of procollagen type I C-peptide (PICP) at different concentrations provided a linear relationship in the range 0–600 ng ml−1 PICP, which is sufficient for clinical estimation of PICP in the blood. The results may provide a new benchmark for a mechatronic antibody immobilization-based capillary flow immunoassay chip. [ABSTRACT FROM AUTHOR]