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Material processing with high repetition rate ultra-short laser pulses with intensities higher than 10 13 W/cm 2 may lead to the X-ray emission dose exceeding allowed dose limits. We have investigated a worse-case exposure scenario, in which laser processing parameters were tuned to maximize X-ray yield. Use of double pulse regime leads to increase of the X-ray yield up to two orders of magnitude compared with the single pulse regime. H ′ (0.07) and H*(10) dose rates were measured using X-ray spectrometer and electronic dosimeter. Maximum dose rates at 35 cm distance from the X-ray source calculated using spectrometer data exceeded 1 Sv/h and 9 mSv/h, respectively. Dose rates measured using the dosimeter were lower. The difference is attributed to narrower X-ray spectral range detectable by dosimeters, which may lead to underestimation of exposure doses in the laser processing laboratories. The presented method might be used as an example to evaluate X-ray yield and optimize measures of radiation protection. [ABSTRACT FROM AUTHOR]

The influence of metal nanoparticles on linear and nonlinear optical properties of surrounding organic molecules has been widely investigated, whereas much less attention has been paid to the influence of molecules on properties of nanoparticles. Here, we employ transient absorption spectroscopy to address the nonlinear optical responses of the resonantly coupled silver nanoparticle-organic dye systems and demonstrate that silver nanoparticles covered with dye molecules show enhanced and spectrally different nonlinear extinction changes from pristine nanoparticles. We identify changes of the plasmon resonance band of nanoparticles induced by excitation of surrounding dye. We attribute these exciton-plasmon coupling effects to the excitation-induced refractive index modifications of the dye layer surrounding a nanoparticle and to the back-transfer of the oscillator strength borrowed by the dye from the nanoparticle. [ABSTRACT FROM AUTHOR]

In this study, we present the development and characterization of Cu plasma X-ray source driven by 20 W average power high repetition rate femtosecond laser in ambient atmosphere environment. The peak Cu- Kα photon flux of 2.3 × 10 photons/s into full solid angle is demonstrated (with a process conversion efficiency of ~ 10), using pulses with peak intensity of 4.65 × 10 W/cm. Such Cu- Kα flux is significantly larger than others found in comparable experiments, performed in air environment. The effects of resonance plasma absorption process, when optimized, are shown to increase measured flux by the factor of 2-3. The relationship between X-ray photon flux and plasma-driving pulse repetition rate is quasi-linear, suggesting that fluxes could further be increased to 10 photons/s using even higher average powers of driving radiation. These results suggest that to fully utilize the potential of high repetition rate laser sources, novel target material delivery systems (for example, jet-based ones) are required. On the other hand, this study demonstrates that high energy lasers currently used for plasma X-ray sources can be conveniently and efficiently replaced by high average power and repetition rate laser radiation, as a way to increase the brightness of the generated X-rays. [ABSTRACT FROM AUTHOR]

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S and ICT are vibrationally distinct species. Identification of the FSRS modes on the S and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction. [ABSTRACT FROM AUTHOR]

Femtosecond laser systems are becoming essential tools in various areas of material processing, medicine and scientific research. In order to tailor the wavelength of radiation to the absorptivity of materials being processed, UV harmonic generators are often employed. At high average powers and high repetition rates, the long warm-up time of harmonic generators, caused by UV absorption in nonlinear crystals, can be a serious obstacle for efficient and fast processing. To increase the speed and reduce the cost of technological processes, new methods are required to speed up the settling of the output power when the harmonic generators are switched on after longer idle period. We have investigated a fourth-harmonic generator of a high-repetition rate femtosecond laser and studied the properties of the output radiation in order to optimize its fast-switching capabilities. Theoretical modeling of thermal effects in the nonlinear crystal allowed us to explain temporal dependencies of temperature and output power after switching the laser on. Based on these results, we were able to optimize the trajectory of nonlinear crystal rotation following the system start-up and reduce the switching-on time from tens of seconds to 50 ms without any negative effect on the output power. [ABSTRACT FROM AUTHOR]

A correction to the article "Investigation of the S/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy" in the October 2016 issue is presented.