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Abstract The effect of the variation of the gas pressure on the high harmonic generation (HHG) from nitrogen molecular gas (N 2) and atmospheric air with ultrashort intense laser pulses is measured. The optimum pressure in the interaction region of a gas jet for maximizing the HHG yield is determined for both gases. Enhancement of the HHG output and its extension to higher harmonic orders are observed around the optimum pressure value of 0.33 bar. Theoretical calculations based on one-dimensional models explain this effect and provide reasonable agreement with experimental results. [ABSTRACT FROM AUTHOR]

Abstract A broadband detection method with coherent laser pulses exciting surface plasmons is investigated. The intensity and phase characteristics of the reflected light in Kretschmann configuration are calculated. It is shown that by mixing in-plane and out-of-plain polarization components of the incident light, the measurements of the changes of the refractive index of the adjacent media can be efficiently performed by observing the shift of the steep slope of the surface plasmon resonance (SPR) reflectance curve. We have demonstrated that such sensing can be performed by observing the angular shift of the SPR curve as well as the spectral shift. The developed model can account for an arbitrary number of layers and is used to simulate the response when an additional functionalizing layer adjacent to the gold film is used. [ABSTRACT FROM AUTHOR]

Time resolved laser techniques enabled the study of the nonlinear evolution of surface acoustic wave (SAW) pulses of very high-amplitudes with acoustic Mach numbers of approximately 0.01. In such waves even shock fronts can be formed during their propagation. Changes of the shape of intense SAW pulses provide information on the nonlinear acoustic parameters and the nonlinear elastic constants of the material. Measurements in polycrystalline stainless steel have shown that a compression of the nonlinear SAW pulse takes place in this metal material yielding a positive parameter of the local nonlinearity. The changes of the SAW pulse shape were calculated using a nonlinear evolution equation and the nonlinear acoustic parameters were determined by fitting the evolution equation to the experimental data. The attenuation of SAWs was determined by measuring low amplitude pulses. In addition, the velocities of longitudinal and shear waves were obtained by registering the precursors of bulk waves at the surface. [Copyright &y& Elsevier]

Highlights • Methane and carbon dioxide in the atmosphere were monitored with an optical path up to 2.6 km. • Wavelength modulation spectroscopy with a reference cell and lock-in detection were implemented. • The measurements revealed the influence of the air turbulence and the concentration variations. Abstract Open-path measurements of greenhouse gases yield valuable data for atmospheric research. We present such a method based on wavelength modulation spectroscopy to measure greenhouse gases. A movable platform was developed capable of detecting atmospheric variations of the concentrations of methane and carbon dioxide. The system was calibrated in the laboratory and then moved to the field for continuous automated measurements over paths of up to 2.6 km. The intensity modulation caused by turbulence was also measured. The continuous monitoring of methane was carried out for more than 30 h. Similar measurements of carbon dioxide were performed for about 10 h. The results show that the detection limits of methane and carbon dioxide are ∼2 ppb and ∼20 ppm with integration times of 60 s and 20 s, respectively. We conclude by discussing the achieved parameters in comparison to results obtained with other techniques, which shows that the developed sensor is a promising tool for monitoring of greenhouse gases. [ABSTRACT FROM AUTHOR]

Abstract: Thin films of the molecular magnet Mn12-acetate, [Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H2O, have been prepared using a laser ablation technique with a nitrogen laser at low laser energies of 0.8 and 2mJ. Chemical and magnetic characterizations show that the Mn12-acetate cores remain intact and the films show similar magnetic properties to those of the parent molecular starting material. In addition, the magnetic data exhibit a peak in the magnetization at 27K indicating the creation of an additional magnetic phase not noted in previous studies of crystalline phases. [Copyright &y& Elsevier]

• A basis for further research on the contacts between DL and absorption spectroscopy. • An end-to-end DNN based DAS gas sensing system is established for the first time. • Higher accuracy and faster computation speed compared with WMS. • Robustness to noisy conditions, laser aging and circuit parameters compared with WMS. A novel direct absorption spectroscopy gas sensing system based on end-to-end deep neural networks was proposed for measurements of gas concentration. One-dimensional convolutional neural network and deep multi-layer perceptron were explored to measure the concentrations of methane and acetylene. The accurate measurement results for both gases demonstrated that deep neural networks based direct absorption spectroscopy technique can be reliably applied to different gas molecules. The developed gas sensing system achieved more precise concentration retrieval compared with that of wavelength modulation spectroscopy, and fast computation speed as well as robustness to noisy conditions, laser aging and circuit parameter variation simultaneously. The combination of deep neural networks and direct absorption spectroscopy provides new ideas for the further research of gas absorption spectroscopy. [ABSTRACT FROM AUTHOR]

• Long-range sensor with wavelength modulation spectroscopy was built and tested. • The signal processing with Kalman filtering and wavelet denoising were compared. • The detection limit of 4.7 ppb with 0.25 s integration time was achieved. A wavelength modulation spectroscopy (WMS) setup for long-range methane measurements in the atmosphere over distances of kilometers was developed. To overcome the influence on the signal from noise, random factors and air turbulence, we applied denoising processing methods, which were analyzed and compared. The real-time digital Kalman filter was selected and utilized, improving the detection limit for methane sensing from 60 ppb with integration time of 60 s to 2 ppb with integration time of 1 s. To our knowledge, this is the first realization of such highly sensitive and yet simple sensor by combining the real-time filter and WMS. This approach opens the possibility to replace much more complicated instruments in sensitive measurements of trace gases when evaluating the experimental data with the Kalman filter. [ABSTRACT FROM AUTHOR]