Rising the signal-to-noise ratio in X-ray spectra of femtosecond laser-produced plasmas using the “mean-median” algorithm

As femtosecond laser pulses increase in intensity and decrease in duration, interaction between the focused laser radiation and a substance is followed by a sharp increase in the intensity of fast-electron, ion, and electromagnetic noise generation. In turn, the noise signals of X-ray detectors grow in amplitude and the signal-to-noise ratio in recording X-ray spectra of multiply charged ions approaches unity. A significant excess of the noise level over the useful signal is observed in plasmas generated by laser pulses with a power density of ≥10¹⁷ W/cm². The most powerful effect of the above factors is exerted on X-ray spectra recorded by such electromagnetic equipment as CCD-based detectors, photoelectron amplifiers, etc. A new “mean-median” algorithm is described, with which it is possible to considerably increase the signal-to-noise ratio of CCD detectors used to measure X-ray spectra of femtosecond laser-produced plasma.     

激光衍射粒径测量中的Chin-Shifrin反演算法

建立了以线阵CCD作为探测组件的激光衍射粒径测量系统用于颗粒群直径分布的测量.在数值计算的基础上,分析了传统Chin-Shifrin积分变换粒径分布反演算法存在的两个问题:小粒径反演结果的发散和多假峰现象,并结合理论分析给出了这两种现象的物理图像.提出了修正积分变换公式以解决小粒径反演结果发散的间题并针对Chin-Sh...     

Quantitative measurement of hard x-ray spectra for high intensity laser produced plasma

X-ray line spectra ranging from 17 to 77 keV were quantitatively measured with a Laue spectrometer, composed of a cylindrically curved crystal and a detector. Either a visible CCD detector coupled with a CsI phosphor screen or an imaging plate can be chosen, depending on the signal intensities and exposure times. The absolute sensitivity of the spectrometer system was calibrated using pre-characterized laser-produced x-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency for specific x-ray line emissions, is derived as a consequence of this work.     

Some aspects of the silicon behaviour under femtosecond pulsed laser field evaporation

Three dimension atom probe analysis of semiconductor materials requires the ability to bring high electric field at the specimen apex to remove atoms. It is shown that, if voltage pulses are used to evaporate doped silicon, the resistivity of the material has to be lower than about 10(2) Omega cm. To overcome this problem, voltage pulses have been replaced by femtosecond laser pulses. The laser pulses give rise to field evaporation by two processes. Both thermal and optical field evaporation have been observed. Thermal evaporation takes place at high laser intensities and with short wavelengths while the evaporation is assisted by the rectification of the optical field for lower intensities and in the infrared domain. Using the optical field evaporation, reproducible and good analyses in term of spatial and mass resolutions could be conducted.     

Errors of measuring picosecond light pulses by picosecond streak cameras

Errors of measuring time dependences of the light intensities of picosecond laser pulses by two electron-optical systems are studied. The first system is the picosecond streak camera made in Russia, and the second system, designed in Germany, consists of a picosecond streak camera coupled with a double spec-trograph. The manufacturers test similar systems by using femtosecond pulses, however, this does not allow one to predict errors of measuring picosecond pulses. Parameters that characterize just these errors are proposed and measured. Their behavior is determined, when the amplitude (energy) of the measured pulses increases, and the influence of the cross-section shape of the light beam on the measurement accuracy is studied.     

Note: Diagnosing femtosecond laser-solid interactions with monochromatic Kα imager and x-ray pinhole camera

An x-ray pinhole camera and a monochromatic K(α) imager are used to measure the interactions of intense femtosecond laser pulses with Cu foil targets. The two diagnostics give different features in the spot size and the laser energy scaling, which are resulted from different physical processes. Under our experimental conditions, the K(α) emission is mainly excited by the fast electrons transporting inside the cold bulk target. In contrast, the x-ray pinhole signals are dominated by the broadband thermal x-ray emission from the hot plasma at the front target surface.     

Terahertz Response from a Silicon Surface with Deposited Nanosized Gold Particles

Terahertz emission spectra of the surface of silicon crystals with different types of conductivity were experimentally recorded upon excitation by femtosecond laser pulses at various temperatures. The observed features in the terahertz spectra of the silicon surface correspond to the energy structure of the impurity centers determining the type of conductivity of the sample. Comparison was made with the results obtained in the case of deposition of gold nanoparticles on a semiconductor surface. The spectral features of the surface with deposited nanoparticles are discussed using the terahertz re-emission mechanism in two-phonon absorption.

     

Study of self-generated magnetic fields in laser produced plasmas using a three-channel polaro-interferometer

Self-generated magnetic fields produced in laser plasmas at moderate laser intensities have been measured using a three-channel polaro-interferometer. The main elements of this device are two birefringent calcite wedges placed between two crossed polarizers. Using this device, the spatial profiles of (a) the rotation angle (polarometry), (b) the electron density (interferometry), and (c) the transmitted probe beam intensity (shadowgraphy) are recorded simultaneously using a digital camera with a large format CCD in a single laser shot. Magnetic fields of 2-4 MG had been estimated in aluminum plasma at laser intensities ~10(13) W/cm(2). It is also possible to use this device in other configurations to get time resolved information.     

Normalization schemes for ultrafast x-ray diffraction using a table-top laser-driven plasma source

We present an experimental setup of a laser-driven x-ray plasma source for femtosecond x-ray diffraction. Different normalization schemes accounting for x-ray source intensity fluctuations are discussed in detail. We apply these schemes to measure the temporal evolution of Bragg peak intensities of perovskite superlattices after ultrafast laser excitation.     

A new TriBeam system for three-dimensional multimodal materials analysis

The unique capabilities of ultrashort pulse femtosecond lasers have been integrated with a focused ion beam (FIB) platform to create a new system for rapid 3D materials analysis. The femtosecond laser allows for in situ layer-by-layer material ablation with high material removal rates. The high pulse frequency (1 kHz) of ultrashort (150 fs) laser pulses can induce material ablation with virtually no thermal damage to the surrounding area, permitting high resolution imaging, as well as crystallographic and elemental analysis, without intermediate surface preparation or removal of the sample from the chamber. The TriBeam system combines the high resolution and broad detector capabilities of the DualBeam(TM) microscope with the high material removal rates of the femtosecond laser, allowing 3D datasets to be acquired at rates 4-6 orders of magnitude faster than 3D FIB datasets. Design features that permit coupling of laser and electron optics systems and positioning of a stage in the multiple analysis positions are discussed. Initial in situ multilayer data are presented.     

Novel x-ray multispectral imaging of ultraintense laser plasmas by a single-photon charge coupled device based pinhole camera

Spectrally resolved two-dimensional imaging of ultrashort laser-produced plasmas is described, obtained by means of an advanced technique. The technique has been tested with microplasmas produced by ultrashort relativistic laser pulses. The technique is based on the use of a pinhole camera equipped with a charge coupled device detector operating in the single-photon regime. The spectral resolution is about 150 eV in the 4-10 keV range, and images in any selected photon energy range have a spatial resolution of 5 microm. The potential of the technique to study fast electron propagation in ultraintense laser interaction with multilayer targets is discussed and some preliminary results are shown.     

An imaging proton spectrometer for short-pulse laser plasma experiments

The ultraintense short pulse laser pulses incident on solid targets can generate energetic protons. In addition to their potentially important applications such as in cancer treatments and proton fast ignition, these protons are essential to understand the complex physics of intense laser plasma interaction. To better characterize these laser-produced protons, we designed and constructed a novel spectrometer that will not only measure proton energy distribution with high resolution but also provide its angular characteristics. The information obtained from this spectrometer compliments those from commonly used diagnostics including radiochromic film packs, CR39 nuclear track detectors, and nonimaging magnetic spectrometers. The basic characterizations and sample data from this instrument are presented.     

Femtosecond broadband stimulated Raman spectroscopy: Apparatus and methods

The laser, detection system, and methods that enable femtosecond broadband stimulated Raman spectroscopy (FSRS) are presented in detail. FSRS is a unique tool for obtaining high time resolution (<100 fs) vibrational spectra with an instrument response limited frequency resolution of <10 cm(-1). A titanium:Sapphire-based laser system produces the three different pulses needed for FSRS: (1) A femtosecond visible actinic pump that initiates the photochemistry, (2) a narrow bandwidth picosecond Raman pump that provides the energy reservoir for amplification of the probe, and (3) a femtosecond continuum probe that is amplified at Raman resonances shifted from the Raman pump. The dependence of the stimulated Raman signal on experimental parameters is explored, demonstrating the expected exponential increase in Raman intensity with concentration, pathlength, and Raman pump power. Raman spectra collected under different electronic resonance conditions using highly fluorescent samples highlight the fluorescence rejection capabilities of FSRS. Data are also presented illustrating our ability: (i) To obtain spectra when there is a large transient absorption change by using a shifted excitation difference technique and (ii) to obtain high time resolution vibrational spectra of transient electronic states.     

Novel diode-pumped infrared tunable laser system for multi-photon microscopy

We report on a novel laser source, emitting high energy (20 nanoJoule) femtosecond pulses, in a broad spectrum (250 nm). This source is easily tuned from 950 to 1200 nm, without any laser adjustment, and delivers sub-300 femtosecond pulses with a 10-nm spectral width.     

Quantitative atom probe analyses of rare-earth-doped ceria by femtosecond pulsed laser

We have investigated the irradiation conditions of femtosecond laser pulses for quantitative atom probe analyses of rare-earth (RE) doped ceria. The influence of laser wavelength, power, pulse frequency, as well as specimen temperature on mass resolution and background noise of atom probe mass spectra were investigated. Furthermore, quantitative atom probe analysis of yttrium distribution in Y-doped ceria was carried out with the optimized evaporation conditions. The distribution of yttrium was found to be uniform within the grains, but they were confirmed to be segregated at grain boundaries.     

ENERGY DEPOSITION AND NON-THERMAL ABLATION IN FEMTOSECOND LASER GROOVING OF SILICON

Ultra-short pulsed laser ablation of crystalline silicon is characterized by a complicated heat diffusion and material removal process. In this research, a computational investigation is undertaken to understand the temperature distribution and heat effect in femtosecond laser grooving of silicon. Energy accumulation and threshold fluence of silicon ablation by femtosecond lasers are estimated through solving coupled energy balance equations. Thermal and optical properties of the material are considered in the calculations. The possible non-thermal ablation process and ablation geometry are analyzed for the case of succession of laser pulses. Thermal-mechanical response induced by temperature gradient is discussed around the laser ablation region. The agreement between the model calculations and experimental results show that this research provides an efficient thermal analysis method of the explosive laser-silicon interaction process, and a feasible way to optimize process parameters with minimum thermal damages.     

Ultrastructure and reproduction behaviour of single CHO-K1 cells exposed to near infrared femtosecond laser pulses

In the present work, the authors investigated ultrastructural changes as well as the reproduction behaviour of preselected single CHO-K1 cells exposed to 170 femtosecond laser pulses at different power output levels in comparison with cells outside the illumination volume. The ultrashort laser pulses were provided by an 80 MHz Ti:sapphire laser at 780 nm. The cells were scanned ten times with a scan rate of 1/16 s(-1). Single CHO-K1 cells exposed to low mean power of 2 mW revealed no significant changes in ultrastructure after laser exposure. In some cases, changes of mitochondria with slight disordering of cristae were found. Cytoplasm was filled with vesicles that seemed to be released from Golgi stacks. Cells irradiated with higher powers demonstrated more dramatic changes in ultrastructure. A considerable number of swollen mitochondria in conjunction with loss of cristae was observed. The main event of mitochondrial changes was the formation of electron dense bodies in the mitochondrial matrix. In addition, lumen of endoplasmatic reticulum was enlarged. Highest applied mean laser power of 12.5 mW lead to complete destruction of mitochondria and their transformation to electron dense structures containing membrane material. Compared with cell targets irradiated with 2 mW mean power, the release of vesicles from Golgi stacks seemed to be rather moderate. Cells localised outside the laser beam revealed no ultrastructural changes. Low mean laser power at 2 mW was unable to impair the reproduction behaviour of CHO-K1 cells. At higher laser power output levels, CHO-K1 cells started to delay cell division. At 12.5 mW, no cell division occurred. The obtained results may be helpful in recommending parameters for safe femtosecond laser microscopy of living specimens.     

Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

A laser-based tabletop approach to femtosecond time-resolved photoelectron spectroscopy with photons in the vacuum-ultraviolet (VUV) energy range is described. The femtosecond VUV pulses are produced by high-order harmonic generation (HHG) of an amplified femtosecond Ti:sapphire laser system. Two generations of the same setup and results from photoelectron spectroscopy in the gas phase are discussed. In both generations, a toroidal grating monochromator was used to select one harmonic in the photon energy range of 20-30 eV. The first generation of the setup was used to perform photoelectron spectroscopy in the gas phase to determine the bandwidth of the source. We find that our HHG source has a bandwidth of 140 ± 40 meV. The second and current generation is optimized for femtosecond pump-probe photoelectron spectroscopy with high flux and a small spot size at the sample of the femtosecond probe pulses. The VUV radiation is focused into the interaction region with a toroidal mirror to a spot smaller than 100 × 100 μm(2) and the flux amounts to 10(10) photons/s at the sample at a repetition rate of 1 kHz. The duration of the monochromatized VUV pulses is determined to be 120 fs resulting in an overall pump-probe time resolution of 135 ± 5 fs. We show how this setup can be used to map the transient valence electronic structure in molecular dissociation.     

掺铒单模光纤飞秒脉冲激光器和放大器

为了获得一种被动锁模掺铒光纤振荡器及功率放大器,数值模拟出超短脉冲在光纤中的传输和演化过程,并基于此搭建了一种被动锁模掺铒光纤飞秒振荡器及功率放大器。实验获得了中心波长1560 nm、重复频率100 MHz、输出功率30 mW、脉冲宽度85 fs超短脉冲。通过采用PPLN晶体进行倍频,进一步获得了输出功率5 mW,中心波长780 nm的飞秒脉冲。该光纤激光器为全保偏光纤结构,具有体积小巧、可靠性高、稳定性好的特点。     

Influence of laser irradiation condition on a femtosecond laser-assisted tomographic atom probe

Influence of femtosecond laser pulse condition on the performance of an energy-compensated optical tomographic atom probe has been investigated. The unstable oscillator makes the mass peaks significantly broadened. Double 80 fs pulse train with 10 ns interval makes the mass peaks slightly shifted to the higher mass side. The mass peak shift corresponds to the fight time of ions triggered by laser pulsing. Chirping ratio for the laser pulses ranging from 80 fs to 10 ps is controlled by the pulse compressor for the fragile specimens such as oxide dispersion strengthen steel or insulator materials. A first-principle calculation for optical dielectric breakdown in diamond has been successfully demonstrated. It is shown that effective conductive increase has appeared at the laser intensity around 10¹³ W/cm².