“PPAS” X-RAY ABSORPTION SPECTROSCOPY IN LASER-PRODUCED ALUMINUM PLASMA

We have developed and successfully demonstrated the "point projection absorption spectroscopy (hereafter abbreviated to PPAS)" technique to record keV X-ray absorption spectra in the coronal region of laser produced aluminum plasma which provides spatial information in the corona. Absorption spectrum of the Al Ⅻ 1s²-1s2p resonant line for time delays up to 250, 500 and 750 ps after the peak of the incident laser pulse have been observed. Analysis of an absorption spectrum should allow a high spatial resolution (< 25μm) history of the absorbing ion population density to be determined. The 1-D hydro-code JB19 has been used to simulate the experiment and the predictions are compared to the observations. Such comparison can be useful in validating parts of the codes used for gain prediction on recombination X-ray laser transitions.     

Processes underlying the laser photochromic effect in colloidal plasmonic nanoparticle aggregates

We have studied the dynamic and static processes occurring in disordered multiparticle colloidal Ag aggregates with natural structure and affecting their plasmonic absorption spectra under pico-and nanosecond pulsed laser radiations, as well as the physical origin responsible for these processes. We have shown that depending on the duration of the laser pulse,the mechanisms of laser modification of such aggregates can be associated both with changes in the resonant properties of the particles due to their heating and melting(picosecond irradiation mode) and with the particle shifts in the resonant domains of the aggregates(nanosecond pulses) which depend on the wavelength, intensity, and polarization of the radiation.These mechanisms result in formation of a narrow dip in the plasmonic absorption spectrum of the aggregates near the laser radiation wavelength and affect the shape and position of the dip. The effect of polydispersity of nanoparticle aggregates on laser photochromic reaction has been studied.     

Stabilization and Shift of Frequency in an External Cavity Diode Laser with Solenoid-Assisted Saturated Absorption

A simple method to realize both stabilization and shift of the frequency in an external cavity diode laser （ECDL） is reported. Due to the Zeeman effect, the saturated absorption spectrum of Rb atoms in a magnetic field is shifted. This shift can be used to detune the frequency of the ECDL, which is locked to the saturated absorption spectrum. The frequency shift amount can be controlled by changing the magnetic field for a specific polarization state of the laser beam. The advantages of this tunable frequency lock include low laser power requirement, without additional power loss, cheapness, and so on.     

Temperature dependence of line parameters of ~(12)C~(16)O_2 near 2.004 μm studied by tunable diode laser spectroscopy

The absorption spectrum of carbon dioxide at 2.004 μm has been recorded at sample temperatures between 218.0 K and room temperature, by using a high-resolution tunable diode laser absorption spectrometer(TDLAS) combined with a temperature controlled cryogenically cooled absorption cell. The self-, N~(2-), and air-broadening coefficients for nine transitions of ~(12)C~(16)O_2 belonging to the 20012←00001 band in the 4987 cm~(-1)–4998 cm~(-1) region have been measured at different temperatures. From these measurements, we have further determined the temperature dependence exponents of the pressure-broadening coefficients. To the best of our knowledge, the temperature dependence parameters of the collisional broadening coefficients are reported experimentally for the first time for these nine transitions. The measured halfwidth coefficients and the air temperature dependence exponents of these transitions are compared with the available values reported in the literature and HITRAN 2012 database. Agreements and discrepancies are also discussed.     

Some new progress on the light absorption properties of linear alkyl benzene solvent

Linear alkyl benzene(LAB) will be used as the solvent in a liquid scintillator mixture for the JUNO antineutrino experiment. Its light absorption properties should therefore be understood prior to its effective use in the experiment. Attenuation length measurements at a light wavelength of 430 nm have been performed on samples of LAB prepared for the JUNO experiment. Inorganic impurities in LAB have also been studied for their possibilities of light absorption in our wavelength of interest. In view of a tentative plan by the JUNO collaboration to utilize neutron capture with hydrogen in the detector, we also present in this work a preliminary study on the carbon–hydrogen ratio and the attenuation length of the samples.     

Q-Switching Pulse Generation with Thulium-Doped Fiber Saturable Absorber

Q-switched pulsed laser systems operating in the eye-safe wavelength region （λ〉 1.4 μm） are promis- ing for applications such as light detection and ranging （lidar）, differential absorption lidar, optical communi- cation, sensor and instrumentations. Fiber lasers are particularly attractive for such applications due to the fact that the beam quality can be independent of output power and they can be robustly engineered. There is an increasing interest in Q-switched erbium- doped fiber lasers （EDFLs）, which can be generated by using either active or passive techniques. Com- pared with the actively Q-switched ones, passively Q- switched EDFLs have attracted much attention for their advantages of compactness, low cost, flexibility and simplicity of design. Different kinds of saturable absorbers, such as the transition metal-doped crys- tals and semiconductor quantum-well structures, have been applied to realize Q-switched EDFLs.How- ever, when they are used in the laser cavity, additional alignment devices, such as lens, mirrors or U-bench units, have to be applied. This may increase the in- sertion loss and the complexity of the laser cavity.     

A new Monte Carlo code for absorption simulation of laser-skin tissue interaction

In laser clinical applications,the process of photon absorption and thermal energy diffusion in the target tissue and its surrounding tissue during laser irradiation are crucial.Such information allows the selection of proper operating parameters such as laser power,and exposure time for optimal therapeutic.The Monte Carlo method is a useful tool for studying laser-tissue interaction and simulation of energy absorption in tissue during laser irradiation.We use the principles of this technique and write a new code with MATLAB 6.5,and then validate it against Monte Carlo multi layer (MCML) code.The new code is proved to be with good accuracy.It can be used to calculate the total power absorbed in the region of interest.This can be combined for heat modelling with other computerized programs.     

Multilayer-based soft X-ray polarimeter at the Beijing Synchrotron Radiation Facility

A compact high precision eight-axis automatism and two-axis manual soft-ray polarimeter with a multilayer has been designed, constructed, and installed in 3W1B at the Beijing Synchrotron Radiation Facility (BSRF). Four operational modes in the same device, which are double-reflection, double-transmission, front-reflection-behind-transmission and front-transmission-behind-reflection, have been realized. It can be used for the polarization analysis of synchrotron radiation. It also can be used to characterize the polarization properties of the optical elements in the soft X-ray energy range. Some experiments with Mo/Si and Cr/C multilayers have been performed by using this polarimeter with good results obtained.     

Optical fiber temperature sensor based on wavelength-dependent detection

Semiconductor fiber temperature sensors have been used widely in many fields, but most of them pick up temperature by measuring the optical intensity of certain fixed narrow-band in absorption spectrum. Furthermore,they are sensitive to the loss of optical intensity and the fluctuation of light source power. The novel temperature measurement system proposed in this paper is based on the semiconductor absorption theory and the spectral analysis of method.To measure temperature,the sensor model detects not the certain narrow-band spectrum but the most spectra of the optical absorption edge.Therefore the measurement accuracy and the stability can be improved greatly.Experimental results are in agreement with theoretical analysis results perfectly.     

Intersubband Transitions of Si δ-Doped GaAs Layer for Different Donor Distribution Models

For different donor distribution types we theoretically investigate the intersubband transitions of single Si δ-doped GaAs structure as dependent on the applied electric field. The diffusion of donor impurities is taken into account in two different models: a triangular distribution and a non-uniform distribution. The electronic properties such as the effective δ-potential, the subband energies and the eigen-envelope wavefunctions have been calculated by solving the Schroedinger and Poisson equations self-consistently. Abrupt changes of the subband energy difference and the absorption peak are realized whenever the applied electric field reaches a certain value. These critical electric field values change dependent on the donor distribution model. The intersubband absorption spectrum shows that redshifts appear up to the critical electric field value for the (1-2) and (1-3) intersubband transitions. This spectrum also shows that blueshifts can occur when the electric fields are higher than certain values. These changing intersubband absorption peaks can be used in various infrared optical device applications.     

Room—Temperature A1GaAsSb／InGaAsSb Multi—Quantum—Well Lasers with High Chraracteristic Temperature

Planar structure A1GaAsSb/InGaAsSb lasers operated at 2.01/zm with high characteristic temperature have been fabricated from a strained multiple quantum-well heterostructure. To decrease the free carrier induced absorption of optical mode in the mid-infrared, we design a broaden waveguide layer in the laser structures to decrease the optical mode distribution in the heavy doped cladding layer, therefore it can be absorbed easily. To enhance the characteristic temperature of laser diodes, A1 constituent up to 80% was applied to the A1GaAsSb cladding layer. The laser diodes with a threshold current density of 1.8 kA/cm2 can be pulsed operating up to 340 K. The characteristic temperature To is 125 K and 90 K in the operating temperature ranges 170-220 K and 230-340 K, resDectivelv. The emission spectrum shows a multiple longitudinal mode.     

Infrared diode laser spectroscopy of O2–N2O van der Waals complex in the ν1 symmetric stretch region of N2O

The rovibrational spectrum of O₂–N₂O van der Waals complex is measured in the v₁ symmetric stretch region of N₂O monomer using a tunable diode laser spectrometer. The complex is generated by a slit-pulsed supersonic expansion with gas mixtures of O₂, N₂O, and He. Both a- and b-type transitions are observed. The effective Hamiltonian for an open-shell complex consisting of a diatomic molecule in a ³Σ electronic state and a closed-shell partner is used to analyze the observed spectrum. Molecular constants in the vibrationally excited state are determined accurately. The band-origin of the spectrum is determined to be 1284.7504(25) cm^-1, red-shifted from that of the N₂O monomer by ～ 0.1529 cm^-1.     

Simulation study of phase retrieval for hard X-ray in-line phase contrast imaging

X-ray phase contrast imaging (XPCI) is a novel method that exploits the phase shift for the incident X-ray to form an image. For light elements such as carbon, hydrogen and oxygen, the phase-shift term can be up to 1000 times greater than the absorption term in the hard X-ray energy region. So XPCI has attracted much attention in recent years. Various methods for XPCI have been proposed and demonstrated on synchrotron devices and other X-ray sources[1―13], particularly the in-line metho…     

Development in Laser Induced Extrinsic Absorption Damage Mechanism of Dielectric Films

Absorption of host and the temperature-dependence of absorption coefficient have been considered in evaluating temperature distribution in films, when laser pulse irradiates on films. Absorption of dielectric materials experience three stages with the increase of temperature： multi-photon absorption; single photon absorption; metallic absorption. These different absorption mechanisms correspond to different band gap energies of materials, which will decrease when the temperature of materials increases. Evaluating results indicate that absorption of host increases rapidly when the laser pulse will be over. If absorption of host and the temperature-dependence of absorption are considered, the maximal temperatures in films will be increased by a factor of four.     

A new method for information retrieval in two-dimensional grating-based X-ray phase contrast imaging

Grating-based X-ray phase contrast imaging has been demonstrated to be an extremely powerful phase-sensitive imaging technique.By using two-dimensional(2D) gratings,the observable contrast is extended to two refraction directions.Recently,we have developed a novel reverse-projection(RP) method,which is capable of retrieving the object information efficiently with one-dimensional(1D) grating-based phase contrast imaging.In this contribution,we present its extension to the 2D grating-based X-ray phase contrast imaging,named the two-dimensional reverseprojection(2D-RP) method,for information retrieval.The method takes into account the nonlinear contributions of two refraction directions and allows the retrieval of the absorption,the horizontal and the vertical refraction images.The obtained information can be used for the reconstruction of the three-dimensional phase gradient field,and for an improved phase map retrieval and reconstruction.Numerical experiments are carried out,and the results confirm the validity of the 2D-RP method.     

Frequency-Shift of a Frequency Stabilized Laser Based on Zeeman Effect

We introduce a new method of frequency-shifting for a diode laser in laser cooling experiments, the method is based on the Zeeman egect of^87 Rb atoms. The laser frequency is stabilized by absorption spectrum line of atoms in magnetic field. We show that a magnetic field can be added up to 10^-2T. The corresponding frequency shift is 10^2 MHz and the response time is about 1ms. The large range of the frequency shift is suffcient for laser-cooling experiments.     

Absolute density measurement of nitrogen dioxide with cavity-enhanced laser-induced fluorescence

The absolute number density of nitrogen dioxide(NO_2) seeded in argon is measured with cavity-enhanced laserinduced fluorescence(CELIF) through using a pulsed laser beam for the first time. The cavity ring down(CRD) signal is acquired simultaneously and used for normalizing the LIF signal and determining the relationship between the measured CELIF signal and the NO_2 number density. The minimum detectable NO_2 density down to(3.6±0.1)10~8 cm~(-3) is measured in 60 s of acquisition time by the CELIF method. The minimum absorption coefficient is measured to be(2.0±0.1)10~(-9) cm~(-1), corresponding to a noise equivalent absorption sensitivity of(2.2±0.1)10~(9) cm.~(-1)Hz~(-1/2). The experimental system demonstrated here can be further improved in its sensitivity and used for environmental monitoring of outdoor NO_2 pollution.     

Fraction and Temperature of Hot Electrons Generated by Laser—Plasma Interaction at the shenguang—Ⅱ Laser Facility

We have developed a three-dimensional electron-photon Monte Carlo transmission (MCT) code.The conversiontransmissivity coefficient of hot electron x-ray,ηct,is analysed theoretically and simulated numerically using the MCT code.Experiments of disc and cylindrical targets have been performed at the Shenguang-Ⅱ laser facility.A Multi-channel filter-fluorescence spectrometer was used to obtain the hard x-ray spectrum.The temperature and energy of hot electrons are induced from the hard x-ray spectrum and the ηct value.The average energy fraction of hot electrons is 14% of incident laser energy for the 1ωNd laser and the spherical target with an average temperature of hot electrons of 36keV,while it is 15% for the 1ωNd laser and the cavity target with an average temperature of 54keV.We have also obtained the results of 5.6% and 13.3keV for the 3ωNd laser and the disc target,4.9% and 17.9keV for the 3ωNd laser and the half cavity target with a thin wall,and 2.1% and 22.4keV for those with a thick wall.The experimental results agree with theory and simulation.     

Nonlinear optical switching and all-figures of merit in Bi_2S_(3-x)Se_x/PMMA nanocomposite films investigated by Z scan under visible CW laser

Bi2S3-xSex/poly(methyl methacrylate)(PMMA)nanocomposite films were prepared using microwave assisted synthesis with different compositions of x.Crystal structure,surface morphology,and optical properties were investigated to characterize the prepared nanocomposite films.The crystallinity and optical band gap of the prepared Bi2S3-xSex/PMMA were affected by x.The prepared samples showed a blue shift in the absorption edge.The laser power dependent nonlinear refraction and absorption of Bi2S3-xSex/PMMA nanocomposite films were investigated by using the Z-scan technique.The optical nonlinearity of the nanocomposite films exhibited switchover from negative nonlinear refraction to positive nonlinear refraction to negative nonlinear refraction effects,and from saturable absorption to reverse saturable absorption to saturable absorption with an increase and decrease in the composition.An interesting all-optical figure of merit was reported to assess the nanocomposite films for a practical device.It was calculated that the device all-figures of merit were based on the nonlinear response,which is important for the all-optical switching device.The results demonstrate that the optimized all-optical figures of merit can be achieved by adjusting the composition and input laser power,which can be used for the design of different all-photonic devices,and the results of nonlinear switching behavior can open new possibilities for using the nanocomposite films in laser Q-switching and mode-locking.