Luminescence excitation spectra and characteristics of luminescence centers with overlapping absorption bands

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F₂ and F₃⁺ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F₃⁺ centers. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 365–371, May–June, 2008.     

Triggering of high voltage discharge by femtosecond laser filaments on different wavelengths

The triggering and guiding of high voltage discharge by plasma filaments generated by 400 nm and 800 nm femtosecond laser were studied experimentally. The detailed diagnostics for characteristics of filaments, such as amount of free electrons, diameter and electron density, were performed using sonographic method, fluorescence imaging and resistivity measurement. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filaments was observed. It is shown that the 400 nm laser pulses demonstrated stronger triggering ability than 800 nm laser pulse under same pulse energy. This behavior of 400 nm femtosecond pulse is connected to the rich population of highly excited particles in filaments.     

单光子和双光子激发的超歧化聚合物的光谱学特性研究

研究了二种新型超歧化聚合物在氯仿和二甲基甲酰胺溶剂中的飞秒激光单光子和双光子激发的光谱学特性．这二种聚合物是以苯为核、以对二甲氧基苯为连接单元，分别以二甲基苯胺为端基或以特丁基苯为端基的超歧化共轭聚合物．结果表明这种超歧化聚合物具有较大的溶剂极性相关的飞秒双光子吸收截面，不同的端基不仅影响分子内的电荷转移，而且导致超歧化分子共轭程度的变化．作为一种潜在的光学非线性材料，利用这种大的双光子吸收截面的超歧化聚合物可实现了飞秒双光子直写的三维高密度数据存储．     

Molecular Dynamics of Biological Probes by Fluorescence Correlation Microscopy with Two-Photon Excitation

We report on the application of fluorescence correlation microscopy under two-photon excitation of fluorophores of biological interest: FITC–dextran (MW, from 20 to 150 kDa), green fluorescent protein (MW, 27 kDa), and fluorescein (MW, 330 Da). Under these experimental conditions, the translational diffusion coefficients of these molecules in aqueous solutions derived from the fluorescence intensity autocorrelation function were determined for the first time and were found to be 24 × 10^–7, 8.2 × 10^–7, and 3 × 10^–7 cm² s^–1 for 150-kDa FITC–dextran, green fluorescent protein, and fluorescein, respectively. These results are discussed in connection with previously reported results obtained by different methods. The great sensibility of the system has been applied to single-molecule detection of the smaller fluorophore, fluorescein.     

Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption

We report the fluorescence upconversion properties of a class of improved pyridinium toluene-p-sulfonates having donor–π–acceptor (D–π–A) structure under two-photon excitation at 1064 nm. The experimental results show that both the two-photon excited (TPE) fluorescence lifetime and the two-photon pumped (TPP) energy upconversion efficiency were increased with the enhancement of electron-donating capability of the donor in the molecule. It is also indicated that an overlong alkyl group tends to result in a weakened molecular conjugation, leading to a decreased two-photon absorption (TPA) cross section. By choosing the donor, we can obtain a longest fluorescence lifetime of 837 ps, a highest energy upconversion efficiency of 6.1%, and a maximum TPA cross-section of 8.74×10⁻⁴⁸ cm⁴ s/photon in these dyes.     

The study of ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film

The time-resolved magneto-optical Kerr spectroscopy technique is used to study the ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film. We study concretely the influence of the different pump fluence and the different external magnetic field on magnetization dynamics of ultrafast demagnetization, magnetization reversal and magnetization recovery. The pump fluence dependence magnetization dynamics shows that the degree of demagnetization, the degree of magnetization reversal and the time of magnetization recovery increase with pump fluence, which can be interpreted by the “three-temperature” model. The external magnetic field dependence magnetization dynamics shows that the rate of magnetization reversal increases with the external field, which accord with the magnetization reversal mechanism based on the reversed magnetic domain nucleation and domain-wall motion.     

Nonlinear absorption of CS2 at the wavelength of 400 nm with femtosecond pulses

Nonlinear absorption of carbon disulfide (CS₂) was investigated by Z-scan technique and time-resolved pump-probe technique with femtosecond pulses at 400 nm wavelength. By the two techniques, we confirmed that the nonlinear absorption of CS₂ arise from a combination of two-photon absorption (TPA) and the excited state absorption induced by TPA under the incident laser pulses with 400 nm wavelength. The coefficient of TPA, the absorption cross-section of low excited state and lifetime of low excited state were obtained by theoretical fitting the experimental results. The results indicated that the CS₂ has good optical limiting capability at 400 nm wavelength.     

Fluorescence of Styryl Dyes-DNA Complexes Induced by Single- and Two-Photon Excitation

The series of novel monomer and homodimer styryl dyes based on (p-dimethylaminostyryl) benzothiazolium residues were synthesized and studied as possible fluorescent probes for nucleic acids detection. Spectral-luminescent and spectral-photometric properties of obtained dyes in the unbound state and in DNA presence were studied. Fluorescence emission induced by two-photon excitation of dye-DNA complexes in aqueous buffer solution was registered. Two-photon absorption cross section values of the studied dyes in DNA presence were evaluated.     

Synthesis of resonant cavities having continuum spectra by genetic algorithm

This paper proposes a novel method for generating continuum spectra with possible applications in WDM access networks. This new method would allow the development of a simple continuum laser structure with better performances in terms of cost and simplicity than those of present supercontinuum sources. In this aim, we will analyze the possibility to broaden the resonant modes of a Fabry–Perot cavity by operating only on the design of one of the cavity's Bragg mirrors. Firstly we considered the conditions which a Bragg grating must satisfy in order to broaden the resonant modes of a resonant cavity. Because an exact solution is not physical possible, the genetic algorithm is the best available tool for the design of such a grating. An active medium is then added inside the cavity and its behavior below threshold is simulated. The spectra thus simulated are continuum over 5–15 nm.     

Deep tissue microscopic imaging of the kidney with a gradient-index lens system

Intravital microscopy using two-photon excitation is proven to be a valuable tool for studying the kidney and associated disease processes. However, routine performance of intravital kidney imaging is limited by the fact that fluorescence signal is attenuated by the tissue and at certain tissue depth lost its strength completely. For most of the animal tissues, this finite imaging depth is limited to a few hundred microns. Currently it is not possible to non-invasively image the kidney beyond the superficial tissue layers of the cortex. This has imposed significant limitations on the animal models one can use for imaging since structure such the glomerulus is typically located below the superficial layer of the cortex that cannot be imaged using a conventional fluorescence microscope. Here we report the use of a needle-like lens system based on gradient-index (GRIN) microlenses capable of transferring high quality fluorescence images of the tissue through a regular microscope objective for deep tissue imaging of the kidney. By combining this GRIN lens system with a Zeiss LSM 510 NLO microscope, we are able to extend the depth for imaging kidney tissues far beyond the few hundred microns limit. This GRIN lens imaging system provides an alternative microendoscopic imaging tool that will enhance current intravital kidney imaging techniques for studying structural and functional properties of local tissues at locations below the superficial layers of the kidney.     

Bragg grating writing through the polyimide coating of high NA optical fibres with femtosecond IR radiation

Bragg gratings are written with ultrafast 800 nm radiation and a phase mask through the polyimide polymer coatings of commercially available high NA fibres that are both unloaded and loaded with high pressure hydrogen gas. For polyimide coated fibres with very high germanium core concentrations, index modulations greater than 1 × 10⁻⁴ are induced. Stable core index modulations 60% of their original value were present after 115 h at 500 °C.     

遗传算法用于傅里叶变换红外光谱的定量解析

本文利用遗传算法对混叠的傅里叶变换红外光谱图进行了定量的解析。文中分析了最多包含了十个组分的大气有毒有机物（苯，四氯化碳，氯苯，硝基苯，苯酚，甲苯，甲醇，邻甲酚，间甲酚和对甲酚）的混合谱图，建立了适当目标函数，使用了自适应变异技术，讨论了交叉和变异概率对结果的影响。结果证明遗传算法有着极好的非线性特征。在适当的条件下，结果的相对误差小于1％。     

飞秒激光烧蚀硅表面产生等离子体的发射光谱研究

对中心波长为800 nm,脉宽为100 fs的激光脉冲烧蚀空气中硅(111)产生的等离子体发射光谱进行了时间和空间分辨研究. 结果表明,在等离子体羽膨胀初期(小于50 ns时间范围内),等离子体发射光谱主要由连续光谱构成,此后连续光谱强度逐渐减弱,线状光谱开始占主导地位;在羽体膨胀过程中离子谱线的存在时间短于原子谱线的存在时间. 由时间分辨发射光谱发现在羽体膨胀过程中等离子体辐射波长存在红移现象,波长红移量随时间演化呈二次指数衰减. 最后给出等离子体发射光谱谱线强度的时空演化规律. 关键词： 飞秒激光 脉冲激光烧蚀 等离子体 发射光谱     

Geometric-optical reconstruction of a wavefront experimental realization with femtosecond laser pulses

The mechanism of wavefront reconstruction by geometric-optical reflection of reconstruction radiation from surfaces with constant phase differences between the object and reference waves has been investigated. The main difference between this mechanism and a holographic one is the absence of diffraction of the reconstructing radiation by the periodic structure and as a consequence the achromatism of the reconstruction process. Incoherent continuous radiation and ultrashort laser pulses were used in the experiments. The effect of achromatic reconstruction has been obtained after recording the interference of counterpropagating 30–40 fs pulses from an Al₂O₃:Ti³⁺ laser in bulk media.     

Investigation of adaptive laser pulse shaping by direct estimation of group delay profile

We investigate the benefits of estimating the group delay spectrum (as opposed to the phase spectrum) on the performance of an adaptive laser pulse shaping experiment. We use both numerical and experimental approaches, and consider different figures of merit such as efficiency, noise robustness, convergence speed, repeatability, fitness sensitivity, and parameter insensitivity. While primarily focused on the process of broadband second harmonic generation, we also study a resonant third order nonlinear process. Our results show an overall superiority of delay-based formulation in the cases under study (compared to a conventional phase-based formulation) for adaptive laser pulse shaping.     

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the $\pi $ spectral phase step.     

飞秒激光空气等离子体发射光谱的实验研究

对强飞秒激光聚焦在空气中所激发的等离子体的发射光谱进行了实验研究.结果表明,光谱特征表现为短波段(截至波长为340 nm)强烈的连续谱和长波段(波长在800 nm附近)强度相对较低的线光谱.在脉冲宽度(50 fs)保持不变而不断调节激光脉冲能量时,等离子体光谱形状的特征基本相似;当激光脉冲能量(1 mJ)保持不变而脉冲宽度从50 fs增加至500 fs和1 ps时,连续谱的峰值(500 nm)显得格外突出,并开始呈现出线光谱特征. 关键词： 飞秒激光 激光空气等离子体 发射光谱 线光谱