Improving the precision of fluorescence lifetime measurement using a streak camera

Streak camera has high temporal resolution and high sensitivity, and is a powerful tool in biomedical study to measure fluorescence lifetime and perform fluorescence lifetime imaging. However, nonuniformity of the gain in the streak tube and nonlinearity of the sweeping speed limit the precision of fluorescence lifetime measurement, particularly when fluorescence lifetimes are short. We have constructed a two-photon excitation fluorescence lifetime measurement system that is based on a synchroscan streak camera and have developed accordingly a method to correct the effect of gain nonuniformity and nonlinearity of sweeping speed on the measurement precision. A continuous-wave laser of high stability is used to calibrate the gain of the streak camera, and a Fabry-Perot etalon is used to calibrate the nonlinearity of the sweeping speed. Fitting algorithms are used to correct the gain of the streak camera and nonlinearity of the sweeping speed respectively, which significantly improves the measurement precision of the system, as characterized through the fluorescence lifetime of the short-lived fluorescence dye, Rose Bengal. Experimental results show that the measurement fluctuation of the lifetime has been improved from more than 10% to 2% after correcting the effects of gain nonuniformity and sweeping speed nonlinearity.     

Adsorption isotherms and kinetics of methylene blue on a low-cost adsorbent recovered from a spent catalyst of vinyl acetate synthesis

A regenerated activated carbon used as catalyst support in the synthesis of vinyl acetate has been tested as a low-cost adsorbent for the removal of dyes. After a thorough textural characterization of the regenerated activated carbon, its adsorption isotherms and kinetics were determined using methylene blue as model compound at different initial concentrations. Both Langmuir and Freundlich isotherm models were developed and then compared. It was found that the equilibrium data were best represented by the Langmuir isotherm model. The kinetic data were fitted to pseudo-first-order, pseudo-second-order and intraparticle diffusion models, and it was found that the best fitting corresponded to the pseudo-second-order kinetic model. The results showed that this novel adsorbent had a high adsorption capacity, making it suitable for use in the treatment of methylene blue enriched wastewater.     

Improved Statistically Optimal Nearfield Acoustical Holography in subsonically moving fluid medium

Statistically Optimal Nearfield Acoustical Holography (SONAH) can be used to reconstruct three-dimensional sound fields by projecting two-dimensional data measured on a “small” aperture that partially covers a composite sound source in a “static” fluid medium. Here, an improved SONAH procedure is proposed that includes the mean flow effects of a moving fluid medium while the sound source and receivers are stationary. The backward projection performance of the proposed procedure is further improved by using a wavenumber filter to suppress subsonic noise components. Through numerical simulations at Mach 0.6, it is shown that the improved procedure can accurately reconstruct sound source locations and radiation patterns: e.g., the spatially averaged reconstruction errors of the conventional and improved SONAH procedures are 15.40 dB and 0.19 dB, respectively, for a monopole simulation and 21.60 dB and 0.19 dB for an infinite-size panel. The wavenumber filter further reduces spatial noise, e.g., decreasing the reconstruction error from 1.73 dB to 0.19 dB for the panel simulation. An existing data measured in a wind tunnel operating at Mach 0.12 is reused for the validation. The locations and radiation patterns of the two loudspeakers are successfully identified from the sound fields reconstructed by using the proposed SONAH procedure.     

Resolution and sensitivity enhancements in strong grating based fiber Fabry–Perot interferometric sensor system utilizing multiple reflection beams

We investigate an asymmetric intensive fiber Bragg grating (FBG) defined Fabry–Perot (F–P) sensor system decoded by a multiple-path-matched Michelson interferometer. The interrogation of higher order reflection beams cannot only solve the problem of the degraded resolution induced by the spectral mismatch of the FBGs, but also amplify the effect of the fiber strain on the phase of the light. We demonstrate multiple reflection beams in the F–P cavity based on the concept of the FBG effective length for constructing respective interrogation interferometers, and present a cost function with optimized system parameters to improve noise properties. The performances of interrogating the second, third and fourth order reflection beams are compared in a strain sensing experiment arrangement. Under the condition of the same optical path length mismatch, the interrogation of the fourth order reflection beam can achieve 9.8 dB sensitivity enhancement and 3 dB resolution promotion compared with the result using the second order reflection beam.     

CS2团簇增强的激光多价电离现象的质谱研究

利用脉宽为25ns的脉冲Nd:YAG 532 nm的激光,在8×1010W/cm2的强度下,用飞行时间质谱对CS2的激光电离过程进行了研究.观察到了较强的C2+和S2+高价离子信号,这些高价离子C2+,S2+的最可几平动能高达144eV,112 eV.不同进样方式,激光延迟以及束源压力的实验结果表明,这些高价离子可能来源于CS2团簇的库仑爆炸过程.多光子电离引发,逆韧致吸收加热-电子碰撞电离模型可能是高价离子产生的机理.     

Limit cycles for two classes of planar polynomial differential systems with uniform isochronous centers

In this article, we study the maximum number of limit cycles for two classes of planar polynomial differential systems with uniform isochronous centers. Using the first-order averaging method, we analyze how many limit cycles can bifurcate from the period solutions surrounding the centers of the considered systems when they are perturbed inside the class of homogeneous polynomial differential systems of the same degree. We show that the maximum number of limit cycles, $m$ and $m+1$, that can bifurcate from the period solutions surrounding the centers for the two classes of differential systems of degree $2m$ and degree $2m+1$, respectively. Both of the bounds can be reached for all $m$.     

高功率隔离器热致退偏分析与补偿设计

针对磁光晶体在大功率光隔离器中的应用,结合实际光学元件参数,利用琼斯矩阵分析了隔离度与入射光功率间的关系。提出基于外置材料,对磁光晶体热致退偏效应的补偿方案:针对两种不同的常见补偿材料,给出了它们的设计参数,并对两种材料的补偿效果进行对比。结果表明,在50 W的光功率下,利用CaF2晶体和SiO2,可分别提高隔离度约15dB和4dB。     

Analysis of detection limit to time-resolved coherent anti-Stokes Raman scattering nanoscopy

In the implementation of CARS nanoscopy, signal strength decreases with focal volume size decreasing. A crucial problem that remains to be solved is whether the reduced signal generated in the suppressed focal volume can be detected. Here reported is a theoretical analysis of detection limit （DL） to time-resolved CARS （T-CARS） nanoscopy based on our proposed additional probe-beam-induced phonon depletion （APIPD） method for the low concentration samples. In order to acquire a detailed shot-noise limited signal-to-noise （SNR） and the involved parameters to evaluate DL, the T-CARS process is described with full quantum theory to estimate the extreme power density levels of the pump and Stokes beams determined by saturation behavior of coherent phonons, which are both actually on the order of ~ 109 W/cm2. When the pump and Stokes intensities reach such values and the total intensity of the excitation beams arrives at a maximum tolerable by most biological samples in a certain suppressed focal volume （40-nm suppressed focal scale in APIPD method）, the DL correspondingly varies with exposure time, for example, DL values are 103 and 102 when exposure times are 20 ms and 200 ms respectively.     

H_2气氛退火处理对Nb掺杂TiO_2薄膜光电性能的影响

采用电子束沉积方法,以钛酸锶(SrTiO3)为衬底制备铌(Nb)掺杂TiO2薄膜并研究后续H2气氛退火处理对其薄膜样品光电性能的影响.结果发现H2气氛热退火处理能有效改善Nb掺杂TiO2薄膜的导电率,最佳电阻率达到5.46×10-3Ω·cm,在可见光范围内的透光率为60%—80%.导电性能的改善与H2气氛退火处理后多晶薄膜的晶粒尺寸变大和大量的氧空位形成及H原子掺杂有关.