Study of sequential two photon absorption and conduction band electrons absorption in ZnSe

Using the Z-scan technique and pump–probe technique with 130 fs laser pulses at 800 nm, we verify that an intraband one-photon absorption follows the interband two photon absorption. Particularly, we find that there is an intraband relaxation interspersed between these two absorption processes for some of the conduction band electrons but not all of them. In this study, we measure the interband two photon absorption coefficient and the absorptive cross sections associated with both excitation pathways within the conduction band. In addition, we estimate the time for relaxation of electrons within the conduction band to be 250 fs.     

可调谐激光吸收光谱应用于气体二维浓度分布重建研究

基于可调谐半导体激光吸收光谱技术(TDLAS),设计了气体浓度二维分布重建系统。将单路可调谐激光分为24路,交叉穿过目标区域,得到24组直接吸收信号(DA),并采用代数迭代法(ART)计算得到目标区域气体浓度二维分布。搭建了二维气体浓度分布测量试验台,利用中心波长在1.653μm的DFB激光器作为光源,选取HITRAN数据库CH4的2v3带R(3)线作为计算谱线进行数值模拟和试验验证,并对重建结果进行了误差分析。结果表明重建系统能很好的实现气体二维浓度分布重建测量。     

The mean probability of photon capture of resonance radiation in atomic absorption spectroscopy

The mean probability of photon capture Ψ, obtained via an approximation to the complementary escape factor Λ, for calcium and zinc resonance lines at 4226.73 and 2138.56 Å have been determined from their atomic absorption measurements. Calculations are performed for Gaussian, Lorentzian, and Voigt spectral line shapes. Tabulations and graphical plots of the mean probability of photon capture, Ψ, with the optical depth in the center of Doppler profile, τ_D, and the number density of free atoms in the ground state N, are provided.     

可调谐半导体激光吸收光谱法监测二氧化碳的通量

含碳温室气体浓度增加所加剧的温室效应是气候变化的重要原因,大面积范围内二氧化碳气体通量的测量对于评价各类陆地生态系统对大气中主要温室气体浓度的贡献具有重要的意义。可调谐半导体激光吸收(TDLAS)光谱技术具有高分辨率、高灵敏度以及快速响应等特点,是痕量气体高灵敏快速监测的新方法。文章以可调谐分布反馈半导体激光器作为光源,通过波长调制方法对1.573μm附近二氧化碳气体某一吸收线的二次谐波信号测量,结合激光分束技术,实现对不同高度层面700多米光程范围内二氧化碳气体浓度的快速在线检测。结合大口径闪烁仪测量出来的莫宁-奥布霍夫长度和特征速度,通过公式计算得到一天内二氧化碳气体的通量在-1.5～2.5mg·(m2·s)-1范围内的波动,突破了目前对近地面痕量气体通量的监测只能提供局地结果的状况,使大面积范围内痕量气体通量的测量成为可能。     

Absorption of laser radiation in ultracold plasma

In this paper, we analyze the spectral behavior(optical thickness, shape and linewidth) of laser radiation absorption under the correlation heating of ions in an ultracold plasma. The Voigt formula is used to find the absorption coefficient.The spectral line width is shown to grow with time while the optical thickness reduces. Our modeling results are in good agreement with the experimental findings reported in the literature.     

可调谐半导体激光吸收光谱监测农田气体通量特性研究

可调谐半导体激光吸收光谱(TDLAS)具有高选择性,高灵敏度等特性,非常适合监测农田痕量气体交换过程。采用开放式光程的可调半导体激光吸收光谱和非分散红外光谱技术,在封丘农田进行了一个月监测试验。选择CO2作为目标气体,用两种不同光谱技术选择不同的通量计算方法,分别获取农田排放通量。根据实验条件,提出了TDLAS的通量印痕模型,对比分析了两种技术的空间代表性特性,同时分析了数据误差来源和不同外界因素对通量测量的影响特性。结果发现在相同的气象条件下,仪器架设高度越高,光学路径越长,开放光路的TDLAS通量贡献区就越大。这些结果对通量监测中仪器安装有很好的指导意义。     

飞行时间质谱探测磁光阱中超冷分子离子的实验研究

利用飞行时间质谱探测了由磁光阱中的俘获光和 再泵浦光光缔合作用形成的超冷基态铯分子, 研究了微通道板工作电压、加速电场强度和加速电场持续时间对铯分子离子信号强度的影响. 实验结果与理论模型的拟合一致; 获得了适合实验条件的最优化实验参数, 为进一步研究超冷分子的光缔合光谱和光电离光谱奠定了实验基础.     

Photoacoustic detection of two photon absorption in formaldehyde using pulsed dye laser

Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper.     

Wide dynamic detection range of methane gas based on enhanced cavity absorption spectroscopy

Integrated cavity output spectroscopy (ICOS) is an effective technique in trace gase detection. The strong absorption due to the long optical path of this method makes it challenging in the application scenes that have large gas concentration fluctuation, especially when the gas concentration is high. In this paper, we demonstrate an extension of the dynamic range of ICOS by using a detuned laser combined with an off-axis integrating cavity. With this, we improve the upper limit of the dynamic detection range from 0.1% (1000 ppm) to 20% of the gas concentration. This method provides a way of using ICOS in the applications with unpredictable gas concentrations such as gas leak detection, ocean acidification, carbon sequestration, etc.     

Giant two‐photon absorption in monolayer MoS2

Strong two‐photon absorption (TPA) in monolayer MoS₂ is demonstrated in contrast to saturable absorption (SA) in multilayer MoS₂ under the excitation of femtosecond laser pulses in the near‐infrared region. MoS₂ in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO₂/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS₂ possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×10³ cm/GW, larger than that of conventional semiconductors by a factor of 10³. As a result of TPA, two‐photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS₂. For the multilayer MoS₂, the SA response is demonstrated with the ratio of the excited‐state absorption cross section to ground‐state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS₂ is ∼97 GW/cm², larger than that of the multilayer MoS₂ of ∼78 GW/cm².

     

基于激光吸收光谱技术天然气管道泄漏定量遥测方法的研究

报道了基于激光吸收光谱学原理的天然气管道泄漏移动遥测技术,通过模拟天然气泄漏实验, 分析了移动遥测的关键技术问题.为了定量遥测天然气管道微量泄漏,引入一个和剩余幅度调制(RAM) 等值反相的信号对偏差进行补偿,降低RAM对谐波信号的影响,提高系统检测灵敏度. 针对遥测回波吸收光谱特征,提出了改进软阈值小波去噪法,就提高系统信噪比而言, 比传统软阈值去噪法高2倍多,同时对二次谐波(2f)信号形状也有很好的保留,通过探测限计算, 系统移动遥测灵敏度达到80 ppm/m.     

高灵敏度光谱测量技术:腔振铃吸收光谱

腔振铃激光吸收光谱技术是近年快速发展的一项新颖的光谱技术,它不仅检测灵敏度高,而且结构简单,不需要高昂的光谱设备,特别适合于测量弱吸收物质,包括气体、固体、液体等稳态粒子和金属化合物、自由基、团簇等瞬态粒子.本文在介绍其基本原理的基础上,介绍了使用脉冲激光与连续激光光源的2种技术方案.     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

铁电体中极化长程涨落的光子关联谱实验研究

从极化团簇的随机涨落出发, 利用维纳过程模型, 推导了铁电体中极化长程涨落的弛豫规律以及光强自相关函数所可能的表现形式. 阐述了微观极化团簇的弛豫过程与宏观测量弛豫规律之间的联系. 通过对原有氦氖激光光子关联谱实验装置进行改进, 观测了BaTiO₃和0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃单晶中极化团簇长程涨落在居里点和立方到四方相变点附近的弛豫过程. 在BaTiO₃中发现极化团簇长程涨落在居里点之上4 K存在双弛豫现象, 此现象与其有序无序相变机理相联系. 在Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃中发现极化团簇长程涨落在相变点两侧都存在双弛豫现象. 利用推导的理论结果很好地拟合了实验结果并提取了极化团簇长程涨落的弛豫时间. 两种样品中极化团簇长程涨落的弛豫时间都在相变点出现突变, 并呈现临界慢化现象.     

Pulsed differential optical absorption spectroscopy applied to air pollution measurement in urban troposphere

Differential optical absorption spectroscopy (DOAS) is useful for various types of trace gas measurements in the lower troposphere. In this paper, we report the DOAS measurement conducted in Chiba, Japan during 2009 and 2010. An optical path length of 5.5 km is employed to measure the average concentration of NO₂ using a xenon aviation obstruction light that flashes every 1.5 s. By virtue of the high stability of the light source intensity, the intensity measurement enables the retrieval of aerosol extinction coefficient averaged over the optical path. The results are compared with the concurrent results of ground sampling and sunphotometer measurements.     

可调谐激光吸收光谱法与气相色谱法监测甲烷的对比研究

甲烷是一种重要的温室气体,它对大气中温室效应的贡献仅次于二氧化碳.针对环境空气中甲烷的测定,从测量原理、测量过程和测量装置等方面比较了可调谐二极管激光吸收光谱和气相色谱法两种测量方法.结果表明用上述两种方法测得的甲烷的日变化趋势具有良好的一致性.相比较而言,后者造价更低,并且更方便.     

可调谐二极管激光吸收光谱测量真空环境下气体温度的理论与实验研究

真空环境不仅会导致热电偶等温度传感器表面材料解吸,而且其传热机理也与常压不同,因此采用常压下校准和溯源的温度传感器测量真空环境下气体温度存在诸多不确定性问题,为此,本文利用可调谐二极管激光吸收光谱技术(TDLAS)测量真空环境下气体温度,探索TDLAS温度测量技术在真空环境下的应用前景,在模拟热真空实验过程中,首先将真空气室浸没于恒温槽中,然后利用TDLAS测量真空气室中气体温度,同时利用一等标准铂电阻测量恒温槽的温度,试验结果表明:TDLAS和一等标准铂电阻测量得到的气体温度和恒温槽温度具有高度的一致性,两者之间的误差在恒温槽温度稳定时不超过±0.2℃。     

连续和准连续激光调制谱分析技术的比对研究

为了在面对不同检测需求时,能够选取适合的调谐激光吸收光谱技术方案,对直接吸收光谱、连续调制谱和准连续调制谱三种方法进行了理论分析和实验比较。在相同实验条件下,通过同一激光器测量不同浓度CO2气体,比较了这三种方法的技术特点、信号特征、检测灵敏度。结果表明,准连续调制谱技术具有与连续调制谱相当的检测灵敏度,但是受激光能量间断和较大的寄生幅度调制影响,检测信号相对于气体吸收谱的线形失真较大,因此不太适合依赖光谱线形和线宽的压力、流速测量。为选取更加适用的激光调制谱技术,提供了参考依据。     

Hierarchical dimensional crossover of an optically-trapped quantum gas with disorder

Dimensionality serves as an indispensable ingredient in any attempt to formulate low-dimensional physics, and studying the dimensional crossover at a fundamental level is challenging. The purpose of this work is to study the hierarchical dimensional crossovers, namely the crossover from three dimensions (3D) to quasi-2D and then to 1D. Our system consists of a 3D Bose–Einstein condensate trapped in an anisotropic 2D optical lattice characterized by the lattice depths V₁ along the x direction and V₂ along the y direction, respectively, where the hierarchical dimensional crossover is controlled via V₁ and V₂. We analytically derive the ground-state energy, quantum depletion and the superfluid density of the system. Our results demonstrate the 3D-quasi-2D-1D dimensional crossovers in the behavior of quantum fluctuations. Conditions for possible experimental realization of our scenario are also discussed.