基于主动差分吸收光谱系统获取大气颗粒物光学特性研究

以主动差分吸收光谱遥感技术为基础,在干净大气环境条件下,结合参考光路设计技术,开展了获取近地面气溶胶颗粒物的光谱新方法研究.针对以宽带氙弧灯为光源的主动差分吸收光谱系统固有宽波段特性,建立了解析颗粒物光学厚度模型,获取了大气颗粒物光学特性.最后通过外场实验,获得测量期间气溶胶颗粒物光学厚度,然后采用对数线性回归获取Angstrom波长指数和混浊系数,以及大气能见度数据.主动差分吸收光谱系统用测量光程范围内的大气总的吸收特性,获得监测区域的颗粒物平均水平,更具有代表性和进一步应用价值.     

Monitoring the reaction between AICl3 and o-xylene by using terahertz spectroscopy

Terahertz time-domain spectroscopy （THz-TDS） is used to study the interaction between AlCl3 and o-xylene in a temperature range from 300 K to 368 K. For comparison, the three isomers of o-, m-, and p-xylene are measured by using THz-TDS. The o-xylene carries out isomerization reaction in the presence of catalyst AlCl3. The absorption coefficient of the mixed reaction solution is extracted and analyzed in the frequency range from 0.2 THz to 1.4 THz. The temperature dependence of the absorption coefficient, which is influenced by both the dissolution of AlCl3 and the production of the two other isomer resultants, is obtained, and it can indicate the process of the isomerization reaction. The results suggest that THz spectroscopy can be used to monitor the isomerization reaction and other reactions in chemical synthesis, petrochemical and biomedical fields.     

Solvent effects on the S0→S2 absorption spectra of β-carotene

Absorption spectra of β-carotene in 31 solvents are measured in ambient conditions. Solvent effects on the 0-0 band energy, the bandwidth, and the transition moment of the S 0 → S 2 transition are analysed. The discrepancies between published results of the solvent effects on the 0-0 band energy are explained by taking into account microscopic solute-solvent interactions. The contributions of polarity and polarizability of solvents to 0-0 band energy and bandwidth are quantitatively distinguished. The 0-0 transition energy of the S 2 state at the gas phase is predicted to locate between 23000 and 23600 cm-1 .     

Intensity and composition-dependent sign reversal of non-linearity in TiO2/CeO2 nanocomposites

CeO2/TiO2 composite nanoparticles with different Ce/Ti molar ratios have been successfully synthesized via sol-gel method. It was found that the band gap of the nanocomposite is tunable by varying Ce/Ti content. The nonlinear response of the sample was studied by using the nanosecond laser pulses from a Q switched Nd：Yag laser employing the Z-scan method. Open aperture Z-scan experiment revealed that with the increase in the CeO2 amount in the nanocomposite, the non-linearity of the composite increases, and it was assumed that this could be due to the modification of TiO2 dipole symmetry by the addition of CeO2. Closed aperture Z-scan experiment showed that when the CeO2 amount increases, positive nonlinear refraction decreases, and this could be attributed to the increase in the two photon absorption which subsequently suppresses the nonlinear refraction.     

Strong and broadband terahertz absorber using Si02-based metamaterial structure

We design and experimentally demonstrate a broadband metamaterial absorber in the terahertz （THz） band based on a periodic array of aluminum （A1） squares with two different sizes. A thin silicon dioxide （SiO2） film rather than a conventional polyimide （PI） layer is used as a dielectric spacer to separate A1 squares from the platinum （Pt） ground plane in our design, which significantly improves the design precision and the feasibility of the device fabrication. The combination of different sizes of AI squares gives rise to an absorption bandwidth of over 210 GHz with an absorption of over 90%. Our results also show that our device is almost polarization-insensitive. It works very well for all azimuthal angles with an absorption of beyond 80%.     

The effect of an optical pump on the absorption coefficient of magnesium-doped near-stoichiometric lithium niobate in terahertz range

The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz~0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN：Mg crystal is approximately in a range of 22 cm- 1_35 cm- 1 in a frequency range of 0.2 THz-0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH- absorption in this crystal.     

Electronic and optical properties of TiO_2 and its polymorphs by Z-scan method

TiO2 is a material which has attracted considerable attention from the scientific community for its innumerable prop- erties. TiO2 is known to exist in nature in three different crystalline structures： rutile, anatase, and brookite. Anatase and rutile TiO2 films have been widely characterized for their potential applications in solar cells, self-cleaning coatings, and photocatalysis. In the present report, the third-order nonlinear susceptibilities of TiO2 and its polymorphs, anatase, and rutile, prepared by the sol-gel technique followed by heat treatment are investigated using the Z-scan technique at a wavelength of 532 nm with a duration of 7 ns. Imaginary and real values of Z（3） for amorphous, anatase, and rutile are also calculated and found to be 5 × 10^-19 m2/V2, 27 × 10^-19 m2/V2, 19 × 10^-19 m2/V2, respectively. It is found that the values of the optical constants of amorphous TiO2 after heat treatment vary considerably. It is assumed that this could be due to the variation in the electronic structure of TiO2 synchronous with the formation of its polymorphs, anatase, and rutile. Amorphous TiO2 is marked by the localization of the tail states near the band gap, whereas its crystalline counterparts are characterized by completely delocalized tail states.     

Tailoring optical properties of TiO2 in silica glass for limiting applications

We present the linear and nonlinear optical studies on TiO2-SiO2 nanocomposites with varying compositions. Opti- cal band gap of the material is found to vary with the amount of SiO2 in the composite. The phenomenon of two-photon absorption （TPA） in TiO2/SiO2 nanocomposites has been studied using open aperture Z-scan technique. The nanocom- posites show better nonlinear optical properties than pure TiO2, which can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites are thermally treated and similar studies are performed. The anatase form of TiO2 in the nanocomposites shows superior properties relative to the amorphous and rutile counterpart. The involved mechanism is explained by rendering the dominant role played by the excitons in the TiO2 nanoparticles.     

The variation of Mn-dopant distribution state with x and its effect on the magnetic coupling mechanism in Znl-xMnxO nanocrystals

Zn1-xMnxO （x = 0.0005, 0.001, 0.005, 0.01, 0.02） nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and its X-ray absorption fine structure. It is found that the solubility of substitutional Mn in a ZnO lattice is very low, which is less than 0.4%. Mn ions first dissolve into the substitutional sites in the ZnO lattice, thereby forming Mn2＋O4 tetrahedral coordination when x ≤ 0.001, then entering into the interstitial sites and forming Mn3＋O6 octahedral coordination when x ≥ 0.005. All the samples exhibit paramagnetic behaviors at room temperature, and antiferromagnetic coupling can be observed below 100 K.     

Effects of shape and dopant on structural,optical absorption,Raman, and vibrational properties of silver and copper quantum clusters: A density functional theory study

We investigate the effects of shape and single-atom doping on the structural, optical absorption, Raman, and vibra- tional properties of Ag13, Ag12CUl, CUl3, and Cul2Agl clusters by using the （time-dependent） density functional the- ory. The results show that the most stable structures are cuboctahedron （COh） for Ag13 and icosahedron （Ih） for CUl3, Agl2CUlcore, and Cul2Aglsur. In the visible-near infrared optical absorption, the transitions consist of the interband and the intraband transitions. Moreover, red shifts are observed as follows： 1） clusters change from Agl2CUlcore to Ag13 to Ag12Culsur with the same motifs, 2） the shapes of pure Agl3 and Agl2CUlcore clusters change from COh to Ih to decahe- dron （Dh）, 3） the shape of Agl2CUlsur clusters changes from Ih to COh to Dh, and 4） the shapes of pure CU13 and Cu12Agl clusters change from Ih to Dh to COb. All of the Raman and vibrational spectra exhibit many significant vibrational modes related to the shapes and the compositions of the clusters. The ranges of vibrational spectra of Ag13, Agl2CUl or CU13, and Cu12Agl clusters become narrower and the vibrational intensities increase as the shape of the clusters changes from Ih to Dh to COh.     

Microwave-Controlled Light-Pulse Propagation in a Driven A-Type Atomic System with Two-Folded Levels

The temporal and spatial dynamics of one weak probe laser pulse, propagating through a A-type atomic medium with two-folded levels under the resonant excitation of one microwave driving field and one strong control field, is investigated in this paper. By numerically solving coupled Bloch-Maxwell equations, it is found that, in the absence of the microwave driving field, the atomic medium is transparent to the probe pulse at line center, which propagates over sufficiently long distances. By contrast, when the microwave driving field is applied, the probe pulse at line center can be rapidly absorbed on propagation. This substantial reduction of probe transmittance caused by the microwave driving field may lead to potential applications in designing a new kind of optical switching.     

Structural distortions and magnetisms in Fe-doped LaMn1-xFexO3 （0 〈 x ≤0.6）

X-ray absorption spectra （XAS） at Mn K-edge and Fe K-edge in LaMnl-xFexO3 show that with the increase of Fe substitution the chemical valence of Mn4＋ decreases, while the chemical valence of Fe3＋ remains unchanged. Structural distortions, such as the rotating and tilting for oxygen octahedron in the unit cell vary with iron content. A phase transition occurs at the Fe content values of 0.2~0.3. The evolutions of rotation and tilting angle of FeO6/MnO6 octahedral may be the vital factors to the structure and magnetism. We believe that the spin configuration of Fe3＋ may vary from the intermediate spin t2g4eg1 （S = 3/2） to the higher spin t2g3eg2 （S = 5/2） near the phase transition.     

Highly selective population of two excited states in nonresonant two-photon absorption

A nonresonant two-photon absorption process can be manipulated by tailoring the ultra-short laser pulse.In this paper,we theoretically demonstrate a highly selective population of two excited states in the nonresonant two-photon absorption process by rationally designing a spectral phase distribution.Our results show that one excited state is maximally populated while the other state population is widely tunable from zero to the maximum value.We believe that the theoretical results may play an important role in the selective population of a more complex nonlinear process comprising nonresonant two-photon absorption,such as resonance-mediated (2+1)-three-photon absorption and (2+1)-resonant multiphoton ionization.     

High microwave absorption performances for single-walled carbon nanotube–epoxy composites with ultra-low loadings

Microwave-absorbing polymeric composites based on single-walled carbon nanotubes(SWNTs) are fabricated via a simple yet versatile method, and these SWNT–epoxy composites exhibit very impressive microwave absorption performances in a range of 2 GHz–18 GHz. For instance, a maximum absorbing value as high as 28 dB can be achieved for each of these SWNT–epoxy composites(1.3-mm thickness) with only 1 wt% loading of SWNTs, and about 4.8 GHz bandwidth,corresponding to a microwave absorption performance higher than 10 dB, is obtained. Furthermore, such low and appropriate loadings of SWNTs also enhance the mechanical strength of the composite. It is suggested that these remarkable results are mainly attributable to the excellent intrinsic properties of SWNTs and their homogeneous dispersion state in the polymer matrix.     

获取大气颗粒物消光系数的差分吸收光谱法研究

针对雾霾天气日益增多,大气污染气体向颗粒物的转化在加快,研究了一种大范围对其监测的差分吸收光谱方法。差分吸收光谱法可以实时、在线、准确同时获取颗粒物光学特性和大气痕量气体浓度。论文首先分析了双光路差分吸收光学遥感系统获取颗粒物绝对光强的原理,然后研究了基于单光路测量大气吸收谱,在干净天气状况下测量参考光谱,利用能见度数据,在550 nm波段处实现系统校准,计算校准参数,从而获得大气绝对吸收光强,然后解析出大气总的消光系数。再从总的大气消光系数中,去除瑞利散射以及大气痕量气体吸收对消光系数影响后,精确解析出颗粒物消光系数。同时基于差分思想获取大气痕量气体的浓度。最后把该方法应用于外场实验,获取大气颗粒物在350～700 nm波段范围内消光系数和大气中NO₂的浓度。研究结果表明颗粒物消光系数的随着波长的增加而减少,符合Angstrom公式。该研究为分析大气气相/粒子非均相化学反应提供有力的技术支持。     

J/ψ-N center of mass energy dependence of nuclear absorption effect on J/ψ production

In this paper, the J/ψ nuclear absorption effect is studied at RHIC and LHC energies with the EKS98 shadowing parameterizations. By assuming that the J/ψ absorption cross section, σ abs , increases with the charmonium-nucleon （J/ψ-N） center of mass energy, s J/ψN , it is found that σ abs should depend on x F （or y） at a certain center of mass energy per nucleon pair,s , especially at LHC energies. The theoretical results with the x F （or y）-dependence of the absorption effect are in good agreement with the experiment data from PHENIX in d-Au collisions and the predicted results will be examined by the forthcoming experimental data from LHC in d-Pb collisions. Finally, we also present baseline calculations of cold nuclear matter effects on J/ψ production in nucleus-nucleus （A-A） collisions and find that the x F （or y）-dependence of absorption effect is very small at both RHIC and LHC energies in A-A collisions.     

xF （or y）-Dependence of Nuclear Absorption and Energy Loss Effects on J/ψ Production

By means of the nuclear parton distribution studied only with lepton deep-inelastic scattering experimental data, the J/ψ ＂normal nuclear absorption＂ and energy loss effects are studied in a GIauber formalism at HERA and RHIC energies. Assuming that the absorption cross section σabs increases with the charmonium-nucleon center of mass energy, the results reveal a significant dependence of the aabs on rapidity g at RHIC energies. The initial-state energy loss effect, which is found important only at HERA energies, is also considered, and its influence should be eliminated when we studied the absorption effect at low collision energies. Finally, we also present the theoretical prediction for LHC.     

Dye-sensitized solar cells: Atomic scale investigation of interface structure and dynamics

Recent progress in dye-sensitized solar cells(DSC) research is reviewed, focusing on atomic-scale investigations of the interface electronic structures and dynamical processes, including the structure of dye adsorption onto TiO2, ultrafast electron injection, hot-electron injection, multiple-exciton generation, and electron–hole recombination. Advanced experimental techniques and theoretical approaches are briefly summarized, and then progressive achievements in photovoltaic device optimization based on insights from atomic scale investigations are introduced. Finally, some challenges and opportunities for further improvement of dye solar cells are presented.     

Scintillation of partially coherent Gaussian–Schell model beam propagation in slant atmospheric turbulence considering inner- and outer-scale effects

Based on the modified Rytov theory and the international telecommunication union-radio （ITU-R） slant atmospheric structure constant model, the uniform scintillation index of partially coherent Gaussian-Schell model （GSM） beam propa- gation in the slant path is derived from weak- to strong-turbulence regions considering inner- and outer-scale effects. The effects of wavelength of beams and inner- and outer-scale of turbulence on scintillation are analyzed numerically. Compar- ison between the scintillation of GSM beams under the von Karman spectrum and that of beams under the modified Hill spectrum is made. The results obtained show that the scintillation index obtained under the von Karman spectrum is smaller than that under the modified Hill spectrum. This study can find theory bases for the experiments of the partially coherent GSM beam propagation through atmospheric turbulence.     

Partially coherent Gaussian–Schell model pulse beam propagation in slant atmospheric turbulence

Based on the extended Huygens-Fresnel principle, a two-frequency, two-point cross-spectral density function of partially coherent Gaussian-Schell model pulse （GSMP） beam propagation in slant atmospheric turbulence is derived. Using the Markov approximation method and on the assumption that （w1 - w2）/（w1 ＋ w2） ≤ 1, the theory obtained is valid for turbulence of any strength and can be applied to narrow-band signals. The expressions for average beam intensity, the beam size, and the two-frequency complex degree of coherence of a GSMP beam are obtained. The numerical results are presented, and the effects of the frequency, initial pulse width, initial beam radius, zenith angle, and outer scales on the complex degree of coherence are discussed. This study provides a better understanding of the second-order statistics of a GSMP beam propagating through atmospheric turbulence in the space-frequency domain.