Two-photon transition and non-linear transmittance in CdSe0.5S0.5

The non-linear cross section for two-photon process has been measured in CdSe_0.5S_0.5 at two different wave-lengths by the single-beam non-linear transmittance technique. The experimental results are compared with the theoretical values.     

Four-photon transitions in ZnS

The non-linear photoconductivity in ZnS has been studied by means of three-photon and four-photon excitation with a ruby and neodymium laser, respectively. By using the experimental results obtained at the two different wavelengths and the previous value of three photon non linear cross section γ₃, a quantitative determination of γ₄ is obtained. The results are compared with three different theoretical calculations: semiclassical perturbation theory, quantum electrodynamic theory and Keldysh's theory.     

Third-order optical nonlinearity of a novel material: [(C<Subscript>4</Subscript>H<Subscript>9</Subscript>)<Subscript>4</Subscript>N][Co(dmit)<Subscript>2</Subscript>]

A novel dmit organometallic complex—[(C₄H₉)₄N][Co(dmit)₂] (dmit^2? = 4,5-dithiolate-1,3-dithiole-2-thione), abbreviated as BuCo, is synthesized and its third-order optical nonlinearity of the acetone solution is characterized using the Z-scan technique with a 40-ps pulse width at 1064 nm. Z-scan curves show that BuCo sample solution possesses the negative nonlinear refraction, exhibiting a self-defocusing effect. In addition, the two-photon absorption has been observed in BuCo. The effective molecular second-order hyperpolarizability γ_eff of the BuCo molecule was estimated to be as large as 5.60 × 10^?31 esu, suggesting BuCo is a potential material for optical device applications.     

Size dependence of non-linear optical properties of SiO2 thin films containing InP nanocrystals

SiO₂ composite thin films containing InP nanocrystals were fabricated by radio-frequency magnetron co-sputtering technique. The microstructure of the composite thin films was characterized by X-ray diffraction and Raman spectrum. The optical absorption band edges exhibit marked blueshift with respect to bulk InP due to strong quantum confinement effect. Non-linear optical absorption and non-linear optical refraction were studied by a Z-scan technique using a single Gaussian beam of a He-Ne laser (632.8 nm). We observed the saturation absorption and two-photon absorption in the composite films. An enhanced third-order non-linear optical absorption coefficient and non-linear optical refractive index were achieved in the composite films. The nonlinear optical properties of the films display the dependence on InP nanocrystals size. Received: 27 June 2000 / Accepted: 27 June 2000 / Published online: 13 September 2000     

Fe2O3纳米微粒溶胶非线性光学特性的Z-扫描研究

利用Z扫描技术在透明区域研究了Fe₂O₃纳米微粒水溶胶和表面包覆有机溶胶的非线性光学特性，给出了非线性折射率γ、双光子吸收系数β、自由载流子折射系数σ_r和自由载流子吸收截面σ_ab等重要物理参数，讨论了自由载流子效应对Fe₂O₃纳米微粒非线性特性的影响 关键词：     

Application of the model-free approach to the study of non-isothermal decomposition of un-irradiated and γ-irradiated hydrated gadolinium acetylacetonate

The non-isothermal decomposition of unirradiated and γ-irradiated hydrated gadolinium acetylacetone with 10² kGy γ-ray absorbed dose was carried out in air and in nitrogen atmospheres and in the temperature range of 25–1000°C. The results indicate that gadolinium acetylacetonate decomposes through four main decomposition steps leading to the formation of intermediate products whose chemical structure is independent of the gas atmosphere applied and on the investigated absorbed dose. The final product at 820°C was found to be Gd₂O₃ irrespective of the gas atmosphere and the irradiation conditions. The non-isothermal data were analyzed using linear Flynn–Wall–Ozawa and non-linear Vyazovkin (VYZ) iso-conversional methods. The results of the application of these free models on the present kinetic data showed that the activation energy, E_a is independent of α in a very wide conversion range (0.1–0.9) indicating that the decomposition process is controlled by a unique kinetic model. The results of the model-fitting analysis showed that the decomposition course of the four decomposition steps of hydrated gadolinium acetylacetone was controlled by the D₃ Jander diffusion model. Pure phase of Gd₂O₃ nanoparticles was obtained by thermal oxidation of γ-irradiated GdAcAc.3 H₂O at 800°C for 6 h. X-ray diffraction, transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques were employed for characterization of the as-synthesized nanoparticles. This is the first attempt to prepare Gd₂O₃ nanoparticles by solid-state thermal decomposition of γ-irradiated hydrated gadolinium acetylacetone.     

Calculation of the 1s–2s two-photon excitation cross-section in atomic hydrogen

The two-photon excitation cross-section of atomic hydrogen is calculated using explicit summation over intermediate states within the framework of dipole approximation. The matrix element for two-photon excitation is transformed into finite sums, consisting of the product of a radial and angular part. Nine intermediate states are employed in the calculation of the transition matrix element. The two-photon excitation cross-section obtained for the transition 1s ²S_1/2–2s ²S_1/2in atomic hydrogen is a good agreement with the literature.     

Measurement of the ββ-decay rate of 100Mo to the first excited 0+ state in 100Ru

A new and independent confirmation of ββ(2ν) decay of ¹⁰⁰Mo to the first 0⁺ excited state in ¹⁰⁰Ru has been made. This was accomplished using a two-photon coincidence counting technique in which two HPGe detectors were used to observe the two emitted γrays (E _γ1 = 590.8 keV and E _γ2 = 539.6 keV) from the daughter nucleus as it deexcites to the ground state (0⁺ → 2⁺ → 0⁺). The half-life of this decay has been estimated as ～(5–8)×10²⁰ yr.     

Two-photon fluorescence measurements in a hexagonal CdS crystal

Measurements of two-photon fluorescence (TPF) in a CdS crystal using a dye laser pumped by a Q-switched ruby laser are reported. The TPF is measured as a function of source intensity and wavelength. The total fluorescence shows linear and non-linear dependence on the source intensity; from these measurements the TPF part is determined. The measured transition of the crystal's fundamental gap is 2.5 ± 0.02 eV and another forbidden one-photon but allowed two-photon transition is found at 3.24 ± 0.02 eV.     

Synthesis and Characterization of New Fluorene-Based Singlet Oxygen Sensitizers

The synthesis, photophysical characterization, and determination of singlet oxygen quantum yields (Φ_Δ) for a class of fluorene derivatives with potential application in two-photon photodynamic therapy (PDT) is reported. It has been demonstrated that these compounds possess the ability to generate singlet oxygen (¹O₂) upon excitation. A photochemical method, using 1,3-diphenylisobenzofuran (DPBF) as ¹O₂ chemical quencher, was employed to determine the singlet oxygen quantum yields (Φ_Δ) of the fluorene-based photosensitizers in ethanol. Φ_Δ values ranged from 0.35 to 0.75. These derivatives may have potential application as two-photon photosensitizers when pumped via two-photon excitation in the near-IR spectral region.     

Specific features of the two-photon absorption of cationic symmetric polymethine dyes

The two-photon absorption of a number of cationic symmetric polymethine dyes based on 3H-indolium and benzothiazolium is experimentally studied upon excitation by nanosecond radiation from a Nd:YAG laser (1064 nm, 12 ns). The two-photon absorption cross section of dye molecules is determined by the two-quantum standard method. The influence of the spectral-luminescent properties and structure of the polymethine dyes on the two-photon absorption cross section is discussed. It is shown that, upon excitation in the range of the long-wavelength band of the dyes studied (the S ₀ → S ₁ transition), the maximum of their two-photon absorption is blue shifted. The reasons for a considerable increase in the two-photon absorption of symmetric polymethine dyes upon their excitation in the range of the S ₂ state are discussed. Using the data of quantum-chemical calculations, it is shown that, along with changes in the selection rules for two-photon transitions, this increase is connected with an increase in the size of the delocalized π electron cloud of HOMOs involved in the S ₀ → S ₂ transition.     

Dipyrrylmetheneboron Difluorides as Labels in Two-Photon Excited Fluorometry. Part I-Immunometric Assays

Seven different two-photon excitable dipyrrylmetheneboron difluoride labels (dipyrrylmethene-BF₂ labels) and a frequently used TAMRA label were conjugated to mouse IgG against α-fetoprotein in variable substitution degrees. Altogether 40 IgG conjugates were prepared, and studied with respect to one-photon absorption and emission properties, and two-photon fluorescence efficiency using 1064 nm laser as illumination source. Performance of the IgG conjugates as tracers in a separation-free immunometric assay of α-fetoprotein was evaluated using two-photon excitation assay technology, ArcDia^TM TPX. The results show that the dipyrrylmethene-BF₂ labels provide subpicomolar sensitivity, which is an order of magnitude better than that of TAMRA label. The effect of chromophore structure and substitution degree of IgG-label conjugates on the assay performance is discussed.     

Thermal accommodation coefficients by high speed vibration of solid samples: VI. O2 and N2 on As(111) and Sb(111) single crystal surfaces

Translational γ_t and rotational γ_r energy accommodation coefficients are measured using a high speed vibrational method described earlier. For N₂ on cleaved As(111) single crystal surfaces γ_t = 0.34 ± 0.03, γ_r = 0.03 ± 0.03; on cleaved Sb(111) single crystal surfaces γ_t = 0.49 ± 0.02, γ_r = 0.06 ± 0.04. For O₂ on As(111) γ_t = 0.39 ± 0.02, γ_r = 0.02 ± 0.02; on Sb(111) γ_t = 0.56 ± 0.05, γ_r = 0.07 ± 0.06. (Uncertainties are statistical estimates of precision. In addition, it is possible that the results are systematically low by up to 10%.) Comparison with N₂ and O₂ accommodation coefficients measured by the same method on other metal substrates suggests that the antimony surface is covered by chemisorbed oxygen but that the arsenic surface is somewhat cleaner. The results are consistent with oxidation of these semimetals occurring by way of physisorbed gas precursor states.     

Two-photon cross-section measurement of meso-tetra-hydroxyphenyl-chlorin using femtosecond laser pulses

Experiments were carried out using the femtosecond laser to compare the fluorescence amplitude produced by two-photon excitation at 800 nm and 400 nm (i.e. to the strong absorption band in mTHPC). Using an experimental arrangement where the fluorescence sampling geometry was kept unchanged between the two exposure wavelengths a value for the two-photon cross-section of mTHPC at 800 nm of ?1.8 × 10^?57 m⁴ s/molecule/photon was derived. This is larger than the value estimated using a simple theoretical model, probably because the calculation neglects ??resonant?? assistance provided by the real state at ??650 nm (S ₀). In terms of order of magnitude the values are in agreement with the two-photon cross-section for other organic species.     

Ac Stark splitting of two-photon spectra

We observe distinct ac Stark splitting of a two-photon transition (the optical Autler-Townes effect) using an atomic beam of sodium and two cw dye lasers. The positions and splittings of the two-photon lines agree closely with the theory for a simplified three-level atom, even though sodium has a more complicated level scheme. This simplicity results when the Rabi nutation frequency is considerably greater than the 190 MHz hyperfine splitting of the 3P_{$\text{1}\text{2}$} intermediate state.     

Absolute determination of the two photon cross section in NADH

Experimental evidence of two photon absorption in NADH coenzyme at ruby frequency is reported. The γ₂ non linear two photon cross section has been measured by means of two different experimental methods. The high absolute value found (γ₂ = 3.4 x 10^-48cm⁴s) confirms that the reaction rate modifications of NADH under laser irradiation are due to non-linear absorption processes.     

Investigation of optical nonlinearities in Pd(po)2 by Z-scan technique

With nanosecond scale at a 532 nm wavelength, we firstly measured the nonlinear optical absorption and refraction coefficients of Pd(po)₂ complex by using Z-scan technique, here Hpo=1-hydioxy-2-pyridone. We describe an empirical expression for the case when nonlinear refraction is accompanied by nonlinear absorption to separately evaluate the nonlinear refraction and the nonlinear absorption by performing straightforward measurements with the aperture removed. The nonlinear optical response of Pd(po)₂ was determined by the linear decreasing irradiance-dependence. The nonlinear absorption originates from the near resonant two-photon absorption while the mechanism of the nonlinear refraction is the near resonant two-photon absorption transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant two-photon absorption.     

Fastscan z-scan system for determining optical non-linearities in semiconductors

Traditional z-scan techniques have been used to determine the non-linear optical coefficients of semiconductors. This is usually carried out by scanning a sample along the optical z-axis using a manual or computer-controlled linear translation stage. We report upon a novel z-scan method involving a fastscanning sampleholder: This gives real-time z-scan traces and greatly improves experimental efficiency and applicability. The technique is used here to study the two-photon absorption coefficient of ZnSe.     

双光子吸收的Franz-Keldysh效应

从实验上证实Hg_0.695Cd_0.305Te 光电二极管空间电荷区中存在双光子吸收的Franz-Keldysh效应.利用一个皮秒Nd:YAG激光器抽运的光学参量产生器和差频产生器作为激发光源,测量了入射波长为λ₀=7.92μm的脉冲激光所激发的光响应随入射光强的变化关系.脉冲光响应峰值强度随入射光强的增大呈现二次幂函数增强趋势.采用等效RC电路模型将脉冲光伏信号峰值与入射光强相关联,得到空间电荷区中强电场下单光束 关键词： Franz-Keldysh效应 碲镉汞 双光子吸收 脉冲光伏信号     

Vacancy ordering in γ-Fe2O3 small particles

The effect of particle size on the formation of vacancy-ordered superstructure in γ-Fe₂O₃ powders has been investigated by using X-ray, Mössbauer and chemical analyses. Powders of γ-Fe₂O₃ with different average particle size were prepared by chemical precipitation and subsequent heat-treatment. The X-ray diffraction intensity of the superlattice lines decreases with the particle size of γ-Fe₂O₃ and finally disappears at a particle-size between 300-175 Å, possibly around 200 Å. Therefore ordering of the cation vacancies in ultrafine γ-Fe₂O₃ particles is ruled out. Although the vacancies do not form an ordered structure, they do exclusively occupy B-sites.