Sheng-Li Guo

Theoretical study on the closed-aperture Z-scan curves in the materials with nonlinear refraction and strong nonlinear absorption

Abstract The features of the closed-aperture (CA) Z-scan transmittance curves have been studied in detail when the third-order nonlinear refraction and absorption, both of which can be independently positive or negative, are simultaneously present. We found that whether the peak–valley or valley–peak structure exists or not, depends on the ratio ρ of the imaginary part χI(3) to the real part χR(3) of the third-order nonlinear susceptibility χ(3) which are caused by nonlinear refraction and absorption respectively. This communication also gives the ρ dependences of the distance between the peak position and valley position, the difference between peak transmittance and the valley transmittance, the ratio of peak height to the valley depth for CA Z-scan, and especially the exact critical value ρc which determines whether the peak–valley or valley–peak structure exists or not, with Gaussian beams for the first time to our knowledge.

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

Summary 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.

Synthesis, properties of fullerene-containing polyurethane–urea and its optical limiting absorption

Based on the typical two step polyurethane – urea synthesis, self-crosslinkable polyurethane – urea compounded of poly(tetramethylene oxide), MDI (4,4 0 -diphenylmethane diisocyanate) and extended by AEAPS (aminoethylaminopropyltrimethoxysilane) were prepared. On the other hand, a C60-amine adducts by reacting C60 with excessive APES (aminopropyltrimethoxysilane) were synthesized and introduced into the polyurethane – urea to form the homogenous and transparent C60 containing polyurethane – urea films. FT-IR, UV, Electron Spectroscopy for Chemical Analysis, Wide-angle X-ray diffractions, dynamic mechanical thermal analysis and mechanical properties of samples were recorded. The optical limiting properties were also measured. The results show that the C60 containing polyurethane – urea is a kind of homogenous material with the ranging of C60 content. The solution state before processing makes it easy to be cast as variety configurations. With the difference in C60 content in polymers, the optical limiting properties are various and the required transmittance and transmitted energy can be altered with the C60 contents in polymers. q 2003 Elsevier Science Ltd. All rights reserved.

Determination of optical nonlinearities in Cu(mpo)2 by Z-scan technique

The optical nonlinearities of Cu(mpo)2 [formula = C10H8N2CuO2S2] complex have firstly been investigated by using the Z-scan technique with a nanosecond-pulsed Nd:YAG laser at its second harmonic (532 nm) radiation. The nonlinear optical absorption and refraction coefficients of Cu(mpo)2 have been measured with the different on-axial peak irradiances I0 at the waist ranging from 0.48 to 5.66 GW/cm2. The nonlinear transmittance characteristics exhibit the near resonant two-photon absorption (TPA) at 532 nm and are explained by a population redistribution model. The nonlinear absorption originates from the near resonant TPA while the mechanism of the nonlinear refraction is the near resonant TPA transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant TPA.

Second Z-scan in materials with nonlinear refraction and nonlinear absorption

The Z-scan technique is a popular method for measuring optical nonlinearities using a single laser beam. We describe a simplified second Z-scan technique for cases where nonlinear refraction is accompanied by nonlinear absorption to separately evaluate the nonlinear refraction and the nonlinear absorption by performing straightforward measurement with the aperture removed. We divide the normalized transmittance of the closed aperture (S≤1) Z-scan by that of the open-aperture (S = 1) Z-scan. It is found that not only the ratio ρ of the imaginary part χI(3) to the real part χR(3) of the third-order nonlinear susceptibility χ(3) but also the linear transmittance S of the aperture has a great influence on the performance of the second Z-scan. Some useful results, which determine whether a second Z-scan can be performed or not, have been obtained for the ρ dependence of the critical linear transmittance Sc of the aperture and S dependence of the critical ratio ρc. These results may be applied to timely adjusted experimental conditions in order to obtain highly accurate data. We demonstrate this method on a CH3OH solution of Co2L3 with 7 ns laser pulses.

An improved calculation method on optical second-order susceptibilities of organic materials

Abstract An improved method to calculate the macroscopic second-order susceptibility of organic crystals is presented. The wave functions are obtained by the ZINDO single excited configuration interaction (ZINDO-MECI) method. The β cell tensor components for a unit cell of a crystal are calculated by the sum-over-states (SOS) method underlying the idea of “supramolecule” included the intermolecular interaction. Six organic compounds, urea, m -dinitrobenzene, 3-methyl-4-nitropyridine-1-oxide (POM), nitro-4-pyridino-2-( l )-prolinol (PNP), ( N )-(4-nitrophenyl)-( l )-prolinol (NPP) and m -aminophenol have experimental macroscopic NLO coefficients, which are used as a check. A reasonable agreement is obtained between calculated nonlinear optical coefficients and experimental values. The reliability of the method is discussed in terms of the UV spectra of the compounds. The results show that the procedure developed in this article provides a new insight into the second-order susceptibilities of organic molecular crystals, which is important in molecular engineering.

Photoluminescence properties of dinuclear lanthanide complexes in visible and near-infrared region

Several dinuclear lanthanide complexes of Ln2(HTH)6TPPHZ (Ln=Eu, Sm, Er, Nd, Yb, Ho; TPPHZ=Tetrapyrido (3,2-a:2′,3′-c:3′,2″-h:2′″,3′″-j) phenazine; HTH=4,5,5,6,6,6-heptafluroro-1-(2-thienyl)hexane-1,3-dione were synthesized and their photoluminescence properties were investigated. After ligand-mediated excitation, all the complexes showed the characteristic luminescence of the corresponding Ln(III) ions in the visible and NIR regions attributed to energy transfer from the ligands to the metal center. For Eu2(HTH)6TPPHZ complex, the quantum efficiency was calculated as 9.6% with the major 5D0 lifetime (477.86 μs) by the equation ϕ=τobs/τR, where τobs was observed luminescence lifetime and τR was radiative (or natural) lifetime (5 ms), and 14.9% with air-equilibrated aqueous [Ru(bpy)3](II)·2Cl− solution as a standard sample (ϕstd=2.8%), in CH2Cl2 solution at room temperature.

The mechanism of ferromagnetic ordering in PrNi3.9Cu1.1

Abstract Mean-field approximation and perturbation theory have been employed in present work to investigate the mechanism of ferromagnetic ordering in PrNi3.9Cu1.1. We find that its magnetic behaviors in ordering phase are mainly governed by the three lowest crystal-electric-field (CEF) levels; the indirect mixtures of the excited CEF levels into the ground state due to Heisenberg exchange helps to enhance the magnetic ordering at very low temperatures; but the new excited magnetic states formed by level mixtures impede the ordering near Curie temperature, thus play very crucial and subtle roles in modulating the magnetic process in the region and especially the phase transition.

INVESTIGATION OF THE INFLUENCE OF FINITE APERTURE SIZE ON THE Z-SCAN TRANSMITTANCE CURVE

For the nonlinear materials, not only the ratio ρ of the imaginary to the real part of the third-order nonlinear susceptibility but also the finite aperture size S exert a great influence upon the appearance of a peak on the Z-scan transmittance curve. This article investigates the ρ dependence of the critical aperture size Sc, which determines whether a transmittance peak occurs or not, and the dependence of the critical value ρc on the parameter S for the thin-sample and low irradiance limits with a Gaussian beam. These results may be applied to the adjustment of experimental conditions at all times and the assessment of the correctness of data analysis. The experiment with CS2 seeking the critical value Sc has verified above results.

Transition temperatures of rare-earth compounds studied with perturbation theory

Abstract The perturbation theory is employed in present Letter to study the transition temperatures of rare-earth magnets. To simplify the model, it is assumed that the ground and first excited crystal-electric-field (CEF) levels are both nonmagnetic singlets, the Heisenberg exchange interaction is only comparable to the energy gap ( Δ ) between them, and other higher levels are well separated from the first excited one, so their contributions to the bulk magnetization are negligible in low-temperature region. By treating the Heisenberg exchange as a perturbation to the CEF interaction near the critical point, explicit expressions for the Curie and Neel temperatures ( T C and T N ) are derived in terms of the Heisenberg exchange constant ( J ), the gap Δ and the matrix element of the total spin moment in the ordering direction between the two lowest CEF levels. For completeness and to verify the correctness of the theory, we have also derived the analytical expressions for the magnetic moments at zero temperature for both rare-earth ferromagnet and antiferromagnet, the conditions for the magnetic orderings, and calculated the spontaneous magnetizations and magnetic specific heats at finite temperatures numerically for a hexagonal ferromagnetic Pr compound with the model. This approach can be generalized to real systems to evaluate their transition temperatures and to investigate the mechanisms of magnetic ordering.