Wang Yu-Xiao

Transformation of Sign of Nonlinear Refraction between Mo(W)/S/Cu Planar Metal Clusters

Optical nonlinear refractive properties of a series of Mo(W)/S/Cu planar square clusters are investigated using the Z-scan technique with the ns laser pulses at the wavelength of532nm. The result shows that the planar metal clusters containing the halogen ligands demonstrate the self-focusing effect, and the other planar metal clusters demonstrate the self-defocusing effect. These facts indicate that the halogen ligands can act as crucial factors in determining the sign of the nonlinear refraction of the Mo(W)/S/Cu planar metal clusters. The analysis of the experimental data shows that the planar clusters with halogen ligands possess greater refraction volume of the excited state than that of the ground state, while the other planar clusters possess the smaller refraction volume of the excited state than that of the ground state.

Nonresonant Saturation Absorption in Electrostatic Self-Assembly Films Containing Methanofullerene

Ultrathin films containing methaofullerene are prepared by electrostatic layer-by-layer self-assembly of a positively charged trifluoroacetic acid salt of monoamino-substituted methanofullerene (TMAF) derivative and a negatively charged poly (sodium 4-styrenesulfonate) (PSS). Absorptive optical nonlinearities of 30 bilayer TMAF/PSS films and TMAF in water solution are measured using the Z-scan technique at 532 nm. Nonresonant saturation absorption is observed in the film sample, whereas reverse saturation absorption was for TMAF in water. The saturation-absorption behaviour is interpreted by a special formula. The saturation intensity is extracted to be 15.2±0.8 MW/cm2. The mechanism of saturation absorption in the TMAF/PSS film is discussed.

Enhanced Excited-State Optical Nonlinearities in a Novel Metallophthalocyanine Compound (C12H25O)8PcPb

Nonlinear absorption in a novel metallophthalocynine compound (C12H25O)8PcPb was investigated by the Z-scan technique with the irradiation of 8 ns laser pulses at the wavelength of 532 nm. The large optical limiting response was observed. The nonlinear absorption cross section was obtained by the simulation with a simplified rate equation model in which excited triplet-triplet state absorption is dominant. The excited-state absorption cross section and its effective ratio to the ground-state absorption cross section are larger than those of C60. The attenuation factor for (C12H25O)8PcPb is 2.5 times larger than that of C60, which is induced by the heavy atom lead interposition.

Excited state optical nonlinearity of inorganic metal cluster WSe4(AgPPh3)3Cl with picosecond pulses

Abstract The excited state absorption and refraction of a novel cubane-like shaped metal cluster WSe 4 (AgPPh 3 ) 3 Cl in DMF solution were studied by using the Z-scan technique with laser pulses of 40 ps pulse-width at the wavelength of 532 nm. The compound possesses a self-focusing performance. The positive nonlinear refraction is attributed to population transitions among singlet states. The cluster displayed a strong excited state absorption, which induced asymmetry in the Z-scan results.

Nonlinear Absorption and Refraction in Multilevel Organic Molecular System

The nonlinear absorption and refraction in a multilevel organic molecular system is described by using the density matrix theory. The total absorptive coefficient of the system in the low-density case is equal to a linear sum of contributions from each energy level. Similarly, the total refractive index is equal to a linear sum of contributions from each energy level plus the refractive index of the vacuum. The absorption coefficient or refractive index due to each level is proportional to the population of that level, where the constant of proportionality is called the absorption cross-section or the refraction volume, respectively. The relation between the absorption cross-section and the refraction volume for each level is also given.

NONLINEAR REFRACTION IN MULTILEVEL MOLECULAR SYSTEMS

The intensity-dependent nonlinear refraction phenomena in a non-resonant multilevel molecular system is studied using rate equations in steady state, including saturable refraction, reverse saturable refraction, and the turnover between these two effects. The relationship between the refraction saturation and the optical Kerr effect is also discussed.

Nonlinear optical properties in double-sided nonlinear media with Z-scan technique based on the Huygens--Fresnel principle

We present a theoretical model to analyse the propagation of a Gaussian laser beam through double-sided nonlinear media. This model is based on the Huygens?Fresnel diffraction integral method. This theoretical model is not only consistent with the cascade structure model for a small nonlinear phase-shift but also can be used for a large nonlinear phase-shift. It has been verified that it is suitable to characterize the double-sided nonlinear media compared with the cascade structure model. A good agreement between the experimental data and the results from the theoretical model is obtained. It will be useful for the design of multi-sided nonlinear materials.

Optical limiting effect in two phthalocyanines observed by picosecond pulsed laser

Optical limiting (OL) properties of two phthalocyanines were investigated by using picosecond laser pulses at 532nm. The relative ratios k of the absorption cross section of the first singlet excited state to that of the ground state were approximately obtained by the analysis of the experimental results in which the reverse saturable absorption model of the three-energy-level scheme was employed. A significant comparison with fullerene C60 was presented for OL. The OL mechanisms have been analysed in detail.

EXCITED-STATE NONLINEAR ABSORPTION AND ITS APPLICATION IN PHOTONIC TECHNOLOGY

The main achievements in our research on the physical phenomena of the excited-state nonlinear absorption in a molecular system and its applications in photonic technology are described. In the first part of this paper, some energy-level models and rate-equations are used to explain various mechanisms of the excited-state nonlinear absorption under different conditions, such as reverse saturable absorption caused by the triplet and/or the singlet excited-state absorption; saturable absorption due to the first and/or the second singlet excited-state absorption; and the excited-state nonlinear absorption induced by two-photon absorption. The experimental results for metal-organic and C60 materials irradiated by ps and ns laser pulses are consistent with the simulated curves of the transmittance versus fluences. In the second part, the applications of excited-state nonlinear absorption in photonic techniques including optical bistability, optical switching, optical limiting, optical modulation, optical logic and optical storage are introduced. The working principles of the photonic devices based on the excited-state nonlinear absorption are presented. The experimental characteristic curves are found in good agreement with the theoretical simulations for these devices.

Measurement of Ultrashort Laser Pulse Width in a Wide Spectrum Range Using Dimethyl Sulfoxide by Optical Kerr Effect Technique

We illustrate that applying the optical Kerr effect technique to dimethyl sulfoxide (DMSO) can determine the width of laser pulses in a wide spectrum range that is transparent. Different from many other simple molecular liquids, such CS2, and toluene, DMSO responds to both 130 fs 800nm laser pulses and 248 fs 400 nm laser pulses with electronic motions only. Therefore, the observed signal as a function of time reflects the temporal distribution of the intensity. We verify our illustration by both the autocorrelation technique and spectra analysis.