Optical nonlinearities of gold nanoparticles protected by ligands have been investigated by Z-scan technique. Absorptive and refractive optical limiting (OL) effects have been measured with 8 ns duration laser pulses at 532 nm. Significant differences have been found experimentally for these two nanoparticles that differ only by their ligands. Such differences indicate that nonlinear optical responses in two nanocomposites are strongly influenced by the surface plasmon resonance of gold nanoparticles and also the nature of ligands. Both of these two nanocomposites show stronger refractive optical limiting effects than C60 in toluene.

A new approach to superior optical limiting materials, utilizing planar 'open' skeleton heterothiometallic clusters, is reported by incorporating heavy atoms and altering cluster skeletons and is found to decrease the limiting thresholds to as low as 0.07 J cm-2, among the best for OL materials.

The excited -state nonlinear refraction of a novel cubane-like shaped metal cluster MoOS3ICu3 (2-Mepy)3 IN DMF solution is studied by using Z-scan measurement technique with laser pulses of 10 ns and 40 ps pulse width at the wavelength of 532 nm, respectively. For picosecond pulses the compound possesses a self-focusing property, the positive nonlinear refraction is attributed to population transitions among singlet states, while for nanosecond pulses it exhibits a self-defocusing property and the negative nonlinear refraction is attributed to population relaxing to triplet states.

The excited state absorption and refraction of a novel cubane-like shaped metal cluster WSe4 (AgPPh3). Cl in DMF solution were studied by using the Z-scan technique with laser pulses of 40 ps pulse-width at the wavelength of 532nm. 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 as ymmetry in the Z-scan results.

The title compounds [Et4N]2[MS4Cu4(SCN)4(2-pic)4] (M =W 1, Mo 2) have been synthesized by the reaction of (Et4N)2MS4, Cu(SCN) and 2-picoline (2-pic, 2-methylpyridine). Single crystal X-ray diffraction data show that the anion clusters [MS4Cu4(SCN)4(2-pic)4]2-have the planar'open' structure with four Cu atoms in three kinds of coordination modes. Nonlinear optical properties of these two clusters are investigated with a 8 ns pulsed laser at 532 nm. The two clusters exhibit large optical limiting performance, with limiting threshold values of 0.3 J cm-2 for 1, 0.5 J cm-2 for 2, and self-defocusing effects, effective nonlinear refractive index n2 =-6.84×10-12 esu (esu =7.162×1013 m5 v-2) 1 and n2=8.48×10-12 esu 2 respectively. Both compounds show reverse saturable absorption: α2 = 3.1×10-6 m W-1 for 1 and α2=3.2×10-6 m W-1 for 2 in 6.98×10 4 mol dm-3 and 7.44×10-4 mol dm 3 DMF solution respectively. The corresponding effective NLO susceptibilities χ(3) are 6.5×10-8 esu 1 and 8.9×10-8 esu 2 while the corresponding hyperpolarizabilities (γ(1)=9.42×10-32 esu and γ(2)=1.29×10- 31 esu) are also reported.

Nonlinear absorption and refraction of novel cubane-like shaped metal clusters WOS3Cu3I(2-Mepy)3 (I) and MoOS3Cu3I(2-Mepy)3 (Ⅱ) in DMF solution are studied by using Z-scan measurement technique with laser pulses of 10 ns pulse-width at the wavelength of 532 nm. The compound (I) possesses a self-focusing property, while compound (Ⅱ) exhibits a self-defocusing property. The refractive nonlinearity of the cluster can be attributed to the excited-state nonlinear refraction e€ect. A theoretical model of the excited-state refraction is presented to analyze the experimental results. The theoretical results are in good agreement with the experimental data. The condition of the sign change of nonlinear refraction is also studied. Ó 2000 Elsevier Science B.V. All rights reserved.

Optical limiting property of the new cluster [WOS3Cu3I(2-MePy)3] is studied. The comparison of the optical limiting performance of this compound with that of C60 fullerene has demonstrated to be superior to the C60 fullerene. The optical limiting behavior of the cluster can be attributed to the excited-state absorption, which is measured by using Z-scan technique.

Singlet excited-state absorption properties and optical limiting behaviors of organochromium and organomolybdenum C60 derivatives (C89H24CrO3P2 and C89H24MoO3P2) were studied by using a double frequency ns/ps Nd:YAG pulse laser system at wavelength of 532 nm and pulse width 21 ps. The experimental results demonstrated that the optical limiting behavior of organochromium C60 derivative is better than both organomolybdenum C60 derivative and fullerene-C60 itself. The experimental results agree well with the singlet excited-state absorption theory.