Nonlinear optical absorption properties of porphyrins confined in Nafion membrane

The nonlinear absorption (NLA) properties of five different types of porphyrins were studied using the Z-scan technique. The porphyrins under investigation were confined into Nafion column matrix membrane in order to protect them from possible degradation. The results of the experiments have indicated that all the porphyrins tested exhibited interesting NLA properties. The nonlinear absorption coefficients (β’s) were determined at different porphyrin concentrations by comparing the Z-scan data with the theoretical functions.     

Microstructure and optical absorption of Au-MgF2 nanoparticle cermet films

The microstructure and optical absorption of Au-MgF2 nanoparticle cermet films with different Au contents are studied. The microstructural analysis shows that the films are mainly composed of the amorphous MgF2 matrix with embedded fcc Au nanoparticles with a mean size of 9.8-21.4nm. Spectral analysis suggests that the surface plasma resonance （SPR） absorption peak of Au particles appears at λ=492-537nm. With increasing Au content, absorption peak intensity increases, profile narrows and location redshifts. Theoretical absorption spectra are calculated based on Maxwell-Garnett theory and compared with experimental spectra.     

Tunable intrinsic optical bistability of Au–Ag alloy nanoparticle: effects of spatial position and Au composition

The intrinsic optical bistability (IOB) of an Au–Ag alloy nanosphere is theoretically investigated. Calculation results based on quasi-static theory and Kerr nonlinearity show that the optical bistable behavior for both spatial average response and local field at the hot location only exists when the local electric field is enhanced and exceed the threshold intensity, which is dependent on the spatial position and gold composition. Our theoretic investigation also presents a physical picture of how the local field enhancement affects the IOB characters of the alloy nanoparticles.     

Nonlinear optical and optical limiting studies of alkoxy phthalocyanines in solutions studied at 532 nm with nanosecond pulse excitation

We report our results on the nonlinear optical and optical limiting properties of two alkoxy phthalocyanines namely 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine and 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) studied at a wavelength of 532 nm using 6 ns pulses. Using the standard Z-scan technique we observed that both the phthalocyanines exhibited negative nonlinearity as revealed by the signature of closed aperture data. The magnitude of the nonlinear refractive index n₂ evaluated from the closed aperture data was ∼ 1.61×10^-11 cm²/W for the free-base phthalocyanine and ∼ 1.56×10^-11 cm²/W for the metallic phthalocyanine. Open aperture Z-scan data indicates strong nonlinear absorption in both the phthalocyanines with measured nonlinear coefficients of ∼ 1650 cm/GW and ∼ 1850 cm/GW respectively. We also report optical limiting properties of these phthalocyanines with limiting thresholds (I_1/2) of ∼ 0.5 J/cm². Our studies suggest that these phthalocyanines are one of the best molecules for nonlinear optical applications studied recently. PACS 42.65.-k; 42.70.Jk, 42.65.Jx     

Nonlinear refraction of silver nanowires from nanosecond to femtosecond laser excitation

In order to investigate the effect of pulse width and solvent on the nonlinear properties of metal nanostructures, silver nanowires were fabricated in a direct current electric field (DCEF) using a solid-state ionic method and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nonlinear refractive index (γ) of silver nanowires suspended in ethanol was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nanosecond) pulse durations. Experimental results indicated that silver nanowires have obvious positive refractive nonlinearities and γ (the Kerr-induced self-focusing) increases as the pulse duration increases from 7.4×10⁻⁸ cm²/GW at 110 fs to 1.6×10⁻⁴ cm²/GW at 8 ns, due to the additional influence of the atomic reorientational Kerr effect in the case of longer pulses. Due to the solvent dependence of the nonlinear behavior of the silver nanowires, the nonlinear absorption and refraction of silver nanowires suspended in de-ionized water are smaller than those of silver samples suspended in ethanol. The thermal nonlinearities are insignificant in our experimental conditions.     

Nonlinear optical and magneto-optical effects in non-spherical magnetic granular composite

The magnetization-induced nonlinear optical and nonlinear magneto-optical properties in a magnetic metal-insulator composite are studied based on a tensor effective medium approximation with shape factor and Taylor-expansion method. There is a weakly nonlinear relation between electric displacement D and electric field E in the composite. The results of our studies on the effective dielectric tensor and the nonlinear susceptibility tensor in a magnetic nanocomposite are surveyed. It is shown that such a metal-insulator composite exhibits the enhancements of optical and magneto-optical nonlinearity. The frequencies at which the enhancements occur, and the amplitude of the enhancement factors depend on the concentration and shape of the magnetic grains.     

Nonlinear optical properties of Au/PVP composite thin films

Colloidal Au and poly（vinylpyrrolidone） （PVP） composite thin films are fabricated by spin-coating method. Linear optical absorption measurements of the Au/PVP composite films indicate an absorption peak around 530 nm due to the surface plasmon resonance of gold nanoparticles. Nonlinear optical properties are studied using standard Z-scan technique, and experimental results show large optical nonlinearities of the Au/PVP composite films. A large value of χ^（3）/α up to 0.56× 10-^10esu·cm is obtained, which is comparable to the best values reported in metal/oxlde composite films.     

K~π=5~- rotational bands in neutron-rich Mo,Ru,and Pd nuclei

The collective rotations of the K~π = 5~- configuration in neutron-rich Mo,Ru and Pd isotopes were systematically investigated by the configuration-constrained cranking shell model based on the Skyrme Hartree-Fock method with pairing treated by shell-model diagonalization.The calculations efficiently reproduce the experimental moments of inertia of both the ground-state and side bands.Rotational bands built on two-particle K~π = 5~- configurations have been the subject of intense study.Possible configurations were assigned to the observed 5~- bands in ~(102-106) Mo,~(108-112) Ru and ~(112-114) Pd.We predict the existence of the 5~-bands in ~(108,110) Mo.These results provide deep insights into the structure of neutron-rich nuclei,and provide useful information for future experiments.     

Models of optical absorption in amorphous semiconductors at the absorption edge — A review and re-evaluation

Davis-Mott and Tauc models of optical absorption at the absorption edge in the high absorption coefficient region (10⁴cm^–1) are carefully reviewed with regard to their theoretical foundations, assumptions, mathematical derivations, and results. The full implications of these models are exploited, and it is found that the Davis-Mott model for negligible matrix elements between localised states could account for the cubic power law behaviour of with photon energy of some amorphous semiconductors such as a-Si. A fractional power law to find the optical band gapE _opt, of the form [ (–E _opt)^r; 2r3] based on Davis-Mott model is proposed in which the indexr can be a function of disorder. The Tauc model has further been extended to the case of negligible matrix elements between localised states, in which the same square power law for vs. with the same meaning of the optical gap as in the original Tauc model has resulted. A consideration of the case of unequal matrix elements for those transitions between localised states and those between extended states is also included. The meaning ofE _opt has been re-assessed and it is emphasized that it is an extrapolation of delocalised states to the zero of the density of states rather than a threshold energy for the onset of some kind of optical transitions.This paper is cordially dedicated to Professor C. A. Hogarth who taught us the Physics of amorphous materials.     

Excited-state absorption and third-order optical nonlinearities in symmetric π-electron organic molecules

Excited-State Absorption (ESA), Two-Photon Absorption (TPA) and the third-order polarizability (;,, – ) have been investigated for a model dichloride derivative of a symmetrically substituted benzylidene analine (SBAC), using a multielectron configuration-interaction procedure. The calculations indicate that SBAC exhibits ESA across the visible region of the spectrum, but that it is not as extensive as for molecules such as the phthalocyanines. The magnitude of the third-order polarizability is dominated by resonance enhancement from a very strongA _g B _u one-photon absorption. The calculated off-resonance value for (;,, – ) suggests that SBAC is a potential candidate for ultrafast switching applications.     

Synthesis, structural, optical and electrical properties of metal nanoparticle–rare earth ion dispersed in polymer film

Cu-nanoparticles have been prepared by ablating a copper target submerged in benzene with laser pulses of Nd:YAG (wavelength: 355, 532 nm and 1,064 nm). Colloidal nanoparticles have been characterized by UV–Vis spectroscopy and transmission electron microscopy. The obtained radius for the nanoparticles prepared using 1,064 nm irradiation lies in the range 15–30 nm, with absorption peak at 572 nm. Luminescence properties of Tb³⁺ ions in the presence and absence of Cu-nanoparticles have been investigated using 355 nm excitation. An enhancement in luminescence of Tb³⁺ by local field effect causing increase in lifetime of ⁵D₄ level of Tb³⁺ ion has been observed. Frequency and temperature-dependent conductivity of Tb³⁺ doped PVA thin films with and without Cu-nanoparticles have been measured in the frequency range 20 Hz–1 MHz and in the temperature range 318–338 K (well below its melting temperature). Real part of the conductivity spectra has been explained in terms of power law. The electrical properties of the thin films show a decrease in dc conductivity on incorporation of the Cu-nanoparticles.     

Nonlinear light bullets in purely lossy,self-focusing media

We report on light bullet propagation supported by a dynamic equilibrium between self-focusing and nonlinear losses. Unlike solitons, self-focusing is not eliminated, but only frozen. Losses are not compensated by a conical energy flux, but only by that arising from self-focusing. These light bullets rebuild after obstacles, a property that was thought to be exclusive of conical waves.     

Dynamic changes of optical characteristics of resonant domains in metal nanoparticle aggregates under pulsed laser fields

A systematic analysis of the impact of laser radiation on disordered metal nanoparticle aggregates has been carried out using the simplest model of a resonance domain in the form of a bisphere. Such aggregates can be considered as a set of elementary domains with different eigen-resonant frequencies. The model is basically a pair of bound spherical nanoparticles having equal radii and covered with a polymer adsorption layer. A significant change in the resonance properties of the domain occurs due to the change of interparticle gaps under applied radiation. We investigated the temporal variation of parameters and hence optical characteristics of coupled Ag nanoparticles under pulsed laser radiation for different wavelengths, intensities and pulse durations and for different initial values of the interparticle gap.     

Bragg suppression for optical absorption in multi-quantum well structures

Direct calculations of absorption spectra for multi-quantum well structures by extracting field distributions at well positions are performed. Results demonstrate the previously reported Bragg suppression, and agree exactly with the indirect calculation by linear dispersion theory. This reveals that Bragg suppression effect in fact originates from the remarkably decreased intensities at well positions by Bragg interference condition, rather than from the formation of supperradiant modes.     

Quasi-phase-matched optical activity effect in “gyroelectric” crystals and its applications

“Quasi-phase matching” is first introduced to the optical activity (OA) effect and a wave coupling theory for the quasi-phase-matched (QPM) OA effect in periodically poled “gyroelectric” crystals is developed. The OA effect is observed clearly even though the propagating direction of light deviates far from the optical axis in the QPM crystal with both optical activity and natural birefringence. The QPM OA effect provides a special way to determine the gyration coefficients that cannot be observed by the normal OA effect, and it provides a principle for building optical filters without external field.     

Large Decay Rate Variation of ^7Be in Pd and Au

Platinum and gold have the similar crystal structure but different electronic affinities, as well as different effective electron densities near the implanted ions. Both the differences favour larger decay rate of ^7 Be in Pd than that in Au. We measured the variation of the decay rate of ^7Be implanted in Pd and Au host materials. We have found that the decay rate of ^7 Be in Pd is larger than that in Au by 0.8%.     

Nonlinear optical effects in Hg1−xCdxTe1

The alloy semiconductor Hg_1−xCd_xTe has widespread use in modern infrared detection equipment. By adjusting the ratio of Hg to Cd, the energy gap can be tailored to any desired value between 0 eV (x = 0.17) to 1.4 eV (x = 1.0). For values of x near 0.2 the electron effective mass is small, giving rise to a large spin-level splitting factor, or ɡ-value. We have examined two nonlinear optical effects which depend upon the high ɡ-value found in small gap Hg_1−xCd_xTe. These are the spin-flip Raman laser and 4-photon mixing. In each case we employ CO₂ lasers as the input source. In order to achieve resonant enhancement, we adjust the energy gap so that the absorption edge at the temperature of operation, 4 K, is near 10 μm; the required composition is Hg_0.77Cd_0.23Te.During operation as a spin-flip Raman laser, we observe 1st Stokes, 2nd Stokes and anti-Stokes signals, with a basic tunability of 3.8 cm⁻¹/kG for the 1st Stokes radiation. In the 4-photon mixing mode, we see resonant production of an output signal at frequency ω₄ = 2ω₂ − ω₁, where ω₂ and ω₁ are the frequencies of the two CO₂ pump lasers. We also observe under some conditions 6 photon mixing, i.e. ω₆ = 3ω₂ − 2ω₁.     

Nonlinear optical absorption in Bi3TiNbO9 thin films using Z-scan?technique

Bi₃TiNbO₉ (BTN) thin films with layered perovskite structure were fabricated on fused silica by pulsed laser deposition. The XRD pattern revealed that the films are single-phase perovskite and highly (00l) textured. Their fundamental optical constants, such as band gap, linear refractive index, and linear absorption coefficient, were obtained by optical transmittance measurements. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The nonlinear optical absorption of the films was investigated by single beam Z-scan method at a wavelength of 800 nm with laser duration of 80 fs. We obtained the nonlinear absorption coefficient β=1.44×10⁻⁷ m/W. The results show that the BTN thin films are promising for applications in absorbing-type optical devices.