Effective Optical Coefficient Research Articles

Nonlinear optical performance of chemically tailored phthalocyanine–polymer films as solid-state optical limiting devices

Linear and nonlinear optical (NLO) investigations into the material response of a selection of solid-state phthalocyanine?polymer films are reported. Using the simple method of sequentially deposited spin cast phthalocyanine doped polymer films, one can characterize the resulting NLO response by means of the Z-scan technique with 6?ns pulses at 532?nm. Effective optical coefficients are calculated with a nonlinear absorption based model and their intensity dependence is investigated. When compared with the phthalocyanine solutions, the phthalocyanine?poly(methyl methacrylate) (PMMA) films exhibit excellent nonlinear extinction effects, outperforming corresponding phthalocyanine solutions by 2?3 orders of magnitude. Molecular engineering of metallophthalocyanine compounds, through both axial and peripheral substitutes, has led to an optimization of the resulting NLO response in the nanosecond regime. Mechanistic implications of the optical dissipation for various substituted phthalocyanines are also discussed and examined.

Effective electric and magnetic susceptibility of dilute systems of dielectric and metallic Mie particles

Based on a recent effective medium theory, applicable to the case of spherical particles of radius comparable or larger than the wavelength, we compare the behavior of the effective optical coefficients of random systems of transparent-dielectric and metallic particles as a function of the particles radius. We show numerical calculations and discuss the appearance of negative imaginary parts in either the effective electric or magnetic susceptibility in a system of Mie particles.

Nonlinear optical response of multiwalled carbon-nanotube dispersions

Experimental measurements of nonlinear optical extinction of nanosecond laser pulses by a set of conjugated copolymer/multiwalled carbon-nanotube composites dispersed in solution are reported here. The polymer poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) and multiwalled carbon-nanotube composites were varied according to nanotube mass content. The experiments were performed with an open-aperture Z scan with 6-ns Gaussian pulses at 532 nm from a frequency-doubled, Q-switched Nd:YAG laser. The nonlinear optical extinction of the incident pulses displays enhanced dissipation of the incident light for lower incident intensities relative to increasing multiwalled carbon-nanotube content. Either the multiwalled carbon nanotubes or the polymer dominates the nonlinear response of the composite depending on the relative mass of polymer to nanotube. Effective optical coefficients with a nonlinear absorption based model are calculated, and their intensity dependence is investigated. Mechanistic implications of the optical dissipation are also discussed.

Electromagnetic response of a random half-space of Mie scatterers within the effective-field approximation and the determination of the effective optical coefficients

We calculate the coherent reflectance of light from a half-space with randomly located spherical particles using the effective field approximation to solve the integral equation obeyed by the scattered fields. We obtain the effective refractive index and the coherent reflection coefficient to first order in the density of particles. Then we use basic principles of continuum electrodynamics to derive expressions for the effective permittivity and permeability tensors of the half-space. The results are not restricted to particles of radius small compared to the wavelength of the incident radiation. It is shown that an effective magnetic permeability arises, even in the case when the particles are non-magnetic. The present work provides a standard framework to pursue higher order approximations for denser media.

Optical characterization and modeling of black pigments used in thickness-sensitive solar-selective absorbing paints

The performance of black pigments used in thickness-sensitive solar-selective absorbing paints for solar thermal collectors depends on the optical properties of the pigments. Knowledge of the intrinsic optical properties of most paint pigments is very limited. Pellets made from either FeMnCuO x or black carbon dispersed in KBr matrix have been used to determine the effective optical coefficients of the pigments. Scattering and absorption cross-sections were derived from reflectance and transmittance measurements at near normal angle of incidence in the wavelength range 0.3 to 2.5 μm. Volume concentrations of the pigments that gave linear dependencies for the coefficients were found to be between 0.053 and 0.53% (FeMnCuO x ) and 0.076 and 0.31% (black carbon). Subsequently, a four-flux model has been used for calculation of reflectance for thickness-sensitive spectrally-selective paints. The paints were obtained from the FeMnCuO x and black carbon pigments embedded in silicone and phenoxy resin, respectively. We have used the experimentally determined scattering and absorption cross-sections of the pigments as input to the four-flux theory and the calculations have been performed for different thicknesses and/or pigment volume fraction of the paints. The calculated reflectances were compared with experiments and the results for thick films fit well.

Influence of a superficial layer in the quantitative spectroscopic study of strongly scattering media

We have experimentally investigated the meaning of the effective optical absorption [mu(a)((eff))] and the reduced scattering [mu(s)?((eff))] coefficients measured on the surfaces of two-layered turbid media, using the diffusion equation for homogeneous, semi-infinite media. We performed frequency-domain spectroscopy in a reflectance geometry, using source-detector distances in the range 1.5-4.5 cm. We measured 100 samples, each made of one layer (thickness in the range 0.08-1.6 cm) on top of one semi-infinite block. The optical properties of the samples were similar to those of soft tissues in the near infrared. We found that the measured effective optical coefficients are representative of the underlying block if the superficial layer is less than ~0.4 cm thick, whereas they are representative of the superficial layer if it is more than ~1.3 cm thick.