摘要:

In low glass-transition temperature photorefractive materials, push-pull chromophores induce photorefractivity essentially via two mechanisms: optical birefringence and Pockels electrooptic effect. A 2-state 2-form description of these molecules indicates how the molecular structure can be optimized to maximize the photorefractive effect. At the optimum structure the two contributions have opposite signs. For the molecular sizes used currently the optical birefringence should dominate photorefractivity. Lengthening the conjugation path leads to a steeper increase of the Pockels electrooptic effect. Yet for the molecular sizes examined in this work, this contribution remains smaller than the optical birefringence at the optimum structure. Huge photorefractive responses could by attained if the molecular structure can be tuned towards the cyanine limit while increasing the molecular size.

ISSN:1058-725X    

DOI:10.1080/10587259808030193

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report about a nematic liquid crystal device suitable for efficient coherent beam amplification. A proposed novel hybrid structure is composed of nematic liquid crystal layer sandwiched between suitable ultra thin photoconducting (Disperse Red 1 functionalized polythiophene) polymeric surface layers serving as orienting as well as space charge generating layers upon light incidence. The gain coefficient measured at incidence angle 45∠ in 10 μm thick liquid crystal layer in a conventional two-wave mixing experiment is one of the highest reported so far for the similar systems and amounts tog= 7. The net exponential gain coefficient is very high Γ 2600 cm−1and is obtained at voltages below 1 V/μm.

ISSN:1058-725X    

DOI:10.1080/10587259808030194

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The bifunctional molecules presented here, allowed the preparation of photorefractive materials with high optical quality and no aging effect. Because of the low glass transition temperature of the compounds, orientation enhancement of photorefractive properties has been observed. 17 cm−1of net photorefractive gain as been demonstrated at 514 nm.

ISSN:1058-725X    

DOI:10.1080/10587259808030195

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Electroabsorption (EA) studies at room temperature on polycrystalline thin films of α-octithiophene (8T), the longest non-substituted model oligomer of polythiophene, are reported. The EA spectra obtained on isotropic 8T films deposited on top of coplanar electrodes exhibit several intense and narrow peaks in the absorption region and on the absorption edge. They are compared with the derivatives of absorption spectrum, and it is shown that the excitation in the longer wavelength absorption presents a Frenkel exciton character. Third-order nonlinear optical susceptibilityx(3)(ωω, 0, 0) is evaluated to be in the order of 10−10esu at its maximum.

ISSN:1058-725X    

DOI:10.1080/10587259808030196

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The state of the art concerning the design of octupoles, a new familly of NLO active molecules is presented here. 2D and 3D based architectures with higher symmetries lead to high efficiencies for SHG.

ISSN:1058-725X    

DOI:10.1080/10587259808030197

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We introduce a simple fabrication technique of nonlinear polymeric optical waveguide patterns based on the holographic method. It is found that the second-order nonlinearity of a poled polymer film is erased by single pulse UV laser irradiation. In order to form the periodical structure in the optical polymer waveguide, the optical system of two-beam interference is arranged and a single pulse of UV laser is exposed directly onto the poled film. We prove that this method can simply fabricate thex(2)grating with the period from submicron to submillimeter.

ISSN:1058-725X    

DOI:10.1080/10587259808030198

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Recent improvement in analyzing molecular orientational distribution function (ODF) at surfaces and interfaces using surface second-harmonic generation (SHG) was reviewed. The anisotropic dielectric constants of SHG-active thin layers were treated as parameters governed by ODF to be determined. The information about the nonpolar axial ordering was added when using the maximum entropy method for analyzing ODF. Importance of the imaginary part of nonlinear susceptibility elements was also pointed out. A spin-coated film of polyimide with SHG-active side chains was chosen as an example and ODFs of the rubbed surface were obtained with and without using these modified methods for comparison.

ISSN:1058-725X    

DOI:10.1080/10587259808030199

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Some new 8, 8-dicyanoheptafulvene derivatives have been synthesized, among which the title compound (la) is found to crystallize in a polar space group ofP1 showing nlo-characteristics. The molecule is composed of a strong acceptor of the dicyanomethylidene group and a strong donor of the dimethylamino group, both of which are combined with the π-conjugated heptafulvene skeleton. Compoundlais therefore typical of an intramolecular CT compound, for which high second-order hyperpolarizability is expected. For this reason, electronic structure has been investigated in solution and in the solid state on the basis of the molecular and crystal structures together with molecular orbital calculations. The solid-state spectrum is found to be strikingly different from the solution spectrum because of the extent of conjugation between the seven-membered ring and the phenyl ring. There are two electronic transitions A (about 450 nm) and B (580–650 nm) in the solid. Band A is due mainly to the 8, 8-dicyanoheptafulvene skeleton while band B is of charge-transfer character due to the dicyanomethylidene and dimethylamino groups.

ISSN:1058-725X    

DOI:10.1080/10587259808030200

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The performance of organic based field-effect transistors (OFETs) has considerably increased recently, and now compares to that of amorphous silicon. The operation mode of OFETs is described. It differs from that of the conventional MISFET in that it operates in the accumulation regime. Another important difference is the charge transport in organic materials, which occurs via hopping of self-localized polarons. As a result, the mobility is thermally activated and electric field dependent. Effects of this feature on the current-voltage characteristics are outlined.

ISSN:1058-725X    

DOI:10.1080/10587259808030201

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Singlewalled carbon nanotubes (SWNT's) prepared by the electric arc method are characterized by means of spectroscopic techniques, mainly Transmission Electron Microscopy and resonance Raman scattering. TEM observations reveal the existence of nïckel particles embedded in an amorphous carbon matrix, but also a large concentration of SWNT's, whose average diameter can be estimated to be near 1.3 nm, arranged in bundles. Raman spectra recorded on the same samples exhibit several features, mainly consisting of two groups. On one hand, in the high frequency region, several bands are observed in the range 1500 - 1600 cm−1, corresponding to a splitting of the E2g2vibrational mode of graphite. On the other hand, several components are recorded between 150 and 240 cm−1, reflecting a distribution in the diameter of the SWNT's. A systematic study of the ≪collaret≫, from which samples are extracted, is also presented.

ISSN:1058-725X    

DOI:10.1080/10587259808030202

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor