G. Tsigaridas

Two-photon absorption properties of novel organic materials for three-dimensional optical memories

Abstract The two-photon absorption (TPA) properties of a novel group of pyrylium-based compounds were studied. The molecules of this group were synthesized by systematically changing the chemical structure of a specific substituent in an initial chromophore. TPA cross-sections as large as 1.8×10 −47 cm 4 s photon −1 and high quantum yields were obtained. The results combined with the low cost and ease of synthesis of these compounds make them candidates for TPA applications. Particularly, three-dimensional data storage in polymer matrices of the synthesized compounds was realized through the photobleaching process. The high efficiency of recording and the submicron resolution reveal the great potential of this group of molecules as memory materials.

Intensity dependent nonlinear absorption of pyrylium chromophores

Abstract The intensity dependence of nonlinear absorption (NLA) of a series of pyrylium chromophores was studied with femtosecond pulses in their two-photon absorption spectral region to assess their potential as optical limiters. The open z-scan technique using different excitation intensities was employed. Fifth-order effects appear above a critical value of the irradiance and consequently of the first excited singlet-state population. Reverse saturable absorption (RSA) based on two-photon (TPA) and excited-state absorption (ESA) was dominant in most cases. The third- and fifth-order NLA coefficients and high ESA cross-sections of the order of 10 −15 cm 2 are presented at 760, 790 and 840 nm.

Novel class of pyrylium dyes with high efficiency in lasing and two-photon absorption fluorescence

Abstract We report the synthesis of a promising class of pyrylium dyes concerning laser and two-photon absorption action. Three representatives of this class, 4-(2,4-dichlorophenyl)-2,6-bisphenylpyrylium tetrafluoroborate, 4-phenyl-2,6-bis(4-methoxyphenyl)pyrylium tetrafluoroborate, and 4-(2,4-dichlorophenyl)-2,6-bis(4-biphenyl)pyrylium tetrafluoroborate, were investigated. Each dye was synthesized by a convenient and inexpensive method. Efficient laser action was shown at ∼500, ∼560, ∼615 nm. Changing the conjugation length i.e. the length of the oligophenyl moiety, or introducing various substituents on the aromatic rings, the emission maximum can be controlled. Two-photon absorption induced fluorescence was also observed. These pyrylium dyes constitute excellent candidates for laser technology and two-photon absorption applications.

Three-photon induced photobleaching in a three-dimensional memory material

The photobleaching properties of a 3D memory material based on pyrylium salts are studied. It is shown that under ultrashort pulsed irradiation photobleaching occurs through two diverse mechanisms. One is major and causes fast bleaching, whereas the other is minor and causes slow bleaching. Furthermore, it is proved that both mechanisms are three- or more-photon processes. This fact gives the capability of significantly increasing data storage density.

Z-scan analysis for near-Gaussian beams through Hermite-Gaussian decomposition

Gaussian decomposition is used as a theoretical infrastructure with which Z-scan experiments are analyzed. This procedure is extended here to the interesting, from a practical point of view, case in which the laser beam used is not perfectly Gaussian. We follow a perturbative approach to obtain the far-field pattern of the beam after the beam passes through a nonlinear sample. The procedure is based on the decomposition of the electric field at the exit plane of the sample to a linear combination of Hermite–Gaussian functions. To a first-order approximation, each mode of the incident beam is decomposed to a linear combination of different-order modes that do not exceed the order of the original mode. Finally, the effects of the simultaneous presence of first and higher-order refractive nonlinearities or first-order refractive nonlinearity and nonlinear absorption are studied.

Three-dimensional data storage in photochromic materials based on pyrylium salt by two-photon-induced photobleaching

Molecular two-photon absorption (TPA) has gained great interest over recent years owning to each application in various fields, including spectroscopy, microscopy, 3-D optical data storage, optical power limitation and microfabrication. The requirement for organic molecules with large TPA cross-sections is therefore essential. In the present work the two-photon absorption properties of a series of pyrylium-based chromophores were investigated. The molecules of this series were synthesized by systematically changing the chemical structure of a specific substituent of an initial, strong two-photon absorbing, molecule. Very large TPA cross-sections (1800 GM) and high quantum yields were achieved in the visible red and NIR region of the spectrum. The relation between the chemical structure of the molecules and their TPA performance was discussed. Furthermore, the photobleaching efficiency of these molecules doped in polymer matrices was investigated. Parallel successive patterns were recorded in a polymer matrix via photobleaching. The accuracy of recorded patterns reveals the great potential of these chromophores as memory materials.

The effect of aggregates on the decay dynamics of a conjugated polymer studied by femtosecond time resolved spectroscopy

We study the excited state dynamics of a blue emitting conjugated polymer in isolated and aggregated chain environments in order to examine its potential applications in light emitting diodes (LEDs) and solid state lasers. The study was realized using a femtosecond time resolved fluorescence spectroscopy which employs the upconversion technique. Polymeric samples where the chains are isolated show an identical excited state lifetime at all emission wavelengths meaning that well defined chromophores exist in the polymer chains. On the other hand, samples where both isolated and aggregated polymer chains exist, exhibit an increase of the excited state lifetime with the emission wavelength, which is attributed to aggregates. However, the emission properties of isolated chains are not affected by the presence of aggregates in the samples since no energy transfer among these two species occurs. This is due to the localization of the excitons in the conjugated segments of isolated chains and renders the polymer an important candidate for use in LEDs and lasers.

Time-resolved spectroscopy of oligothiophenes using the femtosecond fluorescence upconversion technique

In the present work the electronic properties of two oligothiophenes were experimentally studied through the time-resolved fluorescence spectroscopy up-conversion technique. The effect of the substituent at the end of the main chain on the excited state dynamics was investigated. The excited state decay time (τfl), the reorientation time of the emission dipole (τor), the initial fluorescence anisotropy (r0) and the time-dependence of the fluorescence anisotropy r(t) were measured. The results showed that the molecule with the bigger dipole moment has smaller excited state decay time. The other coefficients are almost the same for the two materials because of their similar chemical structure. Also, the angle (α) between the absorption and emission dipoles, μa and μe respectively, was calculated through the anisotropy and found to be 29°. This is attributed to the different geometry of the molecule in its ground and excited state.

High-order photobleaching of pyrylium salts under two-photon excitation

Pyrylium salts were found to be attractive candidates for applications based on two-photon absorption effect. Especially, they have been used for three-dimensional data storage through photobleaching process. In the present work the photobleaching, under two-photon excitation, is investigated for a representative pyrylium salt hosted in a polymethylmethacrylate film. The photobleaching dependence on time, power and wavelength irradiation as well as on the film concentration is presented. Our results reveal the existence of two photobleaching mechanisms and indicate the presence of high-order interactions in these processes.

On the measurement of high-order refractive nonlinearities through the Z-scan technique

High-order refractive nonlinearities play an increasingly important role for the characterization of new materials in relation to applications as optical switching, optical limiting, optical data processing, etc. In this work a novel more accurate method for determining the coefficients related to the higher-order refractive nonlinearities through the Z-scan technique is presented. The general case of an elliptic Gaussian incident beam is considered. The theoretical analysis is based on the Gaussian decomposition method, appropriately extended to higher-order nonlinearities, which are treated as a perturbation. An analytic expression for the electric field pattern of the beam at any distance D from the exit plane of the sample is obtained. This allows the analysis and simulation of Z-scan experiments based on either the measurement of the transmittance through an aperture or the direct measurement of the beam dimensions in the far-field, which has considerable advantages, especially in the case of elliptic beams. For both cases, an experimental procedure for measuring the high-order refractive nonlinearities is suggested.