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Nonlinear optical response of symmetric donor acceptor structures

Abstract

This research work deals with the field of materials for nonlinear optics (NLO). The investigated systems are organic structures where an intra- or inter-molecular charge transfer process takes place. They include an open shell structure, that is a -dimer of molecular radicals characterised by an intermolecular charge transfer (CT) interaction, and other closed shell systems where a one-dimensional or a bi-dimensional intramolecular CT is effective. Making use of simple quantum chemical models, the goals are to investigate the effect of the vibronic coupling on the NLO response of this molecules and to investigate trends in the structure-properties relationship that do not depend on the detailed chemical and structural properties of the system, in particular for the two-photon absorption process. Since our focus is on the role of the CT interactions, from the standpoint of describing their electronic properties, the molecules are treated as point-site systems where the polyelectronic structure of the Donor (D) or of the Acceptor (A) is substituted by a single site which corresponds to the HOMO of D or the LUMO of A. This amounts to neglecting any possible mixing between CT transitions and localized (say, *- type) electronic transitions. All of the systems we considered contain one D and one or more A: so, within our models, the molecule are n-point site- two-electron systems.
In the calculation we work with the density matrix formalism: in particular we make use of a bielectronic density matrix which allow us to take into account correlation effects. Moreover the calculation is performed in the Liouville space, with the “collective electronic oscillators” picture: this technique is expected to provide some computational advantages in the calculation. The coupling with the vibrations is introduced through a linear coupling scheme, where the dependence of electronic energies on the vibrational modes is considered up to the first order. Because the introduction of the vibronic coupling makes the equation of motion not closed, we adopt the Random Phase Approximation to solve it.
The most part of this work is theoretical, but we also perform an experimental characterisation of the dimer of aimed to achieve information on its dynamic parameters. Making use of an amplified Ti - Shapphire laser, which produce pulses in the femto-second regime, we perform a pump and probe experiment: it allows us to achieve an estimate of the lifetime for the first singlet excited state (charge transfer state) of the dimer. Moreover, the trend on the pump and probe experiment confirm the saturation of the charge transfer transition foreseen in the theoretical model.


Mostra/Nascondi contenuto.
2I INTRODUCTION I-1 Background and state of the art It has been known for a long time that the Pockels effect 1 and the Kerr effect 2 are nonlinear phenomena. Nevertheless, progress in the research on the nonlinear optical (NLO) effects, particularly in the experimental field, had been stagnant until the invention of the laser in the 1960s. In fact nonlinear phenomena require high-intensity radiation to take place and lasers are exactly a source of high-intensity coherent light. In brief, a very common way to describe the origin of NLO phenomena is based on a perturbative description of the polarisation (P) induced in a medium by the electric fields (E) which cross it: expanding the response (P) as a function of the components of the applied (external) electric field ( ( ) i E ), the following expression is got out ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ... 321 3 21 2 1 1 +++= EEEEEEP (I-1) ( )1  is the linear (dielectric) susceptibility, whereas ( )2  , ( )3  are the second-and third-order susceptibilities, respectively. At low intensity, ( )1  alone is sufficient to describe the system; within this approximation the well known Lambert and Beer’s law holds and a linear relation between input and output radiation is fulfilled. At high intensities also the other terms must be considered and they are responsible for the appearance of the NLO phenomena. Nowadays, nonlinear optics is expected to play a major role in the technology of photonics [1]. Photonics is the analogue of electronics in that it describes the technology in which photons instead of electrons are used to acquire, store, transmit and process information. Examples of NLO phenomena that are potentially useful in this context are the ability to alter the frequency or colour of light and to amplify one source of light at the expenses of another, switch it, or alter its transmission characteristics through a medium, depending on its intensity. Using these and other related phenomena, one can build devices, such as frequency mixers, that can act as new light sources or as amplification schemes, light modulators for controlling the phase or amplitude of light beams, optical switches , optical logic units, optical information storage devices, optical limiters and numerous way of processing the information content of data or images. Optical processing of information and optical computing is one of the most appealing applications of photonics. One of its potential advantage is a gain in speed in certain types of switching functions, in fact photonic 1 Pockel effect is responsible for the dependence of the refractive index of a medium on the amplitude of an applied static field. 2 Kerr effect is responsible for the dependence of the refractive index of a medium on the intensity of the applied field.

Tesi di Dottorato

Dipartimento: Chimica Fisica

Autore: Roberto Pilot Contatta »

Composta da 125 pagine.

 

Questa tesi ha raggiunto 236 click dal 20/03/2004.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.