Nonlinear optical response of symmetric donor acceptor structures

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.