Optical spatial solitons for all-optical signal processing

Abstract

Nonlinear systems respond to an excitation in a non-proportional way and do not satisfy the superposition principle. Most of the linear models of physical systems (harmonic oscillator is probably the clearest example) are valid approximations only in a perturbative regime; beyond it their intrinsic nonlinear nature must be considered. The nonlinear world is a source of intriguing phenomena (both from theoretical and applicative points of view) and optics is one of the most accessible area where such effects can be studied. When light and matter interact, the former is able to change the medium properties, in particular the refractive index, so affecting its own propagation. Among the nonlinear processes we will consider self-confinement: the ability of a light beam to compensate its natural tendency to spread. When linear spreading is exactly compensated by non-linear self-focusing, a spatial soliton is formed. Spatial solitons preserve their profile during propagation, which makes them suitable candidates to carry and process other signals, just like waveguides. We will deal with spatial solitons in nematic liquid crystals, namely nematicons, in which nonlinearity is enabled by dipolar interactions between molecules and electric fields: the material is chosen on the basis of its high nonlinearity and versatility. This dissertation reports on the all-optical control of nematicons and some representative applications of signal processing. The work is mostly experimental, with some theoretical considerations wherever it is necessary for the comprehension of the observed phenomena. Chapter 1. Fundamentals of nonlinear optics and spatial solitons are firstly summarized. Then we examine Nematic Liquid Crystals and their physical properties, focusing the attention on their nonlinear optical response and introducing nematicons. Chapter 2. We report some experiments in standard samples. First, we examine the propagation of a nematicon in the presence of a tunable nonlinearity. Then we treat the nonlinear control of the interaction between two nematicons. Chapter 3. Here we present experiments in dye-doped liquid crystals. We describe the optical response of dye-doped nematics and two experiments. In the first, we discuss nonlinear self-steering of light, comparing undoped and doped liquid crystals; in the second we deal with the formation of optical interfaces in order to control the nematicon trajectory. Chapter 4. We introduce liquid crystal light valve as a novel environment for the propagation of nematicons. After a preliminary section where we explain the working principles of the valve, we illustrate the propagation of a nematicon in a fully controllable refractive index landscape. We review briefly the theorical approach, proposing and demonstrating the implementation of a reconfigurable set of all-optical signal processors. This activity was mostly carried out at NooEL - Nonlinear Optics and OptoElectronics Lab at the University ROMA TRE. The work on liquid crystal light valve was developed at the INLN (Institut NonLin´aire de Nice), University of Nice - Sophia Antipolis.