Energy transfer between surface plasmon polariton modes with hybrid photorefractive liquid crystal cells

Abstract

In this thesis, a hybrid photorefractive liquid crystal cell structure with the addition of a thin 40nm Gold layer is proposed that demonstrates significant photorefractive control of Surface Plasmon Polaritons (SPP). The photorefractive effects are generated through optically controlling the conductivity of a ~100nm photoconducting poly-N-vinyl-carboxyl (PVK) layer. Therefore, when a potential is applied across the cell, the liquid crystal alignment and the SPP wavevector is able to be controlled with light. The aim for developing this device is for the eventual demonstration of SPP gain to offset the high optical losses and increase the characteristically short propagation length of SPP. The mechanism we intend to use to demonstrate gain is analogous to the asymmetric energy transfer in a wave mixing system for two laser beams used to typically characterise photorefractive materials.

We first characterise the electrical and optical behaviour of the novel photorefractive plasmonic structure proposed with uniform illumination. Our system demonstrates a good photorefractive wavevector shift of 0.207µm^-1 for a 1.24eV SPP; this shift is in excess of the FWHM of the SPP resonance in the attenuated total reflection spectrum (0.154µm^-1). However, the electric behaviour of the system is found to be highly complex and cannot be fully characterised by an equivalent electrical circuit. In addition, due to electronic stability issues, we require a slow AC potential to demonstrate consistent photorefractive effects.

In a step towards realising SPP gain, we then consider the SPP interaction with a refractive index grating written into the liquid crystal layer with the interference pattern of crossed laser beams. We find that a SPP is diffracted into additional SPP modes. Our investigation then determines the ideal parameters that maximise the energy transfer by examining the diffraction efficiency dependence of each variable of the system. The maximum energy transfer observed is 25.3±2.3% for a 1.05eV SPP from a 4µm grating. With the assistance of a numerical simulation of our system we present a series of qualitative and semi-analytical descriptions to describe the mechanisms behind the observed trends. We discover that the diffraction efficiency is dependent of three important effects; the orientation of the grating, the penetration depth of the SPP into the liquid crystal and the magnitude of the periodic electric field in the liquid crystal. In addition, to fully describe the quantitative values observed we must also consider the presence of a thin 100nm region of the liquid crystal near the photoconductor interface that does not strongly respond to the applied electric field due to anchoring forces.

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