Metamaterial Optical Waveguide Sensors

Abstract

Propagation of electromagnetic waves in linear and nonlinear media has received an increasing attention from researchers in the optoelectronics field. A great attention is nowadays paid to optical waveguide sensors because they offer many advantages such as: small size, low price, safe when used in aggressive environments and mechanically stable. Many theoretical studies concerning analysis of dispersion equations were introduced for many planar waveguide structures, and some authors have proposed scaling rules and universal dispersion analysis. Homogeneous sensors are mainly used in concentration measurements while surface sensors are used in detecting of adsorbed layers . The type of waveguides mostly used in chemical and medical sensing is the planner optical waveguide structure with normal asymmetry (i.e. the substrate refractive index being greater than that of the cover ). Sensitivity of measuring the physical or chemical quantities appearing in the cover region depends on the strength and distribution of the evanescent field in that cover. Sensitivity optimization requires suitable choice of the guiding layer thickness and the materials from which sensor layers are constructed, so that the sensor may exhibit its maximum sensitivity . Scientists have proposed various types of these sensors and theoretically analysed them and suggested solutions for constructing highly efficient sensors. In this thesis, we investigate nonlinear waveguide sensors when the guiding layer is a Left- Handed material (LHM) for both transverse electric (TE) and transverse magnetic (TM) waves. We consider the case when the analyte homogenously distributed in the cladding, i.e., homogenous sensing. The proposed structure consists of a left-Handed material (LHM) as a guiding layer sandwiched between a linear substrate and a nonlinear cover with an intensity dependent refractive index. The dispersion relation of the proposed structure is derived and the sensitivity of the effective refractive index to variations in the refractive index of the cladding is obtained. The condition required for the sensor to exhibit its maximum sensitivity is presented. The variation of the sensitivity with different parameters of the structure is studied and explained. The power flow through the sensor layers is also considered because the fraction of total power flowing in the covering medium is related to sensitivity. With respect to planar optical waveguide sensors, the main remarks gained from our investigations can be summarized as follows: • There is a close connection between the fraction of total power propagating in the covering medium and the sensitivity of the sensor. In most cases, they may be regarded as nearly identical thus the enhancement of the fraction of total power flowing in the cladding is essential for sensing applications. • As the nonlinearity of the cladding increases, the wave crest is displaced towards the cladding and as a result the sensitivity of the optical waveguide sensor is enhanced. • Cladding to film permittivity ratio should be as high as possible but substrate to film permittivity ratio should be as low as possible to increase the evanescent field tail in the cladding and to reduce it as possible in the substrate. The inversion of the conventional waveguide symmetry is strongly recommended if possible. In some cases it is not possible especially when the analyte is air.