My PhD Thesis Abstract

ABSTRACT

The present thesis deals with new planar optical waveguide sensors for chemical and biological applications, taking into account nonlinear media in the proposed structures. In the last two decades, planar slab waveguides as optical sensors have received an increasing interest as non-communication application of integrated optics. However, nonlinear waveguides have not been introduced in the literature. We introduce nonlinear waveguides in the field of optical sensing to enhance the sensitivity of waveguide sensors. We also propose a new optical waveguide sensor that uses a new type of metamaterials called left-handed materials with simultaneously negative permittivity and negative permeability. We show by simulation that the sensitivity of conventional linear waveguide sensors can be dramatically enhanced by using nonlinear materials in the cladding only, in the cladding and the substrate, and by inserting a layer of metamaterial at the film-cladding interface.

We first consider a thin dielectric linear film surrounded by a nonlinear cladding and a linear substrate. Self-focusing and self-defocusing nonlinearity are both presented for transverse electric (TE) and transverse magnetic (TM) field. We then consider a linear guiding layer surrounded by a nonlinear cladding and a nonlinear substrate for both TE and TM polarizations. Finally, a waveguide with a left handed material between the guiding layer and the cladding layer is presented. In our study we consider the case when the analyte is uniformly distributed in the covering medium.

In each of these cases, an extensive theoretical analysis is carried out to derive the dispersion relations and rate of change of the effective refractive index due to changes in the refractive index of the cover which is called the sensitivity of the optical waveguide sensor. Computer programs are developed to solve the dispersion relations for the effective refractive index and to calculate the sensitivity. The variation of the sensitivity of the proposed sensors with different parameters of the structures is shown in order to reach the optimum structure that corresponds to the highest sensitivity. The variation of the sensitivity with guiding layer thickness, wavelength of propagating light, power, permittivity of the cladding and substrate, nonlinearity of the media, and other parameters of the structure is presented and discussed. The conditions required for the sensor to exhibit its maximum sensing sensitivity are derived and discussed. Comparison between our results and that of conventional linear waveguide sensors reveals that the proposed sensors could be used for future versatile applications.

With respect to planar optical waveguide sensors, the main remarks gained from our investigations can be summarized as follows:

· In general, nonlinear optical waveguide sensors have sensitivities higher than the corresponding conventional linear sensors.

· Self-defocused nonlinear media, if used in the covering layer, would diminish the sensitivity to very low values.

· The optimum structure can be obtained by using self-focused nonlinear material in the cladding layer and a linear material in the substrate.

· 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 optimum film thickness is close to the mode’s cut-off thickness.

· A very small change in the guiding layer thickness near the optimum thickness would have a great negative impact on the value of the sensitivity. Thus the fabrication techniques should be as accurate as possible to obtain the exact value of the optimum guiding layer thickness.

· TE mode configurations exhibit maximum sensitivity at film thickness corresponds to the proximity of cut-off whereas TM mode configurations exhibit higher sensitivity peak at thicker thicknesses. Thus TMo is recommended.

· The enhancement of the fraction of total power flowing in the cladding by 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.

· In reverse symmetry waveguide sensors, the optimum film thickness is the cut-off film thickness.

· The sensitivity values of optical waveguide sensors do not depend on the light wavelength. Thus there is a freedom in the choice of the wavelength. Increasing the light wavelength would result in shifting the optimum film thickness towards higher values.

· The sensitivity of an optical waveguide sensor can be dramatically enhanced by using a left-handed material with simultaneously negative permittivity and negative permeability between the cladding and the guiding layers.

· Increasing the thickness of the left-handed material will enhance the surface waves excited at the metamaterial-cladding interface and as a result will enhance the power fraction flowing in the cladding and the sensitivity of the optical waveguide sensor.