NanoPhotonic structures for biosensing applications

Photonics -“ science of optics“ - has become one of the emerging sciences in many applications nowadays. The study of light interaction with matter has opened a lot of interesting phenomena that differ in their applications including sensing, modulation, demultiplexing, etc. Sensing applications represent a major part in the photonics field owing to their crucial role in the detecting and diagnosis of diseases in many medical applications. On the other hand, gas sensing is considered an important application in many industrial centers. During the manufacturing of several products, toxic gases may be generated and hence the ability to detect such types of gases becomes a necessity. The first part of this thesis is concerned with sensing applications using plasmonic and photonic structures. Several plasmonic and photonic structures are proposed that are characterized by their ultimate sensitivity and high performance. Other parameters are taken into consideration like the CMOS compatibility of our design and the possibility of being integrated with electronic chips. Beside optical sensing and their important role in biomedical and environmental applications, optical demultiplexers are considered from the main blocks in different communication systems that are based on wavelength division multiplexing (WDM). The need to highly select certain wavelength to carry the data during transmission is increasing. In the second part of the thesis, the design methodology of an optical filter is discussed. The optical filter can fit into many applications including demultiplexing and sensing. An optical demultiplexer is proposed and characterized by its high selectivity of wavelength in the near-infrared range to fit with the telecommunication systems. In addition, the transmission levels are of an acceptable range to ensure high signal to noise ratio. 9 The third and the last part of the thesis is concerned with optical coupling from free-space to guided structures. In the last part, an optical grating coupler is proposed that is characterized by its high transmission levels. The grating coupler couples the light from free-space to a shallow waveguide with a narrow lateral dimension. Such system can fit in many applications including sensing and modulation applications

Numerical modelling of optical micro-cavity ring resonators for WDM networks

Augmenting the level of integration for a lower cost and enhancing the performance of the optical devices have turned out to be the focus of many research studies in the last few decades. Many distinct approaches have been proposed in a significant number of researches in order to meet these demands. Optical planar waveguides stand as one of vital employed approach in many studies. Although, their low propagation loss, and low dispersion, they suffers from high power losses at sharp bends. For this reason, large radius of curvature is required in order to achieve high efficiency and compromise the high level of integration. For the purpose of this research, in this thesis different ways to improve the performance of optical microcavity ring resonators (MRRs) have been thoroughly investigated and new configurations have been proposed. The Multiresolution Time Domain (MRTD) technique was further developed and employed throughout this thesis as the main numerical modelling technique. The MRTD algorithm is used as a computer code. This code is developed and enhanced using self built Compaq Visual Fortran code. Creating the structure and Post-processing the obtained data is carried out using self built MATLAB code. The truncating layers used to surround the computational domain were Uniaxial Perfectly Matched Layers (UPML). The accuracy of this approach is demonstrated via the excellent agreement between the results obtained in literature using FDTD method and the results of MRTD. This thesis has focused on showing numerical efficiency of MRTD where the mesh size allowed or the total number of computed points is about half that used with FDTD. Furthermore, the MRR geometry parameters such as coupling gap size, microring radius of curvature, and waveguide width have been thoroughly studied in order to predict and optimise the device performance. This thesis also presents the model analysis results of a parallel-cascaded double-microcavity ring resonator (PDMRR). The analysis is mainly focus on the extraction of the resonant modes where the effect of different parameters of the structure on transmitted and coupled power is investigated. Also, accurate analysis of 2D coupled microcavity ring resonator based on slotted waveguides (SMRR) has been thoroughly carried out for the purpose of designing optical waveguide delay lines based on slotted ring resonator (SCROW). The SCROW presented in this thesis are newly designed to function according to the variation of the resonance coupling efficiency of a slotted ring resonators embedded between two parallel waveguides. The slot of the structures is filled with SiO2 and Air that cause the coupling efficiency to vary which in turn control both the group velocity and delay time of SCROW structures results from the changing the properties of the bent slotted waveguide modes which strongly depends on the slot’s position. Significant improvements on the quality factor and greater delay time have been achieved by introducing sub-wavelength-low-index slot into conventional waveguide

One-dimensional photonic crystal / photonic wire cavities based on silicon-on-insulator (SOI)

It has been of major interest in recent research to produce faster optical processing for many telecommunications applications, as well as other applications of high performance optoelectronics. The combination of one-dimensional photonic crystal structures (PhC) and narrow photonic wire (PhW) waveguides in high refractive-index contrast materials such as silicon-on-insulator (SOI) is one of the main contenders for provision of various compact devices on a single chip. This development is due to the ability of silicon technology to support monolithic integration of optical interconnects and form fully functional photonic devices incorporated into CMOS chips. The high index contrast of the combination of a silicon core with a surrounding cladding of silica and/or air provides strong optical confinement, leading to the realization of more compact structures and small device volumes. In order to obtain a wide range of device functionality, the reduction of propagation losses in narrow wires is equally important, although there are still performance limitations determined by fabrication processes. Compact single-row PhC structures embedded in PhW waveguide micro-cavities could become essential components for wavelength selective devices, especially for possible application in WDM systems. The high quality factor, Q, and confinement of light in a small volume, V, are important for optical signal processing and filtering purposes, implying large Purcell factor values. In this thesis, one-dimensional photonic crystal/photonic wire micro-cavities have been designed and modeled using both 2D and 3D versions of the finite-difference time-domain (FDTD) approach. These devices were fabricated using electron beam lithography (EBL) and reactive ion etching (RIE) for patterning of the silicon layer. The device structures were characterized with TE polarized light, using a tunable laser covering the range from 1480 nm to 1585 nm. Single-row periodic hole-type PhC mirrors consisting of identical and equally spaced holes were embedded in 500 nm wire waveguides. Two PhC hole mirrors were separated with a cavity spacer varying from 400 nm to 500 nm in length to form a micro-cavity. In contrast, several different cavity arrangements were also successfully investigated, - i.e. extended cavity and coupled micro-cavity structures. The experimental results on photonic crystal/photonic wire micro-cavity structures have demonstrated that further enhancement of the quality-factor (Q-factor) - up to approximately 149,000 at wavelengths in the fibre telecommunications range is possible. The Q factor values and the useful transmission levels achieved are due, in particular, to the combination of both tapering within and outside the micro-cavity, with carefully designed hole diameters and non-periodic hole placement within the tapered sections. On the other hand, a large resonance quality factor of approximately 18,500, together with high normalized transmission of 85% through the use of tapering on both sides of the hole-type PhC mirrors that formed the micro-cavity, has been obtained. For the extended cavity case, the multiple resonances excited within the stop band, together with substantial tuning capability of the resonances obtained by varying the cavity length has been demonstrated, together with a Q-factor value of approximately 74,000 at the selected resonance frequency with a normalised transmission of 40%. In addition, the coupled micro-cavity structures considered in this thesis have formed the basic building block for designing multiple cavity structures where the combination of several cavities splits the selected single cavity resonance frequency into a number of resonances that depends directly on the number of cavities used in the design. The coupling strength between the resonators and the Free Spectral Range (FSR) between the split resonance frequencies of the coupled cavity combination were controlled via the use of different numbers of periodic hole structures – and through the use of different aperiodic hole taper arrangements between the two cavities in the middle section of the mirrors

Simulations of nanograting-assisted light coupling in GaN planar waveguide

The numerical simulations of nanogratings integrated with gallium nitride (GaN) planar waveguides as well as the experimental in-coupling results are presented. A simulation tool based on the eigenmode expansion method and advanced boundary conditions provided a rigorous model of 400-nm-period grating couplers. A full-vectorial Maxwell solver allowed performing a number of simulations with varying grating parameters, where coupling efficiency, reflection and transmission characteristics of device were calculated. Gratings with different etch depths and arbitrary shapes were simulated using a staircase approximation, with an optimized number of steps per single slope. For the first time, an impact of dry etch processing on GaN coupler efficiency was evaluated, due to the inclusion of the sloped sidewalls, with regard to the technological constrains. Finally, the experimental results in the visible spectrum region (lambda = 633 nm), for 400-nm-deep gratings etched in GaN waveguide, were presented together with theoretical data for binary and trapezoidal profiles of a grating, for different optical mode profiles (TE(0) divided by TE(3) modes)

The optical sensor mote, a novel device for enabling next generation Wireless Sensor Networks

Recent advances in micro-electronics and communications have fuelled research in Wireless Sensor Networks (WSNs). WSNs are a collection of low power, low cost, small form factor devices referred to as sensor motes interconnected in a random manner to establish a network. Despite wide ranging research into a range of applications, significant limitations stand in the way of utilizing WSNs to monitor large scale/area environments. Optical sensing techniques are well suited for monitoring a large variety of environmental variables such as temperature, pressure, humidity, and gas concentrations. However, traditional optical sensing techniques rely on bulky solutions including spectroscopic equipment and fibre based approaches. On the other hand, photonic crystals have caused a revolution in integrated optics as they allow functionalities not possible before; however little has been reported on their use as integrated optical sensors. The research work combines the diverse but related fields of WSNs, integrated optics, and Photonic Crystals. A novel platform, the optical sensor mote, is proposed and its key building blocks are experimentally demonstrated as a feasibility study. Specifically, multi-gas sensors based on the slow light phenomenon in photonic crystal waveguides are theoretically and experimentally demonstrated. These sensors can sense multiple gases without the need of any physical changes. They can also be integrated with electronics to yield an optical sensor mote of small form factor which is stable, multi-functional, and cost-effective. The optical sensor mote represents a significant step towards enabling the wide spread use of WSNs to monitor large scale/area environments and providing a highly integrated mote platform amenable to mass production and providing multi-functions.Recent advances in micro-electronics and communications have fuelled research in Wireless Sensor Networks (WSNs). WSNs are a collection of low power, low cost, small form factor devices referred to as sensor motes interconnected in a random manner to establish a network. Despite wide ranging research into a range of applications, significant limitations stand in the way of utilizing WSNs to monitor large scale/area environments. Optical sensing techniques are well suited for monitoring a large variety of environmental variables such as temperature, pressure, humidity, and gas concentrations. However, traditional optical sensing techniques rely on bulky solutions including spectroscopic equipment and fibre based approaches. On the other hand, photonic crystals have caused a revolution in integrated optics as they allow functionalities not possible before; however little has been reported on their use as integrated optical sensors. The research work combines the diverse but related fields of WSNs, integrated optics, and Photonic Crystals. A novel platform, the optical sensor mote, is proposed and its key building blocks are experimentally demonstrated as a feasibility study. Specifically, multi-gas sensors based on the slow light phenomenon in photonic crystal waveguides are theoretically and experimentally demonstrated. These sensors can sense multiple gases without the need of any physical changes. They can also be integrated with electronics to yield an optical sensor mote of small form factor which is stable, multi-functional, and cost-effective. The optical sensor mote represents a significant step towards enabling the wide spread use of WSNs to monitor large scale/area environments and providing a highly integrated mote platform amenable to mass production and providing multi-functions

Effective index approximations of photonic crystal slabs: a 2-to-1-D assessment

The optical properties of slab-like photonic crystals are often discussed on the basis of effective index (EI) approximations, where a 2-D effective refractive index profile replaces the actual 3-D structure. Our aim is to assess this approximation by analogous steps that reduce finite 2-D waveguide Bragg-gratings (to be seen as sections through 3-D PC slabs and membranes) to 1-D problems, which are tractable by common transfer matrix methods. Application of the EI method is disputable in particular in cases where locally no guided modes are supported, as in the holes of a PC membrane. A variational procedure permits to derive suitable effective permittivities even in these cases. Depending on the structural properties, these values can well turn out to be lower than one, or even be negative. Both the “standard” and the variational procedures are compared with reference data, generated by a rigorous 2-D Helmholtz solver, for a series of example structures.\u