Microstrip ring circuits have been extensively studied in the past three decades. A magnetic-wall model has been commonly used to analyze these circuits. Unlike the conventional magnetic-wall model, a simple transmission-line model, unaffected by boundary conditions, is developed to calculate the frequency modes of ring resonators of any general shape such as annular, square, or meander ring resonators. The new model can be used to extract equivalent lumped element circuits and unloaded Qs for both closed- and open-loop ring resonators. Several new bandpass filter structures, such as enhanced coupling, slow-wave, asymmetric-fed with two transmission zeros, and orthogonal direct-fed, have been proposed. These new proposed filters provide advantages of compact size, low insertion loss, and high selectivity. Also, an analytical technique is used to analyze the performance of the filters. The measured results show good agreement with the simulated results. A compact elliptic-function lowpass filter using microstrip stepped impedance hairpin resonators has been developed. The prototype filters are synthesized from the equivalent circuit model using available element-value tables. The filters are evaluated by experiment and simulation with good agreement. This simple equivalent circuit model provides a useful method to design and understand this type of filters and other relative circuits.Finally, a tunable feedback ring resonator oscillator using a voltage controlled piezoelectric transducer is introduced. The new oscillator is constructed by a ring resonator using a pair of orthogonal feed lines as a feedback structure. The ring resonator with two orthogonal feed lines can suppress odd modes and operate at even modes. A voltage controlled piezoelectric transducer is used to vary the resonant frequency of the ring resonator. This tuned oscillator operating at high oscillation frequency can be used in many wireless and sensor systems.

This thesis presents a detailed study of chip based silicon photonic waveguide technologies for chemical sensing applications. The project specifically focuses on the use of strip and slot waveguide based micro-ring resonators (MRRs) integrated with graphene and graphene oxide (GO) as potential functional sensor coatings. The primary objective is to understand the effect of graphene/GO on the optical properties of such a device, to assess performance in bio-/chemical sensing applications and to identify ways in which such a device may be optimised. A detailed analysis of how the MRR cavity optical extinction ratio (ER) varies with the interaction length of surface integrated graphene reveals, for the first time using this technique, the in-plane graphene linear absorption coefficient, αgTE = 0.11 ± 0.01dBμm⁻¹. A model of the MRR cavity optical losses for different graphene lengths and heights (above the waveguide surface) provides a predictive capability for the design rules of optimised performance in sensing and photo-detector based applications. The graphene integrated MRRs were also characterized by a Raman mapping technique from which careful analysis of the graphene G and 2D scattering peak frequencies and relative intensities revealed that the graphene is electrically intrinsic where it is suspended over the MRR yet moderately hole-doped where it sits on top of the waveguide structure. This 'pinning' of the graphene Fermi level at the graphene-silicon/SiO2 interface is the result of 'trapped' ad-charges, the concentration of which may be increased at dangling bond sites after relatively aggressive (O2 plasma) cleaning of the silicon/SiO2 surface prior to graphene transfer. Quantifying this substrate doping effect is critically important when attempting to determine graphene's optical properties and should be taken into account when designing graphene-silicon hetero-structures for opto-electronic devices. The large absorption coefficient determined for the graphene integrated MRR devices means that cavity losses are far too high for practical realisation of refractive index based sensing. However, an alternative approach using GO as the functional layer for improved MRR based refractive index sensors remains a possibility on account of the much lower transmission loss. GO also has distinct advantages over graphene; ease of integration, a high density of surface functional groups and micro-porosity. Transmission spectral analyses of both bare (uncoated) MRRs and those coated with different GO concentrations revealed the in-plane linear absorption coefficient for the GO film to be αGOTE = 0.027±0.02dBμm⁻¹, which is much lower than that for graphene. Construction of a gas cell and integrated 'bubbler' arrangement for delivering variable vapour concentrations to the graphene/GO integrated MRR devices under test is presented. Both bare and GO coated MRRs were exposed to vapours from a series of typical organic solvents; ethanol, pentene and acetone delivered by a carrier gas (N2). Dynamic optical tracking of the MRR cavity resonance wavelength during vapour exposure, at different flow rates (vapour concentrations) reveals the sensitivity of the device(s) to small changes in refractive index. The dynamic response of the GO coated MRRs to the vapours were up to three times faster than the uncoated MRR with similar improvements in sensitivity and limit of detection, largely attributable to the porous nature and molecular binding affinity of the GO. Critically, these experiments reveal that the detection sensitivity and response of the GO is solvent dependent, which may mean that it is capable of providing a degree of selectivity, which is normally difficult to achieve in refractive index based gas sensing.

Integrated ring resonators have attracted a considerable interest in optical communications because of their small size and wide range of applicability. Here we consider several aspects of these devices, beginning with a tunable hybrid ring resonators consisting of a silicon over insulator (SOI) ring covered with a polymer layer in a variable electric field. Varying the field changes the polymer refractive index and consequently the resonance condition of the cavity. This device offers a large degree of optical confinement together with a high modulation speed. Subsequently, we design and present fabrication results for a Wavelength Division Multiplexing (WDM) multiplexer/demultiplexer formed from a series of ring resonators with two channels separated by 50 GHz each that is predicted to exhibit a free spectral range (FSR) of 100 GHz , signal dispersion less than 30 ps/nm and a signal cross-talk less than -23 dB. Finally, we analyze the application of the coupled ring waveguide circuit to rotation sensors based on the Sagnac phase shift. Here, however our analysis indicates that a single ring, of the same area exhibits a higher degree of sensitivity to rotational motion than a multiple ring circuit.

A chalcogenide glass (arsenic-trisulfide, As2S3) optical waveguide is vertically integrated onto titanium-diffused lithium-niobate (Ti:LiNbO3) waveguides to add optical feedback paths and to create more compact optical circuits. Lithium-niobate waveguides are commonly used as building blocks for phase and amplitude modulators in high speed fiber communication networks due to its high electrooptic coefficient and low mode coupling loss to single-mode optical fibers. Although it can easily be modulated using an RF signal to create optical modulators, it lacks the intrinsic trait to create optical feedback loops due to its low core-to-cladding index contrast. Ring resonators are main building blocks of many chip-scale optical filters that require these feedback loops and are already demonstrated with other material systems. We have, for the first time, incorporated As2S3 as a guiding material on Ti:LiNbO3 and fabricated s-bends and ring resonators. We have examined As2S3-on-Ti:LiNbO3 waveguides at simulation, microfabrication, and optical characterization levels.

Integrated ring resonators have attracted a considerable interest in optical communications because of their small size and wide range of applicability. Here we consider several aspects of these devices, beginning with a tunable hybrid ring resonators consisting of a silicon over insulator (SOI) ring covered with a polymer layer in a variable electric field. Varying the field changes the polymer refractive index and consequently the resonance condition of the cavity. This device offers a large degree of optical confinement together with a high modulation speed. Subsequently, we design and present fabrication results for a Wavelength Division Multiplexing (WDM) multiplexer/demultiplexer formed from a series of ring resonators with two channels separated by 50 GHz each that is predicted to exhibit a free spectral range (FSR) of 100 GHz , signal dispersion less than 30 ps/nm and a signal cross-talk less than -23 dB. Finally, we analyze the application of the coupled ring waveguide circuit to rotation sensors based on the Sagnac phase shift. Here, however our analysis indicates that a single ring, of the same area exhibits a higher degree of sensitivity to rotational motion than a multiple ring circuit.

A new equivalent circuit of left-handed (LH) microstrip transmission line loaded with Complementary split-ring resonators (CSRRs) is presented. By adding the magnetic coupling into the equivalent circuit, the new equivalent circuit presents a more accurate cutoff frequency than the old one. The group delay of CSRRs applied with microstrip transmission line (TL) is also studied and analyzed into two cases which are passive CSRRs delay line and active CSRRs delay line. In the first case, the CSRRs TL is analyzed. The group delay can be varied and controlled via signal frequency which does not happen in a normal TL. In the active CSRRs delay line, the CSRRs loaded with TL is fixed. The diodes are added to the model between the strip and CSRRs. By observing a specific frequency at 2.03GHz after bias DC voltages from -10V to -20V, the group delay can be moved from 0.6ns to 5.6ns. A novel microstrip filter is presented by embedding CSRRs on the ground plane of microstrip filter. The filter characteristic is changed from a 300MHz narrowband to a 1GHz wideband as well as suppression the occurrence of previous higher spurious frequency at 3.9GHz. Moreover, a high rejection in the lower band and a low insertion loss of <1dB are achieved.Finally, it is shown that CSRRs applied with planar antenna can reduce the antenna size. The structure is formed by etching CSRRs on the ground side of the patch antenna. The meander line part is also added on the antenna patch to tune the operation frequency from 1.8GHz downward to 1.73GHz which can reduce the antenna size to 74% of conventional patch antennas. By using the previous antenna structure without meander line, this proposed antenna can be tuned for selecting the operation frequency, by embedding a diode connected the position between patch and ground. The results provide 350MHz tuning range with 35MHz bandwidth.

Metamaterials offer the freedom to tune the rich electromagnetic coupling between the constituent meta-atoms to tailor their collective electromagnetic response. Therefore, a comprehensive understanding of the nature of electromagnetic interactions between meta-atoms is necessary for novel metamaterial design, which is provided in the first part of this thesis. The subsequent work in the thesis applies the understanding from the first part to design and demonstrate novel one-dimensional metamaterials that overcome the limitations of metamaterials proposed in literature or exhibit electromagnetic responses not previously observed. Split-ring Resonators (SRRs) are a fundamental building block of many electromagnetic metamaterials. In the first part of the work in this thesis, it is shown that bianisotropic SRRs (with magneto-electric cross-polarisation) when in close proximity to each other, exhibit a rich coupling that involves both electric and magnetic interactions. The strength and nature of the coupling between two identical SRRs are studied experimentally and computationally as a function of their separation and relative orientation. The electric and magnetic couplings are characterised and it is found that, when SRRs are close enough to be in each other's near-field, the electric and magnetic couplings may either reinforce each other or act in opposition. At larger separations retardation effects become important. The findings on the electromagnetic interactions between bianisotropic resonators are next applied to developing a one-dimensional ultra-wideband backward-wave metamaterial waveguide. The key concept on which the metamaterial waveguide is built is electro-inductive wave propagation, which has emerged as an attractive solution for designing backward-wave supporting metamaterials. Stacked metasurfaces etched with complementary SRRs (CSRRs) have also been shown to exhibit a broadband negative dispersion. It is demonstrated through experiment and numerical modeling, that the operational bandwidth of a CSRR metamaterial waveguide can be improved by restricting the cross-polarisation effects in the constituent meta-atoms. The metamaterial waveguide constructed using the modified non-bianisotropic CSRRs are found to have a fractional bandwidth of 56.3\% which, based on a thorough search of relevant literature, is the broadest reported value for an electro-inductive metamaterial. A traditional coupled-dipole toy-model is presented as a tool to understand the field interactions in CSRR based metamaterials, and to explain the origin of their negative dispersion response. This metamaterial waveguide should be of assistance in the design of broadband backward-wave metamaterial devices, with enhanced electro-inductive waveguiding effects. In the final part of the thesis, a one-dimensional metamaterial prototype that permits simultaneous forward- and backward-wave propagation is designed. Such a metamaterial waveguide could act as a microwave analogue of nanoparticle chains that support electromagnetic energy transfer with a positive or a negative dispersion due to the excitation of their longitudinal or transverse dipole modes. The symmetry of the designed hybrid meta-atom permits the co-existence of two non-interfering resonances closely separated in frequency. It is experimentally and computationally shown that the metamaterial waveguide supports simultaneous non-interacting forward- and backward-wave propagation in an overlapping frequency band. The proposed metamaterial design should be suitable for realising bidirectional wireless power transfer applications.

Many interesting ideas have emerged from research on electromagnetic eld interactions with di erent materials. Analyzing such interactions has extracted some essential proper- ties of the materials. For example, extracting constitutive parameters such as permittivity, permeability, and conductivity, clari es a material's behavior. In general, the electromag- netic eld interacts with materials either in the far- eld or near- eld of a source. This study focuses on the principle of near- eld microwave microscopy for detection purposes. Many studies have focused on the use of an electrically small resonator, such as a split-ring resonator (SRR) and a complementary split-ring resonator (CSRR), to act as a near- eld sensor for material characterization and detection. At the resonance frequency, the electric and magnetic energy densities are enhanced dramatically at certain locations in the resonator. Any disturbance of the eld around such a resonator with a material under test causes the resonance frequencies to exhibit a shift that is used as an indicator of the sensor sensitivity. In this thesis, a single CSRR is used as a sensing element for detecting cracks in metal surfaces. Many microwave techniques have been developed for crack detection. However, these techniques have at least one of the following drawbacks: working at high frequencies, measurement setup complexity and cost, and low sensitivity. The rst part of this thesis presents a new sensor based on the complementary split-ring resonator (CSRR) that is used to detect sub-millimeter surface cracks. The sensing mechanism is based on perturbing the electromagnetic eld around an electrically small resonator, thus initiating a shift in the resonance frequency. Investigation of the current distribution on a CSRR at the resonance frequency shows the critical location at which the enhanced energy is concentrated. In addition, the current distribution demonstrates the sensing element in the CSRR. The sensor is simple to fabricate and inexpensive, as it is etched-out in the ground plane of a microstrip-line using printed circuit board technology. The microstrip-line excites the CSRR by producing an electric eld perpendicular to the surface of the CSRR. The sensor exhibits a frequency shift of more than 240 MHz for a 200 m crack. In the second part of this thesis, the sensitivity of the sensor is increased by lling the same crack with a dielectric material such as silicon oil. While using CSRR to scan a block with 200 m wide and 2 mm depth dielectric lled crack, the resonance frequency of the sensor shifts 435 MHz more than a case scanning a solid aluminum. Finally, the total Inductance of a CSRR for miniaturizing purposes is increased using either lumped or distributed elements. In this thesis, the designs and the results are validated experimentally and numerically.

Many interesting ideas have emerged from research on electromagnetic eld interactions with di erent materials. Analyzing such interactions has extracted some essential proper- ties of the materials. For example, extracting constitutive parameters such as permittivity, permeability, and conductivity, clari es a material's behavior. In general, the electromag- netic eld interacts with materials either in the far- eld or near- eld of a source. This study focuses on the principle of near- eld microwave microscopy for detection purposes. Many studies have focused on the use of an electrically small resonator, such as a split-ring resonator (SRR) and a complementary split-ring resonator (CSRR), to act as a near- eld sensor for material characterization and detection. At the resonance frequency, the electric and magnetic energy densities are enhanced dramatically at certain locations in the resonator. Any disturbance of the eld around such a resonator with a material under test causes the resonance frequencies to exhibit a shift that is used as an indicator of the sensor sensitivity. In this thesis, a single CSRR is used as a sensing element for detecting cracks in metal surfaces. Many microwave techniques have been developed for crack detection. However, these techniques have at least one of the following drawbacks: working at high frequencies, measurement setup complexity and cost, and low sensitivity. The rst part of this thesis presents a new sensor based on the complementary split-ring resonator (CSRR) that is used to detect sub-millimeter surface cracks. The sensing mechanism is based on perturbing the electromagnetic eld around an electrically small resonator, thus initiating a shift in the resonance frequency. Investigation of the current distribution on a CSRR at the resonance frequency shows the critical location at which the enhanced energy is concentrated. In addition, the current distribution demonstrates the sensing element in the CSRR. The sensor is simple to fabricate and inexpensive, as it is etched-out in the ground plane of a microstrip-line using printed circuit board technology. The microstrip-line excites the CSRR by producing an electric eld perpendicular to the surface of the CSRR. The sensor exhibits a frequency shift of more than 240 MHz for a 200 m crack. In the second part of this thesis, the sensitivity of the sensor is increased by lling the same crack with a dielectric material such as silicon oil. While using CSRR to scan a block with 200 m wide and 2 mm depth dielectric lled crack, the resonance frequency of the sensor shifts 435 MHz more than a case scanning a solid aluminum. Finally, the total Inductance of a CSRR for miniaturizing purposes is increased using either lumped or distributed elements. In this thesis, the designs and the results are validated experimentally and numerically.

A two-dimensional isotropic metamaterials formed by crossed split-ring resonators (CSRRs) are studied in this paper. The effective characteristic parameters of this media are determined by quasi-static Lorentz theory. The induced current distributions of a single CSRR at the resonant frequency are presented. Moreover, the dependence of the resonant frequency on the dimensions of single CSRR and the spaces of the array are also discussed. / Singapore-MIT Alliance (SMA)