Affordable, sensitive, selective, fast-responding, label-free sensors are currently in high demand for many of today's applications and technologies, particularly in the food industry, bio-sensing applications, and quality control. In addition, modern technologies such as a lab-on-a-chip involve microfluidic analysis, which requires highly accurate and miniaturized sensing systems. These systems can be implemented in biomedical applications such as point-of-care diagnostics, as well as in environmental monitoring, agriculture, biotechnology, and public health and safety. A need, therefore, exists for highly accurate and reliable sensing systems that can meet the requirements of these applications. This dissertation presents electrically-small planar microwave resonators for the design of near-field sensors that can satisfy the needs of the aforementioned applications. This thesis proposes a number of novel concepts related to miniaturization and the enhancement of the sensitivity of electrically-small sensors. In the first part of the thesis, an analysis of the sensitivity of complementary split-ring resonators (CSRRs) with respect to changes in resonator topology is presented. Eigenmode solvers, circuit models, numerical simulations, and laboratory measurements were all employed for the analysis. The results show that the resonance frequency is adjustable and scalable. The second part of the thesis proposed an ultrasensitive near-field sensor for detecting submillimeter cracks in metallic materials. Experimental measurements revealed that a surface crack of 200 um wide and 2 mm deep results in a 1.5 GHz shift in the resonance frequency. The results led to the idea of utilizing CSRRs for designing near-field sensors for crack detection in dielectric materials. The work was further extended to increase the sensitivity of planar CSRRs to detect the presence of dielectric materials. This concept is based on increasing the sensing areas per unit length and on the utilization of multiple, identical, and coupled resonators. Although the electromagnetic energy stored in electrically-small planar resonators is concentrated primarily in an electrically-small volume, most of that energy is located in the host substrate, thus limiting the sensitivity required for detecting changes in the material under test (MUT), which differs from the host substrate. For this reason, a sensor designed for enhancing the EM energy stored in the sensing volume that is exposed to the MUT is proposed. The design concept is based on the use of a three-dimensional capacitor. For validation purposes, a complementary electric-LC resonator (CELCR) and two metallic bars were utilized for designing the sensor for dielectric materials. Furthermore, by adopting the concept of three-dimensional capacitors, microwave sensors based on planar SRRs are introduced in order to 1) enhance the sensitivity, 2) allow for flexible tunability, and 3) create novel sensors for fluidic applications. For validation purposes, an SRR-based sensor was designed and tested using numerical simulation and experiments to detect fluid materials and fluid levels. The SRR with the three-dimensional capacitors was also utilized to design probes for the near-field scanning microscopy. An additional component of this research was, therefore, an exploration of the miniaturization of CELCR sensing areas so that these devices could be loaded with three-dimensional capacitors in order to design a sensitive near-field sensor for microscale-based technologies. The ability of the sensor to detect the presence of magnetic materials was also investigated numerically. For applications in which flatness or compactness is a relevant factor, enhancing sensitivity with the use of three-dimensional capacitors is not an ideal solution. Although classical planar antennas such as patch antennas are subject to a lack of EM energy localization in small areas, the adoption of the split concept, utilized in electrically-small resonators, can improve these antennas for use in designing near-field microwave sensors. This thesis proposed a planar microwave sensor based on an annular ring resonator loaded with a split, thus enabling it to operate at lower frequencies and to enhance the quality factors. The sensor was tested experimentally with respect to characterizing dielectric slabs and detecting the presence of fluidic materials. The last part of the thesis introduced the concept of an intelligent sensing technique based on the modulation of the frequency responses of near-field microwave sensors for the characterization of material parameters. The concept is based on the assumption that the physical parameters being extracted are uniform over the frequency range of the sensing system. The concept is derived from the observation of the sensor responses as multidimensional vectors over a wide frequency range. The dimensions are then considered as features for a neural network. The concept has been demonstrated experimentally for the detection of the concentration of a fluid material composed of two pure fluids.

This monograph focuses on the design, implementation and characterization of a concurrent dual band RF sensor for non-invasive detection of human vital signs. Exclusive title on multiband short range sensors and their biomedical applications, offers detailed analysis of subsystems based on fabricated and measured prototypes and verifies and discusses the system in the real-time environment. Discusses the practical difficulties of the design process and offers case studies based on the design.

Microwave Materials Characterization is an edited book discussing recent researches on basic and innovative measurement techniques for the characterization of materials at microwave frequencies, in terms of quantitative determination of their electromagnetic parameters, namely the complex permittivity and permeability. It is divided into two parts: Part 1, including original contributions on advanced techniques for the characterization of dielectric materials, and Part 2, devoted to the microwave characterization of biological tissues.

Near-field microwave microscopes, which belong to the local scanning probe microscopes family, are considered today as advanced characterization tools in many applications areas including physics, biology and micro and nanotechnologies. The near-field microwave microscope that is used in the work and described in this manuscript is an instrument developed at IEMN owning a great sensitivity in a wide operating frequency band [2-18 GHz]. The potential of the microscope in terms of applications is demonstrated through the characterization of liquids with different modalities of characterization (probe in contact, non-contact and immersed in a liquid). In particular, this instrument is investigated for dielectric spectroscopy of aqueous glucose solutions.This characterization tool that offers sub-wavelength imaging capability is also tested in different situations (surface and subsurface imaging). Imaging resolution and measurement accuracy are evaluated and easily implementable processing methods are proposed to improve the quality of imaging. Finally, a solution towards a larger compactness of the instrument is investigated through the replacement of the network analyzer by a more compact device (six-port reflectometer type).

The work covers a multimodal microscope technology for the analysis, manipulation and transfer of materials and objects in the submicrometer range. An atomic force microscope (AFM) allows imaging of the surface topography and a Scanning Microwave Microscope (SMM) detects electromagnetic properties, both operating in a Scanning Electron Microscope (SEM). The described technology demonstrator allows to observe the region-of-interest live with the SEM, while at the same time a characterization with interacting evanescent near-field microwaves and intermolecular forces takes place. engl.

The development of high speed, high frequency circuits and systems requires an understanding of the properties of materials functioning at the microwave level. This comprehensive reference sets out to address this requirement by providing guidance on the development of suitable measurement methodologies tailored for a variety of materials and application systems. Bringing together coverage of a broad range of techniques in one publication for the first time, this book: Provides a comprehensive introduction to microwave theory and microwave measurement techniques. Examines every aspect of microwave material properties, circuit design and applications. Presents materials property characterisation methods along with a discussion of the underlying theory. Outlines the importance of microwave absorbers in the reduction in noise levels in microwave circuits and their importance within defence industry applications. Relates each measurement technique to its application across the fields of microwave engineering, high-speed electronics, remote sensing and the physical sciences. This book will appeal to practising engineers and technicians working in the areas of RF, microwaves, communications, solid-state devices and radar. Senior students, researchers in microwave engineering and microelectronics and material scientists will also find this book a very useful reference.

Novel characteristics of spin-based phenomena are intensively researched in the hope of discovering effects that could be used to develop new types of high-performance spintronic devices. Recent dynamics studies have revealed new principles for spintronic devices to sense microwaves. The capabilities for detecting both microwave electric field and magnetic field could make the spintronic microwave sensor as ubiquitous as semiconductor devices in microwave applications in the future. In this thesis, the feasibility of spintronic sensors in microwave applications has been researched and developed. Thanks to the high conversion efficiency of microwave rectification in the magnetic tunnel junction (MTJ) based spintronic sensor, it can directly measure the coherent spatially scattered microwave field distribution and detect a hidden object by analyzing the reflected microwave amplitude pattern. To enable the "real-time" vector measurement of the microwave field, a sensor based rapid phase detection technique is also developed. Combining the rapid phase detection technique and the microwave holography principle, a two-dimensional microwave holographic imaging system using a spintronic sensor was built. The high sensitivity of the microwave phase measurement allows the coherent imaging of the target to be reconstructed in noisy environments. By adapting the broadband measurement, not only the shape but also the distance of the target can be determined, which implies that three-dimensional imaging is achievable using a spintronic device. Combining the broadband microwave measurement and a wavefront reconstruction algorithm with a spintronic microwave sensor in circular trajectory, the reconstructed images of targets are obtained. The reconstructed images clearly indicate the targets' positions even when the targets were immersed in a liquid to simulate an inhomogeneous tissue environment. Our spintronic techniques provide a promising approach for microwave imaging, with the potential to be used in various areas, such as biomedical applications, security services, and material characterization.