Metamaterials have been increasingly used in the past few decades in the design of novel microwave circuits and sensor systems. This paper deals with the design of a split-ring resonator (SRR) sensor for microwave nondestructive evaluation (NDE) applications. Building on prior work in this field, a modified design of the sensor is used for imaging composites. The tip of the sensing ring is extended, and a reference ring is introduced to tailor fit the sensor for imaging and NDE applications. A built-in calibration system, integration of multiple sensors (high and low frequencies), super resolution capability and high Q factor are some of the benefits of the sensor. This paper presents the design and development of the sensor system along with experimental results demonstrating the feasibility of the approach for detection of sub-wavelength defects in composites and additive manufactured metals.

Time reversal (TR) relies on the reciprocity nature of the wave equation. Waves diverging away from a source can be time reversed and back-propagated to focus back at the source. The transmitting and receiving antenna arrays utilized for TR are collectively termed as the TR mirror (TRM). TR imaging in the microwave regime has shown its promise in nondestructive inspection and target localization. This paper deals with the design of a microwave TRM for the detection of breast tumors. The first part of this paper discusses TR simulations based on a signal processing algorithm, to obtain the tumor location from the total fields. The processed time-reversed fields can be back-propagated in a numerical model to focus the wave at the tumor locations. Simulation results aid in investigating the feasibility and the robustness of the method. The second part of this paper deals with experimental studies for the breast tumor detection, using a pulsed time-domain system and the back-propagation model. The successful detection of single and multiple tumors embedded in a liquid breast tissue-based phantom is shown using the microwave system.

Microwave far field imaging techniques are well suited to image source, scatterers and anomalies in dielectric or composite materials. However the diffraction limits restrict the resolution of far field imaging. This contribution focuses on the feasibility of designing a metamaterial lens based sensor for enhancing the resolution of microwave far field imaging. Metamaterials have been increasingly used in the past few decades in design of novel microwave circuits and sensor systems. The unique properties of metamterials offer several advantages such as sub-wavelength nature, compact design and super-resolution, that is not found in conventional materials. The feasibility of enhancing imaging resolution using a metamaterial lens along with time reversal processing of far field microwave data is studied in this paper. Specifically, the super resolution capability of the lens for detection of sub wavelength defects inside the composite materials is presented.

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

This work demonstrates a novel optoelectronic device with the potential for use as a high-frequency, high-power RF source or amplifier. The device is a gallium–arsenide coplanar waveguide with a small gap in the signal trace for optical illumination. A confined charge cloud is generated by illumination through an aperture in an opaque mask over this gap. An electric field above the threshold for negative differential mobility (NDM) enables pulse compression, which prevents the charge cloud from spreading temporally during the drift process. Due to the NDM phenomenon, the output electrical pulse is temporally compressed compared to the input optical pulse. This phenomenon is demonstrated using three different experiments with varied laser pulsewidth (28–700 ps) and device geometry (50- and 100- $\mu \text{m}$ -length gaps). A 66% reduction in the full-width at half-maximum of the electrical pulse relative to the input optical pulse was demonstrated. This novel coupled optoelectronic device opens avenues for high-frequency, high-power, compact devices that could enable next-generation satellite communication systems with faster data rates and longer ranges.

The expanding demand for glass fiber composites in aerospace, automotive and civil infrastructure industries has led to large-scale manufacturing and deployment of these materials. This has motivated the development of new nondestructive evaluation techniques that can detect critical damage modes and continuously monitor structural integrity. The advantages of microwaves for signal propagation and resolution in dielectric materials have previously been exploited. However, work has been largely limited to detecting well-confined, non-extended defects. In this paper we demonstrate the feasibility of a microwave time reversal algorithm for imaging extended defects in dielectric composites with the aid of computational simulations. We introduce an improved process for combining time-resolved information from multiple sources to image extended defects. Model-based numerical results from parametric studies indicate that our proposed algorithm can successfully resolve the location and spatial dimensions of different types of defects such as sub-surface voids, oblique cracks and metal-composite disbonds. The algorithm provides the computational foundation for the development of a new microwave nondestructive evaluation method for imaging challenging defect scenarios in dielectric composites.