Metamaterials Based High Resolution 2-D Microwave Imaging

Metamaterials research is a relatively new field that has exhibited tremendous growth since its first successful demonstration in 2000. It even attracted attention in mainstream media, due to the metamaterials promise of novel capabilities in electromagnetics research, such as cloaking, extremely compact antenna design, high spectral resolution remote sensing, non-ionizing radiation protection, and subwavelength resolution imaging. Unlike conventional materials, metamaterials derive their electromagnetic properties through resonant subwavelength sized elements. Through precise engineering of these elements, the electromagnetic properties of the metamaterials can be tailored. While advances have been made in demonstrating metamaterials throughout the electromagnetic spectrum, applications in sensor systems and devices are just beginning to follow. Of particular interest is the metamaterial lens, which has been shown to provide subwavelength resolution. By overcoming the diffraction limit, these lenses lend themselves to high resolution imaging systems.
For microwave imaging devices, resolution has been of concern due to their long wavelengths. Furthermore, the systems are diffraction limited (far-field imaging) or subject to very short stand-off distance requirements (near-field imaging). With microwave metamaterial lenses it is now possible to achieve imaging of subwavelength sized features at larger working distances. Microwave metamaterial lenses are well suited to imaging applications because long wavelengths give the ability to peer inside of materials, while subwavelength resolution will render small details. In addition, microwave imaging is of particular interest to non-destructive testing, as it is non-ionizing, non-contact, and cost effective. This type of imaging system would lend itself to applications in medicine, automotive, aerospace, and security industries.
This dissertation presents the modeling, design, characterization, and validation of metamaterial lenses for high resolution 2-D microwave imaging. A 2-D metamaterial lens based on split ring resonator and rod constituent elements, optimized for 16.65 GHz, has been fabricated for the imaging application, and its imaging properties have been evaluated in a reflection mode configuration. Furthermore, the metamaterial lens capability for 2-D microwave imaging to detect hidden objects has been demonstrated. It was found that MTM lenses were able to image hidden objects at subwavelength resolution of 0.66 λ and sensitivity of 0.12 λ at a stand-off distance of 1.44 λ.
This imaging system was the first of its kind to provide for 2-D microwave images using MTM lenses in reflection mode. Furthermore, the MTM lenses were found to be highly sensitive to the incident polarization, leading to unwanted rotation of the polarization state of transmitted MW. The polarization rotation has been related to the asymmetric unit cell design of typical MTM lenses. The effect of the polarization rotation in MTM lenses on image performance is the first to be discussed in literature. The current state of MTM based imaging system has been expanded upon by using a multi-detector array. This decreased image acquisition time by a factor of 5 while giving the potential of increasing the contrast of the image. The overall system provided for similar image quality as a near-field measurement at much greater stand-off distance than the near-field. This study highlights the capabilities and limitations of the metamaterial lens for imaging systems.