Comparative study of 1-D and 2-D metamaterial lens for microwave nondestructive evaluation of dielectric materials

A novel microwave nondestructive evaluation (NDE) sensor was developed in an attempt to increase the sensitivity of the microwave NDE method for detection of material defects small relative to a wavelength. The sensor was designed on the basis of a negative index material (NIM) lens. Transmission at the resonant frequency through the 1-D lens was determined to be about 10 times higher than that with the 2-D lens. However, the focusing ability of the 1-D lens was found to be slightly lower to the 2-D lens (focus spot size for the 1-D lens was determined to be 0.7λ vs. 0.48λ for the 2-D lens). A fiberglass material sample with a 3 mm (0.037λ) diameter through hole (perpendicular to the propagation direction of the wave) was tested with both lenses. The hole was successfully detected with an 8.2 cm wavelength electromagnetic wave with both lenses, but the image obtained with a 2-D lens was much sharper. Therefore, the choice of the lens to be used in a sensor is prescribed by the specific requirements of the testing system. For example, a 1-D lens should be considered when the simplicity of the testing system is deemed more important than the quality of the image obtained from a defect. A 1-D lens also allows for a longer sample standoff distance and higher transmission.