Microwave, millimeter wave and terahertz (MM) techniques for materials characterization

Electromagnetic waves (EMW) in the frequency range of microwaves, in the millimeter range and in the terahertz range (MMT) can be used to nondestructively characterize many materials. There is no unique definition of the term microwaves. It can encompass decimeter and centimeter waves corresponding to a wavelength from 1 m to 10 cm to 1 cm, respectively. The millimeter waves extend from 1 cm to 1 mm (30-300 GHz), while the terahertz waves (THz) have wavelengths less than 1 mm (300 GHz). The upper limit of the range of THz waves - also termed far infrared - is not clearly defined. In contrast to conventional ultrasound testing, no coupling agent is needed to couple the EMW into the test object to be investigated. Principally the distance between the test object and test device is not limited. The interior of metallic materials cannot be tested by MMT waves since the high electrical conductivity generates a total reflection of the incident waves thus reducting the penetration depth to a few µm or less.