Background-free THz imaging using interferometric tomography

Summary form only given. THz imaging is an emerging technology that permits three-dimensional tomographic imaging of nonmetallic objects. Numerous applications have been demonstrated where THz imaging could be a valuable complement to existing technologies for noninvasive testing, including the detection of faults or delaminations in packaged integrated circuits and the location of air bubbles or cracks within polymer or ceramic parts. In many of these applications, the feature one wishes to detect is subtle, in the sense that its interaction with the single-cycle THz pulse imposes only a small additional distortion on the waveform. A good example is the detection of a delamination or disbonding between two surfaces. In many practical cases, the gap which opens up between the two surfaces is narrower than the coherence length of the THz pulse, and the waveform is little changed as a result. We report on the use of interferometry in combination with THz tomography, for improving the detectability of such subtle features. This idea has analogies to optical coherence tomography, in which the signal pulse, reflected off of the sample, is interfered with a reference wave to provide enhanced sensitivity.