Enhanced depth resolution in terahertz imaging using phase-shift interferometry

We describe an imaging technique for few-cycle optical pulses. An object to be imaged is placed at the focus of a lens in one arm of a Michaelson interferometer. This introduces a phase shift of approximately π between the two arms of the interferometer, via the Gouy phase shift. The resulting destructive interference provides a nearly background-free measurement, and a dramatic enhancement in depth resolution. We demonstrate this using single-cycle pulses of terahertz radiation, and show that it is possible to resolve features thinner than 2% of the coherence length of the radiation. This technique could have important applications in low-coherence optical tomographic measurements.