Influence of noise on the characterization of materials by terahertz time-domain spectroscopy

We analyze the contributions of various error sources to uncertainty in the far-infrared optical constants (refractive index and absorption coefficient) measured by terahertz (THz) time-domain spectroscopy. We focus our study on the influence of noise. This noise study is made with a thick slab of transparent material for which the THz transmitted signal exhibits temporal echoes owing to reflections in the sample. Extracting data from each of these time-windowed echoes allows us to characterize the noise sources. In THz time-domain spectroscopy experiments in which photoswitches are used as antennae, the transmitting antenna constitutes the principal noise source. The uncertainty in the far-infrared optical constants can be strongly reduced when the extraction is performed with THz echoes that have encountered many reflections in the sample.