Abstract

In a terahertz (THz) time-domain spectroscopic system, the THz beam waist radius can be measured by eclipsing the beam waist with a semi-infinite mask while monitoring signal variation (knife-edge technique). However, the obtained beam waist radius is affected by the spatial distribution of the detector sensitivity. This effect was confirmed by calculating the refractive-index corrections required when inserting a sample into the beam waist with and without consideration of the detector sensitivity distribution and comparing them to the experimental results. The real beam waist radius was $\sqrt 2$ times larger than that obtained by simply differentiating the signal variation in a system comprising optics symmetrical about the beam waist. This effect must be considered in obtaining the correct THz beam profile by the knife-edge technique.

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