Decoupling substrate thickness and refractive index measurement in THz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz–TDS) relies heavily on knowing precisely the thickness or refractive index of a material. In practice, one of these values is assumed to be known, or their product is numerically optimized to converge on suitable values. Both approaches are prone to errors and may mask some real features or properties of the material being studied. To eliminate these errors, we use THz–TDS in reflection geometry to accurately and independently determine both thickness and refractive index by illuminating the step-edge of a substrate atop a metal stage. This method relies solely on the relative time delay among three reflected pulses, and therefore forgoes the need for optimization or assumption of substrate parameters.