Time-frequency representation of terahertz time domain spectroscopy signals based on Stockwell transform

In this paper, we apply a new technique for the representation of terahertz time-domain spectroscopy (THz-TDS) data in time-frequency domain based on Stockwell transform (S-Transform). The S-Transform performs a joint time-frequency analysis which is not the case of the classic Fourier transform where only the frequency content of the signal is provided. Because of S-Transform phase characteristic, it can be employed in cross spectrum analysis and feature extraction. The cross S-Transform matrix is calculated and represented for different substances and showed a significant information that can be used to discriminate between different illicit substances.