Essential Limitations of the Standard THz TDS Method for Substance Detection and Identification and a Way of Overcoming Them

Abstract

Low efficiency of the standard THz TDS method of the detection and identification of substances based on a comparison of the spectrum for the signal under investigation with a standard signal spectrum is demonstrated using the physical experiments conducted under real conditions with a thick paper bag as well as with Si-based semiconductors under laboratory conditions. In fact, standard THz spectroscopy leads to false detection of hazardous substances in neutral samples, which do not contain them. This disadvantage of the THz TDS method can be overcome by using time-dependent THz pulse spectrum analysis. For a quality assessment of the standard substance spectral features presence in the signal under analysis, one may use time-dependent integral correlation criteria.

Spurious Absorption Frequency Appearance Due to Frequency Conversion Processes in Pulsed THz TDS Problems

“…The main aim of this investigation is a demonstration of high similarity between the spectral dynamics at these frequencies. As an example, the modulus of such spectral amplitude |P ν (t)|, |P 2ν (t)| evolution (their computation is described in Appendix A and in [44][45][46]) at the frequencies Figure 3. A high degree of similarity between time-dependent spectral amplitudes is observed in all cases (a-d), and we may expect their high correlation.…”

“…Here M denotes the number of time moments in the spectral dynamics |P ν 1 |, |P ν 2 |, which depends on the dynamics construction parameters-the window length T and its shift ∆ along the signal under investigation. With accordance to our previous articles ([ [44][45][46]), these parameters are chosen as T = 2.8 and ∆ = 0.2 dimensionless units.…”

“…| P ν , which depends on the dynamics construction parameters-the window length T and its shift Δ along the signal under investigation. With accordance to our previous articles ( [44][45][46]), these parameters are chosen as T = 2.8 and Δ = 0.2 dimensionless units. The DFG may be obtained by two pairs of Fourier harmonics interaction: between the harmonics with frequencies ν = 1.2 and ν = 0.8; as well as ν = 1.8 and ν = 1.4, respectively: dif ν = 0.4 = 1.2 − 0.8 = 1.8 − 1.4.…”

“…Below we briefly remind the main notations and formulas at the construction of the evolution of the spectral amplitude (dynamics of spectral line) of the signal F(t) at the chosen frequency ν [44][45][46].…”

“…Often, these covers possess both density fluctuations and sample thickness variations in the sub-millimeter scale of length and, therefore, they can be considered to be a disordered photonic structure in the THz frequency range. Consequently, one can appear the false absorption frequencies, which are caused by inhomogeneities, in the spectrum of the broadband THz pulse transmitted through or reflected from these ordinary materials and these frequencies may coincide with those of the dangerous substances [44][45][46]. Thus, the appearance of the false absorption frequencies under the THz TDS is an ordinary phenomenon and recognizing of the true absorption frequencies inherent to a substance under consideration is an urgent problem.…”