High effective THz-TDS method for the detection and identification of substances in real conditions

Abstract

Low efficiency of the standard THz-TDS method for the detection and identification of substances is demonstrated. For this purpose, we use a few examples. In the first example, we model the noisy THz signals transmitted through the amphetamine-type stimulant in real conditions. Namely, with a temperature 18° C, the relative humidity of about 50 % and the distance between the parabolic mirror and the object about of 3.5 meters. We show that the standard THz-TDS method reveals the spectral features of many neutral substances and explosives in the noisy THz signals from the illicit stimulant MA, at the same time this method is not able to detect the presence of this stimulant in the noisy signals. The second example is the detection and identification of plastids with inhomogeneous surface in reflection mode. We show that inhomogeneous surface distorts spectral characteristics of the reflected THz signal main pulse, which cannot be used for the detection and identification of the plastids by means of the THz TDS method. In the last example we show that even under laboratory conditions (at short distance from the receiver), THz TDS detects in the semiconductors the absorption frequencies, which belong to both hazardous and neutral substances. To overcome this disadvantage, we propose to use the time-dependent spectrum of the THz pulse, transmitted through or reflected from a substance. For quality assessment of the presence of the standard substance absorption frequency in the signal under analysis, we use time-dependent integral correlation criteria. The influence of aperture placed in front of the sample on spectral properties of silicon wafers with different resistivity is demonstrated as well.Keywords: reflected and transmitted THz pulse, noisy THz pulse, method of spectral dynamics analysis, integral correlation criteria, identification of explosives and drugs. *vatro@cs.msu.ru; phone 7 495 939-5255; fax 7 495 939-2597

New Possibilities of Substance Identification Based on THz Time Domain Spectroscopy Using a Cascade Mechanism of High Energy Level Excitation

“…In [ 29 , 30 , 31 , 33 , 34 , 35 , 36 , 37 ] we investigated the efficiency of using the integral correlation between the spectral line dynamics of the reflected or transmitted signal E ( t ) and the spectral line dynamics of the standard transmitted signal e ( t ) from database for the substance detection. For this purpose, we introduce the following notations.…”

“…These depend on the dynamics construction parameters—the window length T and its shift Δ along the signal. Accordingly to our previous investigations [ 29 , 30 , 31 , 33 , 34 , 35 , 36 , 37 ] we choose these parameters in the following way: the window length T = 2.8 ps., the window shift Δ = 0.2 ps.…”

“…Under real conditions, factors such as opaque packaging, inhomogeneity of the substance surface, atmospheric humidity and others affect significantly the THz spectroscopy results and distort the spectral features of the substance under investigation [ 23 , 24 , 25 , 26 , 27 , 28 ]. Thus, when observing a THz pulse reflected from a neutral material, we can detect false absorption frequencies, which are not inherent to this material but they may be wrongly detected as dangerous substances [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ]. This serious disadvantage for the standard THz-TDS method and its physical mechanism understanding are challenging problems for the development of real detection systems.…”

“…In the current paper we show the spectral dynamics analysis method (SDA-method) efficiency and integral correlation criteria (ICC) ([ 29 , 30 , 31 , 33 , 34 , 35 , 36 , 37 ]) using for the detection and identification of a substance covered by such a disordered structure. This allows us to demonstrate a good chance to overcome the standard THz TDS method limitations.…”

“…In [ 33 ] the SDA method together with ICCs was applied for substance detection and identification using a high noise THz signal. In [ 34 , 35 , 36 , 37 ], the spectral properties of THz pulses measured at long distances of about 3.5 m were investigated by means of modified integral criteria. The essential limitations of the standard THz-TDS method were demonstrated in [ 30 , 31 , 34 ] as well.…”