Svetlana A. Varentsova

An Effective Method for Substance Detection Using the Broad Spectrum THz Signal: A "Terahertz Nose"

We propose an effective method for the detection and identification of dangerous substances by using the broadband THz pulse. This pulse excites, for example, many vibrational or rotational energy levels of molecules simultaneously. By analyzing the time-dependent spectrum of the THz pulse transmitted through or reflected from a substance, we follow the average response spectrum dynamics. Comparing the absorption and emission spectrum dynamics of a substance under analysis with the corresponding data for a standard substance, one can detect and identify the substance under real conditions taking into account the influence of packing material, water vapor and substance surface. For quality assessment of the standard substance detection in the signal under analysis, we propose time-dependent integral correlation criteria. Restrictions of usually used detection and identification methods, based on a comparison between the absorption frequencies of a substance under analysis and a standard substance, are demonstrated using a physical experiment with paper napkins.

New method for analysis of temporal dynamics of medium spectrum under the action of terahertz pulse

The new method of measurement of instantaneous spectral amplitudes for requiring frequencies under the action of terahertz pulse is proposed. It allows one to investigate the frequency dynamics by using the integral measurements of the optical signal.

Highly effective method for temporal terahertz spectroscopy under the condition of random probe signals

As it is well known, terahertz time-domain spectroscopy is very perspective method for identification of molecules. The main difficulty of its application, on our opinion, is an absent of high effective method for temporal analysis of the medium response spectrum. According to [13] methods, using in time-domain spectroscopy for treatment of experimental results, have some bad features, which lead to absurd results sometimes. In [14,15,20,21] we developed two possible ways to create effective algorithms for the analysis of instantaneous spectral intensities of medium responce under the e action of pulse with a few cycles. It should be stressed that in the first algorithm we used the Gabor-Fourier window transform, investigated its efficiency and formulated requirements to its parameters for reconstruction of spectral dynamics of medium response with high quality. The second algorithm, based on application of SVD method, allows to obtain an information about many spectral lines simultaneously from one series of measurements of medium response characteristics. In both algorithms, their parameters were ideal ones without any random fluctuations. This paper deals with the more realistic situation: we investigate possibility to get dynamics of any spectral lines of medium response when parameters of measurements are random. Our investigation demonstrates the high efficiency of using method based on SVD algorithm. We apply it as for ideal medium response constructed on the base of some ideal oscillators, as for results of physical experiments. Also below the new approach to finding of instantaneous spectral amplitudes for requiring frequencies under the action of terahertz pulse is proposed. It allows to investigate a frequency dynamics due to using of integral measurements of optical signal.

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

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.

Influence of surface of explosive on its detection and identification using the SDA method for analysis of the reflected THz signal

The SDA (Spectral Dynamics Analysis) method is used for the detection and identification of the PWM C4 explosive with the surface having inhomogeneity, caused by action of the sandpaper with different grit on the explosive surface, or with the surface having various curvature of its surface. We show that the SDA-method is good tool for the detection and identification of the explosive using THz signal reflected from the PWM C4 explosive. We propose (see as well [24]) integral criteria for the identification of substances. These criteria allow to detect the explosive despite an influence of its shape on the THz spectrum. Proposed assessments and algorithms for computation of the identification probability show both high probability of the substance identification and a reliability of realization in practice.

Efficiency of the detection and identification of ceramic explosive using the reflected THz signal

The SDA (Spectral Dynamics Analysis) method was applied for the detection and identification of ceramic explosives (a mixture of Al2O3 with Hexogen or Penthryte) hidden under different coverings - thin, rough, thick layers of Polyethylene foils and a layer of cotton. We analyzed THz pulses reflected from the samples at different angles - nearly 90° (Stand-Off reflection) and 45° (Specular reflection). We showed that in some cases the presence of covering can significantly distort the spectral properties of the reflected THz signal. Nevertheless, it is possible to find the identifiers characterizing the presence of explosive under the covering analyzing the spectrograms and dynamics of spectral lines of the main pulses and the sub-pulses following the main pulse.

2D THz signature for substance identification

The method, which gives us a possibility to obtain the unique 2D signature of substance, for its identification in THz frequency range is developed and applied for the treatment of signals, passed through ordinary materials or selected explosives, including those hidden under opaque simulant covers. The method of identification is based on the analysis of spectrum dynamics (spectrogram) of medium response and has the ability not only to detect the presence of the substance in the sample but to identify it by its 2D signature, which is unique for each investigated substance. It allows to trace the dynamics of many spectral lines in one set of measurements simultaneously and to obtain the full information about the spectrum dynamics of the measured signal. We showed that spectrograms of THz pulses, passed through the explosives, hidden under simulant covers, widely differ from spectrograms of simulant themselves despite of a little difference in their Fourier spectra. Therefore, the method allows detecting and identifying the hidden substances with high probability and can be very effective for defense and security applications. The problem of detection of a noisy regular acoustic signal with linear modulation of frequency is examined too.

Detection and identification of drugs under real conditions by using noisy terahertz broadband pulse

We discuss an effective method for detecting and identifying drugs using a high-noise terahertz (THz) signal. We add a noisy THz signal obtained in real conditions to the THz signal transmitted through a sample with the illicit drug methamphetamine. The insufficiency of the standard THz time-domain spectroscopy method is demonstrated, showing that this method detects the spectral features of neutral substances and explosives in a noisy THz signal. The method discussed is based upon time-dependent integral correlation criteria calculated using spectral dynamics of the medium response. We propose a modification of the integral correlation criterion that is less dependent on the spectral characteristics of a noisy signal under investigation.

The method of the spectral dynamics analysis of reflected signal for problem of identification of substance

The method of spectral dynamics analysis (SDA-method) is used for the detection and identification of explosive RDX and harmless substances in pellets with different weight and hidden under the thin layer of polyethylene or cotton by reflected THz signal. The signal was measured on the long-term interval with duration about 180 ps and contains not only the main reflected THz pulse but the subpulse with less intensity. Two cases of the reflection were investigated - at 90° and at 45° from the sample. SDA method allows to obtain the spectrogram - 2D THz signature of the substance, and to analyze the dynamics of many spectral lines of the signal by one set of measurements. The identifiers characterizing the presence of substance in the sample were found in all investigated samples - they contain in the corresponding spectrograms and dynamics of spectral lines. We showed that the analysis of the reflected signal over the short time intervals (less than 50 ps) is insufficient for reliable identification of the substances, especially of hidden ones, because in some cases the characteristic features of the substances can be found only in the subpulses. The influence of window shape in Fourier-Gabor method on the dynamics of spectral lines was discussed too.

2D signature for detection and identification of drugs

The method of spectral dynamics analysis (SDA-method) is used for obtaining the2D THz signature of drugs. This signature is used for the detection and identification of drugs with similar Fourier spectra by transmitted THz signal. We discuss the efficiency of SDA method for the identification problem of pure methamphetamine (MA), methylenedioxyamphetamine (MDA), 3, 4-methylenedioxymethamphetamine (MDMA) and Ketamine.