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

Abstract

Using an experiment with thin paper layers and computer simulation, we demonstrate the principal limitations of standard Time Domain Spectroscopy (TDS) based on using a broadband THz pulse for the detection and identification of a substance placed inside a disordered structure. We demonstrate the spectrum broadening of both transmitted and reflected pulses due to the cascade mechanism of the high energy level excitation considering, for example, a three-energy level medium. The pulse spectrum in the range of high frequencies remains undisturbed in the presence of a disordered structure. To avoid false absorption frequencies detection, we apply the spectral dynamics analysis method (SDA-method) together with certain integral correlation criteria (ICC).

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

“…Therefore, in Figure 16 In our opinion, the appearance of the Soap_BB spectral minima at the frequencies ν = 3.35, 3.65, and 3.95 THz (Figure 14g) can be due to the energy levels transitions that are caused by the photons with the frequencies ν = 3.05 THz and ν = 0.3, 0.6 THz (see the spectral dynamics in (d)), because there is the high correlation between the corresponding spectral line dynamics. The physical mechanism of these energy level transitions is the cascading mechanism that is described in [57,58], which occurs under the action of the broadband THz pulse. We analyzed such a mechanism of the energy level excitation in the framework of one-dimensional (1D) Maxwell's equations describing the pulse propagation and matrix density formalism for the description of the response of a medium.…”

“…It should be stressed that the spurious absorption frequencies that appear under the frequencies up-conversion process (for example, at SHG) may be used as a new effective tool for the detection and identification of the substance. We illustrated such a possibility in [57,58], where a description of the nonlinear response of a medium was provided while using density matrix formalism [59] and the THz pulse propagation is described by one-dimensional (1D) Maxwell's equations. A single substance layer was placed between two coverings, which consisted of linear layers with random dielectric permittivity.…”

A Possible Way for the Detection and Identification of Dangerous Substances in Ternary Mixtures Using THz Pulsed Spectroscopy

“…Non-radiative transitions are responsible for the absorption frequencies of a medium. Radiative energy level transitions lead to appearance of new spectral harmonics which may have frequency either less than the frequency corresponding to the frequency of excited energy level transition or greater than this frequency due to the cascade mechanism of the energy level excitation (we described this mechanism in [53]). Therefore, from an analysis of the pulse spectrum we will see the emission frequencies and the absorption frequencies caused by appearance of the emission of a medium at some frequencies.…”

“…At the same time, in the separated time intervals the absorption frequencies with lower spectral intensities can be more clearly visible and therefore may be detected. Let us remember that, despite lower intensities at high absorption frequencies, the inhomogeneous surface or non-opaque covering less destroy them [53,58]. Therefore, they can be used for the detection and identification, especially under real conditions.…”

Titanium-Based Microbolometers: Control of Spatial Profile of Terahertz Emission in Weak Power Sources

“…Because of non-ionizing nature of THz radiation, imaging in this range can be found as a beneficial tool for medicine and biomedical stuff inspection [2][3][4]. As THz frequencies display the ability to propagate in dielectric medium without strong attenuation, THz imaging and spectroscopy can also serve as valuable instruments for security needs [5][6][7], post packages inspection [8][9][10] or industrial materials control and their identification [11][12][13][14]. However, direct implementation of THz technology still meets severe difficulties related mainly to reliability of active components (emitters and detectors) and issues of precise alignment of the optical system due to the presence, as a rule, of passive bulky optical elements like mirrors, lenses or beam splitters.…”