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

Abstract

The appearance of the spurious absorption frequencies caused by the frequency conversion process at the broadband THz pulse propagation in a medium is theoretically and experimentally discussed. The spurious absorption frequencies appear due to both the frequency doubling and generation of waves with sum or difference frequency. Such generation might occur because of the nonlinear response of a medium or its non-instantaneous response. This phenomenon is confirmed by the results of a few physical experiments provided with the THz CW signals and broadband THz pulses that are transmitted through the ordinary or dangerous substances. A high correlation between the time-dependent spectral intensities for the basic frequency and generated frequencies is demonstrated while using the computer simulation results. This feature of the frequency conversion might be used for the detection and identification of a substance.

Substance Detection and Identification Using Frequency Doubling of the THz Broadband Pulse

“…If the spectral intensity of the incident THz radiation at the frequency 2ν 0 does not change after its transmission through a medium (black and red lines in Figure 1c), then the minimum at the doubled frequency 2ν 0 in the spectrum of the transmitted broadband THz pulse (blue line in Figure 1a) is induced by the frequency doubling process, and it is a spurious frequency. Let us stress that the results of physical experiments provided in such a way for an aspirin tablet, a piece of soap, a paper sheet, a polyethylene bag, a chocolate bar and dangerous materials confirmed this phenomenon [36].…”

“…This allows one to use the doubled frequencies for the detection and identification of substances instead of the spectral intensity evolution at true (basic) frequencies. We emphasize that the conclusion about whether the doubled absorption frequency is inherent in the substance or whether it is a spurious absorption frequency resulting from the frequency up-conversion process can be drawn using the scheme for providing physical experiments proposed in [36]. In Figure 1 we depict this scheme using dimensionless units for both axes.…”

“…Unlike [36], our aim is to show the high degree of similarity between the evolutions of the spectral amplitudes at the basic and doubled frequencies. The evolution of the spectral amplitudes |P ν (t)|, |P 2ν (t)| (their computation is described in [24,25]) at the frequencies ν = (0.8, 1.6), (1.2, 2.4), (1.4, 1.8), (1.8, 3.6) is depicted in Figure 4a-d, respectively.…”

“…The proposed novel method substantially increases the probability of detecting and identifying a substance by increasing the number of used absorption frequencies (basic frequencies and doubled ones), and due to this fact the substance spectrum at high THz frequencies is distorted less than at low frequencies. Thus, the current study can be considered as a continuation of the research started in [36][37][38].…”

“…However, this physical mechanism can also be accompanied by a negative feature. So, in [36] we showed that this mechanism can lead to the appearance of spurious absorption frequencies under the action of a broadband THz pulse. Nevertheless, this feature of the THz pulse's interaction with a substance can be used with success for substance detection and its identification.…”