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.
Highlights
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
We discuss the results of a few physical experiments carried out with broadband THz pulses and narrowband CW radiation, transmitted through the ordinary materials, to verify our theoretical conclusions about the appearance of the spurious absorption frequencies caused by the frequency conversion processes due to the nonlinear or non-stationary process and to demonstrate their appearance
As we mentioned in the Introduction, the process of a broadband THz pulse interaction with a substance because of both non-instantaneous response of a medium, and its nonlinear response, as well as its resonance response, can lead, in particular, to the appearance of both the spurious absorption frequencies and the substance emission frequencies in the spectrum of the pulse transmitted through a medium
Summary
The THz time-domain spectroscopy (THz TDS) is a non-destructive tool that is very popular nowadays for both basic research and the security screening and for industrial applications . 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. We discuss the results of a few physical experiments carried out with broadband THz pulses and narrowband CW radiation, transmitted through the ordinary materials, to verify our theoretical conclusions about the appearance of the spurious absorption frequencies caused by the frequency conversion processes due to the nonlinear or non-stationary process and to demonstrate their appearance (soap, paper, chocolate, and plastic).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
