  The two-phonon scheme of generation of terahertz (THz) photons by gold
nanobars (GNBs) is considered. It is shown that in GNBs, by choosing their
sizes, it is possible to provide conditions for converting the energy of
longitudinal phonons with THz frequencies into the energy of THz photons. The
prospects of designing GNBs-based soft THz radiation sources (frequencies:
0.14; 0.24; 0.41 and 0.70 THz) with a large flow cross-section (diameter ~40
cm) intended for detection of hidden objects under clothing to ensure security
in public places (airports, railway stations, stadiums, etc.) are assessed. The
choice of the above frequencies is a compromise between the requirements of low
absorption of THz radiation by water vapor in air, good penetration through the
fabric of clothing, favoring a sufficient resolution of the imaging system, and
an abundance of corresponding longitudinal phonons, capable of exciting Fermi
electrons in GNBs. Estimates of the characteristics of the
terahertz-to-infrared converter based on gold nanospheres (GNSs), which could
work in tandem with these sources of THz radiation -- as a means of
visualization of hidden objects -- are also given.
