Transmission of THz pulse with a few circles through opaque samples placed at long distance (4-6 metres)

Abstract

Nowadays, the detection and identification of dangerous substances at long distance (several metres, for example) by using of THz pulse reflected from the object is an important problem. The main problem with this technique is the absorption of THz energy by water vapor. However, using THz pulsed radiation is possible at distance of some metres as it is well-known. Below we demonstrate possibility of THz signal measuring reflected from a flat metallic mirror placed about 4 metres from the parabolic mirror. Investigated object is placed before this mirror. Therefore, at present time our measurements contain features of both transmission and reflection modes. The reflecting mirror is used because of weak averaged power of femtosecond laser. This power is about 1 W. Moreover, the laser beam splits many times. Therefore, the averaged power falling on the THz emitter decreases at least 8 times. The pulse duration generated by the femtosecond laser is equal to 68 fs. Measurements were provided at room temperature and humidity about 70%. The aim of investigation was the detection of a substance in real conditions. We discuss new features of the detection of a substance covered under various ordinary materials and possible way for their influence deleting on the detection using reflected THz pulse. We discuss also details of action of THz pulse with a few circles on media. The main feature of such interaction is its dependence from absolute phase of the THz pulse. We demonstrate results of computer simulations as well as physical experiment results for propagation of such laser pulses.