Terahertz Pulsed Light Propagation And Imaging Characteristics

In recent years, terahertz (THz) radiation which occupied an extremely large portion of the electromagnetic spectrum between microwave and infrared has drawn much° attention and interest, since the rapid advances in femtosecond pulsed laser technology and the rich physical and chemical processes that occur in this region. The propagation and diffraction of the THz pulses have attracted much attention recently both theoretically and experimentally. The uncertainty of the shape of THz pulse has baffled the investigation of THz, and it also affected the precision of result. So it is very important to understand the shape changes of THz pulses in free space for the basic and application investigations of THz pulses. The majority of THz systems are based on transmission of free-space THz radiation. In order to fully describe the behavior of THz pulse in an optics system, it is worth to know clearly how the temporal waveform and the spectrum of the pulse are affected as a result of the spatial and temporal transformation. The imaging and diffraction properties in the terahertz region are investigated in this work.The angular spectrum theory was used to analyze the evolution of THz pulse after passing through an aperture, some propagation characteristics of THz pulses have been obtained:(1) The peak of the spectrum of the THz pulse on the axis for different distances is shifted to a higher frequency at larger distance. This is result from greater diffraction loss for lower frequency components as the pulse propagates away from the source.(2) After THz pulse propagates a certain distance, the red shift of the spectrum of the THz pulse becomes larger with the radial distance increases in the observation plane. This is due to the low-frequency components diffract more strongly than the high-frequency components, so the high-frequency components are concentrated around the axis, whereas the low-frequency components predominate with the larger radial distance from the axis. The loss of the highest frequencies in off-axis gives rise to the spectrum red shifting and narrowing.(3) Due to the propagation and diffraction, differential diffraction loss of wide-ranging frequency components can be a major source of temporal distortion.The simulations of the temporal evolution of the THz pulse diffracted from a circular aperture are presented. At earlier times there is light only at the centre of the screen and at latertimes it has a typical ring structure. The size of the ring is time dependent, and the ring expanding with time after the main peak. The propagation of THz pulse passed through a THz len were also simulated, and the dynamic evolution images were obtained.Furthermore, the evolution pattern of a pulsed THz beam after passing though a sample is obtained. Due to the diffraction and the propagation, the evolution pattern obtained on the observation plane cannot reveal the real information of the picture. Therefore, it is necessary to reconstruct the original field distribution in the incident plane. By inversely using the propagation method in this work, the reconstructed images were obtained. The original sample has been well reconstructed and the sharp boundary can been clearly seen. The simulation results confirm the validity of our method.