Focalizing of terahertz waves with a Fresnel lens

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in planes perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on axis but also inhibit focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to be a terahertz filter. Moreover, the terahertz pulses travelling as basic mode of Gaussian beam is discussed in detail.