Diffraction of terahertz waves after passing through a Fresnel lens

The spatiotemporal and spectral characteristics ofultrawide-band terahertz pulses after passing through a Fresnel lensare studied by using the scalar diffraction theory. The simulationshows that the transmitted terahertz waveforms compress withincreasing propagation distance, and the multi-frequency focusingphenomenon at different focal points is observed. Additionally, thedistribution of terahertz fields in a plane perpendicular to theaxis is also discussed, and it is found that the diffraction notonly induces focusing on-axis but also inhibits focusing at off-axispositions. Therefore, the Fresnel lens may be a useful alternativeapproach to being a terahertz filter. Moreover, the terahertz pulsestravelling as a basic mode of a Gaussian beam are discussed indetail.