Spectral Fresnel filter for pulsed broadband terahertz radiation

We proposed a simple and cost-effective method to manipulate the temporal and spectral properties of pulsed terahertz waves. A deep modulation of a pulse spectrum was both numerically and experimentally verified using Fresnel apertures with a radius ranging from several to several tens of the central wavelength of the broadband terahertz radiation. N-fold frequency minima were formed in the spectrum at a specific axial position behind the filter. Non-paraxial properties of this filter were also analyzed. A significant value (35%) of the ratio of the longitudinal to the transverse field component at the filter frequency was obtained. The measured results agree well with the simulation and theoretical predictions. The property of such a diffractive Fresnel notch filter can benefit the generation of longitudinal terahertz fields and relevant applications.