Concentration of terahertz radiation through a conically tapered aperture

We demonstrate that conically tapered cylindrical apertures can be used to efficiently concentrate broadband terahertz (THz) radiation. Keeping the aperture diameter on the input plane fixed, we show that as the diameter of the aperture on the exit plane is decreased, we obtain an increase in the magnitude of the transmitted electric field that varies inversely with the output aperture diameter. Correspondingly, the transmitted THz intensity concentration increases inversely with the square of the output aperture diameter. For the smallest aperture that we fabricated, we obtain a ~50 fold increase in the transmitted THz intensity. We expect further increases in the intensity concentration with smaller output apertures. As the output aperture diameter is decreased with a corresponding increase in the concentration factor, we directly measure an increase in the propagation time delay of a narrowband pulse through the structure. Finally, we demonstrate that further increase in the concentration factor can be achieved by engraving circular grooves around the input aperture.