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We have generated freely propagating, diffraction-limited beams of single-cycle 0.5 THz electromagnetic pulses from a 5-mm-diam coherent source. After propagating 100 cm in air, there was little change in the measured subps pulse shape, even though the signal strength was reduced by 20 times compared to the strength at 10 cm.

We have significantly improved the emission and detection of electromagnetic beams of single-cycle 0.5 THz pulses, through the use of new dipolar antenna structures. The frequency response was extended to well beyond 1 THz, and the beam power was increased by more than 15 times. The antennas were located at the foci of sapphire lenses and were photoconductively driven by ultrafast laser pulses. An additional collimation by a paraboloidal mirror produced a beam with a 25 mrad divergence, and subsequent focusing by a second identical mirror improved the coupling between the transmitting and receiving antenna by orders of magnitude.

We demonstrate an ultrafast 10 μm sized, electric dipole source of terahertz radiation closely coupled to a 1 cm spherical mirror. This optical approach has the advantages of subpicosecond response times with essentially complete collection efficiency. Using this technique, we have generated and detected subpicosecond freely propagating electrical pulses.