The generation transmission, and detection of extremely rapid electromagnetic pulses have been achieved using fast photoconducting materials as time-varying Hertzian dipoles. This approach, which has a measured time response of 1.6 ps, overcomes many of the limitations imposed by transmission line structures, and due to its jitter-free behavior and open geometry is ideally suited for transient electromagnetic measurements of materials.

Photoconducting antennas have been demonstrated which are capable of generating and coherently detecting subpicosecond electrical pulses. These antennas, when illuminated with femtosecond optical pulses, radiate electrical pulses which have frequency spectra that extend from < 100 GHz to > 2 THz. Microscopic dipoles measuring 50, 100, and 200 pm have been fabricated and tested. Integrated photoconductors of radiation-damaged silicon-on-sapphire were used both for impulsive current excitation of the transmitting antennas as well as for gating the receiving antennas.

We have generated electromagnetic beams from a variety of semiconductors. When a bare semiconductor wafer was illuminated by femtosecond optical pulses, electromagnetic waves radiate from the surface and form collinear diffraction-limited electromagnetic beams in the inward and outward directions. The amplitude and phase of the radiated field depend on carrier mobility, the strength and polarity of the static internal field at the semiconductor surface.

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