Tunable narrowband terahertz pulses in the range of 0.3 to 2.4 THz were created via difference frequency generation using a photoconductive antenna (PCA) design developed by researchers at the Helmholtz-Zentrum Dresden Rossendorf (Dresden, Germany). The narrowest pulses (0.2 THz at a 1 THz central frequency) were generated at an efficiency of 5 × 10-4; pulses with a 0.36 THz bandwidth were generated at an efficiency of 1 × 10-3. Antennas based on low-temperature-grown gallium arsenide (LT-GaAs) as well as on semi-insulating GaAs (SI-GaAs) were demonstrated; the latter devices, although lower in power by a factor of four, use a type of GaAs that is inexpensive and easily available.
A regenerative titanium:sapphire laser amplifier with an 8 µJ pulse energy, a repetition rate of 250 kHz, a pulse duration of 35 fs, and a center wavelength of 800 nm served as the ultrafast source. The pulses were stretched and divided into two pulses: one to create the terahertz radiation and the other to analyze it via electro-optic sampling. The generated terahertz light was collected with a gold parabolic mirror and combined with the probe pulse in a zinc telluride crystal. The PCAs had interdigitated-finger structures with 5 µm gap spacings; the SI-GaAs device was 10 mm square, while the LT-GaAs device was 1 mm square. The researchers believe that the reasonably good results with SI-GaAs occurred because the device’s higher mobility counters the long recovery time.
Contact Dominik Stehr at [email protected].