Detection of up to 20 THz with a low-temperature-grown GaAs photoconductive antenna gated with 15 fs light pulses

The generation and detection of terahertz ~THz! radiation using ultrashort optical pulses has been intensively studied during the last decade. The pulse width of commercially available mode-locked Ti:sapphire lasers is approaching 10 fs. With such ultrashort pulses, a wider detection bandwidth is expected to be possible. Coherent detection of THz radiation based on a photoconductive ~PC! antenna, however, was reported to be lower than 7 THz. This limitation has been explained to be a result of the finite carrier lifetime and the RC time constant of the PC antenna. Therefore, interest in ultrafast detection of the radiation has recently shifted to the free-space electro-optic ~EO! sampling technique because EO crystals are assumed to have an instantaneous nonlinear response, and most of them are transparent in the far to midinfrared radiation regime. By exploiting these advantages, ultrabroadband detection of THz radiation based on EO sampling has been reported. To obtain high-frequency response using EO sampling, the EO crystals should be thin enough to reduce the group velocity mismatch between the near-infrared probe beam and the THz radiation. However, even with a PC antenna fabricated on slow carrier lifetime semiconductors, such as semi-insulating ~SI! GaAs or SI InP, the detection of relatively broadband ~;3 THz! THz radiation has been reported. This detection with a slow carrier lifetime was possibly due to the fast-rising edge of the carrier injected by the ultrashort optical pulses. This suggests that the detection bandwidth is not strongly restricted by the carrier lifetime and is possibly extended by using shorter laser pulses. Thus, it is worthwhile to investigate the high-frequency limit of a PC antenna gated with ultrashort optical pulses whose width is close to 10 fs. In this letter, we report on the ultrabroadband detection of electromagnetic radiation, extending up to the midinfrared regime, with a PC antenna fabricated on a low-temperaturegrown GaAs ~LT GaAs! substrate and gated with 15 fs laser pulses. The detected radiation frequency exceeded 20 THz. This is the highest frequency observed by PC antennas reported so far. The 12 fs light pulses were delivered from a modelocked Ti:sapphire laser ~Femtolasers Produktions, Femtosource PRO! at a center wavelength of 800 nm and with a spectral width of 90 nm ~full width at half maximum!. The