Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser

Doping germanium for more terahertz A negligible absorption of far-infrared radiation together with high electron mobility makes germanium a perfect material for photoconductive terahertz emitters. However, a very long 'lifetime' of photoexcited carriers is a serious disadvantage - electrons and holes excited by a laser pulse do not vanish until the next pulse arrives, they accumulate leading to a dramatic device degradation. The research team at the Helmholtz-Zentrum Dresden-Rossendorf in Germany has managed to overcome this limitation by introducing gold impurities into germanium, thus reducing the carrier lifetime by more than thousand times - down to the sub-nanosecond level. In cooperation with colleagues at the University of Konstanz (Germany) they demonstrated a germanium-based photoconductive antenna emitting terahertz pulses of unprecedented bandwidth of 70 THz using a femtosecond fibre laser. Further work on germanium-based THz devices is expected to revolutionize terahertz technologies, with applications in security, communication, and pharmaceuticals. Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy - reaches only up to 7 THz in the regular transmission mode due to absorption by infrared-active optical phonons. Here, we present ultrabroadband (extending up to 70 THz) THz emission from an Au-implanted Ge emitter that is compatible with mode-locked fibre lasers operating at wavelengths of 1.1 and 1.55 mu m with pulse repetition rates of 10 and 20 MHz, respectively. This result opens up the possibility for the development of compact THz photonic devices operating up to multi-THz frequencies that are compatible with Si CMOS technology.