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

Abstract

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 μm 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.

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.