Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Germanium is a nonpolar semiconductor with missing one-phonon absorption. The absence of a Reststrahlen band enables the generation of a gapless THz spectrum spreading up to 13 THz [1], limited only by the duration of the excitation and detection laser pulses. However, in spite of other promising properties including low bandgap and small effective mass, the long, µs-scale recombination time arising from the indirect bandgap of intrinsic germanium has been prohibitive for practical application as photoconductive THz emitters. Although not essential for broadband THz emission, shorter recombination times are necessary to ensure complete carrier recombination between subsequent laser pulses and to make these emitters compatible with standard modelocked laser systems operating at pulse repetition rates up to hundreds of MHz.
By introducing deep traps into germanium via gold implantation, we have reduced the carrier lifetime to sub-nanosecond values. Fabricated on this Au-implanted Ge material, we have demonstrated a photoconductive THz antenna which is compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 m and with pulse repetition rates of 78 MHz [2] and potentially up to several hundreds of MHz. Reaching up to 70 THz bandwidth, which is almost one order of magnitude higher than that of existing state-of–the-art photoconductive THz emitters fabricated on GaAs or InGaAs, our approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.
[1] A. Singh et al., ACS Photonics 5, 2718 (2018).
[2] A. Singh et al., Light Science & Applications 9, 30 (2020).