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

Singh et al. Light: Science & Applications (2020)9:30 https://doi.org/10.1038/s41377-020-0265-4 ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser Abhishek Singh1, Alexej Pashkin 1, Stephan Winnerl1, Malte Welsch1,2, Cornelius Beckh3, Philipp Sulzer3, Alfred Leitenstorfer3, Manfred Helm1,2 and Harald Schneider 1 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; 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. Introduction THz time-domain spectroscopy using broadband THz pulses has emerged as a powerful tool for probing lowenergy excitations in condensed matter at the meV energy scale1–3. The spectrum of potential applications depends on the available spectral bandwidth, signal-to-noise ratio and data acquisition speed. In general, the techniques for THz generation and detection exploit either photoconductivity or optical nonlinearity4,5. Photoconductive techniques for THz emission and detection are widely used due to their simplicity, compactness and possibility of direct coupling to fiber optics. THz emission from photoconductivity was first demonstrated using Si4,6,7; however, the majority of current photoconductive antennas are based on GaAs or InGaAs (in case of the telecom wavelength) due to the high carrier mobility in Correspondence: Alexej Pashkin () or Harald Schneider () 1 Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 2 Cfaed and Institute of Applied Physics, TU Dresden, 01062 Dresden, Germany Full list of author information is available at the end of the article. these materials and well-established schemes for reducing the carrier lifetime8. Optical rectification techniques rely mostly on polar noncentrosymmetric materials with a strong second-order optical nonlinearity, such as ZnTe, GaP, GaSe, or DSTMS9. The polar nature of these materials renders their optical phonons strongly IR-active, leading to reststrahlen bands in the region between 5 and 10 THz. As a result, the spectral bandwidth of many THz emitters is limited to below 7 THz in the regular transmission mode. In particular, for InGaAs-based photoconductive emitters excited at a wavelength of 1.55 µm, gapless THz spectra up to 6.5 THz have been demonstrated10. Thin electro-optic crystals of GaSe and DAST have shown THz emission extending up to more than 100 THz towards the higher frequency end, but the THz intensity near their phonon frequencies is strongly suppressed11–14. Even in the reflection geometry available with photoconductive emitters, strong absorption and emission by polar TO and LO phonons, respectively, hinders their application for spectroscopy around the resonance frequencies15,16. © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Singh et al. Light: Science & Applications (2020)9:30 Page 2 of 7 To satisfy the demand of a gapless ultrabroadband spectrum, novel techniques such as two-color air plasma17 and spintronic THz emission18 have been introduced. THz emission from air plasma achieves a bandwidth of more than 100 THz, but this technique requires high pump-pulse energies of several 100 µJ or higher that can be achieved only by rather complex and expensive laser amplifiers17,19. Spintronic emitters have shown great potential as a gapless broadband emitter reaching a bandwidth up to 30 THz that is compatible with nJ laser pulses from conventional femtosecond oscillators18. Recently, their scalability for the generation of higher THz fields was also demonstrated20. A similar study by Wu et al.21 demonstrated efficient operation of such a THz emitter driven by a pump power as low as 0.15 mW. Nevertheless, THz generation using photoconductive antennas remains important for many applications due to the direct control of the THz field strength and polarity by an applied bias voltage. Moreover, specially designed electrode geometries enable the generation of radial or azimuthal THz polarizations22 and a fully controllable angle of the linear polarization23,24. However, until recently, the bandwidth coverage of photoconductive emitters has been limited by the abovementioned factors. A breakthrough in the generation of a broadband THz spectrum beyond the reststrahlen band of III-V semiconductors was achieved recently by using a Ge-based photoconductive dipole antenna based on pure Ge25. This semiconductor has a direct interband absorption above 0.8 eV, which is very close to its indirect bandgap at 0.66 eV. The effective electron mass in the center of the Brillouin zone of Ge is fairly small, leading to a strong acceleration of photogenerated electrons and, correspondingly, to efficient THz emission. This property gives Ge a clear advantage over Si in applications for a photoconductive THz devices. Moreover, the relatively small bandgap of Ge enables pumping with compact fiber lasers. Finally, Ge is known to be compatible with Si CMOS technology26; thus, it is attractive for integrated on-chip THz solutions for THz signal processing27,28. The absence of polar phonons in Ge enabled the generation of a gapless THz spectrum spreading up to 13 THz, and it has been demonstrated that the bandwidth of a Ge-based THz emitter is limited only by the (...truncated)