Abstract-Optical Kerr nonlinearity and multi-photon absorption of DSTMS measured by Z-scan method

 

Jiang Li, Rakesh Rana, Liguo Zhu, Cangli Liu, Harald Schneider, Alexej Pashkin

 https://arxiv.org/abs/2102.03242

We investigate the optical Kerr nonlinearity and multi-photon absorption (MPA) properties of DSTMS excited by femtosecond pulses at a wavelengths of 1.43 {\mu}m, which is optimal for terahertz generation via difference frequency mixing. The MPA and the optical Kerr coefficients of DSTMS at 1.43 {\mu}m are strongly anisotropic indicating a dominating contribution from cascaded 2nd-order nonlinearity. These results suggest that the saturation of the THz generation efficiency is mainly related to the MPA process and to a spectral broadening caused by cascaded 2nd-order frequency mixing within DSTMS

Abstract-Nonlinear Charge Transport in InGaAs Nanowires at Terahertz Frequencies

Rakesh Rana, Leila Balaghi, Ivan Fotev, Harald Schneider, Manfred Helm, Emmanouil Dimakis, Alexej Pashkin

https://arxiv.org/abs/2004.00484

We probe the electron transport properties in the shell of GaAs/In0.2Ga0.8As core/shell nanowires at high electric fields using optical pump / THz probe spectroscopy with broadband THz pulses and peak electric fields up to 0.6 MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a systematic redshift and a suppression of its spectral weight for THz driving fields exceeding 0.4 MV/cm. This behavior is attributed to the intervalley electron scattering resulting in the increase of the average electron effective mass and the corresponding decrease of the electron mobility by about 2 times at the highest fields. We demonstrate that the increase of the effective mass is non-uniform along the nanowires and takes place mainly in their middle part, leading to a spatially inhomogeneous carrier response. Our results quantify the nonlinear transport regime in GaAs-based nanowires and show their high potential for development of nano-devices operating at THz frequencies.

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

• Abhishek Singh, 
• Alexej Pashkin,  
• Stephan Winnerl, 
• Malte Welsch, 
• Cornelius Beckh, 
• Philipp Sulzer, 
• Alfred Leitenstorfer, 
• Manfred Helm,  
• Harald Schneider, 

Ultrabroadband THz emission from a Ge:Au antenna pumped at 1100 nm.

https://www.nature.com/articles/s41377-020-0265-4

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.

Abstract-Gapless broadband terahertz emission from a germanium photoconductive emitter

Abhishek Singh, Alexej Pashkin, Stephan Winnerl, Manfred Helm,  Harald Schneider

https://pubs.acs.org/doi/10.1021/acsphotonics.8b00460

Photoconductive terahertz (THz) emitters have been fulfilling many demands required for table-top THz time-domain spectroscopy up to 3-4 THz. In contrast to the widely used photoconductive materials such as GaAs and InGaAs, Ge is a non-polar semiconductor characterized by a gapless transmission in the THz region due to absence of one-phonon absorption. We present here the realization of a Ge-based photoconductive THz emitter with a smooth broadband spectrum extending up to 13 THz and compare its performance with a GaAs-based analogue. We show that the spectral bandwidth of the Ge emitter is limited mainly by the laser pulse width (65 fs) and, thus, can be potentially extended to even much higher THz frequencies.

Abstract-Terahertz dephasing of Landau level transitions in graphene

Harald Schneider, Jacob C. König-Otto,  Alexej Pashkin, Yongrui Wang, Alexey Belyanin, Manfred Helm,  Stephan Winnerl

http://ieeexplore.ieee.org/document/8066873/

Using degenerate four-wave mixing (DFWM), we have investigated the coherent polarization between the lowest Landau levels in graphene under resonant excitation with narrowband THz pulses. A pronounced DFWM signal is observed and its dependence on THz field strength and magnetic field detuning is explored and compared with theoretical expectations.

Abstract-Electric and magnetic terahertz nonlinearities resolved on the sub-cycle scale

Alexej Pashkin¹, Alexander Sell¹, Tobias Kampfrath² and Rupert Huber^3,4

^{http://iopscience.iop.org/1367-2630/15/6/065003}

¹ Department of Physics and Center for Applied Photonics, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
² Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
³ Department of Physics, University of Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany
⁴ Author to whom any correspondence should be addressed. 

Table-top sources of intense multi-terahertz (THz) pulses have opened the door to studies of extreme nonlinearities in the previously elusive mid- to far-infrared spectral regime. We discuss two concepts of fully coherent coupling of phase-locked THz pulses with condensed matter. The first approach demonstrates two-dimensional multi-THz spectroscopy of the semiconductor material InSb. By phase- and amplitude-sensitive detection of the nonlinear optical response, we are able to separate incoherent pump–probe signals from coherent four-wave mixing and reveal extremely non-perturbative nonlinearities. While this class of interactions is mediated by the electric field component of the THz pulse, the second approach is complementary, as it demonstrates that, alternatively, the magnetic THz field may be exploited to selectively control the spin degree of freedom in antiferromagnetic NiO.