Abstract-Non-plasmonic improvement in photoconductive THz emitters using nano- and micro-structured electrodes

 

Abhishek Singh, Malte Welsch, Stephan Winnerl, Manfred Helm, Harald Schneider, 

Simulation of electric field amplitude ((Ex2+Ey2+Ez2)1/2 in V/m) distribution on a slice passing through the center of the emitter in yz-plane in the photoconductor when 10 V bias is applied to the electrodes. Field lines are drawn to show the direction of the electric field. A white dashed line is drawn at a depth (∼ 1 µm) of penetration depth of 800 nm in GaAs.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-24-35490&id=442489

We investigate here terahertz enhancement effects arising from micrometer and nanometer structured electrode features of photoconductive terahertz emitters. Nanostructured electrode based emitters utilizing the palsmonic effect are currently one of the hottest topics in the research field. We demonstrate here that even in the absence of any plasmonic resonance with the pump pulse, such structures can improve the antenna effect by enhancing the local d.c. electric field near the structure edges. Utilizing this effect in Hilbert-fractal and grating-like designs, enhancement of the THz field of up to a factor of ∼ 2 is observed. We conclude that the cause of this THz emission enhancement in our emitters is different from the earlier reported plasmonic-electrode effect in a similar grating-like structure. In our structure, the proximity of photoexcited carriers to the electrodes and local bias field enhancement close to the metallization cause the enhanced efficiency. Due to the nature of this effect, the THz emission efficiency is almost independent of the pump laser polarization. Compared to the plasmonic effect, these effects work under relaxed device fabrication and operating conditions.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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-Plasmonic efficiency enhancement at the anode of strip line photoconductive terahertz emitters

Abhishek Singh, Stephan Winnerl, Jacob C. König-Otto, Daniel R. Stephan, Manfred Helm, and Harald Schneider

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=964173647966952561

We investigate strip line photoconductive terahertz (THz) emitters in a regime where both the direct emission of accelerated carriers in the semiconductor and the antenna-mediated emission from the strip line play a significant role. In particular, asymmetric strip line structures are studied. The widths of the two electrodes have been varied from 2 µm to 50 µm. The THz emission efficiency is observed to increase linearly with the width of the anode, which acts here as a plasmonic antenna giving rise to enhanced THz emission. In contrast, the cathode width does not play any significant role on THz emission efficiency.

© 2016 Optical Society of America

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Abstract-Electron dynamics in silicon-germanium terahertz quantum fountain structures

Diego Sabbagh, Johannes Schmidt, Stephan Winnerl, Manfred Helm, Luciana Di Gaspare, Monica De Seta,Michele Virgilio, and Michele Ortolani

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.5b00561

Publication Date (Web): February 1, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00561?journalCode=apchd5

Asymmetric quantum well systems are excellent candidates to realize semiconductor light emitters at far-infrared wavelengths not covered by other gain media. Group-IV semiconductor heterostructures can be grown on silicon substrates and their dipole-active intersubband transitions could be used to generate light from devices integrated with silicon electronic circuits. Here, we have realized an optically pumped emitter structure based on a three-level Ge/Si0.18Ge0.82 asymmetric coupled quantum well design. Optical pumping was performed with a tunable free-electron laser emitting at photon energies of 25 and 41 meV, corresponding to the energies of the first two intersubband transitions 0→1 and 0→2 as measured by Fourier-transform spectroscopy. We have studied with a synchronized terahertz time-domain spectroscopy probe the relaxation dynamics after pumping, and we have interpreted the resulting relaxation times (in the range 60 to 110 ps) in the framework of an out-of-equilibrium model of the intersubband electron-phonon dynamics. The spectral changes in the probe pulse transmitted at pump-probe coincidence were monitored in the range 0.7-2.9 THz for different samples and pump intensity and showed indication of both free carrier absorption increase and bleaching of the 1→2 transition. The quantification from data and models of the free carrier losses and of the bleaching efficiency allowed us to predict the conditions for population inversion and to determine a threshold pump power density for lasing around 500 kW/cm2 in our device. The ensemble of our results shows that optical pumping of germanium quantum wells is a promising route towards silicon-integrated far-infrared emitters.

Abstract-Gouy phase shift of a tightly focused, radially polarized beam

Korbinian J. Kaltnenecker, Jacob C. König-Otto, Martin Mittendorff, Stephan Winnerl, Harald Schneider, Manfred Helm, Hanspeter Helm, Markus Walther, and Bernd M. Fischer

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-3-1-35&id=335319

Radially polarized beams represent an important member of the family of vector beams, in particular due to the possibility of using them to create strong and tightly focused longitudinal fields, a fundamental property that has been exploited by applications ranging from microscopy to particle acceleration. Since the properties of such a focused beam are intimately related to the Gouy phase shift, proper knowledge of its behavior is crucial. Terahertz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. Since the applied terahertz time-domain approach is capable of mapping the amplitude and phase of an electromagnetic wave in space, we are able to directly trace the evolution of the geometric phase as the wave propagates through the focus. We observe a Gouy phase shift of 2𝜋 for the transverse and of 𝜋 for the longitudinal component. Our experimental procedure is universal and may be applied to determine the geometric phase of other vector beams, such as optical vortices, or even arbitrarily shaped and polarized propagating waves.

© 2016 Optical Society of America

Abstract-Plasmonic Superlensing in Doped GaAs

Markus Fehrenbacher , Stephan Winnerl , Harald Schneider , Jonathan Döring , Susanne C. Kehr ,Lukas M. Eng , Yongheng Huo , Oliver G. Schmidt ,Kan Yao , Yongmin Liu , and Manfred Helm

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl503996q

Publication Date (Web): January 13, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl503996q?journalCode=nalefd

We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around lambda=20um, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a lambda/6 sub-wavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude-Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy.