Abstract-Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons

D. B. But, M. Mittendorff, C. Consejo, F. Teppe, N. N. Mikhailov, S. A. Dvoretskii, C. Faugeras, S. Winnerl, M. Helm, W. Knap, M. Potemski & M. Orlita

https://www.nature.com/articles/s41566-019-0496-1

The Landau level laser was proposed long ago as a unique source of monochromatic radiation that would be widely tunable in the THz and infrared spectral ranges using a magnetic field. However, despite many efforts, this appealing concept never progressed to the design of a reliable device. This is because of the efficient Auger scattering of Landau-quantized electrons, an intrinsic non-radiative recombination channel that eventually gains over cyclotron emission in all materials studied so far (conventional semiconductors with parabolic bands, but also in graphene with massless electrons). Auger processes are favoured in these systems because the Landau levels (or their subsets) are equally spaced in energy. Here, we show that this scheme does not apply to massless Kane electrons in gapless HgCdTe, where undesirable Auger scattering is strongly suppressed and sizeable cyclotron emission is observed. The gapless HgCdTe thus appears as a material of choice for future Landau level lasers.

Abstract-Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

H. Plank, J. Pernul, S. Gebert, S. N. Danilov, J. König-Otto, S. Winnerl, M. Lanius, J. Kampmeier, G. Mussler, I. Aguilera, D. Grützmacher, and S. D. Ganichev

https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.2.024202

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three-dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow us to determine the room temperature carrier mobilities in the surface states despite the presence of thermally activated residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30 and 60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.

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Abstract-Asymmetric strip line electrodes for photoconductive terahertz emission

Singh, A.; Winnerl, S.; König-Otto, J. C.; Stephan, D. R.; Helm, M.; Schneider, H.

http://www.hzdr.de/db/!Publications?pNid=head&pSelMenu=0&pSelTitle=26794

Here we studied various combinations (two symmetric and two asymmetric designs) of stripline widths for stripline photoconductive emitters fabricated on semi-insulating GaAs. We found out that the THz emission efficiency depends strongly on the anode width. The wider the anode, the higher is the THz amplitude. Cathode width does not play a significant role in THz emission performance. 

Abstract-Doubly dressed bosons: Exciton-polaritons in a strong terahertz field

B. Piętka, N. Bobrovska, D. Stephan, M. Teich, M. Król, S. Winnerl, A. Pashkin, R. Mirek, K. Lekenta, F. Morier-Genoud, H. Schneider, B. Deveaud, M. Helm, M. Matuszewski, and J. Szczytko

https://journals.aps.org/prl/accepted/c907bY09Q031096c935669a975ce3e5d3c82b8c69

We demonstrate the existence of a novel quasiparticle: an exciton in a semiconductor doubly dressed with two photons of different wavelengths: near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultra-strong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

Abstract-Carrier Dynamics in Graphene: Ultrafast Many-Particle Phenomena

http://onlinelibrary.wiley.com/doi/10.1002/andp.201700038/abstract

Graphene is an ideal material to study fundamental Coulomb- and phonon-induced carrier scattering processes. Its remarkable gapless and linear band structure opens up new carrier relaxation channels. In particular, Auger scattering bridging the valence and the conduction band changes the number of charge carriers and gives rise to a significant carrier multiplication - an ultrafast many-particle phenomenon that is promising for the design of highly efficient photodetectors. Furthermore, the vanishing density of states at the Dirac point combined with ultrafast phonon-induced intraband scattering results in an accumulation of carriers and a population inversion suggesting the design of graphene-based terahertz lasers. Here, we review our work on the ultrafast carrier dynamics in graphene and Landau-quantized graphene is presented providing a microscopic view on the appearance of carrier multiplication and population inversion.

Abstract-Doubly dressed bosons - exciton-polaritons in a strong terahertz field

B. Piętka, N. Bobrovska, D. Stephan, M. Teich, M. Król, S. Winnerl, A. Pashkin, R. Mirek, K. Lekenta, F. Morier-Genoud, H. Schneider, B. Deveaud, M. Helm, M. Matuszewski, J. Szczytko

(Submitted on 21 Apr 2017)

https://arxiv.org/abs/1704.06547

We demonstrate the existence of a novel quasiparticle: an exciton in a semiconductor doubly dressed with two photons of different wavelengths: near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultra-strong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

Abstract-Lifetime-limited, subnanosecond terahertz germanium photoconductive detectors

N. Deßmann¹, S. G. Pavlov², A. Pohl¹, N. V. Abrosimov³, S. Winnerl⁴, M. Mittendorff^4,5, R. Kh. Zhukavin⁶, V. V. Tsyplenkov⁶, D. V. Shengurov⁶, V. N. Shastin^6,7 and H.-W. Hübers^1,2
http://scitation.aip.org/content/aip/journal/apl/106/17/10.1063/1.4918712
1 Department of Physics, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
2 Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
3 Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany
4 Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany
5 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20740, USA
6 Institute for Physics of Microstructures, Russian Academy of Sciences, GSP-105, Nizhny Novgorod, 603950, Russia
7 Nizhny Novgorod State University, 23 Prospekt Gargarina, Nizhny Novgorod 603950, Russia
Appl. Phys. Lett. 106, 171109 (2015); http://dx.doi.org/10.1063/1.4918712

The recombination times of photo-excited free charge carriers in heavily doped and highly compensated germanium are studied by a time-resolved pump-probe experiment at a frequency of ∼3 THz. The dominant dopant in the germanium samples is either antimony (n-Ge:Ga:Sb) or gallium (p-Ge:Sb:Ga) with compensating doping levels close to 100%. The recombination time of the free charge carriers measured by our pump-probe technique varies between 30 and 300 ps. It decreases with increasing pump pulse energy and increasing compensation due to high concentrations of Coulomb recombination centers. The recombination times at low pump powers are up to ten times shorter than those previously obtained for low-compensated n-Ge:Sb and p-Ge:Ga. The photoconductive detector made from this material shows the response time is in the order of its recombination time.

Abstract-Transient increase of the energy gap of superconducting NbN thin-films excited by resonant narrow-band terahertz pulses

M. Beck, I. Rousseau, M. Klammer, P. Leiderer, M. Mittendorff, S. Winnerl, M. Helm, G. N. Gol’tsman, and J. Demsar

http://prl.aps.org/accepted/75075Y06Jb413d3903430035b96813be3c98ad3c4

Observations of radiation-enhanced superconductivity have thus far been limited to a few type-I superconductors (Al, Sn) excited at frequencies between the inelastic scattering rate and the superconducting gap frequency $2\Delta/h$. Utilizing intense, narrowband picosecond THz pulses, tuned to just below and above $2\Delta/h$ of a BCS superconductor NbN, we demonstrate that superconducting gap can be transiently increased also in a type-II dirty-limit superconductor. The effect is particularly pronounced at higher temperatures and is attributed to radiation induced non-thermal electron distribution persisting on a 100 ps timescale.