Karin Herrmann

Bio: Karin Herrmann is an academic researcher from Humboldt University of Berlin. The author has contributed to research in topics: Photoluminescence & Absorption edge. The author has an hindex of 11, co-authored 46 publications receiving 344 citations.

Infrared Photoluminescence in Narrow‐Gap Semiconductors

Abstract: On s'interesse particulierement a la photoluminescence infra-rouge dans des composes II-VI a bande interdite etroite, et plus specialement au Hg 1-x Cd x Te. On etudie egalement la luminescence dans les sels de plomb et a l'influence du dopage, de l'alliage et de la perfection structurale

Recombination in Pb0.83Sn0.17Te at high levels of optical excitation

Abstract: The photoconductivity and the photomagnetoelectric current in low-concentration, high-lifetime Pb0.83Sn0.17Te samples are measured at strong optical excitation using a Q-switched ruby laser. At quantum flux Q > 2 × 1021 cm−2s−1 nonlinear recombination is observed. From the characteristics measured the coefficient of Auger recombination C = (4.5 ± 1) × 10−27 cm6s−1 is obtained. The results are confirmed by lifetime data obtained at low excitation levels. Die Photoleitfahigkeit und der PEM-Kurzschlusstrom bei Anregung mit einem gutegeschalteten Rubinlaser werden an Pb0,83Sn0,17Te-Proben geringer Ladungstragerkonzentration und hoher Lebensdauer gemessen. Bei Quantenflusdichten Q > 2 × 1021 cm−2s−1 wird nichtlineare Re-kombination beobachtet. Aus den Mesergebnissen wird der Koeffizient der Augerrekombination C = (4,5 ± 1) × 10−27 cm6s−1 bestimmt. Die Ergebnisse werden gestutzt durch Lebensdauer-messungen bei schwacher Anregung.

Recent advances in lead-chalcogenide diode lasers

Abstract: The fundamental material properties of Pb1−xSnxTe, PbS1−xSex and Pb1−xSnxSe are reviewed. Expressions for the temperature and compositional dependences of the band parameters and dielectric constants based on recently published data are presented. As far as device technology is concerned, crystal growth techniques and diode fabrication procedures which are in use today are reviewed and compared. A comprehensive summary of laser properties like threshold current density, output power, efficiency, maximum operating temperature and tuning range of different diode structures are presented. Application related aspects such as long term stability are treated. Recent progress in laser theory is applied to explain experimentali th vs.T curves. The various laser applications are reviewed briefly. A new technique for monitoring gas concentrations using pulsed lasers and an integral method for signal processing is discussed and compared with the differential absorption, derivative spectroscopy. A long-path trace-gas monitoring system incorporating this new technique is presented.

Mixed-valence impurities in lead telluride-based solid solutions

Abstract: Experimental data on impurity states in narrow-gap lead telluride based semiconductors are summarized. Theoretical models describing the nontrivial properties of such states are presented. Applications to the design of highly sensitive far-infrared detectors are considered.

Tunable excitons in bilayer graphene

Abstract: Excitons, the bound states of an electron and a hole in a solid material, play a key role in the optical properties of insulators and semiconductors. Here, we report the observation of excitons in bilayer graphene (BLG) using photocurrent spectroscopy of high-quality BLG encapsulated in hexagonal boron nitride. We observed two prominent excitonic resonances with narrow line widths that are tunable from the mid-infrared to the terahertz range. These excitons obey optical selection rules distinct from those in conventional semiconductors and feature an electron pseudospin winding number of 2. An external magnetic field induces a large splitting of the valley excitons, corresponding to a g-factor of about 20. These findings open up opportunities to explore exciton physics with pseudospin texture in electrically tunable graphene systems​.