Modeling of second harmonic generation in hole-doped silicon-germanium quantum wells for mid-infrared sensing

The development of Ge and SiGe chemical vapor deposition techniques on silicon wafers has enabled the integration of multi-quantum well structures in silicon photonics chips for nonlinear optics with potential applications to integrated nonlinear optics, however research has focused up to now on undoped quantum wells and interband optical excitations. In this work, we present model calculations for the giant nonlinear coefficients provided by intersubband transitions in hole-doped Ge/SiGe and Si/SiGe multi-quantum wells. We employ a valence band-structure model for Si1-xGex to calculate the confined hole states of asymmetric-coupled quantum wells for second-harmonic generation in the mid-infrared. We calculate the nonlinear emission spectra from the second-order susceptibility tensor, including the particular vertical emission spectra of valence-band quantum wells. Two possible nonlinear mid-infrared sensor architectures, one based on waveguides and another based on metasurfaces, are described as perspective application

Long intersubband relaxation times in n-type germanium quantum wells

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Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells

n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films

Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.Comment: 18 pages, 10 figure

Keyword based Search over Semantic Data in Polynomial Time

Abstract-In pursuing the development of Yanii, a novel keyword based search system on graph structures, in this paper we present the computational complexity study of the approach, highlighting a comparative study with actual PTIME state-ofthe-art solutions. The comparative study focuses on a theoretical analysis of different frameworks to define complexity ranges, which they correspond to, in the polynomial time class. We characterize such systems in terms of general measures, which give a general description of the behavior of these frameworks according to different aspects that are more general and informative than mere benchmark tests on a few test cases. We show that Yanii holds better performance than others, confirming itself as a promising approach deserving further practical investigation and improvement

Prognostic role of intragastric cytopathology and microbiota in surgical patients with stomach cancer

In the last decade, analysis of malignant cells and flora in gastric lavage (GL) has provided interesting data on pathogenesis of gastric cancer (GC). For this study, combining such two aspects into one cyto-microbiologic category, we tested the prognostic role of the presence/absence of cancer cells (GL1/GL0) and bacterial microbiota (MB1/MB0) in our GC population. Material and Methods: Between April 2012 and August 2019, 79 surgical patients with GC were prospectively investigated with the determination of GL MB. Results: Compared with GL1 MB0, GL1 MB1 strongly correlated with advanced GC, portended poorer overall survival (OS) (45.8 months vs 20.5 months, P = 0.049), and resulted a significant (P = 0.008) and an independent (P = 0.013) prognostic factor unfavorable for OS. Conclusion: In the light of our results, the cyto-microbiologic parameter of GL MB should be used to gain a better prognosis of GC patients. Administration of antimicrobial treatment for MB1 subjects should be entertained because it could reduce the risk of oncogenesis

Terahertz absorption-saturation and emission from electron-doped germanium quantum wells

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters

Terahertz absorption-saturation and emission from electron-doped germanium quantum wells

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters

Room temperature operation of n-type Ge/SiGe terahertz quantum cascade lasers predicted by non-equilibrium Green's functions

n-type Ge/SiGe terahertz quantum cascade lasers are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs quantum cascade laser design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III–V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement