Optical orientation of electron spins and valence band spectroscopy in germanium

We have investigated optical orientation in the vicinity of the direct gap of bulk germanium. The electron spin polarization is studied via polarization-resolved photoluminescence excitation spectroscopy unfolding the interplay between doping and ultrafast electron transfer from the center of the Brillouin zone towards its edge. As a result, the direct-gap photoluminescence circular polarisation can vary from 30% to -60% when the excitation laser energy increases. This study provides also simultaneous access to the resonant electronic Raman scattering due to inter-valence band excitations of spin-polarized holes, yielding a fast and versatile spectroscopic approach for the determination of the energy spectrum of holes in semiconducting materials

A model structure for the Goldman-Millson theorem

By a result of Vallette, we put a sensible model structure on the category of conilpotent Lie coalgebras. This gives us a powerful tool to study the subcategory of Lie algebras obtained by linear dualization, also known as the category of pronilpotent Lie algebras. This way, we recover weaker versions of the celebrated Goldman-Millson theorem and Dolgushev-Rogers theorem by purely homotopical methods. We explore the relations of this procedure with the existent literature, namely the works of Lazarev-Markl and Buijs-F\'elix-Murillo-Tanr\'e.Comment: 20 pages. (v2) fixed formatting of abstract on arXiv, the core text was not touche

Learning-Based Dequantization For Image Restoration Against Extremely Poor Illumination

All existing image enhancement methods, such as HDR tone mapping, cannot recover A/D quantization losses due to insufficient or excessive lighting, (underflow and overflow problems). The loss of image details due to A/D quantization is complete and it cannot be recovered by traditional image processing methods, but the modern data-driven machine learning approach offers a much needed cure to the problem. In this work we propose a novel approach to restore and enhance images acquired in low and uneven lighting. First, the ill illumination is algorithmically compensated by emulating the effects of artificial supplementary lighting. Then a DCNN trained using only synthetic data recovers the missing detail caused by quantization

Calibration of Local Volatility Model with Stochastic Interest Rates by Efficient Numerical PDE Method

Long maturity options or a wide class of hybrid products are evaluated using a local volatility type modelling for the asset price S(t) with a stochastic interest rate r(t). The calibration of the local volatility function is usually time-consuming because of the multi-dimensional nature of the problem. In this paper, we develop a calibration technique based on a partial differential equation (PDE) approach which allows an efficient implementation. The essential idea is based on solving the derived forward equation satisfied by P(t; S; r)Z(t; S; r), where P(t; S; r) represents the risk neutral probability density of (S(t); r(t)) and Z(t; S; r) the projection of the stochastic discounting factor in the state variables (S(t); r(t)). The solution provides effective and sufficient information for the calibration and pricing. The PDE solver is constructed by using ADI (Alternative Direction Implicit) method based on an extension of the Peaceman-Rachford scheme. Furthermore, an efficient algorithm to compute all the corrective terms in the local volatility function due to the stochastic interest rates is proposed by using the PDE solutions and grid points. Different numerical experiments are examined and compared to demonstrate the results of our theoretical analysis

Interlayer exciton mediated second harmonic generation in bilayer MoS2

Second harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also inter-layer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations.Comment: main paper and supplemen

Theoretical and experimental study of (In,Ga)As/GaP quantum dots

International audience(In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k*p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types

Continuous-wave magneto-optical determination of the carrier lifetime in coherent Ge1−xSnx/Ge heterostructures

We present a magneto-optical study of the carrier dynamics in compressively strained Ge1−xSnx films with Sn content up to 10% epitaxially grown on Ge on Si(001) virtual substrates. We leverage the Hanle effect under steady-state excitation to study the spin-dependent optical transitions in the presence of an external magnetic field. This allows us to obtain direct access to the dynamics of the optically induced carrier population. Our approach reveals that at cryogenic temperatures the effective lifetime of the photogenerated carriers in coherent Ge1−xSnx is on the subnanosecond timescale. Supported by a model estimate of the radiative lifetime, our measurements indicate that carrier recombination is dominated by nonradiative processes. Our results thus provide central information to improve the fundamental understanding of carrier kinetics in this advanced direct-band-gap group-IV-material system. Such knowledge can be a stepping stone in the quest for the implementation of Ge1−xSnx-based functional devices

Structural and optical analyses of GaP/Si and (GaAsPN/GaPN)/GaP/Si nanolayers for integrated photonics on silicon

International audienceWe report a structural study of molecular beam epitaxy-grown lattice-matched GaP/Si(0 0 1) thin layers with an emphasis on the interfacial structural properties, and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown onto the GaP/Si pseudo substrates, through a complementary set of characterization tools. Room temperature photoluminescence at 780 nm from the (GaAsPN/GaPN) quantum wells grown onto a silicon substrate is reported. Despite this good property, the time-resolved photoluminescence measurements demonstrate a clear influence of non-radiative defects initiated at the GaP/Si interface. It is shown from simulations, how x-ray diffraction can be used efficiently for analysis of antiphase domains. Then, qualitative and quantitative analyses of antiphase domains, micro-twins, and stacking faults are reported using complementarity of the local transmission electron microscopy and the statistical x-ray diffraction approaches

Structural and optical properties of (In,Ga)As/GaP quantum dots and (GaAsPN/GaPN) diluted-nitride nanolayers coherently grown onto GaP and Si substrates for photonics and photovoltaics applications

San Francisco, California, United StatesInternational audienceLattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as a competition between conduction band states in X and Γ valleys. Structural properties and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are reporte