Control of the Bright-Dark Exciton Splitting Using the Lamb Shift in a Two-Dimensional Semiconductor.

We investigate the exciton fine structure in atomically thin WSe_{2}-based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting Δ between the bright and dark exciton is measured by photoluminescence spectroscopy. We demonstrate that Δ can be tuned by a few meV as a result of a significant Lamb shift of the optically active exciton that arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measure strong variations of the bright exciton radiative linewidth as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field fluctuations. We find a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control we demonstrate here in the weak light-matter coupling regime should apply to any semiconductor structures.

Control of the Exciton Radiative Lifetime in van der Waals Heterostructures.

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as MoSe_{2} in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Observation of High Angular Momentum Excitons in Cuprous Oxide.

The recent observation of dipole-allowed P excitons up to principal quantum numbers of n=25 in cuprous oxide has given insight into exciton states with unprecedented spectral resolution. While so far the exciton description as a hydrogenlike complex has been fully adequate for cubic crystals, we demonstrate here distinct deviations: The breaking of rotational symmetry leads to mixing of high angular momentum F and H excitons with the P excitons so that they can be observed in absorption. The F excitons show a threefold splitting that depends systematically on n, in agreement with theoretical considerations. From detailed comparison of experiment and theory we determine the cubic anisotropy parameter of the Cu(2)O valence band.

Coherence expansion and polariton condensate formation in a semiconductor microcavity.

The dynamics of the expansion of the first order spatial coherence g(1) for a polariton system in a high-Q GaAs microcavity was investigated on the basis of Young's double slit experiment under 3 ps pulse excitation at the conditions of polariton Bose-Einstein condensation. It was found that in the process of condensate formation the coherence expands with a constant velocity of about 10(8) cm/s. The measured coherence is smaller than that in a thermal equilibrium system during the growth of condensate density and well exceeds it at the end of condensate decay. The onset of spatial coherence is governed by polariton relaxation while condensate amplitude and phase fluctuations are not suppressed.

The Landé factors of electrons and holes in lead halide perovskites: universal dependence on the band gap.

The Landé or g-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI3, MAPb(Br0.5Cl0.5)3, MAPb(Br0.05Cl0.95)3, FAPbBr3, FA0.9Cs0.1PbI2.8Br0.2, MA=methylammonium and FA=formamidinium) and all-inorganic (CsPbBr3) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10 T at cryogenic temperatures. Further, we use first-principles density functional theory (DFT) calculations in combination with tight-binding and k ⋅ p approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.

InSe as a case between 3D and 2D layered crystals for excitons.

InSe is a promising material in many aspects where the role of excitons is decisive. Here we report the sequential appearance in its luminescence of the exciton, the biexciton, and the P-band of the exciton-exciton scattering while the excitation power increases. The strict energy and momentum conservation rules of the P-band are used to reexamine the exciton binding energy. The new value ≥20 meV is markedly higher than the currently accepted one (14 meV), being however well consistent with the robustness of the excitons up to room temperature. A peak controlled by the Sommerfeld factor is found near the bandgap (~1.36 eV). Our findings supported by theoretical calculations taking into account the anisotropic material parameters question the pure three-dimensional character of the exciton in InSe, assumed up to now. The refined character and parameters of the exciton are of paramount importance for the successful application of InSe in nanophotonics.

[Expression of drgA gene encoding NAD(P)H:quinone-oxidoreductase in cells of the cyanobacterium Synechocystis sp. PCC 6803].

The gene drgA of the cyanobacterium Synechocystis sp. PCC 6803 encoding soluble NAD(P)H:quinone-oxidoreductase is involved in NADPH oxidation and controls cell sensitivity to nitroaromatic inhibitors as well as resistance to the oxidative stress inducer menadione. The expression of drgA was analyzed by means of Northern blot hybridization and RT-PCR technique. Two transcripts, which gave a positive hybridization signal with a drgA probe were observed in photoautotrophycally grown cells. One of them (0.6 kb) corresponds in size to mRNA read from the drgA gene; another transcript (1.3 kb), to mRNA transcribed from two genes: drgA and slr1718 located upstream of drgA and having homology with genes of the family comB. The expression of genes drgA and slr1718 was repressed during cell incubation in the dark, but the addition of glucose led to a drastically enhanced expression both in the dark and after illumination of cells. Menadione or nitrophenolic herbicide dinoseb did not induce drgA or slr1718 expression. The results obtained suggest that the expression of these genes in the cytoplasm of cyanobacterium cells is regulated by the NADPH content.

Role of NAD(P)H:quinone oxidoreductase encoded by drgA gene in reduction of exogenous quinones in cyanobacterium Synechocystis sp. PCC 6803 cells.

Insertion mutant Ins2 of the cyanobacterium Synechocystis sp. PCC 6803, lacking NAD(P)H:quinone oxidoreductase (NQR) encoded by drgA gene, was characterized by higher sensitivity to quinone-type inhibitors (menadione and plumbagin) than wild type (WT) cells. In photoautotrophically grown cyanobacterial cells more than 60% of NADPH:quinone-reductase activity, as well as all NADPH:dinoseb-reductase activity, was associated with the function of NQR. NQR activity was observed only in soluble fraction of cyanobacterial cells, but not in membrane fraction. The effects of menadione and menadiol on the reduction of Photosystem I reaction center (P700(+)) after its photooxidation in the presence of DCMU were studied using the EPR spectroscopy. The addition of menadione increased the rate of P700(+) reduction in WT cells, whereas in Ins2 mutant the reduction of P700(+) was strongly inhibited. In the presence of menadiol the reduction of P700(+) was accelerated both in WT and Ins2 mutant cells. These data suggest that NQR protects the cyanobacterial cells from the toxic effect of exogenous quinones by their reduction to hydroquinones. These data may also indicate the probable functional homology of Synechocystis sp. PCC 6803 NQR with mammalian and plant NAD(P)H:quinone oxidoreductases (DT-diaphorases).