Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard.

Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.

Evolution of Temporal Coherence in Confined Exciton-Polariton Condensates.

We study the influence of spatial confinement on the second-order temporal coherence of the emission from a semiconductor microcavity in the strong coupling regime. The confinement, provided by etched micropillars, has a favorable impact on the temporal coherence of solid state quasicondensates that evolve in our device above threshold. By fitting the experimental data with a microscopic quantum theory based on a quantum jump approach, we scrutinize the influence of pump power and confinement and find that phonon-mediated transitions are enhanced in the case of a confined structure, in which the modes split into a discrete set. By increasing the pump power beyond the condensation threshold, temporal coherence significantly improves in devices with increased spatial confinement, as revealed in the transition from thermal to coherent statistics of the emitted light.

Photon-Number-Resolved Measurement of an Exciton-Polariton Condensate.

We measure the full photon-number distribution emitted from a Bose condensate of microcavity exciton polaritons confined in a micropillar cavity. The statistics are acquired by means of a photon-number-resolving transition edge sensor. We directly observe that the photon-number distribution evolves with the nonresonant optical excitation power from geometric to quasi-Poissonian statistics, which is canonical for a transition from a thermal to a coherent state. Moreover, the photon-number distribution allows one to evaluate the higher-order photon correlations, shedding further light on the coherence formation and phase transition of the polariton condensate. The experimental data are analyzed in terms of thermal-coherent states, which gives direct access to the thermal and coherent fraction from the measured distributions. These results pave the way for a full understanding of the contribution of interactions in light-matter condensates in the coherence buildup at threshold.

Room temperature strong coupling in a semiconductor microcavity with embedded AlGaAs quantum wells designed for polariton lasing.

We report a systematic study of the temperature and excitation density behavior of an AlAs/AlGaAs, vertically emitting microcavity with embedded ternary Al0.20Ga0.80As/AlAs quantum wells in the strong coupling regime. Temperature-dependent photoluminescence measurements of the bare quantum wells indicate a crossover from the type-II indirect to the type-I direct transition. The resulting mixing of quantum well and barrier ground states in the conduction band leads to an estimated exciton binding energy systematically exceeding 25 meV. The formation of exciton-polaritons is evidenced in our quantum well microcavity via reflection measurements with Rabi splittings ranging from (13.93 ± 0.15) meV at low temperature (30 K) to (8.58 ± 0.40) meV at room temperature (300 K). Furthermore, the feasibility of polariton laser operation is demonstrated under non-resonant optical excitation conditions at 20 K and emission around 1.835 eV.

Experimental Verification of the Very Strong Coupling Regime in a GaAs Quantum Well Microcavity.

The dipole coupling strength g between cavity photons and quantum well excitons determines the regime of light matter coupling in quantum well microcavities. In the strong coupling regime, a reversible energy transfer between exciton and cavity photon takes place, which leads to the formation of hybrid polaritonic resonances. If the coupling is further increased, a hybridization of different single exciton states emerges, which is referred to as the very strong coupling regime. In semiconductor quantum wells such a regime is predicted to manifest as a photon-mediated electron-hole coupling leading to different excitonic wave functions for the two polaritonic branches when the ratio of the coupling strength to exciton binding energy g/E_{B} approaches unity. Here, we verify experimentally the existence of this regime in magneto-optical measurements on a microcavity characterized by g/E_{B}≈0.64, showing that the average electron-hole separation of the upper polariton is significantly increased compared to the bare quantum well exciton Bohr radius. This yields a diamagnetic shift around 0 detuning that exceeds the shift of the lower polariton by 1 order of magnitude and the bare quantum well exciton diamagnetic shift by a factor of 2. The lower polariton exhibits a diamagnetic shift smaller than expected from the coupling of a rigid exciton to the cavity mode, which suggests more tightly bound electron-hole pairs than in the bare quantum well.

Observation of the Transition from Lasing Driven by a Bosonic to a Fermionic Reservoir in a GaAs Quantum Well Microcavity.

We show that, by monitoring the free carrier reservoir in a GaAs-based quantum well microcavity under nonresonant pulsed optical pumping, lasing supported by a fermionic reservoir (photon lasing) can be distinguished from lasing supported by a reservoir of bosons (polariton lasing). Carrier densities are probed by measuring the photocurrent between lateral contacts deposited directly on the quantum wells of a microcavity that are partially exposed by wet chemical etching. We identify two clear thresholds in the input-output characteristic of the photoluminescence signal which can be attributed to polariton and photon lasing, respectively. The power dependence of the probed photocurrent shows a distinct kink at the threshold power for photon lasing due to an increased radiative recombination of free carriers as stimulated emission into the cavity mode sets in. At the polariton lasing threshold, on the other hand, the nonlinear increase of the luminescence is caused by stimulated scattering of exciton polaritons to the ground state which do not contribute directly to the photocurrent.

Spatial coherence properties of one dimensional exciton-polariton condensates.

In this work, we combine a systematic experimental investigation of the power- and temperature-dependent evolution of the spatial coherence function, g^{(1)}(r), in a one dimensional exciton-polariton channel with a modern microscopic numerical theory based on a stochastic master equation approach. The spatial coherence function g^{(1)}(r) is extracted via high-precision Michelson interferometry, which allows us to demonstrate that in the regime of nonresonant excitation, the dependence g^{(1)}(r) reaches a saturation value with a plateau, which is determined by the intensity of the pump and effective temperature of the crystal lattice. The theory, which was extended to allow for treating incoherent excitation in a stochastic frame, matches the experimental data with good qualitative and quantitative agreement. This allows us to verify the prediction that the decay of the off-diagonal long-range order can be almost fully suppressed in one dimensional condensate systems.

Anomalies of a nonequilibrium spinor polariton condensate in a magnetic field.

We observe a strong variation of the Zeeman splitting of exciton polaritons in microcavities when switching between the linear regime, the polariton lasing, and photon lasing regimes. In the polariton lasing regime the sign of Zeeman splitting changes compared to the linear regime, while in the photon lasing regime the splitting vanishes. We additionally observe an increase of the diamagnetic shift in the polariton lasing regime. These effects are explained in terms of the nonequilibrium "spin Meissner effect."

Room temperature polariton light emitting diode with integrated tunnel junction.

We present a diode incorporating a large number (12) of GaAs quantum wells that emits light from exciton-polariton states at room temperature. A reversely biased tunnel junction is placed in the cavity region to improve current injection into the device. Electroluminescence studies reveal two polariton branches which are spectrally separated by a Rabi splitting of 6.5 meV. We observe an anticrossing of the two branches when the temperature is lowered below room temperature as well as a Stark shift of both branches in a bias dependent photoluminescence measurement.

Lessons learned from microsatellite development for nonmodel organisms using 454 pyrosequencing.

Microsatellites, also known as simple sequence repeats (SSRs), are among the most commonly used marker types in evolutionary and ecological studies. Next Generation Sequencing techniques such as 454 pyrosequencing allow the rapid development of microsatellite markers in nonmodel organisms. 454 pyrosequencing is a straightforward approach to develop a high number of microsatellite markers. Therefore, developing microsatellites using 454 pyrosequencing has become the method of choice for marker development. Here, we describe a user friendly way of microsatellite development from 454 pyrosequencing data and analyse data sets of 17 nonmodel species (plants, fungi, invertebrates, birds and a mammal) for microsatellite repeats and flanking regions suitable for primer development. We then compare the numbers of successfully lab-tested microsatellite markers for the various species and furthermore describe diverse challenges that might arise in different study species, for example, large genome size or nonpure extraction of genomic DNA. Successful primer identification was feasible for all species. We found that in species for which large repeat numbers are uncommon, such as fungi, polymorphic markers can nevertheless be developed from 454 pyrosequencing reads containing small repeat numbers (five to six repeats). Furthermore, the development of microsatellite markers for species with large genomes was also with Next Generation Sequencing techniques more cost and time-consuming than for species with smaller genomes. In this study, we showed that depending on the species, a different amount of 454 pyrosequencing data might be required for successful identification of a sufficient number of microsatellite markers for ecological genetic studies.

Prototype of a bistable polariton field-effect transistor switch.

Microcavity exciton polaritons are promising candidates to build a new generation of highly nonlinear and integrated optoelectronic devices. Such devices range from novel coherent light emitters to reconfigurable potential landscapes for electro-optical polariton-lattice based quantum simulators as well as building blocks of optical logic architectures. Especially for the latter, the strongly interacting nature of the light-matter hybrid particles has been used to facilitate fast and efficient switching of light by light, something which is very hard to achieve with weakly interacting photons. We demonstrate here that polariton transistor switches can be fully integrated in electro-optical schemes by implementing a one-dimensional polariton channel which is operated by an electrical gate rather than by a control laser beam. The operation of the device, which is the polariton equivalent to a field-effect transistor, relies on combining electro-optical potential landscape engineering with local exciton ionization to control the scattering dynamics underneath the gate. We furthermore demonstrate that our device has a region of negative differential resistance and features a completely new way to create bistable behavior.

Visualising Berry phase and diabolical points in a quantum exciton-polariton billiard.

Diabolical points (spectral degeneracies) can naturally occur in spectra of two-dimensional quantum systems and classical wave resonators due to simple symmetries. Geometric Berry phase is associated with these spectral degeneracies. Here, we demonstrate a diabolical point and the corresponding Berry phase in the spectrum of hybrid light-matter quasiparticles-exciton-polaritons in semiconductor microcavities. It is well known that sufficiently strong optical pumping can drive exciton-polaritons to quantum degeneracy, whereby they form a macroscopically populated quantum coherent state similar to a Bose-Einstein condensate. By pumping a microcavity with a spatially structured light beam, we create a two-dimensional quantum billiard for the exciton-polariton condensate and demonstrate a diabolical point in the spectrum of the billiard eigenstates. The fully reconfigurable geometry of the potential walls controlled by the optical pump enables a striking experimental visualization of the Berry phase associated with the diabolical point. The Berry phase is observed and measured by direct imaging of the macroscopic exciton-polariton probability densities.

Response of mycorrhizal Norway spruce seedlings to various nitrogen loads and sources.

Norway spruce seedlings were grown under greenhouse conditions in Rootrainers with a vermiculite-peat moss mixture under various N-regimes for 6 months. Either ammonium or nitrate was applied in loads of 100 or 800 kg N ha(-1) year(-1) to seedlings which were either non-mycorrhizal or inoculated with the mycorrhizal fungi Hebeloma crustuliniforme or Laccaria bicolor. The use of increasing N loads enhanced shoot and total biomass, whereas root/shoot ratio, number of short roots and mycorrhization decreased. A significant enhancement of the concentration and content was obvious for the element N, whereas a significant decrease was obvious for P and Zn concentrations. The use of ammonium, as opposed to nitrate, significantly enhanced the biomass and the numbers of short roots, and reduced the root/shoot ratios, but did not influence the mycorrhization. It further significantly enhanced the N concentrations in roots and shoots. Fungal inoculation with H. crustuliniforme or L. bicolor compared to non-inoculated controls significantly enhanced shoot and total biomass, but reduced root/shoot ratios. The mycorrhization further significantly enhanced N and P concentrations and contents, but reduced Mn. Overall, the mycorrhization improved the P nutrition of the seedlings independently on the applied N loads or N sources. Dose response curves using ammonium nitrate as N source with a maximum load of 1600 kg N ha(-1) year(-1) applied on seedlings associated with H. crustuliniforme revealed that the maximum growth was reached at a load of 800 kg N ha(-1) year(-1) with a simultaneous decrease of the mycorrhization. In both shoots and roots, N concentrations increased constantly with increasing N loads, while P, Ca, and Zn concentrations decreased constantly.

Autobiographical memory for emotional events in amnesia.

This study investigated autobiographical memory for emotionally flavoured experiences in amnesia. Ten amnesic patients and 10 matched control subjects completed the Autobiographical Memory Interview and three semi-structured interviews which assessed memory for personal events associated with pain, happiness and fear. Despite retrograde amnesia for autobiographical facts and incidents, amnesics remembered a similar number of emotionally significant personal experiences as control subjects. Their recollections generally lacked elaboration and detail, but pain-related memories appeared to be more mildly impaired than memories associated with happiness and fear. The findings are discussed in relation to recent views on the relationship between affect and memory.

Lasing from active optomechanical resonators.

Planar microcavities with distributed Bragg reflectors (DBRs) host, besides confined optical modes, also mechanical resonances due to stop bands in the phonon dispersion relation of the DBRs. These resonances have frequencies in the 10- to 100-GHz range, depending on the resonator's optical wavelength, with quality factors exceeding 1,000. The interaction of photons and phonons in such optomechanical systems can be drastically enhanced, opening a new route towards the manipulation of light. Here we implemented active semiconducting layers into the microcavity to obtain a vertical-cavity surface-emitting laser (VCSEL). Thereby, three resonant excitations--photons, phonons and electrons--can interact strongly with each other providing modulation of the VCSEL laser emission: a picosecond strain pulse injected into the VCSEL excites long-living mechanical resonances therein. As a result, modulation of the lasing intensity at frequencies up to 40 GHz is observed. From these findings, prospective applications of active optomechanical resonators integrated into nanophotonic circuits may emerge.

Fully informative microsatellite markers for the tetraploid plant Eritrichium nanum (Boraginaceae).

We characterized eight polymorphic, codominant nuclear microsatellite loci in the tetraploid plant Eritrichium nanum. The different allelic configurations occurring under tetrasomic inheritance were fully resolved at all loci. Two natural populations showed high observed heterozygosities, which were in agreement with Hardy-Weinberg expectations. There was no evidence of genetic linkage disequilibrium for any pair of loci. The results suggest that these microsatellite markers are useful for mating system and population genetic analyses in high-alpine E. nanum.

Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis.

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.

Molecular identification of fine roots of trees from the Alps: reliable and fast DNA extraction and PCR-RFLP analyses of plastid DNA.

Fine roots of trees are intensively used as indicators to assess soil alterations, e.g. those owing to atmospheric inputs of acidifying substances, but their identification to species with morphological criteria is difficult. In this study, we established molecular techniques in order to identify fine roots of the 30 most common tree species of the Alps. We developed a protocol for efficient isolation of DNA from fine roots with extraction of DNA in the presence of polyvinylpyrrolidone (PVP) and polyvinylpolypyrrolidone (PVPP). The trnL (UAA) intron of plastid DNA was used as a marker for fine root identification. We amplified and sequenced this intron with plant universal primers. The size of the sequences ranged from 444 to 672 bp. A synoptic key for species identification was designed on the basis of restriction fragment patterns predicted from sequence data. Using the restriction enzyme TaqI as key enzyme, and where necessary HinfI, RsaI and CfoI, 16 taxa, including Picea abies, Larix decidua, Abies alba, and Fagus sylvatica, the dominant tree species of the Alpine region could be identified by agarose gel electrophoresis of restriction fragments. Fourteen taxa could be identified to the genus level, among them Quercus, Salix and Populus species. In a field study, conducted in a 20 x 30 m plot of a mixed forest with five tree species, fine roots of 43 out of 46 samples were identified and their distributions were mapped. These results demonstrate the utility of our DNA extraction method and of the trnL intron for the identification of fine tree roots.

The evolutionary split of Pinaceae from other conifers: evidence from an intron loss and a multigene phylogeny.

The second intron in the mitochondrial gene nad1 was surveyed using PCR, DNA sequencing, or Southern hybridization in 323 species (313 genera, 212 families) of seed plants. The intron was absent in all 22 species (22 genera, 8 families) of non-Pinaceae conifers studied, in Welwitschia mirabilis, and in seven angiosperms. Whereas absence of the intron in seven angiosperms and Welwitschia is likely due to seven independent losses when evaluated against the recently published multigene phylogenies, the lack of the intron in all non-Pinaceae conifers can be best explained by a single loss. These data suggest that the non-Pinaceae conifers represent a monophyletic group. We also conducted a phylogenetic analysis of seed plants using a combined data set of the partial exon and intron sequences of nad1 generated from this study and published sequences of mitochondrial cox1 and small subunit (SSU) rDNA, chloroplast rbcL, and nuclear 18S rDNA. The results supported the split of conifers into two groups: Pinaceae and non-Pinaceae conifers. The Gnetales were sister to Pinaceae, in agreement with the conclusion from other recent molecular phylogenetic studies that refute the anthophyte hypothesis.