Annihilation of exceptional points from different Dirac valleys in a 2D photonic system.

Topological physics relies on Hamiltonian's eigenstate singularities carrying topological charges, such as Dirac points, and - in non-Hermitian systems - exceptional points (EPs), lines or surfaces. So far, the reported non-Hermitian topological transitions were related to the creation of a pair of EPs connected by a Fermi arc out of a single Dirac point by increasing non-Hermiticity. Such EPs can annihilate by reducing non-Hermiticity. Here, we demonstrate experimentally that an increase of non-Hermiticity can lead to the annihilation of EPs issued from different Dirac points (valleys). The studied platform is a liquid crystal microcavity with voltage-controlled birefringence and TE-TM photonic spin-orbit-coupling. Non-Hermiticity is provided by polarization-dependent losses. By increasing the non-Hermiticity degree, we control the position of the EPs. After the intervalley annihilation, the system becomes free of any band singularity. Our results open the field of non-Hermitian valley-physics and illustrate connections between Hermitian topology and non-Hermitian phase transitions.

Doubly Dressed Bosons: Exciton Polaritons in a Strong Terahertz Field.

We demonstrate the existence of a novel quasiparticle, an exciton in a semiconductor doubly dressed with two photons of different wavelengths: a near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultrastrong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

The excited spin-triplet state of a charged exciton in quantum dots.

We report on spectroscopic studies of resonances related to ladder of states of a charged exciton in single GaAlAs/AlAs quantum dot structures. Polarization-resolved photoluminescence, photoluminescence excitation and photon-correlation measurements were performed at low (T = 4.2 K) temperature also in magnetic field applied in Faraday configuration. The investigated resonances are assigned to three different configurations of a positively charged exciton. Together with a singlet ground state and a conventional triplet state (involving an electron from the ground state electronic s-shell), an excited triplet state, which involved an electron from the excited electronic p-shell was identified in single dots. The appearance of an emission line related to the latter complex is due to a partially suppressed electron relaxation in the investigated dots. An analysis of this emission line allows us to scrupulously determine properties of the excited triplet state and compare them with those of the conventional triplet state. Both triplets exhibit similar patterns of anisotropic fine structure and Zeeman splitting, however their amplitudes significantly differ for those two states. Presented results emphasize the role of the symmetry of the electronic state on the properties of the triplet states of two holes + electron excitonic complex.

Polariton condensation in a one-dimensional disordered potential.

We study the coherence and density modulation of a nonequilibrium exciton-polariton condensate in a one-dimensional valley with disorder. By means of interferometric measurements we evidence a modulation of the first-order coherence function and we relate it to a disorder-induced modulation of the condensate density, that increases as the pump power is increased. The nonmonotonic spatial coherence function is found to be the result of the strong nonequilibrium character of the one-dimensional system, in the presence of disorder.

Probing the dynamics of spontaneous quantum vortices in polariton superfluids.

The experimental investigation of spontaneously created vortices is of utmost importance for the understanding of quantum phase transitions towards a superfluid phase, especially for two-dimensional systems that are expected to be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time-resolved near-field interferometry we track the path of such vortices, created at random locations in an exciton-polariton condensate under pulsed nonresonant excitation, to their final pinning positions imposed by the stationary disorder. We formulate a theoretical model that successfully reproduces the experimental observations.

Coherent oscillations in an exciton-polariton Josephson junction.

We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. Condensation of exciton polaritons here takes place under incoherent excitation in a double potential well naturally formed in the disorder. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect. A theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.

Dynamics of long-range ordering in an exciton-polariton condensate.

We report on time-resolved measurements of the first order spatial coherence in an exciton-polariton Bose-Einstein condensate. Long-range spatial coherence is found to set in right at the onset of stimulated scattering, on a picosecond time scale. The coherence reaches its maximum value after the population and decays slowly, staying up to a few hundred picoseconds. This behavior can be qualitatively reproduced, using a stochastic classical field model describing interaction between the polariton condensate and the exciton reservoir within a disordered potential.

Computer identification of neoplastic urothelial nuclei from the bladder.

OBJECTIVE: To introduce computer-based analysis of Feulgen-stained urinary bladder cell nuclei from voided urine to identify neoplastic urothelial nuclei. STUDY DESIGN: Nuclei from 23 healthy people and 33 patients with urinary bladder cancer were analyzed. The nuclei from 9 cancer patients with grade G1 (stage Ta), 17 with grade G2 (stages Ta, T1, T1a and T2) and 7 with grade G3 (stages Cis, Ta + Tis, T1 and T3b) were analyzed. Image analysis was carried out by means of a digital image processing system designed by the authors. Features describing nuclei were selected as the first step of the procedure. Then a multistage classifier was constructed to identify positive and negative cases. RESULTS: The results of this pilot study of a group of 56 patients yielded a 71% correct classification rate in the control group, while a 66% rate was obtained among the cancer patients. The sensitivity of the method was 100% and the specificity was 77%. CONCLUSION: This approach to the identification of neoplastic urothelial nuclei may be sufficiently well developed to be used successfully both in screening high-risk populations and in clinical practice.

Computer automated approach to the extraction of epiphyseal regions in hand radiographs.

Epiphyseal region is the most sensitive region to developmental changes of the skeletal system. Extraction of this area is the very first step in any computerized image analysis. In this report a fully automated analysis of a hand radiograph resulting in extraction of distal and middle regions of the II, III, and IV phalanx is presented. The processing is performed in 3 stages. First, the trend of background is removed from radiograph to obtain a binary hand mask. At this stage a labeling procedure is necessary to eliminate artifacts (markers). Then, II, III, and IV phalanges are identified in the binary image, and the phalangeal axes are drawn. Finally, the intensity profile along each phalangeal axis is analyzed, and, on its basis, distal and middle regions are located. The presented procedure is designed as a part of currently developed system for automatic bone age assessment; however, it also can be as a preprocessing step in other diseases the diagnoses of which may require a computer assistance.