Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Quantized vortices in an exciton–polariton condensate

Abstract: When a superfluid—such as liquid helium—is set in rotation, vortices appear in which circulation around a closed loop can take only discrete values. Such quantized vortices have now been observed in a solid-state system—a Bose–Einstein condensate made of exciton polaritons. One of the most striking quantum effects in an interacting Bose gas at low temperature is superfluidity. First observed in liquid 4He, this phenomenon has been intensively studied in a variety of systems for its remarkable features such as the persistence of superflows and the proliferation of quantized vortices1. The achievement of Bose–Einstein condensation in dilute atomic gases2 provided the opportunity to observe and study superfluidity in an extremely clean and well-controlled environment. In the solid state, Bose–Einstein condensation of exciton polaritons has been reported recently3,4,5,6. Polaritons are strongly interacting light–matter quasiparticles that occur naturally in semiconductor microcavities in the strong-coupling regime and constitute an interesting example of composite bosons. Here, we report the observation of spontaneous formation of pinned quantized vortices in the Bose-condensed phase of a polariton fluid. Theoretical insight into the possible origin of such vortices is presented in terms of a generalized Gross–Pitaevskii equation. Whereas the observation of quantized vortices is, in itself, not sufficient for establishing the superfluid nature of the non-equilibrium polariton condensate, it suggests parallels between our system and conventional superfluids.

Excitonic Linewidth Approaching the Homogeneous Limit in MoS 2 -Based van der Waals Heterostructures

Abstract: The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe2 and WSe2. In this work, we show that encapsulation of ML MoS2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T=4 K. Narrow optical transition linewidths are also observed in encapsulated WS2, WSe2, and MoSe2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity.

Abstract: Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π–π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Very large optical nonlinearity of semiconductor microcrystallites

Abstract: We analyze theoretically the oscillator strength and the third-order optical polarizability X 13, due to excitons in semiconductor microcrystallites. The nonlinear optical polarizability is shown to be greatly enhanced for an assembly of such microcrystallites as the exciton is quantized due to the confinement effect and the excitons in a single microcrystallite interact strongly enough to make the excitons deviate from ideal harmonic oscillators.

Improving the Performance of Doped pi-Conjugated Polymers for Use in Organic Light-Emitting Diodes

Abstract: Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest unoccupied molecular orbital of the adjacent molecules (simultaneously, holes are injected by the anode into the highest occupied molecular orbital). The two types of carriers migrate towards each other and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped pi-conjugated polymer layers improve the injection of holes in OLED devices; this is thought to result from the more favourable work function of these injection layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such polymers, the barrier to hole injection can be continuously reduced. The use of combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electroluminescence (such as polyfluorene-based systems), finding that it is possible to significantly reduce the operating voltage while improving the light output and efficiency.