Exciton

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

Semiconductor-Metal Nanoparticle Molecules: Hybrid Excitons and the Nonlinear Fano Effect

Abstract: Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

Moir\'e Excitons in Van der Waals Heterostructures

Abstract: In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moire superlattice. While it is widely recognized that a moire superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moire potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moire potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.

Random Walks on Lattices. III. Calculation of First‐Passage Times with Application to Exciton Trapping on Photosynthetic Units

Abstract: The following statistical problem arises in the theory of exciton trapping in photosynthetic units: Given an infinite periodic lattice of unit cells, each containing N points of which (N − 1) are chlorophyll molecules and one is a trap; if an exciton is produced with equal probability at any nontrapping point, how many steps on the average are required before the exciton reaches a trapping center for the first time? It is shown that, when steps can be taken to near‐neighbor lattice points only, as N → ∞, our required number of steps is 〈n〉∼{N2/6, linear chain,π−1NlogN, square lattice,1.5164N, single cubic lattice. The correction terms for medium and relatively small N are obtained for a number of lattices.

Employing ∼100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and Charge-Transfer Excited State

Abstract: In principle, the ratio (Φ) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron–hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of Φ > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor–acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high Φ values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m−2 and a maximum external quantum efficiency (ηext) of ∼2.8%. The HLCT state is a new way to realize high-efficiency of EL devices.

Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

Abstract: Cavity spectroscopy measurements elucidate the Fermi polaron nature of the optical excitations in monolayer transition metal dichalcogenides. The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultracold Fermi gases1,2,3,4,5. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation6 and exciton–electron interactions leading to polaron or trion formation7,8. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe2 (ref. 9) exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode10,11. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Simultaneous observation of polariton formation in both attractive and repulsive branches indicates a new regime of polaron physics where the polariton impurity mass can be much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose–Fermi mixtures12,13.