3,528 research outputs found
New concept for organic LEDs: non-radiative electronic energy transfer from semiconductor quantum well to organic overlayer
No full textThe resonant electronic energy transfer from an excited semiconductor quantum well to a nearby strongly absorbing organic medium may occur on time scales of several tens of picoseconds for II-VI semiconductors and of several hundreds picoseconds for III-V semiconductors, which is in both cases significantly less than the semiconductor excitation life-time in the absence of such a transfer. Thus, a large fraction of the semiconductor excitation energy can be transfered to the organic medium. Due to energy transfer the non-radiative processes in the semiconductor quantum well. with characteristic times larger than that of the energy transfer, will be suppressed. The advantage of the hybrid structures would be also the combination of comparatively good transport properties of semiconductors (allowing for electrical pumping) and good light-emitting properties of organic substances thigh quantum yield, colour tuning). Such a favorable combination may be exploited in constructing new type of LEDs. (C) 2001 Elsevier Science B.V. All rights reserved
Efficient optical pumping of organic-inorganic heterostructures for nonlinear optics
No full textWe theoretically consider a hybrid heterostructure made of an inorganic quantum well in close proximity with an organic material overlayer whereby the latter is used to funnel excitation energy to the former in order to exploit the optical nonlinearities of the two-dimensional Wannier excitons. On the one hand, the diffusion length of Frenkel excitons in the organic medium is assumed to be comparable or larger than the corresponding absorption length. On the other hand, the nonradiative energy transfer from the organic to the inorganic subsytem can be very efficient when the Frenkel exciton energy is significantly higher than the band gap of the inorganic semiconductor. We show in this regime that the resonant optical pumping of the Frenkel excitons can lead to an efficient indirect pumping of the Wannier excitons (or electron-hole plasma) in the inorganic quantum well turning on the corresponding nonlinearities
Interaction of quantum well excitons with a resonant localized excitation
No full textWe consider Wannier-Mott exciton in a semiconductor quantum well coupled via Coulomb interaction to a resonant localized excitation (a luminescent molecule or a quantum dot) placed nearby. As a result, the localized excitation can decay into the quantum well, which can be used for indirect pumping of Wannier excitons. On the other hand, Wannier excitons are scattered, and the scattering can be elastic or inelastic, depending on the strength of the dissipation in the molecule. We also show how the inelastic scattering transforms into the Forster picture of energy transfer when many molecules (quantum dots) with strong intrinsic dissipation are present
Electronic energy transfer in a microcavity
No full textWe study the microcavity effect on energy transfer via the electromagnetic field. The modification of the optical propel ties of the microcavity due to the presence of absorbing accepters is taken into account. We analyze different cases that may be realized depending on the characteristics of the acceptor absorption band: the case of strong and broad absorption, when the cavity mode is practically destroyed; the regime of weak absorption, when the cavity mode is still well defined and just acquires some additional broadening; and the strong-coupling regime, when the acceptor absorption has the shape of a strong and narrow peak and two polariton branches appear, formed due to coherent mixing of the acceptor excitations and the cavity mode. In our calculations we use a realistic model for the cavity mirrors (dielectric or metallic), which is important since the mirrors introduce an additional decay channel for the donor excitation besides the transfer to the accepters. The distribution of the energy between different decay channels is analyzed in detail
Excitons and optical nonlinearities in hybrid organic-inorganic nanostructures
No full textWe present a theoretical review of the properties of electronic excitations in nanostructures based on combinations of organic materials with inorganic semiconductors, having respectively Frenkel excitons and Wannier-Mott excitons with nearly equal energies. We show that in this case the resonant coupling between organic and inorganic quantum wells (or wires or dots) may lead to several interesting effects, such as splining of the excitonic spectrum and enhancement of the resonant optical nonlinearities. First, we discuss the properties of hybrid Frenkel-Wanner-Mott excitons, which appear when the energy splitting of the excitonic spectrum is large compared to the width of the exciton resonances (the case of strong resonant coupling). Such peculiar excitations share at the same time both the properties of the Wannier excitons (e.g., the large radius) and those of the Frenkel excitons (e.g., the large oscillator strength). We discuss mainly two-dimensional configurations (interfaces or coupled quantum wells) which are the most extensively studied. In particular, we show that hybrid excitons are expected to have resonant optical nonlinearities significantly enhanced with respect to those of traditional inorganic or organic systems. We also consider analogous phenomena in microcavities where the exciton resonances are close to the cavity photon mode resonance. Next, we consider the case of weak resonant coupling and show the relevance of the Forster mechanism of energy transfer from an inorganic quantum well to an organic overlayer. Such an effect may be especially interesting for applications: the electrical pumping of excitons in the semiconductor quantum well can be used to efficiently turn on the organic material luminescence
Excitons in hybrid organic-inorganic nanostructures
No full textIn two-dimensional heterostructures made of semiconductor and organic layers, when resonance between the Wannier and Frenkel excitons is realized, the dipole-dipole interaction coupling them leads to novel effects. First, we discuss the pronounced nonlinear optical properties of the hybrid Frenkel-Wannier excitons appearing when the energy splitting of the excitonic spectrum is large compared to the exciton linewidths (the case of strong resonant coupling). Next, we consider the case of weak resonant coupling for which the Forster mechanism of energy transfer from an inorganic quantum well to an organic overlayer is of great interest: the electrical pumping of excitons in the semiconductor quantum well could be employed to turn on efficiently the organic material luminescence. (C) 1999 American Institute of Physics. [S1063-7834(99)00605-X]
Kinematic Frenkel biexcitons
No full textWe consider the two-particle spectra of Frenkel excitons in molecular crystals and demonstrate that, even if the dynamical interaction is weak or absent, bound states of two excitons may be formed due to the kinematic interaction induced by their paulion nature. A simple model of a linear molecular chain with two molecules in the unit cell is studied, which may be applicable to J-aggregates. The number of exciton bands turns our to be crucial for the possibility of kinematic biexciton formation. We also estimate the biexciton contribution to the resonant part of the nonlinear optical polarizability and two-photon absorption, and analyze the stability of the kinematic biexcitons with respect to the dynamical exciton-exciton interaction. (C) 2000 Elsevier Science B,V. All rights reserved
Charged Frenkel excitons in organic crystals
No full textIn molecular crystals a charge carrier and a molecular (Frenkel) exciton are attracted to each other. This attraction arises from the increase of the molecular static polarizability upon electronic excitation, and may be responsible for the formation of a two-particle bound state of the exciton and the free charge. Such charged Frenkel excitons are analogous to trions (bound states of a Wannier-Mott exciton and a charge carrier) in inorganic semiconductors. In this paper we develop a theory of charged excitons in molecular crystals, and apply this theory to study the spectra of charged Frenkel excitons in two-dimensional structures, which serve as a rather good approximation for layered crystals like anthracene, tetracene. We show that the binding energy of charged Frenkel excitons can be of the order of several hundred cm(-1). Thus, they may be stable at room temperatures, in contrast to trions in inorganic semiconductors. Particularly interesting is the possibility of forming charged triplet excitons in the crystal of tetracene where the energy of the lowest triplet exciton is much smaller than the energy of the lowest excitation in ions. This circumstance prevents the transformation of charged triplet excitons into an ion excitation with short lifetime and promotes the formation of rather stable charged triplet excitation. We briefly discuss the optical properties of charged Frenkel excitons and the effects of a static electric field
Raman spectroscopy as a versatile tool for studying the properties of graphene.
No full textRaman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene
Entanglement and quantity in quantum space - About quantum measurement (II)
No full textAs a continuation and extension of "quantity in phase space" "quantity in quantum space" is introduced. With that, the disappearing of quantum interference discussed in a previous paper [S. Durr, et al., Nature 395 (1998) 33] is explained in the same spirit as our recent papers [Ren De-Ming, Commun. Theor. Phys. (Beijing, China) 41 (2004) 685, 833].Physics, MultidisciplinarySCI(E)中国科学引文数据库(CSCD)1ARTICLE133-364
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