Nonradiative and radiative Förster energy transfer between quantum dots

We theoretically study nonradiative and radiative energy transfer between two localized quantum emitters, a donor (initially excited) and an acceptor (receiving the excitation). The rates of nonradiative and radiative processes are calculated depending on the spatial and spectral separation between the donor and acceptor states and for different donor and acceptor lifetimes for typical parameters of semiconductor quantum dots. We find that the donor lifetime can be significantly modified only due to the nonradiative Förster energy transfer process at donor–acceptor separations of approximately 10 nm (depending on the acceptor radiative lifetime) and for the energy detuning not larger than 1–2 meV. The efficiency of the nonradiative Förster energy transfer process under these conditions is close to unity and decreases rapidly with an increase in the donor–acceptor distance or energy detuning. At large donor–acceptor separations greater than 40 nm, the radiative corrections to the donor lifetime are comparable with nonradiative ones but are relatively weak.     

Multichromophoric Förster resonance energy transfer

The theory of F?rster resonance energy transfer is generalized for multichromophoric (MC) and nonequilibrium situations. For the first time, it is clarified that the far-field linear spectroscopic information is insufficient for the determination of the reaction rate and that distance dependence of the rate can vary with the disorder and temperature. Application to a light harvesting complex LH2 reveals the important consequences of a MC structure.     

Nonradiative exciton transfer by the Förster mechanism from InAs/AlAs quantum dots to dye molecules in hybrid structures

Förster resonance transfer of excitons from self-assembled InAs/AlAs quantum dots to cyanine dye molecules deposited on the structure surface is investigated. It is shown that the efficiency of the exciton transfer changes upon the interaction of cyanine dye molecules with chloroform due to a change in the molecule dipole moment.     

Binding energy and photoionizaiton cross-section of hydrogen-like impurity in a Pöschl-Teller quantum well

The effect of the donor impurity position and the form of confining potential on the binding energy and the photoionization cross-section in a semiconductor quantum well with the Pöschl-Teller potential is studied. An analytical expression for the photoionization cross-section is obtained for the case when the polarization vector of light wave is directed along the direction of size quantization. It is shown that the photoionization cross-section has a threshold behavior.     

Saturated Förster resonance energy transfer microscopy with a stimulated emission depletion beam: a pathway toward single-molecule resolution in far-field bioimaging

We theoretically demonstrate that the spatial resolution of stimulated emission depletion (STED) microscopy can be substantially enhanced without increasing the intensity of the STED beam. In our scheme, tiny nanobeads codoped with donor and acceptor molecules are used as fluorescent probes, in which F?rster resonance energy transfer (FRET) can occur with an ~100% efficiency between the donors and acceptors. Enhancement of the depletion of acceptors in the nanobeads with the doughnut-shaped depletion beam can lead to an increase of FRET efficiency in the outer area of the excitation spot, which itself is used for deexciting donor molecules and, consequently, enhancing the optical resolution.     

Enhancement of Frster energy transfer from thermally activated delayed fluorophores layer to ultrathin phosphor layer for high color stability in non-doped hybrid white organic light-emitting devices

Fluorescence/phosphorescence hybrid white organic light-emitting devices(WOLEDs) based on double emitting layers(EMLs) with high color stability are fabricated.The simplified EMLs consist of a non-doped blue thermally activated delayed fluorescence(TADF) layer using 9,9-dimethyl-9,10-dihydroacridine-diphenylsulfone(DMAC-DPS) and an ultrathin non-doped yellow phosphorescence layer employing bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)((tbt)_2Ir(acac)).Two kinds of materials of 4,7-diphenyl-1,10-phenanthroline(Bphen) and 1,3,5-tris(2-Nphenylbenzimidazolyl) benzene(TPBi) are selected as the electron transporting layer(ETL),and the thickness of yellow EML is adjusted to optimize device performance.The device based on a 0.3-nm-thick yellow EML and Bphen exhibits high color stability with a slight Commission International de l'Eclairage(CIE) coordinates variation of(0.017,0.009) at a luminance ranging from 52 cd/m~2 to 6998 cd/m~2.The TPBi-based device yields a high efficiency with a maximum external quantum efficiency(EQE),current efficiency,and power efficiency of 10%,21.1 cd/A,and 21.3 lm/W,respectively.The ultrathin yellow EML suppresses hole trapping and short-radius Dexter energy transfer,so that Forster energy transfer(FRET)from DMAC-DPS to(tbt)_2Ir(acac) is dominant,which is beneficial to keep the color stable.The employment of TPBi with higher triplet excited state effectively alleviates the triplet exciton quenching by ETL to improve device efficiency.     

Photon-exchange energy transfer of an electron–hole plasma between quasi-two-dimensional semiconductor layers

Photon-mediated energy transfer is shown to play an important role for transfer of an electron–hole plasma between two quasi-two-dimensional quantum wells separated by a wide barrier. The magnitude and the dependence of the transfer rate of an electron–hole plasma on the temperature, the well-to-well distance, and the plasma density are compared with those of the standard Förster (i.e., dipolar) rate and also with the exciton transfer rate. The plasma transfer rate through the photon-exchange mechanism decays very slowly as a function of the well-to-well distance and is larger than the dipolar rate except for short distances. The transfer rate of plasmas saturates at high densities and decays rapidly with the temperature.     

Förster resonance energy transfer in hybrid associates of colloidal Ag<Subscript>2</Subscript>S quantum dots with thionine molecules

Nonradiative resonance energy transfer in hydrophilic hybrid associates of thionine molecules (TH⁺) with colloidal Ag₂S quantum dots (QDs) with average diameter of 3.5 nm was studied. Photoluminescence spectra and its decay shown that for these systems the supplemental photosensitization of recombination luminescence of Ag₂S QDs (1200 nm) from the region of TH⁺ fluorescence (618 nm) is possible. It was found that the average lifetime of TH⁺ molecules luminescence is shortened during their association with Ag₂S QDs. Approximation of luminescence decay by stretched exponent with value of parameter β =?0.5 indicates on the inductive-resonance dipole-dipole (Förster) mechanism of nonradiative energy transfer (FRET). The efficiency of FRET was 0.29–0.41.     

Relaxation processes and mobility of electrons in a semiconductor quantum well with the modified Pöschl-teller confining potential

Relaxation processes and mobility of electrons in a semiconductor quantum well are studied. The modified Pöschl-Teller potential is used as a confining potential. Scattering rates due to impurity ions, acoustic and piezoacoustic phonons are calculated taking into account the screening of scattering potentials by charge carriers. It is shown that when degenerate electrons are scattered by acoustic phonons, the dependence of scattering rate on electron wave number ν_ac(k) is almost linear. At small k, the acoustic phonon piezoelectric scattering rate of degenerate electrons increases with k, and then it decreases slightly when k > 8 × 10⁷ m⁻¹. The ionized impurity scattering rate of degenerate electrons does not depend on temperature, is directly proportional to the electron density, and decreases with increasing k. Dependences of electron mobility on surface ion density and temperature are studied. It is shown that in the case of non-degenerate or slightly degenerate electron gas, a maximum appears in the temperature dependence of the mobility, and the screening effect is negligible. The screening significantly increases the mobility of electrons in the case of high degeneration. Obtained results are applied to GaAs-based quantum wells.     

Förster-type energy transfer mechanism in PF2/6 to MEH-PPV conjugated polymers

Energy transfer mechanism between Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF_2/6) as a donor and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an acceptor in different solvents has been studied using steady-state emission measurements. Four different solvents namely, tetrahydrofuran (THF), toluene, chlorobenzene (C.B) and benzene have been used in this study. The absorption and luminescence behaviors of the samples are measured at a fixed concentration of donor (0.1 μM) while the concentrations for acceptor are kept in the range of 0.1–1.0 μM. Based on these measurements, the energy transfer properties namely quenching rate constant (k_SV), energy transfer rate constant (k_ET), energy transfer probability (P_DA), transfer efficiency (η) and critical distance of energy transfer (R_o) are calculated. The use of THF resulted in the highest energy transfer. Long range dipole–dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer as proven by the large values of R_o.     

Resonance energy transfer in a dense array of II–VI quantum dots

Förster resonance energy transfer in inhomogeneous dense arrays of epitaxial CdSe/ZnSe quantum dots is demonstrated by time- and space-resolved photoluminescence spectroscopy. The specific feature of this process is the dipole–dipole interaction between the ground exciton levels of small quantum dots and the excited levels of large dots. This interaction brings efficient energy collection and spectral selection of a limited number of emitters. Results of theoretical modeling of optical transitions in spheroidal quantum dots with a Gaussian potential profile agree with the observed features of optical spectra induced by the change of the dominant energy transfer mechanism.     

Nonlocal effects on second-harmonic generation in a Pöschl-Teller quantum well

Nonlocal effects on the energy conversion efficiency of the second-harmonic generation (SHG) for a p-polarized incident field in a P?schl-Teller quantum well (PTQW) are investigated in detail. The numerical results show that the spatial distribution of the second-harmonic field is nonuniform, and that there exist two resonance peaks in the second-harmonic energy reflection spectra, and their positions have a notable blueshift because of the nonlocal effects. A very important property is that a maximum blueshift at the second-harmonic resonance can be obtained by adopting a proper quantum-well width and donor concentration, which may be interesting in future precision experiments.     

Effect of energy transfer from atomic electron shell to an α particle emitted by decaying nucleus

The process of energy transfer from the electron shell of an atom to an α particle propagating through the shell is formulated mathematically. Using the decay of the ²²⁶Ra nucleus as an example, it is demonstrated that this phenomenon increases the α-decay intensity in contrast with other known effects of similar type. Moreover, the α decay of the nucleus is more strongly affected by the energy transfer than by all other effects taken together.     

Near-infrared quantum cutting via resonant energy transfer from?Pr3+ to Yb3+ in LaF3

Experimental evidence of energy transfer from Pr³⁺ to Yb³⁺ were presented by the excitation spectra and emission spectra in LaF₃:Pr³⁺,Yb³⁺. Temperature-dependent infrared emission and fluorescence decay curves have been measured to investigate the quantum cutting mechanism and energy transfer efficiency of the Pr³⁺?CYb³⁺ couple. Upon excitation of Pr³⁺ at 442?nm, Yb³⁺ emits two near-infrared photons around 1000?nm through two resonant energy transfer processes from Pr³⁺ to Yb³⁺ and the maximum total quantum efficiency reaches 161.6%. The excellent luminescence properties of the Pr³⁺?CYb³⁺ codoped LaF₃ indicate its potential application in improving the energy conversion efficiency of the silicon based solar cells by converting one blue photon to two near-infrared ones.     

Is gravitational mass of a composite quantum body equivalent to its energy?

We define passive gravitational mass operator of a hydrogen atom in the post-Newtonian approximation of general relativity and show that it does not commute with energy operator, taken in the absence of gravitational field. Nevertheless, the equivalence between the expectation values of passive gravitational mass and energy is shown to survive for stationary quantum states. Inequivalence between passive gravitational mass and energy at a macroscopic level results in time dependent oscillations of the expectation values of passive gravitational mass for superpositions of stationary quantum states, where the equivalence restores after averaging over time. Inequivalence between gravitational mass and energy at a microscopic level reveals itself as unusual electromagnetic radiation, emitted by the atoms, supported and moved in the Earth gravitational field with constant velocity using spacecraft or satellite, which can be experimentally measured.     

Variational approaches to quantum impurities: from the Fröhlich polaron to the angulon

ABSTRACT

Problems involving quantum impurities, in which one or a few particles are interacting with a macroscopic environment, represent a pervasive paradigm, spanning across atomic, molecular, and condensed-matter physics. In this paper we introduce new variational approaches to quantum impurities and apply them to the Fröhlich polaron – a quasiparticle formed out of an electron (or other point-like impurity) in a polar medium, and to the angulon – a quasiparticle formed out of a rotating molecule in a bosonic bath. We benchmark these approaches against established theories, evaluating their accuracy as a function of the impurity-bath coupling.     

Specifics of the stochastic ionization of a Rydberg collision complex with Förster resonance

The physics of the formation of dynamic nonlinear resonances in an isolated Rydberg collisional complex is described. The development of the stochastic instability of Rydberg electron trajectories due to charge exchange in the complex is considered. The realization of the resonance in external statistic magnetic and electric fields is predicted to be accompanied by a significant narrowing of areas of stochastic motion with a concurrent decrease in the rates of the ionization of real quasimolecular systems proceeding through the migration over the Rydberg crowding of quantum states.