Annihilation Of Excitations Research Articles

Microstructure-driven annihilation effects and dispersive excited state dynamics in solid-state films of a model sensitizer for photon energy up-conversion applications.

Bimolecular processes involving exciton spin-state interactions gain attention for their deployment as wavelength-shifting tools. Particularly triplet-triplet annihilation induced photon energy up-conversion (TTA-UC) holds promise to enhance the performance of solar cell and photodetection technologies. Despite the progress noted, a correlation between the solid-state microstructure of photoactuating TTA-UC organic composites and their photophysical properties is missing. This lack of knowledge impedes the effective integration of functional TTA-UC interlayers as ancillary components in operating devices. We here investigate a solution-processed model green-to-blue TTA-UC binary composite. Solid-state films of a 9,10 diphenyl anthracene (DPA) blue-emitting activator blended with a (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) green-absorbing sensitizer are prepared with a range of compositions and examined by a set of complementary characterization techniques. Grazing incidence X-ray diffractometry (GIXRD) measurements identify three PtOEP composition regions wherein the DPA:PtOEP composite microstructure varies due to changes in the packing motifs of the DPA and PtOEP phases. In Region 1 (≤2 wt%) DPA is semicrystalline and PtOEP is amorphous, in Region 2 (between 2 and 10 wt%) both DPA and PtOEP phases are amorphous, and in Region 3 (≥10 wt%) DPA remains amorphous and PtOEP is semicrystalline. GIXRD further reveals the metastable DPA-β polymorph species as the dominant DPA phase in Region 1. Composition dependent UV-vis and FT-IR measurements identify physical PtOEP dimers, irrespective of the structural order in the PtOEP phase. Time-gated photoluminescence (PL) spectroscopy and scanning electron microscopy imaging confirm the presence of PtOEP aggregates, even after dispersing DPA:PtOEP in amorphous poly(styrene). When arrested in Regions 1 and 2, DPA:PtOEP exhibits delayed PtOEP fluorescence at 580 nm that follows a power-law decay on the ns time scale. The origin of PtOEP delayed fluorescence is unraveled by temperature- and fluence-dependent PL experiments. Triplet PtOEP excitations undergo dispersive diffusion and enable TTA reactions that activate the first singlet-excited (S1) PtOEP state. The effect is reproduced when PtOEP is mixed with a poly(fluorene-2-octyl) (PFO) derivative. Transient absorption measurements on PFO:PtOEP films find that selective PtOEP photoexcitation activates the S1 of PFO within ∼100 fs through an up-converted 3(d, d*) PtII-centered state.

Energy Transport in SiO2 Crystals: Luminescence Excitation Spectra of Stishovite and α-Quartz

Abstract The migration of elementary electronic excitations was studied in a single crystal of stishovite and compared with migration in a crystal of α-quartz and polycrystalline stishovite powder. The research method is based on comparing the transfer of absorbed energy to luminescence centers, used as detectors of quasiparticles, and the near-surface nonradiative annihilation of electronic excitations. A sign of migration is the appearance of some minima in the photoluminescence (PLE) excitation spectrum in the region of maxima in the intrinsic absorption spectrum. The PLE spectrum of stishovite contains the first minimum at 9.8 eV, indicating the migration of electronic excitations and the existence of an intrinsic absorption band in stishovite at 9.8 eV. In α-quartz, the first minimum in the PLE spectrum is located at 10.5 eV and corresponds well to the intrinsic absorption band of the exciton.

Annihilation of Excess Excitations along Phycocyanin Rods Precedes Downhill Flow to Allophycocyanin Cores in the Phycobilisome of Synechococcus elongatus PCC 7942.

Cyanobacterial phycobilisome complexes absorb visible sunlight and funnel photogenerated excitons to the photosystems where charge separation occurs. In the phycobilisome complex of Synechococcus elongatus PCC 7942, phycocyanin protein rods that absorb bluer wavelengths are assembled on allophycocyanin cores that absorb redder wavelengths. This arrangement creates a natural energy gradient toward the reaction centers of the photosystems. Here, we employ broadband pump–probe spectroscopy to observe the fate of excess excitations in the phycobilisome complex of this organism. We show that excess excitons are quenched through exciton–exciton annihilation along the phycocyanin rods prior to transfer to the allophycocyanin cores. Our observations are especially relevant in comparison to other antenna proteins, where exciton annihilation primarily occurs in the lowest-energy chlorophylls. The observed effect could play a limited photoprotective role in physiological light fluences. The exciton decay dynamics is faster in the intact phycobilisome than in isolated C-phycocyanin trimers studied in earlier work, confirming that this effect is an emergent property of the complex assembly. Using the obtained annihilation data, we calculate exciton hopping times of 2.2–6.4 ps in the phycocyanin rods. This value agrees with earlier FRET calculations of exciton hopping times along phycocyanin hexamers by Sauer and Scheer.

Transition Slow-Down by Rydberg Interaction of Neutral Atoms and a Fast Controlled-not Quantum Gate

Exploring controllable interactions lies at the heart of quantum science. Neutral Rydberg atoms provide a versatile route toward flexible interactions between single quanta. Previous efforts mainly focused on the excitation annihilation~(EA) effect of the Rydberg blockade due to its robustness against interaction fluctuation. We study another effect of the Rydberg blockade, namely, the transition slow-down~(TSD). In TSD, a ground-Rydberg cycling in one atom slows down a Rydberg-involved state transition of a nearby atom, which is in contrast to EA that annihilates a presumed state transition. TSD can lead to an accurate controlled-{\footnotesize NOT}~({\footnotesize CNOT}) gate with a sub-$\mu$s duration about $2\pi/\Omega+\epsilon$ by two pulses, where $\epsilon$ is a negligible transient time to implement a phase change in the pulse and $\Omega$ is the Rydberg Rabi frequency. The speedy and accurate TSD-based {\footnotesize CNOT} makes neutral atoms comparable~(superior) to superconducting~(ion-trap) systems.

Absorption and emission of a collective excitation by a fermionic quasiparticle in a Fermi superfluid

We study the process of absorption or emission of a bosonic collective excitation by a fermionic quasiparticle in a superfluid of paired fermions. From the random phase approximation equation of motion of the bosonic excitation annihilation operator, we obtain an expression of the coupling amplitude of this process, which is limited neither to resonant processes nor to the long wavelength limit. We confirm our result by independently deriving it in the functional integral approach using the gaussian fluctuation approximation, and by comparing it in the long wavelength limit to the quantum hydrodynamic result. Lastly, we give a straightforward application of the coupling amplitude we obtain by calculating the lifetime of the bosonic excitations of an arbitrary wave number. We find a mode quality factor that decreases from its maximum at low wave numbers and vanishes when the bosonic branch hits the continuum of fermionic biexcitations.

Speeding up the antidynamical Casimir effect with nonstationary qutrits

The antidynamical Casimir effect (ADCE) is a term coined to designate the coherent annihilation of excitations due to resonant external perturbation of system parameters, allowing for extraction of quantum work from nonvacuum states of some field. Originally proposed for a two-level atom (qubit) coupled to a single cavity mode in the context of nonstationary quantum Rabi model, it suffered from very low transition rate and correspondingly narrow resonance linewidth. In this paper we show analytically and numerically that the ADCE rate can be increased by at least one order of magnitude by replacing the qubit by an artificial three-level atom (qutrit) in a properly chosen configuration. For the cavity thermal state we demonstrate that the dynamics of the average photon number and atomic excitation is completely different from the qubit's case, while the behavior of the total number of excitations is qualitatively similar yet significantly faster.

Observation of the Phononic Lamb Shift with a Synthetic Vacuum

The quantum vacuum fundamentally alters the properties of embedded particles. In contrast to classical empty space, it allows for creation and annihilation of excitations. For trapped particles this leads to a change in the energy spectrum, known as Lamb shift. Here, we engineer a synthetic vacuum building on the unique properties of ultracold atomic gas mixtures. This system makes it possible to combine high-precision spectroscopy with the ability of switching between empty space and quantum vacuum. We observe the phononic Lamb shift, an intruiguing many-body effect orginally conjectured in the context of solid state physics. Our study therefore opens up new avenues for high-precision benchmarking of non-trivial theoretical predictions in the realm of the quantum vacuum.

Raman scattering signatures of the unusual vibronic interaction of molecules in liquid helium-3

Light scattering in quantum liquid helium-3 may involve a unique mechanism – the creation and annihilation of atom excitations across the Fermi level. The density of states of particle–hole excitations in the low-energy limit is strongly enhanced as compared to that of collective excitations of phonons in helium-3. This makes possible to directly observe Fermi excitations in the resonant Raman scattering (RRS) by 3He droplets doped by impurity molecules. The RRS spectra essentially depend on the excitation frequency. In case of excitation in the anti-Stokes side of absorption the first order RRS is directly determined by the particle–hole excitations in the vicinity of the impurity molecule and the contribution of phonons mainly given by the localized spherical vibration. The calculations are made for a 3He droplet doped by a glyoxal molecule.

Charge Ordering in a Pure Spin Model: Dipolar Spin Ice

We study the dipolar spin-ice model at fixed density of single excitations, ρ, using a Monte Carlo algorithm where processes of creation and annihilation of such excitations are banned. In the limit of ρ going to zero, this model coincides with the usual dipolar spin-ice model at low temperatures, with the additional advantage that a negligible number of monopoles allows for equilibration even at the lowest temperatures. Thus, the transition to the ordered fundamental state found by Melko, den Hertog, and Gingras in 2001 is reached using simple local spin flip dynamics. As the density is increased, the monopolar nature of the excitations becomes apparent: the system shows a rich ρ vs T phase diagram with "charge" ordering transitions analogous to that observed for Coulomb charges in lattices. A further layer of complexity is revealed by the existence of order both within the charges and their associated vacuum, which can only be described in terms of spins--the true microscopic degrees of freedom of the system.

Lasing through a strongly-coupled mode by intra-cavity pumping

We demonstrate room temperature lasing through the polaritonic mode of a J-aggregate microcavity in which losses from exciton-exciton annihilation and slow polariton relaxation typical of direct J-aggregate excitation are circumvented via intra-cavity pumping. The pumping scheme utilizes an organic dye layer (DCM) within the cavity with an emission band overlapping the entire lower J-aggregate polariton branch spectrum, hence forcing DCM lasing to occur through the strongly-coupled mode. This cavity architecture, which separates strong coupling and gain into two materials, presents a general and flexible design for polariton devices and allows for the use of a wide range of materials, organic and inorganic, to be integrated into the cavity.

Efficiency scaling of non-coherent upconversion in a one-dimensional model system

A very promising approach to obtain efficient upconversion of light is the use of triplet-triplet annihilation of excitations in molecular systems. In real materials, besides upconversion, many other physical processes take place--fluorescence, phosphorescence, non-radiative decay, annihilation, diffusion--and compete with upconversion. The main objective of this work is to design a proof of principle model that can be used to shed light on the interplay between these processes. Ultimately, we want to establish general principles that may guide experimentalists toward the design of solid state materials with maximum efficiency. Here we show, in a one-dimensional model system, that upconversion can be optimized by varying the ratio between the two molecular species used in triplet-triplet-annihilation based upconversion systems, even in the presence of undesired losses through phosphorescence, non-radiative decay, or annihilation. We derive scaling laws for this ratio and for the maximum efficiency of upconversion, as a function of the diffusion rate J, as well as of the creation and of the decay rate of the excitations.

Luminescence kinetics of two-component molecular systems in porous glasses

The kinetics of bimolecular photoreactions accompanied by a characteristic delayed emission of reactant molecules is studied in nanoporous silica glasses upon selective activation of a two-component luminophore system (erythrosine-anthracene) by a short (10 ns) pulse of a Nd3+:YAG laser (532 nm). Time-resolved luminescence signals with different shapes are recorded in different spectral regions. The emission at a wavelength of 430 nm is identified as the sensitized delayed annihilation fluorescence of anthracene. The long-time luminescence in the region of 570 nm consists of thermally induced delayed fluorescence of erythrosine and an emission resulting from heterogeneous annihilation of triplet excitations of erythrosine and anthracene. The effect of pore sizes on the triplet-triplet energy transfer efficiency is determined.

Morphology and characterization of TiO2 nanoparticles synthesized by arc discharge

High quality titanium dioxide (TiO2) nanoparticles have been successfully synthesized by arc discharge method. A mixture of anatase and rutile phase of TiO2 crystal nanostructure with spherical shape was produced with an average diameter of 50nm (range 10–100nm). Integrated crystal form and high crystallinity were discovered. Besides a dominate UV emission at 409nm originating from the radiative annihilation of excitation, a weak peak at 658nm from the oxygen vacancies related emission is also detected. Due to its simplicity of synthesis, the proposed arc discharge is a promising technique for the fabrication of TiO2 nanoparticles.

Simulation of interaction of oriented J aggregates with resonance laser radiation

The interaction of laser radiation with single J aggregates of cyanine dyes is theoretically analyzed and numerically simulated. The quantum-mechanical calculations of the equilibrium geometry and the energies and intensities of the lowest singlet electronic transitions in pseudoisocyanine chloride and its linear (chain) oligomers are fulfilled. The data of these calculations can serve as parameters of the analyzed model of interaction of J aggregates with radiation in the one-particle density matrix approximation. This model takes into account relaxation processes, the annihilation of excitations at neighboring molecules, and inhomogeneous broadening. Assuming that the inhomogeneous broadening is absent, calculations demonstrate the existence of spatial bistability, molecular switching waves, and dissipative solitons. The effect of the inhomogeneous broadening and the radiation intensity on the effective coherence length in linear (chain) J aggregates is analyzed.

Change in the kinetics of delayed annihilation fluorescence during rearrangement of polymer-chain structure in a nanocavity of a solid adsorbent

Specially derived spatial density distributions of units of the polymer chain adsorbed by the surface of a nanosized spherical or cylindrical cavity were used to describe the kinetics of the bimolecular reaction of cross annihilation of electronic excitation in nanoporous media. The cases of marked attraction of the chain units by cavity walls and the lack of the attraction were considered. The formation of the radial profile of the density of the units was simulated by means of molecular dynamics to compare with the analytical expressions used.

A general approach for non-coherently excited annihilation up-conversion: transforming the solar-spectrum

We demonstrate up-conversion with an external quantum yield of 3.2% realized at ultra-low excitation intensities (∼0.1 W cm−2). The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has substantially different photophysical characteristics when compared with the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc). Thus, we state the general requirements for the materials' parameters of the couple sensitizer/emitter for obtaining triplet–triplet annihilation up-conversion. One of the applications we suggest for active materials compositions is the achievement of energetically stepwise up-conversion of the terrestrial solar spectrum towards blue wavelengths: as such they could be used for improving the efficiency of the solar cells. Up-conversion with non-coherent excitation by sunlight with wavelengths between 550 and 700 nm is demonstrated.

Size effects in triplet-triplet annihilation: II. Monte carlo simulations

The dynamics of triplet-triplet annihilation (TTA) is theoretically studied in linear chains and nanoparticles, modeled as 1D, 2D, and 3D regular lattices, as a function of size M, of the rate of excitation migration W, and of the rate of excitation annihilation V in the diffusion-influenced limit (V ≫ W). It is shown that a sum of two exponentials is usually sufficient for fitting experimental phosphorescence and triplet-triplet absorption decays. The first term describes the decay of domains containing initially one triplet, while the second one reflects the disappearance of domains containing initially two triplets. Monte Carlo calculations were carried out to compute the survival probability of an annihilating pair of triplets, yielding expressions for the dependence of the rate constant of TTA on the parameters M, W, and V in one, two, and three dimensions.

Size effects in triplet-triplet annihilation: I. Standard and statistical approaches

The dynamics of triplet-triplet annihilation is theoretically studied in linear chains and nanoparticles as a function of size M, the rate of excitation migration W, and the rate of excitation annihilation V. It is shown that a sum of two exponentials is usually sufficient for fitting experimental phosphorescence and triplet-triplet absorption decays. The first term describes the decay of domains containing initially one triplet, while the second term reflects the disappearance of domains containing initially two triplets.

Deactivation of the S1 state of a water-soluble porphyrin cation in a complex with DNA studied by the method of picosecond absorption spectroscopy

The deactivation of the excited S1 state of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H2TMPyP4) in a complex with DNA is studied by the methods of kinetic absorption spectroscopy for different concentration ratios r of porphyrin and DNA base pairs. It is found that, when the DNA bound porphyrin is assumed to be in a monomer (non-aggregate) state (r=1:30), the S1 state of porphyrin excited by high-power picosecond pulses is rapidly deactivated, which is manifested as an additional decay component with the decay time 200–250 ps. This effect is interpreted as singlet—singlet annihilation of excitation indicating the presence of sites on the DNA surface with a higher local concentration of porphyrin molecules. The decrease in the relative content of DNA in solution down to r=1:3 further accelerates the deactivation of the S1 state of porphyrin and causes the hypsochromic shift of its absorption spectrum due to the formation of stack-like porphyrin aggregates along the DNA surface. It is assumed that the additional component with the decay time 44±7 ps observed in the decay curve of the S1 state is caused by the annihilation of excitation in porphyrin aggregates.

Identification of chlorophyll molecules in peripheral light harvesting complex LHC II

Chlorophyll site excitation energies of the peripheral plant light-harvesting complex (LHC-II) have been determined by simulating the steady-state absorption and circular dichroism spectra and using new structural data. By applying the genetic algorithm search procedure it has been found that the fit to circular dichroism spectra is critical in narrowing the space of possible solutions. The obtained chlorophyll energy assignment has been verified by the Monte Carlo simulations of the excitation annihilation kinetics. It suggests that the chlorophyll a dimers a611-a612, a613-a614, and a602-a603 constitute the sites for the energy migration across the peripheral plant light-harvesting antenna.