Fast Nonradiative Relaxation Research Articles

Molecular Engineering of a Tumor-Targeting Thione-Derived Diketopyrrolopyrrole Photosensitizer to Attain NIR Excitation Over 850 nm for Efficient Dual Phototherapy.

Photosensitizers with near-infrared (NIR) excitation, especially above 800 nm which is highly desired for phototherapy, remain rare due to the fast nonradiative relaxation process induced by exciton-vibration coupling. Here, a diketopyrrolopyrrole-derived photosensitizer (DTPA-S) is developed via thionation of carbonyl groups within the diketopyrrolopyrrole skeleton, which results in a large bathochromic shift of 81 nm, endowing the photosensitizer with strong NIR absorption at 712 nm. DTPA-S is then introduced with a functional biomolecule (N3-PEG2000-RGD) via click reaction for the construction of integrin αvβ3 receptor-targeted nano-micelles (NanoDTPA-S/RGD), which endows the photosensitizer with a further superlarge absorption redshift of 138 nm, thus extending the absorption maxima to ≈850 nm. Remarkably, thiocarbonyl substitution increases the nonbonding characters in frontier molecular orbitals, which can effectively suppress the nonradiative vibrational relaxation process via reducing the reorganization energy, enabling efficient reactive oxygen species (ROS) generation under 880 nm excitation. Screened by in vitro and in vivo assays, NanoDTPA-S/RGD with high water solubility, excellent tumor-targeting ability, and photodynamic/photothermal therapy synergistic effect exhibits satisfactory phototherapeutic performance. Overall, this study demonstrates a new design of efficient NIR-triggered diketopyrrolopyrrole photosensitizer with facile installation of functional biomolecules for potential clinical applications.

(Invited) Ultrafast Excited Electron Relaxation in Triply Fused Ni(II) Porphyrin-Nanographene Conjugates for Energy Conversion

Porphyrins are appealing building blocks for solar energy applications, although their photosensitization capability is limited by their large optical energy gap, resulting in a mismatch in absorption for efficient harvesting of the solar spectrum. Porphyrin π-extension by edge-fusing with nanographenes can be employed to narrow their optical energy gap, enabling the development of porphyrin-based panchromatic dyes with an optimized energy onset for solar energy conversion in dye-sensitized solar fuel and solar cell configurations. Metalloporphyrin-nanographene conjugates based on open-shell transition metals, such as Ni(II), exhibit panchromatic absporption covering the entire visible and part of the near-IR spectral range. Dispite this interesting property, they exhibit fast excited-state electronic relaxation across the Ni d levels, which hampers harnessing photoexcited electrons to trigger photophysical or photochemical reactions. In this work, we shed light into the underlying mechanism behind fast non-radiative relaxation in these systems. Variable temperature transient absorption and global fit analysis are combined with time-dependent density functional theory to produce a picture of the relevant excited states and underlying relaxation pathways in the conjugates. At room temperature, photoexcitation to the lowest S1* energy level is followed by vibrational cooling in 1-2 ps, and intersystem crossing in > 10 ps, setting a short temporal window wherein a small fraction of relaxed singlets decay to the ground state. Following intersystem crossing and triplet internal conversion, triplets relax rapidly to the ground state (S0) in few tens of picoseconds. By performing measurements at low temperature, we provide evidences for the interplay of a lowest terminal triplet level with > 1 ns lifetime which acts as a bottleneck, and a sloped conical intersection to S0* which is thermally accessible at room temperature from the former energy level. The overall triplet decay is dictated by the contributions that fall along these two photophysical branches. This observation bears significance in understanding how nanographene functionalization leads to a profound effect, opening avenues for potential applications in Optoelectronics and related fields.[1] Garcia-Orrit, S., Vega-Mayoral, V., Chen, Q., Serra, G., Paternò, G. M., Cánovas, E., Narita, A., Müllen, K., Tommasini, M., Cabanillas-González, J., Nanographene-Based Decoration as a Panchromatic Antenna for Metalloporphyrin Conjugates. Small 2023, 19, 2301596.[2] Garcia-Orrit, S. et al., in preparation Figure 1

Scaling Law for Kasha's Rule in Photoexcited Molecular Aggregates.

We study the photophysics of molecular aggregates from a quantum optics perspective, with emphasis on deriving scaling laws for the fast nonradiative relaxation of collective electronic excitations, referred to as Kasha's rule. Aggregates exhibit an energetically broad manifold of collective states with delocalized electronic excitations originating from near-field dipole-dipole exchanges between neighboring monomers. Photoexcitation at optical wavelengths, much larger than the monomer-monomer average separation, addresses almost exclusively symmetric collective states, which for an arrangement known as H-aggregate show an upward hypsochromic shift. The extremely fast subsequent nonradiative relaxation via intramolecular vibrational modes populates lower energy, subradiant states, resulting in effective inhibition of fluorescence. Our analytical treatment allows for the derivation of an approximate scaling law of this relaxation process, linear in the number of available low-energy vibrational modes and directly proportional to the dipole-dipole interaction strength between neighboring monomers.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex.

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Design of Near-Infrared-Triggered Metallo-Photosensitizers via a Self-Assembly-Induced Vibronic Decoupling Strategy.

Development of highly efficient near-infrared (NIR)-excited photosensitizers is hampered by the fast nonradiative vibrational relaxation process regulated by the energy gap law. Here, from the fundamental perspective we propose that the intermolecular coupling of well-designed photosensitizers has the potential to facilitate exciton delocalization and hence reduce the exciton-vibration coupling, thereby boosting their phototherapeutic efficacy via inhibition of the vibrational relaxation pathway. Such conceived NIR-excited metallo-photosensitizers (IrHA1 and IrHA2) were prepared and studied for experimental validation. The resulting iridium complexes exhibited a little singlet oxygen (1O2) production in the monomeric state, but produced substantially enhanced 1O2 generation efficiency via benefiting from the exciton-vibration decoupling in the self-assembly state. Particularly, IrHA2 exhibits an unprecedented high 1O2 quantum yield of 54.9% (FDA-approved NIR dye indocyanine green: ΦΔ = 0.2%) under 808 nm laser irradiation with negligible heat generation, probably attributed to the suppression of vibronic couplings from the stretching mode of the acceptor ligand. In phototherapy, IrHA2-NPs with high biocompatibility and low dark toxicity can induce substantial tumor regression with 92.9% tumor volume reduction in vivo. This self-assembly-induced vibronic decoupling strategy would offer an effective approach to the design of high-performance NIR-excited photosensitizers.

Effect of Dopants on Laser-Induced Damage Threshold of ZnGeP2

The effect of doping Mg, Se, and Ca by diffusion into ZnGeP2 on the optical damage threshold at a wavelength of 2.1 μm has been studied. It has been shown that diffusion-doping with Mg and Se leads to an increase in the laser-induced damage threshold (LIDT) of a single crystal (monocrystal), ZnGeP2; upon annealing at a temperature of 750 °C, the damage threshold of samples doped with Mg and Se increases by 31% and 21% from 2.2 ± 0.1 J/cm2 to 2.9 ± 0.1 and 2.7 ± 0.1 J/cm2, respectively. When ZnGeP2 is doped with Ca, the opposite trend is observed. It has been suggested that the changes in the LIDT depending on the introduced impurity by diffusion can be explained by the creation of additional energy dissipation channels due to the processes of radiative and fast non-radiative relaxation through impurity energy levels, which further requires experimental confirmation.

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Probing the Spatial Heterogeneity of Carrier Relaxation Dynamics in CH3NH3PbI3 Perovskite Thin Films with Femtosecond Time‐Resolved Nonlinear Optical Microscopy

AbstractThe spatial heterogeneity of carrier dynamics in polycrystalline metal halide perovskite (MHP) thin films has a strong influence on photovoltaic device performance; however, the underlying cause is not yet clearly understood. Here, the sub‐micrometer scale mapping of charge carrier dynamics in CH3NH3PbI3 thin films using time‐resolved nonlinear optical microscopy, specifically transient absorption microscopy (TAM) with sub‐picosecond (ps) and time‐resolved photoluminescence (PL) microscopy with nanosecond temporal resolution is reported. To study the influence of physical morphology on charge carrier dynamics, MHP thin films having granular‐ and fibrous structures are investigated. On both types of films, spatial regions with short‐lived transient gain signals (fast nonradiative relaxation within ≈1 ps) typically show slower charge recombination via radiative relaxation, which is attributed to the presence of additional energy states near the band edge. In addition, fibrous films show longer PL lifetimes. Interestingly, the functional contrast shown in TAM images exhibits fundamental differences from the structural contrast shown in scanning electron microscopy images, implying that the variation of trap density in the bulk contributes to the observed spatial heterogeneity in carrier dynamics.

Electronic Structure and Energy Transfer in Europium(III)-Ciprofloxacin Complexes: A Theoretical Study.

The structure and ligand-localized excited states of [Eu(cfqH) (cfq)(H2O)4]Cl2 (cfqH is ciprofloxacin) are studied by XMCQDPT2/CASSCF with full geometry optimization. The complex includes one anionic and one zwitterionic ligand. Two low-lying triplet states, both localized on the anionic ligand, are found. One of them has sufficient energy to transfer to the (5)D1 sublevel of Eu(3+), because its T-S0 vertical transition energy is equal (or very close) to the (7)F0-(5)D1 Eu(3+) excitation energy. The other triplet state has a very small S0-T1 gap, which favors fast nonradiative relaxation. Two other triplet states are localized on the zwitterionic ligand. One low-lying excited singlet state (S1) is localized on the anionic ligand; the other excited singlet is localized on the zwitterionic one. Spin-orbit coupling constants were calculated for the relaxed geometry of each state (ground state, two low-lying triplets, and one low-lying excited singlet) by spin-orbit configuration interaction (CI) with Pauli-Breit Hamiltonian. Large spin-orbit coupling constants between S1 and both triplets together with small energy gaps are indicative of fast intersystem crossing (ISC) from the excited singlet state to the triplet manifold. This ISC process is followed by energy transfer from the ligand-localized triplet states to the (5)D1 sublevel of Eu(3+). However, relatively large spin-orbit coupling constants between S0 and one of the triplet states together with the small T-S0 energy gap shows that this state can decay without transferring its energy to Eu(3+). This mechanism is expected to be common for other Ln(3+)-fluoroquinolone complexes.

Two blinking mechanisms in highly confined AgInS2 and AgInS2/ZnS quantum dots evaluated by single particle spectroscopy.

Ternary AgInS2 quantum dots (QDs) have been found as promising cadmium-free, red-shifted, and tunable luminescent bio-probes with efficient Stokes and anti-Stokes excitations and luminescence lifetimes (ca. 100 ns) convenient for time resolved techniques like fluorescence life-time imaging. Although the spectral properties of the AgInS2 QDs are encouraging, the complex recombination kinetics in the QDs being still far from understood, limits their full utility. In this paper we report on a model describing the recombination pathways responsible for large deviations from the first-order decay law observed commonly in the ternary chalcogenides. The presented results were evaluated by means of individual AgInS2 QD spectroscopy aided by first principles calculations including the electronic structure and structural reconstruction of the QDs. Special attention was devoted to study the impact of the surface charge state on the excited state relaxation and effect of its passivation by Zn(2+) ion alloying. Two different blinking mechanisms related to defect-assisted charge imbalance in the QD responsible for fast non-radiative relaxation of the excited states as well as surface recharging of the QD were found as the major causes of deviations from the first-order decay law. Careful optimization of the AgInS2 QDs would help to fabricate new red-shifted and tunable fluorescent bio-probes characterized by low-toxicity, high quantum yield, long luminescence lifetime, and time stability, leading to many novel in vitro and in vivo applications based on fluorescence lifetime imaging (FLIM) and time-gated detection.

On the photostability of scytonemin, analogues thereof and their monomeric counterparts.

As a part of their sun-protective strategy, cyanobacteria produce the natural UV-screener scytonemin. Its accumulation in the extracellular sheaths allows the bacteria to thrive in inhospitable locations highly exposed to solar radiation. Scytonemin is often referred to as photostable and has been reported to be non-fluorescent. Taken together, these properties indicate inherently fast non-radiative relaxation processes. Despite these interesting traits, the photophysics of scytonemin is as yet almost completely unexplored. In this study, we have compared the steady-state photophysics of scytonemin itself and four derivatives thereof. Furthermore, the in vitro photostability of scytonemin was studied in different solvents using a solar simulation system. Scytonemin and the investigated derivatives demonstrated a more rapid photoinduced decay in comparison with two commercial UV-screening agents. The photostability could be modulated by varying the solvent, with the protic solvent ethanol providing the most stabilizing environment.

Modification of Photon Emission Statistics from Single Colloidal CdSe Quantum Dots by Conductive Materials

Colloidal CdSe/ZnS core/shell quantum dots (QDs) can be used to generate single photons at room temperature for promising applications in quantum information sciences. Despite the existence of biexciton (BX) states, they would undergo fast nonradiative Auger relaxation (AR), leading to pure single-exciton (SX) emission, even under high-power excitation. In this work, the fluorescence properties of single colloidal QDs deposited on the conductive ITO surface have been investigated by combining photon correlation spectroscopy and time-tagged, time-correlated single-photon counting (TT-TCSPC) measurement. The conductive materials can introduce extra nonradiative (NR) processes, and thus can significantly alter pristine emission properties, including emission brightness, fluorescence lifetime, and photon emission statistics. We found that such a NR process can strongly quench SX emission but has less effect on BX emission, leading to the increase of the quantum-yield (QY) ratio between BX and SX emission. As ...

Picosecond non-radiative relaxation via internal conversion in Ca(PO)F:Cr

We use a femtosecond pump-probe technique to directly measure fast non-radiative relaxation following excitation of the AT transition in Ca(PO)F(CFAP) doped with tetravalent chromium. We measure a T relaxation time of 137 ps at 10 K which decreases to 37 ps at room temperature through internal conversion directly to the A ground state. We find rates for quantum mechanical tunnelling and phonon assisted barrier hopping of Hz and Hz respectively, with a barrier height of 630 cm.

Experimental Investigations and Modeling of Auger Recombination in Silicon Nanocrystals

The Auger process provides one of the most important nonradiative recombination channels in semiconductors and strongly enhances in nanostructures. In this paper, we investigate Auger recombination for silicon nanocrystals embedded in a SiO2 matrix by transient-induced absorption. Past experience showed that such investigations are very difficult since, in contrast to nanocrystals of direct bandgap materials, exciton dynamics in silicon nanocrystals are dominated by efficient trapping and/or other fast nonradiative relaxation processes. Using a femtosecond pump–probe technique, we separate these processes and assign them to (i) very efficient nonradiative recombination in so-called “dark nanocrystals” and (ii) formation of a self-trapped excitonic state. Subsequently, we successfully measure the dynamics of Auger interaction between excitons and model this process in terms of exciton–exciton and 3-charge interaction. We find that the 3-charge interaction model provides a suitable framework to describe Auger recombination in silicon NCs.

A Comparative Analysis of Ytterbium-Erbium Media for 1.5 μm Lasers

To increase efficiency of continuous 1.5 μm laser radiation crystal laser media should be used instead of the glass ones. Generation efficiency in major laser crystals with ytterbium and erbium is at a disadvantage in relation to phosphate glasses. A cause of this phenomenon is a reverse energy transfer. Could be found crystals comparable in efficiency with Yb-Er phosphate glasses? At present a number of investigations on the use of ions relaxators Се 3+ in crystals Ca2Al2Si07 with ytterbium and erbium have been carried out to solve the problem of the reverse energy transfer in crystals [1,2]. It has been determined [1-3] that high content of cerium is needed to depress the reverse energy transfer. But cerium and ytterbium are in different ends of a lanthanide series, so silicate crystals with more isomorphic capacity such as Yb,Er,Ce:CaGdSiO (CGCS) have been proposed [3]. A sufficient solubility of Се 3+ ions in these crystals gives fast excitation relaxation on a laser level and guards against the reverse energy transfer from Er3+ to donor ions. Furthermore, a number of crystal media on the base of borate crystals with a developed phonon spectrum have been proposed [4,5]. They lack the reverse energy transfer, fast multiphonon nonradiative relaxation shunts the 4I11/2 – 4I13/2 Er3+ transfer. New crystals on the base of anhydrous borates – calcium-yttrium oxyorthoborates - Ca4YO(BO3)3 (YCOB) with ytterbium appeared recently [6], which showed high efficiency of generation, as well as calcium-barium fluor-orthoborates YbEr:Ca BaFBO3 (CBFB), in which ytterbium ions formed centers of luminescence with excellent characteristics [6]. Unfortunately, erbium ions in YCOB are characterized by high three level parameter [6], therefore the efficiency of generation Er,Yb:YCOB is not high. It has been determined recently that erbium ions in crystals CBFB have outstanding spectroscopic parameters, besides, between ytterbium and erbium ions fast energy transfer of electronic excitation takes place [7]. Since the efficiency of a sensitized laser medium depends on joint characteristics of the Yb and Er centres and the efficiency of their interactions, then available data allow considering crystals CBFB as a considerably promising matrix for an effective 1.5 μm laser. In order to obtain the most promising laser crystals and to answer the question cited above, it should be made their comparative analysis on basic spectroscopic and generative parameters. One of the important parameters related directly to the efficiency of the laser medium is an energy density of the generation threshold. Given spectroscopic parameters of active centers in crystals, the generation threshold of the one-activated laser medium (in an idealized model without passive losses) with a tree-level or quasi-four-level scheme of generation is specified by properties of active centers and can be easily determined by a simple formula [8]. The results obtained by this method are often used for determination of limiting parameters of the three-level laser media, the ytterbium media, for instance, and their comparative analysis [8]. There are known calculations of sensitized media generation parameters, among them the ytterbium-erbium glasses with a tube pumping [9], which although can be used for a diode pumping, nonetheless they are too unwieldy for simple estimation and comparative analysis. Hence there is a demand for simple analytical expressions such as [8] to calculate the lower limit of generation of the sensitized two-activated laser media with a diode pumping. The work reports the growing of single crystals CBFB and YCOB doped with erbium and ytterbium with erbium, and single crystals Yb,Er,Ce:CGS as well. We have made a comparative analysis of an ytterbium-erbium media for 1.5 μm lasers with a diode pumping on the base of simple expressions for limiting generation thresholds of the idealized sensitized media with a three-level scheme of generation, obtained in the approximation of balance equations without considering nonlinear and cumulative processes.

Spectroscopy of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) attached to rare gas samples: Clusters vs. bulk matrices. II. Fluorescence emission spectroscopy

The interaction between 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) molecules and solid rare gas samples is studied by means of fluorescence emission spectroscopy. Laser-excited PTCDA-doped large argon, neon, and para-hydrogen clusters along with PTCDA embedded in helium nanodroplets are spectroscopically characterized with respect to line broadening and shifting. A fast non-radiative relaxation is observed before a radiative decay in the electronic ground state takes place. In comparison, fluorescence emission studies of PTCDA embedded in bulk neon and argon matrices result in much more complex spectral signatures characterized by a splitting of the different emission lines. These can be assigned to the appearance of site isomers of the surrounding matrix lattice structure.

New Heteroleptic Bis-Phenanthroline Copper(I) Complexes with Dipyridophenazine or Imidazole Fused Phenanthroline Ligands: Spectral, Electrochemical, and Quantum Chemical Studies

Two new sterically challenged diimine ligands L(1) (2,9-dimesityl-2-(4'-bromophenyl)imidazo[4,5-f][1,10]phenanthroline) and L(2) (3,6-di-n-butyl-11-bromodipyrido[3,2-a:2',3'-c]phenazine) have been synthesized with the aim to build original heteroleptic copper(I) complexes, following the HETPHEN concept developed by Schmittel and co-workers. The structure of L(1) is based on a phen-imidazole molecular core, derivatized by two highly bulky mesityl groups in positions 2 and 9 of the phenanthroline cavity, preventing the formation of a homoleptic species, while L(2) is a dppz derivative, bearing n-butyl chains in α positions of the chelating nitrogen atoms. The unambiguous formation of six novel heteroleptic copper(I) complexes based on L(1), L(2), and complementary matching ligands (2,9-R(2)-1,10-phenanthroline, with R = H, methyl, n-butyl or mesityl) has been evidenced, and the resulting compounds were fully characterized. The electronic absorption spectra of all complexes fits well with DFT calculations allowing the assignment of the main transitions. The characteristics of the emissive excited state were investigated in different solvents using time-resolved single photon counting and transient absorption spectroscopy. The complexes with ligand L(2), bearing a characteristic dppz moiety, exhibit a very low energy excited-state which mainly leads to fast nonradiative relaxation, whereas the emission lifetime is higher for those containing the bulky ligand L(1). For example, a luminescence quantum yield of about 3 × 10(-4) is obtained with a decay time of about 50 ns for C2 ([Cu(I)(nBu-phen)(L(1))](+)) with a weak influence of strong coordinating solvent on the luminescence properties. Overall, the spectral features are those expected for a highly constrained coordination cage. Yet, the complexes are stable in solution, partly due to the beneficial π stacking between mesityl groups and vicinal phenanthroline aromatic rings, as evidenced by the X-ray structure of complex C3 ([Cu(I)(Mes-phen)(L(2))](+)). Electrochemistry of the copper(I) complexes revealed reversible anodic behavior, corresponding to a copper(I) to copper(II) transition. The half wave potentials increase with the steric bulk at the level of the copper(I) ion, reaching a value as high as 1 V vs SCE, with the assistance of ligand induced electronic effects. L(1) and L(2) are further end-capped by a bromo functionality. A Suzuki cross-coupling reaction was directly performed on the complexes, in spite of the handicapping lability of copper(I)-phenanthroline complexes.

Determination of Judd–Ofelt intensity parameters from the excitation spectra for rare-earth doped luminescent materials

By utilizing the proportional relationship between the excitation and absorption spectra for some special excited multiplets of rare-earth (RE) ions that are followed by a very fast nonradiative relaxation to the monitored level, we propose a new approach to determine the Judd-Ofelt (JO) intensity parameters that are crucial to the evaluation of laser and luminescent materials via excitation spectra. To validate this approach, the JO parameters of NaGd(WO(4))(2) : Er(3+) and YLiF(4) : Nd(3+) crystals are calculated and compared through both the excitation and absorption spectra. The JO parameters derived from this approach are in good agreement with that determined from the conventional method (absorption spectra). Furthermore, the JO intensity parameters of Y(2)O(3) : Er(3+) nanocrystals are derived from the excitation spectra by taking into account the nano-size effects, which are comparable to the values of the crystal counterpart. The proposed approach is of particular importance for those powders or nanophosphors with low RE doping concentration that their quantitative absorption spectra are difficult to measure.

Optical-phonon scattering and theory of magneto-polarons in a quantum well in a strong magnetic field

We report a theoretical study of the carrier relaxation in a quantum cascade laser (QCL) subjected to a strong magnetic field. Both the alloy (GaInAs) disorder effects and the Frohlich interaction are taken into account when the electron energy differences are tuned to the longitudinal optical (LO) phonon energy. In the weak electron-phonon coupling regime, a Fermi's golden rule computation of LO phonon scattering rates shows a very fast non-radiative relaxation channel for the alloy broadened Landau levels (LL's). In the strong electron-phonon coupling regime, we use a magneto-polaron formalism and compute the electron survival probabilities in the upper LL's with including increasing numbers of LO phonon modes for a large number of alloy disorder configurations. Our results predict a nonexponential decay of the upper level population once electrons are injected in this state.

Bright and dark excitons in semiconductor carbon nanotubes: insights from electronic structure calculations

We review electronic structure calculations of finite-length semiconducting carbon nanotubes using time-dependent density functional theory (TD-DFT) and the time dependent Hartree-Fock (TD-HF) approach coupled with semi-empirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and AM1 calculations overbind excitons, inaccurately predicting the lowest energy state as a bright exciton. Changing the AM1 with the ZINDO Hamiltonian in TD-HF calculations predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functionals with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands: the lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ( approximately 0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leading to the rapid population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.