Unusual temperature-dependent optical properties of self-organized InAs/GaAs quantum dots at high excitation power

The temperature-dependent photoluminescence (PL) properties of InAs/GaAs self-organized quantum dots (QDs) have been investigated at high excitation power. The fast redshift of the ground-state and the first excited-state PL energy with increasing temperature was observed. The temperature-dependent linewidth of the QD ground state with high carrier density is different from that with low carrier density. Furthermore, we observed an increasing PL intensity of the first excited state of QDs with respect to that of the ground state and demonstrate a local equilibrium distribution of carriers between the ground state and the first excited state for the QD ensemble at high temperature (T > 80 K). These results provide evidence for the slowdown of carrier relaxation from the first excited state to the ground state in InAs/GaAs quantum dots.

Self-Heating Effect on the Two-State Lasing Behaviors in 1.3-mu m InAs-GaAs Quantum-Dot Lasers

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-mu m self-assembled InAs-GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light-current (L-I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.

Study of multi-quasiparticle excited states in Ce-139

High-spin states in Ce-139 have been populated using the Te-130(C-12, 3n) reaction at beam energy of 50MeV. The level scheme of Ce-139 has been revised and extended greatly up to E=5765.0keV. The level structure of Ce-139 shows typical characteristics of spherical nucleus, and the high-spin states were formed by the excitations of valence nucleons. Energies of the yrast and near yrast high-spin states in Ce-139 have been calculated by the empirical shell model, and the multi-quasiparticle nature of high-spin excited states has been discussed.

Solution-processible multi-component cyclometalated iridium phosphors for high-efficiency orange-emitting OLEDs and their potential use as white light sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole-transporting carbazole and fluorene-based 2-phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the pi-conjugation through incorporation of electron- pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest oc- cupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium-based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo- and electroluminescence properties of these phosphorescent carbazolylfluorene-functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light-emitting diodes (OLEDs) using these complexes as the solution-processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole-transporting layer.I These orange-emitting devices can produce a maximum current efficiency of similar to 30 cd A(-1) corresponding to an external quantum efficiency of similar to 10 % ph/el (photons per electron) and a power efficiency of similar to 14 Im W-1.

VUV-UV excited luminescent properties of calcium borophosphate doped with rare earth ions

The VUV-UV spectra of rare earth ions activated calcium borophosphate, CaBPO5:RE (RE = Ce3+, sm(3+), Eu2+, Eu3+, Tb3+ and Dy3+) were determined. The bands at about 155 nm in the VUV excitation spectra are attributed to the host lattice absorptions. The bands at 166 and 190 nm for the sample CaBPO5:Sm have been considered as related to the f-d transition and the charge transfer band (CTB) of Sm3+ ions, and the band at 169 nm for the sample CaBPO5:Dy is assumed to be connected with the f-d transition of the Dy3+ ions in CaBPO5. The partial reduction of Eu3+ CaBPO5:Eu prepared by high temperature solid state reaction in air is confirmed by the VUV-UV spectra.

Calculations of cross sections of resonant two-photon transitions to the second excited 4f5d state in Pr3+: Y3Al5O12 using density matrix theory

The density matrix resonant two-photon absorption (TPA) theory applicable to laser crystals doped with rare earth ions is described. Using this theory, resonant TPA cross sections for transitions from the ground state to the second excited state of the 4f5d configuration in cm(4)s Pr3+:Y3Al5O12 are calculated. The peak value of TPA cross section calculated is 2.75 x 10(-50) cm(4)s which is very close to the previous experimental value 4 x 10(-50) cm(4) s. The good agreement of calculated data with measured values demonstrates that the density matrix resonant TPA theory can predict resonant TPA intensity much better than the standard second-order perturbation TPA theory.

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally. (c) 2005 American Institute of Physics.

Experimental study of quantum dot and dash lasers

Quantum dashes are elongated quantum dots. Polarized edge-photovoltage and spontaneous emission spectroscopy are used to study the anisotropy of optical properties in 1.5μm InGaAsP and AlGaInAs-based quantum dash lasers. Strain, which causes TM-polarized transitions to be suppressed at the band edge, coupled with carrier confinement and dash shape leads to an enhancement of the optical properties for light polarized along the dash long axis, in excellent agreement with theoretical results. An analysis of the integrated facet and spontaneous emission rate with total current and temperature reveals that, in both undoped and p-doped InGaAsP-based quantum dash lasers at room temperature, the threshold current and its temperature dependence remain dominated by Auger recombination. We also identify two processes which can limit the output power and propose that the effects of the dopant in p-doped InGaAsP-based lasers dominate at low temperature but decrease with increasing temperature. A high threshold current density in undoped AlGaInAs-based quantum dash laser samples studied, which degrade rapidly at low temperature, is not due to intrinsic carrier recombination processes. 1.3μm GaAs-based quantum dots lasers have been widely studied, but there remains issues as to the nature of the electronic structure. Polarized edge-photovoltage spectroscopy is used to investigate the energy distribution and nature of the energy states in InAs/GaAs quantum dot material. A non-negligible TM-polarized transition, which is often neglected in calculations and analyses, is measured close to the main TE-polarized ground state transition. Theory is in very good agreement with the experimental results and indicates that the measured low-energy TM-polarized transition is due to the strong spatial overlap between the ground state electron and the light-hole component of a low-lying excited hole state. Further calculations suggest that the TM-polarized transition reduces at the band edge as the quantum dot aspect ratio decreases.

Spectroscopic measurements of phase-resolved electron energy distribution functions in RF-excited discharges

The reliable measurement of the electron energy distribution function (EEDF) of plasmas is one of the most important subjects of plasma diagnostics, because this piece of information is the key to understand basic discharge mechanisms. Specific problems arise in the case of RF-excited plasmas, since the properties of electrons are subject to changes on a nanosecond time scale and show pronounced spatial anisotropy. We report on a novel spectroscopic method for phase- and space-resolved measurements of the electron energy distribution function of energetic (> 12 eV) electrons in RF discharges. These electrons dominate excitation and ionization processes and are therefore of particular interest. The technique is based on time-dependent measurements during the RF cycle of excited-state populations of rare gases admixed in small fractions. These measurements yield � in combination with an analytical model � detailed information on the excitation processes. Phase-resolved optical emission spectroscopy allows us to overcome the difficulties connected with the very low densities (107�109 cm�3) and the transient character of the electrons in the sheath region. The EEDF of electrons accelerated in the sheath region can be described by a shifted Maxwellian with a drift velocity component in direction of the electric field. The method yields the high-energy tail of the EEDF on an absolute scale. The applicability of the method is demonstrated at a capacitively coupled RF discharge in hydrogen.

The high-resolution infrared spectrum of HOF near 2700 cm-1: the ground and 2v2 states

The a/b hybrid-type ν1 fundamental and 2ν2 overtone bands of HOF were investigated by FTIR spectroscopy with a resolution close to 0.008 cm−1. Improved ground state parameters of HOF were determined from a merge of more than 3000 ground state combination differences formed from ν1 and previously measured ν2 transitions with the reported pure rotational lines. Excited state parameters of the v2 = 2 state, ν0 = 2686.924 6(1) and χ22 = −9.942 4(1) cm−1, were determined employing Watson's A-reduced Hamiltonian up to sixth order in I′ representation. The 2ν2 state was found to be unperturbed, the excited state parameters being closely related to those of ν2.

Syntheses, x-ray structures, photochemistry, redox properties, and DFT calculations of interconvertible fac- and mer-[Mn(SPS)(CO)3] isomers containing a flexible SPS-based pincer ligand

The lithium salt of the anionic SPS pincer ligand composed of a central hypervalent lambda(4)-phosphinine ring bearing two ortho-positioned diphenylphosphine sulfide side arms reacts with [Mn(CO)(5)Br] to give fac-[Mn(SPS)(CO)(3)], This isomer can be converted photochemicaily to mer-[Mn(SPS)(CO)(3)], with a very high quantum yield (0.80 +/- 0.05). The thermal backreaction is slow (taking ca. 8 h at room temperature), in contrast to rapid electrodecatalyzed mer-to-fac isomerization triggered by electrochemical reduction of mer-[Mn(SPS)(CO)(3)]. Both geometric isomers of [Mn(SPS)(CO)(3)] have been characterized by X-ray crystallography. Both isomers show luminescence from a low-lying (IL)-I-3 (SPS-based) excited state. The light emission of fac-[Mn(SPS)(CO)(3)] is largely quenched by the efficient photoisomerization occurring probably from a low-lying Mn-CO dissociative excited state. Density functional theory (DFT) and time-dependent DFT calculations describe the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of fac- and mer-[Mn(CO)(3)(SPS)] as ligand-centered orbitals, largely localized on the phosphinine ring of the SPS pincer ligand. In line with the ligand nature of its frontier orbitals, fac-[Mn(SPS)(CO)(3)] is electrochemically reversibly oxidized and reduced to the corresponding radical cation and anion, respectively. The spectroscopic (electron paramagnetic resonance, IR, and UV-vis) characterization of the radical species provides other evidence for the localization of the redox steps on the SIPS ligand. The smaller HOMO-LUMO energy difference in the case of mer-[Mn(CO)(3)(SPS)], reflected in the electronic absorption and emission spectra, corresponds with its lower oxidation potential compared to that of the fac isomer. The thermodynamic instability of mer-[Mn(CO)(3)(SPS)], confirmed by the DFT calculations, increases upon one-electron reduction and oxidation of the complex.

Excited states of nitro-polypyridine metal complexes and their ultrafast decay. Time-resolved IR absorption, spectroelectrochemistry, and TD-DFT calculations of fac-[Re(Cl)(CO)3(5-Nitro-1,10-phenanthroline)]

The lowest absorption band of fac-[Re(Cl)(CO)(3)(5-NO2-phen)] encompasses two close-lying MLCT transitions. The lower one is directed to LUMO, which is heavily localized on the NO2 group. The UV-vis absorption spectrum is well accounted for by TD-DFT (G03/PBEPBE1/CPCM), provided that the solvent, MeCN, is included in the calculations. Near-UV excitation of fac-[Re(Cl)(CO)(3)(5-NO2-phen)] populates a triplet metal to ligand charge-transfer excited state, (MLCT)-M-3, that was characterized by picosecond time-resolved IR spectroscopy. Large positive shifts of the v(CO) bands upon excitation (+70 cm(-1) for the A'(1) band) signify a very large charge separation between the Re(Cl)(CO)3 unit and the 5-NO2-phen ligand. Details of the excited-state character are revealed by TD-DFT calculated changes of electron density distribution. Experimental excited-state v(CO) wavenumbers agree well with those calculated by DFT. The (MLCT)-M-3 state decays with a ca. 10 ps lifetime (in MeCN) into another transient species, that was identified by TRIR and TD-DFT calculations as an intraligand (3)n pi* excited state, whereby the electron density is excited from the NO2 oxygen lone pairs to the pi* system of 5-NO2-phen. This state is short-lived, decaying to the ground state with a similar to 30 ps lifetime. The presence of an n pi* state seems to be the main factor responsible for the lack of emission and the very short lifetimes of 3 MLCT states seen in all d(6)-metal complexes of nitro-polypyridyl ligands. Localization of the excited electron density in the lowest (MLCT)-M-3 states parallels localization of the extra electron in the reduced state that is characterized by a very small negative shift of the v(CO) IR bands (-6 cm(-1) for A'(1)) but a large downward shift of the v(s)(NO2) IR band. The Re-Cl bond is unusually stable toward reduction, whereas the Cl ligand is readily substituted upon oxidation.

Phosphorescence quantum yield enhanced by intermolecular hydrogen bonds in Cu4I4 clusters in the solid state

Organo-copper(I) halide complexes with a Cu4I4 cubane core and cyclic amines as ligands have been synthesized and their crystal structures have been defined. Their solid state photophysical properties have been measured and correlated with the crystal structure and packing. A unique and remarkably high luminescence quantum yield (76%) has been measured for one of the complexes having the cubane clusters arranged in a columnar structure and held together by N–HI hydrogen bonds. This high luminescence quantum yield is correlated with a slow radiationless deactivation rate of the excited state and suggests a rather strong enhancement of the cubane core rigidity bestowed by the hydrogen bond pattern. Some preliminary thin film deposition experiments show that these compounds could be considered to be good candidates for applications in electroluminescent devices because of their bright luminescence, low cost and relatively easy synthesis processes

Measurement of Rydberg state lifetimes using cold trapped neutral atoms

In this paper, I review some recent high-precision Rydberg state lifetime measurements using a cold-trapped sample of neutral atoms held in a magneto-optical trap. The measurements were performed in rubidium for the S, P and D states varying the principal quantum number from n = 26 to 45 using the field ionization technique. The experimental results were compared with quantum mechanical calculations and good agreement was observed. This is an important demonstration of how cold atomic samples can be used to perform high-precision spectroscopy in the time domain.