Journal of Luminescence

Published by Elsevier
Online ISSN: 0022-2313
Publications
Article
Multidimensional vibrational echo correlation spectroscopy with full phase resolution is used to measure hydrogen bond dynamics in water and methanol. The OD hydroxyl stretches of methanol-OD oligomers in CCl(4) and HOD in H(2)O are studied using the shortest mid-IR pulses (<50 fs, <4 cycles of light) produced to date. The pulses have sufficient spectral bandwidth to span the very broad (>400 cm(-1)) spectrum of the 0-1 and 1-2 transitions. Hydrogen bond population dynamics are extricated with exceptional detail in MeOD oligomers because the different hydrogen bonded species are spectrally distinct. The experimental results along with detailed calculations indicate the strongest hydrogen bonds are selectively broken through a non-equilibrium relaxation pathway following vibrational relaxation of the hydroxyl stretch. The correlation spectra are also a sensitive probe of the fluctuations in water and provide a stringent test of water models that are widely used in simulations of aqueous systems. The analysis of the 2D band shapes demonstrates that different hydrogen bonded species are subject to distinct (wavelength dependent) ultrafast (~100 fs) local fluctuations and essentially identical slow (0.4 and ~2 ps) structural rearrangements. Observation of wavelength dependent dynamics demonstrates that standard theoretical approaches assuming Gaussian fluctuations cannot adequately describe water dynamics.
 
Article
Transparent glass-ceramic containing rare-earth doped halide nanocrystals exhibits enhanced luminescence performance. In this study, a glass-ceramic with Tb doped gadolinium fluoride nanocrystals embedded in an aluminosilicate glass matrix is investigated for X-ray imaging applications. The nanocrystalline glass-ceramic scintillator was prepared by a melt-quench method followed by an anneal. The GdF3:Tb nanocrystals precipitated within the oxide glass matrix during the processing and their luminescence and scintillation properties were investigated. In this nanocomposite scintillator system, the incorporation of high atomic number Gd compound into the glass matrix increases the X-ray stopping power of the glass scintillator, and effective energy transfer between Gd(3+) and Tb(3+) ions in the nanocrystals enhances the scintillation efficiency.
 
Article
Concentration is a key determining factor in the fluorescence properties of organic fluorophores. We studied self-quenching of disodium fluorescein (uranin) fluorescence in polyvinyl alcohol (PVA) thin films. The concentration dependent changes in brightness and anisotropy were followed by a lifetime decrease. We found that at a concentration of 0.54 M, the lifetime decreases to 7 ps. At a concentration of 0.18 M the lifetime was 10 ps with the relatively high quantum yield of 0.002. In these conditions the fluorescence intensity decay was homogeneous (well approximated by a single lifetime). We realized that such a sample was an ideal fluorescence lifetime standard for spectroscopy and microscopy, and therefore characterized instrument response functions for a time-domain technique. We show that self-quenched uranin enables measurements free of the color effect, making it a superior choice for a lifetime reference over scattered light.
 
Article
We demonstrate that DiI and Rhodamine B, which are not easily distinguishable to one-photon measurements, can be differentiated and in fact quantified in mixture via tailored two-photon excitation pulses found by a genetic algorithm (GA). A nearly three-fold difference in the ratio of two-photon fluorescence of the two dyes is achieved, without a drop in signal of the favored fluorophore. Implementing an acousto-optic interferometer, we were able to prove that the mechanism of discrimination is second-harmonic tuning by the phase-shaped pulses to the relative maxima and minima of these cross-sections.
 
Article
Hesperitin, a ubiquitous bioactive flavonoid abundant in citrus fruits is known to possess antioxidant, anti-carcinogenic, hypolipidemic, vasoprotective and other important therapeutic properties. Here we have explored the interactions of hesperitin with normal human hemoglobin (HbA), using steady state and time resolved fluorescence spectroscopy, far UV circular dicroism (CD) spectroscopy, combined with molecular modeling computations. Specific interaction of the flavonoid with HbA is confirmed from flavonoid-induced static quenching which is evident from steady state fluorescence as well as lifetime data. Both temperature dependent fluorescence measurements and molecular docking studies reveal that apart from hydrogen bonding and van der Waals interactions, electrostatic interactions also play crucial role in hesperitin-HbA interactions. Furthermore, electrostatic surface potential calculations indicate that the hesperitin binding site in HbA is intensely positive due to the presence of several lysine and histidine residues.
 
Conference Paper
Summary form only given. Nonreciprocal effects are caused by breaking of time-inversion symmetry. Nonlinear spectroscopy is an excellent tool to study nonreciprocal effects as, for example, antiferromagnetic (afm) ordering, as was recently shown for Cr<sub>2 </sub>O<sub>3</sub>. The sign of the nonlinear optical susceptibility tensor can be measured by second harmonic generation (SHG) if there is at the same time a SH-contribution from a time invariant susceptibility tensor
 
Conference Paper
We have developed Raman gain spectroscopy with femtosecond resolution. The advantage of the method is as follows: (1) There is no disturbance from fluorescence as in spontaneous Raman scattering; (2) The interference of the signal with nonresonant background does not appear. This is extremely advantageous to time-resolved CARS spectroscopy. We have applied this new method to excitons in polydiacetylene (PDA), polythiophenes, and a polyacetylene derivative with only 1.5 ps lifetime. This produces vibrational spectra with the highest time resolution ever reported. Time-resolved resonance Raman spectroscopy has been recognized as a powerful method for studying structures of transient species and electronic excited states. By applying the new methods to a polydiacetylene (PDA) crystal epitaxially grown on a KCl substrate, we could observe Raman spectrum of the exciton state in PDA with only 1.5 ps lifetime
 
Conference Paper
Summary form only given. While ultrafast semiconductor spectroscopy has investigated scattering times, i.e. the time between different scattering events, for many years, quantum kinetics asks for the duration of one scattering process, i.e. for the memory time. For the electron-LO-phonon quantum kinetics in the weak coupling regime (GaAs), quantum kinetics shows up as an oscillation in the coherent four-wave mixing (FWM) signal. This observation was interpreted as an oscillation of a band electron back and forth between its initial and a final state it scatters into (for long times) due to the emission of n=1 phonon. We discuss the strong electron-phonon coupling regime, in which n>1 phonon scattering processes are expected to become important as well. Bulk ZnSe is used as a model system, in which the Frohlich-constant is about one order of magnitude larger than in GaAs. In our experiments, the band edge of an antireflection-coated, 100 nm thin film of ZnSe at low temperatures is excited with blue, transform-limited 13 fs pulses. Two-pulse, three-pulse FWM-experiments and coherent control experiments are performed and discussed
 
Conference Paper
Although it is considered that the generation mechanism for narrow band-gap semiconductors, such as InAs and InSb, originates from the photo-Dember field and the optical rectification besides carrier acceleration by the depletion field, its mechanism is not well understood. To investigate the mechanism, we applied a magnetic field of up to 5 T to InAs and observed saturation of THz-radiation from femtosecond-laser irradiated InAs in a high magnetic field
 
Article
New evidence is given that two classes (A and B) of the transient IR-absorption bands: (A) with max. at 0.15–0.36 eV (in NaCl : I, NaBr, NaI, KCl : I, KBr : I, RbCl : I, RbBr : I), due to “on-centre self-trapped exciton” and (B) with max. at 0.27–0.36 eV (in NaCl, KCl, KBr, RbCl), due to “shallow trapped electrons” or “bound polarons”, are caused by the same defect—shallow trapped electron (e−) at the substitutional (cation: c-site) alkali impurity cation (M+): [M+]c0e−. The A- and B-class IR bands have the same location, similar shape, half-width (exactly coincide for KCl : I and KCl at 80 or 10 K with the same vibration structure). It is established that the same Mollwo–Ivey plot curves E0=a/dn (d—nn anion–cation distance, n—exponent, a—constant) are fulfilled for both IR band classes, if we plot instead the IR band peak energy values the more definite values of the IR band zero-phonon line energy E0 (for NaCl, KCl, KBr, RbCl and KCl : I) and/or the IR band low-energy edge energy E0 (±0.03 eV) values (for NaBr, NaI, NaCl : I, KBr : I, RbCl : I and RbBr : I). These data are significant additional evidence that the A-class IR bands are caused by the same type defects as the B-class IR bands—by the shallow electron traps, i.e., centres [M+]c0e−. Two types of the [M+]c0e− centres are predominant: (i) [Li+]c0e− in NaX host crystals with E0≈29.4/d4.72 and (ii) [Na+]c0e− in KX and RbX host crystals with the relation E0≈6.15/d2.74.
 
Article
Gallium-doped tin oxide (SnO2:Ga) films have been prepared on α-Al2O3 (0 0 0 1) substrates at 500 °C by the pulse mode metalorganic chemical vapor deposition (MOCVD) method. The relative amount of Ga (Ga/(Ga+Sn) atomic ratio) varied from 3% to 15%. Post-deposition annealing of the films was carried out at different temperatures for 1.5 h in ambient atmosphere . The structural, electrical, optical and photoluminescence (PL) properties of the films have been investigated as a function of annealing temperature. All the films have the rutile structure of pure SnO2 with a strong (2 0 0) preferred orientation. A single ultraviolet (UV) PL peak near 337.83 nm was observed at room temperature for the 3% Ga-doped as-grown film and near 336 nm for the 15%-doped film, which can be ascribed to electron transition from the oxygen vacancy and interstitial Ga3+ donor levels to the acceptor level formed by the substitution of Ga3+ for the Sn site. After annealing, the luminescence spectra have changed a little bit, which is being discussed in detail.
 
Article
CdTe films grown by molecular beam epitaxy on (001) InSb substrates at twelve different substrate temperatures, Ts, are examined by means of low temperature photoluminescence (PL) in the energy range 1.61 eV to 140 eV. Luminescence spectra are reported from three different energy regions and are observed for substrate temperatures from 170°C to 285°C. The spectral regions are as follows: 1.61 eV to 1.56 eV with exciton emission, 1.56 eV to 1.51 eV with free-to-bound (FB) and donor-acceptor pair (DAP) transitions and 1.50 eV to 1.40 eV with as yet unidentified lines and broad band structure. The use of luminescence as a means of assessing film perfection is discussed.
 
Article
Anomalies are found in the near-band-edge luminescence properties of Δ2-light-hole indirect excitons in Si1−yCy-based tensilely strained quantum wells (QWs). The experimental spectra exhibit a clear signature of phonon-assisted transitions on the lower energy side of the “no-phonon” transition, which indicates the relevance of “virtual” indirect valleys and in-plane k-dispersion, as opposed to the theoretical prediction that the zone-centered Δ2 valleys take over the conduction band edge. Intervalley scattering between [0 0 1]-Δ2 valleys and in-plane Δ4 valleys is suggested as the underlying mechanism. On the other hand, the experimental evidence was found for “apparently enhanced” quantum-confined Stark red shifts for Si1−yCy-based QWs. However, quantitative estimates are in conflict with the experimental results and predict a blue shift due to exciton weakening which masks the Stark effect as in the case of Δ4-heavy hole excitons in Si1−xGex-based QWs.
 
Article
Two novel complexes of Sm(III) and Dy(III) with mixed oxydiacetate (ODA) and 1,10-phenanthroline (phen) ligands were synthesized and their structure and luminescence properties were characterized. The complexes of [Ln(ODA)(phen)·4H2O]Cl·5H2O [Ln=Sm and Dy] crystallize in the monoclinic space group P21/n with Sm: a=12.3401(14) Å, b=16.821(2), c=12.6847(11) Å, β=107.939(10)°, V=2505.0(5) Å3, Z=4 and ρ=1.841 mg/m3, and with Dy: a=12.289(7) Å, b=16.805(6) Å, c=12.705(4) Å, β=108.144(18)°, V=2493.4(19) Å3, Z=4 and ρ=1.786 mg/m3. The complexes of [Sm(ODA)(phen)·4H2O]+ and [Dy(ODA)(phen)·4H2O]+ excited by UV light produce orange red and lightly white emissions, respectively, via the nonradiative energy transfer from phen to the metals. The quantum yield of the sensitized luminescence of [Dy(ODA)(phen)·4H2O]+ (Q=19%) is much greater than that of [Sm(ODA)(phen)·4H2O]+ (Q=1.4%). The luminescence decay times of the complexes were in a few microsecond range and independent of temperature.
 
Article
Complexes of Eu(III) with mixed macrocyclic azacrown ethers and 1,10-phenanthroline (phen) were synthesized and their luminescence properties measured. The specific azacrown ethers used were 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate (TETA) and 1,4,8,12-tetraazacyclopentadecane ([15]aneN4). The phen-coordinated complexes excited by UV light produced a very bright red emission via an intra-molecular energy transfer from phen to Eu(III). For [Eu(TETA)·(phen)·(H2O)]− and [Eu([15]aneN4)·(phen)2]3+, the quantum yields of sensitized luminescence were 8.4% and 7.8%, respectively, and were much greater than those from non-sensitized luminescence of 1.2% and 4.4%, respectively. The decay times of the corresponding phen-coordinated complexes, as measured at room temperature, were 1.6 and 0.6 ms, respectively, and were much longer than those of the phen-uncoordinated complexes of 0.3 and 0.2 ms, respectively.
 
Article
This work is concerned with the chemiluminescence (CL) of Nd3+, Yb3+ and Pr3+ β-diketonates in solution. Chemiluminescent reaction of adamantylideneadamantane-1,2-dioxetane (AAD) decomposition generating singlet () and triplet () excited adamantanone was used as a source of excited species. AAD chemiluminescence due to emission from is quenched by Ln3+ β-diketonates: (a) by intermolecular singlet-singlet (S-S) energy transfer from to β-diketonate ligand levels of Ln(TTA)3·2H2O and Ln(BTFA)3·2H2O; (b) by complex formation between AAD and Pr(FOD)3 or Pr(DPM)3. Corresponding Stern–Volmer quenching constants or stability constants of the complex were measured. Chemiluminescence spectra of Ln3+ β-diketonates were recorded and relative luminescence quantum yields compared. Yb3+ chelates show higher luminescence yields compared to Nd3+, due to a different efficiency of non-radiative energy degradation. Chemiexcitation of Ln3+ ions in the systems studied occurs by: (a) intermolecular singlet–singlet energy transfer: (where and are the first singlet and triplet excited states of the β-diketonate ligand); (b) intermolecular triplet–triplet energy transfer: ; (c) intracomplex energy transfer from the decomposition of AAD in the complex with Ln3+ β-diketonate. Efficiency of chemiexcitation pathways is different for each Ln3+ β-diketonate and Ln3+ ion.
 
Article
An intense triboluminescence has been observed from crystals of 1,2,5-triphenylphosphole when these are crushed between hard surfaces. It is likely that this luminescence originates from crystal fluorescence from the matches obtained between the solid state photoluminescence, solution photoluminescence, and triboluminescence spectra, and from solid state and solution photoluminescence lifetime measurements. Crystals of this phosphole are non-centrosymmetric, like many organic triboluminescent materials. Other analogous phosphorus heterocycles have been prepared, and these, plus some parallel heterocyclic and hydrocarbon examples, have also been tested. Whilst many of these were strongly photoluminescent, only 1,2-diphenyl-5-p-chlorophenylphosphole and its 1-oxide were also found to be triboluminescent. The previously reported triboluminescence of triphenylphosphine and triphenylphosphine oxide were found to be dependent on crystallizing solvent and/or other factors.
 
Article
The complexes tris(4,4,4-Trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl) Ln(III), Ln(tan)3bipy, where Ln(III)=Eu3+ and Gd3+ have been synthesized, characterized and their photophysical properties (absorption, excitation and luminescence spectra and emission quantum yield) investigated down to 4.2 K. The Eu(tan)3bipy complex has its molecular structure experimentally determined using X-ray crystallography and theoretically using the SMLC/AM1 method as well as their electronic singlet and triplet states were calculated, using the INDO/S-CI method with a point charge model to represent the Eu3+ ion, where two values were adopted, +3.0e and +3.5e, to investigate the imperfect shielding of the 4f shells. The so calculated +3.5e model electronic absorption spectrum and low lying triplet state energies agreed very well with the experimental ones. The emission quantum yield of the Eu3+ complex is quite low at room temperature, namely 7%, probably due to the too low lying triplet state, 19,050 cm–1, and increases by a factor of three when the temperature is lowered to 4.2 K. This strong thermal effect indicates the presence of a channel deactivating the main emitting state, what can be due to a LMCT state possibly lying in the same spectral region, as usually found in Eu3+ compounds.
 
Article
A series of segmented poly(urethane-urea)s containing 1,3,5 triazine in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers in which the chromophore concentration varied from 4.2% to 18.1%. Although triazine emission is located in the UV region, the films with higher content of the chromophore emitted a visible blue light (425 nm) when excited at the very red-edge of the absorption band. The photophysical properties of the materials were strongly dependent on the relative amount of triazine moieties along the main chain. Isolated moieties emit in copolymers with small amount of triazine groups, indicating that even though in solid state, these moieties tend to be apart. Two photophysical consequences were observed when the amount of triazine increases: there is some energy transfer process involving isolated moieties with consequent decrease of the lifetime and an additional red-edge emission attributed to aggregated lumophores. The mono-exponential decay observed for the isolated form is substituted by a bi-exponential decay of the aggregated species. The materials were not strong emitters, but since the N-containing triazine moieties are good electron transport groups, the polymers have potential application as electron transport enhancers in various applications.
 
Article
In this paper, the formation of exciplexes between 1,4-dicyanobenzene with either benzene, naphthalene or anthracene was studied. The capabilities for exciplexes formation were compared for these three systems and their activation energies were found to be 0.48 Kal/mol, 1.07 Kal/mol and 1.22 Kal/mol respectively. From the fluorescence spectra, activation energies and electrnic cloud density distribution, one can see that in addition to the ionization potentials and electron affinities, the excited molecular electronic cloud density and volume effect also influence exciplexes formation. We conclude that the formation capability of exciplxes with 1,4-dicyanobenzene is the highest for benzene, followed by naphthalene and anthracene; the favorable way of overlapping is the interaction between benzene rings.
 
Article
A Cr-activated material, Ba2TiO4:0.1%Cr, exhibits at room temperature line emission at ∼1.2 μm, with an exponential decay constant of ≈80 μs. At low temperatures the decay constant increases to ≈240 μs, and an intensity redistribution takes place within the main emission doublet, with the high-energy component of the doublet drastically losing intensity. At higher Cr formulations (0.5% and 1%) line emission and band emission are simultaneously present. An interesting discontinuous change is noted in the emission/excitation properties at these higher Cr formulations, namely at 1%Cr a new set of emission lines, to the high-energy side of the original set, appears at low temperatures, and this is accompanied by marked changes in the excitation spectra. The Ba2TiO4:Cr materials were post-fired in nitrogen, in an attempt to induce the formation of visible-emitting Cr3+ centers. Instead, this post firing tended to enhance the band-emission components in the ∼1.3 μm region. The line emission is attributed to Cr4+ in four-coordinated sites. In the simple Tanabe-Sugano formalism this implies the choice of Dq/B values exceeding those reported in the literature for Cr-activated försterite. For Dq/B≈2 the lowest excited level is 1E and the observed line emission is assigned to the inter-system transition 1E(e2)→3A2(e2).
 
Article
The luminescence seen from crystals of CaCrO4 and SrCrO4 at ⩽ 4 K is assigned to a spin-triplet state based on the observed zero-field ODMR signals. The zero-field splittings of the emitting triplet state in the two systems have similar |D| values (≈ 18 GHz) but dissimilar |E| values (0.9 and 3.2 GHz for the Ca2+ and Sr2+ salts, respectively).
 
Article
Er–Si–O crystalline compounds, which exhibit superlattice structures and sharp and strong Er-related 1.54 μm photoluminescence (PL) spectra at room temperature have been formed by self-assembling growth mechanism. Oxidation of the starting materials which have Si and Er at an atomic ratio of 2:1 are prepared and then oxidation and succeeding high-temperature annealing in Ar above 1250 °C cause a self-assembled superlattice-structured Er–Si–O crystalline compounds. The control of the ratio of Si and Er, as well as the following oxidation and annealing processes, is found to be sensitive to the crystalline properties, PL spectra and electrical properties. In this study, Er–Si–O crystalline thin films are formed on Si substrates by sol–gel and MOMBE methods, and their crystalline properties such as crystalline orientation and concentration ratio of Er, Si and O are investigated. Crystalline Er–Si–O films of high orientation are successfully grown on Si(1 0 0) and its inclined surface. The PL and excitation spectra, fluorescence decay and the electrical properties are found to be strongly related to the crystalline properties. Excess O causes a broader 1.54 μm PL spectra, slower fluorescence decay, lower carrier-mediated excitation and higher resistivity. A precise control of O is found to be necessary to grow superlattice-structured Er–Si–O compounds, which are semiconducting and are excitable via carrier-mediated excitation mechanism.
 
Article
Sol–gel derived Fe2O3 films containing about 10 wt% of Er2O3 were deposited on porous silicon by dipping or by a spin-on technique followed by thermal processing at 1073 K for 15 min. The samples were characterized by means of PL, SEM and X-ray diffraction analyses. They exhibit strong room-temperature luminescence at 1.5 μm related to erbium in the sol–gel derived host. The luminescence intensity increases by a factor of 1000 when the samples are cooled from 300 to 4.2 K. After complete removal of the erbium-doped film by etching and partial etching the porous silicon, the erbium-related luminescence disappears. Following this, luminescence at 1.5 μm originating from optically active dislocations (“D-lines”) in porous silicon was detected. The influence of the conditions of synthesis on luminescence at 1.5 μm is discussed.
 
Article
Solutions of alpha, beta, and gamma cyclodextrin have been shown to enhance the chemiluminescence of the reaction between hydrogen peroxide and 10,10'-dimethyl-9,9'biacridinium nitrate. A 20-fold increase in chemiluminescence intensity is observed in 10-2 M beta cyclodextrin. The enhancement is attributed to an increase in the excitation efficiency and the rate of the reaction through the inclusion of a reaction intermediate in the cyclodextrin cavity.
 
Article
We describe the basic illumination characteristics of lighting source using 10 cd-class InGaN-based white LED (an efficacy of 15 lm/W) under a driving condition of AC 100 V. 697 white LEDs with series connections were arrayed on a glass epoxy substrate and were driven by adjusting a current of less than 20 mA. The temperature dependence of both the emission spectrum and intensity was measured. The white LED array indicates two distinct electroluminescence peaks at 465 and 555 nm at room temperature, which are related to the radiative recombinations from the InGaN MQW blue LED and from the YAG : Ce phosphor, respectively. We have obtained a maximum luminous intensity of about 30 000 lx and a maximum luminance of 95 000 cd/m2 from the LED array.
 
Article
Single-frequency diode lasers have been stabilized to 200 Hz at 1.5 μm and independently to 20 Hz at 793 nm over 10-ms integration times using narrow spectral holes in the absorption lines of Er3+- and Tm3+-doped cryogenic crystals as frequency references. Kilohertz stability over 100-s integration times is provided by these techniques, and that performance should be extendable to long integration times with further development. The achieved frequency stabilization provides ideal lasers for high-resolution spectroscopy in the time and frequency domains, real time analog optical signal processing based on spatial-spectral holography, interferometry, and other applications requiring ultra-narrow-band light sources or coherent detection. The stabilized lasers have enabled demonstrations of analog optical signal processing in Er3+ materials at 0.5 GHz bandwidths at temperatures of 4.2 K, and they will be important for electromagnetically induced transparency and quantum information demonstrations.
 
Article
Rhodamine-110/sol–gel samples are prepared by sol–gel technique using dip method. Concentration dependent photophysical studies of these samples have indicated about the least possibility of aggregate formation. The lasing action of Rh-110 in silica samples is studied as a function of dye concentration. An efficient laser emission is observed when the samples are transversely pumped at 337.1 nm and 1.5 Hz repetition rate using a nitrogen laser (400 μJ energy/pulse and 4 ns pulse duration). The maximum of 166% laser efficiency of dye doped sol–gel samples compared to Rhodamine-6G (Rh-6G) in methanol is achieved. The photostability is also measured by using N2 laser at 1 Hz and it is found nearly 165 pulses. The possible reasons for the photodegradation of the dye molecules are discussed in detail.
 
Article
We have measured the azimuthal angle dependence of the second harmonic (SH) intensity from Cu nanowires on the faceted NaCl (1 1 0) substrates in air at the fundamental photon energy of 1.17 eV. The SH intensity patterns showed two main lobes for p-in/p-out, s-in/p-out, and s-in/s-out polarization configurations. From the results of the experiment and the pattern analysis we have found that the observed SH light is enhanced by the electric field components along the substrate normal.
 
Article
A thin AlN buffer layer was used to grow ZnO thin film on Si(1 1 1) substrate by atmospheric pressure MOCVD to protect the substrate from being oxidized and to eliminate the mismatch between the epilayer and the substrate. Double crystal X-ray diffraction results indicate that high-crystallinity ZnO film has been obtained. The full-width of half-maximum (FWHM) of ZnO (0 0 0 2) and ZnO ω-rocking curve peaks are 460″ and 1105″, respectively. The crack density Of ZnO surface is 20 strip/cm by optical microscope graph determination. In situ laser reflectance trace shows that a quasi-two-dimension growth mode was obtained when the film growth rate is up to 4.3 μm/h. Free exciton emission and bound exciton emission accompanied by their longitudinal optical phonon replicas can be observed from the photoluminescence spectrum at 10 K.
 
Article
Diode laser frequency stabilization to 500 Hz Allan deviation is demonstrated over 2 ms integration times with drift reduced to 7 kHz/min. This was achieved at 1536 nm in the technologically important communications band by stabilizing external cavity diode lasers to regenerative transient spectral holes in the inhomogeneously broadened 4I15/2(1)→4I13/2(1) optical absorption of Er3+ : Y2SiO5. Spectral diffusion, which currently limits the achievable stabilization performance, has been studied using stimulated photon echoes. Due to spectral diffusion, significant broadening of the homogeneous linewidth at low magnetic fields from a few kHz to tens of kHz develops as the waiting time T between pulses two and three was increased from microseconds up to the T1∼10 ms lifetime of the excited state. This evolution of the homogeneous linewidth has been mapped out as a function of magnetic field. The classic spectral diffusion can be reduced to negligible levels upon application of a magnetic field in a 0.02 atomic percent Er3+ : Y2SiO5 crystal.
 
Article
It is known that MgS : Eu2+/Eu3+ shows the most promising characteristics of any frequency domain optical storage material (Hasan et al., Appl. Phys. Lett. 72 (1998) 2373, 3399). Efficient photon-gated hole burning is observed in 4f7–4f6 5d1 ZPL transition of Eu2+ in MgS. In this study, the dynamics of hole burning and hole erasure is investigated. The dependence of the holedepth and holewidth on the burning time and the burn power levels has been experimentally studied. To explain the experimental data a theoretical model has been developed. The model shows very good agreement with the experimental data. The same model explains the behavior of the erasure and thermal cycling of holes.
 
Article
PbS quantum dots of average size 10 nm are encapsulated in a matrix (polyvinyl alcohol (PVA)) following chemical route. They are irradiated with 160 MeV Ni12+ ion beam with fluences 1012–1013 ions/cm2. Red shift in the absorption response in the optical absorption spectra reveal size enhancement of the quantum dots after irradiation and was confirmed by transmission electron microscopy (TEM). Photoluminescence (PL) study was carried out with excitation wavelength 325 nm on both unirradiated and irradiated samples at different fluences and fluence-dependent surface states and excitonic emission is observed in the PL study. The Huang–Rhys coupling constant decreases significantly after swift heavy ion (SHI) irradiation and shows a decreasing trend with increase in ion fluence.
 
Article
Results are presented on the scintillation properties of LaF3 crystals doped with Nd3+ concentrations ranging from 0.1 to 15 mol%. Attention will be focussed on the absolute light output in photons/MeV contained in the 173 nm emission band, caused by dipole allowed 5d-4f transitions in the Nd3+ ions.
 
Article
The rare-earth compound BaFCl0.5Br0.5:Sm2+ represents a new type of material in between the crystalline and glassy state. The Br and Cl ions randomly occupy the Cl-sites around the Sm2+ impurity so that this material exhibits microscopic disorder in spite of a definite lattice structure. The inhomogeneous line width of the 7F0-5D2-transition of Sm2+ is 38 cm-1 and was found to be independent of temperature between 1.5 and 77 K. Using high resolution laser-spectroscopy we have burnt permanent spectral holes in the inhomogeneous absorption line. The hole width is 86 MHz at 1.5 K and increases to 10.7 GHz at 77 K, so that 50–100 permanent holes could potentially be burnt simultaneously at liquid nitrogen temperatures. Gating light from the 5145 Å Ar+ line enhanced the hole-burning efficiency by a factor 3. Holes could be erased by illuminating the sample with 4880 Å blue light. The hole position as a function of temperature follows a T4 power law, encountered also in homogeneous crystalline systems. The coupling constant, however, was found to be four times smaller than in comparable homogeneous crystalline hosts and is rather comparable to the values measured in glassy systems. In lower spectral resolution (≈0.5 cm-1) holes could be burnt and directly detected up to 183 K, the highest temperature where the phenomenon of spectral hole-burning has yet been observed.
 
Article
The interaction of Na9[SbW9O33]·19.5H2O (SbW) with bovine serum albumin (BSA) is studied by spectroscopic and voltammetric methods. Absorption spectroscopy of BSA and the linear sweep voltammetry of SbW proved the formation of ground-state SbW–BSA complex. Fluorescence quenching of serum albumin by SbW is also found to be a static quenching process. The binding constant Ka is 4.13×104 L mol−1 for SbW–BSA at pH 7.40 Tris–HCl buffer at 295 K. The number of binding sites and the apparent binding constants at different temperatures are obtained from the analysis of the fluorescence quenching data. The thermodynamic parameters determined by the Van’t Hoff analysis of the binding constants (ΔH=−80.01 kJ mol−1 and ΔS=−182.85 J mol−1 K−1) clearly show that the binding is absolutely entropy driven. Hydrogen bonding and van der Waals interaction force play major role in stabilizing the complex. The effect of SbW on the conformation of BSA is analyzed using synchronous fluorescence spectroscopy.
 
Article
The physical phenomena involved in the luminescence of alkali halides are discussed. Particular attention is given to the spectroscopy of luminescent centers, recombination luminescence and quasiparticle treatment of luminescence. An extensive bibiliography is appended.
 
Article
An influence of phenoxy groups for the luminescent and electroluminescent properties of spiro-compounds with pyrazolo [3,4-b] quinoline structure (fluorophore) chromophore has been studied. All the compounds exhibit strong fluorescence in solution and in solid state as well. The prepared compounds were used as dopant chromophore in PVK polymer matrices for electroluminescent (EL) and light-emitting diode (LED) devices with configuration ITO/PEDOT-PSS/PVK/PQ/Ca/Al. Role of the bathochromic shifts and solvent polarity in absorption and photoluminescent maxima is considered. Relation between the number of pyrazoloquinoline chromophore and presence of phenyl group on the fluorescence spectra is explored. Polarizability of the particular pyrazoloquinoline compounds on the solvatochromic effects is investigated. Possible ways of enhancement of the brightness in the light-emitting properties of the mentioned chromophore are discussed.
 
Article
Multitopic ligand, 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (pyterpy), has attracted growing attention because of its unique structural features, optical and electrochemical properties. Here, we report spectroscopic studies of pyterpy and its metal complexes in methanol solution. For the pure pyterpy, the ligand emission intensity increased with its concentration in the dilute solution, but decreased when its concentration was over 1.3×10−5 mol/l due to the concentration quenching. No significant influence on the ligand luminescence was observed for the Zn2+-pyterpy complex but strong luminescence quenching was observed for the electroactive Fe2+- and Co2+-pyterpy complexes. The lanthanide (Sm3+, Eu3+ and Tb3+) complexes of the pyterpy showed both ligand and lanthanide ion emissions, especially for the Tb3+-pyterpy complex, suggesting that the excited energy of pyterpy ligand could be efficiently transferred to the central Tb3+ ions. The luminescence was pH sensitive with the strongest emission in the neutral solution. The results indicated that the multitopic ligand of pyterpy could not only act as linkers for the metal-directed building blocks, but also act as optical materials with its own emission at about 364 nm and as light antenna for the lanthanide ions.
 
Article
The first excited singlet state of 2,2-diphenylethylamine (DPEA) shows a small exciton splitting of about 137 cm-1. The measured lifetimes of the upper and lower excitonic levels are τ1 and τ2, respectively, (τ1 < τ2) and only τ2 is sensitive to polarity of solvents in which the spectrum of DPEA is measured. Flourescence emission mostly takes place from the lower level with longer lifetime which apparently indicates that the upper level is depopulated by rapid internal conversion. It is suggested that the geometry of DPEA, conformation of the NH2 group, as also restricted orientation of the group, are not favourable for intramolecular CT interaction. Analysis of kinetic parameters of DPEA in different solvents in the presence of chloroalkanes at 77 K shows that it is lower excitonic level that is involved in CT complex formation with the heavy atom quenchers and that from this level the triplet excitation is enhanced.
 
Article
The interaction of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) with bovine serum albumin (BSA) has been studied using absorption and steady state fluorescence techniques. Fluorescence spectrum of BSA (λexi=280 nm) in the presence of DBO clearly shows that DBO acts as a quencher. The number of binding sites ‘n’ and apparent binding constant ‘K’ were measured by Stern–Volmer equation. Synchronous fluorescence and absorption spectra were used to study protein conformation. The interaction between DBO and BSA is consistent with static quenching and the conformational changes of BSA observed.
 
Article
The position and intensity of the emission band for 2,3-diphenylbenzo[b]furans 1–16 and phenanthrobenzofurans 17–22 are affected by the nature and position of the substituents, particularly, by those which extend the conjugated system. The major structural factors responsible for the augmentation of the fluorescence quantum yield and the displacement of the fluorescence spectra towards the longer wavelengths, include considerable lengthening of the conjugation system and the electron–donor behaviour of the substituents which upset the distribution of electron density in the molecules of the compounds under study.
 
Article
The fluorescence quenching of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by a series of uracils has been studied in water and acetonitrile solvents using steady-state and time-resolved fluorescence techniques. The steady-state fluorescence quenching technique has been performed in three different pHs (i.e. 4, 8 and 12). The bimolecular quenching rate constant (kq) increases with increase in pH of uracils. In acidic pH, a pure hydrogen atom abstraction is proposed as the quenching mechanism. This is supported by a pronounced solvent deuterium isotope effect. Electron transfer from the anionic form of uracil to the excited state of DBO is proposed as a mechanism for quenching in basic pH on the basis of highly exergonic thermodynamics obtained from the Rehm–Weller equation. The variation of kq is explained on the basis of the electronic effect of substitution in uracils as well.
 
Article
Samples of natural andalusite (Al2SiO5) crystal have been investigated in terms of thermoluminescence (TL) and electron paramagnetic resonance (EPR) measurements. The TL glow curves of samples previously annealed at 600 °C for 30 min and subsequently gamma-irradiated gave rise to four glow peaks at 150, 210, 280 and 350 °C. The EPR spectra of natural samples heat-treated at 600 °C for 30 min show signals at g=5.94 and 2.014 that do not change after gamma irradiation and thermal treatments. However, it was observed that the appearance of a paramagnetic center at g=1.882 for the samples annealed at 600 °C for 30 min followed gamma irradiation. This line was attributed to Ti3+ centers. The EPR signals observed at g=5.94 and 2.014 are due to Fe3+. Correlations between EPR and TL results of these crystals show that the EPR line at g=1.882 and the TL peak at 280 °C can be attributed to the same defect center.Research highlights► TL and EPR spectra have been measured in natural andalusite crystals. ► Andalusite irradiated with different γ-doses shows TL peaks at 150, 210, 280 and 350 °C. ► The EPR spectra show lines due to Fe3+ ions and due to the [TiO4/Na+]0 center.
 
Article
Standard Reference Material (SRM) 2940 is a cuvette-shaped, Mn-ion-doped glass, recommended for use for relative spectral correction of emission and day-to-day performance validation of steady-state fluorescence spectrometers. Properties of this standard that influence its effective use or contribute to the uncertainty in its certified emission spectrum were explored here. These properties include its photostability, absorbance, dissolution rate in water, anisotropy, temperature coefficient of fluorescence intensity, and fluorescence lifetimes. Long and short lifetime components of the fluorescence displayed different emission spectra, making the certified spectrum useful with fluorescence instruments employing continuous excitation only. The expanded uncertainties in the certified spectrum are about 5% around the peak maximum at 620 nm, using an excitation wavelength of 412 nm. The SRM also exhibits a strong resistance to photodegradation, with no measurable decrease in fluorescence intensity even after 17 h of irradiation with the visible light from a Xe lamp.
 
Article
Standard Reference Material® (SRM®) 2941 is a cuvette-shaped, uranyl-ion-doped glass, recommended for use for relative spectral correction of emission and day-to-day performance validation of fluorescence spectrometers. Properties of this standard that influence its effective use or contribute to the uncertainty in its certified emission spectrum have been explored here. These properties include its photostability, absorbance, dissolution rate in water, anisotropy, temperature coefficient of fluorescence intensity, and fluorescence lifetimes. The expanded uncertainties in the certified spectrum are about 4% around the peak maximum at 526 nm, using an excitation wavelength of 427 nm. The SRM also exhibits a strong resistance to photodegradation, with no measurable decrease in fluorescence intensity even after 8 h of laser irradiation.
 
Article
We have constructed a nonlinear spectroscopic system for performing multiresonant four-wave mixing with infrared lasers. The system consists of three coherent sources, two of which are tunable in the infrared region of the spectrum. The sources are tuned to different vibrational resonances and the four-wave mixing output is monitored as a function of the two infrared frequencies. When the frequencies match direct infrared absorption or Raman transitions, the four-wave mixing output is enhanced. A two-dimensional display of the data shows the output intensity as a function of the two infrared frequencies. We observe that cross-peaks appear in the 2D spectra when multiple resonances are excited. We have named the method “doubly vibrationally enhanced four-wave mixing (DOVE-FWM)”. This method represents the long sought optical analogue to 2D nmr. It should provide a method that is complementary to nmr because of the difference in the time scales of the dephasing processes. Spin-lattice interactions fix the dephasing times for NMR measurements at millisecond time scales so nmr senses the ensemble average of a material's structure. Vibrational dephasing times occur on the picosecond time scale so the DOVE–FWM measurement represents a more instantaneous measurement of material structure.
 
Article
Exciton localization in quantum wells (QWs) formed by wide-gap solid solutions with isoelectronic substitution of ZnCdSe and InGaN is discussed. As a first step we discuss the microscopic mechanisms of exciton localization which are common for both 2D and bulk solid solutions. In most important for application solid solutions with good solubility, the exciton localization at small solution concentrations originates from the statistical clusters formed by a few atoms of the narrow-gap component. A theoretical model of absorption and photoluminescence (PL) spectra of diluted solutions is presented. The model explicitly accounts for the statistics of the substitutional atoms over the lattice sites. The parameters of the model can be determined from the analysis of the fine structure of PL spectra from the cluster states with different number of atoms. As a practical application of the model, the description of exciton spectra of diluted bulk InGaN solutions is presented. The effect of the lowering of dimensionality on exciton localization and the conditions for formation of 2D QWs from solid solutions are discussed. Experimental data on exciton localization in ZnCdSe/ZnSe QWs are presented. For diluted solutions the spectra can be described under the assumption of random distribution of Cd atoms along the QW plane. An increase of concentration due to the partial phase separation of ZnCdSe solutions leads to the formation of planar islands strongly enriched in Cd content. The results of optical study of the exciton states in the QWs with such island are reported.
 
Article
Monolayer well-ordered close-packed 2D nanostructures of gold nanoparticles were fabricated without using any capping reagent or surfactant. The area of the 2D nanostructure obtained was as large as several 100 s μm2 to mm2. The individual gold nanoparticles of the 2D nanostructure are not in contact depicting the possible surface active site (i.e. hot site). The SERS-activity was measured by using crystal violet molecules adsorbed on the 2D nanostructure. The estimated enhancement was found as much as 108. Size-dependent SERS intensity of the well-ordered 2D nanostructures is discussed in detail.
 
Article
An unusually large splitting of the 2E state of Cr3+ in kyanite (Al2O3:SiO2) is reported. To account for the thermal quenching of the R-lines it is necessary to recognize that the two components of the doublet may have remarkably different radiative lifetimes (3400 and 480 μs). These lifetimes were obtained from low temperature decay and from the temperature dependence of the intensity ratio of the two components. These separate lifetimes were then set to fit the temperature dependence of the joint effective lifetime and of the steady state intensities of both lines. At higher temperatures the effect of an unobservable quartet had to be postulated. A reasonable agreement between theory and experiment was achieved assuming that the quartet radiative lifetime was 13 μs and its zero-phonon energy 15570 cm-1. A simple approach, based on spin-orbit coupling of split 2E components to 4T2 and a one-configuration-coordinate model is an aid to a qualitative understanding of the large difference between the lifetimes of the R-lines, although quantitative agreement between theory and experiment is far from satisfactory.
 
Top-cited authors
Pieter Dorenbos
  • Delft University of Technology
Andries Meijerink
  • Utrecht University
Oscar Malta
  • Federal University of Pernambuco
Thomas Jüstel
  • FH Münster
Karl W. Krämer
  • Universität Bern