Optical Transition Probabilities Research Articles

Optical Transition Probabilities of Er3+Ions in ErBa3B9O18Crystal

The optical absorption and emission intensity of luminescent and birefringent crystal ErBa3B9O18were examined from optical absorption data based on Judd-Ofelt theory. The three intensity parametersΩt(t=2,4,6) are 3.10 × 10−20, 0.87 × 10−20, and 1.80 × 10−20 cm2, respectively. From the obtained intensity parameters, the radiative probabilitiesAr, radiative lifetimeτf, fluorescence branching ratiosβc, and integrated emission cross sections∑have been calculated. In comparison with other Er-doped luminescent crystals, ErBa3B9O18may find application in thin disk laser.

A simple model for predicting the course of photochemical reactions

The possibility of building and using a simplified model with constant (time-invariant) reaction probabilities for rapid simulation of photochemical processes with the required reliability of the prediction has been shown. A relation between the probabilities of transformations and fundamental molecular characteristics (optical transition probabilities and quantum beat frequencies referring to a nonradiative reaction transition) that can be quantitatively determined with a good degree of certainty has been revealed. For monomolecular isomerization reactions as an example, the attainable level of the accuracy of prediction by the simple model has been estimated, which has been shown to be ∼10% in most cases. It is important that the prediction of the order of magnitude of the key characteristics of photochemical processes, such as quantum yield and the reaction rate, is definitely correct as is required in most applications of photochemistry.

High quality semipolar (11¯02) AlGaN/AlN quantum wells with remarkably enhanced optical transition probabilities

Adjusting the growth conditions from those for c-plane growth realizes high-quality semipolar (11¯02) AlGaN/AlN quantum wells (QWs) with atomically smooth surfaces and abrupt interfaces on AlN substrates. Upon comparing the optical properties to those of c-plane QWs using time-integrated and time-resolved photoluminescence spectroscopy, the estimated internal electric field is much smaller in (11¯02) AlGaN/AlN QWs than in c-plane QWs. Thus, (11¯02) AlGaN/AlN QWs have narrower emission line widths and remarkably faster radiative recombination lifetimes, realizing highly efficient deep ultraviolet emissions.

Visible range colossal magnetorefractive effect in (La1 − yPry)2/3Ca1/3MnO3 films

We report a colossal magnetorefractive effect (MRE) in epitaxial thin films of a classical colossal magnetoresistance (CMR) manganite, (La1 − yPry)2/3Ca1/3MnO3 (y = 0.375 and 0.7). Close to the ferromagnetic (FM) phase transition a moderate applied magnetic field, H ∼ 10 kOe, results in a reduction of the optical reflectance by ∼18% for the photon energy E ∼ 2.7 eV. The MRE spectral behavior with three pronounced maxima at E = 1.6, 2.7 and 4.0 eV points out an inter-site nature of the involved optical transitions. The results are discussed within a phase separation scenario with coexisting FM metallic nanodomains antiferromagnetically coupled by correlated polarons. The probability of MRE optical transitions is maximal for antiparallel alignment of Mn3+/Mn4+-spins realized for the coercive field, Hc ∼ 200–800 Oe, and is suppressed by stronger fields, which favor FM metallic behavior. As a result, both the optical reflectivity and the electrical resistance decrease, yielding a close similarity between the CMR and MRE behavior.

Role of strain in interacting silicon nanoclusters

The possibility of controlling the optical transition probability between neighboring silicon nanoclusters (Si-NCs) constitutes nowadays an attractive prospect in nanophotonics and photovoltaics. In this work, by means of theoretical ab initio calculations, we investigate the effect of strain on the optoelectronic properties of Si-NCs pairs. We consider two sources of strain: the strain induced by an embedding SiO${}_{2}$ matrix and the strain generated by mutual NC-NC forces occurring at small distances. Independently on its source, we observe a fundamental impact of the strain on the orbital localization and, as a consequence, on the transition probability between energy states belonging or not to the same NC. The resulting picture allots to the structural strain a fundamental role in the NC-NC interaction mechanisms, suggesting the possibility of enabling a strain-controlled response in Si-NC ensembles.

Morphological effects on transparency and absorption edges of some semi-alicyclic polyimides

Some novel semi-alicyclic polyimides based on epiclon and three aromatic diamines are investigated. The ellipsometry data reveal that the reduced polarizability provided by the alicyclic dianhydride moieties leads to polyimides with low refractive index, ranging from 1.64 to 1.72. The corresponding dielectric constants decrease from 2.97 to 2.71 as the polarization of the diamine part is lower. The transmittance is of appreciatively 90 % for polyimides containing methylene groups in the diamine residue. The cut-off wavelength decreases, as the backbone conjugation is smaller, being comprised between 305 nm and 317 nm — values comparable to that of a fully aliphatic PI. To obtain the optical energies describing absorption limits, the approach proposed by Tauc and Davis-Mott has been used, due to the similarity of the absorption edges. The values of optical band-gap are found from 3.51 to above 3.78 eV, indicating a low probability of optical absorption transitions and thus a good transparency. The Urbach and Tauc energies changed in the range of 480–806 meV and 1,453–2,786 meV, being smaller as the flexible sequence in the diamine moieties is longer. The novelty of this work consists in relating optical properties with morphological aspects, namely surface defects and entropy of morphology. These new results have a great impact in selecting the proper materials as liquid crystal alignment layers or as cell culture substrates.

Axially symmetrical molecules in electric and magnetic fields: energy spectrum and selection rules

Abstract In this paper we investigate the effects of external electric and magnetic fields on a three-dimensional harmonic oscillator with axial symmetry. The energy spectrum of such a system is non-degenerate due to the presence of the magnetic field. The degeneracy of the energy spectrum in the absence of a magnetic field is discussed. The influence of electric and magnetic fields, as well as the frequencies of the oscillator on the probability distribution function is analyzed. Optical transition probabilities are examined by deriving the selection rules in dipole approximation for the quantum numbers n p, m l and n z. Employing stationary perturbation theory, the effects of deformations of the potential energy function on the oscillatory states are analyzed. Such models have been used in literature in analysis of spectra of axially symmetrical molecules and cylindrical quantum dots.

Optical conductivity of highly mismatched GaP alloys

Highly mismatched alloys are promising for applications to intermediate-band (IB) solar cells. Here, we report first-principles prediction of intermediate bands in GaP on the basis of hybrid-density-functional theory, which enables to handle large supercells including defects with much better accuracy than semilocal functionals. Calculated optical conductivity reveals that the intermediate states due to co-doped Mg and O have sufficiently high optical transition probability. The multiple gaps are robust against thermalization. Intermediate-band states become more delocalized by hybridization with phosphorus-vacancy states, increasing the optical transition probability.

Transparency and absorption edges of disiloxane modified copolyimides

Optical characteristics of two novel copolyimides prepared from an alicyclic or a fluorine-based dianhydride combined with an aliphatic siloxane-based diamine and an aromatic one containing ether linkages, are compared. Ellipsometry data show that the reduced polarizability, provided by the aliphatic moieties, leads to copolyimides with low refractive index (1.53–1.57). The corresponding dielectric constants decrease from 2.71 to 2.57 as the polarization of the dianhydride unit is reduced by the presence of CF3 groups. The transmittance is of approx. 90% and the cut-off wavelengths are comprised between 336 and 380nm. The values of optical band-gap are higher for the fluorine-based copolyimide, namely 3.00eV comparatively to 2.34eV for the sample with alicyclic dianhydride sequences, indicating a lower probability of optical absorption transitions and thus a better transparency. The Urbach and Tauc energies changed in the range of 2295–2809meV and 893–1161meV, being smaller as the polarizability is diminished by the presence of CF3 groups. Optical properties are correlated with morphological aspects, namely surface roughness parameters and entropy of morphology, which varies in agreement with the refractive indices of copolymers.

Dependency of non-homogeneity energy dispersion on absorbance line-shape of luminescent polymers

In this paper, we study the importance of the non-homogeneity energy dispersion on absorption line-shape of luminescent polymers. The optical transition probability was calculated based on the molecular exciton model, Franck–Condon states, Gaussian distribution of non-entangled chains with conjugate degree n, semi-empirical parameterization of energy gap, electric dipole moment, and electron-vibrational mode coupling. Based on the approach of the energy gap functional dependence 1/n, the inclusion of the non-homogeneity energy dispersion 1/n2 is essential to obtain good experimental data agreement, mainly, where the absorption spectra display peaks width of about 65meV. For unresolved absorption spectra, such as those observed for a large number of conjugated polymers processed via spin-coating technique, for example, the non-homogeneity energy dispersion parameterization is not significant. Results were supported by the application of the model for poly (p-phenylene vinylene) films.

Confined electronic states and their modulations in graphene nanorings

Confined electronic states in quantum rings formed by spatially modulated finite Dirac gap (FDGQR) in graphene are systematically studied by series-expansion method, and are compared with those in infinite-mass-boundary and one-dimensional quantum rings. The shape-size effect of FDGQR is illustrated to be distinct from that in graphene quantum dots. The Aharonov-Bohm effect in FDGQR is clearly shown by the energy spectrum and the optical-transition probabilities. The FDGQR coupled with the electrostatic-potential induced nanoring is found useful for modulating the Dirac electronic states and the optical-transition probabilities. These results may help us to understand and to control the quantum behaviors of confined electronic states in graphene.

Selection rules for optical transition probabilities in superlattices

Based on numerical evaluations of the transition probabilities, and using true superlattice eigenfunctions derived from the theory of finite periodic systems, new superlattice selection rules are obtained. We show that the evaluation of the large number N of the possible recombination transitions reduces by a factor of two. We also show that taking into account these selection rules, the theoretical calculations for specific blue emitting GaN⧹(InxGa1-xN⧹InyGa1-yN)n⧹GaN superlattices agree with those reported by Nakamura et al. [1].

Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

The magnetotransmission, magnetoreflection, and magnetoresistance of the La0.7Ca0.3MnO3 and La0.9Ag0.1MnO3 epitaxial films have been investigated. It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures. It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands. The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects. As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits.

MODIFICATION OF OPTICAL TRANSITION PROBABILITY IN SEMICONDUCTOR SUPERLATTICES: EFFECTS OF STRUCTURAL DEFECT AND LAYER THICKNESS

We present the modification of optical transition probability in the superlattices (SLs) due to the presence of structural defect and the variation of layer thickness. The results show that optical transition probability between two bound states is mainly determined by the thicknesses of layer in defect region and its nearest constituent layers for a symmetric structure. When the well layer thickness varies, the variation of transition probabilities between two bound states depend on the parity of the index of minigap n, while those from the bound states to the delocalized scattering is irrelevant to n. It is believed that applying appropriate structure and layer thickness may enhance the optical transition probability in designing optical device.

Energy level diagram, wavefunctions, and probabilities of optical transitions in KCuF3

Using modern concepts of the crystal field in ion-covalent compounds, we calculate the energy level diagram, wavefunctions, and probabilities of optical transitions in the orbital ordering phase of KCuF3. On this basis, we discuss the possible mechanisms by which an optical absorption sideband structure forms, accompanied by the creation of magnons.

Enhanced dipole moments in photo-excited TTF–TCNQ dimers

We have studied the dynamics of electron transfer between the molecules of an organic donor–acceptor pair upon absorption of light. Specifically, we considered the tetrathiafulvalene (TTF)–7,7,8,8-tetracyanoquinodimethane (TCNQ) donor–acceptor pair using time-dependent density functional theory with local-density approximation. The molecular planes of the two components are parallel to each other, and the optical transition probability is found to be highest when the optical electric field is parallel to these planes. Under these conditions, absorption induces additional electron transfer from TTF to TCNQ in the π-orbitals perpendicular to the molecular plane and, consequently, we found that the dimer's dipole moment perpendicular to the molecular axes is enhanced with the increase rate of 1% in 15 fs. This enhancement reflects the fact that photo-excited electron–hole pairs tend to dissociate, i.e. electrons and holes move away from each other. We thus suggest potential photovoltaic devices employing these molecules as building blocks.

Disordering of the electronic structure of YBaCuO amorphous films upon incorporation of crystalline clusters formed in laser-induced plasma into their composition

The effect of crystalline clusters formed in a laser-induced plasma on the optical properties of YBa2Cu3O6 + x amorphous films prepared by pulsed laser deposition has been investigated. It has been demonstrated that an increase in the number of clusters leads to a gradual disappearance of interference fringes inherent in optically homogeneous media. Simultaneously, the incorporation of metallic and insulating clusters into the amorphous medium results in a decrease in the optical band gap E 0 of the YBaCuO amorphous matrix from 1.28 to 1.06 eV and a considerable decrease in the probability of interband optical transitions with charge transfer O 2p → Cu 3d due to the loosening of the structure and generation of local stresses. It has been revealed that there is an additional band gap E 1, which decreases from 0.25–0.30 eV to zero values with a decrease in the optical band gap E 0. The additional gap has been interpreted as an energy gap between localized states that belong to the valence and conduction bands. A decrease in the density of electronic states in the narrow 3d band leads to the overlap of tails of the density of states, so that the band gap E 1 becomes negative.

Simulation of the cyclopropanecarbaldehyde and cyclopropylethanone photoisomerization processes and calculation of the quantum yields of the reactions

The cyclopropanecarbaldehyde → 2-butenal and cyclopropylethanone → 3-pentenone-2 photoisomerization processes were simulated. The calculated quantum yields of the products are in quantitative agreement with experimental data. The validity of the method proposed for the calculation of quantum yields of photochemical reactions was confirmed. It was found that if the condition of relative smallness of the optical transition probabilities as compared to the quantum beat frequency is met, the quantum yields can be quantitatively estimated with satisfactory accuracy directly from the transition probabilities without running the full calculation of the phototransformation kinetics. It was shown that the integral characteristics of photochemical reactions (quantum yields) are highly responsive to the conformational state of the molecules involved in the process.

Optical transition probabilities inEr3 + - andTm3 + -dopedLiLa9(SiO4)6O2 crystals

In this work, Er3 + and Tm3 + -dopedLiLa9(SiO4)6O2 crystals have beengrown from an Li2MoO4 flux in the 1360–940 °C temperature range. Optical absorption spectra have been measured to obtainthe experimental oscillator strengths of the transitions from the ground state tothe excited levels. Judd–Ofelt calculations have been performed to estimate theΩ2,Ω4 andΩ6 intensity parameters. The dynamics of selectedEr3 + andTm3 + manifolds have been investigated under selective pulsed excitation in order to determinethe energy gap law by comparing the observed decay rates with the Judd–Ofelt predictions.

Investigation of Rb D 1 atomic lines in strong magnetic fields by fluorescence from a half-wave-thick cell

It has been experimentally demonstrated that the use of the effect of significant narrowing of the fluorescence spectrum from a nanocell that contains a column of atomic Rb vapor with a thickness of L = 0.5λ (where λ = 794 nm is the wavelength of laser radiation, whose frequency is resonant with the atomic transition of the D1 line of Rb) and the application of narrowband diode lasers allow the spectral separation and investigation of changes in probabilities of optical atomic transitions between levels of the hyperfine structure of the D1 line of 87Rb and 85Rb atoms in external magnetic fields of 10–2500 Gs (for example, for one of transitions, the probability increases ∼17 times). Small column thicknesses (∼390 nm) allow the application of permanent magnets, which facilitates significantly the creation of strong magnetic fields. Experimental results are in a good agreement with the theoretical values. The advantages of this method over other existing methods are noted. The results obtained show that a magnetometer with a local spatial resolution of ∼390 nm can be created based on a nanocell with the column thickness L = 0.5λ. This result is important for mapping strongly inhomogeneous magnetic fields.