Dipole Forbidden Research Articles

Nonlinear cooperative transfer of atomic energy in the vacuum field of the cavity

This paper researches the nonlinear cooperative transfer of atomic energy in the vacuum field of the cavity. Many scientists are concerned about a more detailed study of the cooperation interactions between different subsystems of quantum processes. Based on the theory of resonant interaction with two photons, between two active dipole atoms and a forbidden dipole in resonance with two photons, the author examines for the first time the resonance effect in two ways of the cavity. In some situations, the so-called quantum inseparability needs a new redefinition in order to describe the process that occurs between the photons belonging to these two modes of the cavity. In such a situation, it is better to study the quantum properties of collective modes arising from nonlinear interactions between traditional modes of the electromagnetic field cavity.

Ni3+ -induced semiconductor-to-metal transition in spinel nickel cobaltite thin films

In this paper, we report insights into the local atomic and electronic structure of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films with controlled properties on $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}(001)$ substrates using pulsed laser deposition. Films grown at low temperatures ($<400{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the ${\mathrm{Ni}}^{3+}$ valence state promoted at low growth temperature introduces delocalized $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-derived states at the Fermi level (${E}_{F}$), responsible for the metallic state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$, while the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and $\ensuremath{\sim}+2$. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the ${\mathrm{Ni}}^{2+}$ valence state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$. The $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-related state becomes localized away from ${E}_{F}$, opening a band gap for a semiconducting state. The band gap of the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ is estimated to be $<0.8\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is $d\text{\ensuremath{-}}d$ dipole forbidden, and therefore, the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ still shows reasonable transparency in the infrared-visible light region. The present insights into the role of ${\mathrm{Ni}}^{3+}$ in determining the electronic structure and defect chemistry of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ provide important guidance for use of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ in electrocatalysis and opto-electronics.

Band Gap Measurements of Nano-Meter Sized Rutile Thin Films.

Thin Titanium films were fabricated on quartz substrates by radio frequency magnetron sputtering under high vacuum. Subsequent annealing at temperatures of 600 C in air resulted in single-phase with the structure of rutile, as X-ray diffraction experiment demonstrates. Atomic-force microscopy images verify the high crystalline quality and allow us to determine the grain size even for ultrathin films. Rutile has a direct energy band gap at about 3.0–3.2 eV; however, the transitions between the valence and conduction band are dipole forbidden. Just a few meV above that, there is an indirect band gap. The first intense absorption peak appears at about 4 eV. Tauc plots for the position of the indirect band gap show a “blue shift” with decreasing film thickness. Moreover, we find a similar shift for the position of the first absorbance peak studied by the derivative method. The results indicate the presence of quantum confinement effects. This conclusion is supported by theoretical calculations based on a combination of the effective mass theory and the Hartree Fock approximation.

Dipole forbidden, nuclear electric quadrupole allowed transitions and chirality: The broadband microwave spectrum and structure of 2-bromo-1,1,1,2-tetrafluoroethane

The chirped pulse Fourier transform microwave spectrum of the chiral molecule 2-bromo-1,1,1,2-tetrafluoroethane has been observed in the 6–18 GHz region of the electromagnetic spectrum and is reported. Six isotopologues, including 13C isotopic substitution at each carbon atom in both the 79Br and 81Br parent species have been observed in natural abundance. All rotational constants, quartic centrifugal distortion contants, and nuclear quadrupole coupling tensor components have been experimentally determined and have been reported. In addition, spin-rotation components Maa, Mbb, and Mcc are also determined and reported. The theoretical structure of the species has been determined and reported. The C–C–Br backbone structure has been determined experimentally implementing both Kraitchman single and Rudolph double substitution analysis, the later being explained in the text. 3440 transitions total have been observed with 45 transitions being electric dipole forbidden. These dipole forbidden transitions have been identified as they are planned to be used in microwave three-wave mixing experiments in order to better understand their role, if any, in determining chirality. The determined nuclear electric quadrupole coupling tensor has been diagonalized and compared to similar small alkane species in order to make inferences on the effects of fluorination on the C–Br bond.

Lasing on a narrow transition in a cold thermal strontium ensemble

Highly stable laser sources based on narrow atomic transitions provide a promising platform for direct generation of stable and accurate optical frequencies. Here we investigate a simple system operating in the high-temperature regime of cold atoms. The interaction between a thermal ensemble of $^{88}$Sr at mK temperatures and a medium-finesse cavity produces strong collective coupling and facilitates high atomic coherence which causes lasing on the dipole forbidden $^1$S$_0 \leftrightarrow ^3$P$_1$ transition. We experimentally and theoretically characterize the lasing threshold and evolution of such a system, and investigate decoherence effects in an unconfined ensemble. We model the system using a Tavis-Cummings model, and characterize velocity-dependent dynamics of the atoms as well as the dependency on the cavity-detuning.

Electronic structure andp-type conduction mechanism of spinel cobaltite oxide thin films

This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide (${\mathrm{Co}}_{3}{\mathrm{O}}_{4}, {\mathrm{ZnCo}}_{2}{\mathrm{O}}_{4}$, and ${\mathrm{CoAl}}_{2}{\mathrm{O}}_{4}$) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that ${\mathrm{ZnCo}}_{2}{\mathrm{O}}_{4}$ has a fundamental bandgap of 1.3 eV, much smaller than the generally accepted values, which range from 2.26 to 2.8 eV. The valence band edge mainly consists of occupied $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d\phantom{\rule{0.28em}{0ex}}{t}_{2\mathrm{g}}^{6}$ with some hybridization with O $2p$/Zn $3d$, and the conduction band edge of unoccupied ${e}_{g}^{*}$ state. However, optical transition between the two band edges is dipole forbidden. Strong absorption occurs at photon energies above 2.6 eV, explaining the reasonable transparency of ${\mathrm{ZnCo}}_{2}{\mathrm{O}}_{4}$. A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high $p$-type conductivity of ${\mathrm{ZnCo}}_{2}{\mathrm{O}}_{4}$, and the hole conduction mechanism is described by small-polaron hoping model. The high $p$-type conductivity, reasonable transparency, and large work function make ${\mathrm{ZnCo}}_{2}{\mathrm{O}}_{4}$ a desirable $p$-type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ is further proved to be \ensuremath{\sim}0.8 eV arising from the tetrahedrally coordinated ${\mathrm{Co}}^{2+}$ cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.

Density functional theory study explaining the underperformance of copper oxides as photovoltaic absorbers

Most photovoltaic absorbers are identified using the standard Shockley-Queisser selection principle which relies on optimal band-gap values. However, this criterion has been shown to be insufficient, as many materials with appropriate values still perform badly. Here, we have employed calculations based on the density functional theory to assess three copper oxides as potential photovoltaic materials: ${\mathrm{Cu}}_{2}\mathrm{O}$, ${\mathrm{Cu}}_{4}{\mathrm{O}}_{3}$, and $\mathrm{CuO}$. Despite their promising theoretical solar power conversion efficiency of over 20%, experimental values are found to be far lower. Theoretical evaluation of the electronic and optical properties reveals that certain transitions within the band structures are dipole forbidden, whereas the fundamental band gaps of ${\mathrm{Cu}}_{4}{\mathrm{O}}_{3}$ and $\mathrm{CuO}$ are of indirect nature. These findings correlate to the weak and shifted absorption properties found experimentally, which underpin the inefficient light capture by copper oxides. Based on these results and an applied extended selection metric, we can explain why copper oxides are unable to reach the efficiencies previously proposed theoretically and why we need to revise their maximum conversion values.

Electronic structure and optical properties of Sr2IrO4 under epitaxial strain

We study the modification of the electronic structure in the strong spin–orbit coupled Sr2IrO4 by epitaxial strain using density-functional methods. Structural optimization shows that strain changes the internal structural parameters such as the Ir–O–Ir bond angle, which has an important effect on the band structure. An interesting prediction is the Γ−X crossover of the valence band maximum with strain, while the conduction minimum at M remains unchanged. This in turn suggests strong strain dependence of the transport properties for the hole-doped system, but not when the system is electron doped. Taking the measured value of the Γ−X separation for the unstrained case, we predict the Γ−X crossover of the valence band maximum to occur for the tensile epitaxial strain exx ≈ 3%. A minimal tight-binding model within the Jeff = 1/2 subspace is developed to describe the main features of the band structure. The optical absorption spectra under epitaxial strain are computed using density-functional theory, which explains the observed anisotropy in the optical spectra with the polarization of the incident light. We show that the optical transitions between the Ir (d) states, which are dipole forbidden, can be explained in terms of the admixture of Ir (p) orbitals with the Ir (d) bands.

Optical absorption driven by dynamical symmetry breaking in indium oxide

Indium oxide is a wide band gap semiconductor that provides the platform for most $n$-type transparent conductors. Optical absorption is dominated by a strong edge starting at the optical gap around 3.7 eV, but the material also exhibits a prominent absorption tail starting around 2.7 eV. We use first principles methods to show that this tail arises from interband transitions that are dipole forbidden at the static lattice level, but become dipole allowed via a dynamical symmetry breaking induced by nuclear motion. We also report the temperature dependence of the absorption onset, which exhibits a redshift with heating, driven by a combination of electron-phonon coupling and thermal expansion. We argue that the role of dynamical symmetry breaking in optical absorption is a general feature of semiconductors and that the computational design of novel materials for optical applications, ranging from transparent conductors to solar cells, should incorporate the lattice dynamics of the crystal.

Static polarizabilities and optical absorption spectra of boron clusters (n = 2–20, 38 and 40) using first principles

We critically analyze and discuss the optical properties and static polarizabilities of the boron clusters within the framework of the time-dependent density functional theory. Our calculations show the dependence of the static and anisotropic polarizability, dispersion coefficient (C6) and enhancement factor (f) on the shape, size and dimensionality of the clusters. The deviation of the ‘C6’ and ‘f’ from the linearity explains the anisotropic nature of the clusters. The optical absorption occurs in the visible and ultra-violet region. The majority of clusters distinguish by the plasmon-like excitations in the high energy region. This is accompanied by the blue-shift and enhancement in the intensity of spectral lines in the 3D structures. All the optical transitions occur between π or π∗ → σ and σ → π-type of orbital, which is also confirmed from the natural transition orbital. It observes that the optical gap of all the clusters except B4 belongs in the visible part of the spectrum. It also finds that the oscillator strength is suppressed in the visible region with the exception of a couple of small peaks as the low energy transitions are dipole forbidden.

Enhanced Second-Order Nonlinearity for THz Generation by Resonant Interaction of Exciton-Polariton Rabi Oscillations with Optical Phonons.

Semiconductor microcavities in the strong-coupling regime exhibit an energy scale in the terahertz (THz) frequency range, which is fixed by the Rabi splitting between the upper and lower exciton-polariton states. While this range can be tuned by several orders of magnitude using different excitonic media, the transition between both polaritonic states is dipole forbidden. In this work, we show that, in cadmium telluride microcavities, the Rabi-oscillation-driven THz radiation is actually active without the need for any change in the microcavity design. This feature results from the unique resonance condition which is achieved between the Rabi splitting and the phonon-polariton states and leads to a giant enhancement of the second-order nonlinearity.

A Study on Probe Transmission through an Inverted Y-type Atomic System in Presence of Three Coherent Laser Fields

A four-level inverted Y-type atomic system is subjected to three coherent laser fields simultaneously. The system has two closely spaced ground energy levels, one intermediate energy level and an uppermost energy level. Transition from the two ground levels to the intermediate level as well as from the intermediate level to the uppermost level are dipole allowed. Transition from the two ground energy levels to the uppermost level is dipole forbidden. A weak laser beam, known as the probe beam, acts between one of the ground levels and the intermediate level. A strong laser field, known as the control or pump field, couples the intermediate level with the uppermost level. A repump filed of intensity usually higher than the probe field acts between the other ground level and the intermediate level. The probe transmission through such a medium is investigated at different values of the control and repump Rabi frequencies under Doppler free condition as well as with thermal averaging. The nonradiative population transfer rates between the ground energy levels have been incorporated.

Two-Photon Collective Atomic Recoil Lasing

We present a theoretical study of the interaction between light and a cold gasof three-level, ladder configuration atoms close to two-photon resonance. In particular, weinvestigate the existence of collective atomic recoil lasing (CARL) instabilities in differentregimes of internal atomic excitation and compare to previous studies of the CARL instabilityinvolving two-level atoms. In the case of two-level atoms, the CARL instability is quenchedat high pump rates with significant atomic excitation by saturation of the (one-photon)coherence, which produces the optical forces responsible for the instability and rapid heatingdue to high spontaneous emission rates. We show that in the two-photon CARL schemestudied here involving three-level atoms, CARL instabilities can survive at high pump rateswhen the atoms have significant excitation, due to the contributions to the optical forces frommultiple coherences and the reduction of spontaneous emission due to transitions betweenthe populated states being dipole forbidden. This two-photon CARL scheme may form thebasis of methods to increase the effective nonlinear optical response of cold atomic gases.

Perovskite Sr-Doped LaCrO3 as a New p-Type Transparent Conducting Oxide.

Epitaxial La1-x Srx CrO3 deposited on SrTiO3 (001) is shown to be a p-type transparent conducting oxide with competitive figures of merit and a cubic perovskite structure, facilitating integration into oxide electronics. Holes in the Cr 3d t2g bands play a critical role in enhancing p-type conductivity, while transparency to visible light is maintained because low-lying d-d transitions arising from hole doping are dipole forbidden.

Optically bright p-excitons indicating strong Coulomb coupling in transition-metal dichalcogenides

It is shown that the strong Coulomb coupling in intrinsic suspended semiconducting transition metal dichalcogenides can exceed the critical value needed for an excitonic ground state. The dipole-allowed optical excitations then correspond to intra-excitonic transitions such that the optically bright excitonic transitions near the Dirac points have a p-like symmetry, whereas the s-like states are dipole forbidden. The large intrinsic coupling strength seems to be a generic property of the semiconducting transition metal dichalcogenides and strong Coulomb-coupling signatures in the form of the optical selection rules can be observed even in samples grown on typical substrates like SiO2. For the examples of WS2 and WSe2, excellent agreement of the computed excitonic resonance energies with recent experiments is demonstrated.

Statistical uncertainty of 2.5×10−16for the199Hg1S0−3P0clock transition against a primary frequency standard

High-resolution spectroscopy has been carried out on the ${}^{199}Hg$ ${}^{1}{S}_{0}\ensuremath{-}{\phantom{\rule{0.16em}{0ex}}}^{3}{P}_{0}$ spin and dipole forbidden transition, where the atoms are confined in a vertical one-dimensional optical lattice trap using light at the magic wavelength. We describe various characteristics of the resulting line spectra and assess the strength of the Lamb-Dicke confinement. Through a series of absolute frequency measurements of the ${}^{199}Hg$ clock transition with respect to the LNE-SYRTE primary frequency standard, recorded over a 3-month period, we demonstrate a statistical fractional uncertainty of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$. We include details relating to the generation of ultraviolet light at three wavelengths necessary for the experiment: 253.7 nm for cooling and detection, 265.6 nm for clock transition probing, and 362.570 nm for lattice trapping, along with further aspects related to the magic wavelength evaluation.

Insights into Magnetically Induced Current Pathways and Optical Properties of Isophlorins

The magnetically induced current density of tetraoxa-isophlorin and dioxa-dithia-isophlorin have been studied at the density functional theory (DFT) level using the gauge including magnetically induced current method (GIMIC). The current density calculations show that the studied isophlorins with formally 28 π electrons are strongly antiaromatic, sustaining paratropic ring currents of -48.5 and -58.3 nA/T, respectively. All chemical bonds of the porphyrinoid macroring participate in the transport of the paratropic ring current. Calculations of excitation energies at the time-dependent density functional theory (TDDFT) level and at correlated ab initio levels show that tetraoxa-isophlorin and dithia-isophlorin have small optical gaps of 0.82-1.34 and 0.90-1.25 eV, respectively. The transition to the lowest excited state, which belongs to the Bg irreducible representation, is dipole forbidden and has therefore not been observed in the spectroscopical studies. For dioxa-dithia-isophlorin, the excitation energies of the Q and B bands calculated at the TDDFT level agree rather well with experimental values with deviations in the range of [-0.15, +0.27] eV, whereas corresponding excitation energies obtained at the ab initio levels are 0.14-0.51 eV too large as compared to experimental values. For tetraoxa-isophlorin, the deviations of the TDDFT excitation energies from experimental values are in the range [-0.10, +0.50] eV and the ab initio values are 0.53-0.97 eV larger than the experimental values due to the significant double excitation character of the excited states.

Observation of Forbidden Exciton Transitions Mediated by Coulomb Interactions in Photoexcited Semiconductor Quantum Wells

We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the 1s exciton emission, which we attribute to the terahertz-induced 1s-to-2p excitation. Simultaneously, a pronounced enhancement of the 2s exciton emission is observed, despite the 1s-to-2s transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the 2s and 2p states, yielding an effective terahertz transition between the 1s and 2s populations.

Slow light from sharp dispersion by exciting dark photonic angular momentum states

A photonic angular momentum state (PAMS) with a topological charge of m≠±1 is dipole forbidden at all polarizations of free-space incidence due to the existence of a unique helical phase. We show that by indirectly exciting dark PAMSs through coupling with a bright resonant element, a sharply variant transmission behavior and strong dispersion can be achieved. This behavior can subsequently be utilized in slow light. A metamaterial design, in which a group index n(g) greater than 500 can be achieved, is present.

Ab initioabsorption spectrum of NiO combining molecular dynamics with the embedded cluster approach in a discrete reaction field

We developed a procedure that combines three complementary computational methodologies to improve the theoretical description of the electronic structure of nickel oxide. The starting point is a Car-Parrinello molecular dynamics simulation to incorporate vibrorotational degrees of freedom into the material model. By means of complete active space self-consistent field second-order perturbation theory (CASPT2) calculations on embedded clusters extracted from the resulting trajectory, we describe localized spectroscopic phenomena on NiO with an efficient treatment of electron correlation. The inclusion of thermal motion into the theoretical description allows us to study electronic transitions that, otherwise, would be dipole forbidden in the ideal structure and results in a natural reproduction of the band broadening. Moreover, we improved the embedded cluster model by incorporating self-consistently at the complete active space self-consistent field (CASSCF) level a discrete (or direct) reaction field (DRF) in the cluster surroundings. The DRF approach offers an efficient treatment of electric response effects of the crystalline embedding to the electronic transitions localized in the cluster. We offer accurate theoretical estimates of the absorption spectrum and the density of states around the Fermi level of NiO, and a comprehensive explanation of the source of the broadening and the relaxation of the charge transfer states due to the adaptation of the environment.