Antiparallel Dipole Moments Research Articles

Simulating quantum interference effects in the fluorescence of a V-system with perpendicular dipole moments

We present a scheme that enables the observation of interference effects in the resonance fluorescence of a V-type atom with orthogonal dipole moments. Specifically, we consider the atomic configuration of a J g = 0 to J e = 1 transition driven by a single laser field. By employing polarization-sensitive detection in such a way that the light emitted on the two transitions become indistinguishable, we show that one can simulate the effect of vacuum-induced coherence on the resonance fluorescence of this system. In addition, we demonstrate the possibility of realizing atomic transitions with both parallel and antiparallel dipole moments and their effects on the fluorescence spectrum. The interference induced leads to interesting features in the fluorescence spectrum such as asymmetric spectral peaks, enhancement and suppression of the sidebands, and disappearance of fluorescence in the particular direction of detection. The numerical results are understood in the context of the dressed states of the combined atom–field system.

Metastable bound states of the quasi–bimagnetoexcitons in the lowest Landau levels approximation

Four different spin structures of two electrons and of two holes situated on the lowest Landau levels (LLLs) are taken into account to investigate possible bound states of the two-dimensional magnetic biexciton formed of two magnetoexcitons with opposite wave vectors and antiparallel dipole moments. The singlet and triplet states of the spins of two electrons and of two holes separately, as well as of two para- and two ortho-magnetoexcitons are considered. The general expressions describing the binding energy of the bound states and the normalization conditions characterized by the effective spin parameter for the corresponding wave functions are derived. The most favorable of the four considered spin configurations happened to be the triplet-triplet spin structure of two electrons and of two holes. In its frame a metastable bound state with activation barrier comparable with two ionization potentials of the magnetoexciton is revealed .

Metamaterial made of V-type systems from LiH molecules

We build a V-type system from LiH molecule with antiparallel dipole moments by applying a strong laser field to one of the two transitions in the \(\varLambda \)-type system made of the excited vibrational state \(A^{1}\varSigma ^{+} (v=1,\,j=1)\) and the vibrational states of the ground level \(X^{1}\varSigma ^{+} (v=1,\,j=0)\) and \(X^{1}\varSigma ^{+} (v=0,\,j=0)\). This has been called previously by us as V-type system of the second kind. In this system, we have magnetic dipole-allowed transitions in addition to the electric dipole-allowed transitions due to the fact that the obtained dressed states have mixed parities. We find that a medium with V-type systems of the second kind from LiH molecule can have a negative refractive index and/or can be a left-handed medium and therefore can be a metamaterial. We determine the incoherent pumping rate \(\varLambda \), the probe detuning \(\Delta _{p}\) and the relative phase between the probe and coupling field phases \(\phi \), for which our medium of molecular systems achieves a negative refractive index and/or both negative electric permittivity and magnetic permeability.

Dark spots along slowly scaling chains of plasmonic nanoparticles.

We numerically investigate the optical response of slowly scaling linear chains of mismatched silver nanoparticles. Hybridized plasmon chain resonances manifest unusual local field distributions around the nanoparticles that result from symmetry breaking of the geometry. Importantly, we find localization patterns characterized by bright hot-spots alternated by what we term dark spots. A dark spot is associated to dark plasmons that have collinear and antiparallel dipole moments along the chain. As a result, the field amplification in the dark interjunction gap is extinguished for incident polarization parallel to the chain axis. Despite the strong plasmonic coupling, the nanoparticles on the sides of this dark gap experience a dramatic asymmetric field amplification with amplitude gain contrast > 2×102. Remarkably, also for polarization orthogonal to the axis, gap hot-spots form on resonance.

Charge density of the biologically active molecule (2-oxo-1,3-benzoxazol-3(2H)-yl)acetic acid.

(2-Oxo-1,3-benzoxazol-3(2H)-yl)acetic acid is a member of a biologically active class of compounds. Its molecular structure in the crystal has been determined by X-ray diffraction, and its gas phase structure was obtained by quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory. In order to understand the dynamics of the molecule, two presumably soft degrees of freedom associated with the relative orientation of the planar benzoxazolone system and its substituent at the N atom were varied systematically. Five conformers have been identified as local minima on the resulting two-dimensional potential energy surface within an energy window of 27 kJ mol(-1). The energetically most favourable minimum closely matches the conformation observed in the crystal. Based on high-resolution diffraction data collected at low temperature, the experimental electron density of the compound was determined. Comparison with the electron density established by theory for the isolated molecule allowed the effect of intermolecular interactions to be addressed, in particular a moderately strong O-H...O hydrogen bond with a donor...acceptor distance of 2.6177 (9) Å: the oxygen acceptor is clearly polarized in the extended solid. The hydrogen bond connects consecutive molecules to chains, and the pronounced charge separation leads to stacking between neighburs with antiparallel dipole moments perpendicular to the chain direction.

Bose–Einstein condensation of quasiparticles in graphene

The collective properties of different quasiparticles in various graphene-based structures ina high magnetic field have been studied. We predict Bose–Einstein condensation (BEC)and the superfluidity of 2D spatially indirect magnetoexcitons in a two-layer graphene. Thesuperfluid density and the temperature of the Kosterlitz–Thouless phase transition areshown to be increasing functions of the excitonic density but decreasing functions of amagnetic field and the interlayer separation. The instability of the ground state of theinteracting 2D indirect magnetoexcitons in a slab of superlattice with alternating electronand hole graphene layers (GLs) is established. The stable system of indirect 2Dmagnetobiexcitons, consisting of a pair of indirect excitons with antiparallel dipolemoments, is considered in a graphene superlattice. The superfluid density and thetemperature of the Kosterlitz–Thouless phase transition for magnetobiexcitons in agraphene superlattice are obtained. Moreover, the BEC of excitonic polaritons in a GLembedded in a semiconductor microcavity in a high magnetic field is predicted. While thesuperfluid phase in this magnetoexciton polariton system is absent due to a vanishingmagnetoexciton–magnetoexciton interaction in a single layer in the limit of a high magneticfield, the critical temperature of the BEC formation is calculated. The observation ofthe BEC and superfluidity of 2D quasiparticles in graphene in a high magneticfield would be interesting confirmation of the phenomena we have described.

Quantum interference and phase-dependent fluorescence spectrum of a four-level atom with antiparallel dipole moments

We investigate the effects of spontaneously generated interference on the spectrum of quantum fluctuations in phase quadratures of resonant fluorescence emitted from the transition of j = 1/2 to j = 1/2 of a monochromatically driven four-level atom with antiparallel dipole moments. It is found that the spectrum exhibits obvious characteristics of quantum interference in both cases of weak and strong laser fields. For the Zeeman-degenerate system, strong radiation squeezing in weak atomic excitation can occur at the central frequency of the spectrum, which is quite different from the case without consideration of the interference. Moreover, when we apply an external magnetic field to the atom, we find that the quantum interference can lead to a narrowing spectrum with much stronger squeezing by adjusting the magnetic field strength. These results suggest that the phase-dependent fluorescence spectrum can serve as a probe of the quantum interference effects in a realistic system such as a trapped 198Hg+ ion or charged quantum dots.

Quantum interference between decay channels of a three-level atom in a multilayer dielectric medium

Spontaneous emission of an atom with two close upper levels and one lower level in a multilayer dielectric medium, and quantum interference between transitions from the upper levels to the lower one, are considered. Since the medium is inhomogeneous along the direction normal to the interfaces of the dielectric layers, the vacuum has an anisotropic spatial structure, and quantum intereference between the two decay channels can appear even if the two dipole matrix elements are orthogonal to each other. If the atom is placed in a three-layer dielectric plate cavity, a strong quantum interference similar to that for an atom with two nearly parallel dipole moments in free space arises due to the influence of anisotropic vacuum. Putting the atom in a three-layer dielectric waveguide and a one-dimensional photonic crystal, there appears a strong quantum interference similar to that for an atom with nearly antiparallel dipole moments.

Quantum interference in the fluorescence of a molecular system

It has been observed experimentally [H.R. Xia, C.Y. Ye, and S.Y. Zhu, Phys. Rev. Lett. {\bf 77}, 1032 (1996)] that quantum interference between two molecular transitions can lead to a suppression or enhancement of spontaneous emission. This is manifested in the fluorescent intensity as a function of the detuning of the driving field from the two-photon resonance condition. Here we present a theory which explains the observed variation of the number of peaks with the mutual polarization of the molecular transition dipole moments. Using master equation techniques we calculate analytically as well as numerically the steady-state fluorescence, and find that the number of peaks depends on the excitation process. If the molecule is driven to the upper levels by a two-photon process, the fluorescent intensity consists of two peaks regardless of the mutual polarization of the transition dipole moments. If the excitation process is composed of both a two-step one-photon process and a one-step, two-photon process, then there are two peaks on transitions with parallel dipole moments and three peaks on transitions with antiparallel dipole moments. This latter case is in excellent agreement with the experiment.

Evidence for the High-Energy (Collinear) van der Waals Isomer of the Acetonitrile Dimer

While the stable configuration of the acetonitrile dimer has antiparallel dipole moments, recent calculations [Popelier, et al. Faraday Discuss. 1994, 97, 243] reveal a high-energy isomer in which the dipoles are collinearly aligned. We report direct evidence for the existence of this isomer using a strategy in which we enforce the less-favorable orientation of the neutrals by electrostatically binding them to the iodide ion in the I-·(CH3CN)2 cluster. The collinear dimer is then released by photoexcitation of the dipole-bound excited state of I-·(CH3CN)2, producing the (CH3CN)2- anion as the dominant ionic photofragment. The fragment is mass selected and stripped of the excess electron in a modest external electric field (∼10 kV/cm), indicating that the anion consists of a diffuse electron as expected for a “dipole-bound” species. This behavior suggests that the force field of the neutral “core” complex supports a minimum in the highly dipolar, collinear configuration.

Microscopic Theory of Reentrant Behavior in Liquid Crystals†

Abstract The extraordinary phase sequences, namely nematic(N)→smectic A(SA1)→reentrant nematic (Nre) and N → SA1 → Nre → reentrant smectic A(SAre) occur with decreasing temperature in strongly polar liquid crystals (LC). The period of the smectic density wave in intermediate SA1 phase is not commensurate with the molecular length I but varies for different compounds from 1.1 l up to 1.6 l and the period of the density wave in the SAre phase is almost equal to molecular length (i.e. 0.99 l). In order to explain this behavior it is assumed that the pairing of molecules with antiparallel dipole moments takes place in polar LCs. In the present paper, in a mean field approximation, we considered LCs consisting of dimers (paired molecules) and monomers (unpaired molecules) in which two density waves with periods equal to the length of the dimer (d) and the length of monomer (l), respectively, may exist. The free energy F of such LCs is expressedin terms of amplitudes of these density waves, temperature T and di...

Volume viscosity and structure of n-propanol by ultrasonics

The adiabatic volume viscosities η v and η′ v have been evaluated in n-propanol along with X 2 (the molefraction of molecules arranged in pairs with antiparallel dipole moments), using experimental ultrasonic absorption data in the temperature range 10°C to 80°C. Both η v and η′ v decrease nonlinearly with temperature and the ratio η′ v/η v is found to be nearly unity at all temperatures.

Ultrasonic study of volume viscosity and structure of ethanol as a function of pressure

Structural volume viscosities η v and η v' have been evaluated in ethanol along with Δ V, Δ F and x 2 (the mole fraction of molecules arranged in pairs with antiparallel dipole moments) using experimental ultrasonic absorption data in the pressure range 1–5000 kg/cm 2. Both η v and η v' increase with pressure and the ratio η v/gh v' is found to be nearly unity at all the pressures. x 2 shows an exponential fall with pressure and Δ V shows a minimum at p ≈1000 kg/cm 2.

Critical Störmer Conditions in Quadrupole and Double Ring-Current Fields

A theoretical study has been made of the behavior of the critical Störmer pass points for general axially symmetric magnetic configurations. A topological method has been derived to predict the occurrence of the critical Störmer conditions for charged-particle exclusion. This analytic technique, when applied to geomagnetically interesting fields, should be a useful aid to the understanding of experimental data. The method is applied here to three magnetic geometries: double ring currents with parallel dipole moments and with antiparallel dipole moments, and the axial magnetic quadrupole. The topology of the regions representing allowed motion is treated systematically and the behavior of the critical pass points is illustrated in typical Störmer plots. For the quadrupole and the antiparallel ring system critical pass points are found to occur only out of the equatorial plane. For the parallel ring system, critical points can occur in or out of the equatorial plane. For certain special conditions as many as three simultaneous critical pass points are found, and two simultaneous points occur for a wide range of parameters.