External Optical Field Research Articles

Optical manipulation of photonic defect-modes in cholesteric liquid crystals induced by direct laser-lithography

Manipulation of photonic defect-modes in cholesteric liquid crystals (ChLCs), which are one-dimensional pseudo photonic band-gap materials have been demonstrated by an external optical field. A structural defect in which the pitch length of the ChLC in the bulk and the defect are different was introduced by inducing local polymerization in a photo-polymerizable ChLC material by a direct laser-lithography process, and infiltrating a different ChLC material as the defect medium. When an azobenzene dye-doped ChLC was infiltrated in the defect, the trans– cis isomerization of the dye upon ultraviolet (UV) exposure caused the pitch to shorten, changing the contrast in the pitch lengths at the bulk and the defect, leading to a consequent shifting of the defect-mode. The all-optical manipulation was reversible and had high reproducibility.

Optically Induced Spin-Hall Effect in Atoms

We propose an optical means to realize the spin-Hall effect (SHE) in a neutral atomic system by coupling the internal spin states of atoms to radiation. The interaction between the external optical fields and the atoms creates effective magnetic fields that act in opposite directions on "electrically" neutral atoms with opposite spin polarizations. This effect leads to a Landau level structure for each spin orientation in direct analogy with the familiar SHE in semiconductors. The conservation and topological properties of the spin current, and the creation of a pure spin current are discussed.

Theory and computation of nuclear magnetic resonance parameters

The art of quantum chemical electronic structure calculation has over the last 15 years reached a point where systematic computational studies of magnetic response properties have become a routine procedure for molecular systems. One of their most prominent areas of application are the spectral parameters of nuclear magnetic resonance (NMR) spectroscopy, due to the immense importance of this experimental method in many scientific disciplines. This article attempts to give an overview on the theory and state-of-the-art of the practical computations in the field, in terms of the size of systems that can be treated, the accuracy that can be expected, and the various factors that would influence the agreement of even the most accurate imaginable electronic structure calculation with experiment. These factors include relativistic effects, thermal effects, as well as solvation/environmental influences, where my group has been active. The dependence of the NMR spectra on external magnetic and optical fields is also briefly touched on.

Analysis of some intuitive approaches to the coherent control of state-selected ultracold molecules

Coherent femtosecond optical pulses provide a unique means to manipulate the collision process between a pair of ultracold atoms. The capability to arbitrarily shape strong electric fields gives the possibility of dynamically sculpting the system Hamiltonian on the collisional timescale. In this paper we explore theoretically physically intuitive excitation schemes by which a control field can manipulate the vibronic state populations in an ultracold molecular system, with the goal of bringing atom pairs to short (R < 10 Å) separations where they may form a tightly bound molecule. The microscopic dynamics of individual pairs of colliding atoms under the influence of an external optical field are explored through explicitly time-dependent numerical simulations. The results provide physical justification for the employment of optimal control techniques for the achievement of state-selective formation of ultracold molecules.

Light scattering by nanosized systems with different spatial organizations

The integrodifferential equation method is used to study the spectrum of a nanoparticle colloid for the example of interaction of three arbitrarily arranged dielectric particles made up of nonresonant atoms (the eigenfrequency of the transition is far from the emission frequency) with incorporated barium atoms in an external optical radiation field. The effect on the light-scattering properties of a nanosphere in the ensemble of its distant “neighbors” is studied; an additional peak associated with them is observed as a frequency close to the resonance for an isolated nanosphere, which under certain conditions has higher intensity than the main peak corresponding to optical near-field resonance in a two-particle system. The dependence of the spectrum of the nanosized system on the geometric structure is studied, and it is shown that very precise tuning of the resonance frequency is possible by varying the angular distribution of the particles.

All spins under control

The interaction between electron and nuclear spins is the ultimate limit to using quantum dots as fundamental units for quantum information. Novel results show the way to control this interaction, opening interesting perspectives for quantum computation with solid-state matter. The efforts of scientists around the world to find the ideal physical system to be used for quantum information processing involves the most diverse materials. In this context, solid-state-based applications hold considerable attraction, given the possibility of realizing compact and stable devices that could easily be addressed by external electrical and optical fields.

Phase control of trapped ion quantum gates

There are several known schemes for entangling trapped ion quantum bits for large-scale quantum computation. Most are based on an interaction between the ions and external optical fields, coupling internal qubit states of trapped-ions to their Coulomb-coupled motion. In this paper, we examine the sensitivity of these motional gate schemes to phase fluctuations introduced through noisy external control fields, and suggest techniques to suppress the resulting phase decoherence.

Control of the entanglement of two atoms in an optical cavity via white noise

We employ external optical white-noise fields as the actual driving fields of two atoms inside an optical cavity and investigate how controllable entanglement between two atoms arises in such a situation. Two different action configurations of noise are considered. If two atoms are simultaneously driven by two independent white-noise fields with the same intensity, the entanglement between them is suppressed and eventually completely destroyed by the noise. However, if only one atom is exposed in the white-noise fields, the steady state of the two atoms exhibits entanglement. A stochastic-resonance-like behaviour of steady state entanglement is also revealed. Finally, we examine the Bell violation between atoms and show that the steady state does not violate the Bell inequality even though it is inseparable. The stochastic-resonance-like behaviour cannot be observed in the Bell violation of two atoms during the evolution. The stronger the noise intensity, the more rapidly the Bell violation disappears.

Generation of nonclassical states of light in the Bose-Einstein condensate under electromagnetically induced transparency

A quantum theory of the interaction of the atomic Bose condensate with external optical fields has been developed for a two-beam Λ-scheme close to resonance. Regimes have been obtained where the coefficients of Kerr nonlinearity and nonlinear absorption reach giant values and even become negative, which gives rise to the effect of nonlinear electromagnetically induced transparency. The possibility of the efficient generation of quadrature-squeezed light under the condition of the nonlinear compensation of optical losses has been shown.

Boundary-value problems in near-field optical microscopy and optical size resonances

We have solved a boundary-value problem for a ball probe interacting with a flat dielectric surface in an external optical radiation field. This interaction gives rise to the optical size resonance at frequencies significantly different from the natural frequencies of two-level atoms both in the medium and in the probe with allowance for the local field corrections. These resonances depend significantly on the distance from the probe center to the surface, on the ball probe size, on the concentration of two-level atoms in the probe and in the medium, on the spectral line width, and on the atomic inversion. The field strengths inside and outside the ball probe and a semiinfinite dielectric medium have been calculated in the near-field and wave zones. It is shown that the proposed electrodynamic theory of optical near-field microscopy agrees with the results of experimental measurements.

Collisionless Boltzmann equation with an external periodic traveling force: analytical solution and application to molecular optics.

We present an analytical solution to the collisionless Boltzmann equation for describing the distribution function of molecular ensembles subject to an external periodic traveling force of pulsed optical fields. We apply our solution to study a pulsed standing wave mirror for neutral molecules, recently proposed [P. Ryytty et al., Phys. Rev. Lett. 84, 5074 (2000)]. Using our analytical solution we study the effects of the anharmonicity of optical potential on the reflectivity of the molecular mirror and the corresponding optimal pulse duration. We demonstrate that the reflectivity of the molecular mirror can be significantly improved by optimizing the pulse duration of the external optical fields when taking into account the anharmonicity of molecular motion.

Entangled light from white noise.

An atom that couples to two distinct leaky optical cavities is driven by an external optical white noise field. We describe how entanglement between the light fields sustained by two optical cavities arises in such a situation. The entanglement is maximized for intermediate values of the cavity damping rates and the intensity of the white noise field, vanishing both for small and for large values of these parameters and thus exhibiting a stochastic-resonancelike behavior. This example illustrates the possibility of generating entanglement by exclusively incoherent means and sheds new light on the constructive role noise may play in certain tasks of interest for quantum information processing.

Exciton absorption in semiconductor quantum wells driven by a strong intersubband pump field

Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.

Potential energy surfaces for the collinear H 3+ system

Approximate potential energy surfaces for the two lowest singlet states of collinear H 3 + are computed using the diatomics-in-molecules (DIM) approach. Permanent dipole moments for these states and transition dipole moments between them are computed analytically in this approximation. These potential surfaces and dipole functions are essential for a realistic description of the interaction between the H 3 + and a strong external optical field. The computed results show the correct asymptotic behavior as the inter-nuclear distances in H 3 + become large and are in good agreement with an accurate ab initio calculation but are not currently available as a complete package, either in the literature or from existing ab initio quantum-chemistry softwares.

Adiabatic Approximation in Interaction Between Two-Level Atom in an External Potential and Optical Field

In discussion about the detection of the Bose–Einstein condensation, the adiabatic approximation method was taken. According to the perturbation theory, this approximation is studied in detail by a single atom model in this paper.

Inhibition of spontaneous emission from quantum-well magnetoexcitons.

We propose a method for tailoring the radiative properties of two-dimensional magnetoexcitons. First, we show that the magnetoexciton-photon coupling is purely bilinear in the strong-magnetic-field limit, implying that optical nonlinearities appear through the breakdown of bosonic commutation relations at high densities. The dispersion curve of magnetoexcitons may be modified or engineered using external electric or optical fields. We show that the application of an in-plane electric field will render the ground-state magnetoexcitons stable against radiative recombination, as a result of the momentum conservation. Based on this effect, we propose a particular geometry which should allow for the investigation of condensation effects and superfluidity in direct-band-gap semiconductors. \textcopyright{} 1996 The American Physical Society.

A simplified approach to optimally controlled quantum dynamics

A new formalism for the optimal control of quantum mechanical physical observables is presented. This approach is based on an analogous classical control technique reported previously [J. Botina, H. Rabitz, and N. Rahman, J. Chem. Phys. 102, 226 (1995)]. Quantum Lagrange multiplier functions are used to preserve a chosen subset of the observable dynamics of interest. As a result, a corresponding small set of Lagrange multipliers needs to be calculated and they are only a function of time. This is a considerable simplification over traditional quantum optimal control theory [S. Shi and H. Rabitz, Comp. Phys. Comm. 63, 71 (1991)]. The success of the new approach is based on taking advantage of the multiplicity of solutions to virtually any problem of quantum control to meet a physical objective. A family of such simplified formulations is introduced and numerically tested. Results are presented for these algorithms and compared with previous reported work on a model problem for selective unimolecular reaction induced by an external optical electric field.

Multi-electron ejection of inner-shell electrons through multiphoton excitation of clusters

The existence of coherent electronic motions induced by the presence of a strong external optical field enables the process of direct multi-electron ejection from an inner-shell to occur with high probability. Experimental evidence for this mechanism is present in the observed spectra of five cases studied (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)). Analysis shows that the multiphoton interaction can enter a regime of strong-coupling (Z2a>or=1) in which the rate of multiple electron ejection can become comparable to the removal of a single electron. The enhanced coupling arises from the coherent motion of the (Z) field-ionization electrons whose interactions constructively interfere, thereby behaving like a quasi-particle with a charge Ze. This mechanism consistently explains (1) the unusually high values of Xe M-shell ionization observed in both Xe(M) (Xe36+) and Xe(L) (Xe45+) emissions and (2) the anomalous double 2p vacancy production seen in the Xe(L) spectrum. Clusters of heavy atoms appear as exceptionally favourable systems for the copious production of multiple vacancy hollow atom configurations.

Modulating light with light: A slab waveguide-liquid crystal device

Abstract We present an experiment of modulation of the light propagating in a waveguide-liquid crystal (LC) planar stack, by an external light source. The results confirm that the molecular reorientation in the LC layer, induced by an external optical field, gives rise to a change in the refractive index which in turn modifies the propagation constants of the guided modes, so that the light can be partially leaked off the guide. It is shown how the modulation depends on the physical parameters of the materials and of the light.

Molecular-dynamics simulator for optimal control of molecular motion

In recognition of recent interest in developing optimal control techniques for manipulating molecular motion, this paper introduces a computer-driven electromechanical analog of this process. The resultant molecular-dynamics simulator (MDS) is centered around a linear air track for which the atoms of the controlled molecule are simulated as nearly frictionless carts on the track. Bonds in the simulated molecule are described by precision springs, and the interaction with an external optical field is simulated through a computer-based linear driver. When the MDS is operated in the harmonic regime, it can be used as an exact analog of molecular-scale quantum systems through Ehrenfest’s theorem or, equivalently, as a classical set of coupled oscillators. The tools of optimal control theory currently being applied at the molecular scale are used to design the forcing function for the MDS. Optical encoders are used to measure bond distances for graphic representation of the MDS behavior. Bond breaking can also be simulated by bond-length-sensitive trigger-release mechanisms. The MDS is especially useful as a modeling tool to bridge theoretical studies and eventual laboratory experiments at the true molecular scale.