Search Results (149 total)

We study the biorthogonality between decaying and growing eigenmodes in one-dimensional potential barrier problems. It is shown that Petermann factors K_n of the eigenmodes, a measure of nonorthogonality, are involved in decaying mechanism of an initially confined particle. We also show that the decay tail of the growing modes at an exceptional point (EP), where K_n become infinite, is not exponential, but ∼t²e^−Γ_EPt, Γ_EP the decay rate of the decaying mode at EP. In addition, the geometrical phase near an EP is illustrated by the evolution of wave function.

We present spectroscopic observation of an exceptional point or the transition point between mode crossing and avoided mode crossing of neighboring quasieigenmodes in a chaotic optical microcavity of a large size parameter. The transition to the avoided mode crossing was impeded until the degree of deformation exceeded a threshold deformation owing to the system’s openness also enhanced by the shape deformation. As a result, a singular topology was observed around the exceptional point on the eigenfrequency surfaces, resulting in fundamental inconsistency in mode labeling.

We experimentally studied the evolution of quasieigenmodes as classical dynamics undergoing a transition from being regular to chaotic in open quantum billiards. In a deformation-variable microcavity, we traced all high-Q cavity modes in a wide range of frequency as the cavity deformation increased. By employing an internal parameter, we were able to obtain a mode-dynamics diagram at a given deformation, showing avoided crossings between different mode groups, and could directly observe the coupling strengths induced by ray chaos among encountering modes. We also show that the observed mode-dynamics diagrams reflect the underlying classical ray dynamics in the phase space.

We have used cold target recoil ion momentum spectroscopy to study the continuum correlation between the photoelectron of core-photoionized neon and the subsequent Auger electron. We observe a strong angular correlation between the two electrons. Classical trajectory Monte Carlo calculations agree quite well with the photoelectron energy distribution that is shifted due to the potential change associated with Auger decay. However, a striking discrepancy results in the distribution of the relative angle between Auger and photoelectron. The classical model predicts a shift in photoelectron flux away from the Auger emission direction, and the data strikingly reveal that the flux is lost rather than diverted, indicating that the two-step interpretation of photoionization followed by Auger emission is insufficient to fully describe the core-photoionization process.

We investigate two coupled microdisks with nonidentical radii focusing on the parametric evolution of energy levels and the unidirectional far-field emission. We show that the evolution of energy levels is characterized by the avoided crossing intrinsically associated with the exceptional point or the non-Hermitian degeneracy. These spectral properties explain highly asymmetric near-field intensity pattern of the resonance mode. The observed unidirectional far-field emission is shown to be understood by considering the forbidden interdisk coupling in the ray picture induced by the frustrated total internal reflection near the closest point between two disks when the interdisk distance is small enough.

A study of charge-carrier recombination in intrinsic hydrogenated amorphous silicon (a-Si:H) is presented using pulsed electrically and pulsed optically detected magnetic-resonance spectroscopies in order to measure the influence of spin-dependent recombination on photoluminescence (PL) and photoconductivity (PC). The experiments show band tail state recombination influencing the PL but not the PC which constitutes geminate recombination of correlated charge carriers that do not contribute to charge transport. In contrast, nongeminate recombination through silicon dangling bonds is observed influencing both PL and PC. The experiments presented constitute a direct and unambiguous observation of geminate and nongeminate recombination in a-Si:H.

We performed a search for a light pseudoscalar particle X in the decay K_L⁰→π⁰π⁰X, X→γγ with the E391a detector at KEK. Such a particle with a mass of 214.3  MeV/c² was suggested by the HyperCP experiment. We found no evidence for X and set an upper limit on the product branching ratio for K_L⁰→π⁰π⁰X, X→γγ of 2.4×10^-7 at the 90% confidence level. Upper limits on the branching ratios in the mass region of X from 194.3 to 219.3  MeV/c² are also presented.

It is numerically shown that the Petermann excess noise factor diverges to infinity at an exceptional point (EP) found near an avoided resonance crossing in a stadium-shaped dielectric microcavity. Square-root singularity of eigenvalue at the EP is explicitly demonstrated in two-dimensional parameter space spanned by refractive index and deformation parameter. The results imply that mode-mode interaction makes the Petermann factor large.

We study the control of transport properties in a deterministic inertia ratchet system via the extended delay feedback method. A chaotic current of a deterministic inertia ratchet system is controlled to a regular current by stabilizing unstable periodic orbits embedded in a chaotic attractor of the unperturbed system. By selecting an unstable periodic orbit, which has a desired transport property, and stabilizing it via the extended delay feedback method, we can control transport properties of the deterministic inertia ratchet system. Also, we show that the extended delay feedback method can be utilized for separation of particles in the deterministic inertia ratchet system as a particle’s initial condition varies.

We performed a search for the K_L⁰→π⁰νν̅ decay at the KEK 12-GeV proton synchrotron. No candidate events were observed. An upper limit on the branching ratio for the decay was set to be 6.7×10^-8 at the 90% confidence level.

The vertical-beam emittance in an electron storage ring is mainly determined by two factors: the linear betatron coupling and the spurious vertical dispersion generated by magnet errors. We find that the contribution of spurious vertical dispersion is larger than that generated by the linear betatron coupling. Using the independent component analysis (ICA) method, we develop stop band corrections to reduce the vertical emittance. We demonstrate our method by making ICA and correction to a quadruple-bend achromatic low emittance lattice. Six families of skew quadrupoles can effectively minimize both the vertical dispersion and the linear betatron coupling.

We present both experimental and theoretical evidence for uncertainty-limited turnstile transport in deformed microcavities. As the degree of cavity deformation was increased, a secondary peak gradually emerged in the far-field emission patterns to form a double-peak structure. Our observation can be explained in terms of the interplay between turnstile transport and its suppression by the quantum mechanical uncertainty principle.

Using the ORBIT code we study the sensitivity of electron cloud properties with respect to different proton beam profiles, the secondary electron yield (SEY) parameter, and the proton loss rate. Our model uses a cold proton bunch to generate primary electrons and electromagnetic field for electron cloud dynamics. We study the dependence of the prompt and swept electron signals vs the bunch charge and the recovery of electron clouds after sweeping on the beam loss rate and the SEY. The simulation results are compared with the experimental data measured at the proton storage ring at the Los Alamos National Laboratory. Our simulations indicate that the fractional proton loss rate in the field-free straight section may be an exponential function of proton beam charge and may also be lower than the averaged fractional proton loss rate over the whole ring.

We examine the collective quantum dynamics of photons and atoms driven by Raman transitions inside a low-Q cavity. At short times, we show that most of the Stokes photons can be captured by a single time-varying field mode. We determine an approximate analytical form of the mode function and construct a single-mode effective Hamiltonian. The model allows us to address the structure of the photon-atom state explicitly. In particular, we indicate that photons and atoms exhibit a form of photon-atom squeezing during the initial stage of the evolution.

The first search for the rare kaon decay K_L→π⁰π⁰νν̅ has been performed by the E391a collaboration at the KEK 12-GeV proton synchrotron. An upper limit of 4.7×10^-5 at the 90% confidence level was set for the branching ratio of the decay K_L→π⁰π⁰νν̅ using about 10% of the data collected during the first period of data taking. First limits for the decay mode K_L→π⁰π⁰P, where P is a pseudoscalar particle, were also set.

Two partial Siberian snakes were used to avoid all the spin imperfection and vertical intrinsic resonances in the alternating gradient synchrotron (AGS) at Brookhaven National Laboratory. However, the horizontal betatron motion can cause polarization loss resulting from the nonvertical stable spin direction in the presence of two partial snakes. This type of resonance, called a horizontal intrinsic spin resonance, was observed and systematically studied in the AGS. A simplified analytic model and numerical simulation have been developed to compare with experimental data. Properties of the horizontal intrinsic resonance are discussed.

We examine the mode structures of photons emitted by collective spontaneous decay of N_A>1 excited atoms confined in a small volume. By introducing the notion of dynamical modes, we show how the emission process selects a set of time-dependent mode functions in which photons are optimally occupied. In particular we show that the effective number of such dynamical modes is governed by the separability of the two-time correlation function of atomic dipoles. Due to the nonlinearity of atomic dipoles and quantum fluctuations, more than one mode can be involved even in the large-atom-number limit.

The first dedicated experiment for the rare kaon decay K_L⁰→π⁰νν̅ has been performed by the E391a Collaboration at the KEK 12-GeV proton synchrotron. A new upper limit of 2.1×10^-7 at the 90% confidence level was set for the branching ratio of the decay K_L⁰→π⁰νν̅ using about 10% of the data collected during the first period of data taking.

Systematic nonlinear space-charge resonances may cause substantial emittance growth in the nonscaling fixed-field alternating-gradient (FFAG) accelerators. To avoid systematic nonlinear space-charge resonances, the phase advance of each nonscaling FFAG cell must avoid π/2 and π/3. Using multiparticle numerical simulations, we empirically obtain a minimum tune ramp rate vs the systematic 4th order space-charge resonance strength. We also find that the emittance growth obeys a simple scaling property when the betatron tunes cross the linear half-integer and sum resonances.

Spin-resonance strength produced by a localized rf field has been a focus of recent publications [V. S. Morozov , Phys. Rev. ST Accel. Beams 7, 024002 (2004).; M. A. Leonova (to be published).; T. Roser, in Handbook of Accelerator Physics and Engineering, edited by A. W. Chao and M. Tigner (World Scientific, Singapore, 1999), p. 151.; M. Bai, W. W. MacKay, and T. Roser, Phys. Rev. ST Accel. Beams 8, 099001 (2005).; V. S. Morozov , Phys. Rev. ST Accel. Beams 8, 099002 (2005).]. This paper discusses the debated factor of 2, and provides a formula to calculate the component enhanced by the induced betatron motion.

We study the optical interaction in a coupled dielectric microdisks by investigating the splitting of resonance positions of interacting whispering-gallery modes (WGM’s) and their pattern change, depending on the distance between the microdisks. It is shown that the interaction between the WGM’s with odd parity about the y axis becomes appreciable at a distance less than a wavelength and causes directional emissions of the resulting interacting WGM’s. The directionality of the interacting WGM’s can be understood in terms of an effective boundary deformation in ray dynamical analysis. We also discuss the oscillation of the splitting when the distance is greater than a wavelength.

Faraday rotation based on ac Stark shifts is a mechanism that can entangle the polarization states of photons and atoms. We study the entanglement dynamics inside an optical cavity, and characterize the photon-atom entanglement by using the Schmidt decomposition method. The time dependence of entanglement entropy and the effective Schmidt number are examined. We show that the entanglement can be enhanced by the cavity, and the entanglement entropy can be controlled by the initial fluctuations of atoms and photons.

We study the survival probability time distribution (SPTD) in dielectric cavities. In a circular dielectric cavity the SPTD has an algebraic long time behavior, ∼t⁻² in both the TM and TE cases, but shows different short time behaviors due to the existence of the Brewster angle in the TE case where the short time behavior is exponential. The SPTD for a stadium-shaped cavity decays exponentially, and the exponent shows a relation of γ∼n⁻², n is the refractive index, and the proportional coefficient is obtained from a simple model of the steady probability distribution. We also discuss the SPTD for a quadrupolar deformed cavity and show that the long time behavior can be algebraic or exponential depending on the location of islands.