Akira Shojiguchi

Excitation energy transfer and population dynamics in a quantum dot system induced by optical near-field interaction

Energy transfer and exciton population dynamics in a two-quantum dot system coupled with a phonon heat-bath system are examined using the density matrix formalism. In such a system, optical near-field interactions induce energy transfer between quantum dots, and exciton–phonon interactions guarantee the unidirectional excitation energy transfer. Our theoretical investigation shows that the population dynamics change drastically depending on the coupling strengths due to optical near-field interactions and exciton–phonon heat-bath interactions. The temperature effect promotes frequent energy back-transfer from the heat-bath to the quantum dot system. Applying our theoretical formulation, we numerically calculate the time evolution of populations, and estimate energy transfer time or state-filling time for a CuCl quantum dot system. The estimated time is suitable for the elements in our proposed optical nano-switch and nano-photonic devices.

Superradiance and dipole ordering of an N two-level system interacting with optical near fields

A model is presented for a system of N two-level excitons interacting with each other via optical nearfields represented as localized photons. In a low exciton density limit, quantum dynamics of the dipolemoments or quantum coherence between any two energy levels is linear. As the exciton density becomeshigher, the dynamics becomes nonlinear, and the system has several kinds of quasi-steady states of thedipole distribution depending on the system parameters. These quasi-steady states are classified with thehelp of the effective Hamiltonian that is derived from the renormalization of degrees of freedom oflocalized photons with a unitary transformation. Among them there exist a ‘‘ferromagnetic’’ state(dipole-ordered state), in which all electric dipoles are aligned in the same direction, and an ‘‘anti-ferromagnetic’’ state, where all dipoles alternatingly change the direction. In addition, we show that anarbitrary state can be transformed into a dipole-ordered state by manipulating initial values of thepopulation differences appropriately. For example, if we initially prepare a dipole-forbidden state, whichis similar to the ‘‘anti-ferromagnetic’’ state and cannot be coupled with propagating far fields, and if wemanipulate the distribution of the population differences properly, the initial state evolves into a dipole-ordered state. The radiation property of such dipole-ordered states is examined in detail. Neglectingenergy dissipation by radiation, we find that some of the ordered states show strong radiation equivalentto Dicke’s superradiance. Then by introducing a radiation reservoir, the dissipative master equation isderived. Solving the equation with and without quantum correlations, we numerically show that multiplepeaks in the radiation profile can survive in both cases. The mechanism of this phenomenon is discussed,and a brief comment on an application to photonic devices on a nanometer scale is given.

Dynamical Hierarchy in Transition States of Reactions

Abstract.We present a partial normalization procedure of Lie canonical perturbation theory to elucidate the phase space geometry of the transition state in the multidimensional phase space for a wide range of energy above the threshold. State selectivity and dynamical correlation along the evolution of reactions will also be discussed.

Excitation dynamics in a three-quantum dot system driven by optical near-field interaction: towards a nanometric photonic device

Using density operator formalism, we discuss interdot excitation energy transfer dynamics driven by the optical near‐field and phonon bath reservoir, as well as coherent excitation dynamics of a quantum dot system. As an effective interaction between quantum dots induced by the optical near‐field, the projection operator method gives a renormalized dipole interaction, which is expressed as a sum of the Yukawa functions and is used as the optical near‐field coupling of quantum dots. We examine one‐ and two‐exciton dynamics of a three‐quantum dot system suggesting a nanometric photonic switch, and numerically obtain a transfer time comparable with the recent experimental results for CuCl quantum dots.

Wavelet Analysis and Arnold Web Picture for Detecting Energy Transfer in a Hamiltonian Dynamical System

Our motivation is to understand how, in chemical reactions, the reaction coordinate effectively gains dynamical energy from the other degrees of freedom (i.e., bath coordinates) avoiding thermalization of the redistributed energy. In such a system, the phase space structure should be not homogeneous; i.e., the system is never ergodic. In this study, we introduce a way to capture the inhomogeneity of the phase space and to monitor energy transfers among their partial degrees of freedom in nonergodic systems using wavelet analysis and a picture of the Arnold web. First, we examine several simple energy transfer processes, i.e., a motion on a resonance line, between resonance lines, and around a resonance junction in a simple three-degree-of-freedom (DOF) system and show how the elemental processes of the intramolecular vibrational energy redistribution (IVR) are detected by our tools. We especially note that the structure of the higher order resonance of the system can be detected by wavelet analysis and motion in the action space. Next, we analyze a reaction process in a simple Hamiltonian system of 3 DOF with a double-well potential, i.e., a system with a transition state of the center-saddle-center type, and detect energy transfers in the reactive process. The aim of the study is to propose a way to characterize the inhomogeneity of the phase space, e.g., the reactive doorway, which leads to controllability of the chemical reaction by light, i.e., control of the reaction by selectively preparing an initial state in the reactive doorway by optical excitation.

Coherent Dipole Oscillation Induced by Localized Photons

This paper describes a novel model for the dynamics of the dipole coherence in two-level quantum dots (QDs) interacting with localized photons. The model is based on the fact that the coupling on a nanometer scale between QDs via optical near fields represented by localized photons is much stronger than that via propagating far fields. The model predicts that an intriguing phenomenon such as a collective dipole oscillation occurs if the initial quantum states are locally manipulated. The origin of this kind of collective behaviour is discussed in view of Dickes superradiance.

Linear CDMA Detection Algorithm Based on Statistical Neurodynamics and Belief Propagation and the Stability Conditions

Recently a modified algorithm of code-division multiple-access (CDMA) parallel interference canceler (PIC) has been proposed by Tanaka based on statistical neurodynamics. In this paper we apply the modified algorithm to the linear PIC (LPIC) and investigate its stability. We show that the stable (unstable) fixed points of the modified algorithm correspond to the stable (unstable) replica symmetry solutions with the Gaussian prior. We also show the modified algorithm is a special case of Kabashima’s belief-propagation algorithm with Gaussian prior.

Improving the performance of linear parallel interference cancellation for CDMA using a method of the statistical mechanics

In this paper we apply a modification scheme of the parallel interference cancellation (PIC), recently proposed by Tanaka and Okada based on techniques of the statistical neurodynamics, to the linear PIC (LPIC) with a tentative soft decision function f(x)=ax, and show that the modification enhances its convergence property.

A nanophotonic switch: transient dynamics and switching operation

The operation of a nanophotonic switch was investigated theoretically. An increase in the exciton population with fast vibrations was obtained in the state-filling condition. We also found that the population decay time can be shortened by changing the state-filling time.

Functional operations using a near-field optically coupled quantum-dot system

We investigate the population dynamics in a three quantum-dot system coupled via an optical near field, which consists of a coherent operation part and an output part. We analytically show that a resonance condition between the two parts depends on initial excitation in the coherent operation part. Using this feature, AND- and XOR-gate operations are demonstrated in the case of symmetrically arranged quantum dots. We also evaluate the effects of asymmetric arrangement on these operations, and show that the coherence plays an important role in an asymmetric system.