Yasushi Ohfuti

Theory of resonant SNOM (scanning near-field optical microscopy): breakdown of the electric dipole selection rule in the reflection mode

A theoretical study is reported about SNOM with the use of light in resonance with the quantized levels of a mesoscopic sample. The microscopic variation of electromagnetic field and the coherence of matter state are properly treated by solving the microscopic Maxwell equations and Schrodinger equation self consistently. A remarkable effect, i.e., the breakdown of the dipole selection rule, is shown in the reflection mode where a probe tip is used for both illumination of the sample and collection of the signal. Namely, both dipole-active and higher-multipole-active levels give comparable signal intensities for appropriate probe tip positions.

Study of Scanning Near-Field Optical Microscopy (SNOM) by Nonlocal Response Theory

In the pursuit of higher resolution and signal intensity in SNOM by making use of resonant light, we propose to use a microscopic nonlocal theory of optical response, which allows a microscopic description of EM field down to atomic size. Model calculations are shown for various assemblies of spheres with resonant levels. It is demonstrated that the choice of light frequency at various resonances of the system reveals their characteristic near-field patterns, in contrast to the bias voltage dependence of STM. When the light frequency is resonant to a level of the whole system including probe tip, there can be an extreme distortion of image, because the resonant energy changes quickly as one scans the tip. We make a proposal to make a positive use of this effect as a new type of SNOM.

Nonlocal aspects of the optical responses of mesoscopic systems

Abstract We first outline the framework of microscopic nonlocal theory of optical response which determines the self-consistent motions of radiation field and induced current. Noting the several advantages of this formulation over traditional local theories, we demonstrate various examples of application, which include the cases of an ultrafine particle, its assembly in various forms, and a slab as linear and/or nonlinear responses.

Resonant size enhancement of induced polarization with particular spatial distribution in optical response of mesoscopic systems

Abstract For an incident light resonant with a size quantized material level, the induced polarization with a particular spatial pattern has been predicted to be resonantly size-enhanced, which could lead to a double resonance effect in energy (frequency) and size in pump-probe type nonlinear processes. Two examples are shown for a slab and a finite 1D chain of identical fine particles.

Radiative decay rate of a quantum well exciton in a semiconductor microcavity: Cross-over behavior of exciton- and cavity-modes

Abstract A cross-over behavior was found between microcavity (MC) mode and quantum well (QW)-exciton mode as a function of parallel wave vector k for a QW in a MC, i.e., a 2D structure consisting of two sets of distributed Bragg reflectors (DBR) with a QW at the center of them. The radiative widths of the two modes were calculated as functions of k, the layer number of DBR, and the non-radiative width of the QW-exciton. The radiative decay rate of the QW-exciton mode shows a remarkable enhancement at a critical value of k, which is determined by the relative frequencies of the “empty” MC and “bare” QW-exciton modes. The cross-over behavior is the result of the excessive mixing of the two modes.

Nonlocal response of size-quantized excitons in a semiconductor microsphere

Abstract By using nonlocal response theory which determines the selfconsistent motions of induced polarization and radiation field, the linear response of size-quantized excitons in a semiconductor microsphere has been calculated as a function of the radius of the sphere. The induced polarization for resonant light shows a remarkable size-resonant enhancement.

New method of photonic band calculation and its application

Abstract A new formulation of photonic band calculation is presented. Though it uses plane wave (PW) expansion, an eigenvalue equation is set up for inverse frequency squared, and the convergence behavior is much better than the simple PW method. Thus, it reduces the burden in numerical works to treat various shapes of the unit dielectrics of photonic crystals. A few examples of application are presented.

A soliton in polyacetylene: Optical and dynamical activities

Abstract Doping of a polyacetylene chain is numerically simulated. The Su, Schrieffer, and Heeger model is used for the chain. Two types of doping mechanisms are examined. One is the doping by “site-type” impurities. They randomly modify the electronic energy levels at each site. Other is by “bond-type” impurities. The electronic transfers between two adjacent sites are randomly modified. For each random configuration of impurities, we determine metastable bond configurations, electron concentrations, dynamical conductivity, and infrared active phonon modes. It is found that the doping gives rise to solitons which are pinned by the impurities. Dynamical conductivity shows that the charged solitons, produced by the doping, are playing important roles in the infrared absorption.

Separable nature of nonlocal susceptibilities for general cases of interacting radiation-matter systems

Abstract A revision and an extension has been made to our recent work on the separable nature of nonlocal electric susceptibilities of matter. The separability holds always for resonant processes. It is valid also for nonresonant processes if the spatial variation of the (transverse) vector potential is negligible, which holds approximately in many cases approximately and even exactly for certain cases. This argument can be extended to nonlocal magnetic susceptibilities by including the spin Zeeman term and the spin-orbit interaction in the matter Hamiltonian.

Nonlocal Formulation of Optical Response of an Assembly of Fine Particles

A nonlocal optical response formulation developed recently is applied to an assembly of fine particles with a resonant level. In the theory, the field produced by electron excitation is fed back selfconsistently to the motion of the electrons. The selfconsistency between Maxwell and Schrodinger equations is kept within the linear response for polarization and it gives rise to the radiative lifetime and the Lamb shift within the semi-classical treatment. From the obtained response spectra, the radiative width and the Lamb shift are evaluated as functions of the number of the particles. It is observed that the radiative width deviates from the linear dependence on the system size even for a system size of about one-tenth of the light wavelength.