Hiroharu Tamaru

Resonant light scattering from metal nanoparticles: Practical analysis beyond Rayleigh approximation

We propose a simple analytical formula that can quantitatively predict resonant light scattering from metal nanoparticles of arbitrary shape, whose sizes are too large for Rayleigh approximation to be applicable. The formula has been derived as an empirical extension of Mie’s rigorous calculation for light scattering from spheres. It can very well reproduce the experimental characteristics of light scattering from Au nanorods.

Resonant light scattering from individual Ag nanoparticles and particle pairs

Light scattering by individual Ag nanoparticles and structures have been studied spectroscopically. Individual particles were selected and manipulated with a micromanipulator installed inside a scanning electron microscope (SEM). With typical particle dimensions of some 100 nm, the plasma resonances of particles and the coupled modes of particle pairs were observed in the visible region. The polarization dependence of the resonance frequencies strongly reflects the shape anisotropy; the effect that would be averaged out for experiments on ensembles. With a simple approximation to take the glass substrate into account, the results are in good agreement with the analytical calculations by Mie scattering, and with numerical calculations by the finite-difference time-domain method, both of which are performed with the morphological parameters obtained from the SEM observation for the corresponding particle or particle pair.

Phase control through anisotropic strain in Nd0.5Sr0.5MnO3 thin films

Strain effect in charge- and orbital-ordered state has been investigated for Nd0.5Sr0.5MnO3 thin films deposited on (100), (110), and (111)-oriented substrates of SrTiO3. Films on (001) and (111) substrates have a monotonous temperature dependence for magnetic and transport properties showing no first-order phase transition. On the other hand, films on (110) substrate show a clear ferromagnetic-antiferromagnetic and metal-insulator transition around 170K similar to that in a bulk single crystal, which is a manifestation of the charge and orbital order. Precise control of the hole concentration was also demonstrated around half doping.

Optical properties of two‐dimensional dye aggregate

Absorption and emission spectra of a monolayer J aggregate of a cyanine dye have been studied in detail from room temperature to liquid helium temperature. Various sources of disorder that affects the degree of delocalization of the exciton have been investigated including the hitherto unnoticed effect of He gas. Simple numerical simulations were performed, which reproduce the basic features of the spectra fairly well. The spectral signatures indicate that we have a typical two‐dimensional Frenkel exciton system characterized by large red shift and large cooperation number despite strong disorder, which is in contrast to the well‐studied pseudoisocyanines (PICs) that have been analyzed as one‐dimensional even in the Langmuir–Blodgett monolayers. The disorder stems partly from the exciton–phonon coupling but is dominantly of the static character at low temperatures.

Quantitative evaluation of blinking in surface enhanced resonance Raman scattering and fluorescence by electromagnetic mechanism.

We analyze blinking in surface enhanced resonance Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of rhodamine 6G molecules as intensity and spectral instability by electromagnetic (EM) mechanism. We find that irradiation of intense NIR laser pulses induces blinking in SERRS and SEF. Thanks to the finding, we systematically analyze SERRS and SEF from stable to unstable using single Ag nanoparticle (NP) dimers. The analysis reveals two physical insights into blinking as follows. (1) The intensity instability is inversely proportional to the enhancement factors of decay rate of molecules. The estimation using the proportionality suggests that separation of the molecules from Ag NP surfaces is several angstroms. (2) The spectral instability is induced by blueshifts in EM enhancement factors, which have spectral shapes similar to the plasmon resonance. This analysis provides us with a quantitative picture for intensity and spectral instability in SERRS and SEF within the framework of EM mechanism.

Selection rules for light-induced magnetization of a crystal with threefold symmetry: the case of antiferromagnetic NiO.

We propose Raman-induced collinear difference-frequency generation (DFG) as a method to manipulate dynamical magnetization. When a fundamental beam propagates along a threefold rotational axis, this coherent second-order optical process is permitted by angular momentum conservation through the rotational analogue of the Umklapp process. As a demonstration, we experimentally obtained polarization properties of collinear magnetic DFG along a [111] axis of a single crystal of antiferromagnetic NiO with micro multidomain structure, which excellently agreed with the theoretical prediction. PACS numbers: 78.20.Ls, 42.65.-k, 76.50.+g

Pseudomorphic strain effect on the charge-orbital ordering pattern in Pr0.5Sr0.5MnO3 epitaxial thin films

We accomplished coherent epitaxial growth of a Pr0.5Sr0.5MnO3 thin film on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (110) substrate, which demonstrates a robust charge-orbital ordering (COO) phase transition. In addition to the isotropic transport properties, a drastic recovery of high-resistivity state (>20Ωcm) from a field-cooled metallic state (∼0.5mΩcm) was observed at 5K with a release of a magnetic field. These results indicate a possible modification of COO pattern into CE-type COO state [d(3x2−r2∕3y2−r2)] in the film from A-type stripe COO state [d(x2−y2)] observed in bulk samples by a substrate-induced strain.

Microsphere resonators strongly coupled to a plane dielectric substrate: coupling via the optical near field

A model is proposed that describes the essential optical process in the recently observed resonant light scattering from a microsphere resonator that is strongly coupled to the substrate. The experimentally observed field patterns across the resonance can be reproduced quite well by a numerical calculation taking into account only a few vector spherical waves that are converted from nonpropagating to propagating waves at the substrate surface. Explicit consideration of the multiple-reflection effect is not necessary to reproduce the experimental results. Comparison of the experiment and the calculation suggests the splitting of degenerate resonance modes that have different azimuthal mode numbers within a single broad resonance line. These results are discussed on the basis of the strongly coupled nature of the system.

Fluctuating single sp2 carbon clusters at single hotspots of silver nanoparticle dimers investigated by surface-enhanced resonance Raman scattering

We evaluate spectral changes in surface enhanced resonance Raman scattering (SERRS) of near-single dye molecules in hotspots of single Ag nanoparticle (NP) dimers. During the laser excitation, surface enhance florescence (SEF) of dye disappeared and the number of SERRS lines decreased until finally ca. two lines remained around 1600 and 1350 cm−1, those are evidence of G and D lines of single sp2carbon clusters. Analysis of the G and D line intensity ratios reveals the temporal fluctuation in the crystallite size of the clusters within several angstroms; whereas, broadening and splitting in the lines enable us for identifying directly the dynamics of various defects in the clusters. This analysis reveals that the detailed fluctuations of single sp2carbon clusters, which would be impossible to gain with other microscopic methods.

Observation of a modulation effect caused by a microsphere resonator strongly coupled to a dielectric substrate.

The effect of modulation caused by a microsphere resonator is experimentally investigated with a model system consisting of a microsphere resonator and a plane substrate. We used total internal reflection microscopy (TIRM), which is a combination of conventional optical microscopy and the total internal reflection method, and observed the intensity distribution under the resonator in the evanescent-wave incidence condition. The TIRM patterns drastically change when the wavelength of the incident beam is scanned across a resonance. The response of the system is discussed on the basis of a recent proposal of traveling-wave resonance.