Table of contents

Preface

This volume contains a collection of papers of the 12th International Conference on Excitonic and Photonic Processes in Condensed Matter and Nano Materials (EXCON 2018).

EXCON 2018 was held in the historic city of Nara, Japan, on July 8-13, 2018. The previous eleven EXCON conferences were held in Darwin, Australia (1994), Kurort Gohrisch, Germany (1996), Boston, USA (1998), Osaka, Japan (2000), Darwin, Australia (2002), Kraków, Poland (2004), Winston-Salem, USA (2006), Kyoto, Japan (2008), Brisbane, Australia (2010), Groningen, The Netherlands (2012) and Montréal, Canada (2015). The EXCON series of conferences has provided an interdisciplinary forum for mutual research communications among solid-state physicists, photo-physicists, photo-chemists, photo-biologists, material scientists, and device-oriented researchers from academia and industry.A variety of excitonic and photonic phenomena in diverse materials from bulk materials to micro- and nanostructures have been covered in the conferences, such as quantum dots, metallic antennas, atomic-layer materials, bio-materials and molecular materials. The complete list of topics covered by EXCON 2018 is as follows:

–Coherent excitonic processes, including multi-excitonic processes;

–Excitonic and photonic phenomena in nanostructures;

–Excitonic and photonic phenomena in atomic-layer materials;

–Strong light-matter coupling through microcavities, photonic crystals, and plasmonic structures;

–Exciton-lattice interaction and photo-induced phase transition;

–Excited states in disordered and amorphous systems;

–Excitonic processes in organic materials and polymers;

–High-density excitonic systems and condensation;

–Light-matter interaction in metamaterials;

–Excitonic processes in photovoltaic materials and devices;

–Ultrafast dynamics of excitonic relaxation, energy transport, and charge transfer;

–Applications of excitonic and photonic processes in quantum information processing;

–Optical devices: LEDs and solar cells in inorganic and organic systems;

–Optical manipulation of nanostructures;

–New experimental techniques applied to excitonic and photonic processes;

–New theoretical and computational techniques applied to excitonic and photonic processes.

List of Committees, Conference Chair, International Advisory Committee, Program Advisory Board and Local Organizing Committee are available in this pdf.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Strong-field biasing of a solid with intense lightwaves leads to simultaneous interband excitation and intraband acceleration of electron–hole pairs. These coupled dynamics result in high-harmonic emission from the bulk solid. For a controlled acceleration of quasiparticles with well-defined initial conditions, we prepare coherent electron–hole pairs by a resonant near - infrared pulse before a strong multi-terahertz field accelerates these entities. The ballistic dynamics manifests itself as high-order sidebands to the near-infrared excitation spectrum. This mechanism allows for the implementation of a quasiparticle collider in order to study those entities in close analogy to conventional collision experiments. Accelerating electrons and holes in a monolayer of a transition metal dichalcogenide extends this scheme to internal quantum degrees of freedom. We show how a strong lightwave can transport electron–hole pairs from one valley to the other faster than one oscillation of the carrier wave, effectively switching the valley pseudospin on a sub-cycle scale. This scheme paves the way to ultimately fast valleytronics.

In this work we present the interaction potentials and eigenenergies of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. More precisely, we present the theory of giant-dipole excitons in Cu₂O. We derive an exact formulation of the field-dressed excitonic Hamiltonian and exemplarily calculate the excitonic eigenenergies in an exact diagonalization approach for external field strengths of B = 4T and E = 1 kV/cm. For this particular field configuration, we obtain level spacings between 1.14 μV and 77.6 μeV.

We demonstrate a Kitaev spin liquid in a polyhedral cluster and propose detecting its fractional excitations—Majorana spinons—by magnetic Raman scattering. While little polarization dependence of the Raman spectra at sufficiently low temperatures is usual with quantum spin liquids, the present observations are primarily of geometric origin. The Raman scattering intensity peaks melt with increasing temperature, to be more precise, with increasing number of background gauge-flux excitations.

A chiral nanostructure, which exhibits optical activity, absorbs different amounts of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light. In this work, we report the observation of dissymmetry between two-photon-induced LCP and RCP photoluminescence from plasmonic two-dimensional (2D) chiral Au nanostructures. Under excitation by linearly polarized femtosecond pulses from a mode-locked Ti:sapphire laser with a low incident power of 3 mW, the 2D chiral plasmonic nanostructure yields circularly polarized two-photon-induced photoluminescence (TPIPL) due to resonance with a chiral multipolar plasmon mode of the nanostructure. The handedness of the circularly polarized TPIPL was dependent on the handedness of the chiral plasmonic nanostructure. The chiral nature of TPIPL may find potential applications in optical devices, sensing of chiral molecular environments in biological systems, and so forth.

We report ultrafast electronic relaxation dynamics of yttrium-doped samarium monosulfide, Sm_0.83Y_0.17S, which is one of valence fluctuating compounds, by pump-probe measurements. We observed a large increase of the Drude weight in the reflectivity spectrum by the photo-excitation and a double exponential decay of the relaxation time to a metastable state. This suggests that the photo-induced effect can be explained as the change of carrier density. The metastable state has a long lifetime ( > 1 ns) and the carrier density is slightly higher than that before the photo-excitation.

We conduct a theoretical study of the nonlinear optical dynamics of a 2D super-crystal comprising regularly spaced identical semiconductor quantum dots (SQDs), subjected to a resonant continuous wave excitation. A single SQD is considered as three-level ladder-like systems involving the ground, one-exciton and bi-exction states. We show that the super - crystal reveals a rich nonlinear dynamics, exhibiting multistability, self-oscillations and chaos. The behaviour is driven by the retarded SQD-SQD interactions and bi-exciton binding energy.

We report a stable low-threshold amplified spontaneous emission performance from a new type of all-inorganic perovskite CsPb₂Br₅ microplate with superior crystallization, enhanced stability, and tunability under both one- and two-photon excitation for the first time.

We theoretically discover that the spin angular momentum (SAM) enables to modulate the orbital torque through the inter-particle light-induced force (IP-LIF). Laguerre - Gaussian beam with the orbital angular momentum (OAM) can induce the orbital motion for the optically trapped objects. In addition, the SAM can also accelerate or decelerate the orbital motion due to the IP-LIF. Our discovery provides a new physical aspect, i.e. the IP-LIF plays an important role in the many-body dynamics of nanoparticles via the SAM-OAM coupling.

To study a bi-axial stress effect in a Cu₂O thin crystal sandwiched by paired MgO substrates, we have applied Bayesian spectroscopy to decompose an absorption spectrum with employing a replica exchange Monte Carlo (RXMC) method. The absorption spectrum includes broad bands of an inter-band transition and an exciton continuous state simultaneously with excitonic resonance transitions. However, it had been difficult to decompose correctly because of a bottleneck in optimal solution search efficiency. To relieve the bottleneck, we introduced the RXMC method and succeeded in decomposing all spectral components. Posterior probabilities of the band-gap energy and the exciton binding energy distribute at lower energy side than those in stress-free bulk crystals, which provides statistical evidence to show that the bi-axial stress remains in the Cu₂O thin crystal sandwiched by MgO substrates.

We performed photocarrier injection by two-photon excitation in rubrene crystal using laser pulses of nanosecond-duration at various wavelengths under an external electric field. Based on the excitation spectra, it is revealed that photocarriers are injected at excitation energies corresponding to the intrinsic absorption by two-photon process, in contrast to the midgap-states-mediated injection by one-photon process. This result means that the two-photon excitation method is useful to inject photocarriers free from the surface traps.

We investigated the red afterglow process of CaTiO₃:Pr, Al through photoluminescence and thermoluminescence measurements by using fabricated single crystals. The crystals with several cubic millimeter in size were obtained through a flux method. We found that the afterglow properties varied with the excitation energy. At room temperature, the red afterglow was observed for the charge transfer from Pr³⁺ to Ti⁴⁺ excitation and the direct excitation of Pr³⁺, while not observed for the interband excitation of CaTiO3 due to the relaxation process going through 4d¹5d¹ state of Pr³⁺. For the direct excitation of Pr³⁺ at 80 K, the excited electrons did not reached to trap states for afterglow due to a potential barrier, which can overcome with thermal assistance at room temperature. For the charge transfer excitation at 80 K, the excited electrons could reach the traps with excess energy.

Photoirradiation effect in a ferroelectric cocrystal Hdppz-Hca is investigated in terms of time-resolved second harmonic (SH) generation measurements. By the photoexcitation with a visible pulse (530 nm) causing the intramolecular transition of Hca molecule, the SH intensity was suddenly suppressed by ≈30% on the time scale of picosecond, while the reflectivity scarcely changed. This result is discussed based on reversal of the polarized domain by the photoexcitation.

The carrier dynamics in the nanostructured semiconductors related to the drift and diffusion currents strongly affects the device performance, such as the efficiency of the carrier injection into the active layer. We report on the effects of the photocurrent direction on the THz signals emitted from the GaAs crystal including an interface. The polarity inversion of the signal is caused by the change in photocurrent direction from diffusion to drift. The inversion is not affected by electron lifetimes. These results suggest that measurement of the THz wave is useful to consider the photocurrent direction.

A yellow N-doped TiO₂ powder was prepared by the sol-gel method using titanium tetra-isopropoxide and hydrazine monohydrate. An EPR signal was induced by the irradiation of light with hv ≥ 2.2 eV to the yellow TiO₂ powder and the signal intensity gradually decreased in the dark. The EPR signal was due to thermally trapped carriers interacting with the doped N. The thermal activation energy of the trapped carriers was estimated to be 0.22 eV. XPS measurement was performed on the yellow powder as well as colorless and blue N-doped powders obtained by oxidation and reduction, respectively. The N1s bands at 399.5 eV can be assigned to nitrogen adsorbed on the surface. The atomic valence of nitrogen in the colorless, yellow and blue powders was positive, neutral and negative, respectively. The atomic valence of nitrogen in doped TiO₂ depends on the oxidation-reduction procedure.

It is well-recognized that the Brownian motion of particles in fluids is random. Nevertheless, there can be characteristics depending on the specific physical conditions. We analyze the system of nanoparticle clusters formed by the laser trapping force field at the solid-liquid interface, based on the microscopy movie data. Since the laser trapping force field is basically a function of radial distance from the focal point in the two dimension at the liquid-solid interface, we examine the difference of displacement distributions in the radial and circumferential directions. The results show that the basic characteristics in this system depends on the laser power, and there is an anisotropy in the stochastic motion of the nanoparticles.

Electronic structure of monolayer graphene with metal contacts can be modified by control of interfacial interaction. Here, we show that the strength of local interfacial interaction between graphene and Au(111) surface can be electrochemically controlled by the hydrogen evolution reaction process. Graphene/Au(111) electrodes were prepared by the chemical vapour deposition. Raman spectroscopy indicated existence of monolayer of graphene over Au surface. Local interaction of graphene/Au(111) interaction can be differentiated by intercalation of H₂ molecules at interface. This study shed lights on the possibility of proton penetration through the graphene to modify the strength of interaction between graphene and Au(111) surface.

We observed the dynamics of the electric-field induced magnetization in antiferromagnetic chromium oxide (Cr₂O₃) by the Faraday-rotation measurement using a continuous-wave probe light in the millisecond region and a pulse probe light in the nanosecond region. It was found that the Faraday-rotation amplitude linearly depends on the electric field, decreases with increasing temperature, and disappears above the Néel temperature. In the nanosecond region, nanosecond rise of the electric-field induced Faraday-rotation signal was observed.

Modified modulation photocurrent (MPC) measurements were carried out to investigate the carrier transport process in organic photovoltaics based on polymer and fullerene derivatives. In conventional MPC measurements, a photocurrent component modulating at the same frequency as the modulated incident light is recorded; however, in this study, a component modulating at twice the modulation frequency was detected. A linear process does not contribute to the detected signals; thus, this technique allows for focus on a process that converts incident photons into photocurrent in a nonlinear manner. An analysis using rate equations suggested that the nonlinearity resulted from the carrier density dependence of the internal electric field, which is the driving force of the carriers under the short-circuit condition. Furthermore, features due to both electron and hole transports were more clearly observed in the modified MPC measurements than in the conventional MPC measurements.

A giant microwave response is transiently observed in CH₃NH₃PbI₃ single crystals under pulsed laser excitation at temperatures in both structural phases across the transition at 161 K. The response is caused by photocarriers generated in the specimen mounted in a microwave cavity. This detection technique is capable of measuring the carrier dynamics with a time-resolution of a few nanoseconds. Based on the time-resolved excitation spectrum near the absorption edge, we unveil that the photocarriers are generated by long-lived excitons as well as by fast band-to-band transition.

We investigated the influence of excitation processes of a biexciton on the generation of superfluorescence, which is pulsed radiation from coherently coupled two-level systems, from biexcitons in CuCl quantum dots. Two excitation processes of biexcitons were examined: resonant two-photon excitation of biexcitons, and resonant one-photon excitation of excitons. The excitation density dependence of the time-resolved photoluminescence was measured. The results showed that for one-photon excitation, a stronger peak intensity, longer delay time and wider pulse width were observed compared to resonant two-photon excitation. The high density of the excited dots in the one-photon excitation process results in the strong pulse, while an initial population of excitons results in a suppression of coherent coupling.

We report an experimental study of stimulated Raman scattering in anatase TiO₂ at 4K under both resonance and off-resonance conditions. Efficient first - and higher-order stimulated Raman emission of the lowest optical mode was observed, especially for the resonance case. From the dependence of the first-order Stokes emission on laser intensity, we estimate values of the Raman gain coefficient which are large compared with those reported for other crystalline materials. The large gain values are attributed to the narrow linewidth of the Raman line at low temperatures.

We measured the transient reflection spectra induced by infrared light for a CuCl single crystal using pump-probe spectroscopy. A Stark shift of the 1s exciton was observed using light with a near resonant energy between the 1s and 2p exciton levels. The amount of energy shift due to the Stark effect was obtained from the absorption spectra calculated using the Kramers–Kronig transform. We investigated the energy shift of 1_s excitons with respect to the excitation density. This is the first published report on the Stark effect of 1_s exciton associated with the internal energy levels of the excitons in CuCl crystal.

We have investigated carrier injection routes in high-purity diamond crystals in a wide temperature range using the time-resolved cyclotron resonance method. By analysing the excitation spectra, we find that carriers are injected under optical excitation above the energy threshold for exciton generation with emission of a transverse acoustic phonon at 10 K and 80 K, whereas with absorption of a transverse optical phonon at 300 K. The excitation power dependence clearly shows that the carriers are generated by thermal dissociation of excitons at 300 K.

We have investigated the optical properties of ZnSe quantum dots (QDs) prepared by a hydrothermal method. The photoluminescence (PL)-decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behaviour due to thermal quenching. The temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground state and two excited states of the lower-lying bound-exciton and higher-lying dark-exciton states. The analysis of the temperature dependence of the decay time indicates that the dark-exciton state contributes to the PL-decay process in the ZnSe QDs.

We investigate the emission characteristics of a single CdSe quantum dot on an optical nanofiber at cryogenic temperatures from the viewpoint of quantum photonics. We show that the charged exciton (trion) of the quantum dot is a promising quantum emitter for both aspects of spectral and temporal characteristics. The nanofiber/quantum-dot system may give a promising work bench for the future quantum photonic network.

We have investigated the photoluminescence (PL) dynamics of exciton-exciton inelastic scattering at temperatures from 10 to 60 K. It was found that the energy dependence of the PL decay rate is scaled by that of the group velocity of the photon-like lower polariton (LP) at each temperature, taking account of the broadening factor of the polariton state. This fact demonstrates that the PL decay rate is dominated by the photon-like LP which is the final state of the exciton-exciton inelastic scattering process. The broadening factor is proportional to temperature, which indicates the influence of acoustic phonon scattering on the LP state.

We prepared colloidal ZnSe_xS_1−x alloy quantum dots (QDs) and investigated their optical properties. ZnSe_xS_1−x QDs were successfully prepared by a hydrothermal method, which was confirmed by the results of X-ray structural analysis. When the alloy composition was decreased from x = 1 to 0, the absorption energy continuously shifted to the high energy side. Band-edge photoluminescence (PL) was the main PL band in ZnSe QDs, whereas only defect - related PL band was observed in ZnS QDs. The band-edge PL was clearly observed as the main PL band for x = 0.52 to 1, which shifted to the high energy side by decreasing x.

Herein, we investigated the preparation and optical properties of water-soluble CdSe quantum dots (QDs). CdSe QDs with a narrow size distribution were hydrothermally prepared by reacting Cd²⁺ with NaHSe in the presence of N-acetyl-L-cysteine as ligand. Furthermore, photoluminescence quantum yield increased to ∼47% when a ZnS shell was applied to prepare the CdSe/ZnS core/shell QDs.

We investigated the size dependence of band-edge photoluminescence (PL) dynamics for CdS quantum dots (QDs). The temperature dependence of the PL-decay profiles of CdS QDs with an average diameter of 3.7–6.0 nm was measured. The PL-decay profiles became longer as the temperature increased. Further, it was found that the temperature dependence of the PL-decay profiles depends greatly on the QD size. These experimental results can be understood by considering that the magnitude of the splitting energy between the bright-and dark-exciton states depends on the QD size and becomes larger as the QD size becomes smaller.

We have investigated the photoluminescence (PL) polarization characteristics of the self-trapped exciton (STE) in an undoped β-Ga₂O₃ single crystal at 77 K under three-photon excitation. From analysis of the polarization characteristics, we found that the STE PL is polarized almost parallel to the a-axis. The STE-PL polarization corresponds to the orientation of the self-trapped hole which was investigated in previous works using an electron-paramagnetic-resonance experiment and a first-principles calculation.

The energy relaxation from host crystals to impurity centers has been investigated by measuring the absorption and excitation spectra and the temperature dependence of the luminescence spectrum of NaI:In⁺. In the absorption spectrum, the A, B, and C bands due to the In+ centers are confirmed at 3.8, 4.0, and 4.5 eV, respectively. The excitation spectrum for the A_T luminescence due to the In⁺ centers responds to the energy position of the A, B, and C bands and also the energy region above the exciton transition of NaI. The fact suggests the existence of the energy relaxation from host crystals to the In⁺ centers. The total intensity of the A_T and A_X luminescence due to the In⁺ centers exhibits different temperature dependence between the excitations at the C band and host crystals. Under excitation at the C band, the total intensity of the A_T and A_X luminescence monotonously declines with increasing temperature from 10 to 300 K. On the other hand, under excitation at host crystals, the total intensity of the A_T and A_X luminescence exhibits an increase with increasing temperature from 10 to 50 K, and a decrease with increasing temperature from 50 to 100 K. We discuss the temperature dependence of the total intensity of the luminescence under excitation at host crystals in relation to free excitons and self-trapped excitons.

We have investigated the nonlinear optical responses of biexcitons which show the polarization entanglement by using pump-probe spectroscopy. Measuring all polarization combinations of pump-probe signals and precise analysis using the idea of quantum tomography enable us to extract the specific property of the entanglement. The obtained results indicate the two kinds of the third-order nonlinear response, namely the photo-induced absorption and the optical Kerr effect. From the analysis with the optical Kerr effect signals, the polarization correlations of the optical responses are clearly derived.

Manipulation of small particles can be achieved by using the radiation pressure of a laser beam. The strength and direction of the radiation pressure depends on the dielectric constant of target materials. It can therefore be expected to induce mechanical motions of small materials in optical trap by changing their optical property by chemical reactions. Along this line, we recently demonstrated switching of radiation pressure acting on single optically trapped microparticles by using photochromic molecules (Refs. 3, 4). To explore more flexible mesoscopic motions synchronizing photochemical reactions, we have investigated in the present study the micromechanical motion of optically trapped particles containing a T-type photochromic molecule, naphthopyran (NP), with multiple thermal back-reaction rates. The single microparticle with NP was optically trapped with CW 532-nm laser and the photochromic reaction was induced by UV irradiation, resulting in the modification of the optomechanical responses of the particle under laser trapping.

Poly(N-isopropylacrylamide) (PNIPAM) exhibits phase separation with lower critical solution temperature (LCST). In the 1990s, Masuhara and co-workers reported the first demonstration of optical trapping of PNIPAM forming a micrometer-sized polymer droplet. Since then, this technique has attracted much attention to create a molecular assembly in a microspace. In the present study, we targeted poly(N,N-diethylacrylamide) (PDEA), which has an analogous chemical structure to PNIPAM. We demonstrated that optical tweezers formed the unique micro-morphologies of a phase separated PDEA droplet. Fluorescence microscopic images and Raman spectra of the PDEA droplet showed that a lot of smaller-sized water-rich micro-domains were inhomogeneously formed in the droplet. Such unique phase separation behavior was never observed in steady-state heating of an aqueous PDEA solution above its LCST. Our results indicate that a novel micro-structure can be formed by coupling of an optical gradient force and a local temperature elevation.

We studied the dielectric screening effects by a SiO₂/Si substrate on the excitonic properties of monolayer WS₂ by means of microscopic reflectance and photoluminescence spectroscopy. Through the observation of high-order exciton resonances from 1s to 5s at 10 K, we estimated the band gap and exciton binding energy. Our theoretical calculation using the screened Keldysh potential well reproduced the high-order Rydberg energies. In addition, we discussed the deviation of oscillator strength obtained from experimental results in comparison with theoretical calculation.

We investigated the spatial distribution of the crystallinity for sputtered-MoS₂ films by Raman imaging technique. MoS₂ lattice could not be structured in as-sputtered sample prepared at room temperature. The crystallinity could be improved by thermal annealing at 630 oC in vacuum condition. However, annealed MoS₂ films had two kinds of circular perforated areas on their top surface. In one area, MoS₂ film was evaporated and the substrate was exposed. In another area, MoS₂ films formed a hillock shape and showed tensile strain. In addition, the crystallinity was deteriorated in hillock formed area, which is due the generation of sulfur vacancies by thermal annealing.

The interaction between highly charged ions (HCIs) and the solid surface causes various phenomena due to high potential energy of HCIs. Luminescence is also included in them. In order to investigate the origin of the light emission from the surface irradiated with HCIs, we conducted spectroscopic measurements and then we investigated characteristics of Balmer light emission because potential effect of HCI was remarkable in Balmer light emitted from hydrogen atoms desorbed from the surface. In the present study, the Balmer light intensity was revealed to increase rapidly with the charge state of HCIs.

Quantum dots (QDs) are widely used for enhancing the performance of optical devices. Intermediate-band solar cells that use multistacked QDs can surpass the conversion efficiency of conventional silicon cells. We performed the optical characterization of In_0.4Ga_0.6As/GaAs multistacked QDs without strain compensation. We used the theoretical and experimental techniques prescribed for intermediate-band solar cells, i.e., photoluminescence (PL) spectroscopy and two-color excitations spectroscopy. The interdot spacings were not uniform and were found to be 15 nm and 7 nm. The results verify the formation of intermediate bands by the multistacked QDs. Using the theoretical studies and experimental results, we performed an in-depth study on the mechanism underlying the formation of intermediate bands by the multistacked QDs and the effect of different interdot spacings.

We report on our experimental results for the nondestructive dispersion of CdSe/ZnS core-shell quantum dots and ZnS-AgInS₂ solid solution quantum dots into buffer gases by using a nebulizer. By monitoring scattering light from droplets and fluorescence from quantum dots, we discuss the evaporation and loss of the droplet and the fluorescence quenching of the quantum dot after the evaporation.

We theoretically study a new configuration control of nanoparticles based on an optical force, which enables to trap two kinds of nanoparticles simultaneously in two-dimensional hetero-arrangement. Our proposed scheme utilizes two resonance effects on a localized surface plasmon and an electronic transition of nanoparticles. These resonances change the optical force behavior significantly. We propose periodic rhomboid metallic nanostructures with two linear polarized laser beams, the directions of which polarization are parallel to the two diagonals of rhomboid. The polarization direction fractionates the trapped nanoparticles when the two laser frequencies correspond to the electronic transition energies of two kinds of particles, respectively. We evaluate the feasibility of our proposed scheme numerically. The map of the optical force shows a clear spatial separation of trapping position for the two kinds of particles. Our results demonstrate an essential advantage of the optical manipulation that enables creating novel nanostructures unrealizable by conventional nanofabrication techniques.

Plasmonic optical tweezers (POT) has a high potential for manipulation of nanomaterials due to an enhanced optical force. However, unfavorable thermal effects induced by plasmon excitation have frequently hindered the manipulation. For this issue, we have recently developed a novel non-plasmonic optical tweezers using a nanostructured silicon substrate (B-Si). We called it "Nano-Structured Semi-Conductor-Assisted Optical Tweezers (NASSCA-OT)". In the present study, we trapped pyrene-pendant polymer chains homogeneously dissolved in water for POT or NASSCA-OT. We used plasmonic gold nanopyramidal dimer arrays or B-Si in contact with the aqueous polymer solution. During plasmon excitation with a near-infrared laser light, any sign of optical trapping was never detected in fluorescence micro-spectroscopy. By contrast, trapping of the polymer chains was obviously observed for NASSCA-OT. Upon laser irradiation, pyrene excimer fluorescence was dramatically increased at the focal spot. These results indicate that NASSCA-OT is a powerful tool for manipulation of molecular materials.

Resonant optical manipulation using absorption force so far has been based on the linear (one-photon) absorption by target material, while optical forces due to multiphoton absorption have not been much investigated. As multiphoton absorption obeys different selection rule from that of one-photon absorption, and also shows non-linear dependence on light intensity, a larger variety of photo-mechanical responses of small particles can be expected by using multiphoton absorption force. In this study, we focused femtosecond laser pulses to a single polymer microparticle containing fluorescence dyes to exert multiphoton absorption force on the particle. We successfully observed the three-dimensional motion of the photo-irradiated microparticle due to the multiphoton absorption.

Anti-Stokes fluorescence from chlorophyll a molecules dispersed in diethyl ether has been observed. From the excitation power dependence of the fluorescence spectrum, it is concluded that the anti-Stokes fluorescence appears via the linear optical process. To consider the relationship between the observed spectral shape and vibrational modes, the model spectral density has been determined based on the Raman spectrum and real-time coherent vibrational oscillation signals. The absorption spectrum observed in the experiment was well reproduced by the calculation, indicating the validity of the model spectral density. The involvement of the low-frequency vibrational modes in the anti-Stokes fluorescence process is discussed.

The photo-excited state properties of an organic Mott-insulator β'-(BEDT-TTF)(TCNQ) crystal were investigated by pump-probe measurements. Considerable change in the pump-probe reflection signal with a wide spectral range was observed near the antiferromagnetic (AF) transition temperature of the TCNQ layers. In the time response, approach to the AF temperature induced the appearance of a slow decay signal whose lifetime was estimated to be about 3 msec maximum. These results indicate that a sustained mechanism exists for photo-excited states near the AF transition, which is probably related to a critical slowing down phenomenon.

Nonlinear optical responses of β-carotene were investigated upon excitation using chirped pulses. We especially focus on spectrally resolved transient grating (TG) signals to discuss the influence of the spectral chirp over a wide spectral range. A significant change in the TG signal was observed when negatively chirped pulses were used. The experimental results were qualitatively reproduced by employing the Brownian oscillator model. The relationship between the wave packet motion in the excited state and spectral chirp is discussed. It is concluded that the third order nonlinear optical response reflects the competition between the spectral chirp and the energy separation of the potential curves.

We study magnetic Raman, i.e. inelastic light, scattering of Kitaev's quantum spin liquids realizable in honeycomb nanoribbons. Elementary excitations in the Kitaev spin liquids consist of itinerant Majorana (matter) fermions and localized gauge fluxes (visons). Matter fermions are characterized by the projective point (gauged rotation) group inherent in the lattice. Inelastic light scattering is mediated by a pair of matter fermions without any vison excitation. We can selectively "observe" matter fermions of the same symmetry and those of different symmetries depending on the polarization of incoming and/or outgoing lights. A pair of matter fermions with particular translational quantum numbers ±k cannot be Raman active.

We studied the formation mechanism of in-plane nuclear field by anomalous Hanle effect measurements in single self-assembled In_0.75Al_0.25As/Al_0.3Ga_0.7As quantum dots and InAs/GaAs quantum rings. The observed anomalous Hanle curves indicated quite large width and hysteretic behavior to the externally applied magnetic field. These anomalies appears in both materials whose electron g factors have opposite sign each other, and cannot be reproduced by a traditional spin dynamics model. In this study, we show that a proposed spin dynamics model can explain the formation of a large in-plane nuclear field regardless of the sign of electron g factor. The model is based on the dynamic nuclear spin polarization mechanism including the effects of nuclear quadrupole interaction.

We have studied optical and electrical properties in thin films consisting of the network of silicon nanoparticles modified with mercaptosuccinic acid (MSA). A photocurrent measurement system was set up to observe the photocurrent signal as functions of excitation photon energy and power. In addition, we observed the absorption spectra of the thin film and solution of the nanoparticles to clarify the contribution of the nanoparticles structure to the optical transition property. It was found that the photocurrent has an excitation-energy dependence while the profile is different from that of a bulk silicon photodiode. This behaviour suggests the specific photogenerated-carrier transport originating from the structure of the thin film of MSA-modified silicon nanoparticles.

We have investigated the angle-resolved transmission spectra of the CuCl λ-and λ/2-microcavities composed of cavity layers sandwiched by distributed Bragg reflectors (DBRs) with various numbers of periods of the DBR layer which affect the degree of confinement of the photons in the microcavity, so-called quality factors (Q factors). The angle-resolved transmission spectra of the CuCl microcavities drastically change with the Q factor and the thickness of the cavity layer. The incident angle dependence of the peak energies obtained from the angle-resolved transmission spectra indicates that the cavity polariton modes are observed in the CuCl microcavities, except for the λ-microcavity with 2 periods of the DBR layer. The change of the angle-resolved transmission spectra in the microcavities will result from the difference in the coupling state between the excitons and photons in the microcavities.

The surface-enhanced chiral-optical spectroscopy is based on the interaction of chiral molecules with chiral electromagnetic near field localized on a chiral plasmonic nanostructure. It is of fundamental importance to reveal the spectral characteristics of chiral near fields for maximizing the chiral interaction. Here we investigate relations between near field and far field polarization characteristics of the chiral plasmonic nanostructures, using electromagnetic simulations. We found that spectral features of chiral near fields created by the nanostructures intercorrelate with those of far field optical rotation. This finding may provide us a method to characterize and design the chiral near field.

In this paper, we describe the synthesis and characterization of quantum dot (QD) chains. We used small QD-DNA conjugates, i.e., short-ligand-capped QDs conjugated with short (15-mers) single-stranded DNA, as the building blocks. The small conjugates were separated based on the valence of the DNA, i.e., the amount of DNA per particle, using agarose gel electrophoresis. The conjugates formed self-assemblies composed of closely spaced (inter-particle spacing of 0.6–2 nm) QDs. We report the results of our investigation of the electrophoresis and hybridization of the conjugation.

We focused on Bi³⁺ as a promising candidate for donor impurity in lead halide perovskites and fabricated electron-doped CH₃NH₃PbBr₃ single crystals. Room-temperature optical measurement revealed the increase in Urbach tail and suppression of photon recycling effect by Bi doping, which results in photoluminescence (PL) blue shift. Furthermore, we conducted optical reflectance and PL measurements at cryogenic temperature, where the thermal broadening is supressed. At low temperature, exciton dominates the optical properties and the photon recycling effect is negligible. From the PL intensity and spectral width, we quantitatively evaluated the impact of Bi-doping on crystal inhomogeneity.

Linear as well as time resolved absorption measurements were performed on 40 nm and 170 nm thick MAPbL films with PEDOT:PSS hole extraction layer, spin-coated on quartz substrate. From linear absorption measurements exciton binding energy of 18 – 19 meV and band gap of 1.60 - 1.62 eV was deduced. Transient absorption spectra after the excitation at 1.77 eV showed a strong difference in carrier recombination dynamics for the two MAPbI₃ films of different thicknesses. From the analysis on the decay dynamics, hole population lifetime of 0.3 ns and 3.5 ns for 40 nm and 170 nm films, respectively, are determined. A numerical 1D diffusion model was used to model the carrier relaxation dynamics yielding hole diffusion constants of 0.025 - 0.030 cm²s⁻¹, which results in a hole mobility of 1 cm²(Vs)⁻¹ in these MAPbI₃ films.

Nanoparticles in a cluster trapped by laser-induced force field show Brownian motion at solid-liquid interfaces. The cluster formation means that the particles are highly concentrated. In general, the diffusion coefficients of particles in fluids decrease with substantially high concentration and also in the vicinity of solid walls due to the hydrodynamic effect. The particle trajectory data obtained from the experimental measurements show that the longer time span of observation leads to smaller diffusion coefficient due to the confinement effect. However, they also exhibit higher diffusion coefficient compared to the the bulk condition when evaluated at a sufficiently short time span of the frame interval under the condition of sufficiently high laser powers.

We studied the electric-field induced magnetization caused by the magnetoelectric effect in YIG. The magnetization was observed as the Faraday rotation of a transmitted continuous-wave probe light. From the observed dependence of the Faraday-rotation amplitude on the electric and magnetic fields, it was found that the two components linear and quadratic in the electric field coexist at low temperatures.

We have derived the optical selection rule of monolayer transition metal dichalcogenide (TMD) for various optical vortices. Monolayer TMD, which is two-dimensional direct semiconductor, has a twofold valley degree of freedom. The valley degree of freedom can be controlled by circular polarized light. However, it is not clear how the orbital angular momentum of light relates to the valley degree of freedom. Here we clarified that the orbital angular momentum of light modifies the optical selection rule at valley points and the selection rule reflects the three-fold rotation symmetry of monolayer TMD. We expect that this modified selection rule broadens the research field of two-dimensional layered materials and spin-valleytronics.

We studied on the crystallinity of as-sputtered and annealed MoS₂ thin films by Raman scattering. The samples were prepared by RF magnetron sputtering, and the thermal annealing was carried out under sulfurous atmosphere. Although as-sputtered MoS₂ thin films clearly showed the deterioration of the lattice ordering, it was drastically improved by the thermal annealing due to the sulfurization of the sample. And since the sulfurization occurred remarkably on the top surface of MoS₂ sputtered thin films, it was expected to be an effective method to realize a few-layer MoS₂ sputtered thin films with high crystallinity.

Electromagnetic wave is reflected and refracted at interfaces, satisfying Fresnel-Snell law which is required by conservations of energy and momentum. If the incident angle is lower than the critical angle, we can use this Fresnel-Snell law, but the Fresnel-Snell law is modified in the case of existence of dissipation (ñ = n + ik,k > 0) or in the condition of total internal reflection. In the cases, we have to extend the angle of refraction from real number to complex number ($\theta \to \tilde{\theta }=\theta +i\psi,\psi \ne 0$). In this paper, by using complex-angle approach, we analyse the behaviour of electromagnetic waves in various kind of interfaces: dielectric - dielectric system, dissipative dielectric - dielectric system, and metal - dielectric system. We show that iso-frequency curves in wavenumber space is opened in the case where n, k > 0, and closed either when n → 0 or when k → 0 ('Lifshitz transition' of electromagnetic waves). Excess momentum (wavevector) and anomalous circular polarisation emerging with the transition are also discussed.

Sub-wavelength square array of a triangular Au platelet and its complementary structure (i.e. that of a triangular hole in a square film of Au) are compared in terms of second harmonic generation efficiency for fundamental light in the near infrared and visible region of spectrum for normal incidence. Electric field strength around the convex corners of a triangular particle is at least 10 times larger than the one around the concave corners of triangular hole in the complementary structure. Nevertheless the SHG intensity at the respective resonant frequency is found to be comparable, which are numerically estimated by an overlap integral of nonlinear polarization and electric field at the SHG frequency in the nonlinear optical scattering theory originally proposed by Roke et al. (Phys. Rev. B 70, 115106 (2004)). The reason is due to the large electric field strength at the sides of the triangular hole at the resonance frequency, which compensates the suppressed electric field at the concave corners in the overlap integral.