Takuya Iida

Control of Plasmonic Superradiance in Metallic Nanoparticle Assembly by Light-Induced Force and Fluctuations.

The possibility of simultaneous control of the configuration and optical functions of a metallic nanoparticle (NP) assembly by light-induced force (LIF) and thermal fluctuations has been demonstrated on the basis of self-consistent theory of LIF and nonequilibrium dynamics. It has been clarified that the NPs are arranged parallel to the polarization of the focused laser beam under the balance of LIF and the electrostatic repulsive force due to the ions on the surface of NPs. Particularly, in such a NP assembly consisting of high-density NPs, the light-scattering rate (radiative decay) of localized surface plasmon polaritons (LSPPs) can be drastically enhanced to be greater than 100 meV (10 times that of single NPs), and the spectral width is also greatly broadened due to the superradiance effect. The results will provide a foundation of the principles for designing a NP assembly with controllable light scattering for highly efficient broad-band light energy conversion devices.

Anomalous optical selection rule of an organic molecule controlled by extremely localized light field

We have theoretically demonstrated the drastic enhancement of light-induced electric polarization in molecular nondipole type dark states. Its magnitude can exceed that of bright states, even in a nanoscale molecule, due to the spatial correlation between the wave function of the excited states and the localized light field. Moreover, it was clarified that the direct observation of such an anomalous enhancement of dark states in a metal nanogap is possible through near field spectroscopy under one-photon excitation. The results obtained will open the way to single molecule detection methods to reveal the molecular level scheme including parity-forbidden states.

Fabrication of single-crystalline microspheres with high sphericity from anisotropic materials

Microspheres with high sphericity exhibit unique functionalities. In particular, their high symmetry makes them excellent omnidirectional optical resonators. As such perfect micrometre-sized spheres are known to be formed by surface tension, melt cooling is a popular method for fabricating microspheres. However, it is extremely difficult to produce crystalline microspheres using this method because their surfaces are normally faceted. Only microspheres of polymers, glass, or ceramics have been available, while single-crystalline microspheres, which should be useful in optical applications, have been awaiting successful production. Here we report the fabrication of single-crystalline semiconductor microspheres that have surfaces with atomic-level smoothness. These microspheres were formed by performing laser ablation in superfluid helium to create and moderately cool a melt of the anisotropic semiconductor material. This novel method provides cooling conditions that are exceptionally suited for the fabrication of single-crystalline microspheres. This finding opens a pathway for studying the hidden mechanism of anisotropy-free crystal growth and its applications.

Study of the mechanical interaction between an electromagnetic field and a nanoscopic thin film near electronic resonance

The mechanical interaction between an electromagnetic field and a nanoscopic thin film near electronic resonance is theoretically studied by calculation of Maxwells stress tensor. As a result of numerical demonstrations for both propagating and evanescent incident waves, the following effects that are specific to this condition have been clarified: (1) The force exerted on a nanoscopic thin film is greatly enhanced near the resonance frequency to the same order of magnitude as for a film with macroscopic thickness. (2) The peak position of the gradient force in its spectrum is highly sensitive to the change in nanoscopic thickness that is due to the polaritonic effect. (3) In a total-reflection region a large enhancement of the repulsive force between the two thin films occurs when the films act as an optical cavity.

Fluctuation-Mediated Optical Screening of Nanoparticles

Inspired by biological motors, we propose a guiding principle for selectively separating nanoparticles (NPs) by efficiently using the light-induced force (LIF) and thermal fluctuations. We demonstrate the possibility of transporting metallic NPs of different sizes with a size-selection accuracy of less than 10 nm even at room temperature by designing asymmetric spatiotemporal light fields. This technique will lead to unconventional nanoextraction processes based on light and fluctuations.

Ambient-dependent optomechanical control of cantilever with mechanical nonlinearity by cavity-induced radiation force

Theoretical aspects of the nonlinear dynamics of a cantilever for a scanning probe microscope are studied using the extended Duffing equation incorporating the cavity-induced radiation force (CIRF) and environmental fluctuations. Cantilever vibrations can be significantly damped with negative optical rigidity by mechanical frequency shifts with a near-resonant CIRF depending on the laser intensity. Furthermore, under the fluctuations induced by fast collisions with ambient molecules about several tens of nanoseconds, laser intensities of several hundreds of microwatts are sufficient to attain effective temperatures of 10 mK. Such cooling may be expected to offer a method for super-sensitive detections of ambient molecules.

Development of a rapid bacterial counting method based on photothermal assembling

We developed a rapid bacterial counting method based on the photothermal assembling (PTA). Based on the laser-induced PTA in fluid medium, an initial bacterial concentration was estimated from the number of assembled bacteria. The measuring time of our method is 90 s, which is more rapid than the conventional cultivation method requiring several days at longest. Furthermore, the difference between the estimated concentrations by our method and by the cultivation method is less than 10%, which sufficiently guarantees the availability. The clarified principle will pave the way to a rapid and high throughput bacterial assay useful for medical care and food safety.

Cancelation of thermally induced frequency shifts in bimaterial cantilevers by nonlinear optomechanical interactions

Bimaterial cantilevers have recently been used in, for example, the calorimetric analysis with picowatt resolution in microscopic space based on state-of-the-art atomic force microscopes. However, thermally induced effects usually change physical properties of the cantilevers, such as the resonance frequency, which reduce the accuracy of the measurements. Here, we propose an approach to circumvent this problem that uses an optical microcavity formed between a metallic layer coated on the back of the cantilever and one coated at the end of an optical fiber irradiating the cantilever. In addition to increasing the sensitivity, the optical rigidity of this system diminishes the thermally induced frequency shift. For a coating thickness of several tens of nanometers, the input power is 5–10 μW. These values can be evaluated from parameters derived by directly irradiating the cantilever in the absence of the microcavity. The system has the potential of using the cantilever both as a thermometer without frequen...

Submillimetre Network Formation by Light-induced Hybridization of Zeptomole-level DNA

Macroscopic unique self-assembled structures are produced via double-stranded DNA formation (hybridization) as a specific binding essential in biological systems. However, a large amount of complementary DNA molecules are usually required to form an optically observable structure via natural hybridization, and the detection of small amounts of DNA less than femtomole requires complex and time-consuming procedures. Here, we demonstrate the laser-induced acceleration of hybridization between zeptomole-level DNA and DNA-modified nanoparticles (NPs), resulting in the assembly of a submillimetre network-like structure at the desired position with a dramatic spectral modulation within several minutes. The gradual enhancement of light-induced force and convection facilitated the two-dimensional network growth near the air-liquid interface with optical and fluidic symmetry breakdown. The simultaneous microscope observation and local spectroscopy revealed that the assembling process and spectral change are sensitive to the DNA sequence. Our findings establish innovative guiding principles for facile bottom-up production via various biomolecular recognition events.

Multiple Resonances Induced by Plasmonic Coupling between Gold Nanoparticle Trimers and Hexagonal Assembly of Gold-Coated Polystyrene Microspheres.

Optical properties of a gold nanoparticle trimer assembly coupled with gold-coated hexagonally close-packed polystyrene microspheres were investigated by linear and nonlinear spectroscopy. The observed reflection spectrum shows multiple peaks from the visible to near-infrared spectral regions. The spectroscopic properties were also examined by a finite-difference time-domain simulation. We found that the optical response of plasmons excited in the gold nanoparticle trimers was significantly modulated by strong coupling of the plasmons and the photonic mode induced in the gold-coated polystyrene assembly. Two-photon induced photoluminescence and Raman scattering from the sample were investigated, and both signals were significantly enhanced at the gold nanoparticle assembly. The simulations reveal that the electric fields can be enhanced site-selectively, not only at the interstitial sites in the nanoparticle assembly but also at the gaps between the particle and the gold film due to plasmonic interactions, by tuning the wavelength and are responsible for the strong optical responses.