Two-color light was irradiated to azo-polymer films to investigate the mechanism of light-induced polymer movement. A collimated green light was used for inducing softening of the polymer, while a focused red light was used for inducing optical gradient force. We found from the topology change of the film that polymer movement was induced only when both green and red light were simultaneously irradiated. This finding indicated that the polymer movement was induced by optical gradient force generated by the red light under the condition that the polymer was softened by repeatable photoisomerization by the green light.
DNA changes its conformation by combining a transcription factor or transcription factor complex on specific base sequences. We investigated the conformation changes by using local surface plasmon resonance of two gold nanoparticles linked to each other via the DNA, which compose a nano-dimer. Gap distance of the nano-dimer is reduced due to the DNA conformation change or bending, then the plasmon resonance shifts to longer wavelength. By measuring the plasmon resonant wavelength, gap distance is determined with a calibration curve prepared beforehand. Hence, conformation change of DNA bound with transcription factors is evaluated at nanoscale or sub-nanoscale. For example, a bending angle was determined to be 61.3º when SOX2, one of transcription factors, was bound on a double-stranded DNA having DC5 sequence and the DNA changes conformation. Binding SOX2 and PAX6 together on DC5 sequence, bending angles were evaluated to be 61.3º at SOX2 side and 5.7º at PAX6 side, respectively. When we used DNA having a DC5-con sequence which is a little different from DC5 sequence, bending angles were evaluated to be 61.1º at SOX2 side and 2.3º at PAX6 side. Such small difference in DNA conformations can be distinguished by using the local surface plasmon resonance. We also observed DNA conformation change by binding SOX2 on DC5 in real time and duration for conformation change was determined to be less than 100 msec. Such binding of DNA and transcription factors has possibility for a driving component for nano-machines.
The investigation of light-matter interaction has had growing interest in the field of photonics. In particular, metalinsulator-metal (MIM) sensors are of interest owing to their high sensitivity compared to conventional sensor made by a single metal layer. The high resolution and low concentrations detection is a result of the sharp resonance of the surface plasmon polariton waveguide hybrid mode at the Au/water interface supported by MIM structures. In this context, we have implemented experiments and theoretical simulations to estimate the enhancement of the sensitivity of metal-insulatormetal sensors. By changing the refractive index of an aqueous solution of glucose, we found that the use of a metalinsulator-metal stack improves the figure of merit of the sensor 7.5 times compared to that of a conventional surface plasmon resonance sensor.
When the size of metallic nanoparticles becomes smaller than 1 nm, of which nanostructures are composed of several tens of atoms, the plasmonic effect disappears and the electronic energy levels of the nanoparticles called as nanoclusters are quantized. Then, the nanoclusters can emit fluorescence of which wavelength depends on their size. We investigated synthetic method of Platinum nanoclusters (Pt NCs) that exhibit blue to yellow photoluminescence by a facile one-pot reduction method. They were synthesized from the mixture of H2PtCl6, hyper-branched polyethylenimine (PEI), and L-ascorbic acid, resulting in the formation stabilized with the amino groups in the cavities formed by coiled PEI ligands. The chain conformation of cationic polymer PEI depends on pH of solution. By controlling pH of the synthesis solution, the size of Pt NCs@PEI changes and their fluorescent wavelength can be tuned. Pt NCs@PEI were applied to the labeling of Chemokine receptors of the membrane of cancer HeLa cells and Glutamate receptors of the membrane of neural cells by binding them to an antibody via a conjugate protein for bio-imaging. They showed lower cell cytotoxicity than other nanoparticles such as Q-dots@COOH, indicating that they have better cell viability and great potential for biological applications.
Optical tweezing;by photochemistry is a novel concept in the field of optical manipulation. I discuss it in azo-polymer films through theory and experiments. I will show that optical tweezing can be obseved by a photochemical force, e.g. photoisomerization force which leads to a spring type motion. This force is derived from a harmonic light potential that moves the azo-polymer, and it is parenting to optical tweezers since it occurs in the presence of a gradient of light intensity, but it is quite different in the sense that it requires photoisomerization to occur. The azo-polymer’s motion is governed by four competing forces: the photoisomerization force, and the restoring optical gradient and elastic forces, as well as the random forces due to spontaneous diffusion.
We present experimental evidence of the generation of narrow Fano line shapes in planar multilayer
structures. The Fano line shape originates from coupling between a high loss surface plasmon
polariton mode with a low loss planar waveguide mode. The line shape is shown to depend strongly
on the structural parameters that govern the position of the waveguide mode and the coupling
strength, and to be in good agreement with results of electromagnetic calculations.
Photomigration in azo polymers is an area of research that witnessed intensive studies owing to its potential in optical manipulation; e.g. optical tweezing; the physical mechanism of which remains unsolved since its discovery about two decades ago. In this presentation, a detailed theoretical study that reproduces the phenomena associated with photomigration is presented, including the physical models and the associated master equations. I discuss polarization effects and I give the analytical solutions describing the steady-state and the dynamics of photomigration.
We perform time-resolved observation of living cells with gold nanoparticles using surface-enhanced Raman scattering (SERS). The position and SERS spectra of 50-nm gold nanoparticles are simultaneously observed by slit-scanning Raman microscopy with high spatial and temporal resolution. From the SERS observation, we confirm the attachment of the particles on the cell surface and the entry into the cell with the subsequent generation of SERS signals from nearby molecules. We also confirm that the strong dependence of SERS spectra on the position of the particle during the transportation of the particle through the cell. The obtained SERS spectra and its temporal fluctuation indicate that the molecular signals observable by this technique are given only from within a limited volume in close proximity to the nanoparticles. This confirms the high spatial selectivity and resolution of SERS imaging for observation of biomolecules involved in cellular events in situ.
Raman spectroscopy has been utilized to investigate properties of biomolecules due to its capability of detecting
molecular vibrations that represent molecular species, structures and environmental conditions. In this research, we used
Raman scattering to image dynamics of molecules in living cells. By combining the 532 nm excitation and a slitscanning
detection technique, we observed dynamics of molecular distributions of cytochrome, protein beta sheets, and
lipids in an unstained HeLa cell during mitosis with a frame rate of 5 minutes.
We observed spatial and temporal behaviors of surface enhanced Raman scattering (SERS) signals with gold
nanoparticles in living cells. Gold nanoparticles with the diameter of 50 nm were introduced into macrophage cells
through endocytosis. We performed observation of SERS signals from a macrophage with 785 nm excitation. Strong
SERS signal from the particles in the cell was observed, and spectrum from each particle shows difference in Raman
peaks and intensity. We also observed time-lapse SERS and dark-field image with a frame rate of 3 min. We confirmed
that strong SERS signal from the particle in the cell and the spectral changes with positions of the particles in the cell.
We demonstrate dynamic imaging of molecular distribution in unstained living cells using Raman scattering. By combining slit-scanning detection and optimizing the excitation wavelength, we imaged the dynamic molecular distributions of cytochrome c, protein beta sheets, and lipids in unstained HeLa cells with a temporal resolution of 3 minutes. We found that 532-nm excitation can be used to generate strong Raman scattering signals and to suppress autofluorescence that typically obscures Raman signals. With this technique, we reveal time-resolved distributions of cytochrome c and other biomolecules in living cells in the process of cytokinesis without the need for fluorescent labels or markers.
When Raman scattering is excited from the evanescent light field created by illuminating the apex of a sharp metallic
nano-tip, it achieves new aspects with strong enhancement of scattering efficiencies and super resolving capabilities. The
primary mechanism of tip-sample interaction is electromagnetic, which is based on the excitation of localized surface
plasmon polaritons. However, when the tip is close enough to the sample, typically at molecular distances, the chemical
interactions between the tip and the sample become important. Strong temporal fluctuations of Raman scattering,
including fluctuations of peak frequencies and peak intensities, together with extraordinary enhancement of several
peaks, were observed. These temporal fluctuations, which are typical signature of single molecule detection, were
attributed to the changes of molecular orientations of the sample molecules in the upper layer of the nanocluster, which
got chemically adsorbed at the tip molecules.
Near-field Raman scattering has been successfully utilized to study the interaction between a metal-coated nano-tip and
carbon nanostructures, such as carbon-60 molecules and single walled carbon nanotubes. The enhanced and localized
light field in the vicinity of the tip apex provides high resolution imaging as well as the detection of weak vibrational
modes. Apart from the electromagnetic and chemical interactions, a mechanical interaction between tip molecules and
the sample molecules has also been investigated.
The interactions between an apertureless metal-coated tip and C-60 molecules have been studied by near-field Raman scattering technique. The electromagnetic and the chemical interaction through the local plasmon polaritons generated at the metallized tip apex provide an enhanced scattering and high spatial resolution much beyond the diffraction limits of the probing light. And additional enhancement under the gap-mode configuration, when C-60 molecules were sandwiched between a thin rough metal layer and metallized tip, was observed. Further, the mechanical interaction between the tip and C-60 molecules were studies by pressurizing C-60 molecules uniaxially by the tip. The photopolymerization of C-60 molecules was found to accelerate under high tip-pressure.
Near-field Raman spectroscopy using an apertureless metallic probe has attracted much attention owing to its capability of chemical analysis with high spatial resolution far beyond the diffraction limit. The local plasmon excited at the probe tip amplifies optical near-field in the vicinity of the tip apex, and the local field is used to enhance Raman scattering. The metallic probe contributes not only to the enhancement of the Raman scattering, but also to spectral changes due to the chemical and mechanical interaction between the metal and the molecules. We experimentally and theoretically investigated these two effects in this study. These effects selectively provide vibrational information of the molecules directly adsorbed on the metal, and, therefore, have a potential to improve the spatial resolution. In addition, the metallic probe has also been applied for enhancement of nonlinear Raman scattering. Coherent anti-Stokes Raman scattering (CARS) has been strongly enhanced by the probe, and has provided molecular-vibration images of deoxyribo nucleic acid (DNA) with high spatial resolution.
Optical microscopy that can visualize the molecular vibration with a nanometric spatial resolution has been realized by a combination of near-field optics and coherent anti-Stokes Raman scattering (CARS) spectroscopy. A metallic probe with a sharp tip is used to strongly enhance optical near-field in the local vicinity of the tip owing to the excitation of local surface plasmon polariton. CARS signals of molecules in the local area can be strongly induced by the plasmonic field. We have visualized DNA molecules and single-walled carbon nanotubes (SWNTs) with a spatial resolution far beyond the diffraction limit by the tip-enhanced near-field CARS microscopy.
We report on near-field Raman spectra of a single nanocrystal of adenine molecules using a silver-coated apertureless probe tip. Raman signal is amplified due to surface plasmon polaritons localized at the metallic tip by a factor of more than 2700 times compared with the far-field Raman signal. Plasmonic near-field Raman spectrum showed eight Raman bands assigned to the normal modes of adenine molecule based on the density functional theory calculations. Vibrational frequencies of some Raman bands are observed to have shifted to the values of the corresponding bands, observed using conventional surface enhanced Raman-scattering spectrum. We found that these frequency shifts are caused by the transient states of the adenine-silver complexes by analyzing vibration mode of the complexes; that is, the near-field Raman spectra of adenine agree with Raman spectra of the complexes which are calculated by reducing the bond distance between an adenine molecule and a silver atom. Repulsive forces calculated from reduction of the bond distance were equal to the atomic force applied to the adenine molecule in our Raman NSOM experiment. All results support that the active Raman shift occurs owing to the deformation of adenine molecules pressurized by the silver atoms of the tip.
A metallic nano-probe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. The excitation fields and CARS polarization are enhanced by the tip apex of the nano-probe through the excitation of local surface plasmons. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in the spatial resolution far beyond the diffraction limit of light. Our CARS microscope using a silver-coated probe visualized the DNA network structure at a specific vibrational frequency (~1337 cm-1) of adenine molecules with a spatial resolution of ~15 nm and sufficient sensitivity.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is difficult due to the strict limitation of spatial resolution due to the wave nature of light. While there have been attempts to overcome the diffraction limit by using nonlinear response of materials, near-field optical microscopy could provide better detecting accuracy. In this paper, we present molecular distribution nano-imaging colored by Raman-scattering spectral shifting, which is probed with a metallic tip. The metallic probe tip has been used to enhance the optical field only in the vicinity of probe tip. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS), while in this case a single metallic tip works for the field enhancement in nanometer scale.
We have developed a nanometric fabrication system by manipulating chromium (Cr) atoms with laser beams. We utilized a laser diode with very narrow linewidth (approximately 5 MHz) to generate the photon force on the Cr atoms. The gradient force is exerted on the atoms in a standing wave, and the atoms are deposited in the periodical low potential regions of the standing wave, and a series of Cr lines are formed on the substrate with the periodicity of the standing wave. In order to optimize conditions for this deposition technique, we have performed a numerical analysis of the property of fabrication by tracing the trajectories of the atoms in the potential of light field. We found that the laser power and the degree of the collimation of the atom beam are important to obtain sharp structures. In addition, the longitudinal deceleration of the atom beam reduces strongly the structure size and increases the depth of the focus of the atom beam.
KEYWORDS: Near field scanning optical microscopy, Luminescence, Raman scattering, Near field, Light scattering, Stray light, Photons, Near field optics, Objectives, Rhodamine
We have developed a NSOM which has a metallic probe tip and a highly focused evanescent light field spot. Evanescent illumination effectively rejects the background light, e.g. the stray light from the shaft of the probe. By suppressing the stray light and utilizing the field enhancement generated by the metallic probe, a sudden increment of the fluorescence was observed in the near-field region. We have used this for near-field Raman scattering detection of molecules vibrations with the aid of surface enhanced Raman scattering. One specific stokes-Raman-shifted lines was observed by near-field excitation together with several other lines that were excited by the far-field light.
We developed a laser-beam-controlled atom-manipulation system to deposit nanometric structure of atoms on a substrate. Atom beam of chromium is thermally evaporated and collimated with Doppler cooling technique using frequency-tuned laser diode. The gradient force is exerted on atoms by a standing wave over the substrate, and atoms are trapped in a low potential region of the standing wave. Consequently, a series of lines are formed on the substrate with the same periodicity to the standing wave. We present the analysis of trajectories of atoms in the light force potential to find conditions of the laser power, the sign and the degree of the detuning of the laser frequency from the resonance frequency of the atom, to reduce the width of the structure.
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