Hiromitsu Furukawa

Local field enhancement with an apertureless near-field-microscope probe

Computer-simulated results of an electromagnetic field near the apex of a metallic probe of an apertureless near-field scanning optical microscope are shown under some different conditions. The results indicate that the metallic probe amplifies the electromagnetic field locally under the probe apex with an extent of apex size. The results were obtained through the calculation of Maxwells differential electromagnetic equations in three-dimensional digitized space. Polarization-dependence of illumination on the field is also discussed.

Analysis of image formation in a near-field scanning optical microscope: effects of multiple scattering

We analyze how the interaction between the probe and the field near the sample surface affects the characteristics of the images produced by a near-field scanning optical microscope. The numerical calculation is carried out using the finite-difference time-domain method, and a three-dimensional model that includes a probe of arbitrary shape is considered. Assuming an aperture-type probe (collection-mode near-field microscope), we compare the images produced from the same surface under both the constant-height-scanning and constant-distance-scanning modes. Our results indicate that the probe-sample interaction is critical in defining the characteristics of the generated images. We also investigated image characteristics as function of the polarization state of the illuminating light. When the effects of multiple scattering by the probe tip are taken into account, the calculated intensity image approximates the geometric profile of the sample surface. The dependences of the detected image intensity with the scanning mode utilized, the polarization state of the illuminating light and the sample size are also investigated.

Point spread function of optical microscopes imaging through stratified media

We propose a model for imaging point objects through a dielectric interface or stratified media. The model is applicable to conventional and confocal fluorescence microscopy, with single- or multiphoton excitation. An analytical solution is obtained in the form of readily computable functions. When large mismatches occur in the refractive indices of the media of the objective lens and specimen the illumination and detection point spread functions differ significantly, showing that currently used imaging models may fail to correctly predict imaging properties of optical microscopes.

PICO-NEWTON FRICTION FORCE MEASUREMENTS USING A LASER-TRAPPED MICROSPHERE

We have developed a system for measuring the force of friction by using the technique of laser-trapping. A polystyrene microsphere of a 3 µm diameter is trapped in a water layer using a focused near infrared laser. While the sample is scanned, all of the viscous, friction, and radiation forces are induced in the microsphere. The friction force acting on the microsphere is estimated by the viscous force obtained from the scanning speed of the stage using Stokes law, and the radiation force is obtained by the displacement of the microsphere. We demonstrate experimentally the existence of a linear relationship between the force of radiation and the displacement. We found that the value of the spring constant of the radiation force is 7.2 µN/m. Furthermore, we measured both the friction force acting on the probe on a flat quartz substrate surface and that at the edge of small steps. The magnitude of these forces are 0.88 pN and 1.23 pN, respectively.

Advanced laser-produced EUV light source for HVM with conversion efficiency of 5-7% and B-field mitigation of ions

We propose a new scheme for high conversion efficiency from laser energy to 13.5 nm extreme ultra violet emission within 2 % band width, a double pulse laser irradiation scheme with a tin droplet target. We consider two-color lasers, a Nd:YAG laser with 1.06 µm in wavelength as a prepulse and a carbon dioxide laser with 10.6 µm in wavelength for a main pulse. We show the possibility of obtaining a CE of 5 - 7 % using a benchmarked radiation hydro code. We have experimentally tested the new scheme and observed increase of CE greater than 4 %. We show many additional advantages of the new scheme, such as reduction of neutral debris, energy reduction of debris ions, and decrease of out of band emission. We also discuss debris problems, such as ion sputtering using newly developed MD simulations, ion mitigation by a newly designed magnetic coil using 3-PIC simulations and tin cleaning experiments.

Adaptive Optics with a Liquid-Crystal-on-Silicon Spatial Light Modulator and Its Behavior in Retinal Imaging

An adaptive optics system with a brand-new device of a liquid-crystal-on-silicon (LCOS) spatial light modulator (SLM) and its behavior in in vivo imaging of the human retina are described. We confirmed by experiments that closed-loop correction of ocular aberrations of the subjects eye was successfully achieved at the rate of 16.7 Hz in our system to obtain a clear retinal image in real time. The result suggests that an LCOS SLM is one of the promising candidates for a wavefront corrector in a prospective commercial ophthalmic instrument with adaptive optics.

Light propagation in periodic microcavities

We demonstrated light propagation in a type of optical waveguide, which consisted of periodically arrayed dielectric particles. Although the form is similar to a photonic crystal waveguide, our method used resonance frequencies instead of band-gap frequencies. Advantages provided by this waveguide are that dynamic optical interconnection can be realized and that a multichannel waveguide is easily formed. To fabricate the system of periodic microcavities, we used polystyrene-latex particles that have a uniform size of 15.6 μm. Some discrete frequencies of fluorescence were selected to agree with the phase matching condition and they were propagated through the microspheres. The assembled microspheres can be taken as a multichannel waveguide.

Retinal blood oxygen saturation mapping by multispectral imaging and morphological angiography

We report on an experiment for mapping two- dimensional blood oxygen saturation distribution by measuring multispectral images in the wavelength range from 500 to 650 nm with the resolution of 7 nm. The multispectral images of the retina are acquired with the originally designed imaging system equipped with the tunable spectral filter. To separate retinal blood vessels from other tissue area, morphological image processing is adopted. The small flick motion is also compensated. After prepossessing, the partial least squares regression model for the oxygen saturation is built by sampling typical spectra reflected from artery and vein. Applying the regression model for all the point extracted by the morphological processing allows the two-dimensional oxygen saturation map.

The interaction physics of the fast ignitor concept

The interaction of relativistic electrons produced by ultrafast lasers focussing them on strongly precompressed thermonuclear fuel is analytically modelled. Energy loss to target electrons is treated through binary collisions and Langmuir wave excitation. The overall penetration depth is determined by quasielastic and multiple scattering on target ions. It thus appears possible to ignite efficient hot spots in a target with density larger than 300 g/cc.

Oximetry of Retinal Capillaries by Multicomponent Analysis

Retinal oximetry of capillaries was performed for early detection of retinal vascular abnormalities, which are caused predominantly by complications of systemic circulatory diseases. As the conventional method for determining absorbance is not applicable to capillaries, multicomponent analysis was used to estimate the absorbance spectra of the retinal blood vessels. In this analysis, the capillary spectrum was classified as intermediate between those of the retinal arteries and veins, enabling relative estimation of oxygen saturation in the capillaries. This method could be useful for early recognition of disturbances in the peripheral circulation. Furthermore, a spectroscopic ophthalmoscope system based on the proposed method was developed to examine the human retina. A clinical trial of this system demonstrated that oximetry of the retinal capillaries may be an improvement over the present diagnosis for patients of malignant hypertension.