Yoshinori Iketaki

Generation of a doughnut-shaped beam using a spiral phase plate

To generate a doughnut-shaped beam, i.e., the first order of a Bessel beam, a spiral phase plate with 8 divided etching areas is fabricated with an etching accuracy of better than 6 nm. The etching depth of each area is designed so that the phase distribution of the laser beam passing through has a phase difference of π at a symmetric position with respect to the optical axis. Using a laser beam with a wavefront aberration of 1/10λ, the phase distribution of the beam passing through the plate is measured by a Shack Hartman wavefront sensor. It has been found that the beam has a spiral phase change of 2π along the optical axis. The focused beam has a circular doughnut pattern, as predicted by a theoretical calculation, and we succeeded to generate the ideal first-order of a Bessel beam.

Two-point-separation in super-resolution fluorescence microscope based on up-conversion fluorescence depletion technique.

Pronounced separation (750 nm) between two individual fluorescence spots in a novel super-resolution microscopy based on a two-color up-conversion fluorescence depletion technique has been investigated. This microscopy has the potential to achieve a spatial resolution (<300nm) of 1/2 the diffraction limit.

Investigation of polarization effects for high-numerical-aperture first-order Laguerre-Gaussian beams by 2D scanning with a single fluorescent microbead

The focal intensity distribution of strongly focused (NA=0.9) first order Laguerre-Gaussian doughnut beams is investigated experimentally for three different polarizations: linear, and left-handed circular and right-handed circular. The investigations are done by 2-dimensional scanning the focal plane with of a 100nm diameter fluorescent microbead, and measuring the fluorescence signal. The results are shown to be in excellent agreement with theoretical predictions, and demonstrate the superiority of one of the circular polarizations to achieve a sharp dark central spot.

Two-color far-field super-resolution microscope using a doughnut beam

We have demonstrated a realistic super-resolution scanning fluorescence microscope using conventional nanosecond lasers. This super-resolution microscope is based on the combination of two-color fluorescence dip spectroscopy and shape modulation to a doughnut beam. Only by introducing a doughnut erase beam, the resolution of the laser fluorescence microscope breaks the diffraction limit by two times without using any mechanical probe.

Investigation of the center intensity of first- and second-order Laguerre-Gaussian beams with linear and circular polarization

The vectorial Debye integral shows that tightly focused Laguerre-Gaussian (LG) beams have a residual intensity at the focal point for linear polarization, for a topological charge of m=1 and 2. We measured the shapes of linearly and circularly polarized LG beams and found that a central intensity appeared at m=1 and 2 for linear and right-handed circular polarization, however, it is completely canceled for left-handed circular polarization. In general, when the orbital angular momentum of the LG beam is parallel to the spin angular momentum of the photons, zero intensity is always achieved at the focus.

Theoretical investigation of the point-spread function given by super-resolving fluorescence microscopy using two-color fluorescence dip spectroscopy

The profile of the point spread function (PSF) in superresolution microscopy is studied theoretically. The fluorescence spot profile (i.e., the PSF) is determined by the focused beam patterns of the applied two-color lasers and the optical properties of the fluorescence-depletion process induced by the lasers (the pump and erase beams). In this study, the fluorescence-depletion process for the sample molecule is analyzed using a rate equation for a three-state model. Based on this result, we calculate the PSF for the case where the erase beam is modeled by a first-order Bessel function. In the case of an erase beam with a large photon flux, the obtained PSF has a Lorentzian-like shape, which seldom appears in traditional microscopy. In this work, we also investigated a possible relationship between the PSF and other parameters in the fluorescence-depletion process.

Investigation of the fluorescence depletion process in the condensed phase; application to a tryptophan aqueous solution

Abstract By using a two-color dip spectroscopy, we measured the fluorescence intensity from tryptophan in a water solution. The fluorescence intensity exponentially decreased as the laser intensity for the S n ←S 1 excitation increased. The phenomenon was analyzed by a rate-equation for a three-state model. The analysis shows that tryptophan with the S n state has a radiationless relaxation process without any process through the S 1 state, and that the S n →S 1 internal conversion does not have a 100% yield. The branching ratio of the process is estimated to be 20%. The presented result clarifies in detail the real meaning of Kasha’s rule.

Formation of a doughnut laser beam for super-resolving microscopy using a phase spatial light modulator

The formation of a doughnut-shaped laser beam is presented. To generate the beam, we use an optically addressed parallel-aligned nematic liquid-crystal phase spatial light modulator (PAL-SLM), and observe the shape of the focused beam. By using a compensating technique for wave aberration, the beam has a symmetric doughnut shape with a hole size of 1 µm on the focal plane. The experimental result shows that the generated beam can be expected to be applicable to super-resolving microscopy based on the fluorescence depletion process.

Nanometer scale marker for fluorescent microscopy

To establish a calibration method of optical performance in fluorescence microscopy, we fabricated a fluorescent nanometer-scale marker by combining a dry dye method for polymer film and fine lithography. The marker has a 50 nm line-and-space fluorescent pattern, finer than the optical diffraction limit. A spin-coated poly(methyl methacrylate) thin film on a silicon wafer was densely doped with Rhodamine 6G using a simple vacuum process, named the vapor-transportation method, and then the pattern was formed on the film using electron-beam lithography. The figure accuracy of the fabricated marker was calibrated by electron microscopes. Using this marker, one can quantitatively evaluate the optical properties; i.e., the contrast-transfer function, the point-spread function, magnification, and so on. To show practical use of the marker, we demonstrated the evaluation of a fluorescent microscope system.

Construction of super-resolution microscope based on cw laser light source

We constructed a super-resolution microscope system based on a cw laser light source. Electro-optical modulators convert the cw laser light into a light pulse with a width of 15ns and provide a repetition rate of 100kHz. The performance of the microscope was evaluated using 100nmϕ fluorescence microbeads. The fluorescence signal from the beads can be detected with very low statistical fluctuation. The super-resolution image was obtained with better signal to noise ratio compared with that given by a pulse laser light source with a repetition rate of several tens of hertz. The fluorescent size of the beads was 150nm, which is 2.3 times smaller than the diffraction limit. Experimental results confirmed that the two-point resolution also overcomes the diffraction limit. The constructed system is expected to become a commercial microscope.