Yuichi Kozawa

Generation of a radially polarized laser beam by use of a conical Brewster prism

To generate a radially polarized laser beam we designed and fabricated a new Brewster optical element that consists of convex and concave conical prisms. The lateral surface of the convex conical prism was coated with a dielectric multilayer (SiO2 and Ta2O5) to enhance polarization selectivity. By combining two prisms we obtained a conical Brewster prism without beam divergence owing to refraction. A radially polarized TEM01* (R-TEM01*) mode laser beam was demonstrated when this prism was used inside a Nd:YAG laser cavity.

Sharper focal spot formed by higher-order radially polarized laser beams

The intensity distributions near the focal point for radially polarized laser beams including higher-order transverse modes are calculated based on vector diffraction theory. For higher-order radially polarized mode beams as well as a fundamental mode (R-TEM01*) beam, the strong longitudinal component forms a sharper spot at the focal point under a high-NA focusing condition. In particular, double-ring-shaped radially polarized mode (R-TEM11*) beams can effectively reduce the focal spot size because of destructive interference between the inner and the outer rings with pi phase shift. Compared with an R-TEM01* beam focusing in a limit of NA=1, the full width at half-maximum values of the focal spot for an R-TEM11* beam are decreased by 13.6% for the longitudinal component and 25.8% for the total intensity.

Generation of a radially polarized laser beam by use of the birefringence of a c-cut Nd:YVO4 crystal

We demonstrated the generation of a radially polarized laser beam from an extremely simple laser resonator including a c-cut Nd:YVO4 crystal as a laser medium. The oscillation in the radial polarization was based on the optical path difference between an extraordinary ray and an ordinary ray induced by the birefringence of the crystal. By simply adjusting the distance between two cavity mirrors, only the extraordinary ray became stable for the oscillation, resulting in the generation of a radially polarized beam. The beam was very stable even at low power output and is expected to be a promising radially polarized laser source because of its excellent simplicity.

FOCUSING PROPERTY OF A DOUBLE-RING-SHAPED RADIALLY POLARIZED BEAM

The intensity distributions of a tightly focused radially polarized beam that has a double-ring-shaped transverse mode pattern were calculated based on vector diffraction theory. The distribution of the longitudinal component near the focus varied drastically with the degree of truncation of the incident beam by a pupil. When the ratio of the pupil radius to the beam radius was approximately 1.3, the longitudinal component disappeared at the focal point, owing to destructive interference. This dark area surrounded by an intense light field was of the order of the wavelength, with excellent intensity symmetry.

Calculation of optical trapping forces on a dielectric sphere in the ray optics regime produced by a radially polarized laser beam

We calculated the optical trapping forces on a microscopic particle in the ray optics regime for the case where a radially polarized laser beam is applied. A higher axial trapping efficiency than for a circularly polarized doughnut beam was predicted due to the large p polarization component. Three-dimensional optical trapping was expected for particles with a larger index of refraction and for objectives with a smaller numerical aperture.

Optical trapping of micrometer-sized dielectric particles by cylindrical vector beams

Single-beam optical trapping of micrometer-sized dielectric particles is experimentally demonstrated using radially and azimuthally polarized beams. The axial and transverse optical trapping efficiencies of glass and polystyrene beads suspended in water are measured. The radially polarized beam exhibited the highest trapping efficiency in the axial direction due to the p polarization of the radial polarization on the particle surface. On the other hand, the azimuthally polarized beam had a higher transverse trapping efficiency than the radially polarized beam. These results are consistent with numerical predictions.

Visualizing hippocampal neurons with in vivo two-photon microscopy using a 1030 nm picosecond pulse laser

In vivo two-photon microscopy has revealed vital information on neural activity for brain function, even in light of its limitation in imaging events at depths greater than several hundred micrometers from the brain surface. We developed a novel semiconductor-laser-based light source with a wavelength of 1030 nm that can generate pulses of 5-picosecond duration with 2-W output power, and a 20-MHz repetition rate. We also developed a system to secure the head of the mouse under an upright microscope stage that has a horizontal adjustment mechanism. We examined the penetration depth while imaging the H-Line mouse brain and demonstrated that our newly developed laser successfully images not only cortex pyramidal neurons spreading to all cortex layers at a superior signal-to-background ratio, but also images hippocampal CA1 neurons in a young adult mouse.

Lateral resolution enhancement of laser scanning microscopy by a higher-order radially polarized mode beam.

We demonstrate that the lateral resolution of confocal laser scanning microscopy is dramatically improved by a higher-order radially polarized (HRP) beam with six concentric rings. This beam was generated simply by inserting liquid crystal devices in front of an objective lens. An HRP beam visualized aggregated 0.17 μm beads individually and is also applicable to biological imaging. This method can extend the capability of conventional laser scanning microscopes without modification of the system, with the exception of the addition of the liquid crystal devices in the optical path.

Generation of hollow scalar and vector beams using a spot-defect mirror

Generation of both scalar and vector hollow beams was demonstrated by using a mirror with a low-reflectivity spot to suppress the oscillation of lower order transverse modes. As scalar beams, several hollow Laguerre-Gaussian beams have been observed from a side-pumped Nd:yttrium aluminum garnet laser cavity. The intensity profiles were in an excellent agreement with theoretical ones. The phase front variation around the optical axis was verified to be spiral. Furthermore, both Laguerre-Gaussian and Bessel-Gaussian vector beams have been also observed from the identical cavity. In addition to the verification of intensity profiles, polarization pattern measurement confirmed that the beams had revolving polarization distributions along the azimuthal direction as theoretically predicted.

Hollow vortex beams

Hollow beam formation of radially and azimuthally polarized vortex beams, which has arbitrary topological charge, is analytically discussed under the strong focusing condition. The expressions for the electric fields of the focused vector-vortex beams are obtained based on a vector diffraction theory. The order of the Bessel function of the first kind appearing in the expressions indicates the ability to form hollow beams. Similar discussion is applied for different vortex beams, which are expressed by linear combination of radially and azimuthally polarized beams. Calculations of intensity profiles across the focus are also presented.