Formation, manipulation, and elasticity measurement of a nanometric column of water molecules.

Nanometer-sized columns of condensed water molecules are formed by an atomic-resolution force microscope operated in ambient conditions. An unusual stepwise decrease of the force gradient associated with the ultrathin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights (approximately 0.5 N/m) and plateau lengths (approximately 1 nm). Such "quantized" elasticity is indicative of an atomic-scale stick slip at the tip-water interface. A thermodynamic-instability-induced rupture of the water meniscus (5 nm long and 2.6 nm wide) is also found. This work opens a high-resolution study of the structure and interface dynamics of a nanometric aqueous column.

Diffraction-limited dark laser spot produced by a hollow optical fiber.

By using the diffracted field of the LP(11) mode of a hollow-core optical fiber, we have produced a micrometer-sized, focused dark laser spot in the near field of the fiber. The minimum half-width of the dark spot is less than 1 mum . In particular, by masking the hollow core and metal coating the cladding with a microsphere, we blocked the light propagating in the cladding and obtained a clean dark spot, which may be useful in atom-optical experiments such as with atomic lenses, atom traps, and atom switches.

Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals.

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.

High-resolution confocal detection of nanometric displacement by use of a 2 x 1 optical fiber coupler.

A novel high-resolution confocal measurement of nanometric displacement that uses a simple 2 x 1 optical fiber coupler is presented. The basic principle is to detect interferometrically the change in the phase difference between two adjacent input fibers of the coupler that results from the nonuniform wave-front curvature of the reflected light. Measurement of subnanometer axial modulation, which permits high-resolution displacement sensing by optical means, is demonstrated.

Atom trap in an axicon mirror.

We have realized a novel atom trap in an axicon (conical hollow) mirror, using a frequency-modulated, single-diode laser. Different spatial distributions of trapped atoms such as a ball and a ring are observed. We show that our numerical simulations are consistent with experimental results. In particular, the ring diameter is found to be approximately the separation between the mirror axis and the magnetic field axis. The axicon trap may be useful as a precooled atom source for cold atomic beams, atom funnels, and atom waveguides.

Nonglobal model for a near-field scanning optical microscope using diffraction of the optical near field.

Diffraction of the localized optical near field by a subwavelength-sized probe (metallic circular aperture) is studied by a nonglobal approach. The problem is closely relevant to the basic principle of a near-field scanning optical microscope (NSOM). We obtain analytic expressions of the diffracted far field from which the transfer function of the NSOM can be calculated. In particular, we find a resonance enhancement in the detection efficiency when the sample size is similar to the probe size. By lateral scanning of the aperture position at a constant height, we also obtain a NSOM image of a dielectric sphere sample.

Direct observation of size-dependent features of the optical near field on a subwavelength spherical surface.

We investigate the characteristics of the optical near field of a subwavelength dielectric sphere by using a nanometric fiber tip and a sensitive technique of wavelength conversion. The decay length of the near field induced around the single sphere is directly measured to be 0.3 of its radius, obtained by precise positioning of the probe tip. A numerical calculation of the optical near field based on the Mie scattering theory is in good agreement with the experimental results. The enhancement of f luorescence collection efficiency in the near-field region is also observed.

Single-beam atom trap in a pyramidal and conical hollow mirror.

We present a novel and simple magneto-optical trap in pyramidal and in conical hollow mirrors, using a single beam. A diode laser having modulation sidebands at microwaves is used for cooling, trapping, and repumping of rubidium atoms in a vapor cell. When the laser is circularly polarized and sent into the hollow region, three pairs of counterpropagating beams are automatically produced therein that have the same polarization configuration as that of a conventional six-beam magneto-optical trap. The fluorescence by the trapped atoms and its mirror image are observed simultaneously. This system may be useful for atom-manipulation applications such as gravitational atom traps and atom waveguides.