Kazuyuki Nakayama

Precise intensity correlation measurement for atomic resonance fluorescence from optical molasses.

We measured the intensity correlation of true thermal light scattered from cold atoms in an optical molasses. Using a single-mode fiber as a transverse mode filter, measurement with maximally high spatial coherence was realized, allowing us to observe ideal photon bunching with unprecedented precision. The measured intensity correlation functions showed a definite bimodal structure with fast damped oscillation from the maximum value of 2.02(3) and slow monotonic decay toward unity. The oscillation can be understood as an interference between elastic and inelastic scattering fields in resonance fluorescence.

Waveguide-mode interference lithography technique for high contrast subwavelength structures in the visible region

We explore possibilities of waveguide-mode interference lithography (WMIL) technique for high contrast subwavelength structures in the visible region. Selecting an appropriate waveguide-mode, we demonstrate high contrast resist mask patterns for the first time. TM1 mode in the waveguide is shown to be useful for providing a three-dimensional structure whose cross section is checkerboard pattern. Applying our WMIL technique, we demonstrate 1D, 2D and 3D subwavelength resist patterns that are widely used for the fabrication of metamteterials in the visible region. In addition to the resist patterns, we demonstrate a resonance at 1.9 eV for a split tube structure experimentally.

Saturated absorption spectroscopy of acetylene molecules with an optical nanofiber

We performed saturated absorption spectroscopy of acetylene (C₂H₂) ν₁ + ν₃ band transitions with an optical nanofiber (ONF). Owing to high-intensity light around the ONF, we observed a Lamb dip at relatively low-power laser (~10 mW) without a cavity. Our results showed that the simple ONF spectrometer is advantageous for performing saturation absorption spectroscopy and serves as a practical low-cost wavelength reference in the optical fiber communication band.

X-ray source combined ultrahigh-vacuum scanning tunneling microscopy for elemental analysis

An ultrahigh-vacuum scanning tunneling microscope (UHV-STM) combined with an x-ray source has been developed. STM samples were irradiated with an x-ray beam and an x-ray induced photoemission current was detected with the tip. By using the system, apparent height increases due to the x-ray induced current were successfully observed in STM images of Cr/Cu(111), Au/Cu(111), Si(111)7×7, and Si(111)-5×1 Au surfaces. A bias voltage applied to the tip enhances collection of the x-ray induced current and increases the apparent height. The relative height increase of the Au overlayer to the Cu substrate in the STM images of the Au/Cu(111) surface was found to depend on the x-ray irradiation, suggesting that it may be a promising tool for elemental analysis in a STM.

Functional localization of kinesin/microtubule-based motility system along metallic glass microwires

We report an approach using metallic glass microwires for functional organization of kinesin/microtubule-based molecular motility systems along a quasi-one-dimensional track. The molecular motility system assembled along a metallic glass microwire exhibits the typical kinesin-powered gliding motion of microtubules, while the variance of the gliding direction depends on the wire diameter. As a result of the geometrical boundary condition given by the wire tracks, the angle within which the orientations of gliding microtubules fall becomes narrower for smaller wire diameter. Such behavior supports the feasibility of using microwires as a simple and flexible means of spatial regulation of the molecule-based in-vitro motion. Furthermore, the metallic glass wires interact with microtubules, the negatively charged polyelectrolyte, by creating electric fields. We experimentally demonstrate how the electric field-induced forces act as an additional control parameter in the wire-based manipulation of the molecular...

Polarization property of terahertz wave emission from gammadion-type photoconductive antennas

We have investigated the polarization property of terahertz (THz) wave emission from gammadion-type photoconductive antennas (PCAs). It was experimentally shown that the polarization of emitted THz wave from the gammadion-type PCAs was elliptic. Finite difference time domain simulation reproduces the experimental observation fairly well. From simulation, we find out that the gammadion-type PCAs have two kinds of radiation; i.e., the radiation from the gap of the PCA and the radiation from the bends of electrode. These two kinds of radiation have different polarization and different time delay, which results in the observed elliptic polarization.

Erasable nanometer-scale modification at the Au/Si interface by ballistic electron emission microscopy

We report that local modification and its erasing with a nanometer-scale size can be performed at a Au/Si(111) interface using ballistic electron emission microscopy (BEEM). By applying a negative voltage on the tip, a region was created where no BEEM current flows at the interface and was imaged with BEEM. The modified area can be erased by applying a voltage with the opposite polarity. It is found that the minimum size of writing and erasing corresponds to Au grains, suggesting a method of rewritable memory on a nanometer-scale dimension.

Holographic storage of multiple coherence gratings in a Bose-Einstein condensate.

We demonstrate superradiant conversion between a two-mode collective atomic state and a single-mode light field in an elongated cloud of Bose-condensed atoms. Two off-resonant write beams induce superradiant Raman scattering, producing two independent coherence gratings with different wave vectors in the cloud. By applying phase-matched read beams after a controllable delay, the gratings can be selectively converted into the light field also in a superradiant way. Because of the large optical density and the small velocity width of the condensate, a high conversion efficiency of >70% and a long storage time of >120 micros were achieved.

Theory of the optical-rectification effect in metallic thin films with periodic modulation

We conducted theoretical and numerical investigations of the optical rectification (OR) effect in metallic structures with periodic modulation. A new formulation of the OR effect is presented, and the mechanism by which the OR effect is generated, which has been a controversial issue in previous studies, is clarified. We reveal that the OR effect is strongly enhanced by a combination of spatial variation of the metallic structure and local electric field enhancement. Our theory was numerically evaluated and agreed fairly well with experiment.

Fabrication and terahertz response of “split-tube” arrays

We designed and fabricated split-tube arrays using a maskless exposure system which was converted from a commercial projector. We found that the structures have magnetic response in terahertz region which are affected by mutual inductance.