Atsuo Takayanagi

Scanning Wiener-fringe microscope with an optical fiber tip.

A scanning probe optical microscope using the Wiener fringe is presented. The Wiener fringe is formed by a standing wave between the incident and reflected waves on an optically reflective surface and is detected by inserting an optical fiber tip into the fringe-field region. The detected signal is used to maintain the tip-sample distance constant so that a topographic image of a sample can be obtained by a computer-assisted instrument. A spatial resolution of 200 nm has been achieved by observing a sample of known geometry.

Manipulation of metal nanoparticles using fiber-optic laser tweezers with a microspherical focusing lens

This paper describes the laser manipulation of metal nanoparticles and dielectric particles by fiber-optic laser tweezers with a microspherical focusing lens. In this manner, a small ball lens attached to the end of the core focuses light guided through a single-mode optical fiber. Numerical electromagnetic analysis of the microfocusing structure showed the possibility of metal nanoparticle trapping with this method. An effective focus allows metallic particles located in the path of the focused light to be attracted to the center of the beam path and to be two-dimensionally trapped on a substrate. In the experiment, manipulation of Au nanoparticles with diameters of 40, 100, and 200 nm was demonstrated. In addition to metallic samples, manipulation of dielectric particles with 1 and 3 µm diameters was also demonstrated using the same configuration, showing the extended capability of the fiber-optic laser manipulator.

Charge deposition and imaging on a fluoride thin film using the scanning force microscope

This paper describes a localized charge deposition and imaging on a fluoride thin film with a contact mode scanning force microscope (SFM). The charge was deposited by contacting a tip of the SFM to a sample surface and by applying voltage pulses between the tip and back electrode of the sample. After applying a voltage pulse of 300 V and a pulse width of 1 ms, the attractive force image of 250×700 nm2 was obtained corresponding to the charge deposited region. Also, the stored charge was imaged over a period of 8 h. The cause for charge images to occur in elliptical patterns was estimated to be due to the deformation of the tip apex from contact to the sample surface.

Birefringence imaging with illumination-mode near-field scanning optical microscope

A new optical configuration for a near field scanning optical polarized microscope with an illumination mode is reported. It uses two circularly polarized laser beams with different frequencies which are generated by an axial Zeeman laser. A laser beam is incident on an optical fiber and is launched form the apex of a sharpened fiber probe in order to illuminate the sample. The scattered light on the surface of the sample is collected with an objective lens and goes through the optical elements of the quarter wave plate and the linear polarizer. The light from polarization devices is converted to an electric signal with a photomultiplier and fed into a lock-in amplifier. The quarature components and intensity signal are acquired to computer, and the retardation and the azimuth angle of the birefringence are then calculated via computer. The measurement characteristic of the developed system and image of birefringence material are shown.

Development of a visual encryption device using higher-order birefringence

We propose and demonstrate a novel visual encryption device composed of higher-order birefringent elements. When an optical material with higher-order birefringence is placed between a pair of polarizers and illuminated by white light, it appears only white. In contrast, when it is illuminated by monochromatic light, the transmitted intensity varies depending cosinusoidally on the wavelength. An array of such materials can express information (e.g., letters and/or images) by controlling the birefringence of each pixel. If birefringence phase retardation can be adjusted for a specific wavelength, the information will be clearly displayed when it is illuminated at this wavelength. We denote this wavelength a key wavelength. The encryption device was fabricated by controlling the amount of higher-order birefringence to achieve high contrast only by using polarized illumination at the key wavelength. Thus, the information stored in the encryption device can be decoded only by illuminating it at the key wavelength. To demonstrate the validity of this encryption principle, we constructed a 3 × 3 pixel device in which commercial retarder films were laminated. The device was illuminated by a monochromatic light. When a readout experiment was performed using the monochromatic light at the key wavelength, the stored letter was clearly visible. On the other hand, when pixel brightness was randomly distributed with illumination at the wavelength other than the key wavelength, the letter could not be recognized. Furthermore, the stored information can be easily distributed to multiple physical keys that display arbitrary images. In this case, the birefringence phase retardation is obtained by summing the values of retardation of each pixel of the physical keys. In the experimental device, the observed image was decoded by superimposing the two images using different physical keys.

Simultaneous measurement of linear and circular birefringence with heterodyne interferometry

We present a novel technique to measure both linear and circular birefringence, simultaneously. This technique is based on an optical heterodyne interferometry which is performed by an orthogonally polarized two frequency laser. Two orthogonal components of the optical beat signal are detected by two-phase lock-in amplifier. The two components change sinusoidally with the rotation of azimuth angle of polarization devices. The retardation and the orientation of linear birefringence and the rotation angle of circular birefringence can be calculated by applying a Fourier analysis to the two sinusoidal variations. The measurement sensitivity for this method is verified by using the combination of a Babinet Soleil compensator and a half-wave plate as a sample. It is demonstrated that the birefringence of commercially available twisted nematic liquid crystal cell where a driving voltage is applied can be measured by using the proposed technique.

Fast birefringence measurement using right and left hand circulary polarized laser

A fast and accurate birefringence measurement system has been built to study the in-plane birefringence of a rotating optical disk substrate. The fully automated instrument incorporates an axial Zeeman laser which emits both right and left hand circularly polarized lights, stationary polarization elements and a lock-in amplifier. Measurement results showing the accurate and fast features on the system are presented. It is also demonstrated that the in-plane birefringence mapping in rotating substrate of optical disk can be obtained by use of the ability of fast birefringence measurement.

Direct alignment of nematic liquid crystal molecule using atomic force microscope nano-rubbing

The orientation control of liquid crystal (LC) molecular on the polyimide film has been necessary to fabricate LC devices. Nano-rubbing by atomic force microscope (AFM) has been proposed as the one of methods to control it precisely. In the method, a thin polyimide film was rubbed by a sharpened AFM probe-tip with relatively strong load force. However, the method has some drawbacks; the frictional wear of AFM probe-tip and the difficulty of reorientation after rubbing. In this paper, we have proposed the orientation control of LC on the polyimide film and using direct AFM nano-rubbing method with weak load forces. The change of LC alignment was quantitatively observed by a polarization microscope and birefringence-contrast scanning near-field optical microscope. The effect of scanning density was strong for azimuth angle but the effect of the scanning velocity was weak for both retardation and azimuth angle. An optical switching device was developed utilized isotropic-nematic phase change of liquid crystal which was rubbed in the grating pattern with methyl red dying, and the optical device was operated at the frequency of 0.5Hz. As a result, The proposed method had an effective method to fabricate novel liquid crystal optical devices.

Development of birefringence measurement with double rotating polarization components

A novel birefringence measurement with double rotating polarization elements is proposed. This system includes rotating components of linear polarizer and quarter-wave plate with different rotating speeds. The birefringence is calculated by analyzing the Fourier components for rotation frequency in a detected signal. In this paper, the basic principle and experimental results of the birefringence measurement are described.

Laser manipulation and fixation of metal nanoparticles using optical fiber probe

A laser manipulation technique for metal nanoparticles using an optical fiber has been developed. A micro ball lens adhered onto the flattened end of the optical fiber focuses a light beam propagated through a core. An object is trapped in the focused beam. An electromagnetic field distribution was numerically simulated for validation of the focusing lens. Calculation including an Au nanoparticle indicated that the laser trapping would be possible with this method. In the experiment, trapping of Au particles with diameter of 200nm was achieved by using a light source (Nd-YAG: 1064nm). The maximum trapping efficiency attained in the focal region was estimated to be 5.4fN/mW. Additionally, the fixation of a manipulated particle onto a glass substrate was also demonstrated. With intensifying the laser power, a laser-trapped particle is fixed on the substrate. By repetition of the procedure of laser manipulation and fixation, alignment of Au nanoparticles was achieved.