Longitudinal Mode Number Estimation of External Cavity Diode Laser Using Dual Periodic Grating for Optical Profiler System.

The concept of an optical profiler based on optical resonance was proposed, highlighting the initial requirements for mode number estimation. We proposed a method for estimating the longitudinal mode number of a laser propagating in an external cavity diode laser with high accuracy, utilizing dual-periodic diffraction gratings. These gratings were fabricated using interference lithography. To estimate the mode number, the wavelengths of two different modes are compared. Therefore, the greater the difference between the wavelengths, the higher the accuracy of the mode number determination. While the mode number difference was approximately 35 when using a conventional diffraction grating, this could be increased by a factor of 20 to around 700 using the dual-periodic grating. The relative accuracy achieved was 1.4 × 10-5.

Propagation constant-based diameter measurement technique for a submicrometer-scale optical fiber.

Diameter is a critical parameter for determining the physical properties of a submicrometer optical fiber and requires an accurate measurement. In this study, we proposed, to our knowledge, a novel diameter measurement technique derived from the waveguide theory, utilizing the pitch of a standing-wave near-field light generated by two counter-propagating lights within the submicrometer optical fiber. In a submicrometer optical fiber, the propagating light extends into the surrounding air as near-field light, existing within a range approximately equivalent to one wavelength from the surface of the fiber. By generating the standing-wave near-field light with the incident lights from both ends of the fiber, the pitch of the standing-wave near-field light can be measured by scanning along the fiber's central axis with a scanning near-field optical microscopy probe. The fiber diameter is subsequently acquired by solving the optical fiber eigenvalue equation. Based on the feasibility verification experiment, a high-precision measurement of approximately 0.50 µm was realized for the diameter of the optical fiber.

Manipulation of large, irregular-shape particles using contour-tracking optical tweezers.

While the optical tweezers technique is a promising tool for manipulation of microparticles, its application to large (>50 µm) particles and irregular-shape ones is still a hard task. In this Letter, we propose what is to our knowledge a novel concept of contour-tracking optical tweezers (CTOTs), which extract the contour of the objective particle to form the illumination pattern of the trapping laser into the contour shape in real time. We demonstrated the trapping of polystyrene particles of irregular shape with the size of over 100 µm with CTOTs. Our approach has potential to open the way for expanding the applicability of optical tweezers by enabling manipulation of a variety of samples.

Theoretical model of a subwavelength grating polarization beam splitter.

This study presents a theoretical model of a subwavelength grating polarization beam splitter (SWGPBS) using a combination of the theory of thin-film interference and the effective medium theory for guided-mode resonance. The structural parameters of SWGPBSs at oblique incidence calculated by our theoretical models and electromagnetic wave simulation were in good agreement within the range of the predicted approximation error. Feasibility of the oblique incidence SWGPBSs was verified, and the physical limitations of the SWGPBSs were clarified. Because the design procedure of SWGPBSs was simplified with our theoretical analysis, the range of their applications can be expanded to other fields.

Fabrication of nano/micro dual-periodic structures by multi-beam evanescent wave interference lithography using spatial beats.

We propose an effective method for fabricating dual-periodic structures using the combination of multi-beam interference lithography and evanescent wave exposure. Four-beam evanescent wave interference lithography (EWIL) is used as a prototype to demonstrate the fabrication feasibility of one-dimensional (1D) micro-grating structures covered with nanodots and two-dimensional microdot structures filled with subwavelength fringes by designing reciprocal lattice vectors of interference fringes. We experimentally fabricated 1D nano-/micro-grating structures with periods of 140 nm and 12.5 µm and microdots filled with subwavelength gratings of 450 nm period by four-beam EWIL. These structures are applicable to superlattice photonic crystals and subwavelength structured surfaces.

Microdisplacement sensor using an optically trapped microprobe based on the interference scale.

Positioning technology is one of the most important technologies for developing microsystems. In particular, displacement sensors are necessary for positioning devices with nanoscale accuracy. In this study, we propose a new displacement sensor that uses an interference scale as a linear scale and a laser-trapped microsphere as a sensing probe. This sensor has a wide measuring range, high resolution, and accessibility for narrow target areas. A glass microsphere was optically trapped by means of the laser trapping technique. Between the target surface and the probe, an interference scale was generated along the optical axis. The scale origin was fixed on the target surface. The distance between the probe and the target surface could be measured in terms of the shift in the interference scale. This study investigated the fundamental performance of the sensor. The resolution and accuracy of the sensor were 10 and +/-50 nm, respectively; these values could be improved by using trapping lasers having shorter wavelengths. The measurable range was 250 microm. This sensor can provide useful displacement information from a target area having dimensions smaller than 15 microm. In addition, the displacement sensor can measure the distance even for surfaces inclined at angles less than 15 degrees; thus, a flexible arrangement can be used to carry out measurements. In addition, the direction of displacement can be identified.

Measurement of axial and transverse trapping stiffness of optical tweezers in air using a radially polarized beam.

The trapping efficiency and stiffness of optical tweezers using radial polarization are evaluated; the ray-tracing method and a proposed measurement method are used for numerical and experimental analyses, respectively. The maximum axial trapping efficiency with radial polarization is 1.84 times that with linear polarization, while the maximum transverse trapping efficiency decreases by 0.58 times. Further, the axial and transverse trapping efficiencies are found to be 1.19 times larger and 0.83 times smaller, respectively, than the values with linear polarization. From the experiments, the axial and transverse stiffness values are 1.2 times larger and 0.8 times smaller, respectively, with radial polarization. Hence, radial polarization enhances the axial trapping properties while reducing the transverse trapping properties.

Probing technique using circular motion of a microsphere controlled by optical pressure for a nanocoordinate measuring machine.

A new surface probing technique using the circular motion of an optically-trapped microsphere is proposed for a nanocoordinate measuring system. The probe sphere is oscillated circularly in the plane perpendicular to the probe axis and the circular orbit of the probe sphere is monitored for the detection of the position and normal vector direction of the surface. The principle of detection is based on changes in the circular orbit of the microsphere. When the probe approaches a work surface, the orbit of the probe sphere becomes elliptical. The minor-axis length and the minor-axis angle of the ellipse are then used as parameters to detect the position and normal vector direction of the surface, respectively. In this study, the circular motion probe is shown to have a resolution of position detection of 39 nm, and the accuracy of measuring a normal vector to the surface is on the order of 3 degrees.