Optical Interference Method Research Articles

Brownian motion of tethered nanowires.

Brownian motion of slender particles near a boundary is ubiquitous in biological systems and in nanomaterial assembly, but the complex hydrodynamic interaction in those systems is still poorly understood. Here, we report experimental and computational studies of the Brownian motion of silicon nanowires tethered on a substrate. An optical interference method enabled direct observation of microscopic rotations of the slender bodies in three dimensions with high angular and temporal resolutions. This quantitative observation revealed anisotropic and angle-dependent hydrodynamic wall effects: rotational diffusivity in inclined and azimuth directions follows different power laws as a function of the length, ∼ L(-2.5) and ∼ L(-3), respectively, and is more hindered for smaller inclined angles. In parallel, we developed an implicit simulation technique that takes the complex wire-wall hydrodynamic interactions into account efficiently, the result of which agreed well with the experimentally observed angle-dependent diffusion. The demonstrated techniques provide a platform for studying the microrheology of soft condensed matters, such as colloidal and biological systems near interfaces, and exploring the optimal self-assembly conditions of nanostructures.

Permeability coefficients of minus contact lens studied by optical interference method

Contact lenses frequently used for correction of short-sightedness make a barrier between the cornea and the external lacrimal film. The materials of which they are made must meet special demands not only as to mechanical properties (elasticity, mechanical strength) but mainly as to the compatibility with the eyeball tissue, among others they must permeate oxygen and lacrimal fluid. The problem of permeability of these substances is similar to their permeability through a membrane and is based on mass transportation via diffusion. As the lens power depends on the curvature radii of refracting surfaces, which determines the mean thickness of the lens, in this study the method of optical interference was applied to evaluate the permeability of negative contact lenses towards the main components of lacrimal fluid Na+ and Cl− ions. Permeability parameters of the commercial soft contact lenses SoftLens® made by Bausch & Lomb and made of hilafilcon B, of optical powers from −0.5D to −3.75D were measured. On the basis of a nonparametric test a correlation between the lens power and permeability was evaluated.

A Simple Experimental Technique for Measuring the Poisson's Ratio of Microstructures

In this paper, a simple experimental technique for the measurement of the Poisson's ratio of a small-sized specimen is described. When two equal and opposite bending moments are applied to a rectangular solid structure, the structure deforms like a saddlelike shape on the basis of the linear elastic theory. The contour line on the structure middle portion is represented as a family of hyperbolas, and the angle between the asymptote and the structure's width direction is related to the Poisson's ratio. We applied the principle to a microspecimen. Based on Searle's analysis, the specimen was designed by means of finite element analysis. Single crystal silicon (SCS) was chosen as the first target material, and the specimen was manufactured by conventional micromachining technologies. The test equipment with a loading mechanism and a contour line measurement function using an optical interference method was developed. After precise alignment between the loading point and the center of the loading lever, normal loading was applied. From the obtained optical interference patterns, the mean angle of 14.5° was obtained in the penetration depth ranging from 25 to 100 μm. The angle provided the Poisson's ratio, 0.068, which was in very good agreement with the ideal value of the in-plane Poisson's ratio, 0.064, for SCS(001)[110]. The developed technique has the potential for evaluating the Poisson's ratio of small-sized specimens.

Investigation on cam–follower lubricated contacts

The Investigation on cam–follower contact is one of the most complex lubricated non-conformal contact due to its continuous variation of load, speed, and geometry. The complexity of the phenomena occurring in the camshaft systems makes experimental verifications very important. In this study, an already existing versatile experimental apparatus for the investigation of non-conformal lubricated contacts, able to measure film thickness using the optical interference method and friction force trough a load cell, has been modified in order to test cam–follower contacts. Some tests have been carried out for investigating the behaviour of some fundamental components of the rig. A theoretical/numerical simulation has been performed to investigate the dynamic behaviour of the system and in order to provide some indications for the interpretation of the experimental results. Purposely developed versions of the programme have been also an important support for the design of a new apparatus specifically addressed to the cam–follower contact with increased potentialities capable of more detailed measurements of film thickness and contact forces. The apparatus is able to reproduce a cam–follower mechanism that uses a rocker as a link device between the cam follower set and the valve.

Frictional Contact of Coated Paper and Its Surface Chemical Analysis

Relating to the real contact area against a plane smooth surface, frictional properties of coated and uncoated papers were examined under varying applied pressure. An optical interference method using a scanning laser microscope was adopted to measure real contact area against a plane glass plate. The real contact area of the coated paper increased nearly in proportion to the apparent pressure over a wide range of pressure. From the theory of tribology, this result shows that the contact of coated paper is rather plastic and this fact could explain the constant coefficient of friction with increasing apparent pressure. On the other hand, the contact of uncoated base paper was generally elastic and this fact was one reason for the decrease of the coefficient with increasing apparent pressure, shown in the previous report. In addition, an electron spectroscopy for chemical analysis (ESCA) was employed to examine the thickness of binder on pigment particles from the chemical composition at the exact coating surface, because the frictional behavior and the real contact area seemed to depend on the surface material. The result of ESCA suggested that the binder covered the pigment surface enough to control the frictional behavior, and further that the binder existed mainly at the border region between the pigment particles and partly covered the surface of pigment particles.

Optical Tweezers Array System Based on 2D Photonic Crystals

A simple optical interference method for creating multiple optical tweezers from a single laser beam, using two dimentional photonic crystals (PhCs) as a diffractive beam splitter, was described. To obtained clear periodic traps, all diffracted beams sould be used and the intensity of each splitted beam should be same. So the period and the surface features of PhCs was adjusted in the present study As a demonstration of this technique, using 2D PhCs with 700 nanometer period, hexagonal lattice patterns with one micrometer period have been implemented. The image of periodic intensity gradient of light fabricated by this method is presented.

Direct fabrication of periodic patterns with hierarchical sub-wavelength structures on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) thin films using femtosecond laser interference patterning

A simple optical interference method for the fabrication of simply periodic and periodic with a substructure on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) using femtosecond laser interference patterns is demonstrated. The femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Homogeneous periodic arrays could be fabricated even using a single laser pulse. In addition, multipulse irradiation resulted in reproducible sub-wavelength ripples oriented perpendicularly to the laser polarization with spatial period from 170 to 220 nm (around one-fourth of the laser wavelength). In addition, the observed size of the spatial period was not affected by the number of incident laser pulses or accumulated energy density. Using high energy pulses it was possible to completely remove the PEDOT:PSS layer without inducing damage to the underneath substrate.

Method of determining the exciton diffusion length using optical interference effect in Schottky diode

A general model of calculating the short circuit photocurrent and exciton diffusion length from Schottky diode is given here using optical interference method. This method of estimating the exciton diffusion length can be very useful for those materials that do not give photoluminescence. It also serves as an accurate and simple method to determine the exciton diffusion length, which is an important material property to design devices such as organic solar cells. Our model is verified with the spectral response of indium tin oxide (ITO)/copper phthalocyanine (CuPc)/Al and ITO/CuPc/Ag Schottky diodes.

Optical interference methods of subwavelength-resolution imaging

We present two novel optical interference methods for measuring characteristics of surfaces with subwavelength resolution. One method is based on recording interferograms of the light near field which comprises information on the subwavelength structure of the surface. The idea of the other method is to transform the near field to propagated waves and measure their parameters far from the studied surface. The proposed methods have been tested by numerical simulation.

Measurement of Density Inhomogeneity for Glass Pendulum

The density inhomogeneity of a glass pendulum is determined by an optical interference method. The relative variations of the densities over a volume with sizes of 5 × 5 × 5 mm3 are (0.64 ± 0.97) × 10−5 and (0.99 ± 0.92) × 10−5 for the K9 glass and silica glass pendulum, respectively. These variations of densities contributing to the relative uncertainties of the Newtonian gravitational constant G are 0.20 ppm and 0.21 ppm in our experiment on measurement of G.

Mechanical and tribological characterisation of nanostructured Ti/TiB2 multilayer films

Ti/TiB2 nanomultilayer thin films with different bilayer thicknesses λ were deposited onto unheated Si(100) wafers (for mechanical analyses) and AISI M42 tool steels (for tribological measurements) by unbalanced dc magnetron sputtering. The effects of different λ values on mechanical and tribological properties were investigated. These films were characterised and analysed in terms of their hardness by microindentation measurements, their surface root mean square roughness by AFM, their stress by an optical interference method, and their friction and wear behaviors by Rockwell-C testing, nano-/micro-scratch testing, dynamic impact testing and pin on disc tribometer. It was found that the mechanical and tribological properties of multilayer films (typically 1˙58±0˙10 µm in thickness) were closely related to λ (varied from 1˙1 to 9˙8 nm). For the best multilayer film with λ51˙9 nm, a maximum hardness of ∼32˙5 GPa was achieved and the best cohesive and adhesive strength was evidenced in terms of critical load values of LC1 (∼26 N) and LC2 (∼62 N). Moreover, by dynamic impact testing this multilayer film could endure impact cycles up to 4 × 105 without adhesive failure. However, when the λ was further decreased to 1˙1 nm, the hardness, cohesive and adhesive strength were decreased due to a high level of intermixing and lack of a layered structure. It was also found that the nanoscratch test under single pass and constant load conditions showed that the frictional coefficients decreased with λ and increased with normal load due to the ploughing effect. The enhanced hardness in the multilayer films with small λ values improved the wear resistance and lowered the frictional coefficients. These adhesive properties and wear performance are also discussed on the basis of mechanical properties and wear mechanisms.

Determination of Swelling of Responsive Gels with Nanometer Resolution. Fiber-Optic Based Platform for Hydrogels as Signal Transducers

A novel technique for detection of hydrogel swelling intended for use as a chemical or biological sensor, but also generally applicable for obtaining high-precision hydrogel swelling data, is described. The underlying design principle is that a hydrogel bound to the tip of an optical fiber constituting the environmental sensing element makes up a Fabry-Perot cavity for high-resolution detection of the optical length. The interference of light guided by the optical fiber and reflected at the two interfaces, fiber-gel and gel-solution, enables optical detection of the optical path length within the gel and degree of swelling of the gel. Acrylamide-based hydrogels with various molar fractions of the cationic monomer, N-(3-dimethylaminopropyl)acrylamide, were fabricated at the end of the fiber to demonstrate the feasibility of the approach. These sensors were investigated in solutions of varying ionic strength and pH. Relative gel length changes of the approximately 50-microm half-spherical gels were determined with a precision of approximately 2 nm. Moreover, the combination of good reproducibility and resolution of determination of swelling supports measurements of ionic strength changes in the millimolar range. Kinetic measurements for gel swelling induced by changes in ionic strengths had a time constant of approximately 2 s (half-spherical gel with 60-microm radius), whereas the time constants for gel swelling induced by changes in pH were observed in the range 90-130 s. Thus, different processes dictate the swelling rate in the two different cases. The results show that hydrogel equilibrium swelling and kinetics can be determined by the optical interference method with nanometer resolution, thus providing a unique platform for characterization of hydrogels swelling in general, and using functionalized hydrogels as biological sensors in particular.

MEASUREMENT OF THIN OIL FILM THICKNESS USING ULTRASONIC TECHNIQUE

An application of ultrasonic technique is attempted for the purpose of measuring thin oil film thickness between two surfaces. The amplitude of the wave reflected from the boundary is vary depending on film thickness, because the ultrasonic wave emitted to the interface between two surfaces does multiple reflection and interference in oil film. Quantitative measurement of oil film thickness then can be possible. For instance, it is possible to measure the submicron film thickness which exists near the point contact formed by convex glass and plate with high frequency probe. And it is confirmed that the oil film thickness estimated from the echo height agrees with the film thickness decided by the curvature of the lens or obtained by the optical interference method, even if it is 100 nm. On the other hand, the thickness of oil film between cylinder and piston ring can be easily measured by setting the small ultrasonic probe on the back of piston ring. For example, the influence of the second ring and oil ring for the behavior of an oil film formed on a top ring is able to evaluate quantitatively. As mentioned above, it is cleared that quantitative evaluation of thin film thickness is possible with investigating the echo height obtained by ultrasonic wave pulse reflection method.

The Real-Time Mach-Zehnder Interferometer used in Space Experiment

The optical interference method is a promising technique for measuring temperature, density, and concentration in fluids. The non-intrusive and non-invasive nature of its optical techniques to the measured section are its most outstanding features. However, the adverse experiment environment, especially regarding shaking and vibrating, greatly restricts the application of the interferometer. In the present work, an optical diagnostic system consisting of a Mach-Zehnder interferometer (named after physicists Ludwig Mach) and an image processor has been developed that increases the measuring sensitivity compared to conventional experimental methods in fluid mechanics. An image processor has also been developed for obtaining quantitative results by using Fourier transformation. The present facility has been used in observing and measuring the mass transfer process of a water droplet in EAFP protein solution under microgravity condition provided by the satellite Shi Jian No. 8.

A New Approach for the Measurement of Film Thickness in Liquid Face Seals

Face seals operate by allowing a small volume of the sealed fluid to escape and form a thin film between the contacting parts. The thickness of this film must be optimized to ensure that the faces are separated, yet the leakage is minimized. In this work the liquid film is measured using a novel ultrasonic approach with a view to developing a condition monitoring tool. The trials were performed in two stages. Initially tests were based on a lab simulation, where it was possible to compare the ultrasonic film thickness measurements with optical interference methods and capacitance methods. A direct correlation was seen between ultrasonic measurements and capacitance. Where ultrasonic and optical methods overlap, good correlation is observed; however, the optical method will not record film thickness above ∼ 0.72 μ m. A second set of trials was carried out, where the film thickness was monitored inside a seal test apparatus. Film thickness was successfully recorded as speed and load were varied. The results showed that while stationary the film thickness varied noticeably with load. When rotating, however, the oil film remained relatively stable around 2 μ m. During the normal operation of the seal, both sudden speed and load changes were applied in order to initiate a seal failure. During these events, the measured film thickness was seen to drop dramatically down to 0.2 μ m. This demonstrated the ability of the technique to predict failure in a face seal and therefore its aptitude for condition monitoring.

A novel optical interference technique to measure minute root elongations of Japanese red pine ( Pinus densiflora Seibold & Zucc.) seedlings infected with ectomycorrhizal fungi

This study presents a new technique to measure root elongation of Japanese red pine ( Pinus densiflora Seibold & Zucc.) seedlings with very high sensitivity in the order of sub-nanometer by using a novel optical interference method called statistical interferometry. The principle of the statistical interferometry is based on the statistics of a speckle field, which is generated when a rough surface is illuminated by a laser light. The technique facilitates to obtain minute root elongation measurements in the order of sub-seconds. The root elongation behavior of Pinus densiflora seedlings infected with ectomycorrhizal fungi, Pisolithus sp. ( Ps) and Cenococcum geophilum Fr. ( Cg), was investigated in comparison with that of an uninfected control. In the experiments, two points on a root with the separation of 3 mm were illuminated by laser beams and the elongation was measured continuously by analyzing speckle patterns successively taken by a CCD camera. The root elongation rate (RER), measured as the length of root elongation per second per millimeter (mean ± S.D.) for Ps-infected, Cg-infected and uninfected seedlings were 10.85 ± 2.41, 5.54 ± 1.43, and 2.41 ± 1.01 nm s −1 mm −1, respectively. We found that the RERs of seedlings infected with ectomycorrhizal fungi were significantly higher than that of the uninfected seedlings, and the seedlings infected with Ps fungi showed the highest RER. We conducted another experiment to observe two-dimensional root growth, in which the growth measurements were obtained for 4 months. From this experiment, we observed that root growth of ectomycorrhizal fungi infected seedlings were higher than that of the uninfected seedlings. The evaluation of results from these two techniques proves that the proposed statistical interferometry is much faster and very sensitive technique, where the time required for growth monitoring is 10 7 times less than the other. We can conclude that, at the scale of either very short time or relatively long time, the symbiotic relationship between root and ectomycorrhiza has a positive effect steadily on the root elongation.

Paper Friction -Effect of Real Contact Area-

Relating to real contact area against plane smooth surface, effects of the measuring conditions on paper friction, surrounding atmosphere and applied pressure, were examined. An optical interference method using a scanning laser microscope was adopted to measure the real contact area against a plane smooth surface. The estimated contact area of paper is around 1% of nominal area for common papers. The increase in the contact area with increasing relative humidity is considered as a cause of the increase in the friction coefficient with an increase in relative humidity. The contact area of paper generally increased in a ratio of two-thirds power of the apparent pressure over a wide range of pressure. It suggested that contact of paper was generally elastic and this fact was one reason describing the decrease in the coefficient with increasing apparent pressure. However the decrease in the coefficient is little and the frictional behavior of paper can not be fully described by the contact area against a smooth surface.

Statistical interferometric investigation of nano-scale root growth: effects of short-term ozone exposure on ectomycorrhizal pine (Pinus densiflora) seedlings

This study presents the effects of short-term ozone exposure on the nano-scale growth behavior of the fine roots of Pinus densiflora (Japanese red pine) seedlings. Root elongation measurements were obtained in nanometers for very short (sub-second) time intervals by using the optical interference method called statistical interferometry, developed by the authors. Three categories of P. densiflora seedlings were investigated; two categories were infected with ectomycorrhiza of Pisolithus sp. (Ps) and Cenococcum geophilum (Cg), while the third was without any fungal infection. In experiments, two points on a root with a separation of 3 mm were illuminated by laser beams and the elongation was measured continuously by analyzing speckle patterns successively taken by a CCD camera. The ectomycorrhizal fungi-infected and uninfected seedlings were exposed to ozone at concentrations of 120 and 240 ppb for periods of 1, 3, or 5 h in separate treatments. The root elongations of P. densiflora seedlings were measured before and immediately after the each ozone treatment and then the root elongation rates (RER) were determined for growth-measurement periods of 5.5 s and 9.5 min. From the measurements obtained for 9.5 min, we found that the RERs of uninfected and Cg-infected seedlings were reduced by 42 and 18%, respectively, after 5 h of exposure to 120 ppb ozone compared with that before exposure, while the reduction in RER of Ps-infected seedlings was not significant. When the concentration of ozone was increased to 240 ppb, the RERs of Ps-infected and Cg-infected seedlings were reduced by 32 and 44%, respectively, after exposure for 5 h, while the reduction in RER of uninfected seedlings was 59%. These observations prove that the non-mycorrhizal seedling roots are more sensitive to ozone stress. From this study, we found that the RERs of both mycorrhizal and non-mycorrhizal seedlings apparently fluctuated throughout the measurements, even within a few minutes.

Periodic structures prepared by two-beam interference method

We present an optical interference method as the efficient tool for high-quality one- (1D) and two-dimensional (2D) periodic structure fabrication. Different types of 2D periodic structures are prepared using two-beam interference technique by double exposure. Also, the patterning of Au layer is provided using two-beam interference method. Quite homogeneous 1D and 2D structures with small periodicity were prepared in photoresist and Au layer. The experimentally prepared structures are well in agreement with the theoretically designed ones. According to experimental results, we favor two-beam interference technique using multi-exposure process as a very useful method for fabrication of 2D periodic structures and devices for integrated optoelectronics.

Cuprophane membrane permeability to urea studied by optical interference method

Investigation of the mechanisms of transportation through the biological membranes is of interest mainly in biology and medicine. Besides the cognitive value, the aim of such studies is to find new technologies of producing synthetic membranes of desired characteristics to be used in modern methods of medical therapy. The paper reports on the cuprophane membrane permeability to urea studied by very sensitive method of optical interference.