Nanocantilever signal transduction by electron transfer

Microfabricated cantilever beams promise to bring about a revolution in the field of chemical, physical, and biological sensor development. The resonance frequency of a microfabricated cantilever shifts sensitively because of mass loading from molecular adsorption. The minimum detectable adsorbed mass on a cantilever sensor can be increased by orders of magnitude by changing the dimensions of the device; smaller and thicker cantilevers offer higher resonance frequency and therefore better mass detection sensitivity. Here we describe micromachined silicon cantilevers that are 0.5 to 4 microns in length, fabricated with the use of a focused ion beam (FIB). In addition, we demonstrate a technique for detection of the cantilever resonance frequency that is based on electron transfer.     

Diffraction of combined optical vortices

The diffraction of light on the lens of a Gaussian beam past a uniaxial crystal is equivalent to the beam diffraction on a helicoidal phase grating of two partial beams with different curvatures of the wave front. This diffraction significantly distorts the Gaussian beam profile and an extended region with three beam waists is formed near the focal plane. The beam waist region can be rectified by varying the radii of the primary beam waist and the lens pupil.     

Experimental study on the dynamic behavior of TiNi cantilever beams with rectangular cross-section under transversal impact

The experimental investigation of TiNi alloy cantilever in the PE state and R-SME state, respectively, was conducted under transversal impact by using a modified Hopkinson bar apparatus. For comparison, the impact response of A3 steel cantilevers with the same geometry was also studied. The results show that at an early stage the elastic flexible waves dominate the wave response of the beam. After about 1 ms the dynamic structural response will be the main response. Under impact the “phase transformation hinge (TH)”, which differs from the conventional plastic hinge (PH), may form and undertake the main deformation and energy absorption in a cantilever. The impact position has a great influence on the location, time and number of the formation of TH, hence the response mode of a cantilever. After impact there is no residual deformation for a PE cantilever, there is a small residual deformation for an R-SME cantilever due to the R transition and large residual deformation for steel cantilevers due to the plasticity. The energy absorption efficiency of cantilevers for different materials and impact conditions are discussed.     

Atomic-resolution imaging in liquid by frequency modulation atomic force microscopy using small cantilevers with megahertz-order resonance frequencies

In this study, we have investigated the performance of liquid-environment FM-AFM with various cantilevers having different dimensions from theoretical and experimental aspects. The results show that reduction of the cantilever dimensions provides improvement in the minimum detectable force as long as the tip height is sufficiently long compared with the width of the cantilever. However, we also found two important issues to be overcome to achieve this theoretically expected performance. The stable photothermal excitation of a small cantilever requires much higher pointing stability of the exciting laser beam than that for a long cantilever. We present a way to satisfy this stringent requirement using a temperature controlled laser diode module and a polarization-maintaining optical fiber. Another issue is associated with the tip. While a small carbon tip formed by electron beam deposition (EBD) is desirable for small cantilevers, we found that an EBD tip is not suitable for atomic-scale applications due to the weak tip-sample interaction. Here we show that the tip-sample interaction can be greatly enhanced by coating the tip with Si. With these improvements, we demonstrate atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency. In addition, we experimentally demonstrate the improvement in the minimum detectable force obtained by the small cantilever in measurements of oscillatory hydration forces.     

Polarization-dependent diffraction efficiency of a photorefractive volume grating and suppression of this efficiency

In terms of refractive-index ellipsoid of a uniaxial crystal, the relationship between the diffraction efficiency of a volume grating and the polarization state of a readout beam is theoretically analyzed. The direction of a refractive light beam and the corresponding refractive-index modulation will both be changed by a variation of the polarization state. In the polarization state of the readout beam, which may lead to a strong variation in the diffraction efficiency of the volume grating. This kind of polarization-dependent diffraction efficiency of a volume grating in an anisotropic crystal is extremely disadvantageous for some applications. A method to suppress the polarization-dependent diffraction efficiency by use of double volume gratings is presented, and experiments with LiNbO3:Fe crystal are also demonstrated. The experimental results indicate that this method can well suppress the polarization-dependent diffraction efficiency of a volume grating. Furthermore, the diffraction properties of the double volume gratings are almost independent of the polarization state of the readout beam. The relative values of the diffraction peaks are calculated on the basis of the relationship between index modulation and the state of polarization. The experimental values are in good agreement with the theoretical analyses.     

Amplitude modulation and beam-steering properties of active binary phase gratings with reconfigurable absorption areas

By combining a binary phase grating and materials with a controllable absorption, it is shown theoretically that it is possible to modulate the light in a zero-order diffraction beam and that a high contrast level for the beam modulation can be obtained. The intensity of higher diffraction orders also changes, but it is calculated that high contrasts cannot be achieved for these higher-order beams with the active gratings that we examine. This specific modulator design that we use can be applied both for transmitted and for reflected light. Using the same ideas, one may build a beam-deflection device by varying the period of the grating by selectively changing the absorption levels in the grating. The deflection efficiency of the device can be improved compared with other designs by use of a grating with a reduced intensity of the nondeflected zero-order beam.     

Dynamic strain measurement using an extrinsic Fabry-Perot interferometric sensor and an arrayed waveguide grating device

In this paper, we demonstrate an interrogation system, based on an arrayed waveguide grating, capable of monitoring dynamic strain in a cantilever beam at frequencies up to 5 kHz (limited by the actuator) with a similar precision to resistive strain gauges.     

3D finite element analysis of electrostatic deflection and shielding of commercial and FIB-modified cantilevers for electric and Kelvin force microscopy: II. Rectangular shaped cantilevers with asymmetric pyramidal tips

This paper deals with an application of 3D finite element analysis to the electrostatic interaction between (i) a commercial rectangular shaped cantilever (with an integrated anisotropic pyramidal tip) and a conductive sample, when a voltage difference is applied between them, and (ii) a focused ion beam (FIB) modified cantilever in order to realize a probe with reduced parasitic electrostatic force. The 3D modelling of their electrostatic deflection was realized by using multiphysics finite element analysis software and applied to the real geometry of the cantilevers and probes as used in conventional electric and Kelvin force microscopy to evaluate the contribution of the various part of a cantilever to the total force, and derive practical criteria to optimize the probe performances. We report also on the simulation of electrostatic shielding of nanometric features, in order to quantitatively evaluate an alternative way of reducing the systematic error caused by the cantilever-to-sample capacitive coupling. Finally, a quantitative comparison between the performances of rectangular and triangular cantilevers (part I of this work) is reported.     

Atomic force microscope cantilever calibration using a focused ion beam

A calibration method is presented for determining the spring constant of atomic force microscope (AFM) cantilevers, which is a modification of the established Cleveland added mass technique. A focused ion beam (FIB) is used to remove a well-defined volume from a cantilever with known density, substantially reducing the uncertainty usually present in the added mass method. The technique can be applied to any type of AFM cantilever; but for the lowest uncertainty it is best applied to silicon cantilevers with spring constants above 0.7?N?m(-1), where uncertainty is demonstrated to be typically between 7 and 10%. Despite the removal of mass from the cantilever, the calibration method presented does not impair the probes' ability to acquire data. The technique has been extensively tested in order to verify the underlying assumptions in the method. This method was compared to a number of other calibration methods and practical improvements to some of these techniques were developed, as well as important insights into the behavior of FIB modified cantilevers. These results will prove useful to research groups concerned with the application of microcantilevers to nanoscience, in particular for cases where maintaining pristine AFM tip condition is critical.     

Optical beam deflection by phase-controllable dual-period grating employing liquid crystal

A dual-period grating is proposed that can change the period and the phase difference in the grating structure, allowing the switchable diffraction allocation of transmitted light. Liquid crystal is assumed to be a variable-refractive-index medium confined in the grating grooves. The distribution of the transmitted diffraction efficiencies is analysed using a rigorous diffraction analysis for dual-period gratings and this reveals that the efficiencies can be maximized among the zeroth-, first-, and second-order waves successively according to the refractive index change in the liquid crystal. The analytical model provides insight into the operation of the device and emphasizes its potential application as an optical beam deflector with large deflection angles.     

Micron-sized fracture experiments on amorphous SiOx films and SiOx/SiNx multi-layers

In this study miniaturized monolithic cantilevers of thermally grown silicon oxide and multi-layer cantilevers of plasma enhanced chemical vapor deposited silicon oxide and nitride were mechanically characterized. In order to determine the fracture stress as well as the fracture toughness, un-notched and focused ion beam pre-notched cantilevers were tested. While the thickness of the monolithic cantilevers was varied from 280 nm to 2380 nm, the individual sub-layer thickness of the multi-layer cantilevers was adjusted to 50 nm. Bending experiments reveal a small increase of the fracture stresses with decreasing cantilever thicknesses. For the multi-layer stacks the tensile stress at fracture slightly exceeds the strength values of the corresponding monolithic materials. Furthermore, it is demonstrated that the specimens pre-notched by focused ion beam do not show significant changes in fracture toughness with varying pre-notch size. This makes the applied test a reproducible technique to determine fracture toughness of brittle films.     

Diffractive light attenuators with variable transmission

Abstract

A new application of diffractive optical elements (DOEs) for continuous or multistage adjustment of optical radiation intensity is described. The diffractive attenuators are linear or circular gratings (amplitude or phase) with constant period and diffraction efficiency that varies across the grating. The zero order of diffraction is used as the output and transmitted through the grating without angular deviation. The diffractive attenuators, in distinction to conventional analogues, allow one to change the intensity of the light beam according to predetermined function and have no limitations for power of the regulated light beam. These elements can be used in optical systems as a beam splitter with adjusted splitting coefficient. The experimental results on a circular diffractive attenuator fabricated by direct laser writing on a chromium film are presented. The range of transmission variation was 20 times within a 340° angle of attenuator turn. The possibility to use a phase diffractive attenuator as a light radiation modulator for a powerful technological laser is discussed.     

3D finite element analysis of electrostatic deflection of commercial and FIB-modified cantilevers for electric and Kelvin force microscopy: I. Triangular shaped cantilevers with symmetric pyramidal tips

The investigation of the nanoscale distribution of electrostatic forces on material surfaces is of paramount importance for the development of nanotechnology, since these confined forces govern many physical processes on which a large number of technological applications are based. For instance, electric force microscopy (EFM) and micro-electro-mechanical-systems (MEMS) are technologies based on an electrostatic interaction between a cantilever and a specimen. In the present work we report on a 3D finite element analysis of the electrostatic deflection of cantilevers for electric and Kelvin force microscopy. A commercial triangular shaped cantilever with a symmetric pyramidal tip was modelled. In addition, the cantilever was modified by a focused ion beam (FIB) in order to reduce its parasitic electrostatic force, and its behaviour was studied by computation analysis. 3D modelling of the electrostatic deflection was realized by using a multiphysics finite element analysis software and it was applied to the real geometry of the cantilevers and probes obtained by using basic CAD tools. The results of the modelling are in good agreement with experimental data.     

Optical grating diffraction method: from strain microscope to strain gauge

The grating diffraction method for direct strain measurement is reviewed. Two systems which use this method are presented. The first system is a compact strain microscope. A Leitz optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. Gratings with median density of between 40 and 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed with high image quality on the CCD sensor plane. Software is developed to precisely, quickly and automatically determine the diffraction spot centroids. The second system is a new strain sensor based on a high-frequency grating and two Position Sensor Detectors (PSDs). The grating, attached to the surface of the specimen, is illuminated by a focused laser beam, generally with a frequency of 1,200 lines/mm. The centroids of diffracted beam spots from the grating are automatically determined using two PSD sensors connected to a personal computer. The shift of diffracted beam spots due to specimen deformation is detected. Strain sensitivity of one micro-strain can be obtained, as can a 0.4 mm spatial resolution for strain measurement. The system can be used for both static and dynamic tests.     

Design and analysis of an integrated optical sensor for scanning force microscopies

In this paper, a novel probe for displacement sensing will be introduced. It is based on a conventional GaAs cantilever, integrated with a Bragg grating as a photo-elastic strain sensor. The deflection of the cantilever is measured directly from the intensity modulation of the reflected light. The principle of the experimental setup and the sensor, as well as the theoretical investigation of the force and displacement sensitivity of the probe, is presented. Finite-element method simulations were performed to get the optimum sensor design. Transfer matrix method simulation of the waveguide grating have been described in detail. In order to enhance the sensitivity, different types of grating structures are discussed. Using this new design, it should be possible to achieve sensitivities, defined as the fractional change in detected optical power per unit displacement of the cantilever, as high as 10/sup -4/ /spl Aring//sup -1/ of cantilever deflection.     

The elastic modulus of spruce wood cell wall material measured by an in situ bending technique

Using a novel in situ testing technique, the elastic modulus of wood cell wall material can be determined with great accuracy. The method relies on a focussed ion beam system (FIB) to prepare samples from individual structural components at a length scale which otherwise is hardly, if at all, accessible for testing. To determine the elastic modulus of cell wall material, cantilevers are cut with the FIB from wood cells for beam bending experiments inside the FIB or a scanning electron microscope (SEM). This type of sample preparation is site-specific and, at the same time, minimises the usual sample mounting problems. Once cut, the cantilever is tested by applying a known force with a piezoresistive AFM tip that is mounted on a micromanipulator. The resulting displacement is determined from SEM micrographs taken during the test. The cross-sectional area of the cantilever is determined for a number of positions along its length using the FIB as a cutting tool. Applying this method, we measured the elastic modulus of spruce wood cell wall material to be ∼28 GPa.     

Anisotropy in microdevices produced by micromachining of cold-rolled NiTi sheets

The process of fabrication and characterisation of three types of NiTi microdevice are described: a double-beam cantilever of constant beam width and two designs of cantilever of stress-optimised width profile, with thicknesses ranging from 17 to 95 μm produced by cold rolling. The technology of electrolytic photoetching was used as a versatile tool for micromachining. The transformation behavior of the specimens was investigated by differential scanning calorimetry and electrical resistance measurements. Anisotropies caused by rolling-induced textures have been investigated by beam-bending experiments. Strong anisotropies were found in results on thickness-dependent Young’s modulus in the parent phase as well as in the beam-bending characteristics in the rhombohedral and martensitic phase. For beam directions transverse to the rolling direction, beam-bending experiments revealed a thickness dependence of hysteresis associated with the martensitic transformation, which was not observed in the rolling direction. Cantilevers with stress-optimised width profile display favorable beam-bending characteristics compared with cantilevers of constant beam width due to an optimum use of the shape memory effect.     

带有力反馈控制的三明治式微机械干涉加速度计

设计了一种静电力反馈控制的三明治式微机械干涉加速度计,加速度计由敏感芯片、半导体激光器、光电二极管以及相应的驱动电路和反馈控制电路组成.敏感芯片为玻璃-硅-玻璃3层结构,通过硅-玻璃键合体硅工艺制成.硅质量块由铝梁支撑,底部玻璃基片上有金属光栅和电极,通过在质量块和底部玻璃基片上的电极之间施加电压可以调节质量块与玻璃基片间的间隙.入射激光照射到敏感芯片上的光栅上,产生衍射光束,其光强随质量块与下玻璃的间距而变化.反馈控制电路通过测量衍射光强的变化来改变质量块与底电极之间的电压,使得质量块与底部玻璃基片的距离保持为入射光波长1/8的奇数倍,从而提高输出线性度,改善灵敏度,增大量程.     

Diffraction properties of volume holographic gratings for an ultrashort pulsed beam with different polarization states readout

The diffraction properties of volume holographic gratings are studied when the gratings are illuminated by an ultrashort pulsed beam with different polarization states. The developed coupled wave theory of Kogelnik is used. Considering the dispersion effect of the grating media, solutions for the diffracted and transmitted intensities, diffraction efficiencies and the bandwidths of the gratings are given in transmission volume holographic gratings and reflection volume holographic gratings. The bandwidths of the gratings are reduced by the dispersion effect of the grating media. They also have different influences on the diffraction of an ultrashort pulsed beam with different polarization states. For different values of the ratio of the spectral bandwidth of the input pulse to that of the grating, the changes of the spectral and temporal distributions of the diffracted intensities, as well as the diffraction efficiencies of the gratings are shown.     

Effect of reconstruction beam polarization on the kinetics of anisotropic gratings in bacteriorhodopsin

Anisotropic gratings are recorded on bacteriorhodopsin films by two parallelly polarized beams, and the effect of the polarization orientation of the reconstructing beam on the diffraction efficiency kinetics is studied. A theoretical model for the diffraction efficiency kinetics of the anisotropic grating is developed by combining Jones-matrix and photochromic two-state theory. It is found that the polarization azimuth of the reconstructing beam produces a cosine modulation on the kinetics of the diffraction efficiency, being positive at the peak efficiency and negative for steady state. By adding auxiliary violet light during grating formation, the saturation of the grating can be restrained. As a result, the negative cosine modulation for the steady-state diffraction efficiency changes to a positive one. In addition, the steady-state diffraction efficiency is increased appreciably for all reconstructing polarization orientations.