Index Matching Fluid Research Articles

A New Optical-CT Apparatus for 3-D Radiotherapy Dosimetry: Is Free Space Scanning Feasible?

In this paper, we present a new optical computed tomography (Optical-CT) scanner for the verification of the radiation dose schemes delivered in modern radiotherapy applications. The optical-CT scanner is capable of providing rapid relative 3-D dosimetry with high spatial resolution with the use of normoxic N-Vinylpyrrolidone based polymer gel dosimeter. The scanner employs a diffuse uncollimated light illumination beam, a computer controlled motorized rotation stage and a charge-coupled device (CCD) camera. Various test experiments were performed to determine the performance characteristics of the optical-CT apparatus. Attenuation coefficient (micro) versus dose calibration data were generated from two calibration experiments using gel containers of two different diameters. All irradiations were performed using a 6 MV linear accelerator. A comparison of the reconstructed images between optical-CT scans using refractive index (RI) matching fluid and corresponding scans performed in free space was demonstrated. The dose readout of a test irradiation model was found to be in good agreement with independent readout performed by MR imaging. The findings presented in this study suggest that polymer dosimeters combined with the new optical-CT scanner constitute a potentially feasible method capable of measuring complex 3-D dose distributions with high resolution and in a wide dose range.

One-dimensional long-range plasmonic-photonic structures

We have fabricated line gratings from periodically etched fused silica on which a thin silver film is deposited that is in turn covered with a silica index-matched fluid. This dielectrically symmetric geometry supports an independent long-range surface plasmon-polariton (LRSPP) and a short-range surface plasmon polariton, and the associated plasmonic band structure has been probed. Coupling to external light is achieved via the patterned grating, and an ultrasharp LRSPP linewidth of 4 nm is observed. The experimental results are compared with finite-difference time-domain simulations.

Fabrication of surface plasmon waveguides and integrated components on Cytop

The fabrication and physical characterisation of waveguides and integrated components on Cytop are described. The waveguides consist of a thin narrow Au stripe, on an optically infinite layer of Cytop. The integrated components implemented with this waveguide consist of Y-junctions, Mach–Zehnder interferometers and couplers. The waveguides and components are dimensioned such that they propagate long-range surface plasmon-polariton waves when cladded with an index-matched fluid. The fabrication process flow and individual process steps are described in detail. Physical characterisation was conducted on structures having undergone intermediate process steps and on finished structures. The attenuation of waveguides measured through cut-back is in good agreement with theoretical expectations, and mode outputs observed via an infra-red camera are of very good quality. The waveguides and components are useful for (bio)chemical sensing in aqueous solutions.

Bianisotropic negative-index metamaterial embedded in a symmetric medium

In order to more clearly observe the bianisotropic effects due to fabrication-induced structural asymmetries in negative-index metamaterials based on a fishnet structure, it is necessary to measure the optical properties with symmetric substrate and superstrate bounding layers. This is accomplished in this report using an index-matching fluid and identical substrate and superstrate glass materials.

Quantifying Colorimetric Assays in Paper-Based Microfluidic Devices by Measuring the Transmission of Light through Paper

This article describes a point-of-care (POC) system--comprising a microfluidic, paper-based analytical device (micro-PAD) and a hand-held optical colorimeter--for quantifying the concentration of analytes in biological fluids. The micro-PAD runs colorimetric assays, and consists of paper that has been (i) patterned to expose isolated regions of hydrophilic zones and (ii) wet with an index-matching fluid (e.g., vegetable oil) that is applied using a disposable, plastic sleeve encasement. Measuring transmittance through paper represents a new method of quantitative detection that expands the potential functionality of micro-PADs. This prototype transmittance colorimeter is inexpensive, rugged, and fully self-contained, and thus potentially attractive for use in resource-limited environments and developing countries.

Microcapillary Electrophoresis Chip Device Integrated with Micro Focusing Lens Structures and Its Biomedical Applications

In this paper, we present a micro-electro-mechanical-system based on a microcapillary electrophoresis chip device integrated with optical detection components, including a micro-focusing lens structure and buried optic fibers. This is a promising approach to enhance the optical signal of the laser-induced fluorescence system for biomedical detection applications. This study utilized microcapillary electrophoresis (micro-CE) chips with two specific polymer materials, polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS). Both are capable of performing multiple-wavelength fluorescence detection by using integrated optic components. These include multimode optic fiber pairs and a micro-focusing-lens structure, embedded downstream of the separation channel. For detection purposes, the fluorescence signals are enhanced by positioning micro-focusing-lens structures at the outlets of the excitation fibers and the inlets of the detection fibers. In this study, two types of micro-focusing-lens are proposed—fixed-focal-length and controllable micro-lenses. They are made from different materials—PMMA and PDMS, respectively. With regard to the fixed-focal-length micro-lenses, the profile of the micro-lens curve can be formed by the defined master mold with specific temperatures and pressures. With regard to the controllable micro-lens design, deformations of the two flexible surfaces can be generated after pressurized index-matching fluid is injected into the pneumatic side-chambers. The side-chambers can be deflected as a double convex lens to focus both the excitation light source and the fluorescent emission signal. Experimental results revealed that the power amplitude of the excitation laser light can be enhanced by up to 5.4 fold. Fluorescein isothiocyanate, dye labeled protein samples and DNA markers are then utilized for micro-CE chip testing. The results indicated that signal amplitude can be enhanced from 1.7 to 2.6 fold when compared with cases without the micro-lens. According to the experimental results, the developed device has a great potential to be integrated with other microfluidic devices for further biomedical applications.

Characterization of graded index optical fibers by digital holographic interferometry

For the first time to our knowledge, digital holography is used to determine the distribution of parabolic or nonparabolic refractive index fields in graded index optical fibers. The fiber is embedded in an index matching fluid whose refractive index can be varied to a matching and mismatching index with respect to that of the cladding. In experiments for both cases high precision phase-shifting digital holographic interferometry is applied with numerical evaluation employing the multilayer model for recognition of the refraction. Due to the higher redundancy in the multiple phase-shifted holograms better accuracy can be obtained compared to classical two-beam interferometry. Therefore the holographic method is recommended as a nondestructive and noncontacting method for characterizing graded index optical fibers.

Size segregation and particle velocity fluctuations in settling concentrated suspensions

We investigate the sedimentation of concentrated suspensions at low Reynolds numbers to study collective particle effects on local particle velocity fluctuations and size segregation effects. Experiments are carried out with polymethylmetacrylate (PMMA) spheres of two different mean diameters (190 and 25 μm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labelled PMMA spheres and a particle image velocimetry method. We also report measurements of the interstitial fluid pressure during settling. Experiments show that size segregation effects can occur during the sedimentation of concentrated suspensions of either quasi-monodisperse or bidisperse spheres. Size segregation is correlated to the organisation of the sedimentation velocity field into vortex-like structures of finite size. A loss of size segregation together with a significant decrease of the fluid pressure gradient in the bulk suspension is observed when the size of vortex-like structures gets on the order of the container size. However, the emergence of channels through the settling zone prevents a complete loss of size segregation in very concentrated suspensions.

Size Segregation in a Fluid-like or Gel-like Suspension Settling under Gravity or in a Centrifuge

We investigate size segregation effects in a bidisperse concentrated suspension when slowly settling under gravity or when submitted to a centrifugal field. Experiments are carried out with PMMA spheres of two different mean diameters (190 and 25 microm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labeled PMMA spheres and a particle-image-velocimetry method. Large particles behave as hard spheres in purely hydrodynamic interactions, while small spheres interact through weakly attractive forces. For a small amount of small spheres among large ones, the suspension remains fluid during settling and the organization of the velocity field of particles into finite-sized structures also called "blobs" promotes size segregation. A larger proportion of weakly attractive small spheres in the bidisperse suspension causes a considerable slowdown of the settling process under gravity and the occurrence of a large-scale collective behavior together with a loss of size segregation. When centrifuging the gel-like bidisperse suspension, a shear-induced melting of the particle network induces a spectacular segregation of species. As a consequence, aging tests of soft yielding materials using centrifugation methods are not representative of the shelf-life stability of the products. A tentative model based on the competition between viscous stresses acting upon particles and adhesive stresses gives a correct estimate of the critical stationary acceleration for the destabilization of the particle network and the onset of size segregation in a gel-like suspension.

Measurement of Refractive Index of Solid Medium by Critical Angle Method When Air Gap is Present

A critical angle method was used to measure the index of refraction of a solid medium when an air gap between the prism and the medium is present. The gap effect was analyzed both numerically and experimentally. Since the total internal reflection is severely disturbed by the large gap, determination of the critical angle and the resulting refractive index becomes ambiguous and inaccurate. By using an index matching fluid, we could determine the index of refraction with an uncertainty of ${\pm}2{\times}1^{-3}$ even when the gap is as large as 1 ${\mu}m$ .

Microcapillary electrophoresis chips utilizing controllable micro‐lens structures and buried optical fibers for on‐line optical detection

In this study, a new design of a controllable micro-lens structure capable of the enhancement of LIF detection system has been demonstrated, which can be further integrated with buried optical fibers on a micro-CE chip for sample separation and detection. Two pneumatic side-chambers were placed between a micro-CE channel and an optical fiber channel. The intervals between the side-chamber and the microchannel were used to form two surfaces of the controllable micro-lens structure. Deformations of the two surfaces can be generated after pressurized index-matching fluid was injected into the pneumatic side-chambers. The side-chambers can be deflected as a double convex lens to focus both the excitation light source and the fluorescent emission signal. The profile and the focal length of the micro-lens structure can be actively adjusted by applying different liquid pressures so that biosamples with a low concentration can be detected. Using low-cost polymeric materials such as polydimethylsiloxane, rapid and reliable fabrication techniques involving standard lithography and replication process was employed for the formation of the proposed chip device. Experimental results revealed the controllable micro-lens structure can be successfully deformed as a convex lens to focus the laser light source and the collected fluorescence signal can be enhanced accordingly. The power amplitude of excitation laser light can be enhanced by 5.4-fold. FITC dye and DNA markers were then utilized for micro-CE testing. The results indicated that the signal amplitude could be enhanced 2.5-fold when compared to the case without the activation of the micro-lens. According to the experimental results, the developed device has a great potential to be integrated with other microfluidic devices for further biomedical applications.

A lamp light-emitting diode-induced fluorescence detector for capillary electrophoresis

A light-emitting diode-induced fluorescence detector (LED-FD) for capillary electrophoresis was constructed and evaluated. A lamp LED with an enhanced emission spectrum and a band pass filter was used as the excitation light source. Refractive index matching fluid (RIMF) was used in the detection cell to reduce scattering light and the noise level. The limit of detection (LOD) for fluorescein was 1.5 nM (SNR = 3). The system exhibited linear responses in the range of 1 × 10 −8 to 5 × 10 −6 M ( R = 0.999). Application of the lamp LED-FD for the analysis of FITC-labeled ephedra herb extract by capillary electrophoresis was demonstrated.

A glycerol assisted light-emitting diode-induced fluorescence detector for capillary flow systems

A glycerol assisted light-emitting diode (LED)-induced fluorescence detector (IF) for capillary flow systems was constructed and evaluated. A blue LED was used as the excitation source, and optical fibers (OF) were used to transmit the excitation light and collect the fluorescence. A commercial available 5-port manifold was used as detection cell, where the capillary tube and the OF were fixed into the manifold. The precision of the holes on the manifold ensured a self-alignment of optical path. A refractive index matching fluid (RIMF)-glycerol was used to eliminate the interfaces between the OF and the LED, as well as between the fused silica capillary and the transmitting/collecting fiber. The enhancement of excitation light led to 2.8-folds improvement on the signal-to-noise ratio. The use of RIMF also eliminates focusing effect of the capillary wall and reduces both the excitation light directed to the detection cell and background signal, resulting in reduction in the fluorescence intensity and noise level. The intensity was reduced to 47–63% for laser and 60–77% for LED, respectively, for capillaries with i.d. from 50 to 250 μm; while the noise level was reduced to 1/3 when RIMF was used for both laser and LED on the tested capillaries. About 5.6-fold enhancement in signal-to-noise ratio was obtained in total. The detection limit of the LED-IF for fluorescein isothiocyanate (FITC) was 4 nM. Application of the LED-IF for the analysis of FITC-labeled amino acids by electrophoresis was demonstrated.

3D Imaging of particle motion during penetrometer testing

We present the results of direct observation of material rearrangement due to penetration of a solid rod (penetrometer) through a granular medium. Two different techniques and their advantages are discussed in this paper. We investigate the motion of material within the bulk around the rod. Transparent, polydisperse, and irregularly shaped silica particles immersed in index matching fluid are used for detailed imaging of the interior of a granular pile. Motion of material is observed by confocal microscopy from the bottom boundary up to 100 particle diameters in height. Image analysis indicates that rearrangements spread furthest not directly under the penetrometer but in a ring around the penetrometer. In addition, the direction of preformed stress chains in the material influences the particle rearrangements. Material compressed from one side exhibits anisotropic particle rearrangements under penetrometer testing. Laser sheet scanning allows for direct imaging of individual particle motion with greater accuracy, but works best for spherical particles only.

Direct Flow Visualization of Colloidal Gels in Microfluidic Channels

The behavior of colloidal gels under pressure-driven flow in square microchannels is quantified by microscopic particle image velocimetry (muPIV) and compared to predictions of available rheological models. The gels consist of hydrophobically modified silica microspheres (phi = 0.15-0.33) suspended in a refractive index-matched fluid along with fluorescent tracers to aid visualization. Measured velocity flow profiles show a transition from plug flow to more fluid-like behavior with increasing volumetric flow rate (Q) at all phi. This transition is not captured by theoretical predictions of the flow profile based on the Herschel-Bulkley model. Rather, a model that accounts for gel breakup into a suspension of clusters at elevated shear rates by assuming a finite viscosity at infinite shear is needed to accurately predict the flow behavior of colloidal gels at large Q.

Compact 90° trench-based splitter for silicon-on-insulator rib waveguides

Compact silicon-on-insulator (SOI) rib waveguide 90 degrees splitters based on narrow, high-aspect ratio (~10:1) trenches are designed and experimentally demonstrated. The splitter area is only 11 mum x 11 mum. Splitter optical performance is investigated as a function of both trench width and refractive index of the trench fill material. We examine three trench fill materials, air (n=1.0), SU8 (n=1.57), and index matching fluid (n=1.733), and find good agreement between experimental measurement and three dimensional (3D) finite difference time domain (FDTD) simulation. A splitting ratio of 49/51 (reflection/transmission) is measured for an index fluid-filled trench 82nm wide.

Second Harmonic Generation Confocal Microscopy of Collagen Type I from Rat Tendon Cryosections

We performed second harmonic generation (SHG) imaging of collagen in rat-tendon cryosections, using femtosecond laser scanning confocal microscopy, both in backscattering and transmission geometries. SHG transmission images of collagen fibers were spatially resolved due to a coherent, directional SHG component. This effect was enhanced with the use of an index-matching fluid ( n i = 1.52). The average SHG intensity oscillated with wavelength in the backscattered geometry (isotropic SHG component), whereas the spectral profile was consistent with quasi-phase-matching conditions in transmission geometry (forward propagating, coherent SHG component) around 440 nm ( λ p = 880 nm). Collagen type I from bovine Achilles tendon was imaged for SHG in the backscattered geometry and its first-order effective nonlinear coefficient was determined ( | d eff | ≈ 0.085 ( ± 0.025 ) × 10 − 12 m V − 1 ) by comparison to samples of inorganic materials with known effective nonlinear coefficients (LiNbO3 and LiIO3). The SHG spectral response of collagen type I from bovine Achilles tendon matched that of the rat-tendon cryosections in backscattered geometry. Collagen types I, II, and VI powders (nonfibrous) did not show any detectable SHG, indicating a lack of noncentrosymmetric crystalline structure at the molecular level. The various stages of collagen thermal denaturation were investigated in rat-tendon cryosections using SHG and bright-field imaging. Thermal denaturation resulted in the gradual destruction of the SHG signal.

Organic light-emitting device luminaire for illumination applications

The total light output of an organic light-emitting device (OLED) with a truncated square-pyramid luminaire is shown to be 1.8–2.1 times greater than the forward emission of an OLED without a luminaire. Luminaires increase the amount of outcoupled light by reducing waveguiding in the glass substrate, and they are made from 1.2-cm-thick acrylic and attached to the OLED glass substrate using index matching fluid. Based on experimental results and ray tracing models, the ratio of the OLED total emission to the OLED forward emission is found to be independent of luminaire taper angles between 30° and 80°.

Numerical Analyses of the Roles of Gas Phase and Liquid Phase UV Photochemistry in Conventional and Immersion 193 nm Lithography

We examine the impacts of photochemistry of air and water on resist films during lithographic exposure at 193 nm. The analysis uses a detailed quantitative kinetic model based on available literature mechanistic data and constructed with in-house simulation tools. In conventional 193 nm lithography, both oxidation of the resist polymer due to the UV photolysis of molecular oxygen, and film interaction with strong acids, formed by photo-oxidation of nitrogen and sulfur species in ambient air, have been proposed to lead to degraded resist imaging. We assess the extent to which such reactions can occur under typical lithographic process conditions, and find that while oxidation is minimal, acid deposition into the top of the resist film can be significant and can spread over distances of millimeters. Immersion lithography using 193 nm radiation utilizes a layer of highly purified, degassed water as an index-matching fluid. When water is exposed to 193 nm light, short-lived chemical intermediates are produced by two pathways, neutral and ionic. A quantitative evaluation of this photochemistry during lithographic immersion exposure shows that neither these intermediates nor acid formed by photolysis of leached photoacid generator leads to substantial changes in the composition of the water layer, so resist impacts are not likely to be significant. Organic immersion fluids may undergo significant photolysis, however there are insufficient experimental data to assess any potential impacts at this time.

Improved longitudinal magneto-optic Kerr effect signal contrast from nanomagnets with dielectric coatings

We report on the use of dielectric coatings to improve the contrast of longitudinal magneto-optic Kerr effect signals from submicron magnetic structures. Electron-beam lithography was used to define disks in 22 nm thick Ni films deposited on Si substrates. The structures were measured in four configurations: as-deposited, through a fused silica prism using index-matching fluid, coated with ZnS, and using a prism on top of the ZnS layer. The modified samples show up to 20 times improvement in the MOKE contrast due to admittance matching to the magnetic material and suppression of the substrate reflectance. The behavior is successfully predicted by a model that includes the magneto-optic response of the nickel layer and accounts for the fraction of the beam intercepted by the magnetic structure.