Refractive Index Mismatch Research Articles

Surfactant Effects on Efficiency Enhancement of Infrared-to-Visible Upconversion Emissions of NaYF4:Yb-Er

Infrared-to-visible rare earth doped upconversion phosphors that convert multiple photons of lower energy to higher energy photons offer a wide range of technological applications. The brightness (i.e., emission intensities) and energy efficiency of phosphors are important performance characteristics that determine which applications are appropriate. Optical efficiency can be used as a measure of the upconversion emission performance of these rare earth doped phosphors. In this work, hexagonal-phase NaYF(4):Yb-Er was synthesized using the hydrothermal method in the presence of surfactants like trioctylphosphine, polyethylene glycol monooleate, and polyvinylpyrrolidone. The upconversion emission optical efficiencies of NaYF(4):Yb-Er were measured to quantify and evaluate the effects of surface coatings and accurately reflect the brightness and energy efficiency of these phosphors. Polyvinylpyrrolidone-modified NaYF(4):Yb-Er particles were found to be ~5 times more efficient and brighter than the unmodified particles. The difference in efficiency was attributed to reduced reflectance losses at the particle-air interface via refractive index mismatch reduction between the core NaYF(4):Yb-Er particles and air using polyvinylpyrrolidone as a surfactant.

Fluorescence Correlation Spectroscopy: An Efficient Tool for Measuring Size, Size-Distribution and Polydispersity of Microemulsion Droplets in Solution

Fluorescence correlation spectroscopy (FCS) is an ideal tool for measuring molecular diffusion and size under extremely dilute conditions. However, the power of FCS has not been utilized to its best to measure diffusion and size parameters of complex chemical systems. Here, we apply FCS to measure the size, and, most importantly, the size distribution and polydispersity of a supramolecular nanostructure (i.e., microemulsion droplets, MEDs) in dilute solution. It is shown how the refractive index mismatch of a solution can be corrected in FCS to obtain accurate size parameters of particles, bypassing the optical matching problem of light scattering techniques that are used often for particle-size measurements. We studied the MEDs of 13 different W(0) values from 2 to 50 prepared in a ternary mixture of water, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and isooctane, with sulforhodamine-B as a fluorescent marker. We find that, near the optical matching point of MEDs, the dynamic light scattering (DLS) measurements underestimate the droplet sizes while FCS estimates the accurate ones. A Gaussian distribution model (GDM) and a maximum-entropy-based FCS data fitting model (MEMFCS) are used to analyze the fluorescence correlation curves that unfold Gaussian-type size distributions of MEDs in solution. We find the droplet size varies linearly with W(0) up to ~20, but beyond this W(0) value, the size variation deviates from this linearity. To explain nonlinear variation of droplet size for W(0) values beyond ~20, we invoke a model (the coated-droplet model) that incorporates the size polydispersity of the droplets.

Effective holographic recordings in the photopolymer nanocomposites with functionalized silica nanoparticle and polyurethane matrix

Effective holographic nanocomposites were developed by the surface-functionalized silica nanoparticles and two acrylate monomers/polyurethane (PU) matrix polymer. The functionalization was done with silane compounds carrying long alkyl chain or vinyl group. We evaluated the holographic nanocomposite films by the diffraction efficiency, volume shrinkage, optical loss, and the film morphology. It was found that acrylate monomers/PU system gave higher diffraction efficiency than those of two monomers due to the high refractive index mismatch between the acrylate-rich and PU-rich regions. With the modification of silica particle, up to 35% of particle loading was possible to give a maximum diffraction efficiency of 93.6% for a film of 20 μm in thickness, along with improved refractive index modulation and the sensitivity.

Induced aberrations by combinative convex/concave interfaces of refractive‐index‐mismatch and capability of adaptive optics correction

AbstractAdjustable fluidic adaptive lenses have been extensively studied because of the advantage of changing the geometric shape or refractive index without any mechanical moving parts. However, the induced aberrations were rarely discussed before. We address this issue by first clarifying the aberrations experimentally due to injected fluidic volumes. Experimental results show that under the injected fluidic volume of 0.1 ml, the main aberrations come from Z1 (piston), Z2 (tip), and Z5 (defocus) and Zernike coefficients are 0.97, 0.31, and 1.31 μm, respectively. In what follows, a series of tests specifically designed to explore the convex/concave interfaces and refractive‐index‐mismatch (RIM). Furthermore, we demonstrate the capability of adaptive optics (AO) correction on aberrations induced by combinative effects of multiple layers with convex/concave interfaces and RIM. A microelectro‐mechanical system (MEMS) deformable mirror (DM) with 140 actuators was used in conjunction with Shack–Hartmann wavefront sensor to realize the experimentation. In particular, we consider the aberration introduced by interfaces of RIM between water/oil and glass. After AO correction, we can improve wavefront with root mean square (RMS) of 2.17–0.17 μm for an interface between water and glass. As for the interface between oil and glass, we are capable of improving RMS of 0.24–0.10 μm. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2610–2615, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26323

Single myelin fiber imaging in living rodents without labeling by deep optical coherence microscopy

Myelin sheath disruption is responsible for multiple neuropathies in the central and peripheral nervous system. Myelin imaging has thus become an important diagnosis tool. However, in vivo imaging has been limited to either low-resolution techniques unable to resolve individual fibers or to low-penetration imaging of single fibers, which cannot provide quantitative information about large volumes of tissue, as required for diagnostic purposes. Here, we perform myelin imaging without labeling and at micron-scale resolution with >300-μm penetration depth on living rodents. This was achieved with a prototype [termed deep optical coherence microscopy (deep-OCM)] of a high-numerical aperture infrared full-field optical coherence microscope, which includes aberration correction for the compensation of refractive index mismatch and high-frame-rate interferometric measurements. We were able to measure the density of individual myelinated fibers in the rat cortex over a large volume of gray matter. In the peripheral nervous system, deep-OCM allows, after minor surgery, in situ imaging of single myelinated fibers over a large fraction of the sciatic nerve. This allows quantitative comparison of normal and Krox20 mutant mice, in which myelination in the peripheral nervous system is impaired. This opens promising perspectives for myelin chronic imaging in demyelinating diseases and for minimally invasive medical diagnosis.

SU‐E‐T‐122: Effect of Refraction on Dose Reconstruction in Optical‐CT Gel Dosimetry

Purpose: Optical‐CT gel/radiochromic dosimetry has emerged as a promising candidate for 3D dose verification. One factor that affects the accuracy of dose reconstruction in this technique is the refractive index mismatch between the gel phantom and the matching liquid that prevents the full width of the phantom from being used for dose mapping. The purpose of this study is to investigate the effect of the refractive artifact on dose reconstruction, and to identify optimal designs for accurate gel dosimeters. Methods: Numerical experiments were carried out for a series of model attenuation coefficients of the gel phantom and various degrees of refraction resulting in situations where only projections through certain central sections of the sample are detected. Measured data was obtained as the Radon transform of the model attenuation coefficient. The attenuation coefficient was then reconstructed using the filtered backprojection algorithm. A mathematical study was performed in the framework of the so‐called interior problem of Radon transform inversion. The variation of the amount of data accessible experimentally with the refractive index mismatch was investigated using geometrical optics. Results: It was demonstrated that the reconstruction of the attenuation coefficient within 3% is possible in the central part of the phantom even if only 82.5% of the optical projections are detected. This fraction of available data results from a 1.5% refractive index mismatch between the gel and liquid. Using a priori knowledge of the value of the absorption coefficient at a given point in the sample, it is possible to calculate the correct value at all points. Conclusions: Optical attenuation and dose in an optical CT gel dosimeter can be reconstructed accurately in the central part of the dosimeter even if only limited data are available. A priori knowledge of the attenuation coefficient at a point in the sample enables almost exact reconstruction.

SU‐C‐224‐04: Effect of Refraction on Dose Reconstruction in Optical‐CT Gel Dosimetry

Purpose: Optical‐CT gel/radiochromic dosimetry has emerged as a promising candidate for 3D dose verification. One factor that affects the accuracy of dose reconstruction in this technique is the refractive index mismatch between the gel phantom and the matching liquid that prevents the full width of the phantom from being used for dose mapping. The purpose of this study is to investigate the effect of the refractive artifact on dose reconstruction, and to identify optimal designs for accurate gel dosimeters. Methods: Numerical experiments were carried out for a series of model attenuation coefficients of the gel phantom and various degrees of refraction resulting in situations where only projections through certain central sections of the sample are detected. Measured data was obtained as the Radon transform of the model attenuation coefficient. The attenuation coefficient was then reconstructed using the filtered backprojection algorithm. A mathematical study was performed in the framework of the so‐called interior problem of Radon transform inversion. The variation of the amount of data accessible experimentally with the refractive index mismatch was investigated using geometrical optics. Results: It was demonstrated that the reconstruction of the attenuation coefficient within 3% is possible in the central part of the phantom even if only 82.5% of the optical projections are detected. This fraction of available data results from a 1.5% refractive index mismatch between the gel and liquid. Using a priori knowledge of the value of the absorption coefficient at a given point in the sample, it is possible to calculate the correct value at all points. Conclusions: Optical attenuation and dose in an optical CT gel dosimeter can be reconstructed accurately in the central part of the dosimeter even if only limited data are available. A priori knowledge of the attenuation coefficient at a point in the sample enables almost exact reconstruction.

Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate

We present the use of a liquid crystal spatial light modulator to correct for the refractive-index mismatch induced spherical aberration in a high refractive-index lithium niobate crystal when a low repetition rate amplified laser is used for the direct fabrication of three-dimensional micro-structures. By correcting the aberration based on experimentally determined values, we show that the size of written structures decreases dramatically, which allows the fabrication of high quality micro-structures such as three-dimensional photonic crystals. We demonstrate that, through the use of adaptive optics, the fabrication depth and the stopgap strength in the corresponding photonic crystals are increased by a factor of two to three.

Improved reconstructions and generalized filtered back projection for optical projection tomography

Optical projection tomography (OPT) is a noninvasive imaging technique that enables imaging of small specimens (<1 cm), such as organs or animals in early developmental stages. In this paper, we present a set of computational methods that can be applied to the acquired data sets in order to correct for (a) unknown background or illumination intensity distributions over the field of view, (b) intensity spikes in single CCD pixels (so-called "hot pixels"), and (c) refractive index mismatch between the media in which the specimens are embedded and the environment. We have tested these correction methods using a variety of samples and present results obtained from Parhyale hawaiensis embedded in glycerol and in sea water. Successful reconstructions of fluorescence and absorption OPT images have been obtained for weakly scattering specimens embedded in media with nonmatched refractive index, thus advancing OPT toward routine in vivo imaging.

Experimental evidence of an effective medium seen by diffuse light in turbid colloids

The propagation of diffuse light in turbid media is usually modeled with radiative transfer theory. When diffuse light travelling in a turbid colloid is reflected and transmitted at a flat interface where there is a refractive index mismatch, it is not clear whether one should assume the incident diffuse-light is travelling in a medium with a refractive index equal to that of the background medium (usually referred to as the matrix) or if one should assume it travels in an effective medium. Most authors simply avoid this issue and most often use the refractive index of the matrix. While this might be a good approximation for dilute turbid media one may suspect that for highly scattering materials it may not be the case. In this work we investigate experimentally this issue. Our experimental results provide clear evidence that diffuse light inside the turbid colloid travels in an effective medium and not in the matrix.

IMPROVEMENT OF OPTICAL PROPERTIES OF ZnO-SILICONE NANOCOMPOSITES BY CHEMICAL GRAFTING

N-(3-Trimethoxysilylpropyl)-4-azido-2,3,5,6-tetrafluorobenzamide(PFPA-silane) was used to modify zinc oxide(ZnO) nanoparticles.Then,the modified ZnO particles were grafted on silicone chains by PFPA-silane.There are two different reactive groups in PFPA-silane molecules,one reacts with hydroxyl on the surface of ZnO and the other one reacts with silicone chains by grafting reaction.Group variation during reaction was characterized by infrared spectroscopy(IR),and improvement of transparency of the composite after grafting reaction was recorded with ultraviolet-visible spectroscopy(UV-Vis).Change of refractive index was recorded with an Abbe refractometer.The influence of grafting reaction on transparency of the composite is analyzed with relationship between refractive index and light transmittance of composites.It can be concluded that refractive index of the nanocomposite is changed and the refractive index mismatch between ZnO nanoparticles and silicone matrix is reduced through grafting reaction,which benefits to decrease light scattering of ZnO nanoparticles and increase transparency of the composite.The maximal increase amount of light transmittance is 50% in this experiment.The result shows that the match between inorganic fillers and polymer matrix in refractive index can be adjusted by grafting reaction of coupling agent to polymer.The chemical grafting process is useful in preparation of high transparency inorganics-polymer nanocomposites.

3D deconvolution of spherically aberrated images using commercial software

Refractive index mismatch between the specimen and the objective immersion oil results in spherical aberration, which causes distortion and spreading of the point spread function, as well as incorrect readings of the axial coordinates. These effects have to be taken into account when performing three-dimensional restoration of wide-field fluorescence images. By using objects with well-defined geometry (fluorescently stained Escherichia coli or actin filaments) separated from a cover slip by a layer of oil with known refractive index, we investigated the accuracy of three-dimensional shape restoration by the commercial programs Huygens and Autoquant. Aberration correction available in the software dramatically reduced the axial blur compared to deconvolution that ignored the refractive index mismatch. At the same time, it failed to completely recover the cylindrical symmetry of bacteria or of actin fibres, which showed up to a three to five times larger width along the optical axis compared to the lateral plane. The quality of restoration was only moderately sensitive to the exact values of the specimen refractive index but in some cases improved significantly by assuming a reduced NA of the objective. Because image restoration depends on the knowledge of the vertical scale, we also performed detailed measurements of the axial scaling factor and concluded (in agreement with some previous authors) that scaling is adequately described by the simple paraxial formula, even when high-NA oil-immersion objectives are used.

The effects of spherical aberration on multiphoton fluorescence excitation microscopy

Multiphoton fluorescence excitation microscopy is almost invariably conducted with samples whose refractive index differ from that of the objective immersion medium, conditions that cause spherical aberration. Due to the quadratic nature of multiphoton fluorescence excitation, spherical aberration is expected to profoundly affect the depth dependence of fluorescence excitation. In order to determine the effect of refractive index mismatch in multiphoton fluorescence excitation microscopy, we measured signal attenuation, photobleaching rates and resolution degradation with depth in homogeneous samples with minimal light scattering and absorption over a range of refractive indices. These studies demonstrate that signal levels and resolution both rapidly decline with depth into refractive index mismatched samples. Analyses of photobleaching rates indicate that the preponderance of signal attenuation with depth results from decreased rates of fluorescence excitation, even in a system with a descanned emission collection pathway. Similar results were obtained in analyses of fluorescence microspheres embedded in rat kidney tissue, demonstrating that spherical aberration is an important limiting factor in multiphoton fluorescence excitation microscopy of biological samples.

Laser scanning confocal microscopy of comet material in aerogel

We report nondestructive three-dimensional imaging and analysis techniques for material returned by the Stardust cometary collector. Our technique utilizes three-dimensional laser scanning confocal microscopy (3D LSCM) to image whole Stardust tracks, in situ, with attainable resolutions <90 nm/pixel edge. LSCM images illustrate track morphology and fragmentation history; image segmentation techniques provide quantifiable volumetric and dynamic measurements. We present a method for multipart image acquisition and registration in 3D. Additionally, we present a 3D deconvolution method for aerogel, using a theoretically calculated point spread function for first-order corrections of optical aberrations induced by light diffraction and refractive index mismatches. LSCM is a benchtop technique and is an excellent alternative to synchrotron X-ray computed microtomography for optically transparent media. Our technique, developed over the past 2 years, is a noninvasive, rapid technique for fine-scale imaging of high value returned samples from the Stardust mission, as well as various other samples from the geosciences.

Parallel direct laser writing in three dimensions with spatially dependent aberration correction

We propose a hologram design process which aims at reducing aberrations in parallel three-dimensional direct laser writing applications. One principle of the approach is to minimise the diffractive power of holograms while retaining the degree of parallelisation. This reduces focal distortion caused by chromatic aberration. We address associated problems such as the zero diffraction order and aberrations induced by a potential refractive index mismatch between the immersion medium of the microscope objective and the fabrication substrate. Results from fabrication in diamond, fused silica and lithium niobate are presented.

Implementation of the equation of radiative transfer on block-structured grids for modeling light propagation in tissue

We present the first algorithm for solving the equation of radiative transfer (ERT) in the frequency domain (FD) on three-dimensional block-structured Cartesian grids (BSG). This algorithm allows for accurate modeling of light propagation in media of arbitrary shape with air-tissue refractive index mismatch at the boundary at increased speed compared to currently available structured grid algorithms. To accurately model arbitrarily shaped geometries the algorithm generates BSGs that are finely discretized only near physical boundaries and therefore less dense than fine grids. We discretize the FD-ERT using a combination of the upwind-step method and the discrete ordinates (SN) approximation. The source iteration technique is used to obtain the solution. We implement a first order interpolation scheme when traversing between coarse and fine grid regions. Effects of geometry and optical parameters on algorithm performance are evaluated using numerical phantoms (circular, cylindrical, and arbitrary shape) and varying the absorption and scattering coefficients, modulation frequency, and refractive index. The solution on a 3-level BSG is obtained up to 4.2 times faster than the solution on a single fine grid, with minimal increase in numerical error (less than 5%).

Discontinuous Galerkin method for the forward modelling in optical diffusion tomography

AbstractWe propose a discontinuous Galerkin discretization scheme for the forward modelling in optical diffusion tomography in highly scattering media to facilitate dynamic mesh adaptation for complex domains. In addition, the numerical method is also shown to effectively deal with inhomogeneities in optical properties and refractive index mismatch at material interfaces. The accuracy of the method is demonstrated over a model concentric spherical layers problem where the discontinuous Galerkin method is compared against an analytical solution. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Transparent Infrared-Emitting CeF3:Yb−Er Polymer Nanocomposites for Optical Applications

Bright infrared-emitting nanocomposites of unmodified CeF(3):Yb-Er with polymethyl-methacrylate (PMMA) and polystyrene (PS), which offer a vast range of potential applications, which include optical amplifiers, waveguides, laser materials, and implantable medical devices, were developed. For the optical application of these nanocomposites, it is critical to obtain highly transparent composites to minimize absorption and scattering losses. Preparation of transparent composites typically requires powder processing approaches that include sophisticated particle size control, deagglomeration, and dispersion stabilization methods leading to an increase in process complexity and processing steps. This work seeks to prepare transparent composites with high solids loading (>5 vol%) by matching the refractive index of the inorganic particle with low cost polymers like PMMA and PS, so as to circumvent the use of any complex processing techniques or particle surface modification. PS nanocomposites were found to exhibit better transparency than the PMMA nanocomposites, especially at high solids loading (>/=10 vol%). It was found that the optical transparency of PMMA nanocomposites was more significantly affected by the increase in solids loading and inorganic particle size because of the larger refractive index mismatch of the PMMA nanocomposites compared to that of PS nanocomposites. Rayleigh scattering theory was used to provide a theoretical estimate of the scattering losses in these ceramic-polymer nanocomposites.

Zeeman laser scanning confocal microscope and its ability on reduction of specimen-induced spherical aberration

The spherical aberration induced by refractive-index mismatch results in the degradation on the quality of sectioning images in conventional confocal laser scanning microscope (CLSM). In this research, we have derived the theory of image formation in a Zeeman laser scanning confocal microscope (ZLSCM) and conducted experiments in order to verify the ability of reducing spherical aberration in ZLSCM. A Zeeman laser is used as the light source and produces the linearly polarized photon-pairs (LPPP) laser beam. With the features of common-path propagation of LPPP and optical heterodyne detection, ZLSCM shows the ability of reducing the specimen-induced spherical aberration and improving the axial resolution simultaneously.

Characterization of deformable mirrors for spherical aberration correction in optical sectioning microscopy

In this paper we describe the wavefront aberrations that arise when imaging biological specimens using an optical sectioning microscope and generate simulated wavefronts for a planar refractive index mismatch. We then investigate the capability of two deformable mirrors for correcting spherical aberration at different focusing depths for three different microscope objective lenses. Along with measurement and analysis of the mirror influence functions we determine the optimum mirror pupil size and number of spatial modes included in the wavefront expansion and we present measurements of actuator linearity and hysteresis. We find that both mirrors are capable of correcting the wavefront aberration to improve imaging and greatly extend the depth at which diffraction limited imaging is possible.