Effect Of Refractive Index Mismatch Research Articles

Effects of refractive index mismatch between sample and immersion medium in line-field confocal optical coherence tomography

Line-field confocal optical coherence tomography (LC-OCT) is an optical technique based on low-coherence interference microscopy with line illumination, designed for tomographic imaging of semi-transparent samples with micrometer-scale spatial resolution. A theoretical model of the signal acquired in LC-OCT is presented. The model shows that a refractive index mismatch between the sample and the immersion medium causes a dissociation of the coherence plane and the focal plane, leading to a decrease in the signal amplitude and a degradation of the image’s lateral resolution. Measurements are performed to validate and illustrate the theoretical predictions. A mathematical condition linking various experimental parameters is established to ensure that the degradation of image quality is negligible. This condition is tested experimentally by imaging a phantom. It is verified theoretically in the case of skin imaging, using experimental parameters corresponding to those of the commercially available LC-OCT device.

Effect of refractive index mismatch on forward-to-backward ratios in SHG imaging.

Nonlinear optical imaging in the epi-direction is used to image subresolution features. We find that a refractive index mismatch between the object to be imaged and the background medium can change the far-field intensity image. As an example, we study second harmonic generation (SHG) microscopy where the forward-to-backward (F/B) ratio is used to quantify subresolution features. We show both theoretically and experimentally that the inhomogeneous refractive index in collagen tendon tissue creates near-field effects, which can change the F/B ratio by ∼20%-25%, even though the effect is negligible for most of the individual fibrils in the tissue. This is caused by the sensitivity of the backward signal on phase matching conditions.

Effects of refractive index mismatch on SRS and CARS microscopy.

An inhomogeneous linear refractive index profile, such as that occurring in biological tissues, is shown to significantly alter stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy images. Our finite-difference time-domain simulations show that near-field enhancement and microlensing can lead to an increase of an object's perceived molecular density by a factor of nine and changes in its perceived position by 0.4 μm up to 1.0 μm. Thus the assumption that SRS scales linearly and CARS quadratically with density does not always hold. Furthermore, the inhomogeneous linear index can cause false CARS and AM-SRS signals, even for a homogeneous nonlinear susceptibility.

Effect of host glass matrix on structural and optical behavior of glass–ceramic nanocomposite scintillators

Composite scintillator systems have received increased attention in recent years due to their promise for merging the radioisotope discrimination capabilities of single crystal scintillators with the high throughput scanning capabilities of portal monitors. However, producing the high light yield required for good energy resolution has proven challenging as scintillation photons are often scattered by variations in refractive index and agglomerated scintillator crystals within the composite. This investigation sought to mitigate these common problems by using glass–ceramic nanocomposite materials systems in which nanoscale scintillating crystallites are precipitated in a controlled manner from a transparent glass matrix. Precipitating crystallites in situ precludes nanoparticle agglomeration, and limiting crystallite size to 50nm or less mitigates the effect of refractive index mismatch between the crystals and host glass. Cerium-doped gadolinium bromide (GdBr3(Ce)) scintillating crystals were incorporated into sodium-aluminosilicate (NAS) and alumino-borosilicate (ABS) host glass matrices, and the resulting glass–ceramic structures and luminescence behavior were characterized. The as-cast glass from the ABS system displayed a highly ordered microstructure that produced the highest luminescence intensity (light yield) of the samples studied. However, heat treating to form the glass–ceramic precipitated rare-earth oxide crystallites rather than rare-earth halides. This degraded light yield relative to the unaged sample.

Effect of refractive index mismatch on multi-photon direct laser writing

This work reports how the process of three-dimensional multi-photon direct laser writing (mpDLW) is affected when there is a small mismatch in refractive index between the material being patterned and the medium in which the focusing objective is immersed. Suspended-line microstructures were fabricated by mpDLW in the cross-linkable epoxide SU-8 as a function of focus depth and average incident power. It is found that even a small refractive index contrast of Δn = + 0.08 causes significant variation in feature width and height throughout the depth of the material. In particular, both the width and height of features can either increase or decrease with depth, depending upon how much the average incident laser power exceeds the threshold for writing. Vectorial diffraction theory is used to obtain insight into the origin of the effect and how to compensate for it. We demonstrate that varying the average focused power is a practical means for controlling the variation in feature size with focal depth.

Anisotropic photonic properties of III–V nanowires in the zinc-blende and wurtzite phase

Some critical aspects of the anisotropic absorption and emission properties of quasi one-dimensional structures are reviewed in the context of III-V compound semiconductor nanowires. The unique optical and electronic properties of III-V nanowires stem from the combination of dielectric effects due to their large aspect ratio, and their specific crystallographic structure which can differ significantly from the bulk case. The growth conditions leading to single-crystal nanowires with either zinc blende or wurtzite phase are first presented. Dipole selection rules for interband transitions in common III-V compounds are then summarized for the two different phases, and corroborated by ab initio Density Functional Theory calculations of the oscillator strength. The optical anisotropy is discussed considering both the effect of refractive index mismatch between the nanowire and its surroundings and the polarization of the emitting dipoles set by the nanowire crystallographic structure and orientation. Finite Difference Time Domain simulations are finally employed to illustrate the influence of the emitting dipole orientation and the nanowire diameter on the distribution of radiation in the far-field. The importance of the correlation between structural and optoelectronic properties is highlighted in view of potential applications in future nanowire photonics.

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.

Analysis of second-harmonic generation microscopy under refractive index mismatch

On the basis of the vector diffraction theory and Green's function method, this paper investigates the effects of refractive index mismatch on second-harmonic generation (SHG) microscopy. The polarization distribution and SHG intensity are calculated as functions of the sample radius and probe depth. The numerical results show that refractive index mismatch can result in peak intensity degradation, increase secondary lobes and extension of second-harmonic polarization distribution. Because of the attenuation of polarization intensity, the detected SHG intensity significantly decreases with increasing probe depth, which can limit the imaging depth of SHG microscopy inside thick samples. Forward SHG intensity decays slowly than backward SHG, due to the combination of extension second-harmonic polarization distribution and strong dependency of forward SHG on sample radius.

Bragg Solitons in Quantum-Well Structures

Soliton pulse propagation in a periodic quantum-well structure with a period close to one-half the light wavelength corresponding to the exciton resonance frequency was studied. The various kinds of exciton nonlinearity characteristic of a quantum well (P 3-and EP 2-type nonlinearities, biexciton nonlinearity) were included. The characteristic features of the soliton were studied in each of the cases considered. The effect of refractive index mismatch between the barrier and quantum-well materials on the soliton parameters was analyzed. Soliton solutions to the Maxwell-Bloch nonlinear equations are compared with their plane-wave solutions.

Influence of refractive-index mismatch in high-resolution three-dimensional confocal microscopy.

The effects of the refractive-index mismatch in confocal laser scanning microscopy were extensively studied. The axial aberration induced in the case of fluorescent microspheres was measured. The data were used to take into account the mismatch-induced aberrations and to consider object-size influence. Then we focused on the effect of refractive-index mismatch on the effective system's point-spread function under different mismatch conditions and on depth of focusing. We experimentally verified that the peak of the point-spread function intensity profile decreases and the point-spread function itself progressively broadens as a function of the combined effect of the refractive-index mismatch and of the penetration depth, leading to a worsening of the system's overall performances. We also performed these same measurements by embedding subresolution beads in an oocyte's cytoplasm, which can be considered a turbid medium. We found evidence consistent with the previously developed theoretical model; in particular we found a strong dependence of the intensity peak on the focusing depth.

Experimental verification of the effect of refractive index mismatch on the light fluence in a turbid medium.

Diffusion theory is often used to model the transport of light within tissue. It can be used to calculate the light fluence rate in tissue, for example, during photodynamic therapy, or to measure the absorption and scattering properties of tissue. For both of these applications, the influence of the interface between the tissue and the exterior medium on the fluence rate inside the tissue must be known in order to make accurate calculations. We present an experimental investigation of the effect of the refractive index mismatch at the tissue interface on the internal light fluence rate and on the spatially resolved diffuse reflectance as the boundary conditions of the tissue/external medium are changed. The effects of changing the relative refractive index at the boundary are compared to predictions of diffusion theory. The effect of the refractive index mismatch is predicted correctly by diffusion theory.

Performance of fitting procedures in curved geometry for retrieval of the optical properties of tissue from time-resolved measurements

By use of the solution of the diffusion equation for cylindrical and spherical geometry, two fitting procedures for retrieval of the optical properties from time-resolved measurements have been implemented. The fitting procedures are based on the Levenberg-Marquardt algorithm, in which the fitting parameters are the absorption coefficient, the reduced scattering coefficient, and an amplitude factor. Monte Carlo data generated for cylindrical and spherical geometry were fitted by these fitting procedures, and the retrieved optical properties were compared with those obtained from the inversion procedure with a mismatched geometry of a semi-infinite medium. The effects of refractive-index mismatch and of different boundary conditions of the diffusion equation were also studied, together with the effects of several sources of error that are typically found in time-resolved measurements. The advantages and drawbacks of these fitting procedures, including many details in several situations of interest in the field of tissue optics, are discussed. The results also offer a guideline to understanding the effects of mismatching in curved geometry as functions of source-detector distance and radii of cylinders or spheres.

Effects of refractive-index mismatch on three-dimensional optical data-storage density in a two-photon bleaching polymer

Reported is an investigation into the effect of spherical aberration caused by the mismatch of the refractive indices between the recording material and its immersion medium on the three-dimensional optical data-storage density in a two-photon bleaching polymer. It is found both theoretically and experimentally that spherical aberration can be compensated for by a change in the tube length at which a microscope objective is operated in recording and reading processes. After compensation for the spherical aberration it is possible to achieve a three-dimensional recording density of 3.5 Tbits/cm(3) for a commercial objective with a numerical aperture of 1.4.

Refractive‐index‐induced aberrations in two‐photon confocal fluorescence microscopy

The effect of refractive index mismatch on the image quality in two‐photon confocal fluorescence microscopy is investigated by experiment and numerical calculations. The results show a strong decrease in the image brightness using high‐aperture objectives when the image plane is moved deeper into the sample. When exciting at 740 nm and recording the fluorescence around 460 nm in a glycerol‐mounted sample using a lens of a numerical aperture of 1·4 (oil immersion), a 25% decrease in the intensity is observed at a depth of 9 μm. In an aqueous sample, the same decrease is observed at a depth of 3 μm. By reducing the numerical aperture to 1·0, the intensity decrease can be avoided at the expense of the overall resolution and signal intensity. The experiments are compared with the predictions of a theory that takes into account the vectorial character of light and the refraction of the wavefronts according to Fermat's principle. Advice is given concerning how the effects can be taken into account in practice.