Phase Retardation Research Articles

Structure and function of retinal ganglion cells in subjects with a history of repeated traumatic brain injury.

This study tested whether repeated traumatic brain injuries (TBIs) alter the objective structure or the objective function of retinal ganglion cells (RGCs) in human subjects recruited from an optometry clinic. Case subjects (n = 25) with a history of repeated TBIs (4.12 ± 2.76 TBIs over 0–41 years) and healthy pair-matched control subjects (n = 30) were prospectively recruited. Retinal nerve fiber layer (RNFL) thickness was quantified with spectral-domain optical coherence tomography, and scanning laser polarimetry measured RNFL phase retardation. Measurements of the photopic negative response were made using full-field flash electroretinography. There was no statistically significant difference (p = 0.42) in global RNFL thickness between the case cohort (96.6 ± 9.4 microns) and the control cohort (94.9 ± 7.0 microns). There was no statistically significant difference (p = 0.80) in global RNFL phase retardation between the case cohort (57.9 ± 5.7 nm) and the control cohort (58.2 ± 4.6 nm). There were no statistically significant differences in the peak time (p = 0.95) of the PhNR or in the amplitude (p = 0.11) of the PhNR between the case cohort (69.9 ± 6.9 ms and 24.1 ± 5.1 μV, respectively) and the control cohort (70.1 ± 8.9 ms and 27.8 ± 9.1 μV, respectively). However, PhNR amplitude was more variable (p < 0.025) in the control cohort than in the case cohort. Within the case cohort, there was a strong positive (r = 0.53), but not statistically significant (p = 0.02), association between time since last TBI and PhNR amplitude. There was also a modest positive (r = 0.45), but not statistically significant (p = 0.04), association between time since first TBI and PhNR amplitude. Our results suggest that there were no statistically significant differences in the objective structure or in the objective function of RGCs between the case cohort and the control cohort. Future large, longitudinal studies will be necessary to confirm our negative results and to more fully investigate the potential interaction between PhNR amplitude and time since first or last TBI.

Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride.

Birefringence is a fundamental optical property that can induce phase retardation of polarized light. Tuning the birefringence of liquid crystals is a core technology for light manipulation in current applications in the visible and infrared spectral regions. Due to the strong absorption or instability of conventional liquid crystals in deep-ultraviolet light, tunable birefringence remains elusive in this region, notwithstanding its significance in diverse applications. Here we show a stable and birefringence-tunable deep-ultraviolet modulator based on two-dimensional hexagonal boron nitride. It has an extremely large optical anisotropy factor of 6.5 × 10-12 C2 J-1 m-1 that gives rise to a specific magneto-optical Cotton-Mouton coefficient of 8.0 × 106 T-2 m-1, which is about five orders of magnitude higher than other potential deep-ultraviolet-transparent media. The large coefficient, high stability (retention rate of 99.7% after 270 cycles) and wide bandgap of boron nitride collectively enable the fabrication of stable deep-ultraviolet modulators with magnetically tunable birefringence.

Polarization Rotator between the Linear Polarization and the Circular Polarization Based on the Layered Photonic Structure

AbstractA polarization rotator is investigated using a layered photonic structure composed of nonlinear media with the Kerr and single negative materials including epsilon‐negative media and mu‐negative media. Since the zero effective phase (expressed as zero‐φeff) bandgap formed by the alternating epsilon‐negative media and mu‐negative media has the characteristics of broadband phase retardation, as well as the nonlinear effect of the defect layer, the reflection phase difference and polarization form can be adjusted by the incident angle and light intensity. Studying the effect of those two parameters on reflectivity, reflection phase difference, and the polarization form under the forward and backward incidence, it is found that different incident directions can coherently tune the polarization form. By adjusting the incident angle and light intensity, the linear polarization can be converted to the left‐handed circular polarization in the forward incidence, and it can be converted to the right‐handed circular polarization in the backward incidence, thereby obtaining two different forms of circular polarization. Therefore, the different handed circularly polarized waves with relatively wide bandwidths can be obtained by changing the incident direction of electromagnetic waves, which provides the basis for tunable polarization selectors.

Quantification method to objectively evaluate the fibrous structural status of tendons based on polarization-sensitive OCT.

Histological analysis is widely used to evaluate injured tendons; however, it has the limitation of being semi-quantitative. Hence, we developed a quantification method to objectively evaluate the fibrous structure of tendons, exhibiting the optical property of birefringence, using polarization-sensitive optical coherence tomography (PS-OCT). We used a partial-rupture rat model in which the middle 0.75 cm of the Achilles tendon was cut with a blade. Rats were sacrificed at 2, 4 or 6 weeks after the injury, and PS-OCT and histological analyzes were performed. The PS-OCT phase retardation images and score well represented the structural changes of the injured tendon according to the wound healing state. Therefore, the proposed novel quantification method using PS-OCT can be used to evaluate the fibrous structural status of tendons.

Mapping of corneal birefringence in thin and asymmetric keratoconus corneas with ultrahigh-resolution polarization-sensitive OCT.

To evaluate phase retardation (PR) across healthy eyes and eyes with thin corneas (<500 μm) and with asymmetric and bilateral keratoconus (KC). Narayana Nethralaya Eye Hospital, Bangalore, India. Observational cross-sectional. There were 4 eye groups: healthy eyes (Group 1; n = 10 eyes), eyes with thin corneas and no clinical disease (Group 2; n = 10 eyes), eyes with asymmetric KC (Group 3; n = 5 eyes), and eyes with clinical KC (Group 4; n = 15 eyes). All eyes were imaged with polarization-sensitive optical coherence tomography (PS-OCT), MS-39, and Corvis-ST. Using PS-OCT, PR was analyzed in annular regions. The anterior (A-E) and Bowman (E-B) wavefront aberrations, epithelial Zernike indices (EZI), total corneal thickness, Corvis biomechanical index (CBI), total biomechanical index (TBI), and Belin-Ambrósio overall deviation index (BAD-D) were analyzed. Only CBI, TBI, BAD-D, A-E and E-B aberrations, EZI, and total corneal thickness distributions of Groups 1 (n =10), 2 (n =10), and 3 (n =5) were similar ( P > .05) but not CCT ( P < .05). PR distributions clearly showed that the eyes in Groups 1, 2, and 3 had a normal corneal birefringence unlike Group 4 (n = 10) eyes ( P < .05). The PR map was similar to the preferred orientations of collagen fibers seen in X-ray diffraction ex vivo studies of corneal stroma. PR distributions may eliminate the uncertainty associated with the stromal status of thin and asymmetric KC corneas. Group 2 and 3 eyes appeared as healthy because of normal corneal birefringence at the time of imaging, but a longitudinal follow-up of these eyes with PS-OCT may assist in early detection of onset of disease.

Birefringence effect studies of collagen formed by nonenzymatic glycation using dual-retarder Mueller polarimetry.

.Significance: Nonenzymatic glycation of collagen covalently attaches an addition of sugar molecules that initially were involved in a reversibly reaction with amino groups on the protein. Due to the ultimate formation of stable irreversible advanced glycation end products, the process of glycation leads to abnormal irreversible cross-linking, which ultimately accumulates with age and/or diabetes in the extracellular matrix, altering its organization.Aim: We report the use of dual-retarder Mueller polarimetry in conjunction with phase retardance to differentiate collagen cross-linking in a normal collagen gel matrix from that in tissues with nonenzymatic cross-linking.Approach: A dual-liquid crystal-based Mueller polarimetry system that involves electronic modulation of polarization state generators (PSGs) was employed to produce all types of polarization states without moving any part and enable detection of the signal directly using a Stokes polarimeter. The linear phase retardance response was obtained for the characterization of the solution and gel forms of collagen using differential Mueller matrix analysis.Results: We found that linear phase retardance measurements via differential Mueller matrix polarimetry successfully differentiated collagen gel matrices with different degrees of cross-linking formed by a nonenzymatic glycation process and demonstrated that this technology constitutes a quick and simple modality.Conclusions: This approach has high sensitivity for studying differences in fibrillar cross-linking in glycated collagen. Further, our work suggests that this method of structural analysis has potential clinical diagnostic value owing to its noninvasive and cost-efficient nature.

Single region substitute models for dual-porosity advection–dispersion migration process based on contaminant distribution

Dual-porosity (DP) model is recommended for describing the complex non-Fickian contaminant transport in soil, while advection–dispersion-equation (ADE) model is the most popular transport model due to its simplicity and convenience. In this study, single region substitute approaches for the DP model were established based on the contaminant distribution in the mobile and immobile phases. A time dependent parameter Ω was used to determine the participation degree of the immobile phase in contaminant transport. The single region substitute (SRS) model was modified by combining the mobile phase retardation factor with the adsorbed concentration of immobile phase. Furthermore, dispersion-modified SRS (DSRS) model was established for calculating the equivalent dispersion coefficient of the coupled mobile-immobile mass transfer and advection via Fourier transformation. Correlation analyses of the modified models shown that the SRS model fits DP well when advection velocity or the mobile-immobile mass transfer rate is relatively slow under both Cauchy and Dirichlet inlet boundary, whereas that the DSRS model is applicable when mobile-immobile mass transfer rate is relatively higher. Both SRS and DSRS models can provide better understanding to the contaminant transport process of DP model.

Amplitude and phase modulation of optical feedback in a-cut Nd:YVO4 laser with a birefringent external cavity

Considering only the self-mixing interference taking place, the modulation effect of optical feedback in a single polarization a-cut Nd:YVO4 laser is presented. The output intensity of the laser with anisotropic gain medium is modulated with a wave plate (WP) in the external cavity. The rotation orientation and phase retardation of the WP regularly determinate the amplitude and phase of laser intensity fringe, because the feedback light component at the same polarization direction as the output laser can interact with the light field in the cavity. The corresponding theoretical model is established and the windowed Fourier transform method is utilized to quantitatively explain this phenomenon. By introducing the amplitude and phase modulation coefficients, comparisons of experimental results with simulation analysis show good consistency.

Three-Dimensional Measurement Of A Stress Field In A Rectangular Channel Flow Using A Photoelastic Method

The hydrodynamic stress field of a steady laminar flow in a rectangular channel was experimentally measured by using a photoelastic method. For this purpose, following two issues for a mixed solution of photoelastic material, cellulose nanoctystal (CNC), were extensively investigated. The first is to elucidate whether there are any phenomena (trends) peculiar to a CNC mixed solution that differ from those to the solid case. The second is to elucidate the extent to which the stress-optic law holds for CNC mixed solution. As a verification method for the first question, we measured retardation distributions by using a high-speed polarization camera. We compared distributions of measured retardation of a CNC mixed solution (0.5 wt %) (Newtonian fluid) and theoretical secondary-principal stress difference derived from the analytical solution of steady lam- inar flow in a rectangular channel. The results showed that a retardation, which was not affected by stress-oriented phase retardation, appeared in the CNC mixed solution. The reason is considered to be retardation induced by aggregations of CNC particles. This phenomenon is peculiar to CNC mixed solution and does not occur in solids cases. As a method of verification of the second issue, we compared the secondary principal stress difference distribution with the retardation distribution offsetting the effect of the retardation specific to the CNC mixed solution. The result showed that the spatial intensity distributions of both agreed within a relative error of 3.84 %. This indicates that the stress-optic law holds for the retardation offsetting the retardation specific to the CNC mixed solution. To evaluate the validity of the above validation results, we measured the retardation fields of flows at different flow rates (10, 20 ml/min). The measured results agree with theory with relative errors 5.67 % and 3.24 % for flow rates of 10 and 20 ml/min, respectively.

Terahertz optical properties and birefringence in single crystal vanadium doped (100) β-Ga2O3

We report the terahertz optical properties of vanadium doped (100) β-Ga2O3 using terahertz time-domain spectroscopy (THz-TDS). The V-doped β-Ga2O3 crystal shows strong birefringence in the 0.2-2.4 THz range. Further, phase retardation by the V-doped β-Ga2O3 has been measured over the whole THz range by terahertz time-domain polarimetry (THz-TDP). It is observed that the V-doped β-Ga2O3 crystal behaves both as a quarter waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at 0.74 and 1.94 THz, respectively.

Faraday Conversion in Pair-symmetric Winds of Magnetars and Fast Radio Bursts

We consider the propagation of polarization in the inner parts of pair-symmetric magnetar winds, close to the light cylinder. Pair plasmas in magnetic field is birefringent, a ∝ B 2 effect. As a result, such plasmas work as phase retarders: Stokes parameters follow a circular trajectory on the Poincare sphere. In the highly magnetized regime, ω, ω p ≪ ω B , the corresponding rotation rates are independent of the magnetic field. A plasma screen with dispersion measure DM ∼ 10−6 pc cm−3 can induce large polarization changes, including large effective rotation measures (RMs). The frequency scaling of the (generalized) RM, ∝ λ α , mimics the conventional RM with α = 2 for small phase shifts, but can be as small as α = 1. In interpreting observations, the frequency scaling of polarization parameters should be fitted independently. The model offers explanations for (i) the large circular polarization component observed in FRBs, with right–left switching; (ii) large RM, with possible sign changes (when the observation bandwidth is small); and (iii) time-dependent variable polarization. A relatively dense and slow wind is needed—the corresponding effect in regular pulsars is small.

Error analysis and correction equations for a real-time phase-shifting method using circular polariscope-based photoelasticity

The use of a polarization camera to achieve real-time phase shifting in circular polariscope-based photoelasticity is limited due to quantitative error analysis and correction. In particular, the primary factor limiting this method is the mismatch between the quarter-wave plate and the wavelength of the light source. In this regard, this study proposes error correction equations for the instantaneous phase-shifting method based on theoretical analysis and simulation of a diametrically stressed disc. First, the error distributions of the stress direction angle and phase retardation are discussed, and the correction equations of the isochromatic and isoclinic lines are deduced. Subsequently, we consider the phase ambiguity problem caused by the phase absolutization. Finally, the error analysis and correction methods are verified experimentally on the diametrically compressed disc.

Calibration of the Soleil–Babinet Compensator Based on the Vectorial Optical Field

The Soleil–Babinet compensator (SBC) is a variable retarder and has been used in a variety of application fields. A scheme based on the vectorial optical field is proposed to calibrate the SBC by transforming the change of the phase retardation into the visible rotation of the petal-like pattern. The relationship between the rotation angle of the petal-like pattern and the phase retardation of the SBC is established theoretically. In the experiment, the vector beam is generated by using the spiral phase plate (SPP) and the modified Mach–Zehnder interferometer based on the superposition principle of two orthogonal circularly polarized vortex beams with opposite topological charges. Taking advantage of the image processing method, the rotation angles of the acquired petal patterns are calculated, and the relationship between the phase retardation of the SBC and the displacements of its micrometer screw is determined. The measured phase retardation of the SBC ranges from −277.00° to 516.57°. By linearly fitting the experimental data, the phase sensitivity is 33.076 ± 0.147 °/mm, and the coefficient of determination value that shows the linearity of the experimental data is 0.9995. The experimental results agree well with the theoretical data.

Simultaneous measurement of phase retardation and fast axis azimuth of wave plate based on equivalent component and phase detection

Wave plates (WPs) are key components in optical polarization systems whose phase retardations and fast axis azimuths should be determined accurately. At present many measurement methods are based on light intensity detection which is inevitably affected by light intensity fluctuations so that the measurement accuracy is limited. What’s more, quite a few methods cannot measure WPs’ fast axis azimuths simultaneously. In this paper a method of simultaneously measuring the phase retardation and fast axis azimuth of arbitrary WP is proposed, which is based on equivalent component and phase detection. A rotatable half wave plate (HWP) and a retroreflector are used and its effect is equivalent to measuring an equivalent WP whose phase retardation is twice that of the measured wave plate, which is equivalent to doubling the measurement resolution. Phase detection is used to process the signals which means the measurement accuracy is better than that by usual light intensity detection. What’s more, the presented method eliminates the angle positioning errors of birefringent components in principle which exists in many present WP measurement methods. Finally, the measurement system setup is simple as well as the measurement process. The measurement formulae are deduced and corresponding WP measuring system is established. The error analysis shows that the system measurement uncertainty is about 3.9′ for the phase retardation and 5” for the fast axis azimuth. Experiment results and comparisons of quarter WPs and HWPs show that the presented method is in good agreement with other method. The phase retardation measurement repeatability is also good with a standard deviation about 2′.

Interactions between Plasmonic Nanoantennas and Vortex Beams.

The orbital angular momentum (OAM) of light offers a new degree of freedom for light-matter interactions, yet how to control such interactions with this physical dimension remains open. Here, by developing a numerical method enabling optical OAM simulations, we provide insights into complex plasmon behaviors with the physical dimension of OAM, and we prove in theory that plasmonic nanostructures can function as efficient antennas to intercept and directionally reradiate the power of OAM beams. The interplay between optical OAM and spin angular momentum (SAM) generates novel particle polarizations and radiations, which were inaccessible before. For arrayed nanoparticles, coherent surface plasmons with specific phase retardations determined by OAM of the beams enable directional power radiations, making a phased array antenna. These findings expand our knowledge of nanoplasmonics in the OAM area and are promising for quantum information processing and dynamic sensing of ultraweak biosignals.

Heterogeneous and retarded phase transformation of ferrihydrite on montmorillonite surface: The important role of surface interactions

Abstract The formation of heteroaggregates is critical to controlling the stabilization and transformation of nanominerals and mineral nanoparticles (NMMNs) in nature, but the underlying mechanisms remain to be deciphered. In this work, we study the effect of surface interactions between ferrihydrite (Fh) and montmorillonite (Mnt) within their heteroaggregates on the transformation behaviors of Fh. A series of heteroaggregates composed of Fh and Mnt were synthesized by modulating their mass ratios and synthesis methods, i.e., directly complexing Fh with Mnt (Fh-Mnt) or in situ growing Fh on Mnt (Fh/Mnt). Structural characterization using XRD, TG-DSC, TEM, and FTIR indicated that Fh particles coated more evenly on the Mnt surface within the heteroaggregates synthesized by in situ growing Fh on Mnt and with lower Fh to Mnt ratio, and accordingly these heteroaggregates showed stronger surface interactions between Fh and Mnt. The phase transformation of Fh to hematite (Hem) on the heteroaggregates can be significantly affected during the heating treatment. Compared with that of pure Fh, the transformation of Fh on all of the heteroaggregates was retarded (e.g., slower transformation rate and smaller produced Hem particles), particularly for the samples with stronger surface interactions (e.g., Fh/Mnt with lower Fh to Mnt ratio). Noticeably, the heated heteroaggregates may simultaneously contain pristine Fh, intermediate maghemite, and transformed Hem, showing a heterogeneous transformation behavior of Fh. The strong interactions between Fh and Mnt will enhance the dispersion of Fh and restrict the structural rearrangement of Fh (particularly those at the interface) during the phase transformation process, resulting in retarded and heterogenous transformation of Fh on these heteroaggregates. These findings not only enrich our knowledge of the phase transformation characteristics of Fh but also advance our understanding of the important role of mineral surface interactions in stabilizing NMMNs in nature.

Scalable Fabrication of Nanogratings on GaP for Efficient Diffraction of Near-Infrared Pulses and Enhanced Terahertz Generation by Optical Rectification

We present a process flow for wafer-scale fabrication of a surface phase grating with sub-micron feature sizes from a single semiconductor material. We demonstrate this technique using a 110-oriented GaP semiconductor wafer with second-order nonlinearity to obtain a nanostructured device (800 nm lateral feature size and a 245 nm height modulation) with applications relevant to near-infrared optical diffraction and time-resolved terahertz (THz) technologies. The fabrication process involves a plasma-enhanced chemical deposition of a SiO2 layer on the wafer followed by contact photolithography and inductively coupled plasma reactive ion etching (ICP-RIE). We discuss the required radiation dosage, exposure times, temperatures and other key parameters to achieve high-quality nanogratings in terms of filling ratio, edge profile, and overall shape. The phase-grating properties, such as the pitch, spatial homogeneity, and phase retardation, are characterized with an atomic force microscope, scanning electron microscope and a non-invasive optical evaluation of the optical diffraction efficiency into different orders. We demonstrate an application of this device in a time-domain THz spectroscopy scheme, where an enhanced THz spectral bandwidth is achieved by optical rectification of near-infrared laser pulses incident on the grating and efficiently diffracted into the first orders. Finally, the reported process flow has the potential to be applied to various materials by considering only slight adjustments to the ICP-RIE etching steps, paving the way to scalable fabrication of sub-micron patterns on a large range of substrates.

Generation of hollow Gaussian beams by restoring structured light with meta-optics

Hollow Gaussian beams (HGBs) with annular intensity distributions but uniform phase and polarization distributions have aroused enormous interest due to their advantages in optical trapping, optical limiting, etc. Although extensive HGB generation methods have been performed previously, there are still existing limitations in polarization independence and broadband response, which hinder the application of HGBs. Here, we propose and experimentally demonstrate an HGB generation method by restoring structured light with a pair of cascaded dielectric metasurface q-plates. The metasurface q-plates are based on Pancharatnam-Berry (PB) phase and work as elements of optical spin-to-orbital angular momentum conversion. The first metasurface q-plate is used to introduce orbital angular momentum to an incident circularly polarized beam, and the output light field possesses both annular intensity distribution and spiral phase with reversed circular polarization. Since the phase retardation of the metasurface q-plates isπ, the reversed circularly polarized beam output from the first metasurface q-plate will obtain opposite spiral phase after the second metasurface q-plate. Then, the spiral phase of the output light field is counteracted, but its annular intensity distribution is retained as a new HGB. Arbitrarily polarized Gaussian beams are available for incident beams because they can be decomposed into orthogonal circularly polarized components. Furthermore, this method is applicable to a broad spectrum of visible light owing to the dispersion-free characteristic of the PB phase. This work suggests a polarization-independent and broadband HGB generation method, which may benefit HGB-based fundamental research.

Channeled spectropolarimeter with arbitrary retarder orientation settings.

A channeled spectropolarimeter can simultaneously obtain intensity, spectral, and polarization information. In the traditional model, the retarders must be oriented at specific angles. However, misalignments of the retarders are inevitable during assembly, and the status of the retarders is sensitive to environmental perturbations, which affects the performance of the channeled spectropolarimeter. In this study, a general channeled spectropolarimeter model was derived, in which the retarder orientations can be arbitrary and unknown. Meanwhile, the system is unaffected by environmental perturbation because it can self-calibrate to avoid fluctuations in the retarder orientations and phase retardations. The effectiveness and robustness of the model were verified through simulations and experiments.

Foveal Phase Retardation Correlates With Optically Measured Henle Fiber Layer Thickness.

This study quantified and compared phase retardation distribution in the central macula with the thickness of the Henle fiber layer (HFL). A scanning laser polarimeter (SLP) was used to acquire 20° × 40° macular-centered images, either with fixed corneal compensation or with variable corneal compensation, in two cohorts of clinically normal subjects (N = 36). Phase retardation maps from SLP imaging were used to generate a macular cross pattern (fixed compensation) or an annulus pattern (variable compensation) centered on the macula. Intensity profiles in the phase retardation maps were produced using annular regions of interest at eccentricities from 0.25° to 3°. Pixel intensity was averaged at each eccentricity, acting as a surrogate for macular phase retardation. Directional OCT images were acquired in the horizontal and vertical meridians in all subjects, allowing visualization of the HFL thickness. HFL thickness was manually segmented in each meridian and averaged. In both cohorts, phase retardation and HFL thickness were highly correlated in the central 3° assessed, providing further evidence that the source of the phase retardation signal in the central macula is dominated by the HFL and that the center of the macula on cross sectional imaging corresponds closely with the center of the macular cross on SLP imaging.