Birefringence Changes Research Articles

The piezo-optical effect in Na0.5Bi0.5TiO3 crystals

The work reports some aspects of the effect of applied axial pressure on birefringence in Na0.5Bi0.5TiO3 crystals, which have peculiarities of the macroscopic properties in a large extent due to the coexistence of nanoregions of different phases in a wide temperature range. The birefringence ∆n is investigated by the polarization-optical method in temperature range 25–500 °C. The temperature behavior of spontaneous birefringence in single domain crystals is associated with the structure phase transitions and the “isotropization point” and depend on the way used to switch the sample into the single domain state. Also, the influence of axial pressure on ∆n of samples preliminary switched to the single domain state is studied. It is discovered that application of the axial pressure (6.25–16.6 bar) along [100] direction does not show significant changes of birefringence in temperature range 25–500 °C. While the application of axial pressure (2.5–25 bar) along [110] direction leads to exhibition of significant piezo-optical effect. It is supposed that both the presence of polar nanoregions and changes of their orientation make contribution to the piezo-optical effect.

Strain-optical behavior of polyethylene terephthalate film during uniaxial stretching investigated by Mueller matrix ellipsometry

Polyethylene terephthalate (PET) has gained widespread applications in various flexible devices where its optical properties after drawing process in manufacturing play a great importance in the performance of devices. In this work, the strain-optical behavior of pre-oriented polyethylene terephthalate (PET) film during uniaxial stretching is studied utilizing a proposed anisotropic model and Mueller matrix ellipsometry (MME). In the biaxial anisotropic model, the molecular chain orientation is correlated to the change of refractive indices and strain-induced birefringence, while the rotation of molecular chains is directly characterized by the rotation of principal optical axis. According to this model, the refractive indices, extinction coefficients and Euler angles extracted from measured Mueller matrix spectra can be used to interpret the micro-structural response of PET. A set of tensile tests have been carried out, with loading direction being varied. The results show that the strain-induced birefringence is linearly correlated to strain, while loading direction significantly affects the slope of achieved birefringence-strain curve. Meanwhile, the birefringence exhibits a positive linear relation with the orientation estimated by extinction coefficients. Through the analysis of the Euler angle ϕE, the principal optical axis rotating towards the tensile direction is accurately observed. Our results show that MME can provide a deep and systematic understanding of strain-optical behavior which can give accurate control of the optical properties of polymer films during deformation.

COMPARISON OF IMMUNOHISTOCHEMICAL MARKER WITH SPECIAL STAINS IN THE PROGNOSIS OF THE DIFFERENT GRADES OF ORAL SQUAMOUS CELL CARCINOMA

Objectives: The aim of the study is to compare the efficiency of Ki-67 with special stains in the stroma of different grades of Oral Squamous Cell Carcinoma (OSCC) and to evaluate the influence of these changes in predicting the prognosis of these tumors.
 Materials and Methods: A total of 30 cases of different grades of Oral Squamous Cell Carcinoma and 6 cases of control were sections and stained with Picrosirus red (PR), combined Alcian blue-periodic acid Schiff reagent (AB-PAS) and Immunohistochemical (IHC) marker Ki-67.
 Results: Collagen fibre nature using PR stain and proliferative activity of malignant epithelial cells using IHC marker Ki-67 was found to be statistically significant. Mucin presence using AB-PAS was statistically insignificant.
 Conclusion: Prognosis in different grades of oral squamous cell carcinoma can be accessed by change in collagen fibres birefringence, as the tumour progresses there is change from mature to immature collagen. Ki-67 is a good proliferative marker and shows that there is positive correlation with histological grading of oral squamous cell carcinoma. Presence of acidic or neutral mucin in OSCC needed to be further studied. For assessing the prognosis in different grades of OSCC, special stains can also be used.
 Keywords: Collagen, Mucin, Squamous cell carcinoma, Ki-67.

Microsecond Electro-Optic Switching of Nematic Liquid Crystals with Giant Dielectric Anisotropy

Nematic liquid crystals exhibit a fast optical response when the applied electric field modifies the degree of order but does not change the direction of molecular orientation. The effect requires a relatively high electric field, on the order of 10^8 V/m for a field induced birefringence change of 0.01. To address this detrimental issue, this work explores electrically induced modification of the order parameter in a material with a giant dielectric anisotropy of +200. A relatively weak field 3X10^7 V/m causes a significant change of birefringence, by about 0.04. Both switching on and off times are ~10 microseconds. The effect is called a microsecond electrically modified order parameter (MEMOP) and can be used in electro-optical devices, such as fast electro-optic shutters, phase modulators, and beam-steerers that require microsecond response times.

Study of the Acoustoelastic Effect in an Anisotropic Plastically Deformed Material

The paper discusses the dependence of shear elastic waves birefringence on applied mechanical stress and preliminary plastic deformation of a material. A technique involving precision acoustic pulse echo measurements and software processing of the received time scan of signals from a piezoelectric transducer is described, which makes it possible to determine the propagation time of elastic waves with an accuracy of at least 0.5 ns. The dependences of the intrinsic acoustic birefringence and the acoustoelastic coefficient on plastic deformation in a carbon steel sample are investigated. After 12% plastic deformation of the sample, the average value of the acoustoelastic coefficient changes by 30%; the average value of the intrinsic acoustic birefringence changes by 60%. A correlation is observed between the intrinsic acoustic birefringence and the acoustoelastic coefficient. Using the theory of elastic wave propagation in a solid, the obtained experimental data are explained by manifestation of the acoustoelastic effect and the influence of the structural state on the effective elastic properties. The effect of plastic deformation on the accuracy of determining stress is qualitatively assessed. The error in determining stress during 1% plastic deformation is about 40 MPa, i.e., 13% of the yield strength for this material in the initial state.

Optical fiber magnetic field sensor based on birefringence in liquid core optical waveguide

An optical fiber magnetic field sensor based on the birefringence of magnetic fluid is proposed by filling the magnetic fluid into a hollow-core waveguide. The birefringence characteristics of magnetic fluids with different concentrations tuned by magnetic field were investigated under different magnetic fields when the filled hollow-core waveguide is placed in a Sagnac loop. As the magnetic field increases, the internal chain-like structure of the magnetic fluid increases, resulting in changes in birefringence. The experimental results show that the device has a highly linear response to the variation of magnetic field intensity in the range of 0–300 Oe with the maximum sensitivity of 29.2 pm/Oe and resolution of 0.68 Oe. The proposed magnetic field sensor has such advantages as ease of fabrication, simple structure, low cost and it is expected to find potential applications in the magnetic field measurement with high accuracy, optical fiber gyroscopes, magneto-optic modulators and so on. Due to the external field tunability, the device also provides a promising technology for microfluidics research.

Long-term transmission of entangled photons from a single quantum dot over deployed fiber

Entangled light sources are considered as core technology for multiple quantum network architectures. Of particular interest are sources that are based on a single quantum system as these offer intrinsic security due to the sub-Poissonian nature of the photon emission process. This is important for applications in quantum communication where multi-pair emission generally compromises performance. A large variety of sources has been developed, but the generated photons remained far from being utilized in established standard fiber networks, mainly due to lack of compatibility with telecommunication wavelengths. In this regard, single semiconductor quantum dots are highly promising photon pair sources as they can be engineered for direct emission at telecom wavelengths. In this work we demonstrate the feasibility of this approach. We report a week-long transmission of polarization-entangled photons from a single InAs/GaAs quantum dot over a metropolitan network fiber. The photons are in the telecommunication O-band, favored for fiber optical communication. We employ a polarization stabilization system overcoming changes of birefringence introduced by 18.23 km of installed fiber. Stable transmission of polarization-encoded entanglement with a high fidelity of 91% is achieved, facilitating the operation of sub-Poissonian quantum light sources over existing fiber networks.

Polarization Oscillations in Birefringent Emitter-Cavity Systems.

We present the effects of resonator birefringence on the cavity-enhanced interfacing of quantum states of light and matter, including the first observation of single photons with a time-dependent polarization state that evolves within their coherence time. A theoretical model is introduced and experimentally verified by the modified polarization of temporally long single photons emitted from a ^{87}Rb atom coupled to a high-finesse optical cavity by a vacuum-stimulated Raman adiabatic passage process. Further theoretical investigation shows how a change in cavity birefringence can both impact the atom-cavity coupling and engender starkly different polarization behavior in the emitted photons. With polarization a key resource for encoding quantum states of light and modern micron-scale cavities particularly prone to birefringence, the consideration of these effects is vital to the faithful realization of efficient and coherent emitter-photon interfaces for distributed quantum networking and communications.

High Sensitivity Polarimetric Optical Fiber Pressure Sensor Based on Tapered Polarization-Maintaining and Fiber Bragg Grating.

In this paper, we propose a high sensitivity polarimetric optical fiber pressure sensor (OFPS) using a polarization-diversity loop composed of a polarization beam splitter, polarization controllers, and a sensor head. The sensor head consists of 8-cm-long tapered panda-type polarization-maintaining fiber (PMF) and a fiber Bragg grating (FBG) directly spliced with PMF, and the sensor head is located inside a pressure chamber. A pressure-induced birefringence change due to the photoelastic effect can be greatly enhanced at the tapered section of PMF, thereby increasing the pressure sensitivity of the sensor head. The tapered PMF was fabricated using a fusion splicer, and the tapered length and center waist diameter of the tapered PMF segment were ~350 and ~56.82 μm, respectively. At the polarization-diversity loop, PMF is used as a birefringent element to create an interference spectrum due to polarization interference. A pressure-induced birefringence change of PMF results in a wavelength shift of the interference spectrum. Because the PMF birefringence also has a cross sensitivity to temperature, the FBG is utilized for the compensation of the temperature effect on it. The resonance wavelength of the FBG is sensitive to ambient temperature changes but insensitive to changes in pressure. This spectral response of the FBG can be used to compensate additional ambient temperature changes occurred at the sensor head. The pressure sensitivity of our sensor was measured as approximately -27.70 nm/MPa, and an adjusted R² value representing the sensor linearity was measured as ~0.9903 in a measurement range of 0-0.5 MPa. Our fabricated sensor exhibits the highest pressure sensitivity among previously reported polarimetric OFPS.

Molecular orientational distribution function of a chiral de Vries smectic liquid crystal from birefringence measurements.

An alternative method for determining the orientational distribution function and the order parameter from the electric field-induced birefringence measurements of a chiral liquid crystal compound in its Smectic A* is being introduced. A chiral mesogen based on a 5-phenyl-pyrimidine benzoate core terminated by a trisiloxane group on one side and the chiral alkyloxy chain on its opposite side is designed and synthesized to exhibit the "de Vries" smectic characteristics. The compound exhibits first order Smectic A*-Smectic C* phase transition, evidenced by the results of differential scanning calorimetry. The material is being investigated by electro-optical experiment in its smectic phases. We present a model that incorporates the generalised Langevin-Debye model which includes the Maier-Saupe effective mean-field potential term in order to explain the change in birefringence with the electric field. A good agreement between the experimental results and the predictions from the model leads to the determination of the molecular orientational distribution function in Smectic A phase. Furthermore, the temperature dependency of the Saupe orientational order parameter ⟨P2⟩ is obtained using the parameters of the model. Based on the experimental and theoretical results, we show that de Vries Smectic A* phase exhibits a broad volcano-like tilt angle distribution with the two maxima occurring at finite tilt angles closer to the Smectic A*-Smectic C* transition temperature, and a sugarloaf-like distribution occurs in the tilt for temperatures close to the Isotropic-Smectic A* phase transition.

Beat frequency tuning in dual-polarization distributed feedback fiber laser using side polishing technique.

We propose and demonstrate a method for tuning the beat frequency of a dual-polarization distributed feedback (DFB) fiber laser via fiber side polishing. This process can significantly alter the birefringence in DFB fiber lasers. Beat frequency evolutions in DFB fiber lasers were investigated, and the experimental results showed that the beat frequency tuning was dependent on polished thickness, roughness, and direction. The abrasive paper with a grain size of 1.8 µm was adopted to fine-tune the beat frequency. It was found that the beat frequency of DFB fiber lasers shifted toward higher frequencies with increasing polished thickness. However, the beat frequency shifted toward lower frequencies using a secondary side polishing process in the direction orthogonal to the first polished surface. As a result, the beat frequency of the DFB fiber laser was tuned in a wide frequency range from 475.5 MHz to 2080.4 MHz, which corresponds to a birefringence change of 1.2 × 10-5. Side-polished DFB fiber lasers could provide a novel approach to frequency division multiplexing for a large number of fiber laser sensors.

Critical behavior of the optical birefringence at the nematic to twist-bend nematic phase transition

The critical behavior at the transition from uniform nematic to twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.

Investigation of Multiwavelength Laser Performance based on Temperature Variation of PMF and different SOAs

We demonstrated a multiwavelength performance based on bidirectional Lyot filter at different temperature of polarization maintaining fiber (PMF) and semiconductor optical amplifiers (SOAs). The multiwavelength fiber laser (MWFL) based on bidirectional Lyot filter is not investigated thoroughly on its channel spacing tunability due to birefringence change. A channel spacing of this MWFL is tunable due to birefringence change of the PMF. The birefringence value of is changed by heating the PMF, that leads to narrower channel spacing. From the experimental data, the temperature coefficient based on PMF length of 53.2 m and 10.6 m is 0.49 ×10 -3 nm/°C and 1.35 ×10 -3 nm/°C, respectively, thus shorter PMF is more sensitive to temperature. We also investigated the multiwavelength performance at different SOAs. When the SOA from Qphotonics is changed to the SOA from Alphion, the extinction ratio is reduced from 15 dB to 8 dB. In terms of flatness value, the Qhotonics’s SOA has a flatter multiwavelength spectrum with only 1 dB of peak power difference from 1538 nm until 1541 nm as compared to 2.5 dB when using Alphion’s SOA.

Polymorphism in Simvastatin: Twinning, Disorder, and Enantiotropic Phase Transitions.

Simvastatin is one of the most widely used active pharmaceutical ingredients for the treatment of hyperlipidemias. Because the compound is employed as a solid in drug formulations, particular attention should be given to the characterization of different polymorphs, their stability domains, and the nature of the phase transitions that relate them. In this work, the phase transitions delimiting the stability domains of three previously reported simvastatin forms were investigated from structural, energetics, and dynamical points of view based on single crystal X-ray diffraction (SCXRD), hot stage microscopy (HSM), and differential scanning calorimetry (DSC) experiments (conventional scans and heat capacity measurements), complemented with molecular dynamics (MD) simulations. Previous assignments of the crystal forms were confirmed by SCXRD: forms I and II were found to be orthorhombic ( P212121, Z'/ Z = 1/4) and form III was monoclinic ( P21, Z'/ Z = 2/4). The obtained results further indicated that (i) the transitions between different forms are observed at 235.9 ± 0.1 K (form III → form II) and at 275.2 ± 0.2 K (form II → form I) in DSC runs carried out at 10 K min-1 and close to these values when other types of techniques are used (e.g., HSM). (ii) They are enantiotropic (i.e., there is a transition temperature relating the two phases before fusion at which the stability order is reversed), fast, reversible, with very little hysteresis between heating and cooling modes, and occur under single crystal to single crystal conditions. (iii) A nucleation and growth mechanism seems to be followed since HSM experiments on single crystals evidenced the propagation of an interface, accompanied by a change of birefringence and crystal contraction or expansion (more subtle in the case of form III → form II), when the phase transitions are triggered. (iv) Consistent with the reversible and small hysteresis nature of the phase transitions, the SCXRD results indicated that the molecular packing is very similar in all forms and the main structural differences are associated with conformational changes of the "ester tail". (v) The MD simulations further suggested that the tail is essentially "frozen" in two conformations below the III → II transition temperature, becomes progressively less hindered throughout the stability domain of form II, and acquires a large conformational freedom above the II → I transition. Finally, the fact that these transitions were found to be fast and reversible suggests that polymorphism is unlikely to be a problem for pharmaceutical formulations employing crystalline simvastatin because, if present, the III and II forms will readily convert to form I at ambient temperature.

Excitation of multiple phonon modes in copper metaborate CuB2O4 via nonresonant impulsive stimulated Raman scattering

Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show that the agreement between the two sets of data obtained by these allied techniques in a wide phonon frequencies range of 4-14 THz can be achieved by rigorously taking into account the symmetry of the phonon modes, and the corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We demonstrate analytically that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments, limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes.

Temperature Stability of a Hybrid Polarization-Maintaining Photonic Crystal Fiber Resonator and Its Application in a Resonant Fiber Optic Gyro.

A fiber ring resonator (FRR) constructed using a Panda polarization-maintaining fiber does not effectively solve the problem of temperature-related polarization fluctuation, which considerably limits the detection accuracy of the resonant fiber optic gyro. The polarization-maintaining photonic crystal fiber (PM-PCF) can improve the thermal stability of the FRR. In this study, a structure that can simultaneously detect the polarization fluctuation of two FRRs is designed. We analyzed and verified the polarization phase shift errors of these two types of fibers, which are caused by the thermally induced birefringence changes. Theoretical simulation and experimental results confirm that a PM-PCF can be used to optimize the FRR, which can effectively suppress the polarization fluctuation.

A Dual-Parameter Sensor Based on the Asymmetry of Alcohol Filling the Photonic Crystal Fiber in Sagnac Loop

This paper proposes a novel temperature and stress sensor based on the asymmetric filling of alcohol into the photonic crystal fiber in a Sagnac loop. First, we etched the photonic crystal fiber and collapsed the air. Then, we filled alcohol into the asymmetric photonic crystal fiber. Next, we connected this structure into a Sagnac interference ring. When the ambient temperature changed, the thermal expansion effect of filled alcohol would lead to the change of birefringence of photonic crystal fiber, which made the interference spectrum of the sensor drift with the change of temperature. Furthermore, after adding transverse stress to the photonic crystal fiber, the fiber’s strain changed. With its strain changed, the birefringence of photonic crystal fiber also changed, which made the interference spectrum drift. The experimental results showed that when the temperature or the lateral stress is added, a red shift would occur in the interference spectrum. The temperature sensitivity was found to be 0.1864 nm/°C, and the lateral stress sensitivity was 0.6201 nm/N. The sensor achieved the highly sensitive measurement of temperature and stress. Concurrently, the structure has the advantages of high stability, anti-electromagnetic interference, and easy construction, and it provides a new method for obtaining birefringence of ordinary photonic crystal fiber. In addition, an application system of the sensor has been built. Monochromatic light source with 1554-nm wavelength was used in the experiment. The drift of interference spectrum is transformed into the change of light intensity. The relative error is less than 3%, which indicates a good application prospect.

Single-Shot Optical Anisotropy Imaging with Quantitative Polarization Interference Microscopy.

Optical anisotropy measurement is essential for material characterization and biological imaging. In order to achieve single-shot mapping of the birefringence parameters of anisotropic samples, a novel polarized light imaging concept is proposed, namely quantitative polarization interference microscopy (QPIM). QPIM can be realized through designing a compact polarization-resolved interference microscopy system that captures interferograms bearing sample's linear birefringence information. To extract the retardance and the orientation angle maps from a single-shot measurement, a mathematical model for QPIM is further developed. The QPIM system is validated by measuring a calibrated quarter-wave plate, whose fast-axis orientation angle and retardance are determined with great accuracies. The single-shot nature of QPIM further allows to measure the transient dynamics of birefringence changes in material containing anisotropic structures. This application is demonstrated by capturing transient retardance changes in a custom-designed parallel-aligned nematic liquid crystal-based device.

Comparison of intensity, phase retardation, and local birefringence images for filtering blebs using polarization-sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PS-OCT) allows the recording of depth-resolved polarimetric measurements. It has been reported that phase retardation and local birefringence images can noninvasively detect fibrotic area in blebs after glaucoma surgery. Evaluation of scar fibrosis in blebs is important not only for predicting bleb function, but also for planning revision trabeculectomy. Herein, we characterize the intensity, phase retardation, and local birefringence images of blebs using PS-OCT. A total of 85 blebs from 85 patients who had undergone trabeculectomy were examined. Both phase retardation and local birefringence images detected fibrotic changes in blebs after glaucoma surgery. Phase retardation images detected slight fibrotic change during the early stage after surgery, whereas local birefringence images showed localized fibrotic tissue. There are two main patterns of local birefringence image changes in blebs: plate-like birefringence changes and diffuse changes. The area of plate-like birefringence change was significantly larger in poorly functioning blebs and is thus correlated with bleb function. These data suggest that the plate-like fibrotic change evaluation by PS-OCT may be useful not only for noninvasive evaluation of fibrotic scar tissue in blebs, but also for developing strategies for revision trabeculectomy.

Interaction of preservation methods and radiation sterilization in human skin processing, with particular insight on the impact of the final water content and collagen disruption. Part I: process validation, water activity and collagen changes in tissues cryopreserved or processed using 50, 85 or 98% glycerol solutions.

Current regulatory requirements demand an in-depth understanding and validation of protocols used in tissue banking. The aim of this work was to characterize the quality of split thickness skin allografts cryopreserved or manufactured using highly concentrated solutions of glycerol (50, 85 or 98%), where tissue water activity (aw), histology and birefringence changes were chosen as parameters. Consistent aw outcomes validated the proposed processing protocols. While no significant changes in tissue quality were observed under bright-field microscopy or in collagen birefringence, in-process findings can be harnessed to fine-tune and optimize manufacturing outcomes in particular when further radiation sterilization is considered. Furthermore, exposing the tissues to 85% glycerol seems to derive the most efficient outcomes as far as aw and control of microbiological growth.