Birefringent Structures Research Articles

Airy beams generated by ultrafast laser-imprinted space-variant nanostructures in glass.

We demonstrate a technique to generate accelerating Airy beams with a femtosecond laser-imprinted space variant birefringent structure in silica glass. Our approach enables the generation of dual Airy beams with polarization sensitive beam deflection. The produced beam is used for the glass scribing. After the glass-breaking process, a spontaneous self-detachment of a fiber-like structure occurs that can be exploited as an alternative way for fabricating glass cantilevers.

Birefringent phononic structures

Within the framework of elastic anisotropy, caused in a phononic crystal due to low crystallographic symmetry, we adopt a model structure, already introduced in the case of photonic metamaterials, and by analogy, we study the effect of birefringence and acoustical activity in a phononic crystal. In particular, we investigate its low-frequency behavior and comment on the factors which determine chirality by reference to this model.

An electrically-stabilized liquid-crystalline phase: origin and application.

An azobenzene liquid-crystalline compound possessing two chiral centres at both peripheral ends of the molecular structure exhibits electric-field-induced birefringence in the isotropic liquid phase, which was found to be attributable to the stabilization of the liquid-crystalline organization by the emergence of the polar ferroelectric molecular ordering. The optically isotropic texture changes into the homogeneous birefringent texture by the application of the in-plane electric field, which is a new type of switching mode applicable for the liquid crystal displays. The resulting birefringence can be erased by the irradiation of UV light, due to the photoinduced isomerization of the azobenzene compound, thus a dual controlled birefringent structure, by the irradiation of light and/or by the application of the electric field, is reported for the first time.

Changes of birefringence and fine structure tensor in KSc(MoO4)2 induced by ferroelastic phase transitions

Phase transitions in KSc(MoO4)2 doped with Cr3+ are studied by the electron paramagnetic resonance and birefringence techniques in the temperature range covering the three ferroelastic phases. The study is aimed at showing in what way a morphic birefringence in the ferroelastic phases in KSc(MoO4)2 may relate to the deformation of the coordination surroundings of Sc3+ ion seen in this crystal via changes of the fine structure tensor D. Based on the motion of oxygen atoms of the MoO4 groups constituting the environment of Sc3+ions, a new model of the crystal structure deformation on transition from the ferroelastic phase II to the phase III is built. This model accounts for the morphic birefringence and changes in the orientation of the optical indicatrix axes in KSc(MoO4)2.

Influence of diabetes mellitus on tissue response to MTA and its ability to stimulate mineralization.

To evaluate the influence of diabetes mellitus on the tissue response to mineral trioxide aggregate (MTA) and its ability to stimulate mineralization. Twenty-four male Wistar rats were divided into a non-diabetic control group and another with Alloxan-induced diabetes. Two polyethylene tubes, one kept empty as a control and the other containing Angelus MTA(®) , were implanted into the dorsal connective tissue of all rats for 30 days. Animals in each group received injections of calcein, alizarin, and oxytetracycline on day 7, 14, and 21, respectively. Animals were killed after 30 days; specimens were prepared by staining with hematoxylin and eosin (HE) and von Kossa technique as well as for examination of unstained sections with polarized light and fluorescence microscopy. The inflammatory reaction to the implanted tubes was equally mild in both groups. Structures staining with von Kossa were seen in response to Angelus MTA(®) , as were birefringent structures visualized on polarized light analysis; these had no relation to diabetic condition (P < 0.05). Fluorescence intensity was not changed in diabetic rats either (P < 0.05). Diabetes mellitus did not influence the tissue response to Angelus MTA(®) or the mineralization stimulated by it.

Condenser-free contrast methods for transmitted-light microscopy.

Phase contrast microscopy allows the study of highly transparent yet detail-rich specimens by producing intensity contrast from phase objects within the sample. Presented here is a generalized phase contrast illumination schema in which condenser optics are entirely abrogated, yielding a condenser-free yet highly effective method of obtaining phase contrast in transmitted-light microscopy. A ring of light emitting diodes (LEDs) is positioned within the light-path such that observation of the objective back focal plane places the illuminating ring in appropriate conjunction with the phase ring. It is demonstrated that true Zernike phase contrast is obtained, whose geometry can be flexibly manipulated to provide an arbitrary working distance between illuminator and sample. Condenser-free phase contrast is demonstrated across a range of magnifications (4–100×), numerical apertures (0.13–1.65NA) and conventional phase positions. Also demonstrated is condenser-free darkfield microscopy as well as combinatorial contrast including Rheinberg illumination and simultaneous, colour-contrasted, brightfield, darkfield and Zernike phase contrast. By providing enhanced and arbitrary working space above the preparation, a range of concurrent imaging and electrophysiological techniques will be technically facilitated. Condenser-free phase contrast is demonstrated in conjunction with scanning ion conductance microscopy (SICM), using a notched ring to admit the scanned probe. The compact, versatile LED illumination schema will further lend itself to novel next-generation transmitted-light microscopy designs. The condenser-free illumination method, using rings of independent or radially-scanned emitters, may be exploited in future in other electromagnetic wavebands, including X-rays or the infrared.

Fiber-based polarization-sensitive OCT of the human retina with correction of system polarization distortions

In polarization-sensitive optical coherence tomography (PS-OCT) the use of single-mode fibers causes unpredictable polarization distortions which can result in increased noise levels and erroneous changes in calculated polarization parameters. In the current paper this problem is addressed by a new Jones matrix analysis method that measures and corrects system polarization distortions as a function of wavenumber by spectral analysis of the sample surface polarization state and deeper located birefringent tissue structures. This method was implemented on a passive-component depth-multiplexed swept-source PS-OCT system at 1040 nm which was theoretically modeled using Jones matrix calculus. High-resolution B-scan images are presented of the double-pass phase retardation, diattenuation, and relative optic axis orientation to show the benefits of the new analysis method for in vivo imaging of the human retina. The correction of system polarization distortions yielded reduced phase retardation noise, and better estimates of the diattenuation and the relative optic axis orientation in weakly birefringent tissues. The clinical potential of the system is shown by en face visualization of the phase retardation and optic axis orientation of the retinal nerve fiber layer in a healthy volunteer and a glaucoma patient with nerve fiber loss.

Design and fabrication of birefringent nano-grating structure for circularly polarized light emission

Three different nano-grating structures are designed as phase retarders that can transform linearly polarized light to circularly polarized emission for the wavelengths of 488 nm, 532 nm and 632.8 nm, respectively. Gold based nano-grating structures with various periods are fabricated by utilizing laser interference lithography. The ellipticity of all circularly polarized emission can reach around 90% such that the structure has great potential in the applications of three-dimensional (3D) display. The effects of the slit width and metal thickness modulations are simulated by rigorous coupled wave analysis (RCWA) method. Besides, the field intensity and phase of the transmitted TM and TE waves are also simulated to understand their polarization characteristics.

Aperiodic birefringent photonic structures based on Kolakoski sequence

A novel type of aperiodic porous silicon-based multilayered structures which can be used for the electromagnetic radiation manipulation in the visible and infrared parts of spectrum are proposed in this paper. The one-dimensional aperiodic structures under consideration are arranged by stacking together two anisotropic layers according to the Kolakoski self-generation scheme. The spectral properties of the structures are theoretically investigated by using the electromagnetic field scalarization method in coupling with the finite-difference method. It is shown that the Kolakoski multilayers have both completely transparent states in the transmission spectra and pronounced omnidirectional reflectance which can used in integrated optics, optical fiber telecommunications, vertical cavity surface emitting lasers, laser cavities, etc. Also, it is shown numerically that the anisotropic nature of layered media significantly affects on optical properties of the structures.

Quantitative Assessment of Birefringent Skin Structures in Scattered Light Confocal Imaging Using Radially Polarized Light

The polarization characteristics of birefringent tissues could be only partially obtained using linearly polarized light in polarization sensitive optical imaging. Here we analyze the change in polarization of backscattered light from birefringent structures versus the orientations of the incident polarizations using linearly, circularly and radially polarized light in a cross-polarized confocal microscope. A spatially variable retardation plate composed of eight sectors of λ/2 wave plates was used to transform linearly polarized light into a radially polarized light. Based on the experimental data obtained from ex-vivo measurements on human scalp hairs and in-vivo measurements on hair and skin, we exemplify that the underestimation of the birefringence content resulting from the orientation related effects associated with the use of linearly polarized light for imaging tissues containing wavy birefringent structures could be minimized by using radially polarized light.

Paper No 15.4: A Working Prototype of Flickerless Full‐Screen Resolution Autostereoscopic Display With Simultaneous Reproduction of Two Image Elements in One Display Pixel

AbstractThe design and key components of working prototype of the flat autostereoscopic display with full‐screen resolution for each of the two image views of 3D scene are presented. Simultaneous reproduction of two image elements in one display pixel is implemented using polarization coding by standard liquid crystal (LC) matrices. Polarization decoding of two image elements is implemented by static phase‐polarization parallax barrier based on column‐addressed twist or homogeneous birefringent LC structures. Stereoscopic image flicker is absent even at 60 frame rate. Although autostereoscopic image is observed only by a single user in definite viewing zones, the same display design allows to observe stereoscopic images by many users wearing passive polarization glasses (if to switch off the parallax barrier and to remove the output polarizer sheet).

Advanced multi-contrast Jones matrix optical coherence tomography for Doppler and polarization sensitive imaging

An advanced version of Jones matrix optical coherence tomography (JMT) is demonstrated for Doppler and polarization sensitive imaging of the posterior eye. JMT is capable of providing localized flow tomography by Doppler detection and investigating the birefringence property of tissue through a three-dimensional (3-D) Jones matrix measurement. Owing to an incident polarization multiplexing scheme based on passive optical components, this system is stable, safe in a clinical environment, and cost effective. Since the properties of this version of JMT provide intrinsic compensation for system imperfection, the system is easy to calibrate. Compared with the previous version of JMT, this advanced JMT achieves a sufficiently long depth measurement range for clinical cases of posterior eye disease. Furthermore, a fine spectral shift compensation method based on the cross-correlation of calibration signals was devised for stabilizing the phase of OCT, which enables a high sensitivity Doppler OCT measurement. In addition, a new theory of JMT which integrates the Jones matrix measurement, Doppler measurement, and scattering measurement is presented. This theory enables a sensitivity-enhanced scattering OCT and high-sensitivity Doppler OCT. These new features enable the application of this system to clinical cases. A healthy subject and a geographic atrophy patient were measured in vivo, and simultaneous imaging of choroidal vasculature and birefringence structures are demonstrated.

Complex degree of mutual anisotropy of linear birefringence and optical activity of biological tissues in diagnostics of prostate cancer

We present theoretical fundamentals of polarization and correlation analysis of the optical anisotropy of biological tissues. Results of measurements of coordinate distributions of the complex degree of mutual anisotropy (CDMA) that are formed by birefringent structures of the prostate tissue with benign and malignant changes are compared. Magnitudes and ranges of variation of statistical (the firstto fourth-order distribution moments) and correlation (excess of autocorrelation functions) parameters of the coordinate CDMA distributions of histological sections of the prostate postoperative bioptic material are studied. Objective criteria of the diagnostics of the appearance of pathology and of the differentiation of the degree of its severity are determined.

Ultrafast nanoporous silica formation driven by femtosecond laser irradiation

AbstractA type of glass modifications occurring after femto‐second laser irradiation gives rise to strong (10−2) from birefringence. This form birefringence is thought to be related to index nanostructure (called nanogratings). Analyzing induced tracks in fused silica using scanning electron microscopy (SEM) with nm resolution shows that nanostructures are porous nanoplanes with an average index lower than typical silica (Δn ∼ –0.20). Their origin is explained as arising from fast decomposition of the glass under localized, high‐intensity femtosecond laser radiation where strong nonlinear, multiphoton‐induced photoionization leads to plasma generation. Mechanistic details include Coulombic explosions characteristic of strong photoionization and the production of self‐trapped exciton (STE). Rapid relaxation of these STE prevents recombination and dissociated atomic oxygen instead recombines with each other to form molecular oxygen pointed out using Raman microscopy. Some of it is dissolved in the condensed glass whilst the rest is trapped within nanovoids. A chemical recombination can only occur at 1200 °C for many hours. This explains the thermal stability of such a nanostructure. Precise laser translation and control of these birefringent nanoporous structures allo arbitrarily tuning and positioning within the glass, an important tool for controlling optical properties for photonic applications, catalysts, molecular sieves, composites and more.

Laser induced nanogratings beyond fused silica - periodic nanostructures in borosilicate glasses and ULE™

We report on the ultrashort pulse laser induced formation of birefringent structures in the volume of different glasses: Borofloat 33, BK7 and ULE™. Using polarization contrast and scanning electron microscopy we could prove that this birefringence is induced by nanogratings. We were able to identify the pulse duration as a crucial process parameter for the generation of nanogratings in these glasses. The achieved birefringence in ULE is comparable to fused silica, while borosilicate glasses show much less birefringence (only about 12%). Remarkably, the period of the nanogratings is also dependent on the type of the glass, being 250 nm for ULE and only 60 nm in case of Borofloat 33.

DEGREE OF MUTUAL POLARIZATION OF OSSEOUS TISSUE LASER IMAGES

A study of the degree of a mutual polarization of laser images of biological tissues can be used to diagnose the phase birefringent structure of their physiologically normal and pathologically changed architectonic networks.

Polarized Image Correlation for Large Deformation Fiber Kinematics

The underlying fiber architecture of soft tissues, like bat wing skin, plays an important role in the material’s overall mechanical behavior. The mesoscopic birefringent fiber architecture of the bat wing skin can be visualized directly under polarized light. This inherent property is the key to the new experimental technique developed in this work: polarized image correlation (PIC). PIC is a technique for determining full field material strains and fiber kinematics of mesoscopically resolved fibrous tissues during biaxial mechanical testing. Not only is the material birefringence used to determine fiber kinematics under finite deformations, but it is also used for image correlation and strain field computation. Pure translation tests performed with PIC verify the accuracy of the technique. A segmental image processing method was developed to solve an experimental issue of changing birefringent properties to construct accurate continuous deformation profiles. By integrating PIC with traditional digital image correlation, both surface and subsurface data give additional insight into through thickness tissue behavior. The PIC technique is applicable to semi-transparent tissues with birefringent mesosopic structures; incorporation of microscopy would resolve smaller fiber structures. Future work includes extending the techniques to three dimensions to analyze curved surfaces.

Birefringence and band structure of CdP2 crystals

The spatial dispersion in CdP2 crystals was investigated. The dispersion is positive (nk||с>nk||у) at λ>λ0 and negative (nk||с<nk||у) at λ<λ0. CdP2 crystals are isotropic for wavelength λо=896nm. Indirect transitions in excitonic region Еgx are nonpolarized due to one pair of bands. Minimal direct energy intervals correspond to transitions Г1→Г1 for Е||с and Г2→Г1 for Е⊥с. The temperature coefficient of energy gap sifting in the case of temperature changing between 2 and 4.2K equals to 10.6meV/K and 3.2mev/K for Г1→Г1 and Г2→Г1 band gap correspondingly. Reflectivity spectra were measured for energy interval 1.5–10eV and optical functions (n, k, ε1, ε2,d2ε1/dE2 and d2ε2/dE2) were calculated by using Kramers–Kronig analyses. All features were interpreted as optical transitions on the basis of both theoretical calculations of band structure.

Designing Ultra-High-Birefringent Photonic Crystal Fibers with Circular Air Holes in the Cladding

This article reports a new air-silica photonic crystal fiber structure of high birefringence (~10−2). The birefringence is due to large axial anisotropy introduced in the fiber geometry by a preferred arrangement of circular air holes of different sizes in the cladding structure along orthogonal directions. A series study of birefringence and cut-off properties of the fiber that leads to optimization of geometrical parameters toward maximized birefringence achievable in the single-mode regime through this micro-structure is provided. Also, other inherently related modal properties of the fiber and the stack design toward preform realization are investigated. The findings should be useful in realizing high-performance high-birefringence fiber.

Investigation of Dispersion and Performance Based on Ring Cavity by Birefringent Interleaver for DWDM Transmission Systems

We theoretically investigate a 25 GHz multichannel filter based on ring cavity birefringent optical interleaver for dense wavelength division multiplexing (DWDM) transmission systems. The simulation tool used in this work is the Advanced System Analysis Program (ASAP) optical modeling software. We improve the dispersion performance by employingλ/6 andλ/4 wave plates as birefringent compensators for interleavers. The new structure exhibits a high performance with nearly zero ripple, a channel isolation greater than 102 dB, and a passband utilization of 86% within the C-band. The research results illustrate that our modified scheme can improve the dispersion of more than 76.6% in comparison with the previous studies of optical interleaver with birefringent crystal and ring cavity structures.