Birefringent modifications induced by femtosecond filaments in optical glass

We present a study of the refractive index modifications spontaneously induced by infrared femtosecond filaments propagating in the bulk of transparent solids. It was found that extended modified refractive index channels up to several mm in length can be formed under loose focusing conditions in fused silica. The observed birefringence zone at the beginning of these channels is attributed to the anisotropic stress induced via nonlinear losses in the high intensity region of the filament.     

Methods for increasing the efficiency of nonlinear optical interactions in nanostructured semiconductors

Methods for increasing the efficiency of the optical second-and third-harmonic generation in gallium phosphide and silicon nanostructures formed by electrochemical etching of crystalline semiconductors are discussed. The efficiency of nonlinear optical interactions can be increased by using phase matching in anisotropic nanostructured semiconductors that exhibit form birefringence or by increasing the local field, as in scattering in macroporous semiconductors. The efficiencies of third-harmonic generation in porous silicon and of second-harmonic generation in porous gallium phosphide are found to increase by more than an order of magnitude.     

The role of birefringence in modulation instability fiber ring resonators

We considered a high birefringence fiber ring cavity, which can be represented by a system of two incoherently coupled nonlinear Schrödinger equations with periodic boundary conditions. The stability of the system against time domain periodic perturbations could be strongly conditioned by fiber birefringence. We apply both a perturbative approach and a mean field approximation, in order to highlight the dependence of modulation instability on birefringence and ring detuning. We extend the study of the dynamical properties of the system by means of a phase matching interpretation and a selection of numerical solutions of the governing equations.     

Depth-resolved multiphoton polarization microscopy by third-harmonic generation

We achieve depth-resolved polarization microscopy by measuring third-harmonic generation induced by a tightly focused circularly polarized beam. In crystals exhibiting strong birefringence this signal is dominated by positively phase-matched third-harmonic generation. This process occurs in only optically anisotropic media, in which the birefringence compensates for the phase mismatch between the fundamental and the third harmonic induced by dispersion. Both the intensity and the polarization of the emitted signal provide information on the local optical anisotropy. We demonstrate the technique by imaging biogenic crystals in sea urchin larval spicules.     

Coupling effects between dichroism and birefringence of anisotropic media

The basic polarization properties of media are their dichroism and birefringence. In this work we report the possible consequences of the coupling effects between dichroism and birefringence of anisotropic media. Our analysis is based on a solution of the differential equation of the Stokes four-dimensional vector. It is shown that there exist three types of coupling effects between the components of the dichroism vector and birefringence vector of the medium. The conditions under which the polarization properties of a continuous medium can be calculated are identified. These results are helpful for any study involved an interpretation of the measured Mueller matrix of continuous anisotropic medium.     

External anisotropic feedback effects on the phase difference behavior of output intensities in microchip Nd:YAG lasers

We present the effects of optical feedback from an anisotropic external cavity on the characteristics of the output intensities in a single mode Nd:YAG laser. Due to the birefringence effect, the external cavity modulates laser intensities in the two orthogonal directions with a phase difference, which is twice as large as that of the birefringence element in the external cavity. Experimental results are presented, as well as a theoretical analysis. The potential applications of this phenomenon are also discussed. PACS 42.60.Jf; 42.62.Eh; 42.25.Lc; 42.60.By     

Nonlinear photonic crystals as a source of entangled photons

Nonlinear photonic crystals can be used to provide phase matching for frequency conversion in optically isotropic materials. The phase-matching mechanism proposed here is a combination of form birefringence and phase velocity dispersion in a periodic structure. Since the phase matching relies on the geometry of the photonic crystal, it becomes possible to use highly nonlinear materials. This is illustrated considering a one-dimensional periodic Al0.4Ga0.6As/air structure for the generation of 1.5 microm light. We show that phase-matching conditions used in schemes to create entangled photon pairs can be achieved in photonic crystals.     

Dynamics of topological defects in the Lβ′ phase of 1,2-dipalmitoylphosphatidycholine bilayers

We use the optical birefringence of 1,2-dipalmitoylphosphatidycholine bilayers (DPPC) in the gel (L_β′) phase to study recombination dynamics of topological defects. The birefringence of anisotropic thin films, such as the L_β′ phase of DPPC bilayers, is related to their molecular polarizability, different on the heads and the acyl chains. When the sample is cooled down into the L_β′ phase, a period of rapid recombination (taking place over a few seconds) is followed by slow dynamics with metastable states existing in excess of several minutes. After this, either another metastable state or a truly stable state remains where no further change is observed, although a spatially non-uniform distribution of the orientation of the birefringence remains. We compare our results with a model for the free energy and the dynamics of the lipid bilayer in the gel state, finding good qualitative agreement.     

Control of modulation and soliton polarization instabilities in photonic crystal fibers with birefringence management

Photonic crystal fibers with composite linear birefringence, that is induced by both microstructure geometry and stress applying parts, permit the management of the wavelength dependence of both group and phase birefringence. In this work we investigate the novel nonlinear propagation phenomena that are enabled by birefringence management. Vector modulation instability is enhanced near the zero phase birefringence wavelength. Soliton polarization instability is controlled by Raman self-frequency shifting across the zero phase birefringence wavelength.     

2D-Mueller-matrix tomography of optically anisotropic polycrystalline networks of biological tissues histological sections

A new technique of Mueller-matrix mapping of the birefringent structure of biological preparations of human organs tissues is suggested. The algorithms of reconstruction of average values and magnitude of fluctuations of the phase (birefringence) and amplitude (dichroism) of optically anisotropic structure of myocardium and connective tissue component of the vaginal wall histological section are proposed. The magnitudes and ranges of changes in the statistical moments of the 1st-4th order that characterize the distribution of average values and magnitude of fluctuations of birefringence and dichroism of the myocardium and connective tissue of the vaginal tissues histological sections were determined. Joint studies of distributions of the characteristics of phase and amplitude of the anisotropy of myocardium and connective tissue component of the vaginal wall tissues of different states were performed. The cases of various necrotic changes in the myocardium and pathological conditions of the vagina wall (prolapse of the genitals) are examined. Balanced accuracy of the method of Mueller-matrix polarization-phase and diffuse tomography of optically anisotropic polycrystalline networks in the differentiation of necrotic and pathological changes in human organs is determined.     

多叶应力区扭转光纤的耦合模理论

从线性极化模的耦合模理论出发,分析了双叶应力区扭转光纤,即扭转保偏光纤,和四叶应力区扭转光纤.推导应力引起的在光纤芯区各向异性的介质张量表达式;利用该表达式得到线性极化模在有转角时的多叶应力区光纤中传输的耦合模方程;从而推得多叶应力区扭转光纤中的超本地模,由超本地模的相位常数来确定该光纤的双折射.在双叶、四叶应力区理想对称的条件下,前者可以得到较高的椭圆双折射,而后者则无任何双折射.同样的结论可推广到8叶或16叶的扭转光纤,都无任何双折射.     

Phase-matched four-photon mixing processes in birefringent fibers

We describe several new phase-matching processes for four-photon mixing in polarization-preserving birefringent fibers. The frequency shifts and polarization conditions of the waves involved in this nonlinear interaction are specified for a single-mode fused silica birefringent fiber. Our calculations include the effect of dispersion in the birefringence of the fibers.     

Probing vacuum birefringence by phase-contrast Fourier imaging under fields of high-intensity lasers

In vacuum high-intensity lasers can cause photon–photon interaction via the process of virtual vacuum polarization which may be measured by the phase velocity shift of photons across intense fields. In the optical frequency domain, the photon–photon interaction is polarization-mediated described by the Euler–Heisenberg effective action. This theory predicts the vacuum birefringence or polarization dependence of the phase velocity shift arising from nonlinear properties in quantum electrodynamics (QED). We suggest a method to measure the vacuum birefringence under intense optical laser fields based on the absolute phase velocity shift by phase-contrast Fourier imaging. The method may serve for observing effects even beyond the QED vacuum polarization.     

Effect of photonic crystal structure on the nonlinear optical anisotropy of birefringent porous silicon

Anisotropic photonic crystal structures consisting of birefringent porous silicon layers with alternating porosity were fabricated. The in-plane birefringence formed as a result of anisotropic etching in Si(110) results in unique multilayered structures with two distinct photonic bandgaps for orthogonal light polarizations. Nonlinear optical studies based on the third-harmonic generation from these structures demonstrate variation in the symmetry of the nonlinear optical response.     

Nonlinear electrodynamics and anisotropic space-time

The metrical properties of four-space in the presence of a nonlinear electromagnetic field in vacuum are investigated. It is shown that the metric induced by the nonlinear electromagnetic field has a complicated nature and corresponds to an anisotropic space-time, i.e., to a four-space in which birefringence is observed. A special case of such a space is ordinary Riemann-Euclid space. As an example, the influence on the metric of the radiative corrections that follow from quantum electrodynamics is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 52–55, April, 1980.     

Low‐noise,vibrational phase‐sensitive chemical imaging by balanced detection RIKE

We perform a back‐to‐back comparison between two nonlinear vibrational imaging techniques: stimulated Raman scattering (SRS) and balanced detection Raman‐induced Kerr effect (BD‐RIKE). Using a compact fiber‐based laser system for generation of pump and Stokes signals, we image polymer beads as well as human hepatocytes under the same experimental conditions. We show that BD‐RIKE, despite the slightly lower signal levels, consistently offers an improved signal‐to‐noise ratio with respect to SRS, resulting in significantly higher image quality. Importantly, we observe that such quality is not affected by the static birefringence of the sample, which makes BD‐RIKE a robust and attractive alternative to SRS. We also highlight a unique advantage of the technique, which is its capability to easily access both the real and imaginary parts of the nonlinear susceptibility, thus allowing for vibrational phase imaging. The phase information can be readily obtained from BD‐RIKE with minimal experimental effort and provides an additional chemical selectivity channel for coherent Raman microscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

High-efficiency, simple setup for pulse cleaning at the millijoule level by nonlinear induced birefringence

Nonlinear elliptical polarization rotation is used to improve the contrast of femtosecond pulses by several orders of magnitude. Using nonlinear induced birefringence in air, we produced cleaned pulses with an energy of a few hundreds of microjoules. This technique presents several major advantages, such as convenience and stability of the setup. We investigated the phase profile required for obtaining high-energy pulses. No phase distortion is observed, and the spatial quality of the beam is preserved.     

Effective nonlinear GaSe crystal. Optical properties and applications

We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ?-GaSe. Physical properties of ?-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ～ 0.3 at 700 nm; high transparency range (0.7?18.0 μm) with low absorption coefficient (α ≤ 0.3 cm^?1); very high nonlinear susceptibility χ⁽²⁾ (d ₂₂ ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10^?7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.     

Effect of nonreciprocity in the state of polarization of an electromagnetic wave traveling in an inhomogeneously anisotropic crystal

Nonreciprocity in the state of polarization in an inhomogeneously anisotropic crystal is discovered, and the main characteristics of this effect are studied. The effect consists in the appearance of asymmetry of components of the Stokes vector for radiation traveling in forward and backward directions. It is shown to be caused by the presence of a nonlinear gradient of mechanical stress. The effect is theoretically analyzed using the model of a sample whose inhomogeneity is caused by the presence of sections with linear and circular birefringence placed in a series. The theoretical conclusions qualitatively agree with experimental results obtained for a silicon single crystal using the polarization modulation technique.     

液晶双折射Δn的测量

描述了一种简便、精确测量液晶的双折射Δn 值的方法。种这方法既可以用于测量其分离波长的值,又可以测量其连续波长(紫外、可见光或红外光谱区)的函数值。该方法根据单色线偏振光在折射率各向异性媒质中传播时产生相位差这一原理,给出两种液晶材料 E7和 TEB30的测量结果。