Natural Birefringence Research Articles

Dual LiNbO3 Crystal-Based Batteryless and Contactless Optical Transient Overvoltage Sensor for Overhead Transmission Line and Substation Applications

Advanced high-voltage and overvoltage measurement techniques are required for smart grid construction. The existing overvoltage measurement methods that are currently used for power system measurements are mostly based on the use of electromagnetic voltage transformers and capacitive voltage transformers, which have contradiction in measuring accuracy, measuring distance, antijamming, and system compatibility. A batteryless sensor for contactless measurement of the overvoltage on overhead transmission lines based on a combination of the electrooptic effect in LiNbO3 with stray capacitance in the air is designed in this work. On the basis of this design, a dual-crystal structure-based electrooptic conversion unit is presented that eliminates the natural birefringence and improves the operating stability of the sensor. Testing platforms were set up to measure the characteristics of the sensor in thermal stability. In combination with a data acquisition device, the newly designed sensor was applied to online monitoring of the overvoltage in the ac bus and the overhead transmission lines of a 500-kV transformer station and a ±500-kV convertor station in the China Southern Power Grid.

Langasites as electro-optic materials for high-voltage optical sensors

Electro-optic properties and linear birefringence of langasite (LGS), langataite (LGT), and catangasite (CTGS) single crystals are measured in the temperature range of 115 ~310 K, along with room-temperature refractive indices of CTGS at 1540 nm. Lanthanum-containing LGS and LGT have 5 times larger unclamped electro-optic constant r11 and 1 ~2 orders of magnitude less thermal coefficient for longitudinal half-wave voltage than rare-earth free CTGS. Temperature dependence of natural birefringence in LGS is non-monotonic with a maximum at 158 K and a temperature derivative of birefringence within ± 3.3 × 10−8 K−1 in the range of 115 ~172 K. Rare-earth containing langasites like LGS can be considered as promising electro-optic materials for AC high-voltage optical sensors because of the existence of a longitudinal electro-optic effect with weak temperature dependence.

Growth and magnetooptical properties of anisotropic TbF3 single crystals

This paper investigates the Faraday effect and absorption and luminescence spectra of single-crystal TbF3 measured at 90 K and 300 K. The optical-quality single-phase TbF3 crystals (structural type β-YF3) were grown by the Bridgman technique. Faraday rotation angles were measured at remagnetization along the [100] crystallographic axis. Low temperature optical measurements were carried out along the [100] axis. “Quasi-doublet” sublevels with energy at 0 cm−1, 65 cm−1, and 190 cm−1, and also a singlet sublevel with energy at 114 cm−1 located in the ground 7F6 multiplet were determined from the low temperature luminescence spectra. The Van-Vleck behavior of the magnetic susceptibility χb can be satisfactorily explained by the magnetic mixing of wave functions belonging to the ground and first excited “quasi-doublet” sublevels at 0 and 65 cm−1, respectively. Analysis of the oscillation dependences of the rotation angle showed that the value of the natural birefringence (Δn ≈ 0.0186) remains nearly constant within the wavelength and temperature ranges under investigation. As the temperature decreases, we find significant increases in the oscillation amplitude of the rotation angle and in the Verdet constant V. The spectral dependences V(χ) are linear throughout the temperature range. The magnetooptical activity of TbF3 can be explained by means of the spin- and parity-allowed electric-dipole 4f → 5d transitions in the Tb3+ ions.

Electrically tunable terahertz polarization converter based on overcoupled metal-isolator-metal metamaterials infiltrated with liquid crystals

Large birefringence and its electrical modulation by means of Fréedericksz transition makes nematic liquid crystals (LCs) a promising platform for tunable terahertz (THz) devices. The thickness of standard LC cells is in the order of the wavelength, requiring high driving voltages and allowing only a very slow modulation at THz frequencies. Here, we first present the concept of overcoupled metal-isolator-metal (MIM) cavities that allow for achieving simultaneously both very high phase difference between orthogonal electric field components and large reflectance. We then apply this concept to LC-infiltrated MIM-based metamaterials aiming at the design of electrically tunable THz polarization converters. The optimal operation in the overcoupled regime is provided by properly selecting the thickness of the LC cell. Instead of the LC natural birefringence, the polarization-dependent functionality stems from the optical anisotropy of ultrathin and deeply subwavelength MIM structures. The dynamic electro-optic control of the LC refractive index enables the spectral shift of the resonant mode and, consequently, the tuning of the phase difference between the two orthogonal field components. This tunability is further enhanced by the large confinement of the resonant electromagnetic fields within the MIM cavity. We show that for an appropriately chosen linearly polarized incident field, the polarization state of the reflected field at the target operation frequency can be continuously swept between the north and south pole of the Poincaré sphere. Using a rigorous Q-tensor model to simulate the LC electro-optic switching, we demonstrate that the enhanced light–matter interaction in the MIM resonant cavity allows the polarization converter to operate at driving voltages below 10 Volt and with millisecond switching times.

Electro-optical modulators based on KTP crystals for high-power lasers in the mid-IR region

This paper presents the results of experimental studies on the application of the near-IR and mid-IR ranges of KTP-crystal-based modulators in lasers that use birefringence-compensation setups in which the crystallographic axes of the crystals are identically oriented. The chosen setup makes it possible to compensate for not only the natural birefringence of the crystals, but also the thermal distortion induced by laser heating. This made it possible to create modulators with contrast K≥100 for lasers with high average power.

Mueller-matrix modeling and characterization of a dual-crystal electro-optic modulator.

A general mathematical model based on Mueller-matrix calculation is presented to describe the optical behavior of a dual-crystal electro-optic modulator. The two crystals inside the modulator are oriented at ± 45° with respect to the horizontal, thereby cancelling natural birefringence and temperature-induced birefringence. We describe the behavior of the modulator as a function of the ellipticity of the crystals, the rotation angles of the crystals and the applied voltage. By fitting the measured data with a Mueller-matrix model that uses values for the ellipticity and orientation angles of the crystals, the simulated data and the experimental measurements could be matched. This Mueller-matrix includes physical properties of the thermally compensated electro optic modulator, and the matrix can be used in simulations where these device-specific properties are important, for instance in the modeling of a polarization-sensitive optical coherence tomography system.

X-ray natural birefringence in reflection from graphene

The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon $1s$ absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to ${30}^{\ensuremath{\circ}}$, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to ${17}^{\ensuremath{\circ}}$. Both graphene systems show resonantly enhanced effects at the ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of ${140}^{\ensuremath{\circ}}$ has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the ${\ensuremath{\pi}}^{*}$ resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced ${\ensuremath{\sigma}}^{*}$ excitation energy and a strong tilt of $50%$ of the ${p}_{z}$ orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the ${p}_{z}$ orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting $\ensuremath{\lambda}/4$ or $\ensuremath{\lambda}/2$ plates in the design of future high-intensity light sources.

Temperature characteristics of Pockels electro-optic voltage sensor with double crystal compensation

Voltage sensors based on the Pockels electro-optic effect in LiNbO3 crystals have been applied to practical engineering measurements because of their passive nature, wide operating bands, and low transmission loss. However, the temperature of the measurement environment can greatly affect the dynamic responses of these sensors because the natural birefringence of a single LiNbO3 crystal voltage sensor (SVS) is related to its temperature. To improve the stability of this sensor over a wide temperature range, a double crystal compensation method is introduced in this paper to compensate for the natural birefringence of the SVS. A double LiNbO3 crystal voltage sensor (DVS) was fabricated, and its working point drift characteristics and amplitude-frequency response were investigated over the temperature range from 0°C to 50°C. The effects of two intrinsic parameters of the LiNbO3 crystal were also investigated. Comparison between an existing SVS and the proposed DVS showed that the DVS resisted environmental temperature fluctuations more strongly.

Invivo Pattern Recognition and Digital Image Analysis Shear Stress Distribution in Human Eye

Human eye is made of a number of structural components to deliver vision, and cornea is the front window of the eye. Human cornea is made of soft biological materials. It is unfriendly for exposures to the common energy levels of X-ray scans for repeated probing of its structural architecture. Here we study an alternative imaging methodology by using a normal white-light source. By exploiting the natural birefringent property of cornea, the shear stress distribution pattern and its directional characteristics on the surface of cornea is recognized in vivo. Digital image processing of corneal retardation helps us to locate the stress concentration zones on its surface and to study their features along preferential directions. Such digital image outputs could be used in future to bench mark the health standard of cornea as well as a potential identity signature of people’s eyes.

Temperature dependence of electro-optic effect and natural linear birefringence in quartz measured by low-coherence interferometry.

Temperature dependencies of the half-wave voltage and natural linear birefringence in mechanically free quartz are measured at 1560 nm in the temperature range of 85-310 K. Measurements are carried out using a low-coherence interferometric scheme with a pair of identical quartz crystals independently linked with a scanning Michelson interferometer. Half-wave voltage V11 driven by an electro-optic coefficient r11 grows from 868 kV at 85 K to 923 kV at 310 K. The temperature derivative of natural linear birefringence Δn has nearly linear temperature dependence: ∂Δn/∂T=-7.260×10(-7)-9.93×10(-10) T K(-1). Moderate temperature dependence of the electro-optic effect, along with other properties, makes quartz an appropriate sensing medium for electro-optic voltage sensors for the electric power industry.

Measurement of quadratic electrogyration effect in castor oil

This work presents a detailed analysis of electrogyration measurement in liquids with the usage of an optical polarimetric technique. Theoretical analysis of the optical response to an applied electric field is illustrated by experimental data for castor oil which exhibits natural optical activity, quadratic electro-optic effect and quadratic electrogyration effect. Moreover, the experimental data show that interaction of the oil with a pair of flat electrodes induces a significant dichroism and natural linear birefringence. The combination of these effects occurring at the same time complicates the procedure of measurements. It has been found that a single measurement is insufficient to separate the contribution of the electrogyration effect, but it is possible on the basis of several measurements performed with various orientations of the polarizer and the analyser. The obtained average values of the quadratic electrogyration coefficient β13 in castor oil at room temperature are from −0.92×10−22 to −1.44×10−22m2V−2 depending on the origin of the oil. Although this study is focused on measurements in castor oil, the presented analysis is much more general.

自然双折射晶体中热致双折射效应的研究

自然双折射晶体中热致双折射效应的研究

Babinet's principle in double-refraction systems

Babinet's principle applied to systems with double refraction is shown to involve spatial interchanges between the ordinary and extraordinary patterns observed through two complementary screens. As in the case of metamaterials, the extraordinary beam does not follow the Snell-Descartes refraction law, the superposition principle has to be applied simultaneously at two points. Surprisingly, by contrast to the intuitive impression, in the presence of the screen with an opaque region, we observe that the emerging extraordinary photon pattern, which however has undergone a deviation, remains fixed when a natural birefringent crystal is rotated while the ordinary one rotates with the crystal. The twofold application of Babinet's principle implies intensity and polarization interchanges but also spatial and dynamic interchanges which should occur in birefringent metamaterials.

Electro-optic switching of dielectrically negative nematic through nanosecond electric modification of order parameter

We present experimental studies of nanosecond electric modification of the order parameter (NEMOP) in a variety of nematic materials with negative dielectric anisotropy. The study demonstrates that NEMOP enables a large amplitude of fast (nanoseconds) electro-optic response with the field-induced birefringence on the order of 0.01 and a figure of merit (FoM) on the order of 104 μm2/s; the latter is orders of magnitude higher than the FoM of the Frederiks effect traditionally used in electro-optic nematic devices. The amplitude of the NEMOP response is generally stronger in nematics with larger dielectric anisotropy and with higher natural (field-free) birefringence.

Optical electric-field sensor based on angular optical bias using single β-BaB2O4 crystal

A novel optical electric-field sensor is proposed and demonstrated in experiment by use of a single beta barium borate (β-BaB2O4, BBO) crystal. The optical sensing unit is only composed of one BBO crystal and two polarizers. An optical phase bias of 0.5π is provided by using natural birefringence in the BBO crystal itself. A small angle (e.g., 0.6°) between the sensing light beam and principal axis of the crystal is required in order to produce the above optical bias. Thus the BBO crystal is used as the electric-field-sensing element and quarter waveplate. The ac electric field in the range of (1.4-703.2) kV/m has been measured with measurement sensitivity of 1.39 mV/(kV/m) and nonlinear error of 0.6%. Compared with lithium niobate crystal used as an electric-field sensor, main advantages of the BBO crystal include higher measurement sensitivity, compact configuration, and no ferroelectric ringing effect.

Manipulating polarization of light with ultrathin epsilon-near-zero metamaterials

One of the basic functionalities of photonic devices is the ability to manipulate the polarization state of light. Polarization components are usually implemented using the retardation effect in natural birefringent crystals and, thus, have a bulky design. Here, we have demonstrated the polarization manipulation of light by employing a thin subwavelength slab of metamaterial with an extremely anisotropic effective permittivity tensor. Polarization properties of light incident on the metamaterial in the regime of hyperbolic, epsilon-near-zero, and conventional elliptic dispersions were compared. We have shown that both reflection from and transmission through λ/20 thick slab of the metamaterial may provide nearly complete linear-to-circular polarization conversion or 90° linear polarization rotation, not achievable with natural materials. Using ellipsometric measurements, we experimentally studied the polarization conversion properties of the metamaterial slab made of the plasmonic nanorod arrays in different dispersion regimes. We have also suggested all-optical ultrafast control of reflected or transmitted light polarization by employing metal nonlinearities.

Bottom-up Fabrication and Optical Characterization of Dense Films of Meta-Atoms Made of Core–Shell Plasmonic Nanoparticles

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.

Influence of cubic nonlinearity on accuracy of polarization transformation by means of a quarter-wave plate

We consider the propagation of powerful laser radiation in an anisotropic medium with natural birefringence and cubic nonlinearity. By the example of a quarter-wave plate, we show theoretically and experimentally that, under the simultaneous influence of linear birefringence and nonlinearity, the accuracy of polarization transformation decreases in proportion to squared В-integral.

Efficient passively Q-switched Nd:YLF TEM00-mode laser at 1053 nm: selection of polarization with birefringence

The natural birefringence of the Nd:YLF crystal is utilized to achieve a reliable TEM00-mode linearly polarized laser at 1053 nm in a compact concave-plano resonator. The efficient selection of the polarization relies on the combined effect of the difference in diffraction angle for σ- and π-polarization of a wedged laser crystal and the alignment sensitivity of an optical resonator. We further employ a Cr4+:YAG saturable absorber to perform the passively Q-switched operation. At an incident pump power of 12 W, the maximum average output power is up to 2.3 W with a pulse repetition rate of 8 kHz and a pulse width of 9 ns. The pulse energy and peak power are found to be 288 μJ and 32 kW, respectively.

A BROADBAND METAL-MESH HALF-WAVE PLATE FOR MILLIMETRE WAVE LINEAR POLARISATION ROTATION

We present a polarisation rotator based on a dielectrically embedded metal Mesh Half Wave Plate (MHWP) working in the W- band frequency range (75{110GHz). The device was realised using metallic grids with sub-wavelength anisotropic geometries able to mimic the behaviour of natural birefringent materials. The device was designed using a combination of transmission line codes and flnite- element analysis able to achieve phase accuracy down to a fraction of degree. Very accurate intensity and phase measurements were carried out using coherent radiation from a Vector Network Analyser (VNA). The presented device performs better and it is much thinner than previous devices having reduced the number of grids by a factor two and minimised their inductive losses. The new mesh HWP has excellent performances in terms of difierential phase-shift ∞atness and cross-polarisation respectively 180:4 § 2:9 - and i28dB across a 25% bandwidth.