Field-induced Birefringence Research Articles

Electro-optics of blue phase liquid crystal in field-perpendicular direction

The electro-optic effect is the working principle of blue phase (BP) liquid crystals, and it describes the relationship between the field-induced birefringence of BPs and the field strength. Due to the electrostriction of BP crystals under the electric field, an orthorhombic or tetragonal crystal is usually obtained when the field is applied along one of the twofold axes of a BP crystal, leading to the optical biaxiality under electric field. Such field-induced optical biaxiality of BPs has been predicted and observed, but its dependence on the field strength has not been investigated. In this research, we analyze the electro-optics in the field-perpendicular direction by measuring the birefringence in highly ordered BP I(110) crystals perpendicular to the electric field. Results reveal that BP I crystals in the field-perpendicular direction show an electro-optic coefficient of the order of 10−10 m/V2 that may result from the large lattice deformation of BP crystals perpendicular to the electric field. Our research provides important experimental evidence for the tensorial properties of BP Kerr effect and may have important implications on the engineering of BP electro-optical devices in practical applications.

Quadratic electro-optic effect in the nonconjugated conductive polymer iodine-doped polyethylene terephthalate

Quadratic electro-optic effect has been measured in the nonconjugated conductive polymer iodine-doped polyethylene terephthalate (PET). Films of PET were doped with iodine in the vapor phase using standard procedure. The optical absorption and FTIR spectroscopic results show that iodine doping causes a charge-transfer from the terephthalic ring to iodine, thus forming a metallic quantum dot of subnanometer dimension. Nonlinear optical measurements were performed on the iodine-doped PET samples using field-induced birefringence method at 633 nm with lock-in detection. The magnitude of the Kerr coefficient for varying doping levels was determined. The magnitudes were shown to increase with doping level. For an iodine molar concentration of 0.28, a Kerr coefficient of ∼1.5 × 10−11 m/V2 was measured. For a molar concentration of iodine of 0.83, the measured Kerr coefficient was ∼5 × 10−11 m/V2. These magnitudes are comparable to (slightly smaller than) the Kerr coefficients measured in other iodine-doped nonconjugated conductive polymers representing metallic quantum dots of subnanometer dimensions.

Birefringence induced by antiferroelectric switching in transparent polycrystalline PbZr0.95Ti0.05O3 film

The most characteristic functional property of antiferroelectric materials is the possibility to induce a phase transition from a non-polar to a polar phase by an electric field. Here, we investigate the effect of this field-induced phase transition on the birefringence change of $PbZr_{0.95}Ti_{0.05}O_{3}$. We use a transparent polycrystalline $PbZr_{0.95}Ti_{0.05}O_{3}$ film grown on $PbTiO_{3}/HfO_{2}/SiO_{2}$ with interdigitated electrodes to directly investigate changes in birefringence in a simple transmission geometry. In spite of the polycrystalline nature of the film and its moderate thickness, the field-induced transition produces a sizeable effect observable under a polarized microscope. The film in its polar phase is found to behave like a homogeneous birefringent medium. The time evolution of this field-induced birefringence provides information about irreversibilities in the antiferroelectric switching process and its slow dynamics. The change in birefringence has two main contributions, one that responds briskly (~ 0.5 s), and a slower one that rises and saturates over a period of as long as 30 minutes. Possible origins for this long saturation and relaxation times are discussed.

Influence of ZnO nanoparticles on the electric field induced birefringence of the Blue phase

• Presence of ZnO NP decreases field-induced birefringence. • Transition temperature decreases but the BP exists for a narrow temperature range compared to pure. • The Kerr constant (K) decreases and is temperature-dependent. • K varies with temperature and is essentially doubled for the system with the nanoparticles. We report the influence of a small concentration (0.1 wt%) of ZnO nanoparticles (NPs) on the birefringence of the liquid crystal Blue phase. The measurements carried out using the field-induced birefringence technique show that the inclusion of NP diminishes the magnitude of the Kerr constant. We propose that the field-induced disclination networks act as trapping centres for the NPs and enhance the elastic constant leading to a diminution in the birefringence.

Electrical modification of order parameters and director fluctuations in a dielectrically negative nematic doped with a positive additive

Electric effects of nematic liquid crystals (NLCs) have enabled the revolution of the display industry. Recently, the electro-optic switching of NLCs has been accelerated by applying electric fields in such a way that the field does not change the director orientation, but modifies the optical tensors of NLCs due to three effects: induced biaxial orientational order, enhanced uniaxial orientational order, and quenching of director fluctuations. Till now, these three effects have been explored only in NLCs with negative dielectric anisotropy. Here, we experimentally study the electrical modification of order parameters and director fluctuations in a dielectrically negative/positive composite NLC. The dielectrically positive dopant causes the following changes: (a) a faster nanosecond switching for optical responses attributed to the change of order parameters, (b) an increase of the biaxial order parameter, (c) a reduction of the uniaxial order parameter, and (d) a slight reduction in the field-induced birefringence change. This work provides a basis for further studies in nanosecond electro-optics.

Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence

The fundamental properties of water molecules, such as their molecular polarizability, have not yet been clarified. The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water. The terahertz (THz) Kerr effect technique, as a novel tool, is expected to be useful in exploring the low-frequency molecular dynamics of liquid water. Here, we use an intense and ultrabroadband THz pulse (peak electric field strength of 14.9 MV/cm, centre frequency of 3.9 THz, and bandwidth of 1–10 THz) to resonantly excite intermolecular modes of liquid water. Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film. We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water. We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration, indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes of water is established. The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.

Electric-field-induced transition from SmA to ferroelectric SmC* in MC881, where antiferroelectric SmCA* but not SmC* emerges just below SmA

We studied the electric-field-induced tilt angle in SmA of MC881 and found the field-induced, first-order phase transition to SmC∗ in the SmA temperature region just above temperature-induced antiferroelectric SmCA*. The similar field-induced first-order transition from SmA to SmC* was observed by Bahr and Heppke [1, 2] in a compound called C7, where the temperature-induced first-order SmA to SmC∗ transition occurs. Here in MC881, on the contrary, no temperature-induced SmC∗ emerges just below SmA; instead temperature-induced antiferroelectric SmCA* is produced directly by the first-order phase transition from SmA. So far as we know, this is the first observation of the field-induced first-order phase transition from SmA to SmC∗ in the SmA-SmCA* phase sequence. We also confirm a large increase in the field-induced birefringence in SmA near the transition temperature to SmCA*, which was once ascribed by Manna et al. [3] to the existence of de Vries-type SmA. We are now considerably skeptical about their view because of the clear observation of the first-order field-induced transition from SmA to SmC*.

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.

Quadratic Electro-Optic Effect in Metal Nanoparticles in a Transparent Dielectric Medium

The quadratic electro-optic effect/Kerr coefficients were measured for the first time for metal nanoparticles. In particular, gold nanoparticles in glass were studied. Measurements were made using the field-induced birefringence method at a wavelength near the onset of the surface plasmon resonance. The magnitudes of the Kerr coefficients for different sizes of gold nanoparticles in glass were measured and compared with that of subnanometer size metallic particles in non-conjugated conductive polymers. The magnitude of the Kerr coefficient was found to increase rapidly (about d−3) when the diameter, d, of the nanoparticles was decreased. This is consistent with the existing theories and understanding of nonlinear optics in metal nanoparticles. The results imply a broad range of new applications of metal nanoparticles in electro-optic switching/modulation, low-cost Kerr cells and other uses in optoelectronics.

Study of birefringence and electro-optic effect in SBN60 thin film

Recent advances towards Electro-optic (EO) modulators make use of the Pockel’s effect in non-centrosymmetric crystals which allows the efficient modulation of light beam. The large value of electro-optic (EO) coefficient of Strontium barium niobate has gained advances in field of electro optic devices. In the present work, field-induced birefringence was measured in the transverse geometry for 800 nm thin SBN film deposited using Pulsed Laser Deposition (PLD) technique on the fused silica substrate. A small birefringence was induced in the as-deposited film by the alignment of the incident laser along c-axis in the film plane and applying dc electric field perpendicular to it. The value of induced birefringence was found to vary with applied electric field in a non-linear manner indicating the presence of EO Pockel’s and a weak Kerr effect.

Switchable Plasmonic Metasurface Utilizing the Electro-Optic Kerr Effect of a Blue Phase Liquid Crystal

A switchable metasurface composed of plasmonic split ring resonators and a polymer-stabilized liquid crystal blue phase is developed. Owing to field-induced birefringence (electro-optic Kerr effect), the state of polarization of the incident near infrared radiation changes, when a voltage is applied to the liquid crystal. Thus, different resonant modes of the split ring resonators can be addressed and the transmission spectrum changes accordingly. In comparison with other liquid crystal phases, blue phases have several advantages. For example, they are optically isotropic in the field-off state, so that no alignment layer is required. The results of the present study indicate that the advantages of these mesophases can be utilized for switchable metasurfaces.

Enhanced nanosecond electro-optic effect in isotropic and nematic phases of dielectrically negative nematics doped by strongly polar additive

An electric field can induce or modify optical birefringence in both the isotropic and nematic phases of liquid crystals (LCs). In the isotropic phase, the electric field induces birefringence with an optical axis along the field. The phenomenon is known as the Kerr effect. In the nematic, the analog of the Kerr effect is the change of existing birefringence through nanosecond electric modification of order parameters (NEMOP) that does not require realignment of the optic axis. The utility of both effects for practical applications is challenged by a relatively weak birefringence induced by the field. We address the issue by adding a non-mesogenic additive 2, 3-dicyano-4-pentyloxyphenyl 4′-pentyloxybenzoate (DPP) with a large transverse dipole moment to mesogenic materials in order to enhance their negative dielectric anisotropy. The DPP doping substantially increases the field-induced birefringence in both NEMOP and Kerr effects, up to 0.02. The doping also slows down the switching processes, but this effect can be compensated by rising working temperatures, if necessary. The enhancement of field induced birefringence by the non-mesogenic dopant paves the way for practical applications of nanosecond electro-optic effects.

Near infrared Kerr effect and description of field-induced phase transitions in polymer-stabilized blue phase liquid crystals

Studies of the influence of an electric field E on the effective refractive index of a polymer-stabilized blue phase in the near infrared spectral range reveal a considerable field-induced birefringence. At moderate voltages, the birefringence increases linearly with the square of the field strength as expected for the electro-optic Kerr effect, with an effective Kerr constant of K ≈ 6.3 – 6.9 × 10−10 m V−2. However, for E > ≈7.3 V/μm, the slope of the field-induced birefringence versus E2 increases abruptly, before saturation is reached at E > ≈8.5 V/μm. Based on previous observations on blue phases in the visible wavelength range, the discontinuous change can be attributed to a field-induced phase transition. A modification of the extended Kerr model introduced by Wu and coworkers is suggested to take this additional effect into account. In addition to the promising properties of blue phases for improved liquid crystal displays, the observed field-induced birefringence in the infrared region opens interesting perspectives for telecommunication and other non-display applications.

Kerr effect at high electric field in the isotropic phase of mesogenic materials.

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Thermal Behavior of Electrically Induced Birefringence of La-Doped PMN-PT Ceramics

The thermal evolution of the electric field-induced birefringence, δ(Δn(E)) vs T, obtained by the Kerr and Pockels electro-optical effects, was characterized in the La-doped Pb(Mg1/3Nb2/3)O3 – PbTiO3 ferroelectric relaxor ceramic system. The measurements were conducted over a broad temperature range (from 100 K to 650 K), using the dynamic Senarmont compensator method. Moreover, through a particular approach to the analysis of both electro-optical effects as a function of temperature, it was possible to determine the characteristic temperatures predicted for the transitions between the paraelectric (PE), ergodic ferroelectric relaxor (EFR), nonergodic ferroelectric relaxor (NEFR), and normal ferroelectric (FE) states of this system.

Discovery of Liquid Crystal Mesophases with a Six-Layer Periodicity

In 2006, employing ellipsometry and resonant x-ray diffraction, our research group discovered a liquid crystal mesophase having a six-layer periodicity in a ternary mixture (mixture A) as well as in a binary mixture (mixture B). This phase shows antiferroelectric-like properties. Subsequently, J. K. Vij's group used field-induced birefringence to explore the physical properties of various binary mixtures similar to mixture B. Recently, Y. Takanishi et al. obtained dielectric responses and two-dimensional microbeam resonant x-ray diffraction profiles as a function of temperature from a different binary mixture with one compound of the mixture containing a central bromine atom. They discovered another new mesophase which shows a six-layer structure and displays ferrielectric-like responses along with a different phase sequence. This article will review the sequence of events leading up to the discovery of the new phases with a six-layer periodicity and highlight differences in conclusions about the new phases and an ongoing debate about the existence of a phase with five-layer periodicity.

Invited article: An integrated mid-infrared, far-infrared, and terahertz optical Hall effect instrument.

We report on the development of the first integrated mid-infrared, far-infrared, and terahertz optical Hall effect instrument, covering an ultra wide spectral range from 3 cm(-1) to 7000 cm(-1) (0.1-210 THz or 0.4-870 meV). The instrument comprises four sub-systems, where the magneto-cryostat-transfer sub-system enables the usage of the magneto-cryostat sub-system with the mid-infrared ellipsometer sub-system, and the far-infrared/terahertz ellipsometer sub-system. Both ellipsometer sub-systems can be used as variable angle-of-incidence spectroscopic ellipsometers in reflection or transmission mode, and are equipped with multiple light sources and detectors. The ellipsometer sub-systems are operated in polarizer-sample-rotating-analyzer configuration granting access to the upper left 3 × 3 block of the normalized 4 × 4 Mueller matrix. The closed cycle magneto-cryostat sub-system provides sample temperatures between room temperature and 1.4 K and magnetic fields up to 8 T, enabling the detection of transverse and longitudinal magnetic field-induced birefringence. We discuss theoretical background and practical realization of the integrated mid-infrared, far-infrared, and terahertz optical Hall effect instrument, as well as acquisition of optical Hall effect data and the corresponding model analysis procedures. Exemplarily, epitaxial graphene grown on 6H-SiC, a tellurium doped bulk GaAs sample and an AlGaN/GaN high electron mobility transistor structure are investigated. The selected experimental datasets display the full spectral, magnetic field and temperature range of the instrument and demonstrate data analysis strategies. Effects from free charge carriers in two dimensional confinement and in a volume material, as well as quantum mechanical effects (inter-Landau-level transitions) are observed and discussed exemplarily.

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.

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Unusual behaviour of the dark conglomerate (DC) phase seen in an oxadiazole-based achiral bent-core liquid crystal, which has not previously been reported for the DC phase of other liquid crystals, is described. Under polarising optical microscopy, we see no domains of opposite handedness in the ground state of the DC phase. However, it shows unusual transformations when an electric field is applied to the system. On increasing the electric field, at first the domains of opposite handedness become visible and then they grow in size and slowly the sample transforms to a monochiral or single-handed form which is followed by a nonchiral state at very high fields. The threshold electric fields required to achieve these changes are temperature dependent and the transformations are seen irrespective of the frequency of the applied electric field (100 Hz to 5 kHz), type of the waveform (sine, square and triangular) and the thickness (1.5 μm to 15 μm) or the geometry (planar and twisted) of the device used. Further, there is no field-induced high birefringence texture observed even though sufficiently large electric field (~22 V/μm) has been applied across the devices. The nature of the behaviour is investigated by various techniques such as optical microscopy, conoscopy, circular dichroic and Raman spectroscopies, electro-optics and dielectric spectroscopy. The possible physical phenomena behind these changes are discussed in detail.

Crystals modulated by two parameters and their applications

In the applications of two external fields, such as stresses and electric fields, the optical modulation properties of some crystals are theoretically analyzed using the method of index ellipsoid. Simple mathematical formulas for the calculations of the field-induced principal refractive indexes of some crystals and corresponding azimuthal angles of their principal axes can be deduced from the equation of index ellipsoid if there exists only one nonzero cross term in the equation, e.g. x1x2. According to these simple formulas, we can find out some crystals exhibiting dual transverse electrooptic effect, e.g. crystals of the 6 symmetry point group. Under two simultaneously applied external stresses, elastooptic birefringence of a crystal is proportional to the difference between the two external stresses, and the orientations of their birefringent axes are unchanged. When a stress and an electric field are simultaneously and perpendicularly applied to some crystals such as cubic crystals of 43m point group, the field-induced birefringence of the crystal is proportional to the weighted geometric mean of the applied stress and electric field, and the orientations of their birefringent axes only depend on the ratio of the applied electric field and stress. The above electrooptic and elastooptic modulation properties are useful to the design of novel optical modulators and sensors.