Spectroscopic Mueller matrix polarimeter using four-channeled spectra

A Mueller matrix polarimeter acquired for four-channeled spectra is proposed. Both the polarizing and analyzing optics of this system consist of a linear polarizer and a high-order retarder. The polarizing elements can modulate the polarization states in the wavenumber space. By applying a Fourier transform method to a single-channeled spectrum, nine elements of the Mueller matrix can be deconvoluted without modifying the configuration of either the polarizing or analyzing optics. It is thus possible to determine the wavelength dependence of all the Mueller matrix elements from four-channeled spectra obtained using four different configurations for the polarizing and analyzing optics. The performance of this method is evaluated by measuring polarization properties, such as retardance, azimuthal angle, and linear diattenuation, from the obtained Mueller matrix in wavenumber space.     

Optimization of a snapshot Mueller matrix polarimeter

We report on the optimization of a snapshot Mueller matrix polarimeter performed by using singular-value decomposition. The snapshot technique relies on wavelength polarization coding by four wave plates. The statistical noise on Mueller components is minimized through adjustment of the thickness of each plate. The spectrometer response and its cutoff frequency were considered to find the optimal configurations described here.     

Hybrid complete Mueller polarimeter based on phase modulators

Optical properties of matter as diattenuation, retardance and depolarization can be evaluated using polarimetric techniques. In this paper, the analysis and implementation of a complete Mueller polarimeter is presented. The system is constituted of a polarization state generator (PSG) and a polarization state analyzer (PSA), which are controlled and synchronized through a computer program. The PSG comprises a dual liquid crystal variable retarder system while the PSA is based on a two-photoelastic modulator setup. Using air and common polarizing optics as test samples for calibration at 633 nm, the hybrid instrument met a good precision when the Mueller matrices of those optical elements were measured.     

Snapshot imaging Mueller matrix polarimeter using polarization gratings

A snapshot imaging Mueller matrix polarimeter (SIMMP) is theoretically described and empirically demonstrated through simulation. Spatial polarization fringes are localized onto a sample by incorporating polarization gratings (PGs) into a polarization generator module. These fringes modulate the Mueller matrix (MM) components of the sample, which are subsequently isolated with PGs in an analyzer module. The MM components are amplitude modulated onto spatial carrier frequencies which, due to the PGs, maintain high visibility in spectrally broadband illumination. An interference model of the SIMMP is provided, followed by methods of reconstruction and calibration. Lastly, a numerical simulation is used to demonstrate the system's performance in the presence of noise.     

Optical studies in liquid crystals using the high-accuracy universal polarimeter

The so-called high-accuracy universal polarimeter (HAUP) is introduced as a tool to study optical properties of liquid crystals. It is shown that besides homogeneous crystals, the HAUP method also permits to investigate inhomogeneous materials, provided that the optical parameters vary slowly on a wavelength scale. The technique is especially useful when small effects are sought. Measurements are presented for several twisted nematic samples. Twist angles are measured with high accuracy and non-adiabatic effects for light propagation, such as apparent indicatrix rotations or effective gyrations, are easily detected. The results agree well with the predictions of a first-order perturbation treatment for light propagation through a stratified medium. Finally, the changes of the director profile are analyzed in a planar sample that presents a chiral nematic to smectic-A transition. The experimental data indicate that if the rubbing directions of the cell are not strictly parallel, the molecules at the surfaces undergo small twists in both phases, and the twist parameters are determined. Received 13 September 1999 and Received in final form 25 January 2000     

Electrically controlled broadband liquid crystal photonic bandgap fiber polarimeter

We demonstrate a liquid crystal photonic bandgap fiber based polarizer integrated in a double silicon v-groove assembly. The polarizer axis can be electrically controlled as well as switched on and off.     

Electrical tuning of photonic crystals infilled with liquid crystals

In this paper we perform a complete study of electrical tuning in liquid crystal-infilled two-dimensional (2D) photonic crystals (PCs). The nematic liquid crystal (NLC) is characterized by a full range of bulk and surface elastic parameters. An essentially DC tuning field is applied in the axial direction. By minimizing the total (elastic plus electromagnetic) free energy, the configuration of the NLC directors, as a function of radial distance, is obtained. Three possible configurations are considered: escaped radial, planar radial, and axial. It is found that, in general, the escaped radial configuration is the preferred one. However, for sufficiently large applied fields, a phase transition occurs to the axial configuration. For example, in the case of the NLC 5CB, this transition is realized at about 14 V/μm provided that the cylinder radius is greater than about 50 nm. The configuration of the NLC directors determines the dielectric tensor as function of radial distance and this, in turn, leads to the eigenvalue equation for the PC. We present two such equations: one exact and the other approximate. The exact eigenvalue equation is based on the full anisotropy of the dielectric tensor and does not result in the usual separation of normal modes in a 2D PC. The approximate eigenvalue equation is derived from the average (over the cylinder cross-section) dielectric tensor and leads to modes that are polarized in the directions either parallel (E-mode) or perpendicular (H-mode) to the cylinders. Our calculations of the photonic band structure, by both methods, show that the approximate calculation works very well for the 5CB NLC cylinders in a silicon oxide (silica) host. This allows us to introduce the terminology quasi-E and quasi-H polarizations. We show how the partial photonic band gap in the [1 0 0] direction for these polarizations can be tuned and even completely closed. This behavior could be applied to the design of versatile, tunable polarization filters.     

Polarizing terahertz waves with nematic liquid crystals

A Feussner-type terahertz polarizer with a nematic liquid crystal (NLC) layer between two fused-silica prisms is demonstrated. The polarization factor and extinction ratio of the NLC-based terahertz polarizer can exceed 0.99 and 10(-5), respectively.     

Photosensitive liquid crystals with nanoparticulate internal structure

Stabilization of thermodynamic relaxation of photoinduced cis isomers of azobenzene liquid-crystal molecules is observed in nanoparticulate networks. The phenomenon permits bistability of the phase state (anisotropic and isotropic) of the material and reversible all-optical switching between those states, resulting in strong changes in the light-scattering properties of the material. Recording of complex optical structures with high spatial resolution with the aid of laser beams of different wavelengths is demonstrated.     

Achromatic polarimeter

A polarimeter for measuring complete state of polarization of light in a wide spectral range is investigated. The light beam to be analyzed propagates through a continuously rotating first-phase plate; then through a continuously rotating second-phase plate; and, passing through a stationary linear polarizer, is incident on a detector. The time dependence of the intensity of the light beam that propagated though the polarizer is recorded. Fourier analysis of the periodic time dependence of the intensity yields the state of polarization of the light, which is characterized by four Stokes parameters. It is shown that the relation between the rotational velocities of the two phase plates and their thicknesses can be chosen such that the working range of the polarimeter would be limited only by the transparency range of the birefringent material of the phase plates, the working range of the linear polarizer, and that of the detector.     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Photorefractive polymer-dispersed liquid crystals

We report observation of the photorefractive effect in functionalized polymer-dispersed liquid crystals. The photoconductive properties are provided by the polymer matrix, and the field-dependent refractive-index changes are generated by the dispersed nematic liquid-crystal droplets. A high diffraction efficiency (8%) and a high refractive-index modulation amplitude (Dn = 2 x 10(-3)) are obtained in 53- microm-thick samples with an applied field of 22 V/microm.     

Real-time observation of liquid crystal molecular directions based on a snapshot polarimeter

The polarization state of transmitted light is linked to liquid crystal(LC) molecular distribution. The dynamic behavior of a twisted nematic LC molecule is measured with a home-built 10 k Hz snapshot polarimeter. Only the transient molecule rotations are observed when the external voltage changes, and the molecules return to their original orientations quickly even when high voltage is applied. Our observations cannot be attributed to the traditional electro-optic effect. The invalidation of the static external field indicates the shielding effect of redistributing impurity ions in an LC cell.     

Probing liquid crystals nonlinearly

Optical second-harmonic generation has been used to study the anisotropic orientation behavior of liquid-erystal siloxane molecules embedded in a nematic and/or ferroelectric host matrix. All of the studied guest-host systems show an exceedingly high degree of in-plane anisotropy along the direction of the rubbed interior polyimide layer of the surface-stabilized liquid-crystal sample cell. Alongitudinally applied electrical field leads to susceptibility components exceeding 1 pm/V for a poled sample cell of 5 m thickness containing 10% by weight of the siloxane guest in a nematic matrix.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Optical visualization of deformations with nematic liquid crystals

Nematic liquid crystals are shown to be able to visualize elastic deformations even if no dynamic scattering is present. At low values of the elastic energy density, by observing the excited birefringence it is possible to map the normal modes of the elastic disturbance. At higher energy density a cellular motion (rolls) is produced which, in the homeotropic geometry, gives rise to a domain pattern which allows to visualize the velocity field. Work supported by Gruppo Nazionale di Struttura della Materia (GNSM) of CNR.     

Hydrogen spectroscopy with a Lamb-shift polarimeter

A Lamb-shift polarimeter, which has been built for a fast determination of the polarization of protons and deuterons of an atomic-beam source and which is frequently used in the ANKE experiment at COSY-Jülich, is shown to be an excellent device for atomic-spectroscopy measurements of metastable hydrogen isotopes. It is demonstrated that magnetic and electric dipole transitions in hydrogen can be measured as a function of the external magnetic field, giving access to the full Breit-Rabi diagram for the 2²S_1/2 and the 2²P_1/2 states. This will allow the study of hyperfine structure, g factors and the classical Lamb shift. Although the data are not yet competitive with state-of-the-art measurements, the potential of the method is enormous, including a possible application to anti-hydrogen spectroscopy.     

Nematic disc-like liquid crystals

We present the first example of disc-like mesogens which present at high temperature a highly fluid mesophase with a nematic optical texture. These nematic disc-like compounds are pure simple component models to study the carbonaceous mesophase.