Ferroelectric Liquid-crystal Modulator Research Articles

A Decade of Linear and Circular Polarimetry with the POLISH2 Polarimeter

The POLISH2 optical polarimeter has been in operation at the Lick Observatory 3 m Shane telescope since 2011, and it was commissioned at the Gemini North 8 m in 2016. This instrument primarily targets exoplanets, asteroids, and the Crab Pulsar, but it has also been used for a wide variety of planetary, galactic, and supernova science. POLISH2's photoelastic modulators, employed instead of rotating wave plates or ferroelectric liquid crystal modulators, offer the unprecedented ability to achieve sensitivity and accuracy of order 1 ppm (0.0001%), which are difficult to obtain with conventional polarimeters. Additionally, POLISH2 simultaneously measures the intensity (Stokes I), linear polarization (Stokes Q and U), and circular polarization (Stokes V), which fully describe the polarization state of incident light. We document our laboratory and on-sky calibration methodology and our archival on-sky database, and we demonstrate the conclusive detection of circular polarization of certain objects.

Ferroelectric liquid-crystal modulator with large switching rotation angle for polarization-independent binary phase modulation

In this work a ferroelectric liquid crystal (FLC) modulator with a non-standard large switching rotation angle, close to 90∘, is fabricated and characterized. The modulator acts as a switchable wave-plate with an in-plane rotation of the principal axis under the action of a bipolar voltage. In the ideal situation of half-wave retardance, the device is shown to behave as a binary π phase modulator independently of the input state of polarization. We provide physico-chemical properties of the liquid crystalline mixture used to fabricate the FLC modulator with such large switching angle. The characterization method of the device optical properties is presented, which allows the localization of the two LC stable states, the unambiguous determination of the rotation angle, and the evaluation of the spectral retardance function. We demonstrate the polarization-independent π phase shift using an adapted Young's type interferometer for real-time measurements, where we further analyze the operational frequency limits of the device. This FLC operational mode can be exploited to produce binary-phase polarization-independent diffractive optical elements.

P‐11.4: Concept of Electronic Controlling the Ferroelectric Liquid Crystal Shutter at Unequal Durations of its Open and Closed States

Fast transient times of ferroelectric liquid crystal (FLC) modulators (shutters) provide accurate selection of pulse light fluxes, for example, the active selection of view images in the stereoscopic applications. The problem of the electronic control of the FLC shutter appears in the case of unequal durations of its alternating open and closed optical states. Corresponding unequal portions of control electrical power during positive and negative polarities of the control voltage cause the degradation of the FLC performance. Main reason of the degradation is the inequality of the volume ion charges induced into the FLC layer by positive and negative control voltage. In this paper, the concept of solution to this problem is suggested with the corresponding experimental verification. Our method allows to equalize the positive and negative control electrical power values without affecting the optical response of the FLC shutter by introducing the short time gaps into a longer control voltage waveform to lower the corresponding integral electrical power value.

A high-precision polarimeter for small telescopes

We describe Mini-HIPPI (Miniature HIgh Precision Polarimetric Instrument), a stellar polarimeter weighing just 650 grams but capable of measuring linear polarization to $\sim10^{-5}$. Mini-HIPPI is based on the use of a Ferroelectric Liquid Crystal (FLC) modulator. It can easily be mounted on a small telescope and allows study of the polarization of bright stars at levels of precision which are hitherto largely unexplored. We present results obtained with Mini-HIPPI on a 35 cm telescope. Measurements of polarized standard stars are in good agreement with predicted values. Meaurements of a number of bright stars agree well with those from other high-sensitivity polarimeters. Observations of the binary system Spica show polarization variability around the orbital cycle.

A high-sensitivity polarimeter using a ferro-electric liquid crystal modulator

We describe the HIgh Precision Polarimetric Instrument (HIPPI), a polarimeter built at UNSW Australia and used on the Anglo-Australian Telescope (AAT). HIPPI is an aperture polarimeter using a ferro-electric liquid crystal modulator. HIPPI measures the linear polarization of starlight with a sensitivity in fractional polarization of ~4 x 10$^{-6}$ on low polarization objects and a precision of better than 0.01% on highly polarized stars. The detectors have a high dynamic range allowing observations of the brightest stars in the sky as well as much fainter objects. The telescope polarization of the AAT is found to be 48 $\pm$ 5 x 10$^{-6}$ in the g' band.

Study of ferroelectric liquid crystal modulators for their use in diffractive and polarization optical applications

This paper presents a summary of the PhD thesis with the same name. Specifically, the following issues are addressed: 1) Development of a calibration method of the physical parameters for ferroelectric liquid crystal modulators, 2) use of ferroelectric liquid crystal modulators in diffractive optics and polarization, and 3) use of multiple characterization techniques to assess the quality of the computer generated holograms to be fabricated on silicon wafers

Imaging linear polarimetry using a single ferroelectric liquid crystal modulator

In the field of polarimetry, ferroelectric liquid crystal cells are mostly used as bistable polarization rotators suitable to analyze crossed polarizations. This paper shows that, provided such a cell is used at its nominal wavelength and correctly driven, its behavior is close to that of a tunable half-wave plate, and it can be used with much benefit in lightweight imaging polarimetric setups. A partial Stokes polarimeter using a single digital video camera and a single ferroelectric liquid crystal modulator is designed and implemented for linear polarization analysis. Polarization azimuthal angle and degree of linear polarization are available at 150 frames per second with a good accuracy.

Poincaré sphere analysis of a ferroelectric liquid crystal optical modulator: application to optimize the contrast ratio

The Poincare sphere representation is used to analyze the polarization transformation achieved with a ferroelectric liquid crystal (FLC) optical modulator. This device acts as a switchable wave-plate, in which the orientation of the principal axes rotates under the action of an applied bipolar voltage. In the standard operational mode for intensity switching, the rotation angle of the principal axes is �θ = π/4 and the phase shift is φ = π (half-wave-plate). However, for wavelengths different from the design one, the FLC deviates from the half-wave-plate performance and the optical contrast is diminished. We use the Poincare sphere representation to perform a theoretical analysis of the intensity switch performance of the FLC modulator as a function of the phase shift φ. Using spherical trigonometric relations we derive analytical expressions for the intensity contrast as a function of φ and we show how to compensate its decrease when φ � π by using appropriate elliptically polarized illumination. These situations are experimentally demonstrated using a commercially available FLC cell.

High-speed degree-of-polarization imaging with a ferroelectric liquid-crystal modulator

Images describing polarimetric characteristics of objects are more and more used when studying complex scenes. In some cases, only the depolarization phenomenon, mostly due to scattering, proves of interest and estimating the degree of polarization (DOP) is sufficient. For a linearly polarized beam under normal incidence, when the materials in the scene exhibit neither birefringence nor dichroism, this comes down to an intensity measurement in two crossed directions. Nevertheless, the classical optical setup, consisting mainly of a rotating polarizer used as a polarization state analyzer, gives good-quality images, but cannot provide dynamic information. In this paper, we propose to use a ferroelectric liquid-crystal modulator in an imaging polarimeter. We demonstrate the use of this modulator as a polarization state analyzer for frame rates up to about 400 Hz. Provided a fast camera is used, we show that the polarimeter allows us to surpass the classical video frame rate. We propose a setup using a rapid CCD camera (up to 233 Hz). Images in DOP are of very high quality and hold their own against images obtained with a classical polarizer. With a faster camera, we were able to obtain images at up to 400 Hz.

A spectropolarimeter for studying solar magnetic fields

A spectropolarimeter based on a ferroelectric liquid-crystal modulator is described. An optical system with spatial modulation of the positions of the components of Zeeman splitting is a specific feature of this instrument. In comparison to the familiar instruments, the developed spectropolarimeter utilizes the light flux more efficiently and contains only one photodetector array. An operating spectropolarimeter developed at the Sayan Solar Observatory is considered as an example. Comparative estimates of noises in different operating modes are presented.

Polarimeter with two ferroelectric liquid-crystal modulators attached to the Yunnan solar tower

A polarimeter to be mounted on the Yunnan solar tower is described. It features the ability to simultaneously measure the magnetic fields of the solar photosphere and chromosphere by analyzing the Stokes spectra of those magnetosensitive lines forming in the two regions with very high efficiency of polarization measurement. The polarimeter consists of two ferroelectric liquid crystals and one lambda/4 wave plate before a polarizing beam splitter. The achromatism of the design is emphasized to get the maximum combination efficiency over a spectral range from 5000 to 6000 A. For the used solar absorption lines MgI517.27, FeI525.06, FeI630.15, and FeI630.25 nm, the design gives theoretical efficiencies of polarization measurements, which are 0.999, 1.0, 0.943, and 0.943, respectively. A comparison with other reference polarimeters, such as the Synoptic Optical Long-term Investigation of the Sun, the Tenerife infrared polarimeter, and the La Palma Stokes Polarimeter, is carried out.

Different modes for spectropolarimetric measurements

Spectropolarimetric methods used at the Sayan Solar Observatory for measuring magnetic field strengths are described. The spectropolarimeter consists of a polarization splitter and a ferroelectric liquid crystal (FLC) modulator, allowing for several longitudinal H measurement modes. The report discusses advantages and disadvantages of modulation-based and modulationless measurements using ordinary slow CCD cameras. In some cases combined variants appear to be preferable.

Birefringent filter with amplitude beats

In this work, we show a very simple birefringent filter useful to modulate a broadband light spectrum. We first show the generation of various oscillating spectra using two identical multiple-order waveplates. In particular a sinusoidal broadband spectrum with a rapid oscillation is obtained. This spectrum is amplitude modulated by adding another waveplate to the system. This waveplate has a small phase shift, and with the proper orientation generates amplitude beats on the broadband spectrum. Finally, we show how this system can be voltage controlled by means of a ferroelectric liquid crystal modulator. We experimentally demonstrate this behavior with the aid of a portable spectrophotometer, and we provide a theoretical explanation on the basis of the Jones matrix formalism.

Analysis of the Hysteresis Phenomenon in the Intrinsic Half-V-Shaped Switching Ferroelectric Liquid Crystal Mode

The intrinsic half-V-shaped switching ferroelectric liquid crystal mode (iHV-FLC mode) is known to be able to easily yield excellent moving pictures without blurring, based on the nonholding display by simple AC driving in the ideal condition. However, we have sometimes observed the blurred moving pictures even in the iHV-FLC mode with very short response time. We have conducted the detailed measurement of the electrooptical response. The protracted hysteresis phenomenon could be observed in some cells. This phenomenon had a strong relationship with the internal ionic impurity density. The 1st frame hysteresis phenomenon could be observed even in the purest cells. It is considered to be caused by the c-director position at zero field and its driving scheme.

Pretilt Angle Dependence of the Switching Behavior in the Intrinsic Half-V-Shaped Switching Ferroelectric Liquid Crystal Mode

The intrinsic half-V-shaped switching ferroelectric liquid crystal mode (iHV-FLC mode) which is obtained from a material with isotropic (Iso.), cholesteric (Ch) and chiral smectic C (SmC*) phase transition sequences can easily yield excellent moving pictures. There are very few reports of liquid crystal (LC) molecular alignment during the Ch–SmC* phase transition sequence although it is important to clarify the molecular alignment and switching behavior in order to improve device characteristics such as the contrast ratio. We have conducted a detailed observation of the switching behavior of a cell with a striped texture and examined the pretilt angle dependence of the alignment state.

Dot Inversion Spontaneous Polarization Structure in the Half-V-shaped Switching Ferroelectric Liquid Crystal Mode

The half-V shaped switching ferroelectric liquid crystal mode (half-V FLC mode) can easily yield excellent moving pictures based on the nonholding display by simple AC driving. However, the AC driving in the entire area of the liquid crystal display (LCD) panel (i.e. the field-inversion drive method) raises a concern of the cross talk problem in large-size LCD panels. In this paper, we propose a novel alignment method that is controlled to yield alternating layer directions between neighboring pixels. Such layer structures correspond to the structure of dot inversion spontaneous polarization, and a nonholding display can be realized by the dot-inversion drive method. By this method, we can simultaneously obtain a cross-talk-free display, excellent moving pictures and wider viewing angle. The contrast ratio is expected to be improved by further liquid crystal development and/or the multi-domain process technology.

Novel Ferroelectric Liquid Crystal Mode for Active Matrix Liquid Crystal Display Using Cholesteric–Chiral Smectic C Phase Transition Material

We propose a novel ferroelectric liquid crystal (FLC) mode which is capable of gray-scale display with an active device such as a thin-film transistor (TFT). This technology is based on the monostable FLC mode, which is obtained from a material with cholesteric–chiral smectic C (Ch–SmC*) phase transition sequences and by applying a low DC voltage only near the phase transition temperature from Ch to SmC* for making the layer normal direction uniform. The electrooptical response of liquid crystal molecules under AC voltage is asymmetric between plus and minus voltages. It shows characteristics such as low spontaneous polarization (PS) value, short response time, small temperature dependence and wide tolerance to cell gap nonuniformity. Furthermore, we made TFT prototype panels using this mode. We could confirm this mode, realizing high-quality moving pictures without blurring based on the nonholding display and good color reproducibility based on the field sequential color method.

Material characteristics of an active matrix LCD based upon chiral smectics

We describe the material properties of a fast switching active matrix ferroelectric liquid crystal mode. The mode relies on a monostable monodomain having a high transmission and full grey scale capability. Experimental data are compared with the results of switching model calculations and the switching mechanism is discussed. The mode is suitable for video and multimedia application and compatible with conventional amorphous silicon thin film transistors due to the small value of the spontaneous polarization.

Polarization Modulation Differential Interference Contrast (Pol Mod Dic) Microscopy: An Improvement for Video Microscopy

Differential interference contrast (DIC) light microscopy, particularly when coupled with digital image processing, is a powerful tool for the high-resolution microscopy of unstained, transparent biological specimens and can equally well be applied to semiconductor measurements. We show analytically, and with images of diatoms, plant cells and protoplasts, that switching the polarization of the incident light by 90 degrees, changes the image highlights found in conventional DIC images into shadows and vice versa (1). Using a ferroelectric liquid-crystal modulator, this switching can be done at frame rates, synchronized to the camera. By subtracting alternate frames, a stream of difference DIC images is created. We call this technique Pol Mod DIC. Subtraction of alternate images is carried out efficiently by frame buffer operations and amounts to massively parallel synchronous detection. A similar method has been applied to confocal microscopy (2).

Ferroelectric liquid-crystal waveguide modulation based on a switchable uniaxial–uniaxial interface

Liquid crystals have effective electro-optic coefficients that are orders of magnitude larger than other integrated optical materials such as lithium niobate. However, previous studies of liquid-crystal waveguides have mainly focused on nematic liquid crystals, which exhibit impractically large scattering losses as waveguides. Studies of smectic liquid crystals and liquid crystals under strong confinement suggest the losses in these materials may be more manageable. In this study, the possibility of using ferroelectric liquid crystals in active waveguide modulators is explored through the analysis of several modulator configurations: a cutoff modulator, a deflection modulator, and an input coupler. As a way to study these structures, a mode-matching technique was developed to analyze the effects of a discontinuity in a uniaxial slab waveguide whose optic axis is in the plane of the waveguide. The results from the mode-matching technique were compared with those from simple bulk models. The analysis shows that ferroelectric liquid-crystal modulators have many desirable performance characteristics and could form the basis for practical waveguide modulators.