Ingolf Dahl

Bio: Ingolf Dahl is an academic researcher from Chalmers University of Technology. The author has contributed to research in topics: Liquid crystal & Ferroelectricity. The author has an hindex of 11, co-authored 27 publications receiving 819 citations.

Submicrosecond electro‐optic switching in the liquid‐crystal smectic A phase: The soft‐mode ferroelectric effect

Abstract: A new liquid‐crystal electro‐optic modulating device similar to the surface‐stabilized ferroelectric liquid‐crystal device is described. It uses the same kind of ferroelectric chiral smectics and the same geometry as that device (thin sample in the ‘‘bookshelf ’’ layer arrangement) but instead of using a tilted smectic phase like the C* phase, it utilizes the above‐lying, nonferroelectric A phase, taking advantage of the electroclinic effect. The achievable optical intensity modulation that can be detected through the full range of the A phase is considerably lower than for the surface‐stabilized device, but the response is much faster. Furthermore, the response is strictly linear with respect to the applied electric field. The device concept is thus appropriate for modulator rather than for display applications. We describe the underlying physics and present measurements of induced tilt angle, of light modulation depth, and of rise time.

The soft-mode ferroelectric effect

Abstract: In this paper a presentation is given of some of the basic physics of the soft-mode ferroelectric effect, along with its experimental background. Further, possible applications in electrooptic devices are elucidated. A fast (sub-microsecond) electrooptic switching can be achieved in the A* phase, and in similar orthogonal smectic phases made up of chiral molecules. Instead of using the phase variable Φ, it uses the tilt angle θ, which in principle is a “hard” variable, but is expected to soften on approaching the transition to a lower-lying adjacent tilted smectic phase. However, the switching is efficient in the whole range of the orthogonal phase (in fact, less efficient near the tilting transition) and is observed whether an adjacent tilted phase is present or not. As compared to the surface-stabilized (SSFLC) electrooptic mode, this soft mode (SMFLC) is based on the electro clinic, effect of essentially ferroelectric nature very closely related to the presently more investigated ferroelectric...

Elastic and flexoelectric properties of chiral smectic-C phase and symmetry considerations on ferroelectric liquid-crystal cells

Abstract: The elastic energy expression of de Gennes for the non-chiral and chiral smectic-C phases is reformulated, and it is shown that the maximum size of a uniformly oriented sample is limited, not only by the chiral helix but also by a spontaneous bend of the smectic layers. A description of the flexoelectric effects is given: 9 different vector fields are involved, and they all remain in non-chiral smectic-C phase. These vector fields are connected to the divergence terms in the elastic free energy. The complexity of the boundary conditions for ferroelectric liquid crystals is discussed, and the concepts ‘bookshelf geometry’ and ‘φ-can’ are introduced. It is shown how the φ-can may be used to describe various monostable and bistable configurations of smectic-C cells. The symmetry of the cell can determine the configuration. Some general rules concerning the optical behaviour of smectic-C cells are given.

Polarization and Viscosity Measurements in a Ferroelectric Liquid Crystal by the Field Reversal Method

Abstract: With the aim to develop a standard method for measurements of the polarization of ferroelectric chiral smectic C liquid crystalline materials, we have studied thin, well-aligned samples of MBRA-8 using the field-reversal method. The method also provides an estimate for the rotational viscosity and for the response times.

Liquid crystal devices using a linear electro-optic effect

Abstract: A device for influencing light has a polarizer, a ferroelectric or electroclinic response type liquid crystal halfwave plate whose liquid crystals are in the smectic phase with bookshelf geometry and have molecular axes which are rotatable around a first direction corresponding to the direction of incoming light in response to an electric field applied across the electrodes on either side of the halfwave plate, a quarter wave retarder plate, and a mirror. The polarizer, plates and mirror are arranged such that incoming light is passed from the polarizer through the liquid crystal plate and the retarder plate, and is reflected by the mirror in a second direction opposite to the first direction back through the retarder and the liquid crystal plate so as to provide a reflected polarized light plane to the polarizer. The polarizer has a first polarization direction set parallel to one electrically selectable molecular axis direction of the liquid crystal molecules, which is selected in response to a first electric field level. The retarder plate has its slow axis inclined either 45° or 135° relative to the optic axis of the liquid crystal. The liquid crystal, furthermore, is of a material which produces an angular difference 2θ in optic axis direction in response to a second electric field level whose polarity is opposite that of the first electric field level. This allows the device to rotate the reflected polarized light plane by an angle of 8θ in response to the second electric field level.

Optical code-division-multiplexed systems based on spectral encoding of noncoherent sources

Abstract: Presents a new category of optical CDMA systems which work based on spectral encoding. In such systems, that the authors refer to as frequency-encoded CDMA (FE-CDMA) systems, the coding is done in the frequency domain while in the usual CDMA systems the code multiplies the modulation signal in the time domain. They present a new type of FE-CDMA system, based on encoding noncoherent broadband sources. They discuss the advantages of the system compared to other optical CDMA systems and present its performance. They show that very efficient, low-cost, CDMA systems can be obtained with an aggregate throughput of many gigabits per second. Also, for this system, the spreading gain of CDMA is independent of the modulation bandwidth. Hence, the system can accommodate variable bit rates, naturally. >

Current topics in smectic liquid crystal research.

Abstract: Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).

Polarimetric characterization of light and media - Physical quantities involved in polarimetric phenomena

Abstract: An objective analysis is carried out of the matricial models representing the polarimetric properties of light and material media leading to the identification and definition of their corresponding physical quantities, using the concept of the coherency matrix. For light, cases of homogeneous and inhomogeneous wavefront are analyzed, and a model for 3D polarimetric purity is constructed. For linear passive material media, a general model is developed on the basis that any physically realizable linear transformation of Stokes vectors is equivalent to an ensemble average of passive, deterministic nondepolarizing transformations. Through this framework, the relevant physical quantities, including indices of polarimetric purity, are identified and decoupled. Some decompositions of the whole system into a set of well-defined components are considered, as well as techniques for isolating the unknown components by means of new procedures for subtracting coherency matrices. These results and methods constitute a powerful tool for analyzing and exploiting experimental and industrial polarimetry. Some particular application examples are indicated.