Renfan Shao | University of Colorado, Boulder (original) (raw)
Papers by Renfan Shao
Physics, 2008
The phase difference between the fast and slow axes of a liquid crystal variable retarders (LCVR)... more The phase difference between the fast and slow axes of a liquid crystal variable retarders (LCVR) can be changed continuously by modulating an AC voltage applied on the bi-surface of the LCVR.The polarization of an input light beam can thus be controlled by varying the voltage.The calibration method and results of determining the different phase retardances for given wavelengths are described.The theory and method for measuring the integral Stokes parameters by LCVR are also discussed.
Ferroelectrics, Sep 1, 1991
We investigate the texture which appears as parallel stripes perpendicular to the smectic layers ... more We investigate the texture which appears as parallel stripes perpendicular to the smectic layers in SSFLC cells. When the temperature approaches the smectic A to smectic C* transition from above in the smectic A phase, a DC field induces the chevron structure due to the electroclinic effect and then the stripe texture is formed. We have identified the stripes to be parallel zig-zag walls. We show that a reasonably high DC voltage changes the structure of the cell from chevron to book-shelf in the smectic C* phase and we show how this happens. We have experimentally found that the width of the stripes is approximately equal to the thickness of the cell and we present a theoretical explanation of the equivalence. We have measured the layer tilt angles in the plane of the cell as a function of temperature and have found them to be the same as the chevron tilt angles determined by X-ray scattering. The result confirms the chevron layer structure and stripe texture models and also gives a simple way to measure chevron layer tilt angles.
Bulletin of the American Physical Society, Mar 4, 2014
Liquid Crystals, 1989
The theoretical and experimental developments in the interfacial dynamics and the formation of vi... more The theoretical and experimental developments in the interfacial dynamics and the formation of viscous fingering patterns in Hele-Shaw cells of liquid crystal-air systems are summarized and discussed. These include radial and linear cells with or without grooves engraved on the cell plates. Instabilities of fingers, the role of intrinsic and extrinsic anisotropies, etc., are emphasized. In a linear cell, when
Liquid Crystals, 1987
ABSTRACT
Ferroelectrics, 2002
V-shaped switching was founded in a SmC* ferroelectric liquid crystal with the N*-SmC* phase sequ... more V-shaped switching was founded in a SmC* ferroelectric liquid crystal with the N*-SmC* phase sequence. When the FLC samples were cooled down from the isotropic phase to the SmC* phase without an applied voltage, the SmC* phase adapted a stripe-like quasi-periodic texture, ...
Chemical Science, 2014
All glassware was oven-dried or flame-dried. CH 2 Cl 2 and toluene were distilled from CaH 2 unde... more All glassware was oven-dried or flame-dried. CH 2 Cl 2 and toluene were distilled from CaH 2 under nitrogen; THF were distilled from sodium benzophenone ketyl under nitrogen. Unless specifically mentioned, all chemicals are commercially available and were used as received. Flash chromatography was performed using 60 Å silica gel (37-75 μm). 1 H NMR spectra were recorded at either 400 MHz or 500 MHz, and 13 C NMR spectra were recorded at 100 MHz or 101 MHz in CDCl 3 .Chemical shifts are reported in ppm referenced to residual solvent peaks as follows: CDCl 3 (7.26 ppm for 1 H NMR; 77.23 ppm for 13 C NMR). Phase transition temperatures were determined by DSC using a Mettler Toledo DSC823. POM data was collected using a Nikon-HCS400 microscope with an Instec STC200 temperature-controlled stage. X-ray experiments were temperature controlled by an Instec STC200 hotstage, which was mounted on a Huber four-circle goniometer. The data were collected using a point detector with either of the following: synchrotron radiation at beamline X10A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (High Resolution); or Cu K(α) radiation from a Rigaku UltraX-18 rotating anode generator, operated by the Liquid Crystal Materials Research Center, University of Colorado Boulder (Low Resolution).). Portion of Small-angle X-ray scattering (SAXS) was taken at the Advanced Light Source (ALS) beamline 7.3.3 at Lawrence Berkeley National Lab. A silver behenate sample was used as a standard calibrant. The 2D scattering patterns were collected on a 1M Pilatus detector from Dectris (www.dectris.com). The measured two-dimensional scattering data were calibrated and converted to q-space using the Nika macro for Igor Pro developed by Jan Ilavsky at Argonne National Laboratory, where the scattering wave vector q = 4π sin(θ/2)/λ, λ is the wavelength of the incident X-rays (0.124 nm) and θ is the scattering angle.
Physical Review E, Mar 10, 2014
We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (d... more We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.
Organic thin films with large second order nonlinear susceptibility χ(2) are of potential utility... more Organic thin films with large second order nonlinear susceptibility χ(2) are of potential utility in fast integrated electro-optic modulators. Ferroelectric liquid crystals (FLCs), possessing a thermodynamically stable polar structure, and therefore non-zero χ(2) with infinite "thermal stability," combined with good processibility on semiconductor substrates, seem to provide an excellent approach to creation of χ(2) thin films. Indeed, we have outlined in detail an approach for achieving useful χ(2) values in FLCs by orientation of the p-nitroaniline unit along the FLC polar axis. [1] This approach was later shown to afford an FLC glass with χ(2) values on the order of 5 pm/V. [2] However, to be most useful for fabrication of integrated electro-optic modulators, χ(2) values on the order of 50 pm/V arc required.
Bulletin of the American Physical Society, Mar 4, 2020
We have observed a metastable sequence of optical texture with a batonnet morphology in the de Vr... more We have observed a metastable sequence of optical texture with a batonnet morphology in the de Vries smectic A* (de Vries SmA*) phase formed by silicon-containing molecule. [1] de Vries SmA* liquid crystal (LC) placed on silicon wafer and confined in silicon microchannel was analysed based on depolarized reflection light microscopy (DRLM) to observe the distinctive behaviour with a metastable structure. During rapid cooling from isotropic to de Vries SmA* phase, the batonnet structures was generated as an intermediate and metastable state, and then thermodynamically stable toric focal conic domains (TFCDs) were finally formed. This distinctive behaviour of phase transition was examined at the molecular level using grazing incidence X-ray diffraction (GIXD) techniques. We concluded that aggregation of LC molecules arise from the silicon part in de Vries LC molecule, leading to the formation of layered clusters called cybotactic structures. [2-4] The understanding of unusual phase transition accompanied by a batonnet structure in de Vries SmA* phase can provide a clue to the unsolved properties of de Vries LC and advance the development of optoelectronic devices.
Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals (CD-ROM), Sep 1, 1996
Combining excellent processibility on silicon integrated circuits with a thermodynamically stable... more Combining excellent processibility on silicon integrated circuits with a thermodynamically stable polar structure, ferroelectric liquid crystals (FLCs) provide an attractive potential approach to synthesis of materials for second order nonlinear optics (NLO). In order for this potential to be realized, however, adequate magnitude of the second order susceptibility χ must be obtained. Recent results of experiments demonstrating orientation along the
Journal of the American Chemical Society, Jul 1, 1991
... Dimethylmercury is a linear molecule that has a value of 2J = 22.4 Hz.Io The relatively large... more ... Dimethylmercury is a linear molecule that has a value of 2J = 22.4 Hz.Io The relatively large magnitude of 2J for RCu-(CN)Li suggests a linear ... David M. Walba,* M. Blanca Ros,t Noel A. Clark,$ Renfan Shao,* Michael G. Robinson,f Jian-Yu Liu! ... Communications to the Editor ...
Macromolecular Rapid Communications, Nov 19, 2009
Ferroelectrics, Sep 1, 1991
Abstract We have recently reported the first FLCs designed specifically for large second order no... more Abstract We have recently reported the first FLCs designed specifically for large second order nonlinear optical (NLO) susceptibility χ (2); a series of o-nitro-1-methylheptyloxy biphenylbenzoates and phenyl biphenylcarboxylates. Properties of a stable room-...
Proceedings of the National Academy of Sciences, 2020
We report the experimental determination of the structure and response to applied electric field ... more We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm 2 , the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipol...
Physical Review Letters, 2019
An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a non... more An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a non-polar, achiral de Vries smectic A which transitions to a chiral, ferroelectric state in applied electric fields above a threshold. At lower temperature, a chiral, ferrielectric phase with a periodic, supermolecular modulation of the tilt azimuth, indicated by a Bragg peak in carbon-edge resonant soft X-ray scattering, is observed. The absence of a corresponding resonant Umklapp peak identifies the superlayer structure as a twist-bend-like helix that is only weakly modulated by the smectic layering.
Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on... more Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on the electric field-induced reorientation of independent tilt domains [J. V. Selinger et al., Phys. Rev. E 64, 061705 (2001)] fails to account for the sigmoidal dependence of induced polarization (P) on field (E) seen in some materials. To account for this behavior, we model de Vries smectics as ensembles of small but finite anticlinic tilt domains. Within each domain, interlayer tilt coupling favors anticlinic interfaces, but the finite range of in-layer tilt correlations leads to thermally activated synclinic interfaces and a finite tilt correlation length along the layer normal. This model, equivalent to a generalized one-dimensional XY model in an external field with quadratic and quartic nearest-neighbor interactions, is studied by Monte Carlo simulation and transfer matrix methods. The model successfully reproduces the dependence of P on E for a specific material (W530), and yields...
Science, 2003
Any polar-ordered material with a spatially uniform polarization field is internally frustrated: ... more Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or “splayed.” However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such ph...
Physics, 2008
The phase difference between the fast and slow axes of a liquid crystal variable retarders (LCVR)... more The phase difference between the fast and slow axes of a liquid crystal variable retarders (LCVR) can be changed continuously by modulating an AC voltage applied on the bi-surface of the LCVR.The polarization of an input light beam can thus be controlled by varying the voltage.The calibration method and results of determining the different phase retardances for given wavelengths are described.The theory and method for measuring the integral Stokes parameters by LCVR are also discussed.
Ferroelectrics, Sep 1, 1991
We investigate the texture which appears as parallel stripes perpendicular to the smectic layers ... more We investigate the texture which appears as parallel stripes perpendicular to the smectic layers in SSFLC cells. When the temperature approaches the smectic A to smectic C* transition from above in the smectic A phase, a DC field induces the chevron structure due to the electroclinic effect and then the stripe texture is formed. We have identified the stripes to be parallel zig-zag walls. We show that a reasonably high DC voltage changes the structure of the cell from chevron to book-shelf in the smectic C* phase and we show how this happens. We have experimentally found that the width of the stripes is approximately equal to the thickness of the cell and we present a theoretical explanation of the equivalence. We have measured the layer tilt angles in the plane of the cell as a function of temperature and have found them to be the same as the chevron tilt angles determined by X-ray scattering. The result confirms the chevron layer structure and stripe texture models and also gives a simple way to measure chevron layer tilt angles.
Bulletin of the American Physical Society, Mar 4, 2014
Liquid Crystals, 1989
The theoretical and experimental developments in the interfacial dynamics and the formation of vi... more The theoretical and experimental developments in the interfacial dynamics and the formation of viscous fingering patterns in Hele-Shaw cells of liquid crystal-air systems are summarized and discussed. These include radial and linear cells with or without grooves engraved on the cell plates. Instabilities of fingers, the role of intrinsic and extrinsic anisotropies, etc., are emphasized. In a linear cell, when
Liquid Crystals, 1987
ABSTRACT
Ferroelectrics, 2002
V-shaped switching was founded in a SmC* ferroelectric liquid crystal with the N*-SmC* phase sequ... more V-shaped switching was founded in a SmC* ferroelectric liquid crystal with the N*-SmC* phase sequence. When the FLC samples were cooled down from the isotropic phase to the SmC* phase without an applied voltage, the SmC* phase adapted a stripe-like quasi-periodic texture, ...
Chemical Science, 2014
All glassware was oven-dried or flame-dried. CH 2 Cl 2 and toluene were distilled from CaH 2 unde... more All glassware was oven-dried or flame-dried. CH 2 Cl 2 and toluene were distilled from CaH 2 under nitrogen; THF were distilled from sodium benzophenone ketyl under nitrogen. Unless specifically mentioned, all chemicals are commercially available and were used as received. Flash chromatography was performed using 60 Å silica gel (37-75 μm). 1 H NMR spectra were recorded at either 400 MHz or 500 MHz, and 13 C NMR spectra were recorded at 100 MHz or 101 MHz in CDCl 3 .Chemical shifts are reported in ppm referenced to residual solvent peaks as follows: CDCl 3 (7.26 ppm for 1 H NMR; 77.23 ppm for 13 C NMR). Phase transition temperatures were determined by DSC using a Mettler Toledo DSC823. POM data was collected using a Nikon-HCS400 microscope with an Instec STC200 temperature-controlled stage. X-ray experiments were temperature controlled by an Instec STC200 hotstage, which was mounted on a Huber four-circle goniometer. The data were collected using a point detector with either of the following: synchrotron radiation at beamline X10A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (High Resolution); or Cu K(α) radiation from a Rigaku UltraX-18 rotating anode generator, operated by the Liquid Crystal Materials Research Center, University of Colorado Boulder (Low Resolution).). Portion of Small-angle X-ray scattering (SAXS) was taken at the Advanced Light Source (ALS) beamline 7.3.3 at Lawrence Berkeley National Lab. A silver behenate sample was used as a standard calibrant. The 2D scattering patterns were collected on a 1M Pilatus detector from Dectris (www.dectris.com). The measured two-dimensional scattering data were calibrated and converted to q-space using the Nika macro for Igor Pro developed by Jan Ilavsky at Argonne National Laboratory, where the scattering wave vector q = 4π sin(θ/2)/λ, λ is the wavelength of the incident X-rays (0.124 nm) and θ is the scattering angle.
Physical Review E, Mar 10, 2014
We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (d... more We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.
Organic thin films with large second order nonlinear susceptibility χ(2) are of potential utility... more Organic thin films with large second order nonlinear susceptibility χ(2) are of potential utility in fast integrated electro-optic modulators. Ferroelectric liquid crystals (FLCs), possessing a thermodynamically stable polar structure, and therefore non-zero χ(2) with infinite "thermal stability," combined with good processibility on semiconductor substrates, seem to provide an excellent approach to creation of χ(2) thin films. Indeed, we have outlined in detail an approach for achieving useful χ(2) values in FLCs by orientation of the p-nitroaniline unit along the FLC polar axis. [1] This approach was later shown to afford an FLC glass with χ(2) values on the order of 5 pm/V. [2] However, to be most useful for fabrication of integrated electro-optic modulators, χ(2) values on the order of 50 pm/V arc required.
Bulletin of the American Physical Society, Mar 4, 2020
We have observed a metastable sequence of optical texture with a batonnet morphology in the de Vr... more We have observed a metastable sequence of optical texture with a batonnet morphology in the de Vries smectic A* (de Vries SmA*) phase formed by silicon-containing molecule. [1] de Vries SmA* liquid crystal (LC) placed on silicon wafer and confined in silicon microchannel was analysed based on depolarized reflection light microscopy (DRLM) to observe the distinctive behaviour with a metastable structure. During rapid cooling from isotropic to de Vries SmA* phase, the batonnet structures was generated as an intermediate and metastable state, and then thermodynamically stable toric focal conic domains (TFCDs) were finally formed. This distinctive behaviour of phase transition was examined at the molecular level using grazing incidence X-ray diffraction (GIXD) techniques. We concluded that aggregation of LC molecules arise from the silicon part in de Vries LC molecule, leading to the formation of layered clusters called cybotactic structures. [2-4] The understanding of unusual phase transition accompanied by a batonnet structure in de Vries SmA* phase can provide a clue to the unsolved properties of de Vries LC and advance the development of optoelectronic devices.
Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals (CD-ROM), Sep 1, 1996
Combining excellent processibility on silicon integrated circuits with a thermodynamically stable... more Combining excellent processibility on silicon integrated circuits with a thermodynamically stable polar structure, ferroelectric liquid crystals (FLCs) provide an attractive potential approach to synthesis of materials for second order nonlinear optics (NLO). In order for this potential to be realized, however, adequate magnitude of the second order susceptibility χ must be obtained. Recent results of experiments demonstrating orientation along the
Journal of the American Chemical Society, Jul 1, 1991
... Dimethylmercury is a linear molecule that has a value of 2J = 22.4 Hz.Io The relatively large... more ... Dimethylmercury is a linear molecule that has a value of 2J = 22.4 Hz.Io The relatively large magnitude of 2J for RCu-(CN)Li suggests a linear ... David M. Walba,* M. Blanca Ros,t Noel A. Clark,$ Renfan Shao,* Michael G. Robinson,f Jian-Yu Liu! ... Communications to the Editor ...
Macromolecular Rapid Communications, Nov 19, 2009
Ferroelectrics, Sep 1, 1991
Abstract We have recently reported the first FLCs designed specifically for large second order no... more Abstract We have recently reported the first FLCs designed specifically for large second order nonlinear optical (NLO) susceptibility χ (2); a series of o-nitro-1-methylheptyloxy biphenylbenzoates and phenyl biphenylcarboxylates. Properties of a stable room-...
Proceedings of the National Academy of Sciences, 2020
We report the experimental determination of the structure and response to applied electric field ... more We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm 2 , the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipol...
Physical Review Letters, 2019
An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a non... more An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a non-polar, achiral de Vries smectic A which transitions to a chiral, ferroelectric state in applied electric fields above a threshold. At lower temperature, a chiral, ferrielectric phase with a periodic, supermolecular modulation of the tilt azimuth, indicated by a Bragg peak in carbon-edge resonant soft X-ray scattering, is observed. The absence of a corresponding resonant Umklapp peak identifies the superlayer structure as a twist-bend-like helix that is only weakly modulated by the smectic layering.
Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on... more Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on the electric field-induced reorientation of independent tilt domains [J. V. Selinger et al., Phys. Rev. E 64, 061705 (2001)] fails to account for the sigmoidal dependence of induced polarization (P) on field (E) seen in some materials. To account for this behavior, we model de Vries smectics as ensembles of small but finite anticlinic tilt domains. Within each domain, interlayer tilt coupling favors anticlinic interfaces, but the finite range of in-layer tilt correlations leads to thermally activated synclinic interfaces and a finite tilt correlation length along the layer normal. This model, equivalent to a generalized one-dimensional XY model in an external field with quadratic and quartic nearest-neighbor interactions, is studied by Monte Carlo simulation and transfer matrix methods. The model successfully reproduces the dependence of P on E for a specific material (W530), and yields...
Science, 2003
Any polar-ordered material with a spatially uniform polarization field is internally frustrated: ... more Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or “splayed.” However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such ph...