Research ArticleMATERIALS SCIENCE

Stimulated transformation of soft helix among helicoidal, heliconical, and their inverse helices

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Science Advances  04 Oct 2019:
Vol. 5, no. 10, eaax9501
DOI: 10.1126/sciadv.aax9501
  • Fig. 1 Helix transformation based on a photoresponsive cholesteric LC system by dual stimulation with electric field and light.

    (A) Molecular structures of the photodynamic chiral molecular switch (s,s)-BTTC in the ring-open form and the ring-closed form, acting as the chiral motor for the right-handedness and left-handedness helices, respectively. The ring-open structure transforms into the ring-closed isomer and backward upon irradiation with UV (310 nm) and visible (530 nm) light, respectively. Both isomers are fatigue resistant with no thermal relaxation. (B) Schematic illustration of the transformations among the diverse LC helices under the dual stimulation of electric field and light. The top and bottom panels of helical superstructures are in the PSSvis and PSSuv states, respectively. The electric field across the LC cell is the driving force for the transition among different helical superstructures in the same handedness, while suitable light irradiation imposes a reversible handedness inversion between the two panels. (i) The right-handedness and (xii) the left-handedness helicoidal superstructures with the LC director being perpendicular to the helical axis, E < E1. (ii) The right-handedness and (vii) left-handedness heliconical superstructures, in which the LC director is tilted with an oblique angle of θ to the helical axis, E > E1. The heliconical pitch length P and oblique angle θ decrease as the electric field increases, resulting in a wide spectral range tunability of the reflection spectrum from the NIR to the NUV band for the right-handedness (ii to v) and left-handedness (viii to xi) heliconical superstructures, respectively. (vi and xii) The unwound homeotropic nematic state, E > E2. Here, E1 and E2 are the thresholds of the transition from the helicoidal helix to the heliconical helix and the unwinding electric field of a heliconical helix, respectively.

  • Fig. 2 Performance of soft superstructure transformation with electrical and optical stimuli as driving parameters.

    (A) Texture images of different helical superstructures under the polarizing optical microscope, which indicate the helicoidal superstructure at 0 V/μm, the heliconical superstructures with varying pitches from 0.67 to 1.88 V/μm, and the unwound state at 3.0 V/μm. Some dislocation lines can be observed due to the nucleation process during the pitch adjustment. Scale bar, 300 μm. The insets of (ii), (viii), (x), and (xvi) are the conoscopic images at the corresponding states. (B) Measured reflection spectra from the heliconical superstructures in the NUV, visible, and NIR range. The electric field is the stimulus to drive the helix transformation in the PSSvis state or the PSSuv state for the right-handedness or the left-handedness helix, respectively. On the other hand, UV and visible light irradiation drives the helix transformation downward and upward between the two panels, inverting the handedness. The positive and negative intensity value denotes the reflection from the right-handedness and left-handedness heliconical superstructures, respectively. a.u., arbitrary units. (C) Handedness switching performance of the helical superstructure using light stimulus. Reflection intensity and handedness as a function of the irradiation time using the UV (circles) and visible light (triangles) on the initial PSS right-handedness and left-handedness heliconical superstructures, respectively. The applied electric field was maintained as 1.17 V/μm throughout the whole optical tuning process. The reflection central wavelength merely changes during the switching process. A light source at the central wavelength of 310 nm (1.8 mW/cm2) and 530 nm (5.5 mW/cm2) was used for UV and visible irradiation, respectively.

  • Fig. 3 Wavelength and polarization tunable mirrorless lasing from the heliconical superstructure.

    (A) Illustration of the experimental setup for the characterization of laser performance (see Materials and Methods). NDFs, neutral density filters; BS, beam splitter. Output emission to the sample normal was analyzed using a fiber-coupled spectrometer (FCS) by rotating the polarizer (P), where the residue pump beam was blocked by a spectral filter (SF; cutoff wavelength, <550 nm). A quarter wave plate (QWP) at the wavelength of 632 nm was used to convert the circularly polarized emission into linear polarization. (B) Laser emission spectra from the heliconical superstructures with right-handedness (upper part) and left-handedness (lower part) when the electric field was varied between 1.09 and 1.32 V/μm. The positive and negative emission intensities denote the laser emission with right-handedness and left-handedness of circular polarization, respectively. (C) R-CP and L-CP laser emission intensity as a function of the polarization angle in the polar coordinate system from the heliconical superstructure in the PSSvis state and PSSUV state, respectively, where the polar angle ω stands for the transmission angle of the polarizer and the radius r stands for the transmittance.

  • Fig. 4 Electric tunable micropatterns with prescribed microregions of molecular architectures by photopatterning.

    (A) Reversible photopatterning and photoerasing. (i) The PSSvis state of the uniform heliconical texture exhibiting homogeneous red reflection at an electric field of 1.17 V/μm. (ii) Dosage-controlled UV irradiation (1.8 mW/cm2) through a photomask results in a two-dimensional (2D) circular pattern composed of periodic distribution of nematic and right-handedness heliconical regions. The inset shows the local enlarged micrograph of the 2D pattern. In exposed regions, the twist toque is reduced as a result of the decrease in the effective chirality, leading to the homeotropic state under the electric field. One may refer to the phototuning dynamics in Fig. 2C. (iii) Uniform abundant UV irradiation erased the afore-recorded pattern into the homogeneous left-handedness heliconical superstructure showing red reflection. (iv) Dosage-controlled visible irradiation (5.5 mW/cm2) through a photomask results in a stripe pattern composed of periodic distribution of nematic and left-handedness heliconical regions. (v) Uniform abundant visible irradiation erases the afore-recorded pattern and transforms it into the homogeneous right-handedness heliconical superstructure. (vi) Abundant UV irradiation through a photomask results in 1D line grating composed of periodic left- and right-handedness heliconical regions. Uniform abundant visible irradiation erases the afore-recorded pattern into the homogeneous right-handedness heliconical superstructure state. The electric field is maintained as 1.17 V/μm throughout the whole process. (B) Reflection textures at different applied electric fields for (i) the concentric circles composed of regions of the nematic phase and the right-handedness heliconical superstructure, (ii) the Fresnel lens composed of regions of the nematic phase and the left-handedness heliconical superstructure, and (iii) the 1D line grating composed of regions of the right-handedness heliconical superstructure, the left-handedness heliconical superstructure, and the boundary nematic phase. (C) Diffraction patterns for (i) the circular grating, (ii) the Fresnel lens, and (iii) the 1D line grating at an electric field of 1.17 V/μm measured with an He-Ne laser beam at 632.8 nm. HN, heliconical state; UW, unwound state.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax9501/DC1

    Fig. S1. Schematic illustration of LC director configurations in helicoidal, heliconical, and unwound superstructures as a function of the electric field.

    Fig. S2. Electrical tuning performance of the heliconical superstructure.

    Fig. S3. Simulated selective reflection spectra in the right-handed heliconical superstructure with various applied electric fields in the cases of R-CP incidence and L-CP incidence.

    Fig. S4. Theoretical investigation of the effects of the photoinduced chirality changes on switching performance.

    Fig. S5. Calculated reflection spectra from the heliconical superstructure as a function of the electric field and light stimuli.

    Fig. S6. Chemical structure of the LC dimer CB7CB.

    Fig. S7. Optimization of the cholesteric LC material system.

    Fig. S8. UV-visible changes of (s,s)-BTTC in tetrahydrofuran solution and the thermal stability and fatigue resistance of closed-ring isomers.

    Fig. S9. Characterization of the optical properties of the laser dye DCJ.

    Fig. S10. Emission intensity as a function of pump energy for the right-handed heliconical resonator and the left-handed heliconical resonator.

    References (3339)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Schematic illustration of LC director configurations in helicoidal, heliconical, and unwound superstructures as a function of the electric field.
    • Fig. S2. Electrical tuning performance of the heliconical superstructure.
    • Fig. S3. Simulated selective reflection spectra in the right-handed heliconical superstructure with various applied electric fields in the cases of R-CP incidence and L-CP incidence.
    • Fig. S4. Theoretical investigation of the effects of the photoinduced chirality changes on switching performance.
    • Fig. S5. Calculated reflection spectra from the heliconical superstructure as a function of the electric field and light stimuli.
    • Fig. S6. Chemical structure of the LC dimer CB7CB.
    • Fig. S7. Optimization of the cholesteric LC material system.
    • Fig. S8. UV-visible changes of (s,s)-BTTC in tetrahydrofuran solution and the thermal stability and fatigue resistance of closed-ring isomers.
    • Fig. S9. Characterization of the optical properties of the laser dye DCJ.
    • Fig. S10. Emission intensity as a function of pump energy for the right-handed heliconical resonator and the left-handed heliconical resonator.
    • References (3339)

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