Optical metamaterials are usually based on planarized, complex-shaped, resonant nano-inclusions. Three-dimensional geometries may provide a wider set of functionalities, including broadband chirality to manipulate circular polarization at the nanoscale, but their fabrication becomes challenging as their dimensions get smaller. Here we introduce a new paradigm for the realization of optical metamaterials, showing that three-dimensional effects may be obtained without complicated inclusions, but instead by tailoring the relative orientation within the lattice. We apply this concept to realize planarized, broadband bianisotropic metamaterials as stacked nanorod arrays with a tailored rotational twist. Because of the coupling among closely spaced twisted plasmonic metasurfaces, metamaterials realized with conventional lithography may effectively operate as three-dimensional helical structures with broadband bianisotropic optical response. The proposed concept is also shown to relax alignment requirements common in three-dimensional metamaterial designs. The realized sample constitutes an ultrathin, broadband circular polarizer that may be directly integrated within nanophotonic systems.

/pdf/twisted-optical-metamaterials-for-planarized-ultrathin-2w3in3n43h.pdf

Twisted optical metamaterials for planarized ultrathin broadband circular polarizers

Foreword. Series Editor's Foreword. Preface. 1. Liquid crystal physics.* Introduction.* Thermodynamics and statistic physics.* Orientational order.* Elastic properties of liquid crystals.* Response of liquid crystals to electro-magnetic fields.* Anchoring effects of nematic liquid crystal at surfaces. 2. Propagation of light in anisotropic optical medium.* Electromagnetic wave.* Polarization.* Propagation of light in uniform anisotropic optical media.* Propagation of light in cholesteric liquid crystals. 3. Optical modeling methods.* Jones matrix method.* Mueller matrix method.* Berreman 4x4 method. 4. Effects of Electric field on Liquid Crystals.* Dielectric interaction.* Flexoelectric Effect.* Ferroelectricity in liquid crystals. 5. Freedericksz transition.* Calculus of variation.* The Fredeericksz transition: statics.* The Freedericksz transition: dynamics. 6. Liquid Crystal Materials.* Introduction.* Refractive indices.* Dielectric constants.* Rotational Viscosity.* Elastic constant.* Figure-of-merits.* Refractive index matching between liquid crystals and polymers. 7. Modeling of liquid crystal director configuration.* Electric energy of liquid crystals.* Modeling electric field.* Simulation of liquid crystal director configuration. 8. Transmissive liquid crystal display.* Introduction.* Twisted nematic cells.* In plane switching (IPS) mode.* Vertical alignment (VA) mode.* Multi-domain Vertical Alignment (MVA) Cells.* Optically compensated bend (OCB) cell. 9. Reflective and Trasreflective display.* Introduction.* Reflective liquid crystal displays.* Transflector.* Classification of Transflective LCDs.* Dual-cell-gap Transflective LCDs.* Single-cell-gap Transflective LCDs.* Performance of transflective LCDs. 10. Liquid crystal display matrices, drive schemes and bistable displays.* Segmented displays.* Passive matrix displays and drive scheme.* Active Matrix Displays.* Bistable ferroelectric liquid crystal displays and drive scheme.* Bistable nematic displays.* Bistable cholesteric reflective display. 11. Liquid crystal/polymer composites. * Introduction.* Phase separation.* Scattering properties of liquid crystal/polymer composites.* Polymer dispersed liquid crystals.* Polymer stabilization liquid crystals.* Displays from liquid crystal/polymer composites. 12. Tunable liquid crystal photonic devices. * Introduction.* Laser beam steering.* Variable Optical Attenuators.* Tunable-Focus Lens.* Polarization-Independent LC Devices. Index.

Fundamentals of Liquid Crystal Devices

We analyze the design of ultrathin quarter-wave plates based on plasmonic metasurfaces. After exploring the general theoretical possibilities offered by thin surfaces to manipulate the impinging polarization, we propose optimal designs to realize quarter-wave metasurface plates, analyzing their frequency and angular response. Our designs may provide a large degree of linear polarization output for circularly polarized input over a broad bandwidth in the optical regime. The geometry may be implemented within currently available lithographic techniques and easily integrated with other optical devices for polarization manipulation, detection, and sensing at the nanoscale.

Manipulating light polarization with ultrathin plasmonic metasurfaces

Wide-band tunability, large coherence-area, and in some cases multidirectional emission have made liquid-crystal lasers an attractive light source for applications like miniature medical diagnostic tool and large-area holographic laser displays. This article discusses the scientific origins of the technology of liquid-crystal lasers and reviews the current cutting-edge research.

Liquid-crystal lasers

The review describes the status of the studies and the recent achievements in the field of photoalignment of liquid crystals. An update classification of photoaligning materials and exposure schemes, and analyzes of the relationship between the molecular structure of the materials and characteristics of LC alignment are provided. In addition, bulk mediated photoalignment and combination of photoalignment with other alignment methods are discussed. Along with traditional, recently proposed applications of the photoalignment technique are considered.

Photoalignment of liquid crystals: basics and current trends

We have investigated the physical and optical properties of the left-handed chiral dopant ZLI-811 mixed in a nematic liquid crystal (LC) host BL006. The solubility of ZLI-811 in BL006 at room temperature is ~24 wt%, but can be enhanced by increasing the temperature. Consequently, the photonic band gap of the cholesteric liquid crystal (CLC) mixed with more than 24 wt% chiral dopant ZLI-811 is blue shifted as the temperature increases. Based on this property, we demonstrate two applications in thermally tunable band-pass filters and dye-doped CLC lasers.

Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility.

A scattering-free broadband (~120 nm bandwidth) circular polarizer is demonstrated by stacking three chiral polymer films with different pitch lengths. Using 4×4 matrix method, we have theoretically simulated the transmission spectra of each chiral polymer film and the three stacked films. Simulation results agree well with experiment. A broadband circular polarizer with bandwidth ranging from 400 to 736 nm can be achieved by stacking 8 such chiral polymer films together. Simulation results indicate that if a high birefringence (∆n~0.35) polymer film is employed the number of films can be reduced to three. Potential applications of these circular polarizers for liquid crystal displays, optical communications, and optical remote sensors are discussed.

Broadband circular polarizer using stacked chiral polymer films.

A spatially tunable laser emission of the dye-doped cholesteric liquid crystal (CLC) cell using a one-dimensional temperature gradient is demonstrated. The photoexcitation of dye-doped CLC device using a frequency-doubled pulsed Nd: yttrium–aluminium–garnet laser gives rise to laser emission in the yellow-red spectral range. The lasing wavelength is widely tunable from 577 to 670 nm by shifting the position of the dye-doped CLC cell with respect to the pumping beam. The lowest excitation energy and maximum lasing efficiency occur at λ∼605nm which corresponds to the peak fluorescence emission of the dye.

/pdf/spatially-tunable-laser-emission-in-dye-doped-photonic-51az6433x5.pdf

Spatially tunable laser emission in dye-doped photonic liquid crystals

A low threshold and high efficiency laser based on dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input excitation with the same handedness of circular polarization as the helical structure of the sample at the shorter wavelength band edge of the reflection band. The responsible mechanism originates from the dramatic increase of the optical density of state (DOS) at the band edges. The calculated DOS of the CLC system confirms the authors’ experimental results.

/pdf/low-threshold-and-high-efficiency-lasing-upon-band-edge-10v6qq25if.pdf

Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal

We experimentally and theoretically investigated the optical switching characteristics of bacteriorhodopsin (bR) at λ=633 nm using the pump-probe method. A diode-pumped second harmonic YAG laser (λ=532 nm which is located around the maximum initial Br state absorption) was used as a pumping beam and a cw He-Ne laser (λ=633 nm which is around the peaks of K and O states) was used as a probe. Due to the nonlinear intensity induced excited state absorption of the K, L, M, N, and O states in the bR photocycle, the switching characteristics are sensitive to the intensity of the probe and pump beams. Based on this property, we have demonstrated an all-optical device functioning as 11 kinds of variable binary all-optical logic gates.

Yuhua Huang

Papers

Tuning the photonic band gap in cholesteric liquid crystals by temperature-dependent dopant solubility.

Broadband circular polarizer using stacked chiral polymer films.

Spatially tunable laser emission in dye-doped photonic liquid crystals

Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal

All-optical switching characteristics in bacteriorhodopsin and its applications in integrated optics