David Sparre Hermann

Optical devices based on liquid crystal photonic bandgap fibres

Photonic Crystal Fibers (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are microstructured waveguides, usually in silica, with a large number of air holes located in the cladding region of the fiber. The size and location of these air holes opens up for a large degree of design freedom within optical waveguide design. Further, the existence of air holes in the PCF gives access close to the fiber core and by introducing new materials into the air holes, a high interaction between light and hole material can be obtained, while maintaining the microstructure of the waveguide. In this paper, we describe what we call Liquid Crystal Photonic Bandgap Fibers, which are PCFs infiltrated with Liquid Crystals (LCs) in order to obtain increased fiber functionality. We describe a thermo-optic fiber switch with an extinction ratio of 60dB and tunable PBGs using thermo-optic tuning of the LC. These devices operate by the PBG effect and are therefore highly sensitive to the refractive index distributions in the holes.

All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers

Photonic crystal fibers (PCFs) have attracted significant attention during the last years and much research has been devoted to develop fiber designs for various applications, hereunder tunable fiber devices. Recently, thermally and electrically tunable PCF devices based on liquid crystals (LCs) have been demonstrated. However, optical tuning of the LC PCF has until now not been demonstrated. Here we demonstrate an all-optical modulator, which utilizes a pulsed 532nm laser to modulate the spectral position of the bandgaps in a photonic crystal fiber infiltrated with a dye-doped nematic liquid crystal. We demonstrate a modulation frequency of 2kHz for a moderate pump power of 2-3mW and describe two pump pulse regimes in which there is an order of magnitude difference between the decay times.

Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers.

We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

Highly tunable large-core single-mode liquid-crystal photonic bandgap fiber

We demonstrate a highly tunable photonic bandgap fiber, which has a core diameter of 25 mm, and a bandgap tuning sensitivity of 27 nm/degC at room temperature. The insertion loss is estimated to be less than 0.5 dB.

Langevin Type Alignment in a Smectic Liquid Crystal Mixture Showing V-Shaped Switching As Studied by Optical Second-Harmonic Generation

Second-harmonic generation (SHG) was observed in a homogeneously aligned smectic C*-like liquid crystal (LC) cell showing V-shaped switching during the application of a triangular wave. At normal incidence of light, abnormally strong SHG signals were observed at about zero electric field. It was confirmed that this phenomenon is characteristic in LCs showing the V-shaped switching, and is not observable in typical ferroelectric and antiferroelectric LCs. The dynamic response of SHG was successfully simulated using the two-dimensional Langevin type potential model.

Novel passive polymer waveguides integrated with electro-optically active ferroelectric liquid crystals

We report the preparation and characterization of novel isotropic polymer slab waveguides made by photochemical crosslinking, and their integration with a ferroelectric liquid crystal in an integra ...

A pyroelectric liquid crystal polymer (PLCP) for second-harmonic generation

Abstract In order for a material to show a non-vanishing second order non-linear susceptibility, the material has to have a polar axis. This is hard to achieve in a liquid crystal system because of the strong quadrupolar order along the director. Electrostatic poling of polymers and polymer liquid crystals at high temperature and subsequent quenching to lower temperature only gives a small value of the polar order parameter and, in addition, results in materials which are not in thermodynamic equilibrium and therefore not stable over long times. Starting with a ferroelectric liquid crystal (which has polar order perpendicular to the director) we have succeeded in making a material with true polar order. It is not ferroelectric, although the ferroelectric properties of the starting and intermediate materials are basic for the procedure and for the final product which can be used for frequency doubling, of importance, for instance, in a wide area of optic communication applications.

Fully leaky guided mode study of an orthoconic antiferroelectric liquid crystal cell deviating from perfect horizontal surface stabilization

The optical properties of an antiferroelectric liquid crystal (AFLC) cell slightly deviating from the ideal orthoconic condition, i.e., 45° tilt angle and fully horizontal surface stabilization (the molecular tilt plane parallel to the glass plates), have been investigated using the fully leaky guided mode (FLGM) technique. In contrast to standard optical investigative methods, such as polarization microscopy, the FLGM data give information about the orientation of the index ellipsoid or the molecular tilt plane, not only in the plane of the cell but in three dimensions. The sensitivity of the FLGM technique thus allows a detailed optical characterization on a mesoscopic scale and, in particular, has allowed us to discriminate, in the investigated cell, a tilt out of the substrate plane in the anticlinic state, thus a deviation from the horizontal surface stabilization condition. Orthoconic surface-stabilized AFLCs are, in the ideal case, uniaxial negative with the optic axis perpendicular to the glass pl...

Synthesis and electro-optical behaviour of a new chiral smectic side-chain polysiloxane

SummaryA new chiral side-chain polymer was prepared by hydrosilylation of a dimethylsilane-methyl-hydrosilane copolymer. The chiral group of the mesogenic side-chain was (+)2-chloro-3-(3-nitro-4-hydroxyphenyl)propionic acid butyl ester while the main part of the aromatic nucleus consisted of 4′-hydroxybiphenyl-4-carboxylic acid. Phase behaviour was studied by DSC and polarizing microscopy, and correlated with dielectric measurements. Alignment properties and ferroelectricity were studied in cells with surface-induced alignment and in shear cells. The polymer exhibits chiral C-and A-phases. The spontaneous polarization is 130 nC/cm2 at 60°C and the polymer shows ferroelectric switching at room temperature.

Electro-optic modulation of light by a planar waveguide based on ferroelectric liquid crystals

Abstract We present electro-optic switching from a planar waveguide with a ferroelectric liquid crystal overlayer and discuss the operating principle and the parameters significant for the operation of the device. We have found electro-optic response times of about 20 μs at best as well as a high (5.7%) transmittance in the ON-state of the device. The contrast ratio was 4:1 ON to OFF state.