100 GHz electrically tunable planar Bragg grating via nematic liquid crystal overlay towards reconfigurable WDM networks

Novel liquid crystal-based integrated optical devices with >140GHz electrical tuning are presented for application towards reconfigurable wavelength division multiplexing (WDM) networks. Initial results with Bragg wavelength tuning covering five 25GHz WDM channel spacing have been achieved with 170V (peak-to-peak) sinusoidal voltages applied across electro-patterned ITO-covered glass electrodes placed 60µm apart. These prototype devices were fabricated using direct UV grating writing, with an evanescent field coupling into a liquid crystal overlay through an etched window. Electrically controlled liquid crystal birefringence modifies the waveguide effective index, resulting in Bragg wavelength shift. Merck 18523 nematic liquid crystals are used, exhibiting compatible refractive index values to that of silica (no=1.44, ne=1.49 at lambda=1550nm). Homeotropic alignment of the liquid crystal is provided by application of a surfactant layer.The inherent refractive index sensitivity of our etched direct-UV-written structures allows observation of previously unreported liquid crystal surface-behaviour, such as multi-threshold points during variation of the applied field. Continued optimisation based on evanescent field penetration, electrode layout, and surface interaction will allow implementation towards a variety of novel liquid crystal applications and devices. For example, a cascaded architecture of these integrated liquid crystal devices operating at different Bragg wavelengths would pave the way towards true colorless add/drop modules for dense optical networks

Surface charge layers and beam coupling in photosensitive polymer-liquid crystal structures

We present results on DC field and light-induced photorefractive-like gratings in liquid crystal cells made with photosensitive polymer layers. Surface charge layers that develop on a liquid crystal-polymer interface are responsible for screening the liquid crystal bulk from an external DC field. These layers can be selectively discharged via polymer photoconductivity and lead to liquid crystal reorientation grating. Efficient two-beam coupling gain can arise from the reorientation grating for relatively high DC field (above 30 V). For lower voltages, strong diffraction can be observed that reduces the gain. Two-beam coupling gain and diffraction can also be controlled by experimental geometry and liquid crystal alignment. The simulated profile of electric field shows that its penetration depth depends on grating spacing

Surface screening layers and dynamics of energy transfer in photosensitive polymer-liquid crystal structures

The dynamics of energy transfer in photoconductive polymer liquid crystal structures can contain important information on interface effects and surface electric fields contributing to the strength of liquid crystal reorientation gratings. The characteristic, transient effects observed during switching on and off of incident light or electric field can be explained by the presence of surface screening layers. Screening layers play an important role in the reorientation of liquid crystal director in cells with different alignment layers. Strong screening of external DC field is present not only in cells with a photoconductive polymer (56 V), but in standard cells with thicker (0.3 ?m) polyimide, aligning layers

Photorefractive gratings in liquid crystals with polymer doped C₆₀ layers

Photorefractive-like liquid crystals [1], with their large diffraction efficiencies and nonlinear effects combined with thin film format, are ideal materials for integrated optics applications. Liquid crystals doped with dyes or with added photoconducting polymer layers' show high two-beam coupling gain.The reorientation process that liquid crystal molecules undergo is induced and controlled by the application of light and electric field. There is typically more than one mechanism involved in the reorientation of liquid crystal molecules and, with careful design, the photorefractive space-charge field can play an important role. Moreover, the reorientation can also be strongly influenced by surface-mediated effects and surface-charge modulation, as well as surface anchoring.Photorefractive liquid crystals structures with photoconductive layers on their substrates have shown to be very efficient, in particular using polyvinyl carbazole (PVK) polymer doped with photosensitiser trinitrofluorene (TNF). However, the practical application of this structure is minimised by the toxic nature of the TNF dopant

Nonspecular total internal reflection of spatial solitons at the interface between highly birefringent media

We investigate total internal reflection of nonlocal spatial optical solitons at the interface between two differently oriented regions of a highly birefringent nematic liquid crystal. The solitons survive the interaction with an induced index mismatch and undergo nonspecular reflection, with an emerging angle differing appreciably from the incidence angle

Towards high-speed liquid crystal electrically tunable planar Bragg gratings for integrated optical networks

Liquid crystal-based integrated optical devices offer the potential for high speed and dynamically tunable optical switches in modern telecommunications networks. Here, electrically tunable devices have major advantages over their thermal counterparts, with superior response times and low operating voltages (~100V). Our approach to achieving such devices is to fabricate planar optical waveguides with integrated Bragg gratings via direct UV writing1 into silica-on-silicon samples with evanescent field coupling into a liquid crystal overlay through an etched window (Fig. 1(a)). Such electrically tunable devices work on the principle of shifting the Bragg wavelength by modifying the effective index of a waveguide in a multilayer substrate. Electrically controlled liquid crystal birefringence modifies the waveguide effective index, producing a Bragg wavelength shift. In our early samples, Merck 18523 nematic liquid crystal is used as it has a compatible refractive index to silica (n=1.49 at lambda=1550nm). Homeotropic alignment of the liquid crystal is provided by application of a surfactant layer

Nonlinear shift of spatial solitons at a graded dielectric interface

We investigate total internal reflection of optical spatial solitons at the interface between two regions of nematic liquid crystals with different optical densities. Due to nonlinear molecular reorientation, the solitons experience a penetration depth, hence, a lateral shift that depends on the excitation, with lateral shifts from 0.7 to 1.2 mm as input powers increased from 1.6 to 9.3 mW

Enhanced two-beam coupling in colloids of ferroelectric nanoparticles in liquid crystals

We report on the first, to the best of our knowledge, studies of photorefraction in nematic liquid crystal (LC) doped with nanoferroelectric particles. We found the strong enhancement of two-beam coupling in the colloid of ferroelectric nanoparticles in LC. The effect originated from an increased birefringence of the colloid and a stronger LC reorientation torque. Our measurements allowed us to suggest that increased birefringence is caused by the contribution of polarizability anisotropy of the ferroelectric particles. Stronger reorientation torque is caused by the permanent dipole moment of the particles contributing to the dielectric anisotropy of the colloid a col. The enhancement of two-beam coupling in LCs by doping with ferroelectric nanoparticles at extremely small concentration shows the strong potential of ferroelectric nanoparticles for improving the optical response of LCs, especially for those materials where a method of chemical synthesis has reached its limit

Surface charge screening and boundary conditions for high two-beam coupling gain in pure liquid crystals

We report on asymmetric two-beam coupling and the ways of controlling it in liquid crystals cells with photoconducting polymer layers. The cells had one of the substrates covered with a photoconductive polymer layer, namely PVK, photosensitised with C60 to respond to visible light. Efficient gain was measured in 30 micron thick cells with two incident beams having the same intensity. We present a model of two-beam coupling gain based on the build-up and discharge of surface charge screening layers, spatially modulated due to the photoconductivity of doped PVK. The simulation of electric field distribution inside a liquid crystal cell for different two-beam coupling grating spacing showed different penetration of field into the liquid crystal bulk. The characteristics of dynamics, magnitude of two-beam coupling and the efficiency of diffraction were determined for different values of applied DC field, cell configuration and liquid crystals. We found that the direction of energy flow was determined just by the cell tilt and not by the DC field bias

Voltage-driven in-plane steering of nematicons

Using an external voltage and interdigitated electrodes in a liquid crystalline cell, we demonstrate tunable steering of light beams and spatial solitons without the detrimental walk-off out of the plane of propagation. The results agree well with a simple model describing birefringence and reorientation in uniaxial nematic liquid crystal