Rita Asquini

Guided-wave liquid-crystal photonics

In this paper we review the state of the art in the field of liquid-crystal tunable guided-wave photonic devices, a unique type of fill-once, molecular-level actuated, optofluidic systems. These have recently attracted significant research interest as potential candidates for low-cost, highly functional photonic elements. We cover a full range of structures, which span from micromachined liquid-crystal on silicon devices to periodic structures and liquid-crystal infiltrated photonic crystal fibers, with focus on key-applications for photonics. Various approaches on the control of the LC molecular orientation are assessed, including electro-, thermo- and all-optical switching. Special attention is paid to practical issues regarding liquid-crystal infiltration, molecular alignment and actuation, low-power operation, as well as their integrability in chip-scale or fiber-based devices.

Nematic Liquid Crystal Optical Channel Waveguides on Silicon

We demonstrate the first channel waveguides made of E7 nematic liquid crystal (LC) in SiO2-Si V-grooves. The grooves have been obtained by wet etching n-Si substrates first and then by thermally growing an approximately 2-mum-thick SiO2 cladding layer. Propagation of infrared light at a wavelength of 1550 nm shows a good optical confinement in 10-mum-wide LC waveguides. Modal analysis and beam propagation simulations predict single mode propagation. This is experimentally confirmed by the acquired near field images. The optical waveguide acts as an integrated optic polarizer, since only vertical polarization can propagate due to the orientation of the LC molecules. The horizontal polarization state is suppressed by more than 25 dB

A Switchable Liquid-Crystal Optical Channel Waveguide on Silicon

An integrated optical switch based on liquid crystal on silicon is reported. The switch consists of an optical waveguide, whose core is made of nematic liquid crystal E7 infiltrated in a SiO2/Si V-groove. The electrooptic effect allows us to control the waveguide propagation condition by means of ITO and Si electrodes. A voltage as low as 2 V applied to the driving electrodes turns the waveguide on. Since the output intensity increases with voltage, the waveguide behaves also as a variable optical attenuator. A maximum ON-OFF extinction ratio in excess of 44 dB was measured.

Electro-optic properties of switchable gratings made of polymer and nematic liquid-crystal slices

We report the diffraction properties at wavelengths of 632.8 and 1550 nm for volume transmission gratings made of a sequence of continuously aligned nematic liquid-crystal layers separated by isotropic polymer slices. The gratings are generated by holographically curing a solution of liquid crystal diluted in an isotropic prepolymer by means of a laser beam at a wavelength of 352 nm with a total intensity of approximately 10 mW/cm2. A diffraction efficiency of 98% was measured, and an electric field as low as 5 V/microm switches off the phase grating. Measured angular spectra are fitted by use of the modified coupled-mode theory including the effects of grating birefringence.

All-optical intensity modulation of near infrared light in a liquid crystal channel waveguide

We demonstrate a nonlinear optical channel waveguide made of E7 nematic liquid crystal infiltrated in a silica on silicon groove. Near infrared light at the wavelength of 1560 nm fiber coupled to the core of the liquid crystal waveguide was optically modulated by an optical beam with power below 25 mW by exploiting the optical Freedericks transition. By modeling the optical molecular reorientation in the nematic liquid crystal confined in a waveguiding geometry we are able to reproduce the experimental results.

Integrated electro-optic switch in liquid crystals.

We demonstrate a compact, low-power and broadband electro-optic switch in a simple waveguide geometry with undoped nematic liquid crystals. We experimentally achieve near infrared switching and signal routing with voltage modulations as low as 0.21V and a device length of 0.16mm.

Liquid Crystal Devices for Photonic Switching Applications: State of the Art and Future Developments

Liquid crystal devices to perform optical switching, filtering and to build optical crossconnects for wavelength division multiplexed optical communication systems are reviewed. Basic working principle of both devices already commercialised and those ones still at laboratory stage reported in the recent scientific literature are described. Devices based on liquid crystals and those ones made by using alternative technologies (MEMS, lithium niobate, semiconductors) are compared in terms of advantages and disadvantages. Technology issues to fabricate liquid crystal based photonic devices are discussed. Recent results of both free-space and waveguided based liquid crystal optical devices along with novel device concepts using optical channel waveguides are reported.

Observation of tunable optical filtering in photosensitive composite structures containing liquid crystals

We report on the investigation and characterization of an optically tunable filtering effect, observed in a waveguide grating made of alternated strips of photocurable polymer and a mixture of azo-dye-doped liquid crystal. The grating is sandwiched between two borosilicate glasses, one of which includes an ion-exchanged channel waveguide, which confines the optical signal to be filtered. Exposure to a low power visible light beam modifies the azo-dye molecular configuration, thus allowing the filtered wavelength to be tuned over a 6.6 nm range. Simulations of the filtering response are well described with our experimental findings.

Widely tunable electro-optic distributed Bragg reflector in liquid crystal waveguide

We propose and numerically investigate a versatile and easy-to-realize configuration for a guided-wave voltage-tunable distributed feedback grating based on reorientation in nematic liquid crystal and coplanar comb electrodes. The device has a wide tuning range exceeding 100 nm and covers C and L bands for wavelength division multiplexing.

Electro-optic routing in a nematic liquid-crystal waveguide

We propose a versatile guided-wave geometry encompassing electro-optic control for signal routing. A zero-gap directional coupler in liquid crystal can switch between two output states in the guide plane, permitting signal rerouting with modulation voltages as small as 70 mV. In the absence of an applied bias, no guiding--hence no modal output--is provided by the structure.