Romeo Beccherelli

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.

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.

Evaluation of optical anisotropy in the pretransitional regime in antiferroelectric liquid crystals

By measuring the transmission through an antiferroelectric liquid crystal device (placed between crossed polarizers) as a function of both orientation and applied field in the so-called electroclinic-like, pretransitional regime the behaviour of the effective optic axis tilt angle (psi) and effective optic anisotropy (Delta n) is determined. The relatively poor alignment obtained in antiferroelectric liquid crystal devices is allowed for in the data interpretation and high quality results are presented.

Guided lamb wave electroacoustic devices on micromachined AlN/Al plates

An electroacoustic micro-device based on the propagation of guided acoustic Lamb waves in AlN/Al plate is described. The AlN thin film is deposited by sputtering technique, optimized to achieve a high degree of orientation (rocking curve full-width at half-maximum ¿ 3.5°) of the c-axis perpendicular to the plate surface. The AlN plate is micromachined using anisotropic reactive ion etching (RIE), followed by isotropic RIE to remove the silicon underlayer. Simulation results for the dispersion phase velocity curves and the electromechanical coupling coefficient (K2) are obtained by the matrix method and by the finite element method and compared with experimental data. A delay line is implemented on the structure and tested for the propagation of the first symmetrical Lamb mode (s0) at the frequency of 1.22 GHz. Measurements have shown that the structure is suitable for implementation of arrays of electroacoustic devices on a single chip for application to both sensing devices and signal processing systems.

Tunable terahertz fishnet metamaterials based on thin nematic liquid crystal layers for fast switching.

The electrically tunable properties of liquid-crystal fishnet metamaterials are theoretically investigated in the terahertz spectrum. A nematic liquid crystal layer is introduced between two fishnet metallic structures, forming a voltage-controlled metamaterial cavity. Tuning of the nematic molecular orientation is shown to shift the magnetic resonance frequency of the metamaterial and its overall electromagnetic response. A shift higher than 150 GHz is predicted for common dielectric and liquid crystalline materials used in terahertz technology and for low applied voltage values. Owing to the few micron-thick liquid crystal cell, the response speed of the tunable metamaterial is calculated as orders of magnitude faster than in demonstrated liquid-crystal based non-resonant terahertz components. Such tunable metamaterial elements are proposed for the advanced control of electromagnetic wave propagation in terahertz applications.

Large-Scale Chemical Sensor Array Testing Biological Olfaction Concepts

Biological olfactory systems are characterized by a large number of sensors with broad overlapping specificities. The sensitivity and selectivity of the system may be enhanced by the huge redundancy of the olfactory receptor neurons (ORNs). A European project, NEUROCHEM, was devoted to test computational models of the olfactory system of vertebrates and insects. To test these models, a realistic artifact of the olfactory epithelium was developed as a large sensor array mimicking some features of biological ORNs, in particular, the broad and overlapping selectivity to many odors, the combinatorial response, the high level of redundancy, and the different dynamic ranges exhibited by same types of ORNs. The sensor array is composed of 16 384 elements arranged in four smaller arrays of 64 × 64 interdigitated electrodes deposited on a borosilicate substrate. To mimic the redundancy of the biological ORNs, tens of organic conductive polymers were chosen as active sensing materials because of their broad and diverse, but overlapping, specificity to different classes of volatile organic compounds. These sensors were characterized by their responses to varying concentrations of test analytes. The collected sensor data were processed with standard multivariate techniques and the results are reported in this paper.

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.

Modelling, design and analysis of liquid crystal waveguides in preferentially etched silicon grooves

This paper presents a fully consistent theoretical framework for liquid crystal (LC) channel waveguides that have been experimentally demonstrated in previous publications. We revise the optical design of the LC waveguides in silicon grooves and implement here a vectorial, fully consistent model of the LC waveguide electro-optical behaviour, based on the finite element method. The numerical investigation shows that LC waveguides demonstrate properties of propagation control and switching. They switch on and off with a low applied voltage. We discuss the major design parameters of the device and the effect of loss-inducing control electrodes.