Nina Podoliak

Macroscopic optical effects in low concentration ferronematics

We present a detailed experimental and theoretical study of the optical response of suspensions of ferromagnetic nanoparticles (“ferroparticles”) in nematic liquid crystals (“ferronematics”), concentrating on the magnetic field-induced Frederiks transition. Even extremely low ferroparticle concentrations (at a volume fraction between 2 × 10−5 and 2 × 10−4), induce a significant additional ferronematic linear response at low magnetic field (<100 G) and a decrease in the effective magnetic Frederiks threshold. The experimental results demonstrate that our system has weak ferronematic behavior. The proposed theory takes into account the nematic diamagnetism and assumes that the effective magnetic susceptibility, induced by the nanoparticles, no longer dominates the response. The theory is in good agreement with the experimental data for the lowest concentration suspensions and predicts the main features of the more concentrated ones. The deviations observed in these cases hint at extra effects due to particle aggregation, which we have also observed directly in photographs.

Magnetite nanorod thermotropic liquid crystal colloids: synthesis, optics and theory.

We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)(2) with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10 mol dm(-3) NaOH at 100°C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length ~500 nm and typical diameter ~20 nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field-LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod-liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods.

Elastic constants, viscosity and response time in nematic liquid crystals doped with ferroelectric nanoparticles

We report on the role of ferroelectricity on the physical and electro-optic parameters in suspensions of nanoparticles in cyclohexane based, fluorinated nematic liquid crystals. The dielectric and elastic constants, response time, and viscosity of the suspensions were analyzed experimentally and compared with the undoped samples. Our study shows a decrease in the splay elastic constant and an increase in the dielectric constants, together with an increase in the average rotational viscosity.

Design of dual-core optical fibers with NEMS functionality

An optical fiber with nano-electromechanical functionality is presented. The fiber exhibits a suspended dual-core structure that allows for control of the optical properties via nanometer-range mechanical movements. We investigate electrostatic actuation achieved by applying a voltage to specially designed electrodes integrated in the cladding. Numerical and analytical calculations are preformed to optimize the fiber and electrode design. Based on this geometry an all-fiber optical switch is investigated; we find that optical switching of light between the two cores can be achieved in a 10 cm fiber with an operating voltage of 35 V.

Controlling Stiction in Nano-Electro-Mechanical Systems Using Liquid Crystals

Stiction is one of the major reliability issues limiting practical application of nano-electro-mechanical systems (NEMS), an emerging device technology that exploits mechanical movements on the scale of an integrated electronic circuit. We report on a discovery that stiction can be eliminated by infiltrating NEMS with nematic liquid crystals. We demonstrate this experimentally using a NEMS-based tunable photonic metamaterial, where reliable switching of optical response was achieved for the entire range of nanoscopic structural displacements admitted by the metamaterial design. Being a more straightforward and easy-to-implement alternative to the existing antistiction solutions, our approach also introduces an active mechanism of stiction control, which enables toggling between stiction-free and the usual (stiction-limited) regimes of NEMS operation. It is expected to greatly expand the functionality of electro-mechanical devices and enable the development of adaptive and smart nanosystems.

Design and Fabrication of Suspended Indium Phosphide Waveguides for MEMS-Actuated Optical Buffering

We present the design and fabrication of suspended optical waveguides on indium phosphide platform for use in an optical buffer device with MEMS actuation, in which the optical delay can be achieved by changing the spacing of the waveguides by electrostatic actuation. The optical and mechanical properties of the waveguides and pillar supports are modeled, and different MEMS actuation schemes are simulated. We also present fabrication and characterization results of the epitaxially grown sample structure and of the suspended waveguide device, exhibiting two parallel waveguides with submicron dimensions separated by a 400-nm air gap, and suspended at 40-μm intervals by S-shaped supports.

Optimal liquid crystal modulation controlled by surface alignment and anchoring strength

Spatial modulation of liquid crystals can be controlled and adjusted by light polarization, the degree of pretilt on the substrates, anchoring strength, and the experimental geometry. In particular, strong anchoring can affect the liquid crystal orientation in opposite ways, depending on the polarization of the incident light. Here we present a theoretical model that describes the liquid crystal modulation and how it can be controlled and optimized. The model is valid for electric fields with a uniform component that is large with respect to the spatial modulation, a situation typical of spatial light modulators and photorefractive cells.

Nanomechanical Optical Fiber with Embedded Electrodes Actuated by Joule Heating

Nanomechanical optical fibers with metal electrodes embedded in the jacket were fabricated by a multi-material co-draw technique. At the center of the fibers, two glass cores suspended by thin membranes and surrounded by air form a directional coupler that is highly temperature-dependent. We demonstrate optical switching between the two fiber cores by Joule heating of the electrodes with as little as 0.4 W electrical power, thereby demonstrating an electrically actuated all-fiber microelectromechanical system (MEMS). Simulations show that the main mechanism for optical switching is the transverse thermal expansion of the fiber structure.

MEMS actuation for a continuously tunable optical buffer

We present our work towards development of an optical buffer based on the InP platform that consists of two suspended coupled optical waveguides. A continuously tunable delay in the propagation time can be achieved by varying the spacing between the waveguides using MEMS actuation. Up to 100% variation of the delay time with a driving voltage of 3 V is predicted.

Electrostatic actuation of nanomechanical optical fibers with integrated electrodes

We investigate theoretically and experimentally the possibility of electrostatic actuation of nanomechanical optical fibers with integrated electrodes. The fiber has two optically guiding cores suspended in air by thin flexible membranes. This fiber structure allows for control of the optical properties via nanometer-range mechanical core movements. The electrostatic actuation of the fiber is generated by electrically charged electrodes embedded in the fiber cladding. Fiber designs with one to four electrodes are analyzed and, in particular, a quadrupole geometry is shown to allow for all-fiber optical switching in a 10cm fiber with an operating voltage of 25 - 30V. A multi-material fiber draw technique is demonstrated to fabricate a fiber with well-defined dual core structure in the middle and four continuous metal electrodes in the cladding. The fabricated fiber is analyzed and compared with the modeled requirements for electrostatic actuation.