Voltage controlled scattering from porous silicon Mie-particles in liquid crystals

Abstract

Creating novel liquid crystalline (LC) photonic metamaterial structures is a highly promising approach for the development of tunable photonic devices particularly when nanoporous structures are used in which the LC can infiltrate and modify their properties. In the present work, a tunable scattering device made of partially oxidized porous silicon microparticles mixed with LC is proposed and demonstrated. Being porous in nature, the microparticles trap the LC molecules and enhance the scattering in the voltage OFF state. Due to the random orientational order inside the particles, the averaged refractive index (RI) inside these nanopores is close to that of the isotropic LC RI in the OFF state. Averaging between this LC and the oxidized porous Si brings the average RI of the microparticle down and maximizes the index mismatch with the surrounding LC regions. When the porosity is large enough, a high RI mismatch between the particles and its surrounding in the OFF state is obtained. In the ON state, the molecules orient along the electric field direction, decreasing the RI mismatch and increasing the transparency of the system. The scattering decreases gradually and reversibly with the applied voltage while at zero voltage state it decreases with temperature. At the high voltage state the scattering increases with temperature with some enhancement at the clearing point due to the LC order parameter fluctuations. A slight decrease of the clearing point is observed as a result of weakening the intramolecular interaction and lowering the order parameter. Using fluorescent confocal microscopy the changes in the LC and microparticles domains are monitored showing some reversible dragging of the microparticles towards one of the substrates as the voltage increases. Being a tunable light scattering device, it has a great potential for smart window and light shutter applications.