Aerogels for Optofluidic Waveguides

Abstract

Aerogels—solid materials keeping their internal structure of interconnected submicron-sized pores intact upon exchanging the pore liquid with a gas—were first synthesized in 1932 by Samuel Kistler. Overall, an aerogel is a special form of a highly porous material with a very low solid density and it is composed of individual nano-sized particles or fibers that are connected to form a three-dimensional network. The unique properties of these materials, such as open pores and high surface areas, are attributed to their high porosity and irregular solid structure, which can be tuned through proper selection of the preparation conditions. Moreover, their low refractive index makes them a remarkable solid-cladding material for developing liquid-core optofluidic waveguides based on total internal reflection of light. This paper is a comprehensive review of the literature on the use of aerogels for optofluidic waveguide applications. First, an overview of different types of aerogels and their physicochemical properties is presented. Subsequently, possible techniques to fabricate channels in aerogel monoliths are discussed and methods to make the channel surfaces hydrophobic are described in detail. Studies in the literature on the characterization of light propagation in liquid-filled channels within aerogel monoliths as well as their light-guiding characteristics are discussed. Finally, possible applications of aerogel-based optofluidic waveguides are described.

A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels

“…The performance of optofluidic waveguides can be characterized in terms of the waveguide propagation losses that can in turn be determined from the measurement of the input optical power and the output power observed after the light has propagated a certain distance z along the waveguide [36–38,40]. Our measurements with the same waveguide geometry reported recently in [40] have shown that the propagation losses of multimode aerogel waveguides with aqueous cores are below −1.5 dB cm −1 .…”

“…It has been recently demonstrated that aerogels can be used for efficient optical waveguiding in liquids [36][37][38][39][40]. Aerogels are highly porous nanostructured solids consisting of an interconnected open network of loosely packed, bonded particles separated by air gaps, and they feature very low refractive index and high specific surface area [36,40].…”

“…It has been recently demonstrated that aerogels can be used for efficient optical waveguiding in liquids [36][37][38][39][40]. Aerogels are highly porous nanostructured solids consisting of an interconnected open network of loosely packed, bonded particles separated by air gaps, and they feature very low refractive index and high specific surface area [36,40]. These properties, particularly the low refractive index of approximately 1.05, for which the aerogels are often referred to as 'solid air', make aerogels a remarkable solid-cladding material for optofluidic waveguides that can be used without any additional modification, since almost all liquids have refractive indices exceeding that of aerogels.…”

Photocatalytic transformation in aerogel-based optofluidic microreactors

“…In this approach, the external wall of the microchannel should be made out of a suitable material which confines the reaction medium and also behaves like waveguide cladding. Aerogels are highly porous nanostructured materials that can be produced as macroscopic monoliths with unique properties such as extremely low refractive index, high surface area to volume ratio, and high porosity [14,15]. These properties, particularly the low refractive index of ~1.05, make aerogels a remarkable solid-cladding material without any additional coating since almost all liquids have refractive indices exceeding that of aerogel.…”

“…Following surface treatment, the channels are filled with a suitable liquid that serves as the core liquid without any restriction on the type of liquid and the type of aerogel that can be used, as long as the liquid can be confined inside the aerogel block without penetrating its network. Due to the low refractive index of aerogels, light can be uniformly propagated by total internal reflection (TIR) in the reaction medium inside the channel in an aerogel with low losses similar to conventional optical fibers [14,16]. Photocatalytic reactions are another interesting area for the potential application of aerogel-based optofluidic waveguides.…”