Aerogels in Fibre-Optics

Lead Research Organisation: University of Bath
Department Name: Physics

Abstract

The proposed project is about using silica aerogels in the field of fibre optics. Aerogel is a highly porous form of glass with remarkable optical properties, but has barely been exploited in fibre optics. It is made of the same material as optical fibres, namely silica glass. However, whereas the glass in the fibre is solid, an aerogel is like a sponge: an open porous network of solid matter with air in between. Indeed, most of the volume of an aerogel is air, making it very lightweight. The glass network and the pores are structured on a scale of tens of nanometres or less, much smaller than the wavelength of light. The aerogel is therefore an example of a metamaterial: light sees the aerogel as an averaged-out medium that is mainly air, yet the glass network makes it rigid like a solid. The refractive index of aerogel can be as low as 1.01: almost the same as air and very different from solid glass. Indeed, the lowest index of any common solid or liquid is around 1.27, so aerogel is really the only way to get lower-index materials. Although light and friable, aerogel is quite robust if not abused: it looks like frozen smoke and handles like Oasis floral foam. It has found diverse applications, from thermal insulation to the collection of interplanetary dust.Despite the intriguing properties of this material, few people have tried to use it in fibre optics. We now have a unique opportunity to bring these two technologies together, having recently demonstrated the necessary methods using tools we have developed for supercritical drying and fibre fabrication. The aim will be to optimise the material, allow it to interact with the light in a fibre, and so enable an entirely new class of photonic components that exploit the optical properties of aerogel in a fibre-compatible way. Key applications include high-numerical-aperture waveguides, compact splitters and couplers, fibre-coupled nonlinear sources and new plasmonic devices.

Publications

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Description Aerogel is a highly porous form of silica glass. Whereas glass is normally a solid dense material, an aerogel is like a microscopic sponge: an open porous network of solid matter with air in between. Indeed, most of the volume of an aerogel is air, making it very lightweight. It has found diverse applications, from thermal insulation to the collection of interplanetary dust.



Our project was about using silica aerogel in fibre optics and other optical technologies. The glass network is on a much smaller scale than the wavelength of light, making the aerogel a type of metamaterial: light sees the aerogel as a kind of "average" material that's mostly air, yet it is rigid like a solid. This gives it remarkable optical properties.



We started by discovering how to make highly transparent aerogel with flat surfaces, to be a good optical material, and we developed new techniques to do this routinely. Then we demonstrated two ways to incorporate it into optical fibres.



In the first approach, we narrowed fibres by heating them near the melting point and stretching them. This brings the light in the fibre to the surface, where it can interact with aerogel that we form in situ using our new techniques. The aerogel is like "solid air": it has a similarly low refractive index so that the fibre behaves optically as if still suspended in air, but the aerogel protects it from being degraded by contamination or handling. Examples of fibre components protected like this include fused couplers (fibre-optic beam-splitters found in fibre telecommunication systems), tapered supercontinuum generators (producing intense white light with uses in spectroscopy, time and frequency standards and bio-medical imaging and analysis), gas sensors (exploiting how gases diffuse through the aerogel) and photonic nanowires (to make tiny fibre circuits).



In the second approach, we took a fibre with a hollow core and holes in the cladding and filled it with aerogel, and showed that it acts as a waveguide. This is a remarkable result: no solid has a low enough refractive index to be a cladding for an aerogel core, but our holey air-glass cladding behaves as if it has an even lower index. The fibre allowed interactions of high intensity lasers with the aerogel over relatively long distances.



Finally, we modified aerogel to give it new optical properties. Gold nanoparticles gave the aerogel plasmonic properties, exploiting the highly intense electric fields around small metal particles. Adding ytterbium allowed us to form dense doped glass with applications in lasers and optical amplifiers. Other dopants include silicon nanoparticles, quantum dots, nano-diamonds, metal-organic framework nanoparticles and carbon thin films, and we collaborated with other researchers studying how their optical materials behaved when held fixed in a low index rigid material.



In summary, we proved the potential of silica aerogels in fibre optics. We optimised the material itself, incorporated it within fibres in two complementary ways, and enabled new types of fibre-compatible photonic components. We also facilitated our collaborators studies in their own fields of research.
Exploitation Route 1. Stable, high-temperature packaging of photonic components that require surrounding air as a part of the optical structure. Examples include fused couplers, tapered fibre supercontinuum generators, gas sensors, evanescent input and output couplers for integrated optics, nanofibre circuits and evanescent gas sensors.



2. Long-distance high-intensity interactions between light and dopants in aerogel, for example in nonlinear optical devices.



3. Porous rigid substrates for particle-based sensors. Examples include metal-organic framework nanoparticles, gold nanoparticles, quantum dots and silicon nanoparticles.



4. Optical amplifiers and lasers based on rare-earth doped silica made using aerogel or xerogel techniques.



5. In collaboration with Brunel University we showed that, even with the small-scale unoptimised processes we use to make aerogels, the energy and CO2 burden of manufacture is offset by savings within two years for use as a retrofitted thermal insulator in buildings. 1. Stable, high-temperature packaging of photonic components that require surrounding air as a part of the optical structure. Examples include fused couplers, tapered fibre supercontinuum generators, gas sensors, evanescent input and output couplers for integrated optics, nanofibre circuits and evanescent gas sensors.



2. Long-distance high-intensity interactions between light and dopants in aerogel, for example in nonlinear optical devices.



3. Porous rigid substrates for particle-based sensors. Examples include metal-organic framework nanoparticles, gold nanoparticles, quantum dots and silicon nanoparticles.



4. Optical amplifiers and lasers based on rare-earth doped silica made using aerogel or xerogel techniques.



5. As a rigid air-like substrate for volume distributions of particles, to enable studies of their optical properties.



6. As a substrate for opto-fluidic waveguides, guiding light in aqueous cores, or as reaction media for water and soluble gases.
Sectors Chemicals

Construction

Digital/Communication/Information Technologies (including Software)

Energy

 
Description Aerogel doped with PbSe quantum dots 
Organisation University of Limoges
Country France 
Sector Academic/University 
PI Contribution We produced samples of aerogel doped with PbSe quantum dots provided by the University of Limoges, France, to enable them to study optical interactions with volume distributions of quantum dots.
Start Year 2008
 
Description Aerogel doped with nano-diamond 
Organisation Macquarie University
Country Australia 
Sector Academic/University 
PI Contribution We produced samples of aerogel doped with nano-diamond provided by the MacQuarie University, Australia, to enable them to study emission in nano-diamond in a low-index environment.
Start Year 2008
 
Description Carbon deposition in aerogel 
Organisation University of Bath
Department Department of Chemical Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution We provided aerogel samples for our collaborator in the Chemical Engineering Department at Bath to use as a microporous substrate for the deposition of carbon films.
Start Year 2008
 
Description Energy efficiency of aerogel 
Organisation Brunel University London
Country United Kingdom 
Sector Academic/University 
PI Contribution We collaborated with Brunel University and Buro Happold Ltd to study the energy efficiency of aerogel as a transparent insulating building material.
Start Year 2008
 
Description Energy efficiency of aerogel 
Organisation BuroHappold Engineering
Country United Kingdom 
Sector Private 
PI Contribution We collaborated with Brunel University and Buro Happold Ltd to study the energy efficiency of aerogel as a transparent insulating building material.
Start Year 2008
 
Description Laser-written waveguides in aerogel 
Organisation Heriot-Watt University
Country United Kingdom 
Sector Academic/University 
PI Contribution We provided samples of optical quality aerogel for the laser inscription of waveguides at Heriot-Watt University, UK.
Start Year 2008
 
Description Novel doped glass for fibre fabrication 
Organisation Boston University
Country United States 
Sector Academic/University 
PI Contribution We produced aerogel samples with various dopants, such as ytterbium ions or silicon or gold nanoparticles, for use by our collaborator at Boston University as a core material for optical fibre fabrication.
Start Year 2008
 
Description Positron spectroscopy of aerogel 
Organisation University of Bath
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution We supplied samples of aerogel for positron spectroscopy by our collaborator in the Physics Department at Bath.
Start Year 2008
 
Description Raman spectroscopy of doped aerogel 
Organisation University of Bath
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution We provided samples of aerogel doped with silicon nanoparticles, for our collaborator in the Physics Department at Bath to perform Raman spectroscopy studies of oxygen sensing.
Start Year 2008
 
Description Substrates for metal-organic frameworks 
Organisation University of Bath
Department Department of Chemical Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution We produced aerogel samples doped with metal-organic framework particles supplied by our collaborator in the Chemical Engineering Department at Bath, for use as a toxic gas sensor.
Start Year 2008
 
Description Supercritical carbon dioxide rig 
Organisation University of Bath
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Construction and operation of a shared facility for supercritical carbon dioxide processing of our aerogels and of carbon nanotubes studied by our collaborator in the Physics Department at Bath.
Start Year 2008
 
Description Thermal poling of aerogel 
Organisation University of Sydney
Country Australia 
Sector Academic/University 
PI Contribution We provided samples of aerogel with a thin aspect ratio to investigate whether the material can be thermally poled to give it a second-order optical nonlinearity.
Start Year 2008
 
Description trade magazine reports 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Our research was reported in the News pages of the trade magazine "Laser Focus World" on two occasions, and included in their technology review for 2009.
Year(s) Of Engagement Activity 2008