In recent years, a variety of sol-gel methods
have been developed for the creation of ceramic and glass materials
and coatings with tailored optical, electronic and mechanical properties.
The term 'sol-gel' refers to the transition of liquid 'sol' (tetraethylorthosilicate
or TEOS) into a solid 'gel' phase. Liquid TEOS is either applied,
moulded or cast and then solidifies, forming a ceramic or glass
substance in a wide variety of forms: ultra-fine or spherical shaped
powders, thin film coatings, fibres, microporous inorganic membranes,
monolithic elements or extremely porous aerogel materials. New materials
for optoelectronic applications have also been developed using sol-gel
Extremely fine components
This project set out to develop an improved
low-viscosity sol gel which could be poured into extremely fine
pattern moulds for producing high-precision glass optical elements
for electronics and other applications. Such components would be
particularly attractive in applications related to high-speed and
broadband optical communications.
The partnership, which included industrial manufacturers and suppliers
of sol-gel technologies and products, experts in optoelectronics
and coating techniques, and academic bodies from Germany, Italy
and the UK, was dedicated from the outset not only to getting the
project successfully completed but to producing commercially marketable
Focusing on performance
The new sol material pours easily into a fine
pattern mould. A condensation reaction then forms a solid gel network
from the liquid. This hydrogel exhibits little or no adhesion to
the mould surface and is stable enough to be removed from the mould.
Further drying and high-temperature treatment is then used to harden
and shrink the hydrogel cast so that it forms a smaller solid and
ultra-pure glass form about half the size of the original and possessing
excellent light transmission characteristics.
Materials for optical applications in the 780, 1 300 and 1 500-nanometre
ranges were successfully fabricated using this technique, as were
advanced extremely fine prototype optical gratings.
In addition, a new technology for making glass-amplifying waveguides
was developed using commercially available glasses and glasses prepared
by melting xerogel powders, with very homogeneous distribution of
the rare earth ions in high concentration. And micro-optical elements
such as tiny Fresnel lenses (shaped like the large cutaway lenses
seen in many lighthouses) were produced using organic inorganic
For fine circuit-like components requiring surface patterning, photolithographic
and embossing techniques were used, including embossing with micropatterned
moulds, laser patterning and photolithography.
Each of these major achievements in the project represents a significant
advance in the state of the art, providing the technological basis
for the fabrication of optical components at competitive costs for
a variety of industrial applications.
Using the newly developed sol-gel moulding methodology, bare surfaces
or surfaces loaded with microstructures can be duplicated for the
same cost. It is therefore well suited even to the fabrication of
very complex components, representing an important further step
in miniaturisation. The new method is also fairly inexpensive and
environmentally friendly compared to more conventional deposition
Given the high performance and flexibility
of sol-gel-derived materials, they will undoubtedly take their rightful
place next to classic, traditional materials such as glass, silicon,
lithium niobate, polymers and others in the optoelectronics market.
Solvent casting of sol-gel film is inexpensive compared to equivalent
methods for porous silica film deposition (flame-hydrolysis deposition)
on account of its use of cheaper and more readily available raw
materials, the elimination of chlorine, and lower energy requirements.
Estimates suggest a three-fold cost advantage compared to the same
product made by flame-hydrolysis deposition.
The European market for lens arrays, waveguides and sol-gel-coated
silica wafers has been estimated at more than 14 million euro per
year. Operations are now being scaled up to enable the production
of such elements for the rapidly expanding high-quality telecommunications,
sensing and automotive markets.