Sol-Gel Applications

Sol–gel technology is a very flexible, simple, and low-cost approach to fabricate photonic structures that have applications in various fields. Characterized as a melt-free process, sol-gel is attractive for development of advanced glass-based materials that would be difficult to realize by conventional melt-quenching and vapor phase methods. In fact, sol-gel has a great potential in manufacturing advanced materials as specialty glasses, glass-ceramics, nanocomposites, and hybrids. Final products can have tailored topological structures (porous, mesoporous, and dense) in various sizes and shapes, e.g., powders, core-shells, films, and monoliths. Sol-gel derived photonic structures can lead us to higher performance, to low-cost products, as well as to innovative physics.

CSMFO research group is known for the extensive work on glasses and glass-ceramics doped with rare earth ions for Photonics and Optoelectronics. Below are some highlighted examples of photonic structures derived from sol-gel routes.

Among the most common glasses used in integrated optics, silicates take a significant place. Rare-earth-activated silica-hafnia binary system (SiO2-HfO2) has shown excellent optical and structural properties. Hafnium oxide has a wide bandgap of about 5.5 eV allowing transparency from 300 nm to 10 μm, exhibits high refractive index (~ 1.95 at 1 μm), and a low cutoff phonon energy (700 cm-1). As an example concerning silicate glasses, an integrated waveguide laser has been developed for biomedical sensing and produced by the sol–gel technology on Nd3+ activated silica-hafnia platform as shown below.

Another worth-noting advantage of sol-gel processing is its versatility in obtaining homogeneous coatings on curved and irregular surfaces. In fact, we put in evidence that sol-gel is a useful approach for preparing rare-earth-based coated spherical microresonators or spherical microlasers. Below is an example of Er3+-activated SiO2-HfO2 film coated microspheres, prepared by dip-coating technique with sol-gel route. In this structure, silica microspheres serving as a base resonator structure were made by melting the end of a stripped standard telecommunication fiber (SMF 28).

Rare-earth-activated (RE) photonic structures are fundamental actors in widespread applications such as luminescence sources, optical sensors, quantum technologies, and solar energy conversion. There are two main critical problems to face in the RE photonic systems: (i) the RE low absorption cross-section, (ii) the luminescent quenching processes. To overcome these issues, glass-ceramics are robust approaches. Transparent glass-ceramics combine interesting properties of both amorphous and crystalline phases and offer specific characteristics of capital importance in photonics.

One interesting example is sol-gel derived rare-earth activated SiO2-SnO2 glass-ceramics (SiO2-SnO2:RE3+), which uniquely combines: (i) efficient rare-earth luminescent sources sensitized by SnO2 nanocrystals and (ii) high photorefractive systems, enabling direct UV writing of gratings and channel waveguides.

As a solution processing technology, sol-gel offers promising low-cost and large-area deposition for optical coatings. Furthermore, sol-gel can be employed in printing-based patterning techniques as well as roll-to-roll processing for flexible photonic devices’ manufacture. We have recently shown that sol-gel is a profitable method for obtaining transparent crystalline SnO2:RE3+ coatings on the flexible glass substrate. The obtained SnO2:RE3+ exhibit: (i) a wide transparency window with a transmittance of about 80% covering the 400 nm – 3200 nm range, and (ii) efficient RE3+ luminescence sensitized by SnO2 nanocrystals. Such flexible inorganic structures lead to unprecedented opportunities for a broad spectrum of applications ranging from optical components to sensors for civil infrastructure and environments, to coherent and incoherent light sources.