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Until now, the polymers used for fabrication of DOE microstructure by multi-photon lithography (MPL) or nanoimprint lithography (NIL) were organic or organic-inorganic hybrid materials prepared using the sol-gel technology. The high organic content meant that due to the low laser induced damage threshold (LIDT) they could only be used for prototyping and testing of optical elements for high-power systems such as ultra-short pulse (USP) laser systems.

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Most conventional optical elements, such as lenses, optical filters, prisms, mirrors, bend light rays by refraction or transmission depending on their shape and their refractive index and have traditionally been made from glass materials. Various types of Diffractive Optical Elements (DOEs) have transformed optical elements technology. They work by utilising a microstructure surface relief profile for their optical function. Light transmitted by a DOE can shaped to almost any distribution, simply by diffraction and the subsequent propagation. Due to their design flexibility, DOEs can provide unique optical characteristics. Compared to refractive optical elements, DOEs are typically much thinner and lighter, making them an attractive replacement in numerous applications. The PULSE project partner FORTH has now developed high damage threshold materials for MPL and NIL, using sol-gel technology and thermal annealing. By adopting a new materials synthesis strategy of making organic-inorganic polymeric hybrids with high inorganic content, FORTH has developed novel materials for use with high-power USP laser beams. These new materials remain stable in liquid forms for several weeks, are highly transparent at infrared and optical wavelengths and can be deposited as thin films without cracking or delaminating. The developed organic-inorganic hybrids, based on zirconium and titanium silicates and photo-initiators, have been independently tested and shown an exceptionally high damage threshold for IR pulsed lasers of 12.6 J/cm2 for 10 nanoseconds, and 0.5 J/cm2 for 300 femtosecond pulses. For single pulses the threshold is 33.5 J/cm2 and 1.2J/cm2 respectively. The results mean these materials can be used for high-power applications not possible with previous materials. DOEs manufactured using these new materials could be implemented in high-power laser systems for beam splitting, beam combining, beam shaping and other functions previously only possible using glass materials. The four female inventors of this technology from FORTH are, clockwise from top left, Prof Maria Farsari, Dr Areti Mourka, Dr Elmina Kabouraki and Dr Vasileia Melissinaki. Based on the broad applicability of these devices, the results are now subject of a patent application.


photonics, lasers, materials

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