Hybrid materials for micro-optics

Wafer-scale UV-embossing can be applied to substrates other than glass, for example Si and semiconductor III–V based wafers with prefabricated devices. In these cases, it is often advantageous to use the same hybrid materials in a combined lithographic and embossing mode to produce free-standing micro-optical elements, for example the lenslet on VCSEL elements for fiber coupling (CSEM in collaboration with Avalon Photonics Ltd., CH-Zurich). Fig. 16 shows SEM images of processed microoptical components on VCSEL wafers: (a) diffractive lenses, (b) an array of refractive lenses.

All the Pyramid Optics' collimator arrays shown in Fig. 17 are constructed with a silicon V-groove fiber array mounted with a microlens array. The microlens arrays are replicated in a ORMOCER1 thin film on a BK7 glass substrate.

The new collimator arrays are offered for several wavelength regions: 630–690 nm; 780–850 nm and 1250–1650 nm. Other wavelengths are possible on request. Several of the above used hybrids for optical applications are now licensed by Fraunhofer-ISC to the company Micro Resist Technology GmbH and they are producing them in large scale and marketing them worldwide under the names: ''Ormocore'', ''Ormoclad'' and ''Ormocomp''. As a result, besides the given industries, lots of companies from Japan and South Korea as well as from Europe (Germany, Switzerland, Sweden, and Finland) have integrated these materials mostly in micro-optical products.

Nanohybrids are also used for interference optical coatings. Dielectric mirrors or reflectors can be prepared using interference quarterwave stacks of colloidal-based low-refractive index material and a hybrid dense material as the high refractive index layer. In the literature, a possible high index layer used for laser optical thin films139 is a nanohybrid material prepared from mixing a nanosized-zirconia suspension with a transparent polymer alcoholic solution. Using a hybrid system with a polymeric binder in the high index oxide sol helps to decrease the total layer number required for the same reflection value. The selected polymer must be soluble in the suspending medium, preventing the sol from flocculation, and also needs to be transparent at the wavelength of interest. That is the reason why polyvinylpyrrolidone (PVP) was chosen as possessing the best combination of properties for binder use. Considering the structure of the monomer unit, it is seen to have an amphiphilic character.

Indeed, PVP contains a highly polar amide group conferring hydrophilic and polar-attracting properties, and also apolar methylene (CH2) and methine (CH) groups in the backbone and the ring conferring hydrophobic properties. When added to a colloidal suspension, the amphiphilic character of PVP helps to maintain colloidal stability and to reduce flocculation through a steric stabilization mechanism. As the purpose was to get the maximum refractive index value, the oxide/polymer ratio of the hybrid system needed to be optimized. It was demonstrated that the index value was directly dependent on the PVP/oxide ratio and the optimum ratio was determined in a separate experiment in which the hybrid refractive index variation was plotted regarding the PVP/oxide ratio (see Fig. 18).

The PVP polymer is supposed to ''smother'' the oxide nanoparticles leading to a dense hybrid structure with hydrogenbonding between the amide carbonyl groups of PVP and surface hydroxyl groups of oxide particles (Fig. 19). A similar dense hybrid structure was described by Toki et al.177 for silica nanosized particles.

Because PVP was soluble in alcoholic solvents, multilayer deposition required a UV-curing step to avoid redissolution of the previous deposited polymeric layer. PVP is a photosensitive polymer that could be UV-cured using short wavelength irradiation. In the UV-curing of PVP, FT-IR measurements show that the increase of the hydroxyl-band is correlated to the decrease of the ketone and amide bands. This UV-induced hydroxylation of PVP explains the modification of its solubility because, as for polyvinyl alcohol (PVA), crystallinity of hydroxylated-PVP (OH-PVP) has changed.

Using such nano-hybrid materials, 99.5% reflection coatings have been prepared on a 42 6 46 6 9 cm deformable mirror (BK-7 substrate) for near infrared use (1053 nm wavelength) exhibiting high optical uniformity and low optical losses (as low as 0.35%) (Fig. 20)