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Elastomeric materials for all-optical fabrication of tunable volume diffraction gratings

Presentation held at EOSAM 2016, Berlin, 26-30 September 2016
: Sakhno O.; Raybchun, A.; Wegener, W.

presentation urn:nbn:de:0011-n-4353583 (2.0 MByte PDF)
MD5 Fingerprint: 4ff700727e9e128926f5ee625bc3dfda
Created on: 21.2.2017

2016, 13 Folien
European Optical Society (EOSAM Annual Meeting) <2016, Berlin>
Bundesministerium für Bildung und Forschung BMBF
03V0881; EDEL
Presentation, Electronic Publication
Fraunhofer IAP ()
tunable volume diffraction gratings; PDMS; holographic structuring

Elastomeric material for all-optical fabrication of tunable volume diffraction gratings O. Sakhno, A. Raybchun, W. Wegener Fraunhofer Institute for Applied Polymer Research, Potsdam-Golm, 14476, Germany Summary All-optical fabrication of volume diffraction gratings of different geometrical configurations in polydimethylsiloxane (PDMS) doped with benzophenone (BPh) is presented. The inscribed gratings are able to change their parameters by strain and can be used to create soft tunable optical diffraction elements. Introduction Nowadays one of the important tasks of materials science is the development of controllable and easy processable "smart" materials for optics and photonics. One of the most promising materials for that aim are elastic polymers, such as PDMS which thanks to their huge and reversible elastic deformations and good optical quality are widely used for the production of flexible and stretchable optical diffraction elements and devices: gratings, couplers, filters, lenses, beam steering devices and etc. [1] Most reported PDMS gratings are surface relief gratings fabricated through a replica molding technique which mostly act in the Raman-Nath diffraction regime. The period of such elastic gratings can be easily driven by piezoelectric actuators or by dielectric elastomer actuators [2]. Stretchable volume diffraction (Bragg) gratings which are preferable for some applications because of their specific diffraction performance were not developed up to now. Volume diffraction gratings are formed by a periodically modulated refractive index in th e volume of material without any surface corrugations. In the current work we demonstrate for the first time the approach which allows optical fabricating of elastic volume transmission gratings [3]. Discussion The material method consists in dissolving of BPh in the base component of PDMS at elevated temperature with the following curing. With this approach up to 2 wt.% of BPh can be introduced to PDMS, remaining stable (without crystallization) event after a year of storage at room temperature and providing high optical quality films of 50 -500 µm thickness (d). Transmission volume Bragg gratings have been fabricated using UV holographic technique (exposure wavelength - 364 nm). With the in-line monitoring of the recording process with a He-Ne beam incident at a Bragg angle, it was found that the gratings are formed directly upon holographic exposure. 2D-gratings have been made using a two-step non-coherent UV exposure through the amplitude mask (Fig.1a, insert on bottom). No additional post-exposure steps are required. The grating mechanism was investigated. Diffusion of BPh molecules from the dark to the bright areas of an interference pattern upon exposure and local covalent photo-attachment of BPh to the PDMS-matrix are responsible for the formation of stable refractive index gratings. The diffusion coefficient of BPh in PDMS was estimated as high as ~1.5×10-7 cm2 s-1. The highest refractive index modulation amplitude of 6.9×10-4 was reached over a 400 nm - 2 µm range of the grating period. Not only symmetric transmission gratings but also slanted on es were fabricated in the doped PDMS. The grating period as well as angular and spectral selectivity can be varied by mechanical strain. The diffraction angle of a proper wavelength changes with stretching, i.e. with changing period, according to the Bragg equation. The spectral tuning of PDMS Bragg gratings was demonstrated applying mechanical strain to the grating of the period 796 nm and d=220 µm. The 75% elongation of the grating changes the color of the diffracted beam from violet to deep red, indicating a significant increase in the grating period from 796 nm to 1348 nm, giving a period change of about 552 nm and a shift of the diffraction maximum from 410 up to 700 nm, i.e. for of about 290 nm. At that strain the grating thickness is decreased for 25%. One thousand on-off cycles with a 0% - 30% strain variation reveals repeatable spectral shift of the diffracted beam without fatigue or hysteresis. These results show that the alterations of the grating parameters are reproducible and reversible that is a key point for optical tuning materials. Conclusions Novel material based on conventional PDMS doped with BPh or its derivatives allows holographic and through a mask fabricating of volume diffraction gratings using UV light. Diffusion and local photo-attachment of a BPh to PDMS matrix provide stable refractive index gratings over a broad range of periods and thicknesses. Alterations of the grating parameters upon strain are completely reversible. Symmetric and slanted transmission Bragg gratings, 2D-gratings as well as reflection gratings have been fabricated that demonstrates high flexibility of the method for creation tunable diffractive optical elements.