Fabrication of Flexible Photonic Crystal Slabs
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Fabrication of Flexible Photonic Crystal Slabs Torben Karrock, Julius Schmalz, Yousef Nazirizadeh, and Martina Gerken Institute of Electrical and Information Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstr. 2, 24143 Kiel, Germany
ABSTRACT Two methods for the fabrication of flexible and stretchable photonic crystal slabs are demonstrated and compared. In both cases a periodically nanostructured polydimethylsiloxane (PDMS) membrane is used as substrate. The first method is based on oblique-angle vapor deposition of SiO as a high refractive index material onto the nanostructured membrane. The deposition is made at an angle of 45° to the surface. The grooves of the nanostructure are aligned such that shading effects cause an inhomogeneous layer thickness distribution on the surface. This supports controlled, periodic cracking of the high index layer upon stretching. In the second approach ZnO nanoparticles are spin-coated on the nanostructured PDMS membrane. Here, the membrane can be stretched and serves as a photonic crystal slab without the need of any further treatment. For both types of flexible photonic crystal slabs a shift of the guided mode resonances to longer wavelengths is observed upon stretching. For a 20% strain perpendicular to the grating grooves a resonance shift of more than 50 nm is obtained.
INTRODUCTION A photonic crystal slab is composed of an optical waveguide with a periodic nanostructure. Such a photonic crystal slab supports guided mode resonances, i.e., optical modes in the waveguide that couple to far-field radiation. Flexible photonic crystal slabs are interesting as strain or pressure sensors and can be used for a variety of applications [1-3]. A deformation of the photonic crystal slab shifts the wavelength position of the guided mode resonances as shown schematically in Figure 1. This change may be evaluated remotely by measuring a reflection or transmission spectrum. Here, we present and compare two methods for fabricating flexible photonic crystal slabs. The first method is based on oblique-angle evaporation of a high refractive index material on a periodically nanostructured membrane. The second method employs spin-coating of high refractive index nanoparticles on a nanostructured membrane. As substrates nanostructured polydimethylsiloxane (PDMS) membranes are used. They are obtained by pressing a linearly nanostructured master stamp into uncured PDMS on a poly(methyl methacrylate) (PMMA) surface. When cured and pulled off, a thin nanostructured PDMS membrane is realized. The flexibility of the resulting photonic crystal slab is adjusted by the mixing ratio of the two components of the PDMS. In the next section we discuss the
realization of photonic crystal slabs, first by oblique-angle evaporation and then by spin-coating of nanoparticles onto the nanostructured surface. In the last section the results are compared and conclusions are drawn.
Figure 1. Schematic of the stretching of a flexible photonic crystal slab (PCS). The guided mode resonance’s wavelength shif
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