Project conclusion

Within the NILmeta project anisotropic metamaterials with a negative refractive index - so called Negative Index Materials (NIMs) - were fabricated using Nanoimprint Lithography (NIL). Different structural designs were realised which provide resonances in the infrared (IR). The aim was to demonstrate the fabrication of single layers but also of 3D NIMs using NIL which is a nano-structuring technique scalable for mass production. This work is therefore an important step towards exciting applications of NIMs like perfect lenses and magnification of objects below the diffraction limit. The NIMs were characterised using spectroscopic ellipsometry and compared to theoretical results.

The main focus in the NILmeta project was on three different designs: the split ring resonators (SRRs), the fishnet and "isotropic" fishnet structures. The structure size ranges from 60 nm to 10 µm exhibiting negative refraction in the infrared. Two different NIL based fabrication techniques have been conceived to fabricate the negative index material (NIM) by NIL: Ultra-violet NIL (UV-NIL) and Nano Contact Printing. The advantages and disadvantages of these processes for the fabrication of NIMs were evaluated and the most promising method - in this case UV-NIL combined with a lift-off process - has been optimized. The NIL stamps used for this work were fabricated at the Friedrich Schiller University, Jena or at NILTechnology using e-beam lithography and reactive ion etching.

To achieve 3D NIMs the single NIM layers have to be stacked. First a process using chemical mechanical polishing was optimized and later replaced by a much simpler technique. Here the single layers are peeled off from the substrate by using a UV-curable hybrid polymer Ormocer® as "glue". By continuing this process the single layers can be stacked on top of each other and therefore a 3D material can be achieved. For determining the optical properties of the samples optical characterisation techniques like transmission and reflection measurements have been performed and the refractive index has been determined using RCWA simulations. Further, the anisotropic refractive index was measured using a spectroscopic ellipsometer in the infrared and visible frequency range at oblique incidence. The ellipsometry measurements in combination with the Berreman formalism made it possible to show the restrictions of the validity of an effective medium for NIM structures experimentally.