In this study, we have examined the effects of transfer residue and sample annealing on the optical properties of chemical vapor deposited graphene, transferred onto a sapphire substrate. The optical absorption of graphene was obtained from point-by-point inversion of spectroscopic ellipsometry measurements in the visible and ultraviolet ranges (250–800 nm). Measured spectra were analyzed by optical models based on the Fresnel coefficient equations. The optical models were supported by correlated Raman, scanning electron microscopy, and atomic force microscopy measurements. The obtained data were phenomenologically described by a Fano model. Our results show that a residue layer left on graphene can significantly increase its optical absorption in the visible range, compared to an annealed sample.

We investigate electrically tunable split ring resonators (SRRs) obtained by coupling the SRRs with graphene in terahertz and near-infrared frequency range. Two different geometries are considered: SRRs on homogeneous graphene and SRRs with graphene patches inserted only in SRRs' gap. Graphene conductivity is tuned by changing its Fermi level. This gives tunable absorption in the graphene, which is strongly enhanced by large electric field in the vicinity of SRR resonances. As a result, SRR-graphene systems could be used as deeply subwavelength modulators with amplitude and phase tuning of the reflected field.

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