Milka M. Jakovljević

Influence of transfer residue on the optical properties of chemical vapor deposited graphene investigated through spectroscopic ellipsometry

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.

Spectroscopic ellipsometry of few-layer graphene

The optical properties of few-layer graphene (FLG) films were measured in the ultraviolet and visible spectrum using a spectroscopic ellipsometer equipped with a 50-mu m nominal microspot size. The FLG thickness was found by atomic force microscopy. Measurements revealed that the microspot is larger than the FLG flake. The ellipsometric data was interpreted using the island-film model. Comparison with graphite and recently published graphene data showed reasonable agreement, but with some features that could not be explained. The error margin for the optical constants was estimated to be +/- 10%.

Spectroscopic ellipsometry and the Fano resonance modeling of graphene optical parameters

We investigate the optical response of graphene via spectroscopic ellipsometry in the ultraviolet and visible ranges. The optical conductance of graphene is described by a Fano model. The parameters of this model are extracted from our spectroscopic ellipsometry measurements, and the complex refractive index of graphene is obtained. Graphenes dispersion relation shows that the density of states function has a logarithmic van Hove singularity corresponding to the M point of the Brillouin zone. The exciton binding energy is calculated as the difference between the resonant and the saddle point energies.

Spectroscopic ellipsometry of split ring resonators at infrared frequencies

Using spectroscopic ellipsometry, we have determined the plasmonic resonances of split-ring resonator arrays. The features in the ellipsometric spectra were explained by the analysis of calculated polarized complex reflection spectra. The calculated near-field and current distribution revealed the nature of the resonances. The positions of Rayleigh’s anomalies are calculated and marked in the spectra. We have also shown that oblique incidence enables excitation of plasmonic modes that are not possible to excite at normal incidence.

Oblique incidence reflectometry and spectroscopic ellipsometry of split-ring resonators in infrared

Ellipsometry and infrared polarized reflection spectroscopy at oblique incidence of golden split-ring resonators were simulated and discussed. The ellipsometric spectra were related to the reflection spectra for the two polarization of the incident wave, s- and p-, with electric field being normal and parallel to the plane of incidence, respectively. Near-field and bulk current distribution at the resonances proved that they correspond to the multiple plasmonic modes of the split-ring resonators. The calculated magnetic moment showed that at oblique incidence both magnetic and electric field induce the magnetic resonances.

Polarization-dependent optical excitation of gap plasmon polaritons through rectangular hole arrays

We use variable-angle reflection ellipsometry to investigate the polarization-dependent optical excitation of highly confined gap plasmon polaritons in a fishnet metamaterial with rectangular holes. From the amplitude of features in the ellipsometric spectra and their variation with the angle of incidence, we find that the gap plasmon polaritons supported by the 35 nm thick silica gap layer are much more efficiently excited by light polarized along the short axis of holes. This inference is corroborated by numerical simulations of plasmonic near fields, while the simulated ellipsometric spectra are in excellent agreement with the measurements. By examining fishnet structures with decreasing hole sizes but a fixed aspect ratio, we find that the polarization dependence persists even in the absence of hole resonances suggesting that it can be explained by the quasi-static polarizability of holes.

Ellipsometry and Correlation Measurements

Ellipsometry, in general, is one of the most sensitive spectroscopic techniques nowadays in both macro- and nano-scale research. Advantages like absolute measurements (without need for references), high precision, non-destructive character, easy real-time monitoring and many possible applications make Imaging Ellipsometry and Spectroscopic Ellipsometry unavoidable tools in modern research practice. Still, one of the main disadvantages of ellipsometry concerns the fact that an optical model is required in order to define constituents, the structure and morphology of each sample. In most cases, a good optical model that properly describes the structure is not possible to make without additional information like the number of layers, their structure, surface roughness or type of interfaces. For this reason we need additional measurements that corroborate ellipsometric ones in order to construct a valid optical model. We call these measurements Correlation Measurements and they are the topic of this communication. We describe the most frequently used ones like SEM, AFM, STM, Raman or FTS, and we also mention new trends that combine spectroscopic or imaging ellipsometers with one or more correlation techniques in the same instrument.

Surface plasmon polaritons and negative refraction in fishnet metamaterial

Surface plasmon polaritons (SPP), supporting sub-wavelength confinement of light [1], enable a great potential for nanophotonic applications. SPP modes of flat interfaces, such as insulator-metal-insulator (IMI), and MIM geometries have been well investigated, however, more complicated structures used in metamaterial design need further exploration. In this work we characterize electromagnetic response of a fishnet metamaterial in analogy with the SPP modes of an IMIMI structure and associate its lowest order internal SPP mode to the so-called negative refractive response of the fishnet. The sample, fabricated using nano-imprint lithography, is comprised of 40-20-40 nm sandwich of Ag-SiO 2 -Ag layers on a glass substrate, and perforated by a P = 200 nm period square array of holes. Inset of Fig. 1b schematically shows the experimental setup. The wave-vector of the incident polarized light k i is defined by the angle of incidence relative to the z-axis (θ). In order to measure inplane dispersion of the fishnet Bloch SPPs, k || is swept along the Γ-X direction of the1st Brillouin zone, by varying θ. Figure 1 shows the measured band-structure of the fishnet, mapped by varying θ from 0° to 80° with 2° increments. SPP modes of the corresponding IMIMI geometry folded to the first Brillouin zone of the fishnet lattice are superimposed as dotted lines. Where, eSPP1(m, n) and eSPP2(m, n), represent the external SPP modes at the substrate and air interfaces, respectively, while iSPP1(m, n) shows the internal SPP resonances confined in the SiO 2 spacer region. The calculated values of average electric field magnitudes at cross sections between the holes (orange areas in the inset of Fig. 1) show a strong correlation between resonant field enhancement and features in the transmittance spectra. The modes labelled as iSPP1(−1,0), iSPP1(1,0) and iSPP1(−1,±1) show a significant field enhancement within the gap region, while, eSPP1, and eSPP2 have their peaks at the substrate and air side respectively, confirming the relation of Bloch SPPs with matching folded IMIMI SPPs. The significantly increased confinement and field enhancement is the specific of fishnet gap plasmon inherited from the MIM SPPs [2], and absent in the conventional EOT [3]. Of particular interest is the lowest frequency negative dispersion mode, iSPP1(−1,0), which has the highest field enhancement accompanied by a strongly backward power flux . This mode is usually associated with negative effective index of refraction at normal incidence. In conclusion, we have observed the geometrically designed dispersion of fishnet SPPs, and pointed out their distinct features. In particular, backward gap plasmons lead to negative refraction.