Recommendations and validity domains of techniques for experimental electromagnetic characterization of diï¬€erent nanostructured materials based on their chemical and geometrical structures
As it was shown in our reports the situation in the electromagnetic characterization of nanostructured materialsis not favorablefor the choice of the best technique if the materialis resonant.
First, among the methods of the electromagnetic characterization of nanostructured materials only two methods -Nicolson-Ross-Weir (NRW) retrieval (M.Sucher, Handbook of Microwave Measurements, vol2, BrooklynPolytechnic Press1963,Â V.N. Egorov, Instruments and Experimental Techniques 50 (2007)143) and Variable-Angle-Spectroscopic Ellipsometry(VASE) retrieval (G.E. Jellison, Thin Solid Films 313/314 (1998)193,Â V.S. Merkulov, Optics and Spectroscopy 103(2007)629) -have been developed enough.
The NRW method is based on the measurement of the complex refraction R and transmission T coefficients for anormally incident plane wave. It is used to characterize bulk magnetoelectric layers including thin films in the radio frequency range. With respect to nanostructured materials whose electromagneticproperties refer to the optical region, this method canbe reduced to the measurement of only absolute values of R and T or to the measurement of only complex R. This becomes possible because permeability μ of usual materials in the optical frequency region is equal to 1, and to retrieve two real values, namely real and imaginary parts of the medium permittivity ε is possible from two measured values. The NRW method gives not a full characterization of anisotropic material since is restricted by the normal incidence only.
The VASE method is combination of the ellipsometry and spectroscopy and is applicable to characterize layers of anisotropic (uniaxial) media whose complex permittivity is a tensor. It gives the better accuracy for nanostructured thin films than the NRW method especially in the case of losses.Â
The literature analysis in our reportsÂ clearly showed that these two well-developed methods are suitable only for those nanostructured materials whose nanosized elements are optically densely packed and not resonant. These two methods are not applicable to nanostructured photonic crystals, nanopatterned diffraction gratings. Such optically sparse structures cannot be characterized in terms off em material parameters. These two methods are also not applicable to nanostructured metamaterials,i.e. composites with optically dense package of resonant constitutive nanoelements. Nanostructured metamaterials form an important class of artificial materials, as it is clear from the classification. For the characterization of nanostructured metamaterials more advanced methods are needed.
However, these more advanced methods, whose overviews were presented in our reports, are currently under theoretical study and can not be referred to as real techniques. Therefore in this report we omit these prospective advanced methods and consider limitations to the existing techniques related to the structural and chemicalÂ properties of nanostructured materials.Â
Please, note, the frequency region, for which the characterization is to be done, should satisfy the following rules:
- the arrangement of inclusions should be optically dense;
- should be optically small;
- inclusions should not be arranged in chiral and magnetic arrays;
- the material layer thickness should be significantly larger than the inclusion sizes;
- concentration of inclusions should not strongly vary in the illuminated spatial domain;
- inclusions shouldbepassive(nogenerationinthen);
- inclusions should be non-resonant.
Read more on NRW and VASE and their validity in theÂ full version of this documentÂ and inÂ our overview on all of the methods.Â
See our roadmap for characterization.