Metamaterials and metasurfaces offer many possibilities for controlled absorption of electromagnetic fields (broadband vs. narrow-band, polarization selective, over wide vs. narrow incidence angles, etc.). These absorbers are foreseen for many different applications depending on their absorbing characteristics. For example, extremely narrow-band absorbers can be used for material sensing [1,2], while wide-angle absorbers with a moderate bandwidth can be used to improve thermophotovoltaic devices [3].
Describing efficiently and unambiguously the absorbing capabilities of a given device is complicated by the fact that absorption is a quadratic quantity of the incident fields. Moreover, at infrared and optical frequencies, the incident fields are usually only partially spatially coherent, complexifying the problem. An elegant solution has been proposed by the authors of [4,5], which consists in computing the natural absorption modes of the structure, a decomposition of the incident fields for which the coupling between different modes vanishes. This formalism can accommodate for the partial coherence of the fields [6,7]. One key point of this formalism is that the information can be retreived experimentally using the so-called Energy Absorption Interferometry (EAI) provided that one can measure the total power absorbed by the structrure [8].
We use this formalism to numerically study different plasmonic absorbers. In the cases studied, the peculiar absorbing patterns of the metasurfaces can be explained through surface modes [7, 9]. The complex transverse wave-vector of these modes can be determined from EAI measurements. By playing with the periodicity of the structure, propogative incident plane-wave can couple to these surface modes, so that absorption can be dramatically increased. Long-range surface modes provide an absorption which is selective with respect to the angle of incidence, while localized surface modes improve absorption over large opening angles [9, 10].
[1] D. Zhao et al., “Ultra-narrow-band light dissipation by a stack of lamellar silver and alumina,” Applied Physics Letters, 104, 221107, 2014. [2] L. Meng et al., “Optimized grating as an ultra-narrow band absorber or plasmonic sensor,” Optics Letters, 39(5), 1137-1140, 2014. [3] C. Simovski et al., “Optimization of radiative heat transfer in hyperbolic metamaterials for thermophotovoltaic applications,” Optics Express, 21, 14988-15013, 2013. [4] G. Saklatvala, S. Withington and M.P. Hobson, “Coupled-mode theory for infrared and submillimeter wave detectors,” JOSA A, 24(3), 764-775, 2007. [5] S. Withington and G. Saklatvala, “Characterizing the behaviour of partially coherent detectors through spatio-temporal modes,” Journal of Optics A: Pure and Applied Optics, 9(7), 626-633, 2007. [6] C. Craeye, S. Withington and C.N. Thomas, “Characteristic functions describing the power absorption response of periodic structures to partially coherent fields,” JOSA A, 31(7), 1360-1368, 2014. [7] D. Tihon et al., “Characterization of power absorption response of periodic three-dimensional structures to partially coherent fields,” JOSA A, 33(12), 2459-2469, 2016. [8] C.N. Thomas and S. Withington, “Experimental demonstration of an interferometric technique for characterizing the full optical behavior of multi-mode power detectors,” IEEE Transactions on Terahertz Science and Technology, 2(1), 50-60, 2012. [9] D. TIhon et al., “Improving absorption in periodic plasmonic structures through a scattering interface layer,” to be submitted. [10] C. Argyropoulos et al., “Broadband absorbers and selectrive emitters based on plasmonic Brewster metasurfaces” Physical Review B, 87, 205112, 2013.