Light Source Engineering of Directive Photoluminescent Metasurfaces with the Local Kirchhoff’s Law

Controlling spontaneous is the key to build efficient sources with controlled spectrum, angular emission pattern or polarization. This can be traditionally performed by adding external components, such as filters, lenses or polarizers that shape the wavefront emitted by an external source – but this all results in bulky and expensive system.<br/><br/>Over the past few years, progress in nanophotonics showed the extreme versatility of metasurfaces, designed as thin multifunctional components operating via the interaction between an incident wavefront and periodic elements covering the surface. The shaping of light features comes from the local scattering of a coherent wavefront by meta-atoms. Spatial coherence is therefore critical to engineer directional light sources, and is challenging to maintain when working with ensembles of emitters, as required in order to build bright sources delivering enough sufficient levels of optical power.<br/><br/>In this presentation, we investigate designs of light-emitting metasurfaces, or in other words, nanostructured surfaces covered with emitters. In these systems, the interaction between emitters and the metasurface enables a precise control of the emission properties. In particular, spatially extended modes can be exploited to restore spatial coherence between initially independent emitters, and therefore to shape the angular distribution of emitted light.<br/><br/>A first part of our presentation will comment on our design approach. The standard approach way to proceed is to calculate the field emitted by a dipole placed in the electromagnetic environment of the structure, and by summing and averaging out incoherently over all positions of the dipole. We follow here a radically different approach that exploit reciprocity : we use the local Kirchhoff’s law to optimize the emission features in terms of spectrum, polarization and directivity by optimizing the absorption features of the same structure in the reciprocal picture.<br/><br/>This approach gives us the possibility to access quantitatively to the performances of the structure, in terms of efficiency or total emitted power. The latter for instance can be maximized by maximizing the local absorptivity of the structure within the active layer, and making it as cloSe as possible to unity.<br/><br/>In a second part, we will present devices consisting of a 2D metallic (silver) grating covered with a thin layer of nanoplatelets deposited by spin coating on the metasurface. The samples were designed following the Kirchhoff’s approach, then fabricated in a clean room environment. Our samples provide highly directional photoluminescence at 605 nm, both in s and p polarizations, with an emission cone around 13.4° half angle.<br/><br/>This work paves the way towards new paradigms in metasurface designs and to a new generation of ultra-thin photoluminescent sources providing highly-controlled wavefront all in a device incorporating both the active medium and the electromagnetic environment that shapes the emission.<br/><br/><b>References :</b><br/><br/>E. Bailly, J.-P. Hugonin, B. Vest, and J.-J. Greffet. Spatial coherence of light emitted by thermalized ensembles of emitters coupled to surface waves. Phys. Rev. Res., 3 :L032040, Aug 2021<br/><br/>E. Bailly, K. Chevrier, C. Perez de la Vega, J.-P. Hugonin, Y. De<br/>Wilde, V. Krachmalnicoff, B. Vest, and J.-J. Greffet. Method to measure the refractive index for photoluminescence modelling. Opt. Mater. Express,<br/>12(7) :2772–2781, Jul 2022.<br/><br/>H. Monin, A. Loirette-Pelous, E. De Leo, A. A. Rossinelli, F. Prins,<br/>D.J. Norris, E. Bailly, J.-P. Hugonin, B. Vest, and J.-J. Greffet. Controlling light emission by a thermalized ensemble of colloidal quantum dots<br/>with a metasurface. Opt. Express, 31(3) :4851–4861, Jan 2023