Biological metasurfaces based on tailored Luria Bertani Agar growth medium formulations for photonic applications.

Biodegradable alternatives to classic solid-state components are rapidly taking place in front-end photonic systems like metamaterials, meta-surfaces and photonic crystals. From this point of view, numerous solutions have been proposed involving eco-friendly compounds. Among them, the Luria Bertani agar (LBA) growth medium has been recently proposed as a functional option with the remarkable advantage of allowing the growth of fluorescent protein expressing bacteria. Such a possibility promises to lead to development of a new generation of biological and eco-sustainable optical sources based on meta-surfaces. There is, however, still a main drawback to address, related to the highly scattering nature of these compounds. To ensure adequate nutritive elements for cell growth, LBA hosts several compounds like NaCl, yeast extracts and tryptone. The presence of these components leads to very scattering LBA films, thus hindering its performance as an optical polymer. A trade-off arises between nutritive capacity and optical performance. In this paper, we successfully address this trade-off, demonstrating that a reduction of the basic nutrients (net Agar concentration) of LBA largely enhances the optical properties of the film as a photonic polymer without compromising its cell-viability. We considered two new LBA formulations with two- (LB₂A) and four-fold (LB₄A) reduction of the nutrients and replicated a square-lattice meta-surface used as a benchmark architecture. We demonstrated that both the replica molding performances and the optical properties (absorption, scattering and diffraction efficiency) of LBA formulations increase with decreasing nutrient concentration, without losing their cell-growth capability. To demonstrate this fundamental aspect, we inoculated the most critical case of LB₄A with green-fluorescent-protein-expressing E. coli bacteria, verifying both their vitality and good photoluminescence properties. These results overcome one of the main limitations of LBA as a functional biopolymer for optical applications, unlocking its use in a new generation of biological quantum optical frameworks for all-biological weak and strong light-matter interactions.