Molecular spin-crossover polaritonics in nanocavities – SCOPOL

When molecules are strongly coupled to confined electromagnetic fields within micro- and nano-structures (e.g. Fabry-Pérot or plasmonic nanocavities), new hybrid light-matter states – known as polaritons – are formed; the coupled {molecules + cavity} system having to be thought as a single entity with new energy levels and exhibiting new physico-chemical properties. This field of “molecular polaritonics” has seen a spectacular progress in the past decade and represents today a powerful strategy to explore the synergetic effects that can exist between optical resonant cavities and molecular bistability property. In this context, the SCOPOL project aims at achieving and exploiting light-matter strong coupling of spin-crossover (SCO) molecules, which exhibit reversible switching between their low-spin (LS) and high-spin (HS) electronic configurations, to electromagnetic modes within various optical nanocavities. The aim of the project is twofold. (1) On one hand side, we seek to use the SCO bistability to switch between strong-coupled and uncoupled regimes in the cavity through various external stimuli (temperature, light irradiation, etc.) and then scrutinize possible applications in reconfigurable/adaptive optics. (2) On the other way around, we seek to use the strong-coupling regime, which deeply modifies the energy landscape of the coupled system, to fine tune the phase stability (spin-transition temperature, abruptness of the transition, hysteresis width) and the spin-state switching properties of the SCO molecules. This property could be the “Holy Grail” in the spin-crossover field allowing, for instance, custom-designed spin-transition properties or light-induced spin-state switching at room temperature.

SCOPOL is a strongly interdisciplinary project, which is based on the complementary skills of the host teams at the LCC-CNRS and LAAS-CNRS, combining in-depth knowledge of spin-crossover nanomaterials and a cutting-edge nanotechnology platform for photonic device fabrication. In this frame, conventional SCO-embedded metal-mirror-based Fabry-Pérot (FP) and plasmonic cavities will be fabricated, with purposefully adjusted size and shape, to explore the interplay between SCO and light-matter strong coupling. As a second step, more advanced nano-photonic devices will be developed (FP resonators with 3D cavity mode, sub-wavelength-grating structures), with higher quality factors and further confined cavity volume, which will not only allow for more flexibility in the integration and control of SCO materials, but also provide scope for proof-of-concept applications. In a third step, the tuning of SCO properties will be investigated, more specifically by coupling selected molecular vibrational transitions to cavity modes (in the infrared spectral region), as a means to control the LS-HS energy gap and thus the switching properties of SCO molecules.

SCOPOL is an exploratory fundamental scientific project, which is also motivated by future technological applications. In particular, using a novel paradigm (formation of hybrid light-matter states), this project aims to turn these switchable molecules into active elements in photonic devices, which will open new avenues for a variety of applications, especially in the context of spatial light modulator technologies as well as for photonic integrated circuits.