Multimode vibrational strong coupling in direct laser written mid-IR plasmonic MIM nano-patch antennas

Strong coupling of mid-infrared(mid-IR)vibrational transitions to optical cavities provides a means to modify and control a material’s chemical reactivity and offers a foundation for novel chemical detection technology.Currently,the relatively large volumes of the mid-IR photonic cavities and weak oscillator strengths of vibrational transitions restrict vibrational strong coupling(VSC)studies and devices to large ensembles of molecules,thus representing a potential limitation of this nascent field.Here,we experimentally and theoretically investigate the mid-IR optical properties of 3D-printed multimode metal-insulator-metal(MIM)plasmonic nanoscale cavities for enabling strong light-matter interactions at a deep subwavelength regime.We observe strong vibration-plasmon coupling between the two dipolar modes of the L-shaped cavity and the carbonyl stretch vibrational transition of the polymer dielectric.The cavity mode volume is half the size of a typical square-shaped MIM geometry,thus enabling a reduction in the number of vibrational oscillators to achieve strong coupling.The resulting three polariton modes are well described by a fully coupled multimode oscillator model where all coupling potentials are non-zero.The 3D printing technique of the cavities is a highly accessible and versatile means of printing arbitrarily shaped submicron-sized mid-IR plasmonic cavities capable of producing strong light–matter interactions for a variety of photonic or photochemical applications.Specifically,similar MIM structures fabricated with nanoscopic voids within the insulator region could constitute a promising microfluidic plasmonic cavity device platform for applications in chemical sensing or photochemistry.