  Metal-Insulator-Metal (MIM) plasmonic structures can confine electromagnetic
waves to a deep subwavelength regime, enabling strong light-matter interactions
with potential applications in nonlinear optics and on-chip photonic circuitry.
In addition, strong coupling of mid-infrared (mid-IR) vibrational transitions
to optical cavities provides a way to modify and control a material's chemical
reactivity and may also allow for highly sensitive chemical detection
technology. Here, we experimentally and theoretically investigate the mid-IR
optical properties of 3D-printed, nanoscale, anisotropic, L-shaped MIM
plasmonic cavities. We observe strong vibrational-plasmon coupling between the
two dipolar modes of the L-cavity and the polymer dielectric. The resulting
three polariton modes are well described by a multimode coupled oscillator
model, which we employ to predict the polariton behavior as a function of
cavity arm length. The 3D printing technique offers time and cost reduction
advantages over typical electron beam lithography and represents a highly
accessible and versatile means of printing arbitrary-shaped nanometer-sized
mid-IR plasmonic cavities capable of producing strong light-matter interactions
for a variety of photonic or photochemical applications.
