The use of solid-solid phase transitions to modulate the plasmonic response of metal nanostructures is a promising approach to nanophotonic technologies, including sensors based on signal modulation in confined nanoscale volumes [1]. Consideration of phase-transforming materials has typically focused on composition, whereas relatively little attention has been paid to the question of size dependence in determining stable phases. However, size effects play a crucial role in determining the coupling with mechanical, optical, chemical or thermal input required to effect the phase transformation [2]. With rapid progress in nanofabrication techniques, size-dependent properties become relevant and systematic studies to assess both the role of nucleation in forming a new state and of the nanoscale dynamical effects are needed.

Here, we describe an example that shows how, by systematically varying the gap between the arms of split-ring plasmonic resonators, the in-arm coupling resonance in a split-ring metamaterial can be used to monitor the metal-insulator transition in discrete volumes of the strongly correlated VO2. Moreover, this “plasmonic hysteresis” technique also provides a means to correlate the electronic phase-transition with its structural counterpart which was previously measured using SERS technique [3]. If the number of intrinsic nucleation sites is directly proportional to the interrogated volume (a reasonable assumption) [4], we have effectively shown that well-crafted plasmonic structures with well-understood modes can be a helpful tool to probe size-dependent effect [5]. Full field 3D finite-difference time-domain simulations show that the physical origins of these non-isotropic electron oscillations leads to concentration of the electromagnetic energy for focused interrogation and high sensitivity.

As an additional example, we briefly describe an investigation into the use of similar nanostructures as chemical sensors based on coupling of autocatalytic reactions at the gold-VO2 interface and molecular recognition moieties. In this case, the detection method involves the change in optical transition of a metamaterial array incorporating VO2 when the heat of decomposition is sufficient to initiate the metal insulator transition.