One of the most attractive potentials of plasmonic metamaterials is the amplification of intrinsically weak signals such as molecular infrared absorption or Raman scattering for detection applications. This effect, however, is only effective when target molecules are located at the enhanced electromagnetic field of the plasmonic structures (i.e., hot-spots). It is thus of significance to control the spatial overlapping of molecules and hot-spots, yet it is a long-standing challenge, since it involves the handling of molecules in nanoscale spaces. Here a metamaterial consisting of a nanofluidic channel with a depth of several tens of nanometers sandwiched between plasmonic resonators and a metal film enables the controllable delivery of small molecules into the most enhanced field arising from the quadrupole mode of the structures, forming a plasmon-molecular coupled system. It offers an ultrasensitive platform for detection of IR absorption and molecular sensing. Notably, the precise handling of molecules in a fixed and ultrasmall (10-100 nm) gap also addressed some critical issues in IR spectroscopy such as quantitative measurement and measurement in aqueous solution. Moreover, a drastic change in the reflectance characteristic resulting from the strong coupling between molecules and plasmonic structures indicates that molecules can also be utilized as triggers for actively switching the optical property of metamaterials.

This new method that allowed to separately introduce nanopatterns into multiple interfaces in OPVs cumulatively increased the photocurrent without deterioration of their electronic properties.

The behavior of molecules under nanoconfinement is crucial for understanding the chemical processes in biological and nanomaterial systems. We demonstrated here an infrared spectroscopic method to characterize the molecular structures of molecules confined in several tens of nanometer cavities by employing the plasmonics–nanofluidics hybrid device. This device consists of an array of metal nanostructures and a metal mirror separated by a nanofluidic cavity. Its configuration enables the confinement of both molecules and light energy as localized surface plasmons inside the physicochemically well-defined nanocavity. Exploiting the plasmons–molecular coupling, the vibrational modes of the nanoconfined molecules are selectively detected with a prominent sensitivity. Applying water as a proof-of-concept sample, we have successfully measured the infrared absorption characteristic and elucidated the molecular structures of water confined in a 10 nm cavity. They unveiled the presence of a strong H-bond network with respect to bulk water. Our method was also able to distinguish the subtle differences in the molecular structures, revealing the scaling behavior of confined water in the several tens of nanometer size regime. This effect is also found not being driven by the interaction with the interfaces; yet the constrained geometry itself promotes the intermolecular interactions of water and results in the modification of the H-bond network. This study has offered an ultrasensitive platform for in situ probing of the nanoconfined molecules and chemical reactions in their intact condition, and thus gives us a fundamental insight into the nanoconfinement effects.

Anomalously low refractive index of nanoconfined water.