Studying the Dielectric Spacer Media of Nanophotonic Devices for Molecular Sensing Applications

The optical response of hyperbolic 2D materials to infrared light enables novel nanophotonic devices capable of detecting various biomolecules and environmentally polluting molecules. Controlling this IR light in the form of light-matter waves called phonon-polaritons (PhPs) is key to modulating this optical response for these applications. Current works in the control of PhPs have featured architectures consisting of 2D material layers atop metallic substrates, separated by dielectric spacer media. These spacer media are of particular interest for molecular sensing applications, as they can be used to enhance the interaction between infrared light and analyte molecules by confining them in an electric field. This work focuses on studying the effect of manipulating dielectric spacer thickness and refractive index on the propagation of PhPs. Using experimental, computational, and analytical methods, the PhP propagation distances and momentums were measured and validated for TiO₂, HfO₂, and VO₂ spacer media in between slabs of MoO₃, atop a gold substrate. Agreement between these methods has been observed in architectures with TiO₂ spacers. This work can guide the design of compact molecular sensing devices that offer high spatial resolution and sensitivity even at low concentrations.

When incident IR light couples with optical phonons in biaxial hyperbolic materials like MoO₃, they produce light-matter waves called phonon polaritons (PhPs) that can couple with molecules with vibrational modes at this same IR range (such as methane or CO₂). This coupling, which can generate detectable signals, can be strengthened by enhancing the local electric field intensity. This localized electric field enhancement can be achieved using dielectric spacer media, which separate the substrate from the hyperbolic material. In potential nanophotonic molecular sensing devices, this electric field enhancement would allow for detection of analyte molecules at low concentrations, with high spatial resolution. This inspires this further study of the dielectric spacer media, and their effects on the phonon polaritons that interact with analyte molecules.

Different experimental, computational, and analytical methods are used to study the effect of changing the refractive index and thickness of the dielectric spacer. The PhP propagation distances and momentums were first measured in various MoO₃ thin films on a gold substrate without a dielectric spacer medium, using scattering-type scanning near-field optical microscopy (s-SNOM), as a standard for comparison. Thus far, agreement between our experimental and analytical methods has been observed in these systems, corroborated by the trend of increasing PhP momentum with decreasing thin film thickness. These same s-SNOM measurements were performed in a system TiO₂ spacer (ref. index = 2.61), and will be performed for HfO₂ (ref. index = 1.9) and VO₂ (ref. index = 3.13), for different MoO₃ thin film thicknesses. This difference in refractive index could change the PhP propagation characteristics due to its effect on phase velocity. s-SNOM is also used to observe PhP propagation in systems with TiO₂ spacers of varying thicknesses, to study if the same PhP propagation effects are preserved in these different architectures. Numerical simulations (COMSOL multiphysics) were performed to observe the electric field confinement in these systems.

By studying the effects of dielectric spacer refractive index and thickness, this work provides a comprehensive view of how nanophotonic device architectures can be optimized for molecular sensing applications.