Far-field controlling of phonon polaritons in biaxial natural hyperbolic van der Waals crystal for molecular sensing

Mid-IR light is spectrally overlapped with the molecular fingerprint region of the electromagnetic spectrum, as many biomolecules exhibit their signature vibrational modes in this regime. Traditional IR spectroscopic methods for sensing present challenges in signal to noise ratio, impairing their sensitivity to trace levels of biomolecules. These signatures are enhanced significantly by coupling of light with charged particles to form quasi-particles referred to as polaritons. This enhancement is possible in the infrared by coupling between molecular vibrations and IR-active polaritonic modes. In this work, we show efficient detection of biomolecules using polaritons supported by in-plane vdW natural hyperbolic material, where the principal components of the dielectric permittivity have opposite signs at the same frequency. These vdW materials support low-loss hyperbolic modes resulting from the formation of phonon polaritons(PhP) which is due to the hybridization of crystal vibrations (phonons) with light. This results in deep sub-wavelength confinement of infrared light, amplifying interactions between light and matter. Nanostructures of such vdW crystals can support localized PhP modes in the far-field, dramatically enhancing coupling between light with the vibrational resonances of a molecule adsorbed on the surface. Due to the enhanced light-matter interaction with the hyperbolic PhPs supported by the vdW nanopatterns, we enhance the sensitivity to the vibrational modes of the biomolecules, enhancing their detection to point-of-care levels.