  Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates, as
a new frontier in the field of SERS, are hindered by their poor electromagnetic
field confinement, and weak light-matter interaction. Metasurfaces, a class of
2D artificial materials based on the electromagnetic design of nanophotonic
resonators, enable strong electromagnetic field enhancement and optical
absorption engineering for a wide range of semiconductor materials. However,
the engineering of semiconductor substrates into metasurfaces for improving
SERS activity remains underexplored. Here, we develop an improved SERS
metasurface platform that leverages the combination of titanium oxide (TiO2)
and the emerging physical concept of optical bound states in the continuum
(BICs) to boost the Raman emission. Moreover, fine-tuning of BIC-assisted
resonant absorption offers a pathway for maximizing the photoinduced charge
transfer effect (PICT) in SERS. We achieve ultrahigh values of BIC-assisted
electric field enhancement (|E/E0|^2 ~ 10^3), challenging the preconception of
weak electromagnetic (EM) field enhancement on semiconductor SERS substrates.
Our BIC-assisted TiO2 metasurface platform offers a new dimension in
spectrally-tunable SERS with earth-abundant and bio-compatible semiconductor
materials, beyond the traditional plasmonic ones.
