Plasmon Nanofocusing for Tip-Enhanced Raman Spectroscopy: Principles, Strategies, and Applications

Abstract

Plasmon nanofocusing has emerged as a powerful optical technique for achieving extreme confinement of electromagnetic fields at the nanometer scale by adiabatically compressing propagating surface plasmon polaritons along tapered metallic waveguides. Compared to traditional methods of confining propagating light into a nanometric volume using optical nanoantennas, this alternative approach offers several advantages, including a background-free light source directly at the nanoscale and broadband capabilities, enabling diverse applications in nano-optics and nanoscopy. This review covers the physical principles of plasmon nanofocusing, along with experimental strategies for efficient coupling, with a particular emphasis on grating-assisted and single-slit-assisted excitation, which enable broadband nanofocusing. Its integration with near-field scanning optical microscopy and tip-enhanced Raman spectroscopy, techniques that have revolutionized nanoscale vibrational imaging, has been revisited. We examine progress across various implementations, considering spatial resolution, bandwidth, and field enhancement, and highlight emerging opportunities in ultrafast spectroscopy, single-molecule detection, and quantum plasmonics. Challenges such as tip fabrication precision and coupling efficiency are also discussed.