Strong‐Coupling‐Enhanced Raman Spectroscopy (SCERS) in a Cascade Microsphere‐Cavity

Abstract Surface‐enhanced Raman spectroscopy (SERS) is a powerful optical technique with high sensitivity to identify analytes down to a single molecule by fingerprinting vibrational information. The strong coupling in microcavities for supermodes with promoted quality factors demonstrates the capability to boost the interaction between photons and molecules for nonlinear effects. Here a strong‐coupling cascade microsphere‐cavity (MC) achieving spontaneous Raman enhancement is reported, for the first time. The cascade MC is composed of two size‐mismatched microspheres, both of which support optical whispering‐gallery modes (WGMs) with significantly different free‐space ranges. The strong coupling between the supported WGMs in the microspheres can be achieved when the quasi‐vernier effect of resonant modes occurs, by which the energy splitting is up to 4 meV. The fast energy transfer between the size‐mismatched MCs therefore increases the quality factor up to 2.1 × 10³ for the upper supermode branch. The strong‐coupling cascade MC structure provides an extra enhancement channel for the Au‐based SERS substrate, demonstrating an enhancement factor of Raman intensity (EFRI) up to 2.6 × 10¹⁰ for the limit of detection (LoD) down to 10⁻¹² M of 4‐aminothiophenol (4‐ATP). The strong‐coupling cascade MC substrates also exhibit the versatility for identification of multiple analytes for high‐performance Raman trace‐detection applications.