A whispering-gallery scanning microprobe for Raman spectroscopy and imaging.

Optical whispering-gallery-mode microsensors are a promising platform for many applications, such as biomedical monitoring, magnetic sensing, and vibration detection. However, like many other micro/nanosensors, they cannot simultaneously have two critical properties - ultrahigh sensitivity and large detection area, which are desired for most sensing applications. Here, we report a novel scanning whispering-gallery-mode microprobe optimized for both features and demonstrate enhanced Raman spectroscopy, providing high-specificity information on molecular fingerprints that are important for numerous sensing applications. Combining the superiorities of whispering-gallery modes and nanoplasmonics, the microprobe exhibits a two-orders-of-magnitude sensitivity improvement over traditional plasmonics-only enhancement; this leads to molecular detection demonstrated with stronger target signals but less optical power required than surface-enhanced-Raman-spectroscopy substrates. Furthermore, the scanning microprobe greatly expands the effective detection area and realizes two-dimensional micron-resolution Raman imaging of molecular distribution. The versatile and ultrasensitive scanning microprobe configuration will thus benefit material characterization, chemical imaging, and quantum-enhanced sensing.