Microsphere-assisted Raman scattering enhancement by photonic nanojet: the role of the collection system

The use of a microlens to create a strong and narrow beam of light called a photonic nanojet (PNJ) has attracted a lot of attention in the scientific community lately. Especially important is its use for Raman scattering enhancement. However, its mechanism is still not clear. While research in the literature has so far concentrated on the excitation part of the problem (PNJ), we investigate the role of the collection system for the enhancement. Detailed and systematic vertical Raman imaging measurements were performed. By experimental optimization of the collection system parameters, up to 19.29× enhancement was achieved. A simple theoretical model is created that calculates the effective numerical aperture (NA_eff) of the microsphere-objective system, from which it computes the enhancement. The comparison between the experimentally measured enhancement and the calculated enhancement from the model shows that the collection system plays a major role. The model surprisingly well matches the experimental data, and with possible improvements in the future, it could open a new door in explaining and predicting microsphere-assisted Raman enhancement.