Single molecule detection and analysis play important roles in many current biomedical researches. The deep-nanoscale hotspots, being excited and confined in a plasmonic nanocavity, make it possible to simultaneously enhance the nonlinear light-matter interactions and molecular Raman scattering for label-free detections. Here, we theoretically show that a nanocavity formed in a tip-enhanced Raman scattering (TERS) system can also achieve valid optical trapping as well as TERS signal detection for a single molecule. In addition, the nonlinear responses of metallic tip and substrate film can change their intrinsic physical properties, leading to the modulation of the optical trapping force and the TERS signal. The results demonstrate a new degree of freedom brought by the nonlinearity for effectively modulating the optical trapping and Raman detection in single molecule level. This proposed platform also shows a great potential in various fields of research that need high-precision surface imaging.

In recent years, light beams containing phase or polarization singularities, such as optical vortices (OVs) and cylindrical vector beams (CVBs), have contributed to significant applications including optical orbital angular momentum (OAM) communications, particle trapping and manipulation, and super-resolved imaging. However, traditional methods for detecting the phase and polarization singularities of light suffer from drawbacks, such as large device size, complicated optics, and limits in detection function. Here, we propose an alternative method for detecting simultaneously phase and polarization singularities based on a spin-multiplexing metasurface. Both numerical and experimental results demonstrate that the metasurface device can be used to measure accurately the topological charge of OVs and the polarization order of CVBs individually or simultaneously, and exhibit beneficial attributes such as a broadband response, compactness, and system simplification. This method offers great potential in applications such as singular optical beam shaping and high-capacity OAM/CVB multiplexing communication.