Effect of Ag film roughness in a dielectric nanoparticle-on-mirror dark-field microscope.

A dielectric nanoparticle-on-mirror system can enhance the scattering signal and modulate the scattering direction of a low-index sample placed on the mirror, so that more of the signal sample can be collected by the objective lens. We find that the surface roughness of the Ag film increases as the deposition rate increases, and the scattering intensity of the sample increases and then decreases as the root-mean-square (RMS) roughness (deposition rate) increases. Under dark-field illumination, the scattering intensity of an individual polystyrene (PS) nanoparticle placed on a 50 nm thick Ag-film-coated substrate with an RMS roughness of 1.1, 5.3, 9.9, and 16.6 nm can be enhanced by about 5.9, 8.0, 7.2, and 6.7 times when compared to that placed on a glass slide. For the Ag-film-coated substrate with an RMS roughness of 5.3 nm, more specimen details can be observed when a biological sample is placed on this substrate under a conventional optical microscope, and a hexagonal close-packed monolayer of 250 nm diameter PS nanoparticle monolayer can be resolved using microsphere-assisted microscopy only when it is placed on it.