Silver nanoparticles (AgNPs) with radius of similar to 20 nm were optically trapped and immobilized on thiophenol (TP)-covered Ag films under a gap mode resonance with extremely weak power density of 1 mu W/mu m(2) at 532 nm. Intensity of Raman scattering from TP markedly increased with the accumulation of AgNPs. Trapping efficiency of AgNPs for p-polarization was 2-4 times higher than that for s-polarization. The observed optical trapping and immobilization were theoretically rationalized using a dipole-dipole coupling under a gap mode and van der Waals interaction between AgNPs and Ag films, which facilitate to fabricate versatile substrates for surface enhanced Raman scattering. (C) 2016 Elsevier B.V. All rights reserved.