Enhanced radiation of a dipole placed between a metallic surface and a nanoparticle

Enhanced dipole radiation in the presence of a flat metallic surface and a metal nanoparticle is considered on the basis of Maxwell's equations. For the case of axi-symmetrical illumination the initial problem is reduced to a system of boundary integral equations for the angular component of the magnetic field and its normal derivative. A boundary element method is used to solve the system of integral equations. The scattering of convergent cylindrical electromagnetic waves from a nanoparticle placed near a surface is calculated. The dipole placed between the nanoparticle and the surface is excited by the enhanced field in the gap and re-radiates electromagnetic waves of the same frequency into space. This dipole radiation in turn is enhanced by the nanoparticle/surface system. Two intensity enhancement factors are calculated: (1) the enhancement of the local electric field at the dipole position by the nanoparticle/surface system; and (2) the increase in dipole radiation due to the presence of a metallic nano-object. For very small gaps (1 nm) between the surface and nanoparticle, these factors reach very large values. At some frequencies the enhancement factors exhibit large resonance peaks which can be explained as plasmon resonances in the nanoparticle/surface system. For various shapes of metal nanoparticles and for different distances in the particle/dipole/surface configuration, the total intensity enhancement factor (the product of the two factors described above) is calculated using the developed model. The very large enhancement factors obtained in our calculations can be considered as a theoretical basis for single molecule Raman spectroscopy.