Strength of the electric field in apertureless near-field optical microscopy

Enhancement gamma of the electrical field at the end of a tip relative to t he incident field in a focused radiation beam is calculated by the finite-e lement time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with an alytical solutions. The calculations on these test structures also reveal t hat phase retardation effects substantially modify gamma when the size of t he structure is larger than approximately lambda /4, lambda being the radia tion wavelength. For plasmon resonance, in particular, phase retardation se verely reduces the resonance and the expected field enhancement for a gold tip. The small value of gamma =4 calculated by FETD is about an order of ma gnitude smaller than the value found in recent published work. Resonance ef fects can be recovered for special tips, which have a discontinuity or a di fferent material composition at the end of the tip. Some tuning of the disc ontinuity dimension is needed to maximize the resonance. Under optimal cond itions for plasmon resonance, an enhancement in the electric field of about 50 is calculated at the end of a small gold protrusion mounted on a wider silicon or glass tip. (C) 2001 American Institute of Physics.