Abstract

The work is devoted to study interference effects in systems with several coherently excited surface plasmon polariton (SPP) beams. Two-beam excitation is experimentally realized using a Michelson-interferometer, allowing to control the intensity and the polarization state in both beams independently. The initial laser beam is divided into two replicas which are focused by a single microscope objective and irradiate a gold surface at different positions. SPPs are excited by localized scattering of the laser light on laser-fabricated nanostructures. The surface nanostructures are produced by two-photon polymerization of spin-coatable epoxy-based resist on a metal covered glass slide [1]. Moreover, the obtained dielectric structures can be combined with spherical gold nanoparticles of arbitrary size produced by the method of laser-induced transfer [2]. The particles are considered as additional SPP scattering centres and can be used for manipulation of the SPP intensity. SPP interference and scattering effects are investigated by leakage radiation microscopy (LRM). The influence of the excitation conditions and nanostructure configurations on the interaction between coherent propagating and intersecting SPP beams is demonstrated and discussed [Fig. 1a-f]. Due to interference between two SPP beams with a controllable phase difference, excited on parallel dielectric ridges, the SPP energy can be concentrated in the local region between two ridges, thus acting as a cavity for SPPs. Changing the inter-ridge distance one can change the SPP energy density in this cavity. The excitation of SPPs with two polarization- and phase-controlled light beams, furthermore, allows the selective excitation of different modes of plasmonic waveguides. The application of in-phase or out-of-phase light beams for excitation of different modes in dielectric and bandgap-confined SPP waveguides is suggested and demonstrated. The experimental results are supported by theoretical modelling using the Green’s tensor approach and FDTD simulations. As an example, a strong focusing effect of two coherent SPP beams in circular nanoparticle structure is shown in Fig. 1g,h. Application of the observed effects to metrology and sensing are discussed.

© 2011 Optical Society of America