Light from plasmonic lenses stimulated by tunnel electrons

A plasmonic lens is a metallic nano- or microstructure designed to control the propagation of surface plasmon polaritons (SPPs), i.e., electromagnetic waves that are coupled to electronic density oscillations at a... [ view full abstract ]

A plasmonic lens is a metallic nano- or microstructure designed to control the propagation of surface plasmon polaritons (SPPs), i.e., electromagnetic waves that are coupled to electronic density oscillations at a metal-dielectric interface. Plasmonic lenses are among the most versatile optical microcomponents since they may be used to (i) convert light into SPPs and vice versa, (ii) focus and collimate SPPs or light, and (iii) couple the optical nearfield of a nanoscale emitter to farfield radiation. The integration of plasmonic lenses with nanoelectronics on a chip is desired, e.g., for applications in optical communications. Such integration requires that electrically driven nanosources of light or SPPs be incorporated into the design of plasmonic lenses, thus raising a number of fundamental and technical issues. In particular, the generation of light and SPPs from electrical current generally yields a broad power spectrum, whereas plasmonic lenses are often designed for specific energies or wavelengths. Their optical response to an electrical, spectrally broad, excitation has rarely been addressed.

Here we report on the emission properties of a plasmonic lens driven by low-energy electrons. The plasmonic lens consists of concentric circular slits etched in a thick gold film on a glass substrate. A local, broadband, electrical excitation is applied using the inelastic effects of the tunnel current from the tip of a scanning tunneling microscope (STM). The excited SPPs scatter at the circular slits. This yields light that we collect below the substrate using the objective lens of an optical microscope. The angular and spectral distributions of the emitted light are measured using optical imaging in Fourier space and optical spectroscopy. As well, we use numerical methods to simulate the electrical excitation of a plasmonic lens with tunnel electrons and the resulting far-field radiation in the substrate. The geometry of the plasmonic lens (diameter, period and number of slits) and the excitation site strongly influence both the angular emission pattern and the emission spectrum. By optimizing these parameters, we can obtain a broadband, electrical microsource of cylindrical vector beams, whose angular divergence is < 4 degrees.