Free-Space Excitation of Propagating Surface Plasmon Polaritons

Lukas Novotny, Institute of Optics, University of Rochester

Surface plasmon polaritons (SPPs) are electromagnetic excitations bound to a metal's surface. Because of their strong localization and resonant properties they find a broad range of applications, ranging from biosensing to optoelectronics. A unique feature of surface plasmon polaritons (SPPs) is that their momentum is larger than the momentum of free-propagating photons of the same energy. Therefore, it is believed that they can be excited only by evanescent fields created by total internal reflection or by local scattering. In this presentation, we will demonstrate the free-space excitation of surface plasmons by means of nonlinear four-wave mixing. The process involves the vectorial addition of the momenta of three incident photons, making it possible to penetrate the light cone and directly couple to the SPP dispersion curve. Using this technique, surface plasmons can be launched on any metal surface by simply overlapping two beams of laser pulses incident from resonant directions. The excitation scheme is also applicable to other bound modes, such as waveguide modes, surface phonon-polaritons, and excitations of two-dimensional electron gases.

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