Parametrically resonant surface plasmon polaritons

The surface electromagnetic waves that propagate along a metal-dielectric or a metal-air interface are called surface plasmon polaritons (SPPs). These SPPs are advantageous in a broad range of applications, including in optical waveguides to increase the transmission rates of carrier waves, in near field optics to enhance the resolution beyond the diffraction limit, and in Raman spectroscopy to amplify the Raman signal. However, they have an inherent limitation:  as the tangential wavevector component of propagation is larger than what is permitted for the homogenous plane wave in the dielectric medium, this poses a phase-matching issue. In other words, the available spatial vector in the dielectric at a given frequency is smaller than what is required by SPP to be excited. The most commonly known technique to bypass this problem is by using the Otto and Kretschmann configurations. A glass prism is used to increase the available spatial vector in dielectric/air. Other methods are the evanescent field directional coupling, optical grating, localized scatterers, and coupling via highly focused beams. However, even with all these methods at our disposal, it is still challenging to couple SPPs that have a large propagation constant. 

As SPPs apply to a wide range of purposes, it is vitally important to overcome the SPP excitation dilemma. Presented here is a novel way to efficiently inject power into SPPs via temporal modulation of the dielectric adhered to the metal. In this configuration, the dielectric constant is modulated in time using an incident pump field. As a result of the induced changes in the dielectric constant, we show that efficient phase-matched coupling can be achieved even by a perpendicularly incident uniform plane wave. This novel method of exciting SPPs paves the way for further understanding and implementation of SPPs in a plethora of applications. For example, optical waveguides can be investigated under such excitation. Hence, this technique opens new possibilities in conventional plasmonics, as well as in the emerging field of nonlinear plasmonics.