Experimental demonstration of plasmon-soliton coupling
Résumé
Merging the fields of plasmonics and nonlinear optics authorizes a variety of fascinating and original physical
phenomena. In this work, we specifically study the combination of the strong light confinement ability of surface
plasmon polaritons (SPP) with the beam self-trapping effect in a nonlinear optical Kerr medium. Although this idea of
plasmon-soliton has been the subject of numerous theoretical papers [1], up to now, no experimental evidence had been
revealed. In the present study, a proper architecture has been designed and fabricated allowing the first experimental
observation of hybrid coupling between spatial solitons and SPP.
To be able to trigger the nonlinearity at moderate light power and simultaneously allow propagation over several
millimeters, a metal-dielectric structure was first designed. It consists of a four–layer planar geometry made of a Kerr
dielectric layer placed on a lower refractive index substrate and covered with a thin dielectric layer followed by a metallic
film. Performed numerical simulations show that the designed planar waveguide exhibits an enhanced Kerr self-focusing
for the TM0 mode thanks to the plasmonic effect.
The experimental analysis consists in injecting a typical 4 × 30 μm2 (FWHM) elliptical beam at 1.55μm from a
femtosecond laser into the 5mm long waveguide. The output beam distribution evolution is then monitored versus input
light intensity. Figure 1 shows the large trapping enhancement observed for a TM mode in presence of the plasmonic
structure (Fig. 1d,e) compared to configurations (Fig. 1b,c and Fig. 1f,g) where plasmonic effect is absent. For an input
intensity of 1.2 GW/cm² the beam is self-confined to a 12μm FWHM (Fig. 1e) while the FWHM is twice (Fig. 1c,g)
without plasmonic effect. The proposed plasmonic structure definitely exposes an enhanced self-focusing nonlinearity.
The strong light confinement is due to the presence of the hybrid coupling between a plasmon and a soliton waves.
Different configurations have also been tested that unambiguously reveal this plasmon-soliton coupling.