In this Optics Express paper, Lee et al propose a new type of 3-dimensional nanoscale metallic funnel waveguide that can be used to significantly boost the strength of the electromagnetic field by up to 400 times with the help of SPPs. For an optimal design, the authors perform a series of finite difference time domain simulations, and eventually discover that a tapered hollow core with elliptical shape can be an efficient geometry to increase the field strength without an increase in pulse duration when the polarization of fs laser pulse is aligned parallel to the minor axis of the ellipse. Furthermore, to verify this enhancement experimentally, the funnel waveguide is fabricated on a thick Ag film using focused ion beam processes, and the enhanced two-photon luminescence following fs pulse excitation manifests the strong field enhancement in the funnel waveguide.
In summary, the authors successfully devise a 3 dimensional nanoscale metallic funnel waveguide in which the fs laser pulse can be spatially confined during its propagation with nearly no broadening of its duration, something that is critical for maintaining the enhanced field strength. As the authors note, this waveguide can be potentially useful for performing strong field experiments such as nonlinear optical frequency upconversion, since the intensity of the field can reach a few tens of TW/cm2.
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