Observation of strongly enhanced ultrashort pulses in 3-D metallic funnel-waveguide
Spotlight summary: One of the most challenging topics in the field of ultrafast optics is the understanding of unusual phenomena arising only under ultra-intense electromagnetic fields, because with other currently available technologies it is impossible to reach such a high field strength generated with femtosecond (fs) lasers. Two strategies are commonly used to enhance the strength of the electromagnetic field from fs lasers: one can either amplify the energy of the fs laser pulse by using a series of amplifiers, or one can concentrate or confine fs laser pulses within a very small volume. The former approach is generally not preferable at small–scale laboratories due to the high price of amplifiers, their high maintenance cost, and amount of space required. The latter option, on the other hand, is more cost-effective, and does not require giving up the high repetition rate of fs laser oscillators, whereas the use of amplifiers often does. Moreover, it has been demonstrated that electromagnetic waves can be confined within nanoscale metallic structures by using surface plasmon polaritons (SPPs), providing an easy way to achieve tremendous field strength enhancements.
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.
--Taek Yong Hwang
Technical Division: Light–Matter Interactions
ToC Category: Ultrafast Optics
|OCIS Codes:||(190.4180) Nonlinear optics : Multiphoton processes|
|(190.7110) Nonlinear optics : Ultrafast nonlinear optics|
|(320.7120) Ultrafast optics : Ultrafast phenomena|
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