Control of Ultrafast Surface Plasmon Coupled onto a Gold Tapered Tip and its Nonlinear Emission by Shaping Femtosecond Laser Pulses

Yuta Masaki Department of Electronics and Electrical Engineering, Keio University, Yokohama, Japan Kazunori Toma Department of Electronics and Electrical Engineering, Keio University, Yokohama, Japan Miyuki Kusaba Department of Electronics and Electrical Engineering, Keio University, Yokohama, Japan Kenichi Hirosawa Department of Electronics and Electrical Engineering, Keio University, Yokohama, Japan Fumihiko Kannari Department of Electronics and Electrical Engineering, Keio University, Yokohama, Japan

Nanofocusing of surface plasmon polaritons (SPPs) on a conical metal tip has actively been studied to achieve optical excitation in subwavelength scales. Ropers et al. demonstrated efficient localized optical excitation at the apex of a nanostructured metal taper by grating-coupled SPPs excited by femtosecond laser pulses [1]. The temporal characteristics of SPPs coupled on an Au taper have been experimentally analyzed by Berweger et al. using second harmonics (SH) generated at the apex [2].

In this paper, we employed cross-correlation dark-field image microscope to analyze SPP pulses coupled on an Au taper as shown in Fig. 1. This scheme is a powerful tool to measure ultrafast SPP time histories both in the phase and the amplitude without any nonlinear optics. We use an Au taper with a tip edge radius of ~20 nm and an opening angle of 15°. Once the temporal response function representing the SPP coupling, propagation and re-emission is obtained, the temporal plasmon characteristics can be deterministically designed by shaping excitation pulses. For example, a Fourier transform limited plasmon pulse can be generated at the apex. On the other hand, we shaped incident wave front to enhance the efficiency of nanofocusing of SPPs [3]. We optimized incident wave front using optimization algorithm since it’s difficult to define the wave front of a broadband laser pulse for complex waveguide geometries. The intensity of SPPs at the apex was enhanced by 5.9 with this optimization. We controlled localized SH light at the apex by shaping the spectral phase of the femtosecond excitation pulse based on the response function. When third-order dispersions are designed for the SPP pulse, the peak of SH spectrum peak exactly shifted to the inflection point of the third-order dispersion curve. Similar control with spectral phase shaping was applied for coherent anti-stokes Raman scattering at the apex.

Fig. 1 Experimental setup of cross-correlation dark-field image measurement. Insets are excitation laser spectrum and measured plasmon response functions and SEM pictures of Au tapered tip.

Fig. 1 Experimental setup of cross-correlation dark-field image measurement. Insets are excitation laser spectrum and measured plasmon response functions and SEM pictures of Au tapered tip.

[1] C. Ropers, et al. Nano Lett. 7, 2784-2788 (2007).

[2] S. Berweger, et al. Nano Lett. 11, 4309-4313 (2011).

[3] S. Schmidt, et al. Opt. Express 21, 26564-26577 (2013).

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