Femtosecond optical control on the nanoscale

. We demonstrate a generalized route to generate nanometer spatially confined ultrafast optical pulses with arbitrary deterministic femtosecond waveform control using surface plasmon polarition nanofocusing in 3D tapered noble metal tips. nanometer-resolved ultrafast and nonlinear near-field imaging.


Introduction
Plasmonic nanostructures as optical antennas have been proposed to overcome the size and mode mismatch between far-field propagating light and nanoscale quantum systems. Previous approaches to achieve nanometer control over ultrafast fields have relied on the use of appropriately shaped pulses to achieve spatial localization [1]. However, as the nanofocusing mechanism itself relies on the spectral phase and polarization characteristics, this fundamentally limits the control of the optical transient that can be obtained on the nanoscale.
Here we present an optical antenna concept capable not only of efficient nanoscale light concentration, but that also enables the simultaneous and independent control of the ultrafast optical transients via pulse shaping. This approach utilizes surface plasmon polarition (SPP) nanofocusing into the apex of 3D noble metal tips [2,3], which we have previously demonstrated to allow for nanometer spatial confinement of optical fields at the apex [4]. As shown in Figure 1a), this effect arises from the increase in index of refraction with decreasing tip radius experienced by propagating SPP's, which in combination with the associated decreasing group velocity and improved mode confinement leads to the continuous transformation of quasi-planar modes into a nanoconfined excitation at the tip apex [5,6].
We demonstrate the compatibility of SPP nanofocusing with ultrafast techniques enabled by the inherent broadband capability of the process [7]. As illustrated in Figure 1b), in combination with frequency domain pulse shaping we excert deterministic control over femtosecond pulses at the tip apex by generating transform-limited pulses as well as arbitrary waveforms. The resulting nanometer-femtosecond spatiotemporal control at the end of a scanning probe tip can be used to perform all-optical nanometer-resolved ultrafast and nonlinear near-field imaging.

Methods
SPP's are launched onto Au tips via grating coupling as illustrated in Figure 1b). The grating is fabricated via focused ion beam milling using a fan-shaped design to optimize broadband performance. The output of a Ti:Sa oscillator is shaped using a pulse shaper with a folded 4f geometry and focused onto the grating. The launched SPP pulses are nanofocused into the tip apex where in addition to the fundamental emission, the broken apex symmetry allows for secondharmonic generation (SHG) that is detected using an imaging spectrograph coupled to N 2 (l) -cooled   CCD camera in a 90°-detection geometry. Access to the spectral phase provided by SHG is used for full tip-apex pulse characterization through interferometry frequency-resolved optical gating (IFROG) and pulse duration optimization using a multiphoton intrapulse interferometric phase scan (MIIPS) algorithm. Due to sensitive grating-coupling conditions pulse-pairs are generated using the pulse shaper to ensure collinearity and thus identical coupling.

Results
As previously shown, SPP nanofocusing can generate a nanoconfined excitation with 10's of nm spatial confinement where nanofocusing efficiencies as high as 9% enable efficient background-free imaging and spectroscopy [8,9]. Under sufficiently high illumination conditions SHG is generated locally at the tip apex that is used through a MIIPS algorithm to produce transform-limited pulses.

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Measurement of pulses is performed via tip-apex IFROG generated using nanofocused pulses after MIIPS optimization with the reconstructed temporal and spectral profile in Figure 2a) and b), respectively. The spectral profile with a flat spectral phase and spectral FWHM = 60 nm corresponds to a transform-limited pulse with a temporal duration as short as 16 fs [7].
The spectral characteristics of an arbitrarily engineered pulse are shown in Figure 2c) with the spectrum (red) and phase (blue) reconstructed from an XFROG measurement, showing good agreement between the applied phase of 200 fs 2 (black dashed) and the reconstructed phase. The temporal characteristics in panel d) show the correspondingly chirped pulse with a broadened profile and a varying instantaneous frequency as seen from the electric field transient (green) [7]. The high degree of waveform engineering achievable arises from the inherently broadband nanofocusing process with no dependence on the spectral phase and only a weak wavelength dependence.
These results show the compatibility of this antenna concept with ultrafast techniques, allowing not only for the optimization of pulse duration, but also for the arbitrary control and generation of optical waveform transients as illustrated in Fig. 2e). The resulting qualitatively new control of the light-matter interaction on simultaneous nanometer -femtosecond scales control will open up applications ranging from quantum coherent control to high harmonic generation. Furthermore with spatial localization occurring at the apex of a scanning probe tip, this spatiotemporal control will open up new avenues to study the nature of the fundamental electronic and vibrational excitations of matter on their characteristic length and time scales simultaneously.