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M. B. Raschke et al.
femtosecond pulse at the apex is shown in Fig. 7.19b, with reconstructed amplitude
and phase (c) corresponding to a 16 fs transform limited pulse. Arbitrary waveform
generation and full deterministic control of the re-emitted apex radiation is possible
through feedback on the nonlinear response of the tip. This capability is demonstrated here by applying a 200 fs 2 chirp to one of the pulses at the apex, as shown in
Fig. 7.19 d) and e) with the reconstructed amplitude and phase characteristics from
cross-correlation FROG (XFROG).
The localized plasmon resonance for the tips in these experiments was red-shifted
relative to the laser bandwidth, so that the tips were non-resonant and had an almost
instantaneous response. A tip with a plasmon resonance close to the laser wavelength
would provide higher field enhancement, but with achievable minimum pulse duration now limited by the plasmon dephasing time to a few 10s of fs, rather than the
SPP coupling bandwidth. The adiabatic nanofocusing process is necessarily accompanied by a decrease in the SPP group velocity on approaching the apex of the tip
[69]. This SPP slow-down could provide a further increase in the nonlinear response
of the tip-apex.
The grating-coupled tips demonstrate how the combination of the ultrafast optical properties of metals and intrinsic and geometry-related SPP behavior allow for
spatio-temporal nano-imaging in a scanning probe configuration. Together with the
nonlinear optical response of the tips and associated symmetry selectivity, this opens
the door for deterministic few-femtosecond optical control on the nanoscale.
More generally, the design and optimization of optical antennas for nonlinear
applications requires the ability to accurately characterize field enhancement and
mode distribution properties within an antenna. Both electron-based techniques and
photon-based techniques have been used for antenna characterization. Conventional
far-field optical characterization can provide information about the interaction between an optical antenna and propagating light, such as the relationship between
device geometry and resonant frequency. While this can be applied over a broad
frequency range, it suffers from comparatively low spatial resolution, and the linear
response of an antenna does not necessarily predict the nonlinear response, due to,
for example, coupling between the driven plasmon and surrounding dielectric resonances. SHG, two-photon photoluminescence, or FWM can provide slightly higher
spatial resolution and more accurate determination of nonlinear spectral properties,
but do not provide knowledge of the underlying resonant modes and their associated
spatial field distribution within an antenna. Electronic techniques, such as electron
energy loss spectroscopy (EELS), transmission electron microscopy (TEM), and
cathodoluminescence can facilitate the extraction of spatially detailed information,
with nanometer resolution of modes and plasmonic field enhancement.
In recent years, near-field optical techniques such as scattering-scanning nearfield optical microscopy (s-SNOM) have been utilized for high spatial resolution
mapping of linear and nonlinear antenna properties, offering information about the
local optical electric field magnitude and phase and interactions of modes in coupled nano-optical plasmonic and optical antenna structures. With these techniques
it is possible to spatially and spectrally probe the microscopic electric field distribution, and correlate details of the field with geometrical features. Such measurements
M. B. Raschke et al.
femtosecond pulse at the apex is shown in Fig. 7.19b, with reconstructed amplitude
and phase (c) corresponding to a 16 fs transform limited pulse. Arbitrary waveform
generation and full deterministic control of the re-emitted apex radiation is possible
through feedback on the nonlinear response of the tip. This capability is demonstrated here by applying a 200 fs 2 chirp to one of the pulses at the apex, as shown in
Fig. 7.19 d) and e) with the reconstructed amplitude and phase characteristics from
cross-correlation FROG (XFROG).
The localized plasmon resonance for the tips in these experiments was red-shifted
relative to the laser bandwidth, so that the tips were non-resonant and had an almost
instantaneous response. A tip with a plasmon resonance close to the laser wavelength
would provide higher field enhancement, but with achievable minimum pulse duration now limited by the plasmon dephasing time to a few 10s of fs, rather than the
SPP coupling bandwidth. The adiabatic nanofocusing process is necessarily accompanied by a decrease in the SPP group velocity on approaching the apex of the tip
[69]. This SPP slow-down could provide a further increase in the nonlinear response
of the tip-apex.
The grating-coupled tips demonstrate how the combination of the ultrafast optical properties of metals and intrinsic and geometry-related SPP behavior allow for
spatio-temporal nano-imaging in a scanning probe configuration. Together with the
nonlinear optical response of the tips and associated symmetry selectivity, this opens
the door for deterministic few-femtosecond optical control on the nanoscale.
More generally, the design and optimization of optical antennas for nonlinear
applications requires the ability to accurately characterize field enhancement and
mode distribution properties within an antenna. Both electron-based techniques and
photon-based techniques have been used for antenna characterization. Conventional
far-field optical characterization can provide information about the interaction between an optical antenna and propagating light, such as the relationship between
device geometry and resonant frequency. While this can be applied over a broad
frequency range, it suffers from comparatively low spatial resolution, and the linear
response of an antenna does not necessarily predict the nonlinear response, due to,
for example, coupling between the driven plasmon and surrounding dielectric resonances. SHG, two-photon photoluminescence, or FWM can provide slightly higher
spatial resolution and more accurate determination of nonlinear spectral properties,
but do not provide knowledge of the underlying resonant modes and their associated
spatial field distribution within an antenna. Electronic techniques, such as electron
energy loss spectroscopy (EELS), transmission electron microscopy (TEM), and
cathodoluminescence can facilitate the extraction of spatially detailed information,
with nanometer resolution of modes and plasmonic field enhancement.
In recent years, near-field optical techniques such as scattering-scanning nearfield optical microscopy (s-SNOM) have been utilized for high spatial resolution
mapping of linear and nonlinear antenna properties, offering information about the
local optical electric field magnitude and phase and interactions of modes in coupled nano-optical plasmonic and optical antenna structures. With these techniques
it is possible to spatially and spectrally probe the microscopic electric field distribution, and correlate details of the field with geometrical features. Such measurements