50
M. I. Stockman
spatial point of the excitation. This pre-modulates the departing SPP wave packets
in such a way that they reach the required point at the sharp edge of the nanowedge
in phase, with equal amplitudes forming a nanofocus where an ultrashort pulse with
required temporal shape is generated. This system constitutes a “nanoplasmonic portal” connecting the incident light field, whose features are shaped on the microscale,
with the required point or features at the nanoscale.
1.4.6 Experimental Demonstrations of Coherent Control
on the Nanoscale
The ideas of the coherent control of the nanoscale distribution of ultrafast optical
fields both space and in time, which have been introduced theoretically in Refs. [148,
195, 210, 214, 218, 236, 237], have been investigated and confirmed experimentally.
Using the full phase and amplitude modulation of the excitation-pulse wavefront in
both polarizations (the so-called polarization pulse shaping), the experiments have
achieved both spatial control [123, 215] and spatiotemporal control [217] on nm–fs
scale.
Recently spatiotemporal nanofocusing via the adiabatic concentration along the
lines of ideas presented above in Sect. 1.4.5 has been successfully demonstrated
experimentally [21]. In this work, a shaped femtosecond pulse has been coupled by
a grating to a TM 0 SPP mode on the surface of an adiabatically-tapered nanocone.
The spatiotemporal concentration of optical energy in space to a ∼10 nm region and
in time to a 15 fs duration (Fourier-transform limited, i.e., the shortest possible at a
given bandwidth). Indeed the position of the nanofocus in Ref. [21] is always the tip
of the nanocone; so the possibility of moving the nanofocus in space is not available.
The ideas of employing the spatial modulation of the excitation wavefront [210]
described above in Sect. 1.4.5 have been experimentally tested and confirmed for
continuous wave (CW) excitation [211, 212]. We will present some of these experimental results below in this section.
We start with experiments on polarization-shaping coherent control that we adapt
from Ref. [215]. The corresponding experimental approach is schematically illustrated in Fig. 1.22. Polarization-shaped ultrashort laser pulses illuminate a planar
nanostructure, with two-photon photoemission electron microscopy (PEEM) [238]
providing the feedback signal from the nanoscale field distribution that is essential
for adaptive near-field control.
The spatial resolution of two-photon PEEM (∼50 nm) is determined by its electron
optics and is, thus, independent of the electromagnetic light-field diffraction limit.
The sensitivity of the two-photon PEEM patterns to the optical field intensities arises
from the nonlinear two-photon photoemission process whose intensity is proportional
to the time-integrated fourth power of the local electric-field amplitude. With these
elements in place, a user-specified nanoscopic optical field distribution is realized by
processing recorded photoemission patterns in an evolutionary algorithm that directs
the iterative optimization of the irradiating laser pulse shape.
M. I. Stockman
spatial point of the excitation. This pre-modulates the departing SPP wave packets
in such a way that they reach the required point at the sharp edge of the nanowedge
in phase, with equal amplitudes forming a nanofocus where an ultrashort pulse with
required temporal shape is generated. This system constitutes a “nanoplasmonic portal” connecting the incident light field, whose features are shaped on the microscale,
with the required point or features at the nanoscale.
1.4.6 Experimental Demonstrations of Coherent Control
on the Nanoscale
The ideas of the coherent control of the nanoscale distribution of ultrafast optical
fields both space and in time, which have been introduced theoretically in Refs. [148,
195, 210, 214, 218, 236, 237], have been investigated and confirmed experimentally.
Using the full phase and amplitude modulation of the excitation-pulse wavefront in
both polarizations (the so-called polarization pulse shaping), the experiments have
achieved both spatial control [123, 215] and spatiotemporal control [217] on nm–fs
scale.
Recently spatiotemporal nanofocusing via the adiabatic concentration along the
lines of ideas presented above in Sect. 1.4.5 has been successfully demonstrated
experimentally [21]. In this work, a shaped femtosecond pulse has been coupled by
a grating to a TM 0 SPP mode on the surface of an adiabatically-tapered nanocone.
The spatiotemporal concentration of optical energy in space to a ∼10 nm region and
in time to a 15 fs duration (Fourier-transform limited, i.e., the shortest possible at a
given bandwidth). Indeed the position of the nanofocus in Ref. [21] is always the tip
of the nanocone; so the possibility of moving the nanofocus in space is not available.
The ideas of employing the spatial modulation of the excitation wavefront [210]
described above in Sect. 1.4.5 have been experimentally tested and confirmed for
continuous wave (CW) excitation [211, 212]. We will present some of these experimental results below in this section.
We start with experiments on polarization-shaping coherent control that we adapt
from Ref. [215]. The corresponding experimental approach is schematically illustrated in Fig. 1.22. Polarization-shaped ultrashort laser pulses illuminate a planar
nanostructure, with two-photon photoemission electron microscopy (PEEM) [238]
providing the feedback signal from the nanoscale field distribution that is essential
for adaptive near-field control.
The spatial resolution of two-photon PEEM (∼50 nm) is determined by its electron
optics and is, thus, independent of the electromagnetic light-field diffraction limit.
The sensitivity of the two-photon PEEM patterns to the optical field intensities arises
from the nonlinear two-photon photoemission process whose intensity is proportional
to the time-integrated fourth power of the local electric-field amplitude. With these
elements in place, a user-specified nanoscopic optical field distribution is realized by
processing recorded photoemission patterns in an evolutionary algorithm that directs
the iterative optimization of the irradiating laser pulse shape.