52
M. I. Stockman
nanoscopic field distribution. We have already discussed such an approach above—
see Fig. 1.10 [123] and the corresponding discussion in Sect. 1.3.6.
The nanostructure used consists of circular Ag disks with 180 nm diameter and
30 nm height, fabricated by electron-beam lithography on a conductive, 40-nm-thick
indium-tin oxide (ITO) film grown on a quartz substrate. The disks are arranged into
three dimers that form the arms of a star-like shape (Fig. 1.22a, lower right). The
whole nanostructure is about 800 nm across, while the gap between two of the dimer
disks is ∼10 nm wide. After inspection by scanning-electron microscopy (SEM),
the sample is mounted in the ultrahigh-vacuum PEEM set-up. The deposition of
a small amount of caesium (∼0.1 monolayers) reduces the work function of the
Ag nanostructure to about 3.1 eV, that is, just below the threshold for two-photon
photoemission with 790 nm photons.
The PEEM pattern obtained after maximization of the photoemission from the
upper two arms of the Ag nanostructure in shown in Fig. 1.22c. It shows strong
emission from these two upper arms and almost no emission from the bottom arm.
Analogously, the photoemission after minimization of the upper part PEEM brightness (Fig. 1.22e) occurs mainly in the lower area while the contribution from the
upper two arms is extremely weak. The adaptively determined solution to each optimization problem has been proven to be robust with respect to slight imperfections
in the experimental nanostructures. These successful optimizations demonstrate that
polarization pulse shaping allows adaptive control of the spatial distribution of photoelectrons on a subwavelength scale, and thus of the nanoscopic optical fields that
induce photoemission.
The optimally polarization-shaped laser pulses after adaptive maximization and
minimization described above are shown in Figs. 1.22b, d, respectively, as determined by dual-channel spectral interferometry [239, 240]. In this representation, the
shape of the quasi-three-dimensional figure indicates the temporal evolution of the
polarization state of the electric field, with the color representing the instantaneous
oscillation frequency. Contributions from both transverse polarization components
are visible in each of the two cases. Whereas the upper-region photoemission maximization is achieved with a comparatively simple time evolution, the corresponding
minimization requires a more complex field with varying degrees of ellipticity, orientation and temporal amplitudes.
Our idea [210] of the coherent control on the nanoscale by spatial modulation
(shaping) of the excitation waveform has been developed theoretically [237] and
experimentally [211, 212]. The coherent control of nanoscale distribution of local
optical fields based on CW excitation aimed at achieving a deterministic control of
plasmonic fields by using the spatial shaping of high order beams such as HermiteGaussian (HG) and Laguerre-Gaussian (LG) beams has been carried out in Ref. [211].
It has been shown experimentally that the spatial phase shaping of the excitation
field provides an additional degree of freedom to drive optical nanoantennas and
consequently control their near field response.
An example of such a deterministic coherent control is illustrated in Fig. 1.23. It
shows a double gap antenna formed by three 500 nm aligned gold bars forming two
identical 50 nm air gaps separated by 500 nm. For reference, in panel (a) it displays
M. I. Stockman
nanoscopic field distribution. We have already discussed such an approach above—
see Fig. 1.10 [123] and the corresponding discussion in Sect. 1.3.6.
The nanostructure used consists of circular Ag disks with 180 nm diameter and
30 nm height, fabricated by electron-beam lithography on a conductive, 40-nm-thick
indium-tin oxide (ITO) film grown on a quartz substrate. The disks are arranged into
three dimers that form the arms of a star-like shape (Fig. 1.22a, lower right). The
whole nanostructure is about 800 nm across, while the gap between two of the dimer
disks is ∼10 nm wide. After inspection by scanning-electron microscopy (SEM),
the sample is mounted in the ultrahigh-vacuum PEEM set-up. The deposition of
a small amount of caesium (∼0.1 monolayers) reduces the work function of the
Ag nanostructure to about 3.1 eV, that is, just below the threshold for two-photon
photoemission with 790 nm photons.
The PEEM pattern obtained after maximization of the photoemission from the
upper two arms of the Ag nanostructure in shown in Fig. 1.22c. It shows strong
emission from these two upper arms and almost no emission from the bottom arm.
Analogously, the photoemission after minimization of the upper part PEEM brightness (Fig. 1.22e) occurs mainly in the lower area while the contribution from the
upper two arms is extremely weak. The adaptively determined solution to each optimization problem has been proven to be robust with respect to slight imperfections
in the experimental nanostructures. These successful optimizations demonstrate that
polarization pulse shaping allows adaptive control of the spatial distribution of photoelectrons on a subwavelength scale, and thus of the nanoscopic optical fields that
induce photoemission.
The optimally polarization-shaped laser pulses after adaptive maximization and
minimization described above are shown in Figs. 1.22b, d, respectively, as determined by dual-channel spectral interferometry [239, 240]. In this representation, the
shape of the quasi-three-dimensional figure indicates the temporal evolution of the
polarization state of the electric field, with the color representing the instantaneous
oscillation frequency. Contributions from both transverse polarization components
are visible in each of the two cases. Whereas the upper-region photoemission maximization is achieved with a comparatively simple time evolution, the corresponding
minimization requires a more complex field with varying degrees of ellipticity, orientation and temporal amplitudes.
Our idea [210] of the coherent control on the nanoscale by spatial modulation
(shaping) of the excitation waveform has been developed theoretically [237] and
experimentally [211, 212]. The coherent control of nanoscale distribution of local
optical fields based on CW excitation aimed at achieving a deterministic control of
plasmonic fields by using the spatial shaping of high order beams such as HermiteGaussian (HG) and Laguerre-Gaussian (LG) beams has been carried out in Ref. [211].
It has been shown experimentally that the spatial phase shaping of the excitation
field provides an additional degree of freedom to drive optical nanoantennas and
consequently control their near field response.
An example of such a deterministic coherent control is illustrated in Fig. 1.23. It
shows a double gap antenna formed by three 500 nm aligned gold bars forming two
identical 50 nm air gaps separated by 500 nm. For reference, in panel (a) it displays