1 Nanoplasmonics: From Present into Future
47
propagation toward the nanofocus and the concurrent adiabatic concentration [12,
230, 231].
The coupling of the external radiation to SPPs and their nanofocusing have been
observed—see, e.g., Refs. [232, 233]. The second component of our approach, the
spatio-temporal coherent control of such nanofocusing has been developed [228,
229]. The third component, the adiabatic concentration of SPPs also has been
observed and extensively studied experimentally [13–16, 18, 19, 22].
The adiabatic concentration (nanofocusing) is based on adiabatic following by a
propagating SPP wave of a graded plasmonic waveguide, where the phase and group
velocities decrease while the propagating SPP wave is adiabatically transformed
into a standing, localized SP mode. A new quality that is present in this approach
is a possibility to arbitrary move the nanofocus along the nanoedge of the wedge.
Moreover, it is possible to superimpose any number of such nanofoci simultaneously
and, consequently, create any distribution of the nanolocalized fields at the thin edge
of the wedge.
To illustrate this idea of the full spatiotemporal coherent control, now let us turn to
a wedge that contains a line of nanosize scatterers (say, nanoparticles or nanoholes)
located at the thick edge and parallel to it, i.e. in the x direction in Fig. 1.20. Consider
first monochromatic light incident on these nanoparticles or nanoholes that scatter
and couple it into SPP wavelets. Every such a scatterer emits SPPs in all directions;
there is, of course, no favored directionality of the scattering.
At this point, we assume that the excitation radiation and, correspondingly, the
scattered wavelets of the SPP are coherent, and their phases vary in space along
the thick edge, i.e., in the x direction. Then the SPP wavelets emitted by different
scatterers will interfere, which in accord with the Huygens-Fresnel principle leads
to formation of a smooth wavefront of the SPP wave at some distance from the scatterers in the “far SPP field”, i.e., at distances much greater than the SPP wavelength
2π/k S P P .
Such wavefronts are shown in Fig. 1.20 with concave black curves. The energy of
the SPP is transferred along the rays, which are the lines normal to the wavefronts,
shown by the colored lines. By the appropriate spatial phase modulation of the
excitation radiation along the line of scatterers (in the x direction) over distances of
many SPP wavelengths, these wavefronts can be formed in such a way that the rays
intersect at a given point, forming a nanofocus at the thin (sharp) edge of the wedge,
as shown schematically in Fig. 1.20. Diffraction of the SPP waves will lead to a finite
size of this focal spot.
By changing the spatial phase profile of the excitation radiation, this focal spot can
be arbitrarily moved along the thin edge. This focusing and adiabatic concentration,
as the SPPs slow down approaching the sharp edge, will lead to the enhancement
of the intensity of the optical fields in the focal region. This dynamically-controlled
concentration of energy is a plasmonic counterpart of a large phased antenna array
(also known as an aperture synthesis antenna), widely used in radar technology
(synthetic aperture radar or SAR) and radio astronomy [234].
Now we can consider excitation by spatiotemporally shaped ultrashort pulses
independently in space. Such pulses are produced by spatio-temporal modulators
47
propagation toward the nanofocus and the concurrent adiabatic concentration [12,
230, 231].
The coupling of the external radiation to SPPs and their nanofocusing have been
observed—see, e.g., Refs. [232, 233]. The second component of our approach, the
spatio-temporal coherent control of such nanofocusing has been developed [228,
229]. The third component, the adiabatic concentration of SPPs also has been
observed and extensively studied experimentally [13–16, 18, 19, 22].
The adiabatic concentration (nanofocusing) is based on adiabatic following by a
propagating SPP wave of a graded plasmonic waveguide, where the phase and group
velocities decrease while the propagating SPP wave is adiabatically transformed
into a standing, localized SP mode. A new quality that is present in this approach
is a possibility to arbitrary move the nanofocus along the nanoedge of the wedge.
Moreover, it is possible to superimpose any number of such nanofoci simultaneously
and, consequently, create any distribution of the nanolocalized fields at the thin edge
of the wedge.
To illustrate this idea of the full spatiotemporal coherent control, now let us turn to
a wedge that contains a line of nanosize scatterers (say, nanoparticles or nanoholes)
located at the thick edge and parallel to it, i.e. in the x direction in Fig. 1.20. Consider
first monochromatic light incident on these nanoparticles or nanoholes that scatter
and couple it into SPP wavelets. Every such a scatterer emits SPPs in all directions;
there is, of course, no favored directionality of the scattering.
At this point, we assume that the excitation radiation and, correspondingly, the
scattered wavelets of the SPP are coherent, and their phases vary in space along
the thick edge, i.e., in the x direction. Then the SPP wavelets emitted by different
scatterers will interfere, which in accord with the Huygens-Fresnel principle leads
to formation of a smooth wavefront of the SPP wave at some distance from the scatterers in the “far SPP field”, i.e., at distances much greater than the SPP wavelength
2π/k S P P .
Such wavefronts are shown in Fig. 1.20 with concave black curves. The energy of
the SPP is transferred along the rays, which are the lines normal to the wavefronts,
shown by the colored lines. By the appropriate spatial phase modulation of the
excitation radiation along the line of scatterers (in the x direction) over distances of
many SPP wavelengths, these wavefronts can be formed in such a way that the rays
intersect at a given point, forming a nanofocus at the thin (sharp) edge of the wedge,
as shown schematically in Fig. 1.20. Diffraction of the SPP waves will lead to a finite
size of this focal spot.
By changing the spatial phase profile of the excitation radiation, this focal spot can
be arbitrarily moved along the thin edge. This focusing and adiabatic concentration,
as the SPPs slow down approaching the sharp edge, will lead to the enhancement
of the intensity of the optical fields in the focal region. This dynamically-controlled
concentration of energy is a plasmonic counterpart of a large phased antenna array
(also known as an aperture synthesis antenna), widely used in radar technology
(synthetic aperture radar or SAR) and radio astronomy [234].
Now we can consider excitation by spatiotemporally shaped ultrashort pulses
independently in space. Such pulses are produced by spatio-temporal modulators