444
S. V. Boriskina
particle creates localized temperature gradient in the substrate material, and can be
used for efficient nanohole processing (Fig. 12.8b) [80].
12.5 Molding the River of Light in Vortex Nanogear
Transmissions
As shown in the previous sections, nanostructure-induced interference of the
evanescent-field components of the electromagnetic field may generate complex
spatial distributions of the time-averaged Poynting vector in the near-field region,
including creation of nanoscale vortices of optical powerflow. This mechanism of
light trapping offers a new way to efficiently focus and store light within nanoscale
volumes. However, it calls for more work to formulate rational strategies for vortexpinning nanostructures engineering and to predict new physical effects. It has long
been realized that variations of amplitude and phase of interacting propagating waves
can be used to create and control free-space vortex fields. Experimental techniques
for the free-space vortices generation include the use of “forked” holograms, lenses
and spiral phase plates as well as their recently proposed plasmonic analogs [82–84].
Complex vortex topologies such as loops and knots can be created by superposing
waves with correctly-weighted amplitudes and phases [85, 86]. A similar theoretical framework for generation and manipulation of the powerflow vortices via the
interference of evanescent waves in plasmonic nanostructures is still under development [49, 67, 87], and below I will review several successful realizations of the
vortex-pinning nanostructures.
The first example to be discussed is a celebrated silver nanolens design for efficient light focusing in the gap between the particles arranged into a showman-like
structure (shown in Fig. 12.9a). This configuration was theoretically introduced over
a decade ago by Li, Stockman and Bergman [89], and has generated over 500
follow-up papers since publication. However, the origin of the observed phenomenon has not been understood until recently [67], and can be revealed through the
concept of coupled-vortex formation. To illustrate the underlying physical effects
of light focusing in the nanolens, in Fig. 12.9b the on-resonance optical powerflow pattern is overlapped with the electric field intensity distribution. The structure is illuminated by a linearly-polarized plane wave (propagating from bottom
up in Fig. 12.9b). Figure 12.9b reveals a complex picture of powerflow through the
nanolens focal point, with the reversal of flow direction at the near-field intensity
resonance, which is caused by the formation of coupled counter-rotating vortices
inside the nanoparticles. The powerflow pattern in Fig. 12.9b resembles a multiplegear transmission (schematically illustrated in Fig. 12.9c), which is composed of
coupled vortex nanogears made of light and arranged into a linear sequence. To
reflect this analogy, the nanostructures engineered to couple counter-rotating optical
vortices into transmission-like sequences have been termed Vortex Nanogear Transmissions (VNTs) [67, 87]. As seen in Fig. 12.9b, light circulation through VNT drives
S. V. Boriskina
particle creates localized temperature gradient in the substrate material, and can be
used for efficient nanohole processing (Fig. 12.8b) [80].
12.5 Molding the River of Light in Vortex Nanogear
Transmissions
As shown in the previous sections, nanostructure-induced interference of the
evanescent-field components of the electromagnetic field may generate complex
spatial distributions of the time-averaged Poynting vector in the near-field region,
including creation of nanoscale vortices of optical powerflow. This mechanism of
light trapping offers a new way to efficiently focus and store light within nanoscale
volumes. However, it calls for more work to formulate rational strategies for vortexpinning nanostructures engineering and to predict new physical effects. It has long
been realized that variations of amplitude and phase of interacting propagating waves
can be used to create and control free-space vortex fields. Experimental techniques
for the free-space vortices generation include the use of “forked” holograms, lenses
and spiral phase plates as well as their recently proposed plasmonic analogs [82–84].
Complex vortex topologies such as loops and knots can be created by superposing
waves with correctly-weighted amplitudes and phases [85, 86]. A similar theoretical framework for generation and manipulation of the powerflow vortices via the
interference of evanescent waves in plasmonic nanostructures is still under development [49, 67, 87], and below I will review several successful realizations of the
vortex-pinning nanostructures.
The first example to be discussed is a celebrated silver nanolens design for efficient light focusing in the gap between the particles arranged into a showman-like
structure (shown in Fig. 12.9a). This configuration was theoretically introduced over
a decade ago by Li, Stockman and Bergman [89], and has generated over 500
follow-up papers since publication. However, the origin of the observed phenomenon has not been understood until recently [67], and can be revealed through the
concept of coupled-vortex formation. To illustrate the underlying physical effects
of light focusing in the nanolens, in Fig. 12.9b the on-resonance optical powerflow pattern is overlapped with the electric field intensity distribution. The structure is illuminated by a linearly-polarized plane wave (propagating from bottom
up in Fig. 12.9b). Figure 12.9b reveals a complex picture of powerflow through the
nanolens focal point, with the reversal of flow direction at the near-field intensity
resonance, which is caused by the formation of coupled counter-rotating vortices
inside the nanoparticles. The powerflow pattern in Fig. 12.9b resembles a multiplegear transmission (schematically illustrated in Fig. 12.9c), which is composed of
coupled vortex nanogears made of light and arranged into a linear sequence. To
reflect this analogy, the nanostructures engineered to couple counter-rotating optical
vortices into transmission-like sequences have been termed Vortex Nanogear Transmissions (VNTs) [67, 87]. As seen in Fig. 12.9b, light circulation through VNT drives