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Z. Wang and B. Luk’yanchuk
Figure 15.6 shows the calculation results of electric field distribution in the artificial
media illustrated by a plane wave (λ = 550 nm). Clear field confinements are observed
within particles gaps, confirming the ability of the composite media to modulate and
confine incident radiation at the nanoscale, as shown in Fig. 15.6b. Since TiO 2 is lossfree at visible wavelengths, this near-field coupling effect can effectively propagate
through the media over long distances, forming an arrayed “patterned illumination”
landscape on the substrate surface, as shown in Fig. 15.6c. Note these illumination
spots are evanescent, containing high–spatial frequency components. Their sizes
are mainly determined by the particle size, having a full width at half maximum
(FWHM) resolution of ~8 nm, as shown in Fig. 15.6d. Therefore, it is expected that
nanoparticle composite media will have the intriguing ability to transform incident
illumination into a large array of nanoscale evanescent wave illumination focused
on the object surface within the nearfield region. This novel nanophotonic effect has
similarity with that of near-field scanning optical microscopy (NSOM), in which resolution is not limited by the incident wavelength but by the aperture size [106, 107].
However, NSOM suffers from several limitations, such as low optical throughput,
long scanning time, and insufficient contact between aperture probe and object surface. In contrast, an array of TiO 2 nanoparticles can act as parallel array of near-field
probes to simultaneously (thousands) illuminate the sample surface, and the strength
of the focused evanescent wave illumination can be maximized owing to the near
perfect solid immersion of imaging object by TiO 2 nanoparticles. Moreover, theoretically, the size of the evanescent wave illumination spots can be further reduced
by using smaller anatase TiO 2 nanoparticles or higher refractive index rutile (n =
2.70) TiO 2 nanoparticles, and this near-field illumination may also be useful for other
applications such as nanoscale light harvesting and sensing.
According to the reciprocal principle [108], the conversion process in nanoparticle composite media (Fig. 15.7) from propagating waves to evanescent waves can
be optically reversed. In other words, an array of evanescent wave source located
on the bottom surface of the nanoparticles composite media will be converted back
into propagating waves by the media. This is confirmed by two–point source calculation. For comparison purposes, both conventional media (homogeneous anatase
TiO 2 material) and our metamaterial media (stacked 15 nm anatase TiO 2 nanoparticle) were simulated. Using slab geometry, we demonstrate in Fig. 15.7 that evanescent
waves behave differently when interacting with conventional media, Fig. 15.7a, and
composite metamaterial media, Fig. 15.7b, d. As shown in Fig. 15.7c, in homogeneous media the evanescent waves decay exponentially as expected when the distance to sources increases. The loss of evanescent waves causes reduced resolution.
Figure 15.7e demonstrates the inability of conventional media to resolve two-point
sources (separated by 45 nm) in far-field zone (e.g. z = 650 nm > λ = 550 nm). The
two points are only resolvable in near-fields when distance to source is extremely
small, typically smaller than 50 nm in present case. In nanoparticle composite media,
however, the evanescent waves experience strong interaction with TiO 2 nanoparticles, which causes effective conversion of evanescent waves into propagation waves.
The converted waves are mainly guided through the gaps between particles. As shown
in Fig. 15.7d, electric field received at far-field region (z > λ) is about |E| ~ 0.45 (since
Z. Wang and B. Luk’yanchuk
Figure 15.6 shows the calculation results of electric field distribution in the artificial
media illustrated by a plane wave (λ = 550 nm). Clear field confinements are observed
within particles gaps, confirming the ability of the composite media to modulate and
confine incident radiation at the nanoscale, as shown in Fig. 15.6b. Since TiO 2 is lossfree at visible wavelengths, this near-field coupling effect can effectively propagate
through the media over long distances, forming an arrayed “patterned illumination”
landscape on the substrate surface, as shown in Fig. 15.6c. Note these illumination
spots are evanescent, containing high–spatial frequency components. Their sizes
are mainly determined by the particle size, having a full width at half maximum
(FWHM) resolution of ~8 nm, as shown in Fig. 15.6d. Therefore, it is expected that
nanoparticle composite media will have the intriguing ability to transform incident
illumination into a large array of nanoscale evanescent wave illumination focused
on the object surface within the nearfield region. This novel nanophotonic effect has
similarity with that of near-field scanning optical microscopy (NSOM), in which resolution is not limited by the incident wavelength but by the aperture size [106, 107].
However, NSOM suffers from several limitations, such as low optical throughput,
long scanning time, and insufficient contact between aperture probe and object surface. In contrast, an array of TiO 2 nanoparticles can act as parallel array of near-field
probes to simultaneously (thousands) illuminate the sample surface, and the strength
of the focused evanescent wave illumination can be maximized owing to the near
perfect solid immersion of imaging object by TiO 2 nanoparticles. Moreover, theoretically, the size of the evanescent wave illumination spots can be further reduced
by using smaller anatase TiO 2 nanoparticles or higher refractive index rutile (n =
2.70) TiO 2 nanoparticles, and this near-field illumination may also be useful for other
applications such as nanoscale light harvesting and sensing.
According to the reciprocal principle [108], the conversion process in nanoparticle composite media (Fig. 15.7) from propagating waves to evanescent waves can
be optically reversed. In other words, an array of evanescent wave source located
on the bottom surface of the nanoparticles composite media will be converted back
into propagating waves by the media. This is confirmed by two–point source calculation. For comparison purposes, both conventional media (homogeneous anatase
TiO 2 material) and our metamaterial media (stacked 15 nm anatase TiO 2 nanoparticle) were simulated. Using slab geometry, we demonstrate in Fig. 15.7 that evanescent
waves behave differently when interacting with conventional media, Fig. 15.7a, and
composite metamaterial media, Fig. 15.7b, d. As shown in Fig. 15.7c, in homogeneous media the evanescent waves decay exponentially as expected when the distance to sources increases. The loss of evanescent waves causes reduced resolution.
Figure 15.7e demonstrates the inability of conventional media to resolve two-point
sources (separated by 45 nm) in far-field zone (e.g. z = 650 nm > λ = 550 nm). The
two points are only resolvable in near-fields when distance to source is extremely
small, typically smaller than 50 nm in present case. In nanoparticle composite media,
however, the evanescent waves experience strong interaction with TiO 2 nanoparticles, which causes effective conversion of evanescent waves into propagation waves.
The converted waves are mainly guided through the gaps between particles. As shown
in Fig. 15.7d, electric field received at far-field region (z > λ) is about |E| ~ 0.45 (since