Ellipsometry Techniques and Its Advanced Applications …
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behind the predicted blue shift in Gans theory. However, the present case exhibits a
change in the shape of particles across the ripple only, albeit, interparticle LSPR field
coupling is more prominent compared to their shape effect. In addition, increasing
width of particles with their aspect ratio reduces the separation between particles
perpendicular to the ripples. Hence, interparticle field coupling dominates over the
size of particles and as a result, transverse LSPR mode also exhibits a red shift. It
is a key point that the red shift is dominated in longitudinal (~150 nm) LSPR mode
compared to LSPR in transverse direction (~40 nm) by increasing aspect ratio (from
2 to 5).
The present results are in accordance with literature that a stronger field coupling is
exhibited parallel to the long axis of particles which also depends on their interparticle
gap [41]. Only due to this reason, a large difference in energy shift is observed in
experiments in comparison to the same from Gans theory.
Finally, it is also observed that with increasing aspect ratio, the LSPR energy shift
also increases in the direction from p- to s-polarization (Fig. 12c). However, particles are mostly elongated along the ripple and their width remains almost constant
owing to the self-organized and preferential growth of nanoparticles along the ripple
patterns. Thus, there is a more prominent red shift for p-polarization compared to
s-polarization with elongation of particles (i.e. increasing aspect ratio) along the
ripples. Therefore, this tunable red shift leads to a significant difference in the experimental findings and the results obtained from Gans theory and it is inferred that the
self-organization of particles during their growth on ripple patterns results in LSPR
coupling along and across the ripples.
In another study, as-prepared and post-growth annealed gold nanoparticle arrays
on ripple patterns grown at 65° (with respect to specimen normal) show well-defined
LSPR (see Fig. 13) while measuring the reflection along and across the ripples.
However, it is also observed that LSPR peak corresponding to E || (708 nm) is red
shifted compared to E ⊥ (578 nm) which is owing to the lesser separation between
nanoparticles parallel to the ripples in comparison to perpendicular to ripples. This
observation confirms the presence of in-plane plasmonic anisotropy. Moreover, it
is also noted that the LSPR peak becomes sharper (i.e. lesser peak broadening)
after annealing of nanoparticles which is attributed to the Ostwald ripening-induced
narrowing of the size distribution of nanoparticles. Thus, optical anisotropy arises due
to tunable interparticle gap between Au-nanoparticles along and across the ripples.
3.3 Ellipsometric Investigations on Isotropic and Anisotropic
Specimen
As it is discussed earlier, anisotropic specimen have different physical properties in
different directions. In fact, non-diagonal components of Jones matrix (r ps and r sp )
are nearly zero for an isotropic specimen. However, for an anisotropic specimen,
incident p-polarized light is converted into s-polarized light upon reflection and vice
563
behind the predicted blue shift in Gans theory. However, the present case exhibits a
change in the shape of particles across the ripple only, albeit, interparticle LSPR field
coupling is more prominent compared to their shape effect. In addition, increasing
width of particles with their aspect ratio reduces the separation between particles
perpendicular to the ripples. Hence, interparticle field coupling dominates over the
size of particles and as a result, transverse LSPR mode also exhibits a red shift. It
is a key point that the red shift is dominated in longitudinal (~150 nm) LSPR mode
compared to LSPR in transverse direction (~40 nm) by increasing aspect ratio (from
2 to 5).
The present results are in accordance with literature that a stronger field coupling is
exhibited parallel to the long axis of particles which also depends on their interparticle
gap [41]. Only due to this reason, a large difference in energy shift is observed in
experiments in comparison to the same from Gans theory.
Finally, it is also observed that with increasing aspect ratio, the LSPR energy shift
also increases in the direction from p- to s-polarization (Fig. 12c). However, particles are mostly elongated along the ripple and their width remains almost constant
owing to the self-organized and preferential growth of nanoparticles along the ripple
patterns. Thus, there is a more prominent red shift for p-polarization compared to
s-polarization with elongation of particles (i.e. increasing aspect ratio) along the
ripples. Therefore, this tunable red shift leads to a significant difference in the experimental findings and the results obtained from Gans theory and it is inferred that the
self-organization of particles during their growth on ripple patterns results in LSPR
coupling along and across the ripples.
In another study, as-prepared and post-growth annealed gold nanoparticle arrays
on ripple patterns grown at 65° (with respect to specimen normal) show well-defined
LSPR (see Fig. 13) while measuring the reflection along and across the ripples.
However, it is also observed that LSPR peak corresponding to E || (708 nm) is red
shifted compared to E ⊥ (578 nm) which is owing to the lesser separation between
nanoparticles parallel to the ripples in comparison to perpendicular to ripples. This
observation confirms the presence of in-plane plasmonic anisotropy. Moreover, it
is also noted that the LSPR peak becomes sharper (i.e. lesser peak broadening)
after annealing of nanoparticles which is attributed to the Ostwald ripening-induced
narrowing of the size distribution of nanoparticles. Thus, optical anisotropy arises due
to tunable interparticle gap between Au-nanoparticles along and across the ripples.
3.3 Ellipsometric Investigations on Isotropic and Anisotropic
Specimen
As it is discussed earlier, anisotropic specimen have different physical properties in
different directions. In fact, non-diagonal components of Jones matrix (r ps and r sp )
are nearly zero for an isotropic specimen. However, for an anisotropic specimen,
incident p-polarized light is converted into s-polarized light upon reflection and vice