Biomedical Engineering Reference
In-Depth Information
the second realizes the so-called adiabatic compression [4, 36]
representing the SPPs minimal loss propagation solution. In Ref.
[37], an effective way for the realization of a TM
0
mode by means
of no-radial sources was also provided. Devices providing adiabatic
compression are demonstrated by the number of publications in
fields such as SNOM [38] or tip-enhanced Raman spectroscopy [39,
40], and nonlinear spectroscopy applications [5] in which conical
structures play a fundamental role. Figure 15.2f shows the full
device simulation.
15.2.3.1 Optical singularity in adiabatic compression regime
Adiabatic compression allows concentrating energy and inducing
high intensity electric and magnetic fields in narrow regions.
Metallic conical nanostructures have been investigated and have
been proved supporting this regime [3, 4, 41]. The localized near
field obtained at the apex of such structures is extremely useful
for sensing applications: devices exploiting adiabatic compression
werefabricatedandtestedgivingreliableresults[39,40].Therefore,
it is important to study the electromagnetic field behavior in such
extreme conditions. In fact, the total electric field above the tip end
is, in general, given by different contributes: the incident light, the
scattered light by the whole structure, and the near field generated
by SPPs” decay at the apex. In such conditions, the local fields may
undergo optical singularities [42, 43]. In particular for a conical
metallic nanostructure, it was found that destructive interference
between incident and scattered field leads to a phase singularity,
see Fig. 15.2a.4. The figure shows how a tilted incident wave (at 37
◦
in this case) can induce adiabatic compression while a wave at 0
◦
cannot. The tilting is measured as the angle between the cone's axis
and the direction of propagation of the incident light starting from
the base of the cone. In order to show the different behavior in the
two cases, a parameter was introduced:
(
|
E
(37
◦
)
| − |
E
(0
◦
)
|
)
(
|
E
(37
◦
)
| + |
E
(0
◦
)
|
)
=
U
(15.2)
where
E
is the electric field calculated at the edge of the cone, from
the base to the apex for the two incident angles. From Fig. 15.3a,
