Biomedical Engineering Reference
In-Depth Information
Figure 15.4
Scanning electron micrographs of the device illustrating the
fabrication steps. The grating, used for nonlocal excitation of the tip apex,
and the sharp cone were prepared by focused ion beam milling and
induceddepositiontechniques,leadandfollowedbygoldsputteringcoating
procedure. Unit bars indicate the 10 m length. Zoomed-in image from the
apex of the coated cone demonstrate a 25 nm tip radius.
15.2.5
Characterization
To characterize the plasmonic properties of the device, we adopted
an optical methodology. In particular, we analyzed the polarization
state of the outcoming far field light from the apex of the cone with
the optical setup sketched in Fig. 15.5a. It makes use of a linearly
polarized laser source (670 nm—the one that match the grating
for the SPP coupling) a half wave retarder to fix the polarization
state of the incoming light in respect to the axis of the device. Two
microscopyobjectiveswereusedforfocalizethelightonthegrating
and to collect it form the tip focal plane, a linear polarizer was
used to analyze the radiated light. We identified the fingerprint of
radial mode in the two lobes spot of Fig. 15.4e,f. Quite similar to
the spherical emitting source case, these images show symmetric
patterns in respect to the polarization analyzer axis, although the
polarizer can be rotate arbitrarily around its axis. The far field
optical propagation properties are inherited from the polarization
stateoftheSPPmodeinthetaperedstructurethatpropagatetoward
the dielectric bulk. This observation lets identify the propagated
SPP with a TM
0
mode. This is illustrated in Fig. 1.4 d-f, where we
reported the spots imaged on a CCD when the tip was observed
through (or without) a polarization analyzer in two mutual normal
configurations,namelyVandH(Uindicatesthenonpolarizedcase).
These results were obtained when excitation was polarized in the
vertical (V) plane. For a horizontal (H) laser polarization it is still
