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5. NANOMEMS APPLICATIONS: PHOTONICS 207

by combining two Bragg mirrors symmetrically with respect to a nanowire
used for launching SPs, see Fig. 5-11.

5.3.4 Semiconducting Nanowire-Based Nanophotonics

In addition to the SP-based nanophotonics approach, an approach based
on using active nanowire waveguides has been advanced by Lieber’s group
[216]. This approach is motivated by an attempt to circumvent the loss
limitations exhibited by passive waveguides, such as SP-based devices,
which may hinder their applicability for manipulating light over the extent of
integrated photonic systems.
Early examples of semiconducting nanowires include nanoscale lasers
[223], in which a sub-wavelength diameter nanocavity is created by
exploiting the high refractive index contrast between a nanowire and its
surroundings. The active waveguide concept pursued by Lieber’s group
[216] involves utilizing cavities such as these as waveguides. The feasibility
of the concept was investigated by quantitatively characterizing the losses
through straight and sharply bent CdS nanowires, of sub-wavelength (200
nm) diameter, by scanning optical microscopy. In particular, the experiments
recorded spatial maps of the intensity of light emitted from one end of the
nanowire, as a function of the position of a diffraction-limited laser spot with
energy greater than the CdS band gap. In this context, the laser energy
absorbed by the CdS nanowire was re-emitted via photoluminescence and
subsequently guided by it. The experiment indicated that active CdS
nanowires are capable of efficient guiding over straight and sharp and acute
angle bends, with typical losses of about 1-2dB in an abrupt bend. In
addition, by studying the characteristics of junctions between two nanowires
it was found that light may be coupled efficiently through sub-wavelength
bends defined by them. Finally, by applying a variable electric field across a
nanowire, it was demonstrated that it is possible to modulate the intensity of
the light exiting the nanowire ~25% at a field of ~ 4.2 × 10 5 V / cm .

5.4 Detection of Surface Plasmons

The detection of SPs relies on their conversion to light, and the
subsequent detection of this light. In this context, one can mention two
detection schemes. In one scheme, detection is effected by monitoring the
light emitted by fluorescent molecules covering the entire device; such was
the approach employed in Section 5.3.2.5 to show direct evidence of SP
propagation in a plasmon waveguide [211]. This approach is more of a
diagnostic tool and does not seem amenable to utilization in actual signal
processing systems where one is interested in detecting the output at the exit