Biomedical Engineering Reference
In-Depth Information
Figure 6.6
Effect of the nanowire input end geometry. (a) Calculated
emissionintensityatthedistalendasafunctionoftheincidentpolarization
angle
90
◦
(blue) and 45
◦
(red). The polarization
angle at which the emission intensity reaches maximum is defined as
θ
max
.
(b) The dependence of
θ
, for cutting angle
α
=
θ
max
(black dot) and the maximal emission power
(redsquare)onthecuttingangle
.Inset:Thegeometryofthecutinputend
with the sharp tip edge smoothed by a 5 nm curvature. The radius of the
nanowire is
R
α
=
λ
0
=
60 nm and the incident wavelength is
632.8 nm.
θ
max
decreases to minus values and goes up again when
α
Then,
is
α
further decreased. As the tip becomes sharper, especially when
is
45
◦
,thecouplingintothenanowirebecomesweaker
so that the maximal transmission intensity is reduced. Detailed
correlation of the in-coupling and out-coupling polarization state
can be found in Ref. [31], in which the influence of the geometry of
the tip is emphasized.
The period of the helical pattern is determined by
=
∼
smallerthan
2
π
Re
k
0,
−
k
1,
−
1
,where
k
0,
and
k
1,
are the propagation
constants of the
m
=
0and
m
=
1 mode. According to Fig.
6.1(a), the real part of the propagation constant of the
m
=
0
mode decreases as the radius of nanowire increases, whereas the
propagation constant of the
m
=
1 mode increases. Therefore, the
pitch of the helical pattern is getting larger for thick wire. This
behavior applies for nanowires of different metals, surrounding
media with different refractive indexes and different wavelengths,
as shown in Fig. 6.7. For a nanowire in the same matrix, the helix
period is larger for 632.8 nm vacuum wavelength than that for 532
nm. At a given wavelength, the helix period is larger in air than that
