Biomedical Engineering Reference
In-Depth Information
It is evident that the feature of sharp tail is lost due to resolution
limitation,andthereforethetailofeachTaijimarkdeviatesfromthe
idealTaijipattern.AccordingtotheSEMobservationresults,thebest
fitting curve for the tail feature of the fabricated Taiji pattern is a
tangentcirclebetweenanellipse(longaxisD1
=
275nm,shortaxis
D2
250 nm). The geometrical
size of more than 20 Taiji marks is carefully measured and the
average radius of the tangent circle is determined to be around
30 nm with a standard deviation of nearly 2 nm. The numerical
simulation model was subsequently set up according to these SEM
observations.
=
250 nm) and a circle (diameter
=
13.2.3
Measurement and Simulation Results
Figure 13.3 shows both the experimental and simulated transmit-
tance spectra of the Taiji array illuminated at normal-incidence
with a linearly polarized light source along
x
-direction and
y
-
direction, respectively. The transmission spectra from wavelength
λ
=
900-1700 nm were measured using a Fourier-transform
infraredspectrometerwithaninfraredmicroscope(15
×
Cassegrain
=
objective, numerical aperture NA
0.4, near-infrared polarizer, and
an InGaAs detector). An iris was used to collect the incident light to
asquareareaofabout50
×
50
μ
m
2
. The transmission spectra are
normalizedby those ofthe fused silicawafer.
The simulation spectra were obtained with the finite-element
method (FEM). For simulating an array structure of a Taiji pattern,
the boundary conditions are set as periodic boundary conditions
with normal-incidence. The simulated Taiji structure has a 60 nm
diameter tangent circle at the tail of the Taiji pattern, in accordance
with our SEM observations. The refractive index of the fused silica
glass substrate is set as 1.4584. The permittivity of gold was
describedbytheDrude-Lorentz modelusingadampingconstantof
0.14 eV with a plasma frequency of 8.997 eV.
There are two dips and one peak in the experimental as well as
simulated spectra in Fig. 13.3; these features of the transmission
spectra are similar under
x
-and
y
-polarized illumination in Fig.
13.3aand13.3b.Thesecondbroadertransmittancedipinbothcases
of
x
-and
y
-polarized illumination shows a particular interesting
