Biomedical Engineering Reference
In-Depth Information
air (
n
2
= 1), since the integer diffraction order
m
, which can also be a neg-
ative number, gives us a large degree of freedom for our choice of coupling
configuration.
In practice, coupling gratings are very shallow and the core layer of the
dielectric slab waveguide is rather thin. Typical values are 100-200 nm for the
core thickness, 300-500 nm for the grating period, and 5-50 nm for the grating
depth [1].
Waveguides with End-Face Coupler
Since the core of the waveguide is so thin, the “obvious” end-face coupling of
light with a focusing lens, as illustrated in Fig. 12.6, is very di
cult to achieve.
The alignment tolerances and the mechanical stability of the optical setup
must be below 100 nm, which is quite di
cult to achieve in practice, where
optical biochips should be disposable, quick to read out, and mechanically
very robust. Therefore, end-face coupling is not used in commercial optical
biosensors.
Resonance Condition for Evanescent Wave Sensing
From (12.4) and (12.5) it becomes clear how measurements are carried out in
optical biosensors: The presence of the analyte subtly changes the refractive
index in the sample volume. This change is sensed by the evanescent wave,
since it implies a change in
n
eff
of the guided wave. A change in
n
eff
alters the
coupling condition, which can be adjusted either with the in- or out-coupling
angle
Θ
or with the wavelength
λ
. For this reason, evanescent wave sensing
in optical biochips as described above is a resonance method: The amount of
light in the guided wave is maximum for the combination of
Θ
and
λ
that
fulfills the coupling condition for the present value of
n
eff
which is dependent
on the analyte concentration.
The most sensitive methods for measuring this resonance condition are
capable of resolving changes in the refractive index of about 10
−
7
[1, 2]. This
corresponds to a mass detection sensitivity of nearly 100 fg mm
−
2
, and the
molar concentration of the analyte can be determined with a sensitivity of
about 10
−
11
, i.e., the concentration sensitivity is 10
−
11
mol l
−
1
.Notethatitis
important to stabilize for temperature variations at these high measurement
sensitivities, since
∂n
/
∂T ≈
10
−
4
K
−
1
for water at room temperature [17].
12.4.2 Fluorescence Sensing
Since fluorescence (or luminescence) sensing requires the imaging of two-
dimensional distributions of excited or self-luminous light patterns, the usual
sensing technique is fluorescence (or luminescence) microscopy [4]. For this
purpose, either conventional optical microscopes or scanning (confocal) op-
tical microscopes are employed, where the sample is typically illuminated
