something adsorbs to its surface—for example, when an antigen binds to a
surface-bound antibody.
8.4.3 Dual polarization interferometry
Unlike other surface characterization techniques such as QCM-D and SPR
whose measurements offer only indirect estimations of the mass, density,
or thickness of a thin film, dual polarization interferometry (DPI) is an
optical technique that can provide accurate, simultaneous measurements
of multiple film parameters. DPI uses two independent measurements of
the effective refractive index of a thin nanofilm to simultaneously determine the mass, density, and thickness of that nanofilm. A variety of other
waveguide spectroscopic techniques exist that are similar to DPI, such as
optical waveguide lightmode spectroscopy (OWLS) and coupled plasmon
waveguide resonance (CPWR) spectroscopy, but only DPI is discussed
here.
8.4.3.1 Waveguide basics
As may be recalled from our discussion of the critical angle of reflection, if
light is shone on the boundary between two substances, the light is totally
reflected within one substance if it is shone at an angle greater than the
critical angle (q critical ) and if the refractive index of the second substance is
less than that of the first (Equation 8.21). If multiple interfaces are present,
this phenomenon can be exploited to confine light within one layer of the
system.
Consider a layer of a substance with one refractive index sandwiched
between two layers of a substance with a lower refractive index. In such a
setup, we would expect that light shone at an angle greater than q critical
could undergo total internal reflection inside the middle layer, alternately
reflecting off the top and bottom layers as it passed through. Indeed, such
behavior is observed and the setup is called a waveguide, with the middle
layer termed the waveguide core and the top and bottom layers called the
cladding regions, as shown in Figure 8.15. When light is shone on the
edge of a waveguide, total internal reflection can occur and the light
passes through the core and emerges from the other side of the waveguide. A common application of total internal reflection is in fiber optic
cables, which transmit light over long distance by confining it within a
high refractive index fiber surrounded by a cladding layer with a lower
refractive index.
CHAPTER 8: Surface Characterization and Imaging Methods
284
surface-bound antibody.
8.4.3 Dual polarization interferometry
Unlike other surface characterization techniques such as QCM-D and SPR
whose measurements offer only indirect estimations of the mass, density,
or thickness of a thin film, dual polarization interferometry (DPI) is an
optical technique that can provide accurate, simultaneous measurements
of multiple film parameters. DPI uses two independent measurements of
the effective refractive index of a thin nanofilm to simultaneously determine the mass, density, and thickness of that nanofilm. A variety of other
waveguide spectroscopic techniques exist that are similar to DPI, such as
optical waveguide lightmode spectroscopy (OWLS) and coupled plasmon
waveguide resonance (CPWR) spectroscopy, but only DPI is discussed
here.
8.4.3.1 Waveguide basics
As may be recalled from our discussion of the critical angle of reflection, if
light is shone on the boundary between two substances, the light is totally
reflected within one substance if it is shone at an angle greater than the
critical angle (q critical ) and if the refractive index of the second substance is
less than that of the first (Equation 8.21). If multiple interfaces are present,
this phenomenon can be exploited to confine light within one layer of the
system.
Consider a layer of a substance with one refractive index sandwiched
between two layers of a substance with a lower refractive index. In such a
setup, we would expect that light shone at an angle greater than q critical
could undergo total internal reflection inside the middle layer, alternately
reflecting off the top and bottom layers as it passed through. Indeed, such
behavior is observed and the setup is called a waveguide, with the middle
layer termed the waveguide core and the top and bottom layers called the
cladding regions, as shown in Figure 8.15. When light is shone on the
edge of a waveguide, total internal reflection can occur and the light
passes through the core and emerges from the other side of the waveguide. A common application of total internal reflection is in fiber optic
cables, which transmit light over long distance by confining it within a
high refractive index fiber surrounded by a cladding layer with a lower
refractive index.
CHAPTER 8: Surface Characterization and Imaging Methods
284