Optical Coatings

Fundamental Optics
Material Properties Optical Specifications Gaussian Beam Optics
Optical Coatings
BROADBAND REFLECTION COATINGS
The design procedure for a broadband reflection coating should
now be apparent. Two design techniques are used. The most obvious
approach is to use two quarter-wave stacks with their maximum
reflectance wavelengths separated on either side of the design wavelength.
This type of coating, however, tends to be too thick and
often has poor scattering characteristics. The basic design is very
useful for dichroic high reflectors, where the peak reflectances of
two stacks are at different wavelengths.
A more elegant approach to broadband dielectric coatings is
to use a single modified quarter-wave stack. In this modified stack,
the layers are not all of the same optical thickness. They are graded
between the quarter-wave thickness for two wavelengths at either
end of the intended broadband performance region. The optical
thicknesses of the individual layers are usually chosen to follow a
simple arithmetic or geometric progression. Using designs of this
type, multilayer, broadband, high-reflectance coatings are possible
with reflectances in excess of 99% over several hundred nanometers.
The greatest impact of improved broadband reflector design
and manufacturing technology has almost certainly been on dye laser
design and applications. In many of these scanning systems, high
reflectance over a large wavelength region is absolutely essential. In
many non-laser instruments, all-dielectric coatings are favored over
metallic coatings because of their high reflectance. Multilayer
broadband coatings are available with high-reflectance regions
spanning almost the entire visible spectrum. Such films are effective
for both s- and p-polarization components, and over a wide range
of incidence angles. At oblique incidence, reflectance is markedly
reduced.
Because of the materials chosen for the multilayer, durability
and abrasion resistance of such films are superior to those of metallic
films. Although the reflectance of dielectric coatings can easily be
made to exceed the highest metallic reflectances over very large
wavelength intervals, metallic coatings are still superior in terms of
usable ranges of incidence angles and wavelengths for a single coating.
POLARIZATION EFFECTS
When light is incident on any optical surface at angles other
than normal incidence, there is always a difference in the reflection/
transmission behavior of s- and p-polarization components. In
some instances, this difference can be made extremely small. On
the other hand, it is sometimes advantageous to design a thin-film
coating that maximizes this effect (e.g., thin-film polarizers).
Polarization effects are not normally considered for antireflection
coatings since these are nearly always used at normal incidence
where the two polarization components are equivalent.
High-reflectance or partially reflecting coatings are frequently
used away from normal incidence, particularly at 45 degrees, for
beam steering or beam splitting. Polarization effects can therefore
be quite important for these types of coating.
effective broadband high-reflection coating
incident
wavelength l 0
NOTE: If at least one component is totally
reflective, the coating will not transmit
light at that wavelength.
noneffective broadband antireflection coating
incident
wavelength l 0
NOTE: Unless every component is totally
nonreflective, some reflection losses will occur.
totally reflective component for l 0
partially reflective component for l 0
totally nonreflective component for l 0
Figure 5.41 Schematic multicomponent coatings with
only one component exactly matched to the incident
wavelength, l . (the high-reflection coating is successful; the
antireflection coating is not).
At certain wavelengths, a multilayer dielectric coating shows a
remarkable difference in its reflectance of the s- and p-polarization
components (see figure 5.42).
The basis for the effect is the difference in effective refractive
index of the layers of film for s- and p-components of the incident
beam, as the angle of incidence is increased from zero. This should
not be confused with the phenomenon of birefringence in certain
crystalline materials, most notably calcite. Unlike birefringence, it
does not require the symmetric properties of a truly crystalline
phase. It arises from the difference in magnetic and electric field
asymmetries for s- and p-components of an electromagnetic wave
at oblique incidence.
The resultant difference in reflectance of the two polarization
components is always in the same sense. Maximum s-polarization
reflectance is always greater than the maximum p-polarization
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