Brewster Angle Microscope, BAM
Definition:
The Brewster Angle Microscope is an non-invasive technique that
enables the characterization of one molecular thick floating monolayers
at the air/water interface (e.g. a monolayer with typical thickness of
ca 2 nm). The BAM allows the direct observation of ultra-thin films on
air/water interfaces or on dielectric substrates.
Summary/Description:
In the beginning of 19th century Sir David Brewster, a Scottish
physicist, discovered the polarization phenomenon of light reflected at
specific angles. In his studies on polarized light, Brewster discovered
that when light strikes a reflective surface at a certain angle, the
light reflected from that surface is polarized into a single plane i.e.
plane-polarized. This angle is commonly referred to as Brewster's angle, and can be easily calculated utilizing the following equation for a beam of light traveling through air:
n = sin(θi)/sin(θr) = sin(θi)/sin(θ90-i) = tan(θi)
The principle behind the Brewster Angle Microscope (BAM) makes use
of the zero reflectance of an air/water interface or dielectric
substrate for vertically linearly polarized light at the Brewster Angle
of incident. As the equation above shows the Brewster angle is
determined by the refractive indexes of the substrates involved for
example for air/water (refractive index of 1.333), air/glass
(refractive index of 1.515), and air/diamond (refractive index of
2.417) interfaces the critical (Brewster) angles are 53, 57, and 67.5,
respectively. When a condensed phase of a (mono)layer with different
refractive index is spread or deposited on the interface of interest a
measurable change in reflectivity will occur. The reflected light can
then be used to form a high contrast image of the lateral morphology of
the spread or deposited layer. For example, a monolayer spread on an
air/water interface is extremely thin, approximately 0.5 % of the
wavelength of visible light. The relative effect it has on the electric
field reflected from a water surface is therefore very small and the
monolayer is under normal conditions quite invisible. However, if the
water surface is illuminated with pure vertically linearly polarized
light at the Brewster angle before spreading the monolayer at the
air/water interface, there is no reflection from the water surface. The
background is then completely dark and after spreading of the monolayer
and compressing it the tiny effect of the monolayer can be visualized.
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