Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 182
( n
− n
)
R f
2
1 2
= 2
( n1
+ n2
)
(6.20)
where ni is the refractive index of substance i. For an air-glass interface, such as that seen
in the ellipsometry experiment, n2 (refractive index of air) is assumed unity in equation
(6.20), as shown by equation (6.21).
2
( n −1)
R f = (6.21)
2
( n + 1)
A circular spot of light from the instruments source is focused onto the sample surface by
viewing through a microscope eyepiece, to ensure the incident light is focused onto the
sample surface region (i.e. the sample height is adjusted). The sample orientation is then
adjusted to be flat relative to the microscope eyepiece using a second circular spot, by
moving the sample stage (i.e. the angle of sample can be adjusted in two planes). The
ellipsometer obtains values Ψ and ∆, by analysing the polarisation of the reflected light,
and displays these values on a screen built into the instrument. These are recorded, and
subsequently input into a program which uses equation (6.21) and (6.19) to calculate n
for the sample, from values of Ψ and ∆.
There are a number of advantages to ellipsometry. The technique is accurate and
reproducible, as it measures the ratio between two values of R (the s and p component of
the reflected beam). No reference is needed, except to calibrate the equipment for
accuracy, and sample preparation is simple. This is because a flat surface is needed for
the measurement rather than a prism. The technique is not sensitive to scatter and