P7 – Scattering of Surface Plasmon Polaritons by Gold ... - repetit.dk

Chapter 4
Experimental Setup
Contents
4.1 ATR Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 LRM Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
In this chapter the experimental setups are described along with the principle behind leakage
radiation microscopy and the Fourier filter.
4.1 ATR Setup
In Fig. 4.1 the setup for measuring the laser beam reflection from the gold film on the prism
as a function of incident angle is shown. The setup consists of two polarizers, a laser and two
coincident circular disks connected to a step motor through two rubber bands.
The smaller of the two disks is placed below the other as shown by the dashed circle in the
figure. Because the inner disk is half the circumference of the other the rotation speed of the
inner disk is twice that of the outer disk. Because the detector is mounted on the inner disk
and the prism is placed on top of the outer disk, the rotation speed of the detector is twice that
of the prism. This is necessary because when the prism has rotated an angle of e.g. 45 ◦ the
reflected beam has rotated an angle of 90 ◦ . If the detector does not rotate twice the speed of
the prism the reflected beam will only hit the detector in the very beginning of the experiment.
The laser used in the experiment was a HeNe laser at a wavelength of 632 nm. The two
polarizers in the setup are to enable control of the intensity of the incident light, but also to
be able to exclude p-polarized light in order to check whether the observations are really due
to SPPs.
The experiments were made by first positioning the prism so that the laser hits the gold film at
an angle of 45 ◦ (Fig. 4.2(B)). The prism was then rotated clockwise about 5 ◦ so that the setup
reaches the position shown in Fig. 4.2(A). From this position the experiment starts and the
prism is rotated until the position shown in (C) is reached, with the reflection being recorded
and saved for each step. 3600 steps corresponds to 90 ◦ rotation so that the ∼10 ◦ rotation
results in ∼400 data points, enough to obtain an acceptable resolution.
It is important that the laser beam hits the gold film at the position which is exactly above
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