Kansas State University Physics REU 2011

The Setup

In order to examine the Faraday rotation spectrum exhibited by gold nanoparticle solutions, a probing method is used in which a flash of white light probes the optical rotation caused by the magnetic field. When the magnetic field is on, a polarized optical signal is passed through a nanoparticle solution contained within a special sample cell in the electromagnet as shown below.

SolidEdge rendering of setup.

The rotation in the plane of polarization of the optical signal is measured through use of the analyzer and recorded by a spectrometer. This signal and a signal taken when the field is off provide data from which the rotation angle can be calculated via Malus’ Law.

Actual setup seen in 3D rendering.

The Experiment

The system is probed twice, once when the magnetic field is on and again when the field is off. From these two data sets the rotation can be calculated with respect to wavelength via Malus’ Law:

I/I0 = cos2(rotation)

From this form of the dataset, the rotation can be converted to a Verdet constant spectrum by dividing by the length of the sample and the magnitude of the magnetic field. This is possible due to the equation:

rotation = v*B*L

v = rotation / B*L

According to published theory, a peak in the Verdet spectrum is expected at 520nm in the same location as the plasmon peak as seen in the following figure.

After conversion, the final Verdet spectrum is qualitatively a good match for the above theory. The peak seen below occurs at the same location and is of approximately of the same width as theory. In the below figure, the rotation has not been converted to a Verdet value but it is clearly seen that in the optimum range for the analyzer used (425nm - 625nm) the shape of the acquired data closely approximates the above figure.