A Novel Method for Measuring Low Energy Excitations with BLS

Abstract: This report presents the work done on an optical setup combining Raman and BLS spectroscopy and discusses the achieved improvements of the system. After some general optimization and minor redesigning, a new alternative beam path was implemented. This new beam path allows to freely adjust all three volume Bragg grating filters in the setup, which are used to suppress the elastic line and which feature an exceptionally narrow filter window. This enables measurement of BLS signal closer to the elastic line, with a frequency separation of only 30 GHz from it achieved during the project. For measuring below this threshold, additional suppression is needed. As a first technique to achieve this, the Fabry-Pérot interferometer provided by the BLS spectrometer was partially put to use. At the point of this report, this has not been completed successfully and the setup also suffers from other adverse effects in the form of signal fluctuations and spurious measurements close to the elastic line. Nonetheless, the setup has showed promising intermediate results and can be expected to perform well once these challenges have been overcome.