How do I control the pump beam polarization? Magic angle, depolarizer, and anisotropy measurements.

In the standard configuration, we use a broadband depolarizer for the pump. This way you don’t have to worry about the magic angle. The achromatic depolarizer used is either DPU25 with -A coating from ThorLabs or DPP-25-A (specified wavelength range is 350-700 nm). You can use any type of depolarizer required for your experiment.

When using UV pump, control the polarization with the optics supplied with the Helios (the polarizing beamsplitter and waveplate) is limited. The problem here is that there are no UV achromatic waveplates. Therefore you need a separate waveplate for every wavelength in the UV. To overcome this we have a depolarizing element in the excitation beam. This ensures that the signal you are measuring is isotropic. Even though this solution does not work for anisotropy measurements it is still useful because most transient absorption experiments are performed to obtain isotropic signals.

We have a separate extension for measuring the absorption anisotropy, which uses different waveplates or a Berek compensator for pump polarization control.