Optical Polarization Microscopy of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$

We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of matter at low temperature and high fields. As a first application of the instrument, we are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We report initial results from polarization analysis and polarization microscopy with sample temperatures below 150 mK and applied magnetic fields from 0 T to 9 T.