Optically Probing Quantum Materials at milliKelvin Temperatures*

Scanning confocal microscopy offers plentiful mesoscale material information that bridges atomic-scale scanning probes and bulk characterization techniques. MilliKelvin (mK) microscopes have become indispensable tools for the description of phases and quantum critical points with critical temperatures that are inaccessible in helium cryostats. Here, we demonstrate Raman microscopy at mK on quantum paraelectric strontium titanate. Resolved TO phonon modes are consistent with the recent claim that strontium titanate is intrinsically microscopically polar. Additionally, with magneto-optical Kerr imaging, we spectrally resolve the magnetic response from yttrium iron garnet (YIG) as a step toward magneto-optical imaging at mK temperatures with YIG indicator crystals.