In some aspects, polarization of a spin ensemble can be increased using cavity-based techniques. A sample contains a first spin ensemble and a second spin ensemble. A drive field couples the first spin ensemble with a cavity, and the coupling increases the polarization of the first spin ensemble. Polarization is transferred from the first spin ensemble to the second spin ensemble. In some examples, the polarization is transferred from a solvent or an abundant species to a solute or a dilute species in the sample.