All-optical cryogenic thermometry based on NV centers in nanodiamonds*

Nitrogen vacancy (NV) centers in nanodiamonds (NDs) have been shown to provide an excellent nanometer-scale high-sensitivity thermometry platform. Here we demonstrate a cryogenic-compatible, all-optical thermometry technique based on the emission spectrum of an ensemble of NV centers in NDs that operates from room-temperature to liquid nitrogen temperatures. The sensitivity is found to be slightly improved at cryogenic temperatures, in contrast to the conventional thermometry technique based on the temperature-dependent zero-field splitting of NV centers. We use this all-optical thermometer at T=170 K to measure the surface temperature of a ferromagnetic insulator, yttrium iron garnet (YIG), over tens of microns with the use of an array of NDs on a flexible polydimethylsiloxane (PDMS) sheet, where the YIG is thermally driven by a resistive heater. We directly observe a thermal gradient over micrometers in YIG, indicating that the technique is independent of magnetic noise and microwave resonances.