All-Optical Cryogenic Thermometry Based on Nitrogen-Vacancy Centers in Nanodiamonds

The nitrogen-vacancy (N-V) center in diamond has been recognized as a high-sensitivity nanometer-scale metrology platform. Thermometry has been a recent focus, with attention largely confined to room-temperature applications. Temperature sensing at low temperatures, however, remains challenging as the sensitivity decreases for many commonly used techniques, which rely on a temperature-dependent frequency shift of the N-V center's spin resonance and its control with microwaves. Here we use an alternative approach that does not require microwaves, ratiometric all-optical thermometry, and demonstrate that it may be utilized to liquid-nitrogen temperatures without deterioration of the sensitivity. The use of an array of nanodiamonds embedded within a portable polydimethylsiloxane sheet provides a versatile temperature-sensing platform that can probe a wide variety of systems without the configurational restrictions needed for applying microwaves. With this device, we observe a temperature gradient over tens of microns in a ferromagnetic-insulator substrate (yttrium iron garnet) under local heating by a resistive heater. This thermometry technique provides a cryogenically compatible, microwave-free, minimally invasive approach capable of probing local temperatures with few restrictions on the substrate materials.