  The nitrogen-vacancy (NV) 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.
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 NV
centers 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 portably polydimethylsiloxane (PDMS) 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 (YIG) 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.
