  The measurement of temperature with nanoscale spatial resolution is an
emerging new technology and it has important impact in various fields. An ideal
nanothermometer should not only be accurate, but also applicable over a wide
temperature range and under diverse conditions. Furthermore, the measurement
time should be short enough to follow the evolution of the system. However,
many of the existing techniques are limited by drawbacks such as low
sensitivity and fluctuations of fluorescence. Therefore, Plank's law offers an
appealing relation between the absolute temperature of the system under
interrogation and the thermal spectrum. Despite this, thermal radiation
spectroscopy is unsuitable for far-field nanothermometry, primarily because of
the power loss in the near surroundings and a poor spatial resolution.
