Nanoscale thermoreflectance with 10mK temperature resolution using stochastic resonance

We present 2D temperature measurements with 250nm spatial and 10mK temperature resolution using thermoreflectance microscopy. We measure the temperature-induced reflectivity change with an accuracy better than ΔR/R=2·10^-6 using a 12bit CCD, which has a quantization limitation of ΔR/R=2.5·10^-4. The dynamic range is thus expanded from 72dB to 114dB, equivalent to more than 18 effective bits. We quantitatively explain this dramatic improvement using the concept of stochastic resonance. In addition, we optimize the thermoreflectance calibration coefficient K≡R^-1· R/T by matching the illumination wavelength to a combination of the thermoreflectance coefficient spectrum R/T and the reflectivity spectrum R. For gold illuminated with a 467nm LED, we obtain the extraordinarily large value κ =3.3·10^-4 K^-1. This calibration coefficient yields a temperature resolution of better than 10mK.