Two-Photon Excitation of Silicon-Vacancy Centers in Nanodiamonds for All-Optical Thermometry with a Noise Floor of 6.6 mKmiddotHz-1/2

In this paper, we introduce ultrasensitive all-optical nanothermometry using high figure-of-merit nanodiamonds with silicon-vacancy (SiV) centers. In contrast to the commonly used single-photon process, we have adopted a two-photon approach to achieve efficient excitation of SiV centers in nanodiamonds. Based on the developed intensity-projected scheme, we have experimentally achieved a noise floor of 6.6 mK center dot Hz-1/2 using nanodiamonds. This serves as a new record of temperature resolution in the relevant field. Particularly, this simplified method allows us to reflect the temperature-induced spectral shift without wavelength scanning via a spectrometer. Furthermore, we demonstrate that the two-photon excitation of SiV centers has been thermally activated, as indicated by an elevated fluorescence intensity accompanied by a temperature increase. Our findings offer an opportunity for harnessing SiV-based nanothermometry in an easy-to-use manner and open the road for the development of practical applications in complicated environments such as living cells.