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All-optical nanothermometry in challenging environments using NV centers in individual nanodiamonds

Dinesh Bommidi, PhD Qualifying Exam, Advised by Professor Andrea Pickel

Wednesday, December 15, 2021

Temperature sensing has been an ever-evolving technology for the past few centuries. One of its latest branches, nanothermometry, deals with the measurement of temperature at hotspots confined to small regions with dimensions ≤100 nm. Nanoelectronics and nanoscale components inside novel devices, like in LEDs and advanced hard drives, can reach high temperatures that lead to rapid thermal deterioration or unreliable operation. Therefore, development of such devices in recent years has increased the demand for thermometry with sub-100 ns resolution coupled with sub-100 nm spatial resolution. However, with currently existing contact-based methods or diffraction-limited optical nanothermometry techniques, either the accuracy of the measurement or the spatial resolution must be compromised.

To overcome these challenges, we have developed an all-optical single-point thermometry technique where we can accurately place a nanosensor at the critical point of interest through atomic force microscope-based nanomanipulation. This achieves spatial resolution equivalent to the nanosensor size, which is far below the optical diffraction limit. With their temperature-dependent fluorescence lifetime on the order of tens of nanoseconds, nitrogen vacancy (NV) centers in individual nanodiamonds (NDs) are excellent candidate sensors for achieving our goal of nanothermometry with sub-100 ns temporal resolution. While thermometry signals based on shifts in NV center spectral features or optically detected magnetic resonance frequency are challenging to detect above ~700 K, through time correlated single photon counting (TCSPC), it is expected that lifetime decay curves of single NDs can be measured at high temperatures. We performed TCSPC measurements of individual NDs on silicon and glass substrates between 300 K and ~500 K. We found that the lifetime of NV center emission becomes longer in a low refractive index environment and decreases with increasing temperature. We also observed a variation in room-temperature lifetime values among individual NDs from the same batch. At 500 K, the average decrease in lifetime values was 32 ± 7.0% for NDs on silicon and 35 ± 8.3% for NDs on glass in relative to their 300 K values. Additionally, most of the NDs emit > 1000 counts/second at 500 K, which is ~10x more than our system detection limit. Both these results support our primary goal of performing single point nanothermometry with sub-100 ns temporal resolution even at high temperatures that are not accessible with existing all-optical nanothermometry methods. We also demonstrated nanomanipulation by making arbitrary microscale patterns with NDs and moving an ND ~20 nm from its original position.

We are also using single point nanothermometry to experimentally verify decades-old predictions about sub-continuum heat dissipation from a nanoscale heat source. Optically levitated individual NDs containing NV centers are employed as a combined nanoheater/sensor where the fluorescence spectra can be used to measure the temperature of the NDs. Since heterogeneity among the NDs’ temperature response can result in inaccurate measurements, we employ optical Debye Waller Factor (DWF) thermometry, a more suitable all-optical temperature dependent signal for this purpose. The DWF vs. temperature calibration is performed outside the optical trap. Subsequently, these DWF values are used to determine the temperature of optically levitated NDs from the same batch. Overall, both the DWF and lifetime nanothermometry techniques will have broad applications with minimal effects on the sample surface and its environment