Using quantum light to measure temperature at the nanoscale
Quantum sensing exploits unique quantum effects—e.g. single-photon emission and spin manipulation—to measure quantities beyond the limits of classical physics, and promises to revolutionize the way we see and interact with the world around us. It is allowing us to measure time, chemical changes, temperature and electric or magnetic fields with unprecedented precisions and at miniscule scales, where everything is billionths of metres small.
Now scientists at the University of Technology Sydney have developed a novel, non-invasive technique that uses quantum light to measure temperature at the nanoscale (a billionth of metre), with extremely high sensitivity. The method finds immediate applications both in fundamental research and industry. Examples range from accurately measuring the temperature changes inside a cell due to e.g. specific biological processes or sickness, to monitoring—in real time—the dramatic temperature increase in high-power, micro- and nano-electronic components, and how this affects their performance.
The study accepted for publication in the journal Science Advances, was led by a team of researchers at the University of Technology Sydney (AU) and involved international collaborators from the Russian Academy of Science (RU), Nanyang Technological University (SG) and Harvard University (US). To measure the temperature at such a small—nanometric—scale the team of researchers used diamond nanoparticles. These are extremely small particles (up to 10,000 times smaller than the width of a human hair) that fluoresce when illuminated with a laser. “Pure diamond is transparent, but it usually contains imperfections such as inclusions of foreign atoms that beyond giving it different colours (yellow, pink, blue, etc.) emit light at specific wavelengths (i.e. colours) when probed with a laser beam,” says Dr Tran, lead author of the study.
The researchers found that there is a special regime—referred to as Anti-Stokes—in which the intensity of the light emitted by these diamond colour impurities depends very strongly on the temperature of the surrounding environment. Because these diamond nanoparticles can be as small as just a few nanometres they can be used as tiny nano-thermometers. “We immediately realized we could harness this peculiar fluorescence-temperature dependence and use diamond nanoparticles as ultra-small temperature probes,” comments Dr Bradac, senior author of the study. “This is particularly attractive as diamond is known to be non-toxic—thus suitable for measurements in delicate biological environments—as well as extremely resilient—hence ideal for measuring temperatures in very harsh environments up to several hundreds of degrees,” he added.
An important advantage is that the technique is all-optical. The measurement only requires placing the nanoparticles (e.g. placing a droplet of the nanoparticles-in-water solution) in contact with the sample and then measuring—non-invasively—their optical fluorescence as a laser beam is shone on them. Recently, similar all-optical approaches using nanoparticles have been realized to measure temperatures at the nanoscale, but none has been able to achieve both the sensitivity and the spatial resolution of the technique developed by the researchers at UTS. “Our sensor can measure temperatures with a sensitivity which is comparable—or is outright superior—to that of the current best all-optical micro- and nano-thermometers, while featuring the highest spatial resolution to date,” Dr Tran concluded. “We are excited about this new technique as it is not just a proof-of-concept realization. The method is immediately deployable. We are currently using it for measuring temperature variations both in biological samples and in high-power electronic circuits whose performance strongly rely on monitoring and controlling their temperature with sensitivities and at a scale hard to achieve with other methods,” concluded Dr Bradac.
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