Taking the temperature of a sample can be tricky—especially in the regimes of the very small and the very cold. The smaller the sample, the greater the influence the thermometer itself may exert on the measurement. And in the ultracold regime, even tiny amounts of heat can significantly alter the temperature reading.

To combat both problems, Victoria Esteso, Rocco Duquennoy (both at Italy’s National Institute of Optics), and their colleagues have developed molecular thermometers that are both extremely sensitive and noninvasive. Specifically, they use a few molecules of the aromatic hydrocarbon dibenzoterrylene embedded in nanocrystals grown from solution. At cryogenic temperatures, the organic molecules are essentially two-level systems that fluoresce from a single excited state to a ground state when irradiated by red laser light. The induced fluorescence has a spectral linewidth that broadens reproducibly as the nanocrystals' temperature rises. Precisely measuring the width of the light emission determines their temperature in the 3–20 K range.

o test their new thermometric method, Esteso and coworkers measured the temperature of nanocrystals placed on a patterned silicon membrane. They used a technique known as excitation spectroscopy, outlined in the figure. Primarily interested in the transfer of heat from one spot to another, they used one laser to heat a series of spots on the silicon at various distances from the nanocrystals and another laser to excite the nanocrystals. A confocal microscope then collected and plotted the fluorescence signal. The researchers’ plot of temperature versus distance confirmed numerical simulations of heat propagation in very thin patterned membranes.

So long as the hydrocarbon-containing nanocrystals can be attached to a sample of interest, there’s no restriction to the kinds of things one can investigate. The researchers say that nanotechnology companies have already expressed interest in commercial applications. (V. Esteso et al., PRX Quantum 4, 040314, 2023.)

Image: A tunable excitation laser (red beam) interrogates molecular thermometers (red disks) on a patterned silicon membrane while another laser (yellow beam) heats the membrane. The thermometers are aromatic hydrocarbon nanocrystals that emit single photons (red dots) at the transition frequency ω_L (the zero-phonon line, ZPL) between the hydrocarbons’ ground and excited states, ∣S₀〉 and ∣S₁〉. As the temperature increases, the energy levels of the ground and excited states—and therefore the fluorescence intensity I_S as a function of the excitation frequency ω_L—broaden in a reproducible way. Credit: Adapted from V. Esteso et al., PRX Quantum 4, 040314 (2023)

Source: Physics Today. Editor's note, 27 October: The figure caption has been updated to correct the definition of ω_L.