Abstract
Thermal control of liquids with high (micrometric) spatial resolution is required for advanced research such as single molecule/cell studies (where temperature is a key factor) or for the development of advanced microfluidic devices (based on the creation of thermal gradients at the microscale). Local and remote heating of liquids is easily achieved by focusing a laser beam with wavelength adjusted to absorption bands of the liquid medium or of the embedded colloidal absorbers. The opposite effect, that is highly localized cooling, is much more difficult to achieve. It requires the use of a refrigerating micro-/nanoparticle which should overcome the intrinsic liquid heating. Remote monitoring of such localized cooling, typically of a few degrees, is even more challenging. In this work, a solution to both problems is provided. Remote cooling in D2 O is achieved via anti-Stokes emission by using an optically driven ytterbium-doped NaYF4 microparticle. Simultaneously, the magnitude of cooling is determined by mechanical thermometry based on the analysis of the spinning dynamics of the same NaYF4 microparticle. The angular deceleration of the NaYF4 particle, caused by the cooling-induced increase of medium viscosity, reveals liquid refrigeration by over -6 K below ambient conditions.
Highlights
Thermal control of liquids with high spatial resolution is required the design and development of remotely activated heating and cooling units of for advanced research such as single molecule/cell studies
In most of the cases, the presence of nonon the micro- and nanoscales. Such an advanced spa- radiative transitions, by multi-phonon relaxation (MPR), results tial control over temperature is necessary in numerous fields in relevant heating of the NaYF4: Ln system through vibrations ranging from the study of cell dynamics[1,2] to the development of ligands and solvent molecules.[21,22]
The critical difference between the two lasers used in this work is their operating wavelength that is correspondingly shorter or longer than the averaged emission wavelength of ytterbium ions in NaYF4
Summary
This means that only a small (q/Pl = 4 × 10−4) fraction of incident laser power is involved in the cooling process This was, expected due to the low absorption coefficient characteristic of lanthanide ions (owing to the forbidden character of the optically active f–f transitions) and due to the fact that 1020 nm radiation is far from the maximum absorption of ytterbium ions (see Figure S2, Supporting Information).[47] The absorption coefficient of ytterbium ions at 1020 nm in crystals is typically close to 1 cm−1 for a 5 at% doping level— this is approximately fourfold lower than found at 976 nm wavelength where the maximum is found.[48] According to the Lambert–Beer law and for a 2.5 μm thick particle, we estimate a single particle absorbance (the fraction of pump power that is absorbed by the microparticle) to be 2.4 × 10−4. The higher the two values will be, the more pumping light will be absorbed and brighter emission will be achieved, which are beneficial for the optical cooling because the emitted photons simultaneously ‘carry’ excess of thermal energy (owing to phonon assistance) out of the luminescent crystal
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