Temperature at the mesoscale is important in many fields due to its key role in, e.g., cell mechanics, quantum ground state studies and hydrodynamics. In levitated optomechanics, measuring temperature is challenging at the same time as necessary to understand the dynamics of the optically trapped particle. Generally, the particle’s temperature has been directly correlated to its centre-of-mass (CoM) motion (i.e. translational dynamics). This, together with the rotational dynamics, encompasses the particle’s external degree of freedom which is affected by the external temperature. However, the particle presents an internal structure that is at another internal temperature. Generally, the CoM temperature is experimentally measured and compared to a theoretically calculated internal temperature. The rotation rate (i.e. rotational dynamics) has also been correlated to an experimentally measured internal temperature for thermometric studies. Despite its importance, the temperature of these three degrees of freedom had never been simultaneously measured and correlated. We developed a tripartite method able to independently measure both the internal temperature of the particle (through temperature-dependent luminescence) and the external temperature (through the rotational rate and trap stiffness). We found that, even though they are strongly coupled, the external and internal degrees of freedom present distinct temperatures. This study gives new insight into thermometry at the mesoscale where the appropriate parameter should be carefully chosen for an accurate characterisation of temperature. Moreover, experiments attempting to cool levitated particles to the quantum ground state, in which all degrees of freedom must be independently controlled and characterised, will also benefit from this advance.
Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques.
Luminescence of a single upconverting particle (NaYF4:Er3+,Yb3+) can be used to determine the optical trap temperature due to the partial absorption of the trapping beam either by the medium (water) or the optically trapped particle itself. This fact is an important drawback can be reduced by shifting the trapping wavelength out of the water absorption band, or by using time-modulated laser trapping beams. Both approaches have been studied and the results have shown that the thermal loading due to the trapping radiation can be minimized.
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