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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.
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