Theoretical seminar | 06 July 2022

Optomechanical systems based on levitated nanoparticles are of interest to researchers for the possibility of both studying fundamental problems of quantum mechanics and practical problems of creating a new type of sensor and measuring devices. Interest in such systems from a fundamental point of view is associated with determining the conditions under which the transition from classical to quantum and, conversely, from quantum to classical behavior of a nanoparticle, which is a condensed state of matter with a characteristic size of tens of nanometers, occurs. The manifestation of wave properties by a nanoparticle opens up new ways of interaction of such an object with an object of a quantum-mechanical nature, for example, to make entanglement of them. The transition of a nanoparticle to a quantum state occurs in optical or radio-frequency traps in a vacuum during its cooling under the action of light to the ground vibrational state. In this regard, the efforts of researchers in recent years have been directed, first of all, to the development of methods and mechanisms for reducing the translational temperature of levitated nanoparticles. At the same time, a solid-state nanoparticle also has an internal temperature, which increases during the implementation of translational cooling mechanisms, which is an obstacle to achieving a quantum state. Thus, in order to achieve the quantum state of a levitated nanoparticle, in addition to translational cooling, the use of internal cooling mechanisms is also required. In addition, it seems extremely interesting from a fundamental point of view the problem of obtaining a nanoparticle cooled both with respect to internal and external degrees of freedom, i.e. absolutely cold quantum nanoparticle, and studying its properties.
Up today, the main mechanism for optical cooling of solid-state materials is a mechanism based on anti-Stokes fluorescence. Despite the fact that this mechanism can theoretically be realized in various solid-state materials, including semiconductors, only crystals doped with ytterbium ions can be experimentally cooled due to anti-Stokes fluorescence. However, even in the case of doped crystals, cooling cannot be performed below 50 K (theoretical limit), which makes it impossible to obtain an absolutely cold nano-object.
New mechanisms for optical cooling of crystalline nanoparticles - nanocrystals, semiconductor quantum dots and wires under the conditions of the dynamic Stark shift are presented in the presentation. Exposure of nanoparticles to pulses of both a strong electric field and powerful electromagnetic radiation leads not only to a rearrangement of the electronic energy spectrum, but also to a change in the vibrational states of the crystal lattice of the material. The proposed approach considers the processes leading to a decrease in the energy of the crystal lattice due to the non-adiabatic interaction of the electronic and vibrational subsystems of the crystal, rearrange by an external influence. A feature of the approach is the possibility of implementing controlled optical cooling by monitoring the parameters of electric or electromagnetic fields. The proposed mechanisms can be used both to reduce the internal temperature of a levitated nanoparticle in combination with translational cooling mechanisms, and separately as mechanisms that implement dual (translational and internal) cooling simultaneously.

Main paper/arXiv, related to the seminar, and other references

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