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Fluctuation-Induced Interactions at the Interface between Photons, Atoms and Solids

Final Report Summary - INPHAS (Fluctuation-Induced Interactions at the Interface between Photons, Atoms and Solids)

Fluctuation-induced interactions have their origin in classical and quantum fluctuations. Paradigmatic examples are the Casimir effect and the van der Waals force. The exact knowledge of these interactions is rapidly becoming important for the characterization of modern experimental set-ups and for the opportunities and challenges that they offer to nanotechnology. Recent theoretical and experimental investigations have shown that such interactions are tunable in strength and sign, opening new perspectives to investigate aspects of quantum field theory and condensed-matter physics. The goal of the INPhAS project was to carry out an intensive theoretical and computational study of equilibrium and nonequilibrium fluctuation-induced interactions for fundamental and nanotechnological applications.
One of the project’s strength is its multidisciplinary aspect. Different but complementary lines of research have been investigated. The most relevant examples are plasmonics, open quantum system, thermodynamics, atom-surface interactions, geometry-induced and dynamical effects. All these topics are intimately related within the framework of fluctuation-induced interactions, allowing for a complete perspective and a detailed understanding of the physics underlying these phenomena. INPhAS developed over three years of activity over which the project fulfilled most of its objectives and led to advancement beyond the state of the art in the field of fluctuation-induced interactions. It produced a total of 10 publications in renowned peer-review journals, 7 invited talks, 5 contributed talks and several highlights in scientific outlets.
Initially the work focused on tailoring electromagnetic fluctuation-induced interactions using nano-structured materials. Since the behavior of a field strongly depends on the boundary conditions it has to fulfill, changing the system's geometry can lead to a modification of phenomena such as the Casimir effect and the spontaneous decay (Purcell effect). Quantitatively, this is also indicated by a change in the system's local density of states, which shows enhancement in some ranges of frequencies (e.g. resonances) or suppression in others (band gaps). Examples of this behavior were addressed in three papers describing different experimentally relevant configurations. In the first paper (Davids et al., Opt. Express 2014) a metallic grating-planar mirror cavity configuration was considered, and the mixing of genuine (related to the material properties) and spoof (induced by the geometry) surface plasmons was investigated. The key-role played by the plasmons and the geometry in fluctuation-induced phenomena was also discussed by analyzing the dynamics of an emitter embedded in periodic nanostructures made by alternating metal-dielectric layers (Intravaia & Busch, PRA 2015). In this context, it was shown that the nonlocal properties of the metal can affect the spontaneous decay of the emitter, which also clearly carried a signature of the model used for the description. In the third paper, the dynamics of defects embedded in an amorphous material with different dimensional features was addressed focusing not only on the impact of the fluctuations of the electromagnetic field but also on the effects induced by elastic vibrations and by the presence of other surrounding defects (Behunin et al., PRB 2016).
The interplay between material properties of the objects composing the system and fluctuation-induced interaction was one of the main focuses of the INPhAS project. It was demonstrated that, by analyzing their spontaneous decay, atomic-like systems can be very precise and accurate sensors for the investigation of graphene's plasmonic and opto-electronic properties in realistic scenarios (Werra et al., PRB Rapid Comm. 2016). These same properties are extremely relevant for describing the behavior of light in waveguiding systems that include graphene (Werra et al., J. Opt. 2016).
Recent years have also witnessed a growing research activity around non-equilibrium systems. In fluctuation-induced interactions, nonequilibrium physics can come into play, for example, when thermal gradients are induced, lasers are present or even just when one of the system's constituents is moving along a prescribed trajectory. Despite the presently intense interest, the theoretical framework is incomplete and, at least within the theoretical description of fluctuation-induced interactions, nonequilbrium physics still represents a largely uncharted territory. This has led to the proliferation of contradicting results as well as the emergence of controversies. Providing a consistent description of nonequilibrium fluctuation-induced phenomena was among the main targets of the INPhAS project. In this context a full quantum theory was developed to show how a thermal- or laser-induced population gradient in the plasmonic contribution modulates the atom-surface interaction, producing trapping configurations and other interesting effects (Bartolo et al., PRA 2016). Several investigations focused also on quantum friction, a drag force acting between two neutral non-touching objects in relative motion in vacuum. Using general results of quantum statistics it was proven that two of the most popular approaches of quantum optics (the Markov approximation) and nonequilibrium physics (the local thermal equilibrium approximation) fail in accurately describing the frictional force (Intravaia et al., PRA 2016 and PRL 2016). Interestingly, although these approximations have been used in several contexts to describe equilibrium and nonequilibrium configurations, their quality was never quantitatively assessed. Perhaps the most emblematic example is represented by the local thermal equilibrium (LTE) approximation. In general, indeed, the detailed quantitative description of nonequilibrium systems is rather challenging and therefore most common approaches rely on the assumption that corrections to the associated equilibrium properties are relatively small. Starting from this premise, the LTE approximation treats interacting subsystems within a nonequilibrum system as being locally in equilibrium with their immediate environment, allowing for a treatment that relies on general equilibrium results. However, calculations on an exactly solvable model have shown that the LTE approximation fails dramatically when applied to the study of quantum friction and leads to an underestimation of the interaction by almost 80%. Considering that this approximation has been the workhorse for the theoretical description of many nonequilibrium phenomena, ranging from thermal energy transport to non-equilibrium dispersion forces, these results demonstrate that LTE-based calculations lack rigorous justification and have to be re-examined. The importance of this result was acknowledged by a publication in Physical Review Letters and by coverage of these results in science, research and technology news media such as These investigations also revealed interesting information about the role of the trajectory followed by the flying objects and about fine cancellations in the power bookkeeping of the quantum frictional process (Intravaia et al., J. of Phys: Cond. Mat. 2015).
All these outcomes are also favoring the emergence of consensus around the description of quantum friction and also allowed to establish a connection among different approaches used in the literature. In addition, the increase in the understanding of the physics of the quantum frictional process has allowed to safely go beyond common descriptions. In deed, recently it was shown that spatial dispersion (nonlocality) in the material composing the objects can enhances drag between two objects in relative motion by three order of magnitude with respect to a local description (Reiche et al., PRB 2017).
Many other interesting activities have developed through the whole INPhAS project and, although they did not yet lead to publications, they have open the way to many promising investigations with an impact which will surely perpetrate in future work. As an example, relevant progress has also been made in the writing and testing of a numerical code that allows the calculation of Casimir force between arbitrarily shaped objects made of a generic material, offering considerable flexibility in the estimation of the interaction which can be achieved neither by analytic or semi-analytic calculations nor by the present numerical approach. Such a tool can be extremely relevant in the design of future nano-scale devices, with profound scientific and economic impact.
In addition to publications in international peer-reviewed journals and many papers in preparations, the success of the INPhAS project can be measured in terms on the numerous invited and contributed talks at international conferences and renewed institutions. An important element for this outstanding result has been the independence and the stimulating environment provided by the Theoretical Optics & Photonics group at the Humboldt University and Max Born Institute in Berlin. With the support of the group leader (Prof. K. Busch), the fellow has applied for and in some cases already won research grants, and has started the process which will allow him to obtain his habilitation degree. This, in conjunction with establishing a subgroup consisting of two PhD students, a bachelor students as well as one adjunct postdoctoral researcher (see the project’s webpage at has represented for the fellow a concrete and decisive step towards obtaining a stable, promising long term position.