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Tunable self-assembled Casimir microcavities and polaritons


Spontaneous formation of ordered structures—self-assembly—is ubiquitous in nature and observed on different length scales, ranging from atomic and molecular systems to micrometre-scale objects and living matter1. Self-ordering in molecular and biological systems typically involves short-range hydrophobic and van der Waals interactions2,3. Here we introduce an approach to micrometre-scale self-assembly based on the joint action of attractive Casimir and repulsive electrostatic forces arising between charged metallic nanoflakes in an aqueous solution. This system forms a self-assembled optical Fabry–Pérot microcavity with a fundamental mode in the visible range (long-range separation distance about 100–200 nanometres) and a tunable equilibrium configuration. Furthermore, by placing an excitonic material in the microcavity region, we are able to realize hybrid light–matter states (polaritons4,5,6), whose properties, such as coupling strength and eigenstate composition, can be controlled in real time by the concentration of ligand molecules in the solution and light pressure. These Casimir microcavities could find future use as sensitive and tunable platforms for a variety of applications, including opto-mechanics7, nanomachinery8 and cavity-induced polaritonic chemistry9.

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Fig. 1: The self-assembled microcavity system and the physical mechanism behind its operation.
Fig. 2: Self-assembled trimer cavities.
Fig. 3: Self-assembled cavities in nanoflake-on-static-mirror configuration and the formation of polaritons.
Fig. 4: Actively tunable microcavities and polaritons.

Data availability

The set of experimental and calculated optical spectra, SEM images, optical images and numerical codes are available through with the identifier Additional data are available from T.O.S. upon request.


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The authors acknowledge K. Eliasson for help with Raman measurements, as well as E. Tornéus, A. B. Yankovich, P. Erhart and M. Käll for stimulating discussions. The authors acknowledge financial support from the Swedish Research Council (under the VR Miljö project, grant no. 2016-06059 to T.O.S; and the VR project, grant no. 2017-04545 to T.O.S), the Knut and Alice Wallenberg Foundation (project no. 2019.0140 to T.O.S.), and the Chalmers Excellence Initiative Nano.

Author information




B.M. and T.O.S. conceived the idea. B.M. fabricated the samples. B.M. and A.C. performed optical measurements. B.M. and B.K. investigated active tuning of the structures. D.G.B. performed theoretical analysis of the experimental data. B.M., D.G.B. and T.O.S. wrote the manuscript with input from all co-authors. T.O.S. supervised the study.

Corresponding author

Correspondence to Timur O. Shegai.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Jeremy Munday and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

This file contains Supplementary Notes 1 – 5, Supplementary Text, Supplementary Figures 1 – 25, Supplementary Tables 1 – 3, legends for Supplementary Videos 1 – 6 and Supplementary References.

Supplementary Video 1

A stable gold nanoflake dimer as well as formation of a dimer.

Supplementary Video 2

Relative displacement of top and bottom nanoflakes within a self-assembled dimer along x and y directions as a function of time. Note that while flakes can move and rotate with respect to each other, their relative displacement is always small in comparison to the lateral size of the flakes. This indicated the dimer stability not only in vertical, but also in lateral directions.

Supplementary Video 3

A stable gold nanoflake trimer.

Supplementary Video 4

Relative displacement of top, middle, and bottom nanoflakes within a trimer along x and y directions as a function of time. This video shows that the self-assembled trimer exhibits an equilibrium not only in vertical direction, but also in lateral directions.

Supplementary Video 5

Additional examples of stable self-assembled multi-stacks of gold nanoflakes.

Supplementary Video 6

Additional examples of stable self-assembled multi-stacks of gold nanoflakes.

Peer Review File

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Munkhbat, B., Canales, A., Küçüköz, B. et al. Tunable self-assembled Casimir microcavities and polaritons. Nature 597, 214–219 (2021).

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