Tunable self-assembled Casimir microcavities and polaritons

Abstract

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

Tunable critical Casimir forces counteract Casimir-Lifshitz attraction

“…1a. The gold flakes are fabricated using a wet-chemical synthesis method 34 that produces cetrimonium bromide ligand molecules (CTAB) stabilized single crystalline gold flakes (diameter, d = 3 ± 2 µm) with average thickness of b = 34 ± 10 nm in aqueous solution 12 and of various geometrical shapes (SEM image in Fig. 2a, details in Methods IV B).…”

“…In fact, a microcavity is formed between the flake and the gold substrate, of which we measure the reflectivity spectrum to determine the spatial variation of the cavity thickness using a spectrometer (Methods IV E and Supplementary Fig. S3) 12 . The measured cavity modes in the reflection spectrum at different positions were analyzed by the standard transfer-matrix method 12 .…”

“…This has motivated several recent studies to search for repulsive Casimir-Lifshitz forces both in vacuum [5][6][7] and in media 8,9 . However, these repulsive forces have only been obtained for specifically-engineered systems, e.g., by a careful choice of the materials of the two interacting surfaces and the separating liquid 10 , by using metamaterials with negative refractive index that are not readily available 5 , by coating one of the surfaces with a low-refractive-index material 11 , or by adjusting the concentration of ligands in solution 12 . Recently, experimental studies have shown that a combination of attractive and repulsive Casimir-Lifshitz forces can be used to levitate particles away from a surface 11 , and that they can trap freely floating particles near surfaces leading to the formation of Fabry-Pérot cavities when multiple particles assemble on-top of each other 12 .…”

Analysis of the Classical Trajectory Treatment of Photon Dynamics for Polaritonic Phenomena

“…This has led to the establishment of a new field of research in chemistry in the past decade, known as polaritonic chemistry, or molecular polaritonics. The possible applications are everincreasing and just within the last year we have seen experimental demonstrations of new phenomena in different directions, including tunable self-assembled polaritons in microcavities [5,6], solar light-harvesting enabled via cavity-coupling [7], and large enhancement of ferromagnetism from collective strong-coupling [8]. In most cases, to achieve strong light-matter coupling a large number of molecules is required, exploiting the collective nature of the coupling that scales as the square-root of the number of molecules.…”

Switching and amplifying three-body Casimir effects

“…Recently, the Casimir effect was used to increase the quality factor of a mechanical resonator [12] and couple two separate mechanical resonators [13,14]. In addition, the Casimir effect has been utilized to realize nonlinear oscillation [15], quantum trapping and self-assembling [16,17]. While the paradigm of Casimir effect between two objects has been extensively explored [18][19][20][21][22][23][24][25], the Casimir force between three macroscopic objects has not been detected yet.…”