A New Signature for Strong Light–Matter Coupling Using Spectroscopic Ellipsometry

Abstract

Light–matter interactions can occur when an ensemble of molecular resonators is placed in a confined electromagnetic field. In the strong coupling regime the rapid exchange of energy between the molecules and the electromagnetic field results in the emergence of hybrid light–matter states called polaritons. Multiple criteria exist to define the strong coupling regime, usually by comparing the splitting of the polariton bands with the line widths of the uncoupled modes. Here, we highlight the limitations of these criteria and study strong coupling using spectroscopic ellipsometry, a commonly used optical characterization technique. We identify a new signature of strong coupling in ellipsometric phase spectra. The combination of ellipsometric amplitude and phase spectra yields a distinct topological feature that we suggest could serve as a new criterion for strong coupling. Our results introduce the idea of ellipsometric topology and could provide further insight into the transition from the weak to strong coupling regime.

All-optical control of phase singularities using strong light-matter coupling

“…After 400 seconds of UV exposure two distinct modes appear; as the MC concentration increases, the mode splitting increases. This behaviour is consistent with the transition from the weak to strong coupling regime [30]. The minimum value of Ψ drops dramatically in the strong coupling regime, with one point on each branch going below 0.1 • (comparable with experimental error).…”

“…Figure 2c shows ρ at four times: t = 10 s, when the film is almost entirely composed of SPI molecules; t = 220 s, when some SPI has been converted into MC but the system remains in the weak coupling regime; t = 510 s, coincident with the upper polariton phase singularity, when enough SPI has been converted to MC to place the system within the strong coupling regime; and t = 1100 s, the final measurement when most SPI has been converted into MC. The evolution of ρ during the transition from weak to strong coupling resembles what we have observed in other strongly coupled systems [30]: the initial TM 2 leaky mode is represented by an arc; in the weak coupling regime this arc is perturbed by the presence of MC; in the strong coupling regime a new loop is observed. As the MC concentration increases, the loop both increases in size and touches the point ρ = 0 for t = 510 s (green loop) and t = 470 s (not plotted for clarity).…”

“…The minimum value of Ψ drops dramatically in the strong coupling regime, with one point on each branch going below 0.1 • (comparable with experimental error). In Figure 2b we plot ∆, which shows the same phase behaviour between the two polariton branches that we have previously interpreted as a signature of strong coupling [30]. Shortly after entering the strong coupling regime, we observe two phase singularities: the first, at t = 470 s, on the lower polariton branch; the second, at t = 510 s, on the upper polariton branch.…”

Topological engineering of terahertz light using electrically tunable exceptional point singularities

“…The light component in these interactions is usually in the form of electromagnetic modes confined in a resonator, whereas the matter component involves a single or a mesoscopic number of oscillators. Changing the number of oscillators coupled to a resonator is one route for achieving strong or weak light-matter coupling ( 1 ); however, this is not desirable in many practical settings as it does not lend itself to tunable and finely controllable platforms that can enable study of both weak and strong coupling regimes as well as transitions between them. The alternative is to keep the number of oscillators fixed while tuning the coupling strength and loss imbalance between the oscillators and the resonator such that the coupled oscillator-resonator system is steered between the weak and strong coupling regimes.…”

Cavity-Free Ultrastrong Light-Matter Coupling

“…Ellipsometry is an ideal tool for our experiment. 28 It allows one to clearly distinguish between TM leaky modes (when r p → 0, tan(Ψ) → 0 and so Ψ → 0°) and TE leaky modes (when r s → 0, tan(Ψ) → ∞ and so Ψ → 90°) in the same amplitude spectrum. Each ellipsometry spectrum can be used to both characterize any strong coupling and determine the thickness and optical constants of the sample under interrogation.…”