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

The work investigates whether strong light-matter coupling in a simple, cavity-free thin film of organic molecules can generate and allow all-optical control of phase singularities in photonic dispersion plots. Phase singularities—points of zero reflectance amplitude with undefined phase—are valuable for sensing and flat optics but typically require carefully engineered nanostructures or multilayers and are observable only under narrow conditions. The authors propose using leaky electromagnetic modes supported by a thin molecular film to strongly couple to the film’s own molecular resonance, thereby creating polariton branches that host phase singularities. By employing photochromic molecules to tune the coupling strength optically, they aim to demonstrate controllable creation of phase singularities without complex nanofabrication.

Singular optics has largely focused on real-space optical vortices, but phase singularities also emerge in parameter-space dispersion plots, enabling sharp phase jumps advantageous for sensing and flat optics. Phase singularities have previously been realized via Brewster angle effects, surface plasmon resonances, plasmonic lattices, transition metal dichalcogenides, optical Tamm states, and Fabry–Pérot microcavities. These typically require lithography, self-assembly, or multilayer stacks and are observable only under specific conditions (e.g., discrete angles). Strong coupling is commonly achieved with microcavities or plasmonics; recent advances show cavity-free strong coupling can occur where material-supported modes couple to the material’s own resonance. Prior work has used photochromic switching to control strong coupling or lasing; however, using strong coupling specifically to create and control phase singularities in dispersion plots had not been reported.

- Materials and film fabrication: Spiropyran (SPI) embedded in a PMMA matrix (SPI:PMMA weight ratio 3:2) was dissolved in toluene and spin-coated (three layers at 2000 rpm) onto silicon wafers, yielding film thicknesses from 84 to 681 nm. SPI is photochromic: under UV irradiation it converts to merocyanine (MC), which exhibits a strong molecular resonance near 2.22 eV; visible light reverses MC to SPI. - Optical characterization: Spectroscopic ellipsometry (J.A. Woollam M-2000XI) measured Ψ and Δ from 210–1690 nm. Ellipsometry records ρ = tan(Ψ) e^{iΔ} = r_p / r_s, comparing p- and s-polarized reflection coefficients. The xenon source provides low-intensity UV, enabling in situ SPI→MC conversion while monitoring optical response. - Optical constants and modeling: SPI permittivity modeled as a Cauchy dielectric; MC as a sum of a Lorentz oscillator resonant at E_L ≈ 2.215 eV and a UV pole with background ε_∞. Parameters were obtained via ellipsometric fitting. Fresnel calculations simulated dispersion (Ψ, Δ) versus energy and thickness; MC concentration was modeled by varying Lorentz oscillator strength. - Strong coupling analysis: Leaky transverse electric (TE) and transverse magnetic (TM) modes in SPI/MC films on Si generate dispersion features (maxima in Ψ for TE, minima for TM). Anticrossing with MC resonance indicates strong coupling. A 2N coupled-oscillator model was used to fit polariton branches and extract coupling strengths g for TE and TM modes. Strong-coupling resolution was assessed by comparing g to the linewidths of the molecular resonance (γ_MC) and leaky modes (γ_mode). - Experimental protocol for phase singularities: Time-resolved ellipsometry at fixed angle θ = 65° tracked the evolution of Ψ, Δ, and complex ρ as SPI converts to MC. Phase singularities were identified where Ψ approached ~0 and ρ passed through the origin, and topological charge C was determined by the phase winding of Δ around singularities in (energy, time) or (energy, thickness) parameter space. - Dispersion mapping: Δ (and Ψ) were measured across films with L from 84–680 nm at multiple UV exposure times (e.g., t = 10, 330, 440, 1100 s) to map creation of singularities across TM1–TM3 leaky modes and validate with Fresnel modeling using varied MC oscillator strength.

- Strong coupling without external cavities: Thin SPI/MC films on Si support leaky modes that strongly couple to the MC resonance at E_MC ≈ 2.22 eV, yielding polariton branches with clear anticrossings (especially TE modes and TM2, TM3). - Coupling strengths and linewidths: From coupled-oscillator fits, TE branches show g ≈ 225 meV; TM2 and TM3 show g ≈ 185 meV and 200 meV, respectively. Reported linewidths: γ_MC ≈ 360 meV; γ_TM2 ≈ 350 meV; γ_TM3 ≈ 240 meV; γ_TM1 ≈ 430 meV. TM2 and TM3 satisfy the stated strong-coupling resolution criterion, while TM1 does not show clear anticrossing. - All-optical control via photoisomerization: UV irradiation converts SPI to MC, increasing oscillator strength and driving transition from weak to strong coupling. In a 407 nm film at θ = 65°, as UV exposure increases, a single TM2 leaky mode splits into upper and lower polariton branches; Ψ minima drop below 0.1° at singularity points. - Creation of phase singularities: Two phase singularities emerge shortly after entering strong coupling: at t ≈ 470 s on the lower polariton and at t ≈ 510 s on the upper polariton. They correspond to points with Ψ ≈ 0 and ρ ≈ 0. The phase winding yields topological charges C = +1 and C = −1, preserving total topological charge. - Parameter-space topology of ρ: As MC concentration increases, the trajectory of ρ evolves from a perturbed arc (weak coupling) to a loop (strong coupling); when the loop crosses ρ = 0, two singularities are guaranteed. - Thickness-dependent mapping: Across films with 84 nm < L < 680 nm at θ = 65°, phase singularities first appear with the best-confined TM1 mode (splitting observed at intermediate times) and then with TM2; clear singularities tied to TM1 are not always unambiguous at the highest MC levels, consistent with limited anticrossing. Fresnel simulations reproducing experimental Δ maps confirm that increasing MC oscillator strength generates pairs of singularities at each anticrossing with charges +1 and −1. - Practical implication: The phase sensitivity of thin films can be tuned by incident angle and molecular concentration, enabling a simple route to phase-singularity-based functionalities (e.g., sensing, flat optics) without complex nanostructures.

The study demonstrates that entering the strong light-matter coupling regime in a simple, cavity-free thin film can deterministically generate phase singularities in dispersion (parameter) space. By exploiting photochromic SPI→MC conversion, the number of coupled molecules—and thus the coupling strength—can be controlled all-optically, enabling the timed creation of pairs of singularities with opposite topological charges on the polariton branches. These singularities are analogous to those previously engineered in complex plasmonic or multilayer systems, but here they arise from intrinsic coupling between leaky modes and a molecular resonance within the same film. The findings extend singular optics by showing that phase singularities can be manipulated in the time domain over seconds and in parameter space, complementing prior spatiotemporal singularity observations on femtosecond scales. This approach offers a versatile, low-complexity platform to achieve large phase responses for sensing and flat-optics applications, with the prospect of reversible control via visible-light back-conversion to SPI.

Molecules in a simple thin film can strongly couple to the film’s own leaky modes, and the onset of strong coupling creates pairs of phase singularities with opposite topological charges on the resulting polariton branches. Using photochromic molecules enables all-optical control over the coupling strength, permitting the creation and tuning of phase singularities across film thicknesses and modes without external cavity structures. This provides a new application of strong coupling and a streamlined, versatile platform for singular optics with potential impact on sensitive phase-based detection and flat optical components. Future work could exploit reversible MC→SPI switching for dynamic creation/annihilation of singularities, explore different incident angles, polarization configurations, and materials systems (e.g., other resonant organics or 2D materials), and integrate the platform into practical sensing architectures.

- Only the forward photoisomerization (SPI→MC) was demonstrated; reversible visible-light back-conversion was not implemented during measurements. - Clear strong-coupling anticrossing and associated singularities were not observed for all modes (e.g., TM1 remained ambiguous under some conditions), limiting universality across modes and thicknesses. - Measurements focused on a fixed incident angle (θ = 65°); broader angular mapping would further generalize the findings. - Identification of singularities relied on ellipsometric detection of very small Ψ (≈0.1°) and ρ ≈ 0; while ellipsometry is sensitive, experimental noise and instrument UV output may constrain temporal control and detection thresholds. - The strong-coupling criterion and extracted parameters depend on model assumptions (optical constants, oscillator-strength mapping to MC concentration, and coupled-oscillator fits), which may introduce model-dependent uncertainties.