Phase singularities, points where the amplitude of reflected light is zero and the phase undefined, are crucial in singular optics for applications like sensing and flat optics. Traditional methods for generating these singularities involve complex nanostructures requiring lithography or self-assembly, limiting their versatility and applicability. This research explores a novel approach leveraging strong light-matter coupling, a phenomenon where energy exchange between electromagnetic modes and molecular ensembles surpasses dissipative processes, offering potential advantages in device miniaturization and control. The study investigates the feasibility of using strong light-matter coupling, specifically a cavity-free design, to generate phase singularities within a simple thin film of organic molecules. The chosen photochromic molecules allow for dynamic control over the phase singularities through all-optical means, enabling reversible creation and annihilation. This offers a significant advance over conventional methods, promising a simpler, more versatile, and potentially more scalable technology for manipulating phase singularities in various applications.

Numerous studies have explored phase singularities using diverse methods, including the Brewster angle, surface plasmon resonances, plasmonic lattices, transition metal dichalcogenides, optical Tamm states, and Fabry-Pérot microcavities. However, these approaches often rely on intricate nanostructure fabrication techniques, such as lithography, self-assembly, or multilayer deposition, making them complex and limiting their practical applications. Strong light-matter coupling, while explored for applications like lasing and chemical process modification, has not previously been used to create phase singularities. Existing strong coupling experiments typically utilize external structures such as microcavities or plasmonic nanostructures to confine electromagnetic fields. This work builds upon recent advancements in cavity-free strong coupling, demonstrating that such structures are not always necessary for achieving strong light-matter interaction and subsequent phase singularity generation.

The researchers used silicon substrates coated with a thin film of spiropyran (SPI) molecules, a photochromic molecule that can be reversibly switched between transparent SPI and strongly absorbing merocyanine (MC) forms using UV and visible light irradiation, respectively. The optical constants of both SPI and MC were determined using spectroscopic ellipsometry. Spectroscopic ellipsometry, employing a J.A. Woollam Co. M-2000XI ellipsometer, measured the polarization changes of reflected light to characterize the sample's optical properties. The parameters ψ (amplitude ratio of p- and s-polarized reflection coefficients) and Δ (phase difference between the coefficients) were used to construct dispersion plots, revealing the presence of phase singularities. A 2N coupled oscillator model was employed to fit the experimental data and extract relevant parameters such as coupling strengths. The photochromic nature of SPI allowed for in-situ control of the molecular concentration (and thus coupling strength) by exposing the film to UV light, enabling the observation of the transition between weak and strong coupling regimes. The film thickness was varied (84 nm to 680 nm) to study the impact on the generation and behavior of phase singularities. Fresnel calculations were conducted to corroborate experimental findings. The SPI/MC film fabrication involved spin-coating PMMA solutions containing SPI onto silicon wafers.

The study successfully demonstrated the creation of phase singularities through cavity-free strong light-matter coupling in a thin film of photochromic molecules. Clear anticrossing behavior in dispersion plots, a hallmark of strong coupling, was observed for both TE and TM leaky modes. The coupled oscillator fit revealed coupling strengths exceeding the relevant loss rates, satisfying the strong coupling criterion. Specifically, the TM2 and TM3 modes showed coupling strengths of 185 meV and 200 meV, respectively. The study observed the formation of phase singularity pairs with topological charges of +1 and -1, maintaining total topological charge conservation. The all-optical control, achieved by photoisomerization of SPI to MC using UV irradiation, allowed for the reversible tuning and detuning of the system into and out of the strong coupling regime, thereby creating and annihilating the phase singularities. The phase singularities were not observed in real space but in parameter space (dispersion plots) as a function of energy and film thickness. The experimental results were well-matched by theoretical simulations using a Fresnel approach by varying the Lorentz oscillator strength. The observations highlight the dynamic control over phase singularities within a simple, easily fabricated platform.

The findings address the research question by showcasing a novel approach for generating and controlling phase singularities using strong light-matter coupling in a straightforward thin-film system. The use of cavity-free strong coupling significantly simplifies the fabrication process compared to previous methods, potentially leading to more scalable and versatile singular optic devices. The all-optical control mechanism further enhances the practicality and tunability of the system. The results highlight a new application for strong light-matter coupling, pushing its boundaries beyond established uses. The qualitative similarities between the observed phase singularities and those reported in spatiotemporal studies underscore the universal nature of these phenomena.

This study successfully demonstrates the creation and all-optical control of phase singularities using strong light-matter coupling within a simple, cavity-free thin film of photochromic molecules. This novel approach offers a significant simplification in the generation of phase singularities, opening new avenues for singular optics applications. Future research could focus on exploring different photochromic molecules with varying properties, investigating the potential for enhanced sensitivity in sensing applications, and exploring the integration of this technology into more complex photonic devices.

The study primarily focuses on the forward photochemical reaction (SPI to MC). While the reversibility of this process is noted, a comprehensive investigation of the reverse reaction's impact on phase singularity control is warranted. The current analysis primarily uses a 2N coupled oscillator model. Further investigation may benefit from more sophisticated modeling to fully capture all aspects of the complex system. The range of film thicknesses studied might need to be expanded to encompass a wider range of conditions, providing a more comprehensive understanding of the phenomenon.