Raman Probing the Local Ultrastrong Coupling of Vibrational Plasmon Polaritons on Metallic Gratings.

Strong coupling of molecular vibrations with light creates polariton states, enabling control over many optical and chemical properties. However, the near-field signatures of strong coupling are difficult to map as most cavities are closed systems. Surface-enhanced Raman microscopy of open metallic gratings under vibrational strong coupling enables the observation of spatial polariton localization in the grating near field, without the need for scanning probe microscopies. The lower polariton is localized at the grating slots, displays a strongly asymmetric line shape, and gives greater plasmon-vibration coupling strength than measured in the far field. Within these slots, the local field strength pushes the system into the ultrastrong coupling regime. Models of strong coupling which explicitly include the spatial distribution of emitters can account for these effects. Such gratings enable exploration of the rich physics of polaritons, its impact on polariton chemistry under flow conditions, and the interplay between near- and far-field properties through vibrational polariton-enhanced Raman scattering.

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

Strong light-matter coupling occurs when the rate of energy exchange between an electromagnetic mode and a molecular ensemble exceeds competing dissipative processes. The study of strong coupling has been motivated by applications such as lasing and the modification of chemical processes. Here we show that strong coupling can be used to create phase singularities. Many nanophotonic structures have been designed to generate phase singularities for use in sensing and optoelectronics. We utilise the concept of cavity-free strong coupling, where electromagnetic modes sustained by a material are strong enough to strongly couple to the material's own molecular resonance, to create phase singularities in a simple thin film of organic molecules. We show that the use of photochromic molecules allows for all-optical control of phase singularities. Our results suggest what we believe to be both a new application for strong light-matter coupling and a new, simplified, more versatile means of manipulating phase singularities.

Polariton assisted photoemission from a layered molecular material: role of vibrational states and molecular absorption.

The way molecules absorb, transfer, and emit light can be modified by coupling them to optical cavities. The extent of the modification is often defined by the cavity-molecule coupling strength, which depends on the number of coupled molecules. We experimentally and numerically study the evolution of photoemission from a thin layered J-aggregated molecular material strongly coupled to a Fabry-Perot microcavity as a function of the number of coupled layers. We unveil an important difference between the strong coupling signatures obtained from reflection spectroscopy and from polariton assisted photoluminescence. We also study the effect of the vibrational modes supported by the molecular material on the polariton assisted emission both for a focused laser beam and for normally incident excitation, for two different excitation wavelengths: a laser in resonance with the lower polariton branch, and a laser not in resonance. We found that Raman scattered photons appear to play an important role in populating the lower polariton branch, especially when the system was excited with a laser in resonance with the lower polariton branch. We also found that the polariton assisted photoemission depends on the extent of modification of the molecular absorption induced by the molecule-cavity coupling.

Cavity-Free Ultrastrong Light-Matter Coupling.

Strong coupling between light and matter can occur when the interaction strength between a confined electromagnetic field and a molecular resonance exceeds the losses to the environment, leading to the formation of hybrid light-matter states known as polaritons. Ultrastrong coupling occurs when the coupling strength becomes comparable to the transition energy of the system. It is widely assumed that the confined electromagnetic fields necessary for strong coupling to organic molecules can only be achieved with external structures such as Fabry-Pérot resonators, plasmonic nanostructures, or dielectric resonators. Here we show experimentally that such structures are unnecessary and that a simple dielectric film of dye molecules supports sufficiently modified vacuum electromagnetic fields to enable room-temperature ultrastrong light-matter coupling. Our results may be of use in the design of experiments to probe polaritonic chemistry and suggest that polaritonic states are perhaps easier to realize than previously thought.

Strong Coupling beyond the Light-Line.

Strong coupling of molecules placed in an optical microcavity may lead to the formation of hybrid states called polaritons; states that inherit characteristics of both the optical cavity modes and the molecular resonance. Developing a better understanding of the matter characteristics of these hybrid states has been the focus of much recent attention. Here, as we will show, a better understanding of the role of the optical modes supported by typical cavity structures is also required. Typical microcavities used in molecular strong coupling experiments support more than one mode at the frequency of the material resonance. While the effect of strong coupling to multiple photonic modes has been considered before, here we extend this topic by looking at strong coupling between one vibrational mode and multiple photonic modes. Many experiments involving strong coupling make use of metal-clad microcavities, ones with metallic mirrors. Metal-clad microcavities are well-known to support coupled plasmon modes in addition to the standard microcavity mode. However, the coupled plasmon modes associated with a metal-clad optical microcavity lie beyond the light-line and are thus not probed in typical experiments on strong coupling. Here we investigate, through experiment and numerical modeling, the interaction between molecules within a cavity and the modes both inside and outside the light-line. Making use of grating coupling and a metal-clad microcavity, we provide an experimental demonstration that such modes undergo strong coupling. We further show that a common variant of the metal-clad microcavity, one in which the metal mirrors are replaced by distributed Bragg reflector also show strong coupling to modes that exist in these structures beyond the light-line. Our results highlight the need to consider the effect of beyond the light-line modes on the strong coupling of molecular resonances in microcavities and may be of relevance in designing strong coupling resonators for chemistry and materials science investigations.

Vibrational Strong Coupling with Surface Plasmons and the Presence of Surface Plasmon Stop Bands.

We demonstrate strong coupling between surface plasmon resonances and molecular vibrational resonances of poly(methyl methacrylate) (PMMA) molecules in the mid-infrared range through the use of grating coupling, complimenting earlier work using microcavities and localized plasmon resonances. We choose the period of the grating so that we may observe strong coupling between the surface plasmon mode associated with a patterned gold film and the C=O vibrational resonance in an overlying polymer film. We present results from experiments and numerical simulations to show that surface plasmon modes provide convenient open cavities for vibrational strong coupling experiments. In addition to providing momentum matching between surface plasmon modes and incident light, gratings may also produce a modification of the surface plasmon properties, notably their dispersion. We further show that for the parameters used in our experiment surface plasmon stop bands are formed, and we find that both stop-band edges undergo strong coupling.