Mid-infrared (fingerprint region) -  Molecular vibrational strong coupling   

•  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 lineshape, 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 form a new system for exploring the rich physics of polaritons and the interplay between their near- and far-field properties through polariton-enhanced Raman scattering (PERS). 

• Probing Vibrational Strong Coupling of Molecules with Wavelength‐Modulated Raman Spectroscopy 

We use a powerful new variant of Raman spectroscopy, wavelength-modulated Raman spectroscopy (WMRS), to investigate molecular resonances in photonic nano- and micro-structures. Specifically, we study strong-coupling between the vibrational modes in a polymer film and two types of confined light field, the fundamental mode of a metal‐clad microcavity, and the surface‐plasmon mode of a metallic grating.

• • • 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. 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. 

• • • Hybridization of Multiple Vibrational Modes via Strong Coupling Using Confined Light Fields

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. This is possible for both excitonic and vibrational molecular resonances. Previous work has shown that strong coupling may be used to hybridize two different excitonic resonances; this can be achieved when more than one molecular species is included in the cavity. Here it is shown that under suitable conditions three different molecular vibrational resonances of the same molecular unit may also be coupled together, the resulting polariton having characteristics of all three vibrational resonances. These results suggest that strong coupling might be used to manipulate vibrational resonances in a richer and subtler way than previously considered, opening a path to greater control of molecular systems and molecular processes via vibrational strong coupling. 

• • • Strong Coupling of Vibrational Modes

Visible- Excitonic strong coupling

• Beyond the Cavity: Molecular Strong Coupling using an Open Fabry-Perot Cavity 

The coherent strong coupling of molecules with confined light fields to create polaritons – part matter, part light – is opening exciting opportunities ranging from extended exciton transport and inter-molecular energy transfer to modified chemistry and material properties. In many of the envisaged applications open access to the molecules involved is vital, as is independent control over polariton dispersion, and spatial uniformity. Existing cavity designs are not able to offer all of these advantages simultaneously. Here we demonstrate an alternative yet simple cavity design that exhibits all of the the desired features. We hope the approach we offer here will provide a new technology platform to both study and exploit molecular strong coupling. Although our experimental demonstration is based on excitonic strong coupling, we also indicate how the approach might also be achieved for vibrational strong coupling. 

• Molecular strong coupling: an exploration employing open, half-and full planar cavities 

We explore the modification of photoluminescence from dye-doped open, half and full optical microcavities. For each configuration, an analysis of the reflectivty data indicates the presence of strong coupling. We find that an open cavity, i.e. a dye-doped dielectric slab placed on a silicon substrate, shows little if any modification of the photoluminescence spectrum. For the half-cavity, in which a thin metallic film is placed between the dye-doped dielectric and the substrate, we find a limited degree of modification of the photoluminescence. For the fullcavity, for which the dielectric layer is bound both above and below by a metallic mirror, we find very significant modification to the photoluminescence emission, the photoluminescence clearly tracking the lower polariton. To learn more about the photo-physics we compare reflectivity, absorption and photoluminescence dispersion data, making use where appropriate of numerical modelling. We discuss our results in the context of harnessing open cavities to modify molecular photo-physics through molecular strong coupling. 

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

Strong light-matter coupling occurs when the coupling strength between a confined electromagnetic mode and a molecular resonance exceeds losses to the environment. 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 a new application for strong light-matter coupling and a new, simplified, more versatile pathway to singular phase optics. 

• 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. 

• 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. 

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

Strong light-matter coupling occurs when the coupling strength between a confined electromagnetic mode and a molecular resonance exceeds losses to the environment. 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 a new application for strong light-matter coupling and a new, simplified, more versatile pathway to singular phase optics.

2D research

• High-harmonic generation enhancement with graphene heterostructures 

We investigate high-harmonic generation in graphene heterostructures consisting of metallic nanoribbons separated from a graphene sheet by either a few-nanometer layer of aluminum oxide or an atomic monolayer of hexagonal boron nitride. The nanoribbons amplify the near-field at the graphene layer relative to the externally applied pumping, thus allowing us to observe third- and fifth-harmonic generation in the carbon monolayer at modest pump powers in the mid-infrared. We study the dependence of the nonlinear signals on the ribbon width and spacer thickness, as well as pump power and polarization, and demonstrate enhancement factors relative to bare graphene reaching 1600 and 4100 for third- and fifth-harmonic generation, respectively. Our work supports the use of graphene heterostructures to selectively enhance specific nonlinear processes of interest, an essential capability for the design of nanoscale nonlinear devices