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Unlike conventional lasing, polariton lasing does not require population inversion, instead needing only strong coupling between the exciton state and a cavity photon mode. Ultra-low energy thresholds are, therefore, possible. To this end, we have identified perylene dyes as ideal materials that offer high Rabi splitting (evidence of strong light matter coupling) > 140 meV while still retaining high quantum yield > 70%. By using molecularly insulated perylenes dispersed in polystyrene matrix sandwiched between DBR mirrors (Q factor ~ 600), we demonstrate polartion lasing evidenced by threshold behavior, strong directionality, and blue-shift in emission.

Collaborators: Wallace Wong (University of Melbourne), Tim Schmidt (University of New South Wales)

One method to reduce laser thresholds in traditional lasing is to incorporate small emissive dye molecules into a guest–host matrix. Injection of charges into the host matrix would result in the formation of excitons that could then be transferred to host dye molecules coupled to the cavity, enabling coherent emission from the lower polariton band. We demonstrate that a polariton system can exhibit efficient host (F8BT)-to-guest (DPP) energy transfer while maintaining strong exciton–polariton coupling and emission. We expect that energy funnelling will become an important tool to drive down polariton laser thresholds in organic systems.

Collaborators: Ebinazar Namdas and Shih-Chun Lo (University of Queensland)

In this space, our focus has been on identifying device geometries that can enable electrical injection. To obtain low lasing thresholds, light must be confined in the vicinity of excitons, i.e., the recombination zone, to achieve light amplification. Using Limerical simulations and experiments, we demonstrated improvements to optical confinement in OFETs using high refractive index cladding layers. We show that introducing this layer does not introduce any optical losses, which may further attenuate the confined light compared to the standard structures. 

Industry partners: BluGlass Pty Ptd (Sydney)

Thermally activated delayed fluorescence (TADF) emitters suffer from molecular aggregation that limits their applicability in light-emitting devices. Aggregation-induced excimer formation often leads to a more significant Stokes shift, broader emission spectrum, and reduced emission quantum yields, limiting emitter dye loading to a few weight percent in organic light emitting devices. We show that under a strong light-matter coupling regime, prompt and delayed emission through excimer states is suppressed due to efficient energy transfer to the lower polariton (LP) states, demonstrated by the emission spectrum’s blue shift and the emission linewidth’s narrowing. We also observe an increase in reverse intersystem crossing (RISC) rate constants up to 33%, which we attribute to a decrease in activation energy by ≈2kT. This work highlights that strong light-matter interactions can be exploited to overcome aggregation-induced excimer losses, providing a pathway towards efficient organic light-emitting diodes with high colour purity and organic semiconductor polariton lasing.

Collaborators: Wallace Wong and Ken Ghiggino (University of Melbourne)

We show strong coupling in solution-processed donor:acceptor bulk-heterojunction organic solar cells (OSCs) can effectively modify the device and photophysics in OSCs, opening a new pathway for engineering more efficient OSC. Our results of combining transient photovoltage decay measurements and nanosecond transient absorption spectroscopy reveal that the effective charge carrier lifetimes are longer in cavity devices due to reduced bimolecular recombination.

Collaborators: Tim van der Laan (CSIRO)

Together with Stefan Meskers (TU Eindhoven) we worked out a theoretical framework that describes the reflection and refraction beyond the traditional Fresnel equations unifying the optics of designer and conventional materials. We calculated the reflection and refraction of light from a surface of oriented Lorentz oscillators for frequencies near the resonance of the oscillators by solving the controversial additional boundary conditions for exciton polaritons using vector potentials rather than fields. Reflection of light from a uniaxial material results in a spectrum featuring a characteristic minimum in the middle of the reflection band that is in agreement with experiments (as shown in figure below). The minimum in reflection is related to the excitation of polaritons in the crystal.

Molecules whose chiroptical signal can be controllably and reversibly altered are of great interest in emerging optoelectronic technologies. Using a range of chiroptical, structural, electrochemical, and spectroelectrochemical techniques, we show that redox reactions of chiral molecular analogues of naphthalimide, ferrocenes and metal organic framework (MOFs) cause substantial changes in their optical and chiroptical properties, enabling them to perform as highly sensitive and reversible redox-modulated chiroptical switches.

Collaborators: Deanna D’Alessandro (University of Sydney), Bun Chan (Nagasaki University)

Faraday Rotation is a type of magneto-optical phenomenon that rotates the polarization of light in the presence of magnetic field that is parallel to the propagation direction of light. Faraday rotators are used at the source of broadband and other communication technologies, blocking reflected light that would otherwise destabilise lasers and amplifiers. They are also used in optical switches and fibre-optic sensors. The global optical switches market alone is worth more than $US4.5 billion and is growing. We demonstrated, for the first time, Faraday rotation in lead bromide perovskites. We showed that they can rival commercial TGG standards for certain wavelengths in the visible spectrum. We carried out temperature dependent studies and found that degree of optical rotation obtained in methylammonium lead bromide perovksites is less dependent on temperature compared to TGG.

Collaborators: Udo Bach (Monash University), Anita Ho-Baillie (University of Sydney) and Makhsud Saidaminov (University of Victoria, Canada)

Singlet fission (SF), the conversion of one singlet exciton into two triplets, offers a promising pathway to overcome thermalization losses and surpass the detailed balance limit. Insights into the SF process have been developed in the last few years led by investigations of the excited state dynamics in designer molecular systems where SF-based chromophores are covalently linked by a bridging molecule. However, little is known about chromophore-bridge interactions that is crucial towards the rational design of efficient SF-based chromophores. We study a designer pentacene dimer with chiral binaphthyl unit acting as the bridge. With the help of ground and excited state circular dichroism (CD), we sensitively probe SF chromophore-bridge interactions and demonstrate that a significant contribution from the bridge to the high-energy singlet transitions. We propose that the wavefunction overlap between the pentacene units and the axially chiral (binaphthyl) bridge aids in the significant suppression of the triplet-triplet recombination and emphasise the role of the frontier molecular orbitals of the bridge on the decay dynamics of the TT state. Our work underlines the promising role of chirality and chiroptical techniques to sensitively investigate chromophore-bridge interactions and the impact on multiexciton dynamics.

Collaborators: Lüis Campos (Columbia University), Akshay Rao (Cambridge University), Dane McCamey and Murad Taybjee (University of New South Wales) and Stavros Anthanasopoulos (Madrid University).

Supramolecular gels are an important class of soft materials composed of hierarchically ordered three-dimensional networks of individual gelator molecules useful for a wide ranging of applications in catalysis, sensing and drug delivery. Insights into the nature of molecular packing across different length scales are crucial to achieve control over the final assembled gel structure. Using a combination of chiroptical techniques – CD, FDCD and MCD – we comprehensively demonstrate stepwise hierarchical self-assembly of perylene imides to gels. This study offers insight into gelation mechanism, which will be relevant to the formation of fibrillar networks and other large supramolecular structures in general.

Collaborators: Pall Thordarson (University of New South Wales), Markus Müllner and Asaph Widmer-Cooper (University of Sydney).

Together with Stefan Meskers (TU Eindhoven), we have work towards developing models to understand the origin behind the chiroptical phenomena in different materials. In the past, we have used a classical coupled oscillator model to explore the possibility of a general theory that can unify two century-old standalone theories on exciton coupling at a molecular level and liquid crystals at a mescoscopic length scale. Bringing together elements from spectroscopy and  theory, we showed that the crossover in optical properties at different length scales can be explained in a phenomenological way using an anisotropic dielectric tensor. This  contribution was featured in J. Phys. Chem. A (116, 1121) with a front cover.