Polaritons are half-light/half-matter quasiparticles formed when a photon strongly couples with an electric dipole which occurs in bulk molecular semiconductors in the form of excitons. In our research, we aim to exploit strong exciton-photon coupling in organic semiconductor Fabry Perot cavities on various photophysical processes, such as coherent emission (lasing), energy transfer, singlet fission, and device physics of solar cells. This research is a dedicated program within ARC Centre of Excellence in Exciton Science.

Exciton polartions in organic semiconductors and lasing

New dyes and optically pumped lasing: 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.

Host-Guest energy transfer: In traditional lasing, one method to reduce laser thresholds is to incorporate small emissive dye molecules into a guest–host matrix. Injection of charges into the host matrix would result in formation of excitons that could then be transferred to host dye molecules that are coupled to the cavity, enabling coherent emission to occur from lower polariton band. To this end, we show DPP dyes embedded in F8BT

Towards electrically injected lasers: In this space, our focus has been on identifying device geometries that can enable electrical injection. To obtain low lasing thresholds, it is essential to confine light in the vicinity of excitons, i.e. recombination zone to achieve light amplification. Using Lumerical 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. 

Collaborators: Wallace Wong (Uni Melb), Tim Schmidt (UNSW), Ebinazar Namdas and Shih-Chun (Lawrence) Lo (Uni Queensland)

Industry partners: BluGlass Pty Ptd (Sydney)

Cavity solar cells

Polaritons have been shown to modify the energy landscapes and allow energy to be harvested over longer distances. We aim to study its impact on device physics of organic solar cells, in particular, on charge photo generation and charge transport.

Reflection of light by anisotropic molecular crystals including exciton polaritons

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.

Selected publications

1. Y.Li, R.P. Sabatini, S.K.K. Prasad, E.T. Hockings, T.W. Schmidt, G. Lakhwani Improved optical confinement in ambipolar field-effect transistors toward electrical injection organic lasers Appl. Phys. Lett. 119, 163303 (2021)
2. T. Yang, C. Jin, J. Qu, A. Asadpoordarvish, R. Sabatini, X. Zhang, H. Chen, S.P. Ringer, G. Lakhwani, F. Li, J. Cairney, X. Liu, R. Zheng Solution Epitaxy of Halide Perovskite Thin Single Crystals for Stable Transistors ACS Appl. Mater. Interfaces, 13, 37840 (2021)
3. R.P. Sabatini, F. Maasoumi, S.K.K. Prasad, B. Zhang, C. Clark, T.W. Schmidt, W.W.H. Wong, G. Lakhwani Organic polariton lasing with molecularly isolated perylene diimides Appl. Phys. Lett. 117, 041103 (2020)
4. M.S. Rahme, R.P. Sabatini, S. McGregor, R. Wawrzinek, E. Namdas, S.-C. Lo, G. Lakhwani Strong coupling and energy funnelling in an electrically conductive organic blend J. Mater. Chem C 8, 8953 (2020)
5. R. P. Sabatini, B. Zhang, A. Gupta, J. Leoni, W.W. H. Wong, G. Lakhwani, Molecularly Isolated Perylene Diimides Enable Both Strong Exciton-Photon Coupling and High Photoluminescence Quantum Yield, J. Mater. Chem C 7, 2954 (2019)
6. S.C.J. Meskers, G. Lakhwani, Reflection of light by anisotropic molecular crystals including exciton-polaritons and spatial dispersion. J. Chem. Phys. 145, 194703 (2016)