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), optical switches, energy transfer, singlet fission, and device physics of solar cells.

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.

Collaborators: Wallace Wong (Uni Melb), Tim Schmidt (UNSW)

Host-Guest energy transfer

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 (Uni Queensland)

Multi-resonance TADF in optical cavities

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 (Uni Melb)

Organic solar cells under strong light-matter coupling regime

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)

Device geometries for lasing

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)

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. Strong Light–Matter Coupling Leads to a Longer Charge Carrier Lifetime in Cavity Organic Solar Cells Y Tang, AN Stuart, T van der Laan, G Lakhwani ACS Photonics 11, 1627 (2024)
2. Multi-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states I Cho, WJ Kendrick, AN Stuart, P Ramkissoon, KP Ghiggino, WWH Wong, G Lakhwani J Mater Chem C 11, 14448 (2023)
3. Molecular Energy Transfer under the Strong Light–Matter Interaction Regime DJ Tibben, GO Bonin, I Cho, G Lakhwani, J Hutchison, DE Gómez Chem. Rev. 123, 8044 (2023)
4. Distributed feedback lasers up to the 400th Bragg order with an organic active layer Y. Li and G. Lakhwani, Appl. Phys. Lett. 122, 021108 (2023) – Editor’s Pick
5. Active waveguide Bragg lasers via conformal contact PDMS stamps – Y. Li and G. Lakhwani, Sci. Rep. 12, 22189 (2022)
6. 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)
7. 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)
8. 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)
9. 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)
10. 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)