Visualizing Chiral Plexcitons at Room Temperature Using Tip-Enhanced Photoluminescence

Chirality is a fundamental property of nature, arising naturally in many systems, ranging from the structure of molecules to the spiral arms of galaxies. Structural chirality leads to circular dichroism, where photons of right or left handedness experience contrasting refractive indices for a particular molecular enantiomer. This phenomenon plays a crucial role in the chemical identification of molecules in the food and drug sciences. In the field of quantum optics, chiral single photon sources have gain increased interest due to their inherent non-repriciprocity¹, allowing for realization of novel optical devices such as single photon isolators and circulators². A requirement in many quantum optical applications is the realization of polariton states through strong coupling of a cavity optical field with a material dipole excitation. In this regime, substantial recent work has focused on plexcitons, strongly coupled excitons with nanocavity plasmon-polaritons, due their ultra-confined cavity volumes ( &lt;10 nm³). These small volumes allow for strong coupling to be readily achieved at elevated temperatures, with Rabi splittings of 50 – 400 meV in a wide array of excitonic systems ranging from quantum dots^3,4, monolayer transition metal dichalcogenides⁵, to molecular J-aggregates⁶. However, despite the explosion of the scientific efforts on the topic, chirality in plexcitonic systems has yet to receive significant experimental attention.<br/>In this presentation I will show our recent experimental efforts on imaging chiral plexcitonic states formed with commercial CdSe/ZnS quantum dots with a gap-mode plasmonic cavity. Using tip-enhance photoluminescence (TEPL), we are able to map out the nanocavity-quantum dot response, revealing a naturally asymmetric spatial profile for nanocavities formed with commercially available silver nano-optical probes and gold substrates, a common cavity geometry. Our nano-optical measurements further show positional control of the quantum dot emission energy, with an upper and lower polariton branches forming in the TEPL spectrum depending on the probe-dot separation and orientation, demonstrating the plexciton nature of the emission. Using a novel nano-optical imaging method that integrates TEPL with single photon avalanche photodiodes (SPADs) and time correlated single photon counting hardware, we map out the degree of circular polarization as a function of probe-dot position, showing polarization ratios as large as 80%, that is controllable by varying the x,y,z position of the nano-optical probe. Our results demonstrate the ability to imprint chiral behavior on quantum-emitter-based polaritons using plasmonic nanocavities, and provide important insight into development of chiral quantum optical devices.<br/><br/>1. Lodahl, P. et al., <i>Nature</i> <b>541</b>, 473-480 (2017).<br/>2. Chen, D., He, R., Cai, H., Liu, X. & Gao, W., <i>ACS Nano,</i> <b>15</b>, 1912-1916 (2021).<br/>3. Park, K.-D. et al., <i>Science Advances</i>, <b>5</b>, eaav5931 (2018).<br/>4. Groß, H., Hamm, J. M., Tufarelli, T., Hess, O. & Hecht, B., <i>Science Advances</i> <b>4</b>, eaar4906 (2018)<br/>5. Qin, J. et al., <i>Physical Review Letters</i> <b>124</b>, 063902 (2020).<br/>6. Wersäll, M., Cuadra, J., Antosiewicz, T. J., Balci, S. & Shegai, T., <i>Nano Letters</i> <b>17</b>, 551-558 (2017).