Strong Light-Matter Coupling with Single-Walled Carbon Nanotubes

Abstract

Single-walled carbon nanotubes (SWCNTs) are a promising material for strongly coupled optoelectronic devices, due to their outstanding electrical properties in combination with their narrowband excitonic absorption and emission in the near-infrared. The rich SWCNT photophysics allow to study the interaction of exciton-polaritons with a range of other quasi-particles, such as phonons and biexcitons, as well as with synthetic, luminescent sp3 defects at room temperature. However, the ultimate goal of polariton condensation has not been achived with SWCNT exciton-polaritons so far, and hence understanding their specific polariton population mechanism with respect to their unique photophysical properties is crucial. Here, time-dependent fluorescence and transmission measurements are used to track the exciton-polariton population in strongly coupled metalclad microcavities, identify the dominant relaxation pathways and transitions, use luminescent sp3 defects to increase the polariton population by radiative pumping, and manipulate the SWCNT absorption edge by strong coupling in hybrid organic photodiodes. By investigating the fluorescence decay of SWCNT exciton-polaritons, it is shown, that the dominant population mechanism in this system is radiative pumping. To overcome the thusly imposed limitation of the polariton population by the low SWCNT photoluminescence quantum yield, the SWCNTs are functionalized with luminescent sp3 defects, leading to a population increase up to 10-fold for highly emissive detunings (photon fractions > 90%). By changing the substituents and the binding pattern, tuning of the defect emission could be further employed to access application-relevant near-infrared wavelengths and improve the conditions for polariton condensation. Furthermore, the SWCNT exciton-polariton dynamics are studied in the ultrafast regime by transient transmission spectroscopy. The results reveal a polariton-mediated biexciton transition, that is threefold more efficient than in weakly coupled SWCNTs. The polariton to biexciton transition under off-resonant polariton excitation also indicates fast population transfer from dark to bright polaritons beyond the exciton and photon dephasing times. The efficient biexciton transition of strongly coupled SWCNTs may enable to study correlated many-body states at room temperature, that are predicted for excitonic molecules in strongly coupled high quality cavities. Lastly, strongly coupled SWCNT hybrid organic photodiodes are presented, demonstrating how exciton-polaritons enable light-detection far beyond the intrinsic SWCNT absorption edge. For equal external quantum efficiency, photocarrier generation was observed 200 nm further into the near-infrared as compared to previously reported strongly coupled photodiodes. Thus, representing the first step towards efficient and tuneable polariton-mediated photocurrent generation by SWCNT hybrid organic photodiodes at application-relevant wavelengths