Invited Presentation: Prolonged Spontaneous Emission and Dephasing of Excitons in Air-Bridged Swcnts
Authors: Sarpkaya Ibrahim , Zhang Zhengyi , Walden-Newman William , Hone James , Wong Chee-Wei , Strauf Stefan 
Published: February 2020

Carbon nanotubes have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices. To date, the majority of experiments reveal however low quantum efficiencies and strong dependencies on the environment and chemical processing. This is very detrimental for optoelectronic device applications dominant non-radiative optical recombination processes lead to strongly reduced optical emission rates of nanoscale light sources. Likewise, non-radiative recombination leads to reduced light-to-current conversion efficiencies in photodetectors and solar cells.
The question arises to what extent previous experiments have so far probed for the true intrinsic properties of the exciton dynamics. In particular, theorists predicted about a decade ago spontaneous emission (SE) lifetimes of excitons of about 10-100 ns while most measurements found orders of magnitude faster lifetimes varying from 20 to 200 ps. In earlier work we have demonstrated how polymer capping of SWCNTs can be used to strongly reduce detrimental environmental interaction effects such as spectral diffusion and blinking [1]. Although these samples showed higher exciton emission efficiencies, the corresponding SE lifetime remained with 30 ps much faster than theoretical predictions, implying that they are still dominated by nonradiative exciton recombination.
To overcome these limitations we have combined sophisticated growth of ultra-clean SWCNTs bridging an air gap over pillar posts with time-resolved photoluminescence measurements of individual SWCNTs over 14 orders of magnitude. In this talk we will discuss our recent finding of the onset of a remarkable new regime of intrinsic exciton photophysics with prolonged SE times up to 18 ns, about two orders of magnitude longer than reported before [2]. While these intrinsic optical lifetimes had been predicted by theorists about a decade ago, they had never been reported due to the masking by extrinsic effects of defects and unintentional doping.
Measurements of exciton emission linewidth versus pump power demonstrated that air-bridged SWCNTs have significantly narrower linewidths than polymer-embedded SWNTs. The later display a rather large linewidth up to 10 meV, while the exciton linewidth of SWCNTs bridging an air gap reaches down to 220 μeV at low pump powers, if they are grown for short times of only 2 minutes. It appears that our best air-bridged samples are only at the onset of this intrinsic photophysics regime. Nanosecond long SE lifetimes have very recently also independently confirmed by Hofmann et al. [3], who also report even narrower spectral linewidth down to 40 meV. Future work has thus much room for improvements to reach deeper into the intrinsic regime and explore the intrinsic limits.
With these supposedly ultraclean air-bridged samples we also carried out first experiments on exciton dephasing times for individual SWCNTs in the time-domain. The interferometric measurements demonstrated exciton dephasing times up to 2 ps, about four-fold longer than in previous ensemble studies (500 fs). The ultra-narrow linewidth and prolonged exciton dephasing times suggest that the acoustic phonon coupling giving rise to exciton dephasing is significantly weaker than previously believed, which is promising news for device applications, and raises the question about the ultimate limit for SE lifetimes and exciton dephasing in the intrinsic regime.

References

[1] W. Walden-Newman, I. Sarpkaya, and S. Strauf, Quantum light signatures and nanosecond spectral diffusion from cavity-embedded carbon nanotubes, Nano Letters 12, 1934 (2012).
[2] I. Sarpkaya, Z. Zhang, W. Walden-Newman, X. Wang, J. Hone, C.W. Wong, and S. Strauf, Prolonged spontaneous emission and dephasing of excitons in air-bridged carbon nanotubes, Nature Communications 4, 2152 (2013), doi:10.1038/ncomms3152
[3] M. S. Hofmann, J. T. Glückert, J. Noé, C. Bourjau, R. Dehmel, A. Högele, Bright, long-lived and coherent excitons in carbon nanotube quantum dots, Nature Nanotechnology 8, 502-505 (2013), doi:10.1038/nnano.2013.119