AP Seminar - Chiroptical Spectroscopy of Two-dimensional Heterostructures and its Interaction with Plasmonic Nanocavity

The optical engineering of transition metal dichalcogenides heterostructures has shown great potential in the research of valleytronic devices and quantum optics. Here, we investigate the valley circular polarization of interlayer excitons in electrically controlled heterobilayers. We demonstrate the dependence of both polarization switching and degree of valley polarization on the moiré period, and also elucidate the experimentally observed twist angle-dependent valley polarization of interlayer excitons. The manipulation of the valley polarization of interlayer excitons by tunning twist angle in electrically controlled heterostructures could be useful in electrically controlling the valley degrees of freedom in twistronic devices [1]. Then, we investigate the coupling of single localized defect excitons in a two-dimensional layer with chiral plasmonic nanocavity. By designing a chiral plasmon mode with high quality factor and coupling it with a WSe₂ monolayer, we have successfully achieved cavity-dependent photon output from a single quantum emitter. The solution of the dynamics model of this coupled system shows that the chiral photon output mainly comes from the spontaneous emission of the quantum emitter which is strongly modified by the chiral plasmonic nanocavity. This also proves that the valley-dependent optical selection rules are absent in such quantum emitters [2].

The heterostructures consisting of two-dimensional layer and plasmonic nanocavity also have the potential to realize a coupling interaction at the level of single quanta. We fabricated a compact bowtie nanocavity and successfully realized a strong coupling of MoS₂ excitons and bowtie nanocavity with high robustness by using gold-assisted mechanical exfoliation and nondestructive wet transfer techniques. The number of excitons involved in the coupling is reduced to about 40-48, which is one of the lowest values for this type of work [3]. By constructing a hybrid cavity with a one-dimensional photonic crystal cavity and a plasmonic nanocavity, we effectively reduce the mode volume of bowtie nanocavity by coupling with Bloch surface waves [4]. The number of excitons contributing to the strong coupling has been reduced down to the single-digit level.

References

[1] D. Dai, B. Fu, J. Yang et al., Twist angle-dependent valley polarization switching in heterostructures, Science Advances, 10, eado1281 (2024).

[2] L. Yang, Y. Yuan, B. Fu et al., Revealing broken valley symmetry of quantum emitters in WSe₂ with chiral nanocavities, Nature Communications, 14, 4265 (2023).

[3] L. Yang, X. Xie, J. Yang et al., Strong light-matter interactions between gap plasmons and two-dimensional excitons under ambient conditions in a deterministic way, Nano Letters, 22, 2177-2186 (2022).

[4] Bowen Fu, Wenshuo Dai, Longlong Yang, et al., Enhanced Light–Matter Interaction with Bloch Surface Wave Modulated Plasmonic Nanocavities. Nano Letters (2025).