Photoexcitation of moiré-trapped interlayer excitons via chiral phonons

Abstract

Moiré superlattices in transition-metal dichalcogenide semiconductor heterobilayers enable the quantum confinement of interlayer excitons with large out-of-plane permanent electric dipoles and spin-valley control. Here, we report a novel phonon-assisted excitation mechanism of individual moiré-trapped interlayer excitons in 2H-stacked MoSe$_2$/WSe$_2$ heterobilayers via chiral $E^{\prime\prime}$ in-plane optical phonons at the Γ-point. This excitation pathway preserves valley-selective optical selection rules and enables deterministic generation of individual interlayer excitons with defined helicity, emitting within a spectrally narrow energy spread. Through photoluminescence excitation spectroscopy in both the ensemble and quantum emitter regimes, we identify a fixed phonon energy of $\sim$23 meV mediating the process. First-principles calculations corroborate the symmetry and energy of the relevant phonon mode and its coupling to interlayer excitons, providing microscopic support for the observed valley-selective phonon-assisted excitation mechanism. Our results highlight the utility of chiral phonons as a tool for controlled excitation of quantum emitters in TMD moiré systems, opening new opportunities for valleytronic and quantum photonic applications.

Figures (5)

• Figure 1: (a) Sketch of a twisted 2H-MoSe$_2$/WSe$_2$ heterostructure encapsulated by hBN. (b) Schematics of the type-II electronic band-edge alignment and spin-valley configuration in 2$H$-MoSe$_2$/WSe$_2$ at the corners of the hexagonal Brillouin zone corresponding to $K$ in WSe$_2$. The double-ended vertical blue and red arrows denote optical transitions corresponding to intralayer bright exciton states in WSe$_2$ and MoSe$_2$, respectively. The double-ended diagonal purple and green arrows represent optical transitions and optical selection rules arising from spin-singlet and spin-triplet neutral interlayer excitons pinned to a moiré site with an atomic registry $H^h_h$yu2018brightenedseyler2019signatureszhang2019highlybrotons2020spinbaek2020highlybrotons2021moire. The helicity of the optical transitions is reversed for optical transitions at the opposite corner of the Brillouin zone. (c) PL spectra of interlayer excitons in the 2$H$-MoSe$_2$/WSe$_2$ sample under resonant excitation of the intralayer A exciton in monolayer MoSe$_2$ for excitation powers of $P=24$$\mu$W (left) and at $P=7$ nW (right). The different color-shaded Lorentzian peaks indicate ensemble PL emission from different interlayer exciton resonances.
• Figure 2: (a) False colour-plot of the IX PL spectra recorded with respect to the CW-laser excitation energy ($E_{exc}$ under a constant power of 25 $\pm$ 3 $\mu$W on the sample. (b) Representative IX PL spectrum (gray line, left axis) superposed with the combined integrated PL intensity of the IX$^0_T$ and IX$^-_T$ ensemble peaks as a function of excitation energy (green dots, right axis). The black solid line represents a four-Lorentzian-peak fit of the integrated PL intensity. (c) Zoom in of panel (b) in the energy range 1.41 - 1.44 eV, as indicated by the red square in (b). (d) Excess energies measured for IX$^0_T$ and IX$^-_T$ as a function of E$_{exc}$.
• Figure 3: (a) Schematic representation of the $E^{\prime\prime}$ phonon mode in the MoSe$_{2}$/WSe$_{2}$ heterobilayer. The arrows indicate the displacement pattern of the Se atoms associated with this vibrational mode. (b) Absorbance spectrum of the pristine MoSe$_{2}$/WSe$_{2}$ heterobilayer, with the main excitonic resonances labeled. A broadening of 0.03 eV was employed for the intralayer excitons, while a smaller broadening of 0.001 eV was used for the interlayer excitons in order to properly resolve their weak absorption features. The inset displays the differential absorbance between the pristine structure and the configuration distorted according to the $E^{\prime\prime}$ phonon eigenvector. A pronounced variation at the IX$^{0}_{T}$ peak reveals strong exciton--phonon coupling between the interlayer exciton and this phonon mode. (c) Illustration of the different excitonic states (orange area) mapped onto the layer-resolved band structure, confirming the origin and composition of each labeled exciton.
• Figure 4: Helicity-resolved photoluminescence spectra of interlayer excitons in the ensemble regime under different resonant excitation conditions. (a)–(c) Circularly polarized PL spectra recorded under $\sigma^{+}$ excitation resonant with the WSe$_2$ intralayer exciton X$_W^0$ (a), the MoSe$_2$ intralayer exciton X$_M^0$ (b), and the interlayer singlet exciton IX$_S^0$ (c), respectively. Panels (a)–(c) illustrate excitation pathways that do not involve phonon angular momentum transfer and therefore follow the optical selection rules expected for direct electronic transitions in 2H-MoSe$_2$/WSe$_2$. (d) Helicity-resolved PL spectrum under excitation resonant with IX$_{ph}$, corresponding to chiral-phonon–assisted excitation. The resulting cross-circularly polarized emission reflects the transfer of pseudo-angular momentum from the $\Gamma$-point chiral phonon to the excitonic system. (e),(f) Schematic illustrations of the excitation–recombination pathways for resonant excitation of IX$_S^0$ (e) and phonon-assisted excitation of IX$_{ph}$ leading to IX$_T^{-}$ formation (f).
• Figure 5: Excitation resolved photoluminescence resolved in polarization. Excitation is $\sigma^+$ circularly polarized and PL is detected in either $\sigma^-$ (a) or $\sigma^+$ (b) polarization. Red dotted line corresponds to $E_{exc}-$30 meV and the white dotted line corresponds to $E_{exc}-$23 meV. Single emitter energies follow better the red dotted line indicating the formation of trions. (c) Valley polarization as a function of the excitation energy. (d) Example of single emitter spectrum for $E_{exc}=1.4226$ eV.