Intrinsic circularly polarized exciton emission in a twisted van der Waals heterostructure

We report the emergence of a significant degree of intrinsic circular polarization of exciton photoluminescence in a twisted ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructure upon nonresonant driving with a linearly polarized laser. The effect is not related to the polarization of the incident light. Moreover, it is present at zero magnetic field, and reacts perceptibly to a perpendicularly applied magnetic field that, unexpectedly, can strongly diminish this effect. The giant magnitude of the polarization, which cannot be explained by natural optical activity or circular dichroism of the twisted lattice, suggests a kinematic origin arising from an emergent pyromagnetic symmetry in our structure, which we exploit to gain insight into the microscopic optical processes of our device.

Figure 1

Description of the sample and determination of the heterostructure twist angle. (a) Microscope image of the sample, the hBN/${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$ flake limits are marked in a dashed yellow/green/red line. (b) Linear polarization analysis of the second harmonic generated light from freestanding monolayer regions of ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ (see green and red dots in panel (a), respectively, where the measurements are performed). The polar plots correspond to the normalized intensities versus the analyzed linear polarization orientation. The twist angle is (5.3 ± 1.4)°.

Figure 2

Photoluminescence of intra- and interlayer excitons. (a) Comparison of low temperature photoluminescence of interlayer and intralayer (see labels ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$) excitons. The red dots (right vertical axis) represent the IX PLE spectrum, revealing two resonances (highlighted in a green and magenta band and spectrally resonant to the ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$-X energies, respectively) where carrier transfer occurs. (b) [left] Plot of the optical resonances obtained from the calculated band structure. [center] Corresponding IX band-structure. [right] Experimental spectrum reproduced from panel (a) presenting with Lorentzian fits, matching the resonance energies and dispersion at the $K$ points of the corresponding reduced Brillouin zone.

Figure 3

Circular polarization analysis of the IX spectrum. (a) The sample is driven by a linear polarized laser, and luminescence is recorded under ${\sigma }^{+}$ and ${\sigma }^{-}$ circular-polarization detection (orange and blue traces, respectively). The lower panel depicts the corresponding DCP as a function of the emission energy. Three energy regions I, II, and III are marked in green, magenta, and cyan colors. The laser excitation wavelength is 730 nm and the continuous wave pump power is 0.53 μW, the measurement is taken under zero magnetic field. (b),(c) DCP dependence versus magnetic field from the energy regions II and III. The DCP values are averaged at each energy in a spectral interval of 20 meV around the marked energy regions. (d) and (e) DCP simulation of excitons as a function of ${\mathrm{\Omega }}_{\mathrm{B}}\tau$, represented according to Eq. (3) for the angles between the eigenframes of the UES and LES doublets $\alpha ={45}^{\circ }$ and $5^{\circ }$ and ${\mathrm{\Omega }}_{\mathrm{ex}}\tau =0.1,0.3,1.0$ (see panel legends).

Figure 4

Microscopic model of kinetic spin polarization of excitons. (a) Exciton alignment along $x$ in the longitudinal-transverse-split upper exciton state (UES) followed by quantum beats in the rotated longitudinal-transverse-split lower exciton state (LES) leads to exciton spin polarization. (b) DCP simulation as a function of $\alpha$ (angle between the $x$ and $x^{\prime }$ axes) and ${\mathrm{\Omega }}_B\tau$ (spin precession frequency in the external magnetic field) for ${\mathrm{\Omega }}_{ex}\tau =1.$