High-Q chiroptical cavities enabled strong valley-selective emission from monolayer MoSe2 at room temperature and revealed how photonic resonances can control exciton recombination.
Valleytronics uses the electronic valleys of two-dimensional materials as an additional degree of freedom for information processing. In monolayer transition metal dichalcogenides, the K and K′ valleys couple selectively to left-handed and right-handed circularly polarized light. At room temperature, however, rapid intervalley scattering and dephasing usually weaken this selectivity. This limits the development of practical valleytronic and chiral photonic devices.
Pan et al., Nature Communications (2026)1, addressed this problem by integrating monolayer MoSe2 with a chiral silicon metasurface that supports high-quality-factor quasi-bound states in the continuum, known as q-BIC modes.
A Chiral Cavity for Valley-Selective Emission
The silicon metasurface consisted of periodically arranged nanostructures with broken mirror and inversion symmetries. This geometry created a chiral optical resonance with a strong helicity-dependent near field. The resonance wavelength was designed to overlap with the A-exciton emission of MoSe2.
When the two resonances were spectrally aligned, the metasurface modified the optical local density of states experienced by the excitons. As a result, radiative transitions associated with one valley were enhanced more strongly than transitions from the opposite valley. The fabricated metasurfaces reached experimentally measured quality factors between approximately 200 and 450, substantially increasing the interaction between the MoSe2 excitons and the chiral optical mode.
Valley Selectivity at Room Temperature
The researchers measured the left-handed and right-handed circularly polarized components of the photoluminescence across temperatures from 100 K to 294 K.
At room temperature, the degree of optical circular polarization (DOP) reached values as high as 0.5. Bare MoSe2 monolayers on glass showed almost no comparable polarization contrast.Importantly, the effect did not require circularly polarized excitation. Similar polarization selectivity was observed for different excitation polarization states, indicating that the metasurface controlled the emission process rather than simply preserving an initially prepared valley population.
The strongest polarization contrast occurred when the chiral q-BIC resonance approached the MoSe2 exciton resonance. This demonstrates that both optical chirality and precise spectral alignment are required for effective valley-selective emission. Momentum-resolved measurements further showed that the metasurface controlled not only the polarization but also the angular distribution of the emitted light.
Time-Resolved Measurements Support the Mechanism
Spin- and time-resolved photoluminescence (TRPL) measurements were used to investigate whether the polarization contrast was connected to different recombination dynamics. The two circular polarization channels showed different decay times due to Purcell-Enhancement, with the contrast becoming most pronounced at room temperature. At selected positions, one polarization component decayed almost twice as fast as the other, showcasing spin-dependent near-field coupling of excitons with the q-BIC states. The emission lifetime of MoSe2 on the metasurface was also shorter than that of MoSe2 on glass. At 294 K, the reported lifetimes were approximately 0.189 ns and 0.398 ns, respectively.
These measurements support the interpretation that the chiral q-BIC mode enhances radiative recombination selectively for one valley. Based on assumptions regarding the nonradiative decay contribution, the authors estimated an approximately 13-fold increase in the radiative transition rate compared with MoSe2 on glass.
Toward Integrated Chiral Light Sources
The study demonstrates that planar silicon nanostructures can control the polarization, direction, and recombination dynamics of light emitted by a two-dimensional semiconductor. This approach could contribute to the development of compact chiral emitters, valleytronic devices, polarized light sources, and future quantum photonic systems.
The results also show why combining steady-state spectroscopy with time-resolved measurements is valuable. Steady-state photoluminescence reveals the polarization contrast, while TRPL provides additional evidence for the underlying modification of the recombination process.
Instrumentation Used in This Study by PicoQuant
Prima
PicoQuant’s picosecond laser Prima was used for time-resolved photoluminescence measurements.
The experimental configuration included:
- 640 nm pulsed excitation
- 100 ps pulse width
- 20 MHz pulse repetition rate
- 0.21 µW average excitation power
Prima enables pulsed and continuous-wave excitation at selectable visible wavelengths and supports repetition rates up to 200 MHz. It is designed for fluorescence and photoluminescence lifetime measurements in materials science and life science applications.
PicoHarp 300
Photon arrival times were recorded using PicoQuant’s PicoHarp 300 time tagger and TCSPC unit. Its current successor, the PicoHarp 330, offers 1 ps base resolution, up to four detector channels, and high throughput time tagging for TCSPC and time resolved photoluminescence applications.
1 Reference: Feng Pan, Xin Li, Amalya C. Johnson, Scott Dhuey, Ashley Saunders, Meng-Xia Hu, Jefferson P. Dixon, Sahil Dagli, Sze-Cheung Lau, Tingting Weng, Chih-Yi Chen, Jun-Hao Zeng, Rajas Apte, Tony F. Heinz, Fang Liu, Zi-Lan Deng & Jennifer A. Dionne. Room-temperature valley-selective emission in Si-MoSe2 heterostructures enabled by high-quality-factor chiroptical cavities. Nature Communications 17, 20 (2026). DOI: 10.1038/s41467-025-66502-4.
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