Electrically tunable dipolar polaritons with giant nonlinearity in a homobilayer microcavity

Active control over strong optical nonlinearity in solid-state systems is central to unlocking exotic many-body phenomena and scalable photonic devices. While exciton-polaritons in transition metal dichalcogenides (TMDs) offer a promising platform, their practical utility is often impeded by fixed interaction parameters and an intrinsic trade-off between nonlinearity and oscillator strength. Here, we report electrically tunable dipolar polaritons in a dual-gated bilayer MoS2 microcavity, demonstrating in situ reshaping of the dispersion and modulation of the light-matter coupling strength via the quantum-confined Stark effect. Crucially, this architecture enables a giant polariton-polariton interaction strength tunable by a factor of seven. This nonlinearity enhancement arises from a synergistic interplay, in which the electric field amplifies the microscopic dipolar repulsion while simultaneously optimizing the macroscopic excitonic Hopfield coefficient. Furthermore, electrostatic doping serves as an independent control knob to switch the system between strong and weak coupling regimes. Our findings bridge the gap between strong optical coupling and giant dipolar nonlinearities, establishing the TMD homobilayer as a versatile platform for engineering programmable correlated many-body states on a chip.

💡 Research Summary

The authors demonstrate electrically tunable dipolar polaritons in a dual‑gated bilayer MoS₂ microcavity. A natural 2H‑MoS₂ homobilayer is encapsulated in hBN and sandwiched between top and bottom graphene gates, allowing independent control of the out‑of‑plane electric field and carrier density. The heterostructure is placed inside a planar cavity formed by a nine‑period DBR bottom mirror and a 50 nm silver top mirror. Reflectivity measurements at 9 K reveal strong A‑ and B‑intralayer excitons and an interlayer exciton (IE) with ~35 % of the A‑exciton oscillator strength. Applying a vertical electric field splits the IE into two branches (IE_L and IE_H) with opposite dipole orientations, producing a characteristic X‑shaped Stark splitting. The permanent dipole moment grows from ~0.40 e·nm at zero field to ~0.48 e·nm at 0.6 V nm⁻¹, indicating that the exciton wavefunction becomes more interlayer‑like (interlayer weight rises from ~61 % to ~72 %).

Ultrafast pump‑probe experiments, resonant with the B‑exciton, show that the IE_L resonance blueshifts with increasing pump fluence. At zero field the shift is modest (~1.4 meV), whereas at 0.6 V nm⁻¹ it reaches ~5.2 meV. By extracting the shift versus exciton density, the polariton‑polariton interaction strength g increases from 0.29 µeV µm² to 1.81 µeV µm²—a seven‑fold enhancement. This giant nonlinearity originates from two synergistic effects: (i) the electric field enlarges the permanent dipole, strengthening dipole‑dipole repulsion, and (ii) the Stark‑induced splitting lifts the degeneracy between IE_L and IE_H, causing the exciton population to concentrate in the lower‑energy IE_L branch. The latter suppresses the attractive interaction between opposite‑dipole species that otherwise compensates the repulsion, thus unmasking the full dipolar repulsion.

Carrier‑density control via electrostatic doping shows a pronounced asymmetry: electron doping rapidly quenches both the A‑exciton and IE due to Pauli blocking and screening in the K‑valley, while hole doping leaves them largely intact because injected holes occupy the Γ‑valley, which is energetically far from the optical K‑valley transitions. Consequently, the device can be switched electrically between strong‑coupling (dipolar polariton) and weak‑coupling (bare photon) regimes.

Time‑resolved measurements reveal that higher electric fields not only increase the initial blueshift magnitude but also prolong the recovery time, reflecting reduced electron‑hole overlap and longer radiative lifetimes of the more dipolar excitons.

Overall, the work provides a versatile platform where both the light‑matter coupling strength (via the Hopfield coefficient) and the polariton‑polariton interaction can be programmed in situ. This capability opens pathways toward reconfigurable polaritonic lattices, on‑chip quantum simulators, deterministic polariton blockade, and scalable photonic logic devices.