From Single-Photon to Superradiant Light Emission in Dense WSe2 Quantum Emitter Arrays

Technical Summary: From Single-Photon to Superradiant Light Emission in Dense WSe2 Quantum Emitter Arrays

Problem Statement
Cooperative quantum light emission phenomena, particularly superradiance, have historically been difficult to realize in solid-state materials. The primary obstacles are emitter inhomogeneity and unavoidable mechanisms for dephasing and nonradiative decay. While progress has been made with epitaxial quantum dots, defect centers in wide-bandgap materials, and van der Waals heterostructures, achieving the necessary scalability, controllability, and uniformity of indistinguishable emitters in close proximity remains a significant challenge. Specifically, creating dense arrays of quantum emitters with sufficient strain uniformity and density to transition from isolated single-photon behavior to cooperative emission has been a major hurdle.

Methodology
The authors developed a platform utilizing strained monolayer tungsten diselenide (1L WSe2) suspended over dense nanocavity arrays. The key innovation lies in the method of strain generation and control:

• Device Fabrication: Hexagonal nanocavity arrays were fabricated on SiO2/Si substrates using nanosphere lithography. Two distinct array pitches were created: 370 nm (cavity diameter 185 nm) and 200 nm (cavity diameter 100 nm), with a uniform cavity depth of 50 nm.
• Electrostatic Gating: Exfoliated 1L WSe2 was transferred onto these substrates. A vertical electric field was applied via a back-gate bias ($V_g$) to the Si substrate, inducing an attractive electrostatic force that deflects the suspended monolayer downward into the cavities. This deflection generates localized tensile strain.
• Strain Control: The magnitude of the strain is dynamically controlled by varying the back-gate voltage. AFM measurements confirmed that voltages of 10 V and 27.5 V induced deflections of 2 nm and 3.5 nm, corresponding to tensile strains of 0.1% and 0.3%, respectively.
• Experimental Design: The study compared two regimes based on the ratio of inter-emitter spacing ($d$) to emission wavelength ($\lambda$). Computational studies suggest a critical threshold ($d_{critical} \approx 0.3\text{--}0.4\lambda$) for superradiance. The authors fabricated devices with $d/\lambda = 0.5$ (expected to show isolated behavior) and $d/\lambda = 0.26$ (expected to show cooperative behavior).

Key Results
The study successfully demonstrated the transition from isolated single-photon emission to superradiant emission by manipulating inter-emitter spacing and electrostatic gating:

1. Isolated Single-Photon Emission ($d/\lambda = 0.5$):

   • In the 370-nm pitch array, emitters behaved as isolated defects.
   • Photoluminescence (PL) showed sharp localized peaks (e.g., at 1.63 eV) with a linewidth of 2.33 meV.
   • Time-resolved PL revealed a slow decay component of $\tau_2 = 2.14 \pm 0.18$ ns, characteristic of independent radiative recombination.
   • Second-order correlation measurements confirmed strong antibunching with $g^{(2)}(0) = 0.24 \pm 0.03$, meeting the criterion for single-photon emission.

2. Superradiant Emission ($d/\lambda = 0.26$):

   • In the 200-nm pitch array, where spacing was below the critical threshold, the system exhibited signatures of cooperative emission.
   • Lifetime Reduction: The radiative lifetime dropped dramatically to the picosecond range ($117\text{--}128$ ps), compared to the nanosecond range of isolated emitters. Based on the scaling law $\tau_{coupled} \propto \tau_{uncoupled}/N$, the authors estimated approximately 17 emitters were participating in the cooperative burst.
   • Photon Bunching: Second-order correlation measurements showed a transition from antibunching to photon bunching, with $g^{(2)}(0)$ increasing from $1.43 \pm 0.03$ at $V_g = 0$ V to $1.67 \pm 0.02$ at $V_g = 20$ V.
   • Intensity Enhancement: Peak emission intensity increased significantly compared to isolated emitters.

3. Electrostatic Control and Exciton Redistribution:

   • Applying a positive back-gate bias ($V_g > 0$) increased the brightness of low-energy emitters (1.48 eV) while decreasing the intensity of higher-energy emitters (1.63 eV).
   • This behavior was attributed to exciton redistribution: electrostatic gating modulates local strain and potential well depths, funneling photogenerated excitons toward energetically favorable, deeply bound defect states.
   • The low-energy emitter at 1.48 eV also exhibited a Stark shift of 9.23 meV, indicating sensitivity to the electric field due to a larger out-of-plane dipole moment.
   • Increased gate voltage enhanced the cooperativity, evidenced by a further reduction in lifetime and an increase in the bunching peak intensity.

Significance and Claims
The paper claims to provide a precise and controllable approach for creating arrays of quantum emitters based on two-dimensional materials. By suspending 1L WSe2 over nanocavity arrays and utilizing electrostatic interactions, the authors achieved:

• Dynamic Strain Engineering: The ability to create and control uniform local strain distributions across different nanocavities without permanent physical deformation.
• Observation of the Transition: Experimental evidence of the transition from isolated single-photon behavior to cooperative superradiant emission by tuning inter-emitter spacing relative to the emission wavelength.
• Tunability: A mechanism to enhance superradiant emission through exciton redistribution via electrostatic gating, effectively increasing the population of emitters participating in the cooperative state.

The authors conclude that this platform offers a versatile architecture for investigating and engineering cooperative quantum light emission, potentially serving as a foundation for future quantum photonic and optoelectronic devices that require densely packed, interacting quantum emitters. They note that while the critical distance regime for superradiance was observed between $0.26\lambda$ and $0.5\lambda$, further investigation is required to precisely define this threshold.