Transition metal dichalcogenides (TMDCs) have emerged as a promising platform for quantum emitters, particularly single-photon sources, due to their unique electronic and optical properties at the monolayer limit. These materials, with the general formula MX2, where M is a transition metal (Mo, W, etc.) and X is a chalcogen (S, Se, Te), exhibit strong excitonic effects and valley-selective optical transitions. Quantum emitters in TMDCs can be broadly categorized into defect-induced emitters, strain-localized excitons, and those coupled to photonic structures, each offering distinct advantages for quantum photonic applications.
Defect-induced quantum emitters in TMDCs arise from localized states created by atomic vacancies, substitutional dopants, or adatoms. These defects break the translational symmetry of the crystal lattice, leading to discrete energy levels within the bandgap that can trap excitons. For example, sulfur vacancies in MoS2 have been shown to create deep-level states that act as single-photon emitters with narrow linewidths at cryogenic temperatures. The emission wavelength of these defect centers typically ranges between 1.6 and 1.8 eV, depending on the specific defect configuration and host material. The quantum efficiency of such emitters is influenced by the defect density and the surrounding dielectric environment, with encapsulation in hexagonal boron nitride (hBN) often improving stability and emission properties. Charge state control of these defects remains an active area of research, as it directly impacts the brightness and spectral stability of the emitted photons.
Strain engineering provides another powerful method to create quantum emitters in TMDCs. By applying localized strain, either through substrate patterning or nanopillar arrays, the band structure can be modified to form potential wells that confine excitons. The strain-induced bandgap modulation can exceed 100 meV per percent strain in monolayer TMDCs, enabling precise control over emitter wavelengths. Strain-localized emitters often exhibit higher brightness compared to defect-based emitters, as they do not rely on non-radiative defect sites. However, the spectral diffusion and linewidth broadening caused by strain fluctuations pose challenges for applications requiring indistinguishable photons. Recent advances in strain engineering techniques, such as atomic force microscope indentation and transfer onto pre-stretched substrates, have improved the reproducibility of these emitters.
Coupling TMDC quantum emitters to photonic structures enhances their performance by increasing photon extraction efficiency and enabling Purcell enhancement of the emission rate. Plasmonic nanocavities, dielectric resonators, and photonic crystal cavities have all been demonstrated to modify the emission properties of TMDC quantum emitters. The Purcell factor, which quantifies the enhancement in spontaneous emission rate, can reach values of 10 to 100 in optimized plasmonic systems. For dielectric cavities, quality factors exceeding 1000 have been reported, leading to significant linewidth narrowing of the coupled emitters. Waveguide integration allows for on-chip routing of single photons, a critical requirement for scalable quantum photonic circuits. The coupling efficiency between emitters and photonic modes depends strongly on the spatial overlap and spectral matching, with deterministic positioning techniques such as electron-beam-induced deposition showing promise for improving yield.
The spin-valley degree of freedom in TMDCs adds another dimension to quantum emitter functionality. Circularly polarized optical excitation can selectively address emitters in specific valleys, enabling polarization-encoded single-photon emission. The valley polarization lifetime varies from picoseconds at room temperature to nanoseconds at cryogenic temperatures, depending on the material and defect environment. Magnetic fields can further manipulate the valley states through Zeeman splitting, providing a means to tune emitter frequencies and control photon indistinguishability.
Temperature plays a crucial role in the performance of TMDC quantum emitters. At room temperature, phonon-induced dephasing typically limits the coherence time to sub-picosecond scales, resulting in broad emission lines. Cooling to liquid helium temperatures reduces phonon scattering, yielding linewidths as narrow as 100 μeV in some cases. The temperature dependence of emission energy follows a power-law behavior due to electron-phonon coupling, with coefficients varying between different TMDC materials.
Scalability remains a key challenge for TMDC-based quantum emitters. While individual emitters can exhibit high purity with g(2)(0) values below 0.1, achieving uniform arrays of identical emitters requires advances in defect engineering and strain patterning. Heterostructure engineering, where TMDC monolayers are combined with other 2D materials, offers additional control over the emitter environment through dielectric screening and charge doping.
Future developments in TMDC quantum emitters will likely focus on improving operating temperatures, increasing photon collection efficiency, and developing electrical excitation schemes. The integration of these emitters with other quantum systems, such as superconducting qubits or atomic ensembles, could enable hybrid quantum devices that leverage the advantages of different platforms. As fabrication techniques continue to mature, TMDC-based single-photon sources may find applications in quantum communication, computation, and sensing, benefiting from the material's inherent flexibility and compatibility with diverse photonic integration schemes.