Fig. 1. Schematics for some fundamental properties of 2D semiconductors. (a) Schematic of atomic structure of TMD MX2[13]; (b) upper: triangular prismatic structure and honeycomb lattice structure of monolayer MoS₂; lower: hexagonal structure of valley band of degenerate states (K and K') and selection rule of valley band transition^[36]; (c) 2H phase of monolayer TMD hexagonal lattice and SHG under symmetry breaking of spatial inversion^[37]; (d) schematic of single photon properties of defect states in 2D materials^[38]

Fig. 2. Optical control for 2D semiconductors. (a) Relationship between PL intensity of charged exciton and exciton X⁰ and gate voltage Vg[72]. Solid line is model curve predetermined by theory; (b) electrical control of SHG resonance intensity with gated voltage^[49]; (c) schematic of Zeeman effect under magnetic field for degenerate valley electronic energy band^[80]; (d) schematic of Stark effect under ultrafast optical field for degenerate valley electronic energy band^[81]

Fig. 3. Fundamental features of 2D heterostructures. (a) Configuration of energy band of type-II heterostructures. Magnified image depicts spatially indirect interlayer exciton^[98]; (b) schematic of twist angle-dependent Moiré lattice in 2D heterostructures

Fig. 4. Schematics of various micro/nano-cavities. (a) Schematic of FP optical cavity with two DBRs; (b) schematic of WGM cavity formed by total reflection along ring perimeter; (c) schematic of PCC with photonic crystal defect nanocavity^[6]; (d) schematic of PCC of BIC mode^[122]; (e) schematic of plasmonic nanocavity^[126]

Fig. 5. Schematics of CQED principles. (a) Schematic of coupling process between excitons and cavity photons; (b) schematic of fluorescence enhancement of Purcell effect in weak coupling regime; (c) schematic of anti-crossed exciton polaritons in strong coupling regime. P₊ (P_-) represents upper (lower) polaritons

Fig. 6. Weak coupling regime of 2D semiconductors and dielectric micro/nano-cavities. (a) Purcell effect of weak coupling between monolayer MoS₂ and PCC nanocavity. Purcell effect has polarization dependence on optical cavity^[141]; (b) schematic of ultra-low threshold laser of monolayer WSe₂ in PCC nanocavity^[143]; (c) schematic of monolayer WS₂ microdisk laser^[144]; (d) schematic of CW near-infrared laser of monolayer MoTe₂ nanobeam cavity at room temperature^[145]; (e) structural diagram of monolayer WS₂ VCSEL^[146]

Fig. 7. Weak coupling regime of 2D semiconductors and plasmonic nanocavities. (a) Purcell effect of weak coupling between plasmonic nanocavity and 2D semiconductor for enhancing Raman and fluorescence intensities^[149]; (b) control of valley degree of freedom through coupling between plasmonic nanocavity and 2D semiconductor^[140]; (c) weak coupling of quantum emitters with defect in monolayer WSe₂ and plasmonic nanocavities^[69]

Fig. 8. Strong coupling regime of 2D semiconductors and plane FP microcavities. (a) Structural diagram for first demonstration of strong coupling between monolayer MoS₂ and FP microcavity at room temperature^[155]; (b) structural diagram for strong coupling between monolayer MoSe₂ and open FP microcavity^[156]; (c) schematic of 2D polarized exciton polaritons with valley degree of freedom in strong coupling regime; (d) schematics of nonlinear optical principles of 2D polarized exciton polaritons in strong coupling regime^[161]; (e) optical valley Hall effects based on nonlinear optical response of 2D exciton polaritons in strong coupling regime^[162]; (f) structural schematic of open FP microcavity for control of 2D exciton polariton and polaron via carrier concentration in strong coupling regime^[163]; (g) structural schematic of sample for realization of electrically pumped 2D exciton polaritons in strong coupling regime^[164]

Fig. 9. Strong coupling regime of 2D semiconductors and micro/nano-cavities with different structures. (a) Sample schematic for realizing strong coupling between monolayer WSe₂ and plasmonic nanocavity^[173]; (b) sample microscopic image with strong coupling between monolayer MoSe₂ and plasmonic array^[178];(c) sample schematic for the strong coupling between monolayer TMD and one-dimensional PCC^[135]; (d) sample schematic with strong coupling between monolayer WS₂ and BSW mode in DBR substrate^[183]

Fig. 10. Laser generated by coupling between PCC nanocavity and interlayer excitons of 2D semiconductor heterostructures. (a) Sample schematic for lasing actions based on coupling between 2D semiconductor WSe₂/MoSe₂ heterostructure and one-dimensional PCC nanocavity^[184]; (b) schematic for lasing principles based on coupling between 2D semiconductor WSe₂/MoS₂ heterostructure and two-dimensional PCC nanocavity^[185]; (c) left: schematic for measurements of Michelson interferometer; right: interference pattern of indirectly exciton lasing, which indicates spatial coherence of interlayer excitonic laser^[184]