Optical Absorption Line Shape of a Λ-Type Three-Level Semiconductor Quantum Dot: Roles of Transition Dipole Moment and Relaxation Rate

The optical absorption of an isolated Λ-type three-level semiconductor quantum dot under continuous-wave excitation was investigated theoretically using a semiclassical density-matrix approach. The equations of motion were derived from the master equation within the rotating-wave approximation and solved in the steady-state regime to obtain the optical susceptibility and absorption spectrum. The roles of the transition dipole moment and relaxation rate were examined systematically in both weak- and strong-field regimes. In the weak-field regime, the spectrum was dominated by the ∣1⟩↔∣3⟩ transition, while a weaker secondary feature associated with the ∣2⟩↔∣3⟩ transition emerged through relaxation-assisted population redistribution. Increasing the relaxation rate broadened and suppressed the dominant peak, whereas increasing the transition dipole moment mainly broadened the weaker secondary feature. In the strong-field regime, the two absorption channels merged into a broadened composite profile due to the combined effects of stronger coherent driving, power broadening, and relaxation. These results clarify how transition strength and dissipation govern the absorption line shape of an elementary multilevel semiconductor quantum-dot system.