Lasing with Resonant Feedback in Weakly Scattering Random Systems

Laser action in active random media in the weak scattering regime far from Anderson localization is investigated by coupling Maxwell’s equations with the rate equations of a four-level atomic system. We report systematic lasing action with resonant feedback and show that the lasing modes mostly consist of traveling waves spatially extended over the whole system. Next we address the question of the origin of the feedback mechanism in such a system where no disorder-induced long-lived resonances are available, and present strong evidence that they correspond to the quasimodes of the passive system. This in turn provides an original way to access the spatial distribution of the quasimodes of a non-Hermitian system.

Figure 1

For the spatial arrangement of the scatterers shown in the inset: (a) Dynamics of the field amplitude. (b) Emission spectrum in the steady state regime with one mode (thick line) and with several modes at higher pumping rate (thin line) compared to the gain profile (dotted line). (c) Spatial distribution of the field amplitude.

Figure 2

(a) Probability distribution of the phase and (b) spatial correlation function of the field $C_{\mathcal{E}}(t_0,t)$ for a lasing mode in the localized (dashed line) and in the diffusive (full line) regime.

Figure 3

Spatial correlation function of the field $C_{\mathcal{E}}(t_0,t)$ (full line) and total energy of the system (dashed line) as a function of time when excitation of the system is turned off. The initial field is (a) the laser field and (b) the field created by an arbitrary set of monochromatic sources at the laser frequency.