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Abstract
The model based on the method of finite-difference time-domain is applied to calculate the general feature of localized modes in active random media. The evolution of the localized modes from passive random media to active ones is investigated. By varying the gain lineshape for an active random medium, we have observed a selective excitation of localized modes, including both single-mode and multi-mode excitations. When the eigenfrequencies for some localized modes are near the centre of the gain spectrum, the optical gain offers coherent amplification to them so that they can obtain more amplification and have stronger field amplitude than other modes. The pumping level dependence of the field amplitude of the localized modes exhibits a threshold behaviour.
Acknowledgments
The National Natural Science Foundation of China under Grant No. 60378001 and Hubei Natural Science Foundation of China under Grant No. 2001ABA003 have supported this research.