Laser action in the regime of Anderson localization is investigated by coupling Maxwell's equations in a strongly scattering medium with the rate equations of a four-level atomic system. We find that the localized modes of the passive medium are not modified by the presence of gain and serve as resonant microcavities of the random laser. The spectrum of laser emission is shown to depend on the location of the external pump. These results are similar to the recent experimental observations in semiconductor powder [Cao et al., Phys. Rev. Lett. 82, 2278 (1999)]. We show that local pumping of the system allows selective excitation of individual localized modes.
Laser action in the regime of Anderson localization is investigated by coupling Maxwell's equations in a strongly scattering medium with the rate equations of a four-level atomic system. We find that the localized modes of the passive medium are not modified by the presence of gain and serve as resonant microcavities of the random laser. The spectrum of laser emission is shown to depend on the location of the external pump. These results are similar to the recent experimental observations in semiconductor powder [Cao et al., Phys. Rev. Lett. 82, 2278 (1999)]. We show that local pumping of the system allows selective excitation of individual localized modes.
Light propagation in matter is described by vibration eigenstates, called modes, which characterize the light-matter interaction. In the specific case of random media, according to the strength of the disorder, the modes can be either extended over the whole system or spatially localized. This disorder-based confinement is called Anderson’s localization.In the first part, we introduce basic notions used along this manuscript. In particular the light-matter interaction requires a semiclassical approach: The electromagnetic field is described by Maxwell’s equations while the quantum nature of matter must be considered. In this thesis open media are studied. In such systems the modal description requires a specific analytic treatment different from closed problems. In the second part, we focus on Anderson-localized modes in open passive random media. Any change of the disorder induces modifications of modes. Therefore, it enables the control over the light properties. Moreover, when inserting an emitter inside an Anderson-localized mode, strong light-matter interaction regimes can be reached. In the third part, active random media, commonly called random lasers, are introduced. Using our experimental achievements, characteristics of random lasers are presented. The notion of mode enables us to describe complex mechanisms involved in the lasing emission. Last, we demonstrate both experimentally and numerically that a non-uniform excitation of random lasers can lead to a control of the properties of the emission. In particular a multimode spectrum for a uniform pumping can be turned into single-mode using an adapted pumping.
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.
Based on the time dependent theory of random lasers, which was built via combining Maxwell equations with the rate equations of a four-level atomic system, the output intensity of a lasing mode varying with the pumping rate is simulated for a two-dimensional random active medium by use of the finite difference time domain method. Results show that the pumping rate dependence of the output intensity presents not only a threshold property, but also a saturation property, just like that of lasing modes in conventional lasers. Charactering the mode's localization extent via the mode's area indicates that the stronger the mode's localization extent is, the lower the mode's threshold is. However, such a relation is not a one-to-one correspondence, meaning that the mode's localization extent is an important fact to determine the mode's threshold, but is not alone.
A model of random laser with semi-classical theory is presented.It couples Maxwell's equations with density matrix of excited atom in a disordered system.So Maxwell-Bloch equations for random laser are obtained.A finite-difference time-domain method is used to solve Maxwell-Bloch equations directly.We studied the pattern of field and localized modes of random laser.A critical pumping rate W~c_p exists for the appearance of the lasing peaks in random system.The number of lasing modes increases with the pumping rate and it is saturated finally.
In the last decade Anderson Localization of Light and Random Lasing has attracted a variety of interest in the community of condensed matter theory. The fact that a system so inartificial as a thin layer consisting of dust particles, which have amplifying properties, can produce coherent laser emission, is of a simple elegance, which promises new insights in fundamental physics. There is no need of an external feedback mechanism, like in the system of a conventional laser. Whereas several attempts try to enter the subject numerically by considering cavity approximations, this thesis is concerned with building a microscopically self-consistent theory of random lasing. Designing this method, we had to study light localization effects in random media including absorption and gain. Incorporating interference effects by means of Cooperon contributions we derived Anderson localized states for passive media. Mapping this on a system which consists of laser active Mie-scatterers in a passive medium, we found that by incorporation of the Cooperon, we loose the Anderson localization, but the system is weakly localized, which is sufficient for population inversion and stimulated emission.
Based on the time dependent theory of random lasers, which was established via combining Maxwell equations with the rate equations of a four-level atomic system, the output intensity of lasing modes varying with the pump rate and area as well as the medium's size and shape is simulated for two-dimensional random media by use of the finite difference time domain method. Results show that the pump-rate dependence of the output intensity presents not only a threshold property, but also a saturation property, just like that of the lasing modes in conventional lasers. Based on the dependence of mode property on the medium's and pump's parameters, the mode-selection approaches are proposed for two-dimensional random lasers, which are quite different from those performed in conventional lasers.