A random laser is a system formed by a random assembly of elastic scatterers dispersed into an optical gain medium 1 . The multiple light scattering replaces the standard optical cavity of traditional lasers and the interplay between gain and scattering determines the lasing properties. All random lasers studied to date have consisted of irregularly shaped or polydisperse scatterers, with a certain average scattering strength that was constant over the frequency window of the laser [2][3][4] . In this letter we consider the case where the scattering is resonant. We demonstrate that randomly assembled monodisperse spheres can sustain scattering resonances over the gain frequency window, and that the lasing wavelength can therefore be controlled by means of the diameter and refractive index of the spheres. The system is therefore a random laser with an a priori designed lasing peak within the gain curve.In recent years the interest in random lasing has grown very rapidly, particularly following the observation of this phenomenon in powdered laser crystals 5 , ceramics 6 , organic composites 7 , and even biological tissue 8 . The necessary condition for a random laser is that the material is multiply scattering light, which means that the transport mean free path (the average distance over which the scattered light direction is randomized) ' t ( L, where L is the sample size. The other fundamental quantity is the gain length ' g , which represents the path length over which the intensity is amplified by a factor e þ1 . The interaction between gain and scattering determines the unique properties of the random laser and, in particular, defines the critical thickness for the sample (in slab geometry) to lase,. Unlike in ordinary lasers, the resulting light emission is multidirectional, but the threshold behaviour 3 , the photon statistics 10,11 and relaxation oscillations 12,13 are very similar to those of standard lasers. The spectral output of a random laser system contains narrow emission spikes 4 , which for large spectral width can merge into a smooth peak with an overall narrowing of the spectrum in most experimental configurations 3,14 , like the one considered in this paper.Wavelength tunability is a crucial property of lasing devices. In regular lasers this is easily achieved by tuning the resonance frequency of the resonator. The same principle has also been applied in more complex cavity structures, such as distributed feedback lasers and photonic crystals lasers, in which the cavity modes are the Bloch modes associated with the periodic structure. Tuning the lattice constant then provides a simple tool to tune the laser for high-quality photonic crystals 15,16 or with localized periodicity 17,18 . These tricks do not work in random structures due to the absence of periodicity. Here we will show, however, that even in a completely random system with no periodicity, resonant tunability can be achieved based on singleparticle resonances.A random system, composed of particles of arbitrary shape and size, has a...

We report on the observation of nonlocalized modes or necklace states of light waves in disordered systems in the Anderson localized regime. The samples consist of positional-disordered binary multilayer systems. Anderson localized modes manifest themselves as narrow high-transmission peaks in the transmission spectrum, whereas the average of the logarithm of the transmission coefficient decreases linearly with thickness. Optical necklace states are observed as modes with a characteristic multiresonance time response and relatively fast decay time.

We report on random lasing in a disordered system in which the multiple scattering feedback mechanism can be switched from a three-dimensional random walk to a quasi-two-dimensional type of transport. The emission from this system is anisotropic, extraordinary polarized, and is controlled via an external electric field. The phenomenon is observed in dye-doped polymer dispersed liquid crystals and makes use of the strong scattering anisotropies in these materials.

We present the experimental observation of multiple resonance transport of light waves, due to necklace states, in disordered one-dimensional systems. Transmission phase measurements allow us to identify these states unambiguously and investigate their statistical properties. A theoretical model is developed to describe the resonance statistics and the frequency dependance of the localization length.

We report on the observation of nonlocalized modes or necklace states of light waves in disordered systems in the Anderson localized regime. The samples consist of positional-disordered binary multilayer systems. Anderson localized modes manifest themselves as narrow high-transmission peaks in the transmission spectrum, whereas the average of the logarithm of the transmission coefficient decreases linearly with thickness. Optical necklace states are observed as modes with a characteristic multiresonance time response and relatively fast decay time.