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...
Numerous optical technologies and quantum optical devices rely on the controlled coupling of a local emitter to its photonic environment, which is governed by the local density of optical states (LDOS). Although precise knowledge of the LDOS is crucial, classical optical techniques fail to measure it in all of its frequency and spatial components. Here, we use a scanning electron beam as a point source to probe the LDOS. Through angular and spectral detection of the electron-induced light emission, we spatially and spectrally resolve the light wave vector and determine the LDOS of Bloch modes in a photonic crystal membrane at an unprecedented deep-subwavelength resolution (30-40 nm) over a large spectral range. We present a first look inside photonic crystal cavities revealing subwavelength details of the resonant modes. Our results provide direct guidelines for the optimum location of emitters to control their emission, and key fundamental insights into light-matter coupling at the nanoscale.
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