A random laser is a non-conventional laser whose feedback mechanism is based on disorder-induced light scattering. Depending on whether the feedback supplied by scattering is intensity feedback or amplitude feedback, random lasers are classified into two categories: random lasers with incoherent feedback and random lasers with coherent feedback. A brief survey of random lasers with incoherent feedback is presented. It is followed by a review of our recent experimental work on random lasers with coherent feedback, including measurement of the lasing threshold, lasing spectra, emission pattern, dynamical response, photon statistics, speckle pattern and the investigation of relevant length scales. Large disorder leads to spatial confinement of the lasing modes, that is the foundation for the micro random laser. Some theoretical models of random lasers with coherent feedback are briefly introduced. The study of random lasers improves our understanding of the interplay between light localization and c...

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Lasing in random media

The building block modular approach that lies behind coordination polymers (CPs) and metal–organic frameworks (MOFs) results not only in a plethora of materials that can be obtained but also in a vast array of material properties that could be aimed at. Optical properties appear to be particularly predetermined by the character of individual structural units and by the intricate interplay between them. Indeed, the “design principles” shaping the optical properties of these materials seem to be well explored for luminescence and second-harmonic generation (SHG) phenomena; these have been covered in numerous previous reviews. Herein, we shine light on CPs and MOFs as optical media for state-of-the-art photonic phenomena such as multi-photon absorption, triplet–triplet annihilation (TTA) and stimulated emission. In the first part of this review we focus on the nonlinear optical (NLO) properties of CPs and MOFs, with a closer look at the two-photon absorption property. We discuss the scope of applicability of most commonly used measurement techniques (Z-scan and two-photon excited fluorescence (TPEF)) that can be applied for proper determination of the NLO properties of these materials; in particular, we suggest recommendations for their use, along with a discussion of the best reporting practices of NLO parameters. We also outline design principles, employing both intramolecular and intermolecular strategies, that are necessary for maximizing the NLO response. A review of recent literature on two-, three- and multi-photon absorption in CPs and MOFs is further supplemented with application-oriented processes such as two-photon 3D patterning and data storage. Additionally, we provide an overview of the latest achievements in the field of frequency doubling (SHG) and tripling (third-harmonic generation, THG) in these materials. Apart from nonlinear processes, in the next sections we also target the photonic properties of MOFs that benefit from their porosity, and resulting from this their ability to serve as containers for optically-active molecules. Thus, we survey dye@MOF composites as novel media in which efficient upconversion via triplet energy migration (TEM) occurs as well as materials for stimulated emission and multi-photon pumped lasing. Prospects for producing lasing as an intrinsic property of MOFs has also been discussed. Overall, further development of the optical processes highlighted herein should allow for realization of various photonic, data storage, biomedical and optoelectronic applications.

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Nonlinear optical properties, upconversion and lasing in metal–organic frameworks

Random distributed feedback fibre lasers

Although ‘random lasing’ in disordered optical media was first demonstrated a decade ago, the mechanism by which it occurs is disputed. New evidence of random lasing in conjugated polymers strongly supports the notion that it is generated within random optical cavities that naturally occur within disordered media.

Naturally occurring resonators in random lasing of π -conjugated polymer films

The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.

Random lasing and weak localization of light in dye-doped nematic liquid crystals.

Short (>300-ps) pulses of stimulated emission were found from powders of NdAl3(BO3)4, NdSc3(BO3)4, and Nd:Sr5(PO4)3F laser crystals during 532- and 805-nm excitation. Study of stimulated emission in the mixture of two powders has shown that the different components influence each other. That implies a collective behavior of emitting particles. The main features of experimentally observed stimulated emission are described with a simple model accounting for 4F3/2 excited-state concentration and emission energy density. The threshold of stimulated emission in powders is shown to be inversely proportional to the small-signal amplification along the photon path in the pumped volume.

Short-pulsed stimulated emission in the powders of NdAl 3 (BO 3 ) 4 , NdSc 3 (BO 3 ) 4 , and Nd:Sr 5 (PO 4 ) 3 F laser crystals

Short (>300 ps) pulses of stimulated emission were found from powders of NdAl3(BO3)4, NdSc3(BO3)4, and Nd:Sr5(PO4)3 laser crystals under 532 nm and 805 nm excitation. Study of stimulated emission in the mixture of two powders has shown that different components influenced each other. The main features of experimentally observed stimulated emission are described with a simple model accounting for 4F_3/2 excited state concentration and emission energy density.

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T. T. Thompson

Papers

Short-pulsed stimulated emission in the powders of NdAl 3 (BO 3 ) 4 , NdSc 3 (BO 3 ) 4 , and Nd:Sr 5 (PO 4 ) 3 F laser crystals

Stimulated Emission without Cavity in Powders and Single Crystals of Nd Doped Materials