Imaging polymer jets at an ultra-high rate of 10 000 frames per second unveils the onset of electrospinning instabilities.

Random lasers are based on disordered materials with optical gain. These devices can exhibit either intensity or resonant feedback, relying on diffusive or interference behaviour of light, respectively, which leads to either coupling or independent operation of lasing modes. We study for the first time these regimes in complex, solid-state nanostructured materials. The number of lasing modes and their intensity correlation features are found to be tailorable in random lasers made of lightemitting, electrospun polymer fibers upon nanoparticle doping. By material engineering, directional waveguiding along the length of fibers is found to be relevant to enhance mode correlation in both intensity feedback and resonant feedback random lasing. The here reported findings can be used to establish new design rules for tuning the emission of nano-lasers and correlation properties by means of the compositional and morphological properties of complex nanostructured materials. Disordered materials with optical gain are the building blocks of random lasers, whose operation is based on the scattering properties of light. 1 These devices can be useful for a wide range of applications, which include diagnostic 2 and speckle-free 3 imaging, spectroscopic tools for monitoring biomaterials and structural deformations, 4 new laser projector schemes and optical tomography. 5 A number of experimental 6-9 and theoretical 10-13 approaches have been developed to rationalize the behaviour and to tailor the performances of random lasers. In this framework, two main classes of devices are distinguished and experimentally observed. In so-called intensity (or incoherent) feedback random lasing (IFRL), the propagation of light can be described as a diffusional process with amplification, neglecting interference effects. 10,14 This mechanism leads to smooth and narrow emission peaks, with full width at half maximum (FWHM) of a few nm. Instead, in resonant feedback random lasing (RFRL), interference upon multiple scattering plays a major role, which might lead to some degree of spatial localization of the light modes, and to very narrow (FWHM down to sub-nm) lasing peaks, as found since 1999 6 in numerous systems at the solid state. The RFRL to IFRL transition has been recently investigated by reducing the directionality of pumping photons. 9,15 In this way, this transition could be associated to the coupling of modes which determines a condensation-like process, namely the onset of collective oscillations from µm-sized titania clusters in a rhodamine solution.Extending the study of this transition to solid-state devices would be highly important to understand how the emission features and mode interactions of random lasers can be tailored. Indeed, regimes with different feedback or inter-mode coupling should be obtained not only by shaping the excitation beam as in previous reports, but also by varying the degree of disorder, through the composition and morphology of materials showing gain properties. 16 These include a large variety of syste...

Abstract:Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm -2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 µm-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.

Electromechanical coupling through piezoelectric polymer chains allows the emission of organic molecules in active nanowires to be tuned. This effect is evidenced by highly bendable arrays of counter-ion dye-doped nanowires made of a poly(vinylidenefluoride) copolymer. A reversible redshift of the dye emission is found upon the application of dynamic stress during highly accurate bending experiments. By density functional theory calculations it is found that these photophysical properties are associated with mechanical stresses applied to electrostatically interacting molecular systems, namely to counterion-mediated states that involve light-emitting molecules as well as charged regions of piezoelectric polymer chains. These systems are an electrostatic class of supramolecular functional stress-sensitive units, which might impart new functionalities in hybrid molecular nanosystems and anisotropic nanostructures for sensing devices and soft robotics

Electrospinning technologies for the realization of active polymeric nanomaterials can be easily up-scaled, opening perspectives to industrial exploitation, and due to their versatility they can be employed to finely tailor the size, morphology and macroscopic assembly of fibers as well as their functional properties. Light-emitting or other active polymer nanofibers, made of conjugated polymers or of blends embedding chromophores or other functional dopants, are suitable for various applications in advanced photonics and sensing technologies. In particular, their almost onedimensional geometry and finely tunable composition make them interesting materials for developing novel lasing devices. However, electrospinning techniques rely on a large variety of parameters and possible experimental geometries, and they need to be carefully optimized in order to obtain suitable topographical and photonic properties in the resulting nanostructures. Targeted features include smooth and uniform fiber surface, dimensional control, as well as filament alignment, enhanced light emission, and stimulated emission. We here present various optimization strategies for electrospinning methods which have been implemented and developed by us for the realization of lasing architectures based on polymer nanofibers. The geometry of the resulting nanowires leads to peculiar light-scattering from spun filaments, and to controllable lasing characteristics.