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Basic Physics and Recent Developments of Organic Random Lasers
Published in Marco Anni, Sandro Lattante, Organic Lasers, 2018
Ilenia Viola, Luca Leuzzi, Claudio Conti, Neda Ghofraniha
A random laser that sustains a large number of modes that are strongly coupled and compete for the available gain can show a fluctuating behavior in its temporal and spectral response: A random laser can have a different spectrum each time it is excited [22]. Mode coupling is particularly strong for extended modes, and it takes place through the gain mechanism, which means that modes of different wavelengths can still be coupled, even if they are not spectrally overlapping. It was found that in these situations, the emission spectra are then completely uncorrelated from shot to shot and being sure that the scattering particles are not moving and all other conditions are kept perfectly constant. The fluctuations have been described by means of mode repulsion [23], Levy statistics [24–27], and the first explanation has been given by replica symmetry breaking theory [28–30].
Spatially and electrically tunable random lasing based on a polymer-stabilised blue phase liquid crystal-wedged cell
Published in Liquid Crystals, 2020
Ruochen Liao, Xiyun Zhan, Xiaowan Xu, Yanjun Liu, Fei Wang, Dan Luo
The mechanism of the formation of a random laser is multiple scattering in the medium [11]. The multiple scattering not only comes from the randomly distributed BP platelets and the discontinuous boundaries, but also the index mismatch between the polymer and the refilled LC molecules. The light waves that are emitted from the laser dye travel along the BP platelets, leading to multiple scattering. Then part of the light waves goes back to the beginning position. The light experiences both loss and amplification along the closed-loop path during scattering. In that case, random lasing appears when gain is higher than loss. The scattering mean free path is a key factor for random lasers emission and influences the wavelength of the emission laser. In a scattering system, the random lasing wavelength is closely related to the scattering mean free path ls [25]. Hu et al. demonstrated that the random lasing wavelength with short ls exhibited blue-shift effect compared to that with long ls in the tunable random polymer fibre laser [26]. Herein, scattering occurs at the boundaries between BP platelets, and between general defects in the BPLC structure. As the PS-BPLC material inside the cell is compressed into thinner regions of the wedged cell, the spaces between the scattering centres will decrease. Therefore, the emitted light has a shorter dwelling time in the thin region than the thick region, which leads to the reduction of the value of the scattering mean free path ls and the blue-shifted laser emission.
Nanosecond illumination source for speckle-free liquid crystalmicroscopy
Published in Liquid Crystals, 2021
A. V. Ryzhkova, U. Jagodič, Igor Muševič
Quite recently random lasers [24,25] were proposed for sub-nanosecond imaging, because they generate very bright and short illumination pulses. In a random laser, light is generated within a positional disordered system, such as a random dispersion of small dielectric spheres in a solvent with a fluorescent dye. The dye provides optical amplification and the laser resonator is, in fact, a virtual resonator, formed by multiple scattering of photons from micro-particles. The resonance condition is fulfilled for a given closed path generated by a sequence of scattering of photons from the dispersed particles. Such a multimode laser exhibits high brightness but suffers from partial coherence, an inherent property of amplification of light by stimulated emission.