Random Lasers (RLs) are realized in disordered media with gain; the feedback for stimulated emission of light is given by the scattering and no external cavity is needed. The cavity is given by multiple scattering. Therefore, when light rays penetrate these materials they interfere with each other because of scattering and, by different material-dependent mechanisms, they establish standing modes.
In a RL the multiple scattering process defines optical modes with a certain central frequency and bandwidth, lifetime and a rich spatial profile. Recently promising methods for the fabrication of planar lasers are referred to RLs and are based on active molecular layers in which defects, aggregates or external beads behave as scattering centers. To build a RL it is important to create strong enough scattering for the material to become optically thick. However, due to the intrinsically randomness of the scattering centers, conventional methods for the fabrication of RLs do not allow for a careful control of the device geometrical parameters, and in turn of the lasing properties. We describe the use of semiconductor organic materials for RL devices in term of physical-chemical properties, characteristics and advantages. Non-conventional self-organization and lithographic processes have been used for the realization of nanoscale organic random lasers. For the first time, the glassy nature of RL spectral intensity fluctuations  was experimentally demonstrated in a solid disordered system.

Highlights:

We demonstrate the random laser emission from scattering nano-aggregates of an organic thiophene-based molecule, obtained in a controlled way by a simple soft lithography technique. The use of surface-tension driven (STD) lithographic processes allows to obtain organic RLs with desired shapes and in which the scattering centers are thiophene aggregates formed by spontaneous molecular self-assembly.

The optimization of the deposition procedure and process kinetics lead to tailor the coherent emission properties by controlling the distribution and the size of the random scatterers.

We reported on the first realization of lasing devices from flexible sheets of common and biodegradable paper, without the presence of any optical cavity and by creating on the cellulose fibres microfluidic porous channels in which a lasing dye can flow by capillarity.

Such a paper-based RL device attests a geometry induced transition in RLs: from a non-resonant RL where the feedback mechanism is solely given by the scattering effect of paper to a resonant RL where the same material, constrained in micro-channels with defined walls and acting as cavity, shows a laser-like behaviour.

We investigate pulse-to-pulse fluctuations in random lasers, we introduce and measure the intensity fluctuation overlap (IFO), the analogue of the Parisi overlap in independent experimental realizations of the same disordered sample, i.e., the experimental realization of  mathematical replicas.

We find that the IFO distribution function yields evidence of a transition to a glassy light phase compatible with a replica symmetry breaking. In an amorphous crystal of thiophene-based oligomer, whose optical behavior under external pumping can be properly represented by a spin-glass theory,  we measure the IFO parameter distribution function and we find a behaviour akin to the one theoretically representing the spin-glass phase (at high pumping) and the paramagnetic/fluorescence phase (at low pumping), and we clearly identify the transition between them, i.e., the lasing threshold.

Nanotec @ Lecce

S.Li.M. Lab @ Roma

1. Ghofraniha, I. Viola, F. Di Maria, G. Barbarella, G. Gigli, L. Leuzzi, C. Conti, Experimental evidence of replica symmetry breaking in random lasers, Nat. Comm. 6, 6058 (2015), doi:10.1038/ncomms7058.
2. Ghofraniha, I. Viola, F. Di Maria, G. Barbarella, G. Gigli, C. Conti,  Random laser from engineered nanostructures obtained by surface tension driven lithography, Laser & Photonics Rev. 7, 432-438, (2013), doi: 10.1002/lpor.201200105.
3. Viola, N. Ghofraniha, A. Zacheo, V. Arima, C. Conti, G. Gigli, Random laser emission from paper-based device, J. Mater. Chem. C 8, 8128-8133,  (2013), doi: 10.1039/C3TC31860E.
4. Ghofraniha, I. Viola, A. Zacheo, V. Arima, G. Gigli, C. Conti, Transition from  non-resonant to resonant random lasers by the  geometrical confinement of  disorder, Opt. Lett. 38, 5043-5046 (2013), doi: 10.1364/OL.38.005043.