(A) At preliminary sage of CARLITO project, we have focused our research goal on developing an experimental setup at UniPA for measuring the random laser (RL) spectra generated by different active and passive disordered system. A tunable pulsed laser system (5 ns, 10 Hz, OPOTEK VIBRANT) is used as pump source. Depending on the purpose of analysis, spectral shape of the light emitted from the RL medium was recorded at different pump energies using an optical fiber connected to (i) a one-box spectrometer (Thorlabs, CCS200) with a spectral resolution of < 2 nm, or (ii) a detection system consisting in a monochromator with 0.15 nm spectral resolution dispersing over an intensified CCD camera (PI-MAX Princeton Instruments) synchronized to the pump source. Thereafter, performance several semiconductor scatterers for RL action in presence of eighter luminescent carbon dots (CDs) or commercial dye as gain media is studied at optimized concentrations (vide-infra). Notably, generation of RL emission from CDs using semiconductor scatterers is some time hard to be achieved because of the strong charge transfer mediated photoquenching effects.
(B) During the execution of CARLITO, we demonstrate RL emission at ∼565 nm from green emitting CDs (gCDs), and we use the emitted light as a light source for the speckle-free microscopy of biological tissues and microparticles [doi.org/10.1021/acsphotonics.4c00279]. The gCDs have been mixed with TiO2 (passive scatterer) to produce a colloidal disordered TiO2-gCDs nanocomposite to be tested for RL. The emission of the CD-based RL is thoroughly studied as a function of the experimental conditions, and well-established mathematical tools in the field are used to perform a detailed statistical study of the lasing output. The CD based RL displays ultranarrow (∼0.70 nm) emission lines over a comparatively broader (∼10 nm) background. These two emissions are due to the so-called coherent and incoherent RL. Their relative weight can be controlled by an appropriate choice of experimental conditions, allowing us to tune the characteristics of RL light. Next, we have studied the role of TiO2 as scatterer for generating RL emission from red emitting CDs both in liquid and solid state.
(C) We demonstrate RL emission using a novel type of red-emitting CDs (R-CDs) obtained via a solvothermal synthesis pathway starting from Neutral Red (NR) dye as a precursor [doi.org/10.1021/acsnano.3c05566]. Prior to the application, a clear discrimination between structural and optical properties of R-CDs and the NR dye is done. In particular, R-CDs display comparable emission quantum efficiency to NR dyes, but significantly enhanced photostability, which provides a crucial advantage for the fabrication of light emitting devices such as a RL. On these grounds, we test the RL emission capability in the red spectral region of the colloidal disorder system consisting of R-CDs mixed with hydrophobic TiO2 used as passive scatterer. We achieve narrowband omnidirectional RL emission at ~640 nm at a threshold pump energy of 30 mJ/cm2 under the excitation wavelength of 532 nm. Interestingly, the degree of coherence of the obtained RL emission found to be dependent on the experimental conditions. Notably, we have also tried to incorporate the R-CD based disorder system inside a polymer matrix (PVA) and we have seen RL emission in solid state, which is unstable. This result has provoked us to investigate incorporation of CDs inside a solid matrix for photostable RL emission.
(D) Taking advantage of the properties of citric acid pyrolysis, which facilitates the formation of a fluorescent carbon matrix. This matrix successfully incorporates dye molecules without affecting their emission properties, allowing their use in solid-state applications. These R-CDs serve as a gain medium for developing silica-based materials functioning as RL systems. The RL performance is controlled by appropriately introducing different scatterers (TiO2, and gold nanoparticles) during the photopolymerization of the silica host. By utilizing the unique properties of R-CDs and the polymerization method, we employed a novel strategy to create a medium for random lasing applications. The CD embedded SiO2 monoliths lase in the region between 580 and 610 nm and exhibit dye loading and concentration-dependent properties of scattering centers. The optical response is compatible with an incoherent random lasing regime, characterized by a low lasing threshold and a spectral emission width of less than 5 nm.
(E) Halloysite nanotubes (HNTs) obtained from natural sources are used as scatterers to generate RL emission when combined to Rhodamine B (RhB) dye. We report for the first time intense RL emission from such composites, in colloidal form [doi.org/10.1021/acsanm.3c02840]. The RL emission is at ~ 598 nm with a linewidth of ~4 nm. Moreover, Moreover, a statistical analysis of shot-to-shot correlations among RL modes reveals an obvious photonic paramagnetic to spin-glass transition with increasing pump energy, which is a unique statistical fingerprint of RL. Interestingly, besides acting as scatterers, the HNTs are also found to improve the photostability of RhB molecules. Additionally, some of the tested HNTs, when UV-excited, are also found to behave as photocatalysts capable to degrading the organic dyes, which are charge selectively adsorbed on the surfaces of HNTs [doi.org/10.3390/photochem4020009].
(F) ZnO microcabbage (ZnOMC) and multiple dyes embedded in a polymer disordered system have been geometrically coupled via an external cavity for producing a color tunable RL emission. Moreover, the degree of color tunability can be controlled just by changing the position of the pump focal point along the depth of the coupled polymer disordered systems. The loosely correlated dual band RL emission is further utilized for processing of unclonable sequence of binary numbers. In principle, such a single wavelength pump compact disordered system may be used for both producing color tunable RL emission as well as creating unclonable security tags.
