Switching and amplification in disordered lasing resonators

Controlling the flow of energy in a random medium is a research frontier with a wide range of applications. As recently demonstrated, the effect of disorder on the transmission of optical beams, may be partially compensated by wavefront shaping, but losing control over individual light paths. Here we report on a novel physical effect whereby energy is spatially and spectrally transferred inside a disordered active medium by the coupling between individual lasing modes. We show that is possible to transmit an optical resonance to a remote point by employing specific control over optical excitations. The observed nonlinear transport bears some analogies to a field-effect transistor for light, which acts as a switch and as an amplifier

The mode-locking transition of random lasers

The discovery of the spontaneous mode-locking of lasers, i.e., the synchronous oscillation of electromagnetic modes in a cavity, has been a milestone of photonics allowing the realization of oscillators delivering ultra-short pulses. This process is so far known to occur only in standard ordered lasers with meter size length and only in the presence of a specific device (the saturable absorber). Here we demonstrate that mode-locking can spontaneously arise also in random lasers composed by micronsized laser resonances dwelling in intrinsically disordered, self-assembled clusters of nanometer-sized particles. Moreover by engineering a novel mode-selective pumping mechanism we show that it is possible to continuously drive the system from a configuration in which the various excited electromagnetic modes oscillate in the form of several, weakly interacting, resonances to a collective strongly interacting regime. By realizing the smallest mode-locking device ever fabricated, we open the way to novel generation of miniaturized and all-optically controlled light sources

Tunable degree of localization in random lasers with controlled interaction

We show that the degree of localization for the modes of a random laser (RL) is affected by the inter mode interaction that is controlled by shaping the spot of the pump laser. By experimentally investigating the spatial properties of the lasing emission we infer that strongly localized modes are activated in the low interacting regime while in the strongly interacting one extended modes are found lasing. Thus we demonstrate that the degree o localization may be finely tuned at the micrometer level

Template-free, surfactant-mediated orientation of self-assembled supercrystals of metal-organic framework particles

Mesoscale self-assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self-assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template-free method to self-assemble (111)-, (100)-, and (110)-oriented face-centered cubic supercrystals of the metal-organic framework ZIF-8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors

Simulations of micro-sphere/shell 2D silica photonic crystals for radiative cooling

Altres ajuts: the CERCA Program/Generalitat de Catalunya.L'article s'ha publicat sota la OSA Open Access Publishing Agreement https://www.osapublishing.org/submit/review/pdf/OSACopyTransferOAAgrmnt(2017-09-05).pdfPassive daytime radiative cooling has recently become an attractive approach to address the global energy demand associated with modern refrigeration technologies. One technique to increase the radiative cooling performance is to engineer the surface of a polar dielectric material to enhance its emittance atwavelengths in the atmospheric infrared transparency window (8-13 ìm) by outcoupling surface-phonon polaritons (SPhPs) into free-space. Here we present a theoretical investigation of new surface morphologies based upon self-assembled silica photonic crystals (PCs) using an in-house built rigorous coupled-wave analysis (RCWA) code. Simulations predict that silica micro-sphere PCs can reach up to 73 K below ambient temperature, when solar absorption and conductive/convective losses can be neglected. Micro-shell structures are studied to explore the direct outcoupling of the SPhP, resulting in near-unity emittance between 8 and 10 ìm. Additionally, the effect of material composition is explored by simulating soda-lime glass micro-shells, which, in turn, exhibit a temperature reduction of 61 K below ambient temperature. The RCWA code was compared to FTIR measurements of silica micro-spheres, self-assembled on microscope slides

Tunable emission in dye-doped truxene-based organogels through RET

Organic systems comprising a truxene-based organogel doped with an organic dye have been fabricated and their photophysical properties examined in the search for an organic matrix with tunable luminescent properties. The addition of the organic dopant has been observed to introduce changes in the morphology of the gel which alters the ratio between monomer and excimer species. Further, the luminescent properties of the doped organogel have been studied and their evolution with dopant concentration explained in terms of resonant energy transfer between the excimer species (acting as a donor) and the organic dopant (acceptor). The interplay between blue, green and red emission bands associated with monomers, excimers and organic dopants allows tuning the luminescence of the system within the visible region reaching white light emission under certain conditions. The origin of the energy transfer is found to be the aggregation of the molecules upon solvent evaporation, the more stable xerogel phase being the extreme case which constitutes a technologically relevant approach where solvent evaporation is not an issue.Peer reviewe

Dynamics of phase-locking random lasers

Laser modes may coalesce into a mode-locked state that enables femtosecond pulse compression. The nature of the interaction and the interaction time play fundamental roles in the onset of this collective state, but the investigation of the transition dynamics is technically challenging because phases are not always experimentally accessible. This is even more difficult for random lasers, a kind of disordered laser in which energies in play are much smaller than in the ordered macroscopic case. Here we investigate experimentally and numerically the dynamics of the phase-locking transition in a random laser. We developed an experimental setup able to pump individual modes with different pulse durations and found that the mode-locked regime builds only for quasicontinuous pumping, resulting in an emission linewidth dependent on the pump duration. Numerical simulation confirms experimental data. © 2013 American Physical Society

Non-locality and collective emission in disordered lasing resonators

Random lasing is observed in optically active resonators in the presence of disorder. As the optical cavities involved are open, the modes are coupled, and energy may pour from one state to another provided that they are spatially overlapping. Although the electromagnetic modes are spatially localized, our system may be actively switched to a collective state, presenting a novel form of non-locality revealed by a high degree of spectral correlation between the light emissions collected at distant positions. In a nutshell, light may be stored in a disordered nonlinear structure in different fashions that strongly differ in their spatial properties. This effect is experimentally demonstrated and theoretically explained in titania clusters embedded in a dye, and it provides clear evidence of a transition to a multimodal collective emission involving the entire spatial extent of the disordered system. Our results can be used to develop a novel type of miniaturized, actively controlled all-optical chip