We analyze the process of informational exchange through complex networks by measuring network efficiencies. Aiming to study nonclustered systems, we propose a modification of this measure on the local level. We apply this method to an extension of the class of small worlds that includes declustered networks and show that they are locally quite efficient, although their clustering coefficient is practically zero. Unweighted systems with small-world and scale-free topologies are shown to be both globally and locally efficient. Our method is also applied to characterize weighted networks. In particular we examine the properties of underground transportation systems of Madrid and Barcelona and reinterpret the results obtained for the Boston subway network.

We report on the preparation and characterization, under optical pump, of second-order one-dimensional distributed feedback (DFB) lasers based on polystyrene films doped with a perylenediimide derivative, as active media. The DFB gratings were engraved on the substrates (SiO2) by thermal nanoimprint lithography, followed by reactive ion etching. Laser emission wavelength was tuned from 554 to 583 nm by changing film thickness (h) between 240 and 1200 nm. The effect on the performance (emission wavelength, threshold, slope efficiency, number of modes, and spectral shape) of varying the grating depth (d) from 30 to 240 nm, for the whole range of h values, has been investigated. Although there is extensive work in the literature aiming to tune the emission wavelength of organic DFB lasers by h variation, the effect of changing d systematically has not been previously studied. Experimental results have been interpreted by models that take into account the presence of the grating by averaging either h or the effective refractive index. Single-mode emission (λ0) was observed for h < 1000 nm, while for thicker films lasing appeared at two different wavelengths (λ0 and λ1). Models indicate that λ0 and λ1 correspond to the TE0 and TE1 waveguide modes, respectively. It was found that d plays an important role in determining the DFB thresholds and slope efficiencies for two h regimes: (i) For h < 350 nm, lowest thresholds and highest slopes efficiencies were obtained with the shallower gratings; and (ii) for h > 1000 nm, d affects significantly the losses associated with the TE1 mode, so single mode emission was achieved at λ0 or at λ1 for deep and shallow gratings, respectively. Finally, the shape of the emission spectra, both below and above threshold, has also been analyzed in order to clarify the physical mechanisms responsible for the existence of gain. Bragg dips were observed in the spectra below threshold only for devices with d/h larger than around 0.3 and their width increased with increasing d/h. In these cases, single-mode DFB emission appeared at the long-wavelength edge of the Bragg dip, indicating that index-coupling modulation contributes significantly to the gain process. On the other hand, for smaller d/h values, Bragg dips became too small to be detected, so gain coupling becomes the dominant mechanism accounting for the presence of gain.

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The marked difference between the optical absorption of dissolved PTCDA monomers and crystalline films is analysed in terms of the deformation of an effective internal vibrational mode of the molecule and transfer of Frenkel excitons between different molecular sites in the crystal. It is shown in detail that the coupling between equivalent molecules in different crystal unit cells, especially among stack neighbours, dominates over exciton transfer between the two different molecules in the crystal basis. The first type of interaction leads to a pronounced deformation of the expected Poisson progression over subsequent vibrational levels while the latter is responsible for a small Davydov splitting and, consequently, different absorption lineshapes along different crystal axes.

We report on the fabrication of efficient organic distributed feedback (DFB) lasers with thermally-nanoimprinted active films, emitting between 565 and 580 nm. The use of thermal-NIL has allowed, as opposed to room temperature or solvent-assisted techniques, high grating quality and excellent modulation depth. The 155°C heat exposure of the NIL process, does not significantly affect the thermal and optical properties of the active material (polystyrene films doped with a perylenediimide derivative). These devices combine a simple and low-cost preparation method with good laser characteristics, i.e. thresholds of 1 μJ/pulse, single-mode emission with linewidths below 0.2 nm and photostability half-lives of ~ 10⁵ pump pulses under ambient conditions. In comparison to more standard DFBs with gratings on the substrate, their fabrication is much easier, while they show a similar laser performance.