The aim of this work was to improve the laser performance, in terms of threshold and operational lifetime, of lasers based on polymer films doped with perylenediimide (PDI) derivatives as active media. For such purpose, we first investigated the amplified spontaneous emission (ASE) properties of perylene orange (PDI-O), when doped into polystyrene (PS) films. Lower ASE thresholds and larger photostabilities than those of similar films containing another PDI derivative (PDI-C6), recently reported in the literature, have been measured. Results have been interpreted in terms of the photoluminescence efficiency of the films, which depends on the type of 10 molecular arrangement, inferred with the help of nuclear magnetic resonance experiments. We also show that PS films have a better ASE performance, i.e. lower thresholds and larger photostabilities, than those based on poly(methyl methacrylate), which was recently highlighted as one of the best matrixes for PDI-O. Finally, a 1D second-order distributed feedaback laser using PS doped with PDI-O, was fabricated and characterized. This device has shown a threshold significantly lower (by around one order of magnitude) than that of a similar laser based on PDI-C6-doped PS.15

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.

An efficient, low-cost, and highly photostable second-order distributed feedback ͑DFB͒ laser, fabricated by thermal nanoimprint lithography and based on a polymer active film containing a perylenediimide derivative, is reported. It shows a photostability half-life of 3.1ϫ 10 5 pump pulses ͑Ͼ8h ͒, when pumped at the same spot of the film. This value is the highest reported to date for organic DFB lasers measured under ambient conditions. The device emits at 573 nm, matching the second low-loss transmission window of poly͑methylmethacrylate͒͑ 460-590 nm͒, thus offering potential for applications in data communications based on polymer optical fibers.

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.

We have studied by means of polarization, tilt and broadband dielectric spectroscopy the liquid crystal (R)-1-methylheptyl-6-͑4Ј-decyloxybenzoyloxy͒-2-naphthalene carboxylate. This compound together with a ferrielectric and two antiferroelectric phases shows a broad range, 6°C, of Sm-C ␣ * phase. The dielectric study shows the presence of a Goldstone mode in the latter mesophase, which in turn would confirm its tilted and helical nature. Other collective modes, such as the soft mode and a mode related to the antiferroelectric ordering and the helical structure, have also been characterized. With respect to the molecular modes, the rotation around the molecular long axis has been studied from the isotropic to the low temperature antiferroelectric phase. This study shows that the molecules are quite rigid and that the dynamics of the motions around the molecular long axis is not greatly influenced by the transitions among the different smectic mesophases. In particular, the thermal activation energy of the process is the same in the paraelectric Sm-A phase and in the complex Sm-C ␣ * phase. However, the frequency shows a jump at the Sm-C ␥ *→Sm-C A * phase transition, which could be due to an increase of the order of the molecular short axes.

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It is shown that the optical pump power (or energy) density thresholds required to obtain lasing from organic second-order distributed feedback lasers, increase when the excitation area (A) is smaller than a certain value (A crit ). So, in order to obtain the minimum possible thresholds and to ensure that they constitute adequate quantities for comparison purposes, the condition A > A crit should be fulfilled. Results also indicate that when A < A crit (A crit $ 0.1 mm 2 for the devices studied here), the operational device lifetime, which depends mainly on the pump power (or energy) density, becomes drastically reduced. 1,2 The interest in OSLs increased with the discovery of stimulated emission in optically pumped semiconducting polymer films, 3,4 since they opened the possibility of using electrical excitation. The goal of obtaining laser diodes was initially the main motivation to decrease the laser thresholds of OSLs, so many works focused in improving the active materials and the resonators. Although diode lasers have not been demonstrated yet, thanks to all these efforts, laser thresholds have been decreased so much that today it is possible to pump with cheap inorganic diode lasers 1,5 and even with light emitting diodes.6 Therefore, these low-cost and compact optically pumped lasers are by themselves useful for applications. 1,2Among the various types of OSLs reported in the literature, distributed feedback (DFB) lasers have been particularly successful.1,2 So today they are being used to develop applications in the fields of telecommunications, 2 biosensing, and chemical sensing. 7,8 In DFB lasers, the active material is deposited as a thin film over an appropriate substrate so it constitutes a waveguide. Feedback is achieved by the incorporation of periodic nanostructures (obtained by modulating either the refractive index or the gain) that Bragg-scatter the light, thus, avoiding the need of good-quality end facets. In a one-dimensional (1D) DFB laser, the wavelength that satisfies the Bragg condition (k Bragg ) given bywhere m is the order of diffraction, n eff is the effective refractive index of the waveguide, and K is the grating period, constitutes the resonant wavelength in the cavity, which will then be diffracted in the grating in different directions.For second-order DFBs (m ¼ 2 in Eq. (1)), light is coupled out of the film in a direction perpendicular to the waveguide film, by first-order diffraction. DFB resonators can be easily integrated into planar organic waveguides, which is a clear advantage from the fabrication point of view, as compared to other types of laser cavities. In general, OSLs demand very intense pumping conditions due to the short photoluminescence (PL) lifetimes of the active materials (typically 1 ns in the case of fluorescent materials). So, excitation is performed by tightly focusing the pump beam through the gain medium provided by a pulsed laser source. Many of the milestones in achieving low thresholds (expressed as energy per pulse) reported in the literature have...