José M. Villalvilla

Tuneability of amplified spontaneous emission through control of the thickness in organic-based waveguides

The thickness dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene films containing 15 wt % of the luminescent and hole-transporting organic molecule N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) is reported. It is observed that the position of ASE can be tuned between 404 and 417 nm by changing the film thickness from 100 to 200 nm. By measuring and modeling the waveguide modes, we demonstrate that the calculated cut-off thickness for the propagation of one mode determine the thickness below which ASE disappears although they do not perfectly correlate with the observed wavelength shifts. Results show that the position of ASE depends on the shape of the photoluminescence spectrum, that is composed of various vibronic peaks whose relative intensities change with film thickness due to the existence of the cut-off thickness. Finally, it is also observed that both the position and the threshold of ASE depend on the different confinement of the propagation modes due to th...

Highly photostable organic distributed feedback laser emitting at 573 nm

An efficient, low-cost, and highly photostable second-order distributed feedback (DFB) 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 3.1×105 pump pulses (>8 h), 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 poly(methylmethacrylate) (460–590 nm), thus offering potential for applications in data communications based on polymer optical fibers.

Improved performance of perylenediimide-based lasers

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 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 one-dimensional second-order distributed feedback 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.

Film thickness and grating depth variation in organic second-order distributed feedback lasers

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, 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.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 e...

Carbon-bridged oligo(p-phenylenevinylene)s for photostable and broadly tunable, solution-processable thin film organic lasers

Thin film organic lasers represent a new generation of inexpensive, mechanically flexible devices for spectroscopy, optical communications and sensing. For this purpose, it is desired to develop highly efficient, stable, wavelength-tunable and solution-processable organic laser materials. Here we report that carbon-bridged oligo(p-phenylenevinylene)s serve as optimal materials combining all these properties simultaneously at the level required for applications by demonstrating amplified spontaneous emission and distributed feedback laser devices. A series of six compounds, with the repeating unit from 1 to 6, doped into polystyrene films undergo amplified spontaneous emission from 385 to 585 nm with remarkably low threshold and high net gain coefficients, as well as high photostability. The fabricated lasers show narrow linewidth (<0.13 nm) single mode emission at very low thresholds (0.7 kW cm−2), long operational lifetimes (>105 pump pulses for oligomers with three to six repeating units) and wavelength tunability across the visible spectrum (408–591 nm).

Influence of the excitation area on the thresholds of organic second-order distributed feedback lasers

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 (Acrit). So, in order to obtain the minimum possible thresholds and to ensure that they constitute adequate quantities for comparison purposes, the condition A > Acrit should be fulfilled. Results also indicate that when A < Acrit (Acrit ∼ 0.1 mm2 for the devices studied here), the operational device lifetime, which depends mainly on the pump power (or energy) density, becomes drastically reduced.

Determination of the glass transition temperature of photorefractive polymer composites from photoconductivity measurements

A method to determine the glass transition temperature (Tg) of photorefractive (PR) polymer composites, based on photoconductivity measurements, is proposed. The composite Tg is identified as the temperature at which a clear change in slope in the Arrhenius plot representation of the photoconductivity is obtained. We demonstrate that for PR polymers, this method is more appropriate than the one generally used, based on differential scanning calorimetry (DSC). For PR composites based on the hole transporting polymer poly(N-vinylcarbazole), the Tg value determined from photoconductivity data is around 30°C higher than that obtained by DSC.

Concentration dependence of amplified spontaneous emission in two oligo-(p-phenylenevinylene) derivatives

Amplified spontaneous emission (ASE) in optically pumped polystyrene (PS) films doped with two different oligo-(p-phenylenevinylene) derivatives (OPVs), with three (3-OPV) and five (5-OPV) monomer units is reported. It is observed that there is a maximum content of oligomer (25wt.% for 3-OPV and 20wt.% for 5-OPV) that can be introduced in the films, due to degradation (in 3-OPV) and ASE quenching (in 5-OPV). Optimal concentrations (with minimum pump-intensity thresholds for the observation of ASE) of 15wt.% and 9wt.% are measured for the trimer and the pentamer, respectively. The concentration dependence of the ASE threshold is discussed in terms of the optical absorption spectra. Gain coefficients are obtained by analyzing the dependence of the ASE with the length of the pump stripe. The results show that films based on 5-OPV appear to be more efficient and stable than those based on 3-OPV.

Perylenediimide-based distributed feedback lasers with holographic relief gratings on dichromated gelatine

One dimensional second-order distributed feedback (DFB) lasers consisting of polystyrene (PS) films doped with a perylenediimide laser dye, deposited over dichromated gelatine (DCG) photoresist layers with solvent resistant relief gratings recorded by holographic lithography, are reported. The advantage of using the grating on DCG in the final device is that the fabrication process is simplified and the grating pattern better preserved, since no etching methods to transfer the grating to another substrate are needed. A very simple model, proposed to explain the experimental waveguide properties, has allowed identifying the waveguide mode at which DFB emission appears, which was the key to optimize the device performance. In the frame of this model, the thickness of the PS and DCG films could be adjusted in order to minimize the laser threshold and to control its wavelength tuneability. The performance of these lasers is comparable to that of recently reported devices based on the same active material, but...

Millisecond photorefractivity with novel dicyanomethylenedihydrofuran-containing polymers

New multifunctional copolymers containing carbazole units and high loads of dicyanomethylenedihydrofuran (DCDHF) were synthesized and used to prepare blends for photorefractive (PR) purposes. The materials response, which is strongly dependent on the glass-transition temperature (Tg), was thoroughly analyzed by holography, conductivity and ellipsometry measurements in order to both determine the limiting factors and optimize the performance. Materials that have a Tg around room temperature show strongly hindered chromophore orientation, which is avoided by lowering the Tg down to 6 °C, without compromising the PR effect or the material stability. A further DCDHF-containing homopolymer without carbazole was synthesized and characterized, showing an inferior PR response, which is attributed to a beneficial role for charge generation and transport of the attached carbazole in the copolymers. The new blends strongly improve the structural properties of previous DCDHF-based materials, allowing application of fields well above 100 V μm−1 and preventing beam fanning. Outstanding PR performance was achieved, with fast buildup and erasure times of a few tens of milliseconds (even at low recording intensities), large refractive index modulation (over 10−2) and two-beam coupling gain (above 350 cm−1). Such performance is among the best reported for PR materials based on multifunctional and nonlinear polymers and comparable to standard PR composites.