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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 (25 wt. % for 3-OPV and 20 wt. % 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 15 wt. % and 9 wt. % 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. [ABSTRACT FROM AUTHOR]

Laser action at 420 nm is observed in photopumped polymer films of polystyrene (PS) doped with the hole-transporting organic molecule N,N′-bis(3-methylphenyl)-N,N′-dyphenylbenzidine (TPD). Measurements of the dependence of signal on the length of the pump stripe show that the mechanism responsible for the observed spectral collapse is amplified spontaneous emission (ASE). By fitting the evolution of the output intensity at 420 nm with the excitation length, gain coefficients up to 16 cm-1 are measured. [ABSTRACT FROM AUTHOR]

Surface‐emitting organic distributed feedback (DFB) lasers are attractive devices for label‐free sensing applications. Of particular interest for cost‐reduction purposes are devices in which both active material and resonator are based on solution‐processable polymeric materials. Here, the capability for label‐free refractive index sensing of DFB devices consisting of top‐layer resonators (1D relief gratings) of dichromated gelatin layers is reported, deposited on top of active films of polystyrene doped with perylene orange. The devices show successful operation for both transversal electric and transversal magnetic modes, with a larger sensor sensibility for the latter (Sb = 38 nm RIU−1). The overall device operation (emission laser wavelength, laser threshold in addition to the sensitivity) has been optimized through the adjustment of the active film thickness and the grating period. This is possible thanks to the versatility offered by the holographic procedure used to fabricate the resonator, and to the top‐layer resonator geometry. Remarkably, the fact that the resonator is based on gelatin (a food‐grade biopolymer), offers great potential for the development of wearable and bio‐compatible sensing applications. [ABSTRACT FROM AUTHOR]

Graphene‐like molecules with multiple zigzag edges are emerging as promising gain materials for organic lasers. Their emission wavelengths can vary widely, ranging from visible to near‐infrared (NIR), as the molecular size increases. Specifically, rhombus‐shaped molecular graphenes with two pairs of parallel zigzag edges, known as [n]rhombenes, are excellent candidates for NIR lasers due to their small energy gaps. However, synthesizing large‐size rhombenes with emission beyond 800 nm in solution remains a significant challenge. In this study, we present a straightforward synthesis of an aryl‐substituted [4]rhombene derivative, [4]RB‐Ar, using a method that combines intramolecular radical‐radical coupling with Bi(OTf)3‐mediated cyclization of vinyl ethers. The structure of [4]RB‐Ar was confirmed through X‐ray crystallographic analysis. Bond length analysis and theoretical calculations indicate that aromatic sextets are predominantly localized along the molecule's long axis. Significantly, [4]RB‐Ar demonstrates narrow amplified spontaneous emission at around 834 nm when dispersed in polystyrene thin films. Moreover, solution‐processed distributed feedback lasers employing [4]RB‐Ar as the active gain material display tunable narrow emissions in the range of 830 to 844 nm. [ABSTRACT FROM AUTHOR]

The optical and mechanical properties of 2D semiconductors make them excellent candidates for the active components of plastic optoelectronic devices. Here, the integration of single‐layer WSe2 (1L‐WSe2) into a polystyrene (PS) film containing dispersed perylene orange (PDI‐O) molecules is investigated. The findings reveal a notable enhancement in the light emission of 1L‐WSe2, which occurs exclusively upon PDI‐O excitation and scales with the concentration of molecules in the PS film. Moreover, the increase in 1L‐WSe2 photoluminescence coincides with a quenching of the PDI‐O light emission intensity and a decrease in its lifetime. These results point to efficient long‐range interactions, such as Förster energy transfer, between PDI‐O (acting as the donor) and 1L‐WSe2 (acting as the acceptor), as the mechanism responsible for the enhanced light emission in the latter. These findings are of great interest for the development of flexible optoelectronic devices integrating active 2D materials; the polymeric matrix plays a dual role, serving as both physical support and host for organic dopants that can optimize the light emission properties of the active 2D material. [ABSTRACT FROM AUTHOR]

Perylenediimide (PDI) compounds are widely used as the active units of thin-film organic lasers. Lately, PDIs bearing two sterically hindering diphenylphenoxy groups at the 1,7-bay positions have received attention because they provide a way to red-shift the emission with respect to bay-unsubstituted PDIs, while maintaining a good amplified spontaneous emission (ASE) performance at high doping rates. Here, we report the synthesis of a series of six PDI derivatives with different aryloxy groups (PDI 6 to PDI 10) or ethoxy groups (PDI 11) at the 1,7 positions of the PDI core, together with a complete characterization of their optical properties, including absorption, photoluminescence, and ASE. We aim to stablish structure-property relationships that help designing compounds with optimized ASE performance. Film experiments were accomplished at low PDI concentrations in the film, to resemble the isolated molecule behaviour, and at a range of increasing doping rates, to investigate concentration quenching effects. Compounds PDI 10 and PDI 7, bearing substituents in the 2′ positions of the benzene ring (the one contiguous to the linking oxygen atom) attached to the 1,7 positions of the PDI core, have shown a better threshold performance, which is attributed to conformational (steric) effects. Films containing PDI 11 show dual ASE. [ABSTRACT FROM AUTHOR]

Large graphene‐like molecules with four zigzag edges are ideal gain medium materials for organic near‐infrared (NIR) lasers. However, synthesizing them becomes increasingly challenging as the molecular size increases. In this study, we introduce a new intramolecular radical‐radical coupling approach and successfully synthesize two fused triangulene dimers (1 a/1 b) efficiently. X‐ray crystallographic analysis of 1 a indicates that there is no intermolecular π‐π stacking in the solid state. When the more soluble derivative 1 b is dispersed in polystyrene thin films, amplified spontaneous emission in the NIR region is observed. Using 1 b as the active gain material, we fabricate solution‐processed distributed feedback lasers that exhibit a narrow emission linewidth at around 790 nm. The laser devices also exhibit low thresholds with high photostability. Our study provides a new synthetic strategy for extended nanographenes, which have diverse applications in electronics and photonics. [ABSTRACT FROM AUTHOR]

Large graphene‐like molecules with four zigzag edges are ideal gain medium materials for organic near‐infrared (NIR) lasers. However, synthesizing them becomes increasingly challenging as the molecular size increases. In this study, we introduce a new intramolecular radical‐radical coupling approach and successfully synthesize two fused triangulene dimers (1 a/1 b) efficiently. X‐ray crystallographic analysis of 1 a indicates that there is no intermolecular π‐π stacking in the solid state. When the more soluble derivative 1 b is dispersed in polystyrene thin films, amplified spontaneous emission in the NIR region is observed. Using 1 b as the active gain material, we fabricate solution‐processed distributed feedback lasers that exhibit a narrow emission linewidth at around 790 nm. The laser devices also exhibit low thresholds with high photostability. Our study provides a new synthetic strategy for extended nanographenes, which have diverse applications in electronics and photonics. [ABSTRACT FROM AUTHOR]

Located in western Cuba, the Jaruco sub-basin is one of the most important karstic aquifer systems in the country. The imposed exploitation regime and the periods of drought that frequently affect the territory, cause the decrease of its groundwater levels and the need to locate artificial recharge wells that allow the water recovery of this aquifer. The processing and interpretation of the geophysical methods applied in the study area, showed that the longitudinal conductance is an effective parameter to identify the most favorable zones where to inject water, from the correlation obtained between this parameter and the hydraulic diffusivity of the aquifer. The results show that the northwest region of the investigated area presents the best conditions to locate artificial recharge wells. [ABSTRACT FROM AUTHOR]

Zigzag edged nanographenes such as peri‐acenoacenes are promising materials for organic lasers, but the effects of heteroatom doping on the electronic properties and gain medium performance remain unclear. Herein, the facile synthesis of a new 1,2‐oxaborine (BO) doped peri‐tetracenotetracene derivative, the bis(1,2‐oxaborine)peri‐tetracenotetracene (BOTT‐Mes), is reported. X‐ray crystallographic analysis confirms the BO‐doped planar structure and the nonexistence of intermolecular π–π stacking in solid state. Compared with the all‐carbon peri‐tetracenotetracene derivative (TT‐Ar), the BO‐doped BOTT‐Mes exhibits more disrupted π‐conjugation at the BO sites, a lower‐lying highest occupied molecular orbital, and a larger energy gap. Due to its rigid skeleton and nonaggregative feature, it displays well‐resolved absorption and emission spectra with a small Stokes shift (8 nm) and high photoluminescence quantum yield (80%) when it is dispersed in a polystyrene (PS) thin film. Notably, 1,2‐oxaborine doping improves the film amplified spontaneous emission (ASE) performance, with a lower ASE threshold (Eth‐ASE = 66 µJ cm–2) as compared to the TT‐Ar doped PS film (Eth‐ASE = 100 µJ cm–2). Furthermore, a low threshold (22 µJ cm–2) solution‐processed distributed feedback laser is fabricated, indicating the feasibility of using BOTT‐Mes as gain medium for practical laser applications. [ABSTRACT FROM AUTHOR]

This paper aims to review the recent advances achieved in the field of organic solid-state lasers with respect to the usage of semiconducting organic molecules and oligomers in the form of thin films as active laser media. We mainly focus on the work performed in the last few years by our research group. The amplified spontaneous emission (ASE) properties, by optical pump, of various types of molecules doped into polystyrene films in waveguide configuration, are described. The various systems investigated include N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), several perilenediimide derivatives (PDIs), as well as two oligo-phenylenevinylene derivatives. The ASE characteristics, i.e., threshold, emission wavelength, linewidth, and photostability are compared with that of other molecular materials investigated in the literature. [ABSTRACT FROM AUTHOR]

Abstract: We analyse the optimized geometric structure and energies of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) neutral molecule in ground and excited states. Ab initio calculations are performed at the Hartree–Fock (HF) or single-excitation (CIS) level with 6-31G basis set using Gaussian 98 program. We got a good qualitative agreement with the results for the neutral molecule in the ground state obtained by density functional theory methods, which were previously reported in the literature. TPD in the ground state has a pronounced twisted geometry, with both central and peripheral dihedral angles of about 40°. On the other hand, the structure of excited state is more planar, as the central dihedral angle becomes smaller. We estimated the reorganization energies associated with the optical transitions, corresponding transition dipole moments, as well as HOMO and LUMO. On the basis of ab initio calculations, we developed a semi-empirical model for the observed absorption and photoluminescence and interpreted the large Stokes shift of about 0.4eV. [Copyright &y& Elsevier]

The search of compounds emitting in the near‐infrared (NIR) has been accelerated owing to their use in biomedical and telecommunications applications. In this regard, nanographenes (NGs) are attractive materials adequate for integration with other technologies, which have recently demonstrated amplified spontaneous emission (ASE) and lasing across the visible spectrum. Here, the optical and ASE properties of four‐zigzag edged NGs of the [m,n]peri‐acenoacene family are reported, whose size is increased through conjugation extension by varying n (from 3 to 5) while keeping m = 2. Results show that such 1D conjugation extension method is more efficient in terms of shifting the photoluminescence (PL) to the infrared (PL at 710 nm in the larger compound, PP‐Ar) than through 2D conjugation extension as in previously reported NGs (PL at 676 nm with the largest compound FZ3, with n = 3 and m = 4). Additionally, PP‐Ar shows dual‐ASE (at 726 and 787 nm), whose origin is elucidated through Raman and transient absorption spectroscopies. These compounds' potential for red and NIR lasing is demonstrated through the fabrication of distributed feedback lasers with top‐layer resonators. This study paves the way towards the development of stable low‐cost all‐plastic NIR lasers. [ABSTRACT FROM AUTHOR]

All‐solution processed surface‐emitting organic distributed feedback lasers are attractive devices for low‐cost applications. Here, lasers emitting in the spectral region between 375 and 475 nm, in which both active material and resonator (1D relief gratings) are based on solution‐processable polymer films, are reported. Ten different organic compounds dispersed in polystyrene are used as active layers of the prepared devices. They include various carbon‐bridged oligo(p‐phenylenevinylene) (COPVn, with n = 1,2) derivatives and two terfluorene compounds. The synthesis and complete optical and amplified spontaneous emission properties of one of the COPV1 compounds, COPV1(Me)‐t‐Bu, designed for deep‐blue emission, are also included. The feasibility of the resonator fabrication, performed by holographic lithography with a dichromated gelatine photoresist over the active film, is successfully demonstrated for all devices. Remarkably, no resolution limitations are found even for the lowest grating period (235 nm) required for the fabrication of the laser based on COPV1(Me)‐t‐Bu. It is also demonstrated that the rectangular grating profile with duty cycle 0.75:0.25 (hill:valley) is very convenient to optimize the resonator efficacy. [ABSTRACT FROM AUTHOR]

Abstract: We analyse the optical spectra of -diphenyl--bis(3-methyl-phenyl)-(-biphenyl)--diamine (TPD) doped polystyrene films. The aim of the present paper is to give a microscopic interpretation of the significant Stokes shift between absorption and photoluminescence, which makes this material suitable for stimulated emission. The optimized geometric structures and energies of a neutral TPD monomer in ground and excited states are obtained by ab initio calculations using Hartree–Fock and density functional theory. The results indicate that the second distinct peak observed in absorption may arise either from a group of higher electronic transitions of the monomer or from the lowest optical transitions of a TPD dimer. [Copyright &y& Elsevier]

Perylene‐fused, aggregation‐free polycyclic aromatic hydrocarbons with partial zigzag periphery (ZY‐01, ZY‐02, and ZY‐03) were synthesized. X‐ray crystallographic analysis reveals that there is no intermolecular π–π stacking in any of the three molecules, and as a result, they show moderate‐to‐high photoluminescence quantum yield in both solution and in the solid state. They also display the characteristic absorption and emission spectra of perylene dyes. ZY‐01 and ZY‐02 with a nearly planar π‐conjugated skeleton exhibit amplified spontaneous emission (ASE) when dispersed in polystyrene thin films. Solution‐processed distributed feedback lasers have been fabricated using ZY‐01 and ZY‐02 as active gain materials, both showing narrow emission linewidth (<0.4 nm) at wavelengths around 515 and 570 nm, respectively. In contrast, ZY‐03 did not show ASE and lasing, presumably due to its highly twisted backbone, which facilitates nonradiative internal conversion and intersystem crossing. [ABSTRACT FROM AUTHOR]

Perylene‐fused, aggregation‐free polycyclic aromatic hydrocarbons with partial zigzag periphery (ZY‐01, ZY‐02, and ZY‐03) were synthesized. X‐ray crystallographic analysis reveals that there is no intermolecular π–π stacking in any of the three molecules, and as a result, they show moderate‐to‐high photoluminescence quantum yield in both solution and in the solid state. They also display the characteristic absorption and emission spectra of perylene dyes. ZY‐01 and ZY‐02 with a nearly planar π‐conjugated skeleton exhibit amplified spontaneous emission (ASE) when dispersed in polystyrene thin films. Solution‐processed distributed feedback lasers have been fabricated using ZY‐01 and ZY‐02 as active gain materials, both showing narrow emission linewidth (<0.4 nm) at wavelengths around 515 and 570 nm, respectively. In contrast, ZY‐03 did not show ASE and lasing, presumably due to its highly twisted backbone, which facilitates nonradiative internal conversion and intersystem crossing. [ABSTRACT FROM AUTHOR]

The efficient synthesis and electronic properties of two large‐size cove‐edged nanographenes (NGs), CN1 and CN2, are presented. X‐ray crystallographic analysis reveals a contorted backbone for both molecules owing to the steric repulsion at the inner cove position. Noticeably, the dominant structures of these molecules contain four (for CN1) or six (for CN2) localized C=C double bonds embedded in nine (for CN1) or twelve (for CN2) aromatic sextet rings according to Clar's formula, which is supported by bond length analysis and theoretical (NICS, ACID) calculations. Furthermore, Raman spectra exhibit a band associated with the longitudinal CC stretching mode of olefinic double bonds. Owing to the existence of the additional olefinic bonds, both compounds show a small band gap (1.84 eV for CN1 and 1.37 eV for CN2). They also display moderate fluorescence quantum yield (35 % for CN1 and 50 % for CN2) owing to the contorted geometry, which can suppress aggregation in solution. [ABSTRACT FROM AUTHOR]

The efficient synthesis and electronic properties of two large‐size cove‐edged nanographenes (NGs), CN1 and CN2, are presented. X‐ray crystallographic analysis reveals a contorted backbone for both molecules owing to the steric repulsion at the inner cove position. Noticeably, the dominant structures of these molecules contain four (for CN1) or six (for CN2) localized C=C double bonds embedded in nine (for CN1) or twelve (for CN2) aromatic sextet rings according to Clar's formula, which is supported by bond length analysis and theoretical (NICS, ACID) calculations. Furthermore, Raman spectra exhibit a band associated with the longitudinal CC stretching mode of olefinic double bonds. Owing to the existence of the additional olefinic bonds, both compounds show a small band gap (1.84 eV for CN1 and 1.37 eV for CN2). They also display moderate fluorescence quantum yield (35 % for CN1 and 50 % for CN2) owing to the contorted geometry, which can suppress aggregation in solution. [ABSTRACT FROM AUTHOR]

The standard procedure to identify the hole- or electron-acceptor character of amorphous organic materials used in OLEDs is to look at the values of a pair of basic parameters, namely, the ionization potential (IP) and the electron affinity (EA). Recently, using published experimental data, the present authors showed that only IP matters, i.e., materials with IP > 5.7 (<5.7) showing electron (hole) acceptor character. Only three materials fail to obey this rule. This work reports ab initio calculations of IP and EA of those materials plus two materials that behave according to that rule, following a route which describes the organic material by means of a single molecule embedded in a polarizable continuum medium (PCM) characterized by a dielectric constant ε. PCM allows to approximately describe the extended character of the system. This "compound" system was treated within density functional theory (DFT) using several combinations of the functional/basis set. In the preset work ε was derived by assuming Koopmans' theorem to hold. Optimal ε values are in the range 4.4–5.0, close to what is expected for this material family. It was assumed that the optical gap corresponds to the excited state with a large oscillator strength among those with the lowest energies, calculated with time-dependent DFT. Calculated exciton energies were in the range 0.76–1.06 eV, and optical gaps varied from 3.37 up to 4.50 eV. The results are compared with experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Thin‐film organic lasers are attractive light sources for a variety of applications. Recently, it is reported that carbon‐bridged oligo(p‐phenylenevinylene)s (COPVn with repeating unit n = 1–6) function as unique laser dyes which combine high fluorescence efficiency, wavelength tunability, and both thermal and photostability, making them ideal for use in organic semiconductor lasers. However, in order to obtain such excellent properties, COPVn require blending in a matrix, such as a thermoplastic polymer, thus leading to miscibility issues, limited absorption, and charge transporting properties. Here, high‐performance lasers with a novel active polymer poly‐COPV1, based on the basic unit of COPV1 and prepared as a high‐quality neat film, are reported which overcome the trade‐off between the device performance and durability. The prepared lasers show thresholds 30 times lower and operational lifetimes 300 times longer than devices based on COPV1 dispersed in polystyrene. The low threshold operation allows the poly‐COPV1 lasers to be pumped by a nitride diode laser. [ABSTRACT FROM AUTHOR]

Carbon-bridged bis(aminoaryl) oligo( para-phenylenevinylene)s have been prepared and their optical, electrochemical, and structural properties analyzed. Their radical cations are class III and class II mixed-valence systems, depending on the molecular size, and they show electronic couplings which are among the largest for the self-exchange reaction of purely organic molecules. In their dication states, the antiferromagnetic coupling is progressively tuned with size from quinoidal closed-shell to open-shell biradicals. The data prove that the electronic coupling in the radical cations and the singlet-triplet gap in the dications show similar small attenuation factors, thus allowing charge/spin transfer over rather large distances. [ABSTRACT FROM AUTHOR]

Carbon-bridged bis(aminoaryl) oligo( para-phenylenevinylene)s have been prepared and their optical, electrochemical, and structural properties analyzed. Their radical cations are class III and class II mixed-valence systems, depending on the molecular size, and they show electronic couplings which are among the largest for the self-exchange reaction of purely organic molecules. In their dication states, the antiferromagnetic coupling is progressively tuned with size from quinoidal closed-shell to open-shell biradicals. The data prove that the electronic coupling in the radical cations and the singlet-triplet gap in the dications show similar small attenuation factors, thus allowing charge/spin transfer over rather large distances. [ABSTRACT FROM AUTHOR]

Solution-processed polymer films are used in multiple technological applications. The presence of residual solvent in the film, as a consequence of the preparation method, affects the material properties, so films are typically subjected to post-deposition thermal annealing treatments aiming at its elimination. Monitoring the amount of solvent eliminated as a function of the annealing parameters is important to design a proper treatment to ensure complete solvent elimination, crucial to obtain reproducible and stable material properties and therefore, device performance. Here we demonstrate, for the first time to our knowledge, the use of an organic distributed feedback (DFB) laser to monitor with high precision the amount of solvent extracted from a spin-coated polymer film as a function of the thermal annealing time. The polymer film of interest, polystyrene in the present work, is doped with a small amount of a laser dye as to constitute the active layer of the laser device and deposited over a reusable DFB resonator. It is shown that solvent elimination translates into shifts in the DFB laser wavelength, as a consequence of changes in film thickness and refractive index. The proposed method is expected to be applicable to other types of annealing treatments, polymer-solvent combinations or film deposition methods, thus constituting a valuable tool to accurately control the quality and reproducibility of solution-processed polymer thin films. [ABSTRACT FROM AUTHOR]

The human epidermal growth factor receptor 2 (ErbB2) protein plays an important role in human malignancies. Its overexpression has been recognized as a feature of a malignant cancerous phenotype in breast cancer cell lines, and has become one of the most widely investigated clinical indicators of breast, ovarian, gastrointestinal and lung cancers. In this work a vertically emitting organic distributed feedback (DFB) laser has been used to detect the ErbB2 protein. This DFB laser consists of a polystyrene (PS) film containing a perylenediimide laser dye, deposited over a second-order one dimensional grating fabricated on fused silica by thermal-nanoimprint lithography and subsequent reactive ion etching processes. Specificity of the system to ErbB2 protein biomarker, achieved by functionalizing the PS with anti-ErbB2 monoclonal antibodies, is demonstrated. A concentration limit of detection for ErbB2 protein of 14 ng/mL has been obtained, and no cross-reactivity has been observed with bovine serum albumin (BSA) and tumor necrosis factor alpha (TNFα) proteins. These findings open the possibility of using this type of biosensors in clinical applications. [ABSTRACT FROM AUTHOR]

The refractive index sensing capabilitity of distributed feedback (DFB) lasers based on highly photostable (under ambient conditions) perylenediimide-based active films, are reported. The sensor bulk sensitivity is determined from changes in the laser emission wavelength upon exposure to different liquids. The role of the active film thickness ( h f ) on the sensor sensitivity and on the laser parameters is studied. Sensors based on very thin films ( h f = 160 nm) show the highest sensitivities, but their laser thresholds are relatively high and their operational durabilities moderate. The use of thicker films ( h f = 850 nm) allows reducing the laser threshold and increasing the durability by two orders of magnitude. In this case, a higher sensitivity is achieved when the sensor operates at the wavelength corresponding to the first-order TE 1 mode, instead of at the TE 0 one. Finally, it is also shown that the inclusion of a high refractive index TiO 2 layer on top of the sensor structure improves the sensitivity by around two times. [ABSTRACT FROM AUTHOR]

Amplified spontaneous emission (ASE) in dye‐doped mesostructured silica films, deposited directly over fused silica substrates, is demonstrated with low thresholds. Due to functionalization of the silica network with phenyl groups, the refractive index of the active layer is sufficiently increased to impart waveguiding properties that lead to highly efficient ASE. Such functionalization eliminates the need for a separate low‐refractive‐index layer between the active film and the substrate, as is often used to achieve waveguiding properties with silica mesostructured materials, though which is typically accompanied by significant optical losses that reduce overall ASE performance. By forming a high‐quality waveguide without the need for a separate low‐refractive‐index layer, significantly improved properties are obtained, with ASE thresholds as low as 8 kW cm−2. Solid‐state 2D NMR spectroscopy measurements establish the compositions and structures of the active mesostructured film at a molecular level, in particular that phenyl functionalization of the mesochannel surfaces promotes the homogeneous dispersion of the laser dye species over nanoscale dimensions in the materials. [ABSTRACT FROM AUTHOR]

Highlights: [•] Emission spectra of DFBs under excitation with different spot diameters are studied. [•] High quality PDI-based DFB devices have been obtained by direct T-NIL. [•] Direct T-NIL shows clear advantages to obtain low cost DFB devices. [ABSTRACT FROM AUTHOR]

Amplified spontaneous emission (ASE) is observed, under optical pump, in polystyrene films doped with two pentathienoacene derivatives functionalised with thienyl‐S,S‐dioxide groups (compounds 2 and 3). The effect of the dioxide groups on the ASE properties is analysed by comparing the performance with that of its corresponding non‐oxidized analogue (1). Films containing either 2 or 3 show ASE at 511 and 574 nm, respectively, when excited directly (at 435 nm) on their absorption bands, showing thresholds and linewidths larger than those obtained from films doped with 1, pumped at 355 nm. ASE is also observed under excitation at 355 nm, in samples containing 1 (host) and either 2 or 3 (guests), due to energy transfer from host to guest. For the blends with 3, the ASE threshold is lower than that obtained when the films are excited directly. Results are interpreted in terms of the photophysical parameters such as absorption capacity, fluorescence efficiency, singlet‐to‐triplet intersystem crossing leading to triplet‐triplet re‐absorptions, bimolecular energy‐transfer efficiency, efficiency of internal conversion process, etc. State‐of‐the‐art quantum chemical calculations are used in the interpretation of the experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The capability of phthalocyanines (Pc’s) to efficiently sensitize low glass transition temperature (Tg) poly(N-vinylcarbazole)-based composites for high-performance photorefractivity (PR) is reported for the first time. Gratings exhibiting total diffraction and two-beam-coupling coefficients as large as 350 cm−1are recorded in the millisecond range. The performance is compared to that of standard composites sensitized with C60showing advantages like much larger phase-shifts, highly convenient for applications based on optical gain. The dependence of the PR performance on the Pc concentration is studied in detail, revealing that only a very small amount of Pc is needed to obtain high efficiency, which allows us to minimize absorption losses and aggregation. The investigation of two different Pc derivatives provides relevant knowledge to design phthalocyanines with improved sensitization properties. [ABSTRACT FROM AUTHOR]

The molecule N,N′-bis(3-methylphenyl)-N,N′-dyphenylbenzidine (TPD) has been widely used in optoelectronic applications, mainly for its hole-transporting properties, but also for its capability to emit blue light and amplified spontaneous emission, which is important for the development of organic lasers. Here, we report deep-blue-emitting distributed feedback (DFB) lasers based on TPD dispersed in polystyrene (PS), as active media, and dichromated gelatin layers with holographically engraved relief gratings, as laser resonators. The effect of the device architecture (with the resonator located below or on top of the active layer) is investigated with a dye (TPD) that can be doped into PS at higher rates (up to 60 wt%), than with previously used dyes (<5 wt%). This has enabled changing the index contrast between film and resonator, which has an important effect on the laser performance. With regards to thresholds, both architectures behave similarly for TPD concentrations above 20 wt%, while for lower concentrations, top-layer resonator devices show lower values (around half). Remarkably, the operational durability of top-layer resonator devices is larger (in a factor of around 2), independently of the TPD concentration. This is a consequence of the protection offered by the resonator against dye photo-oxidation when the device is illuminated with pulsed UV light. [ABSTRACT FROM AUTHOR]

Abstract: The photorefractive (PR) properties, via photoconductivity, birefringence and two-beam-coupling experiments, of a composite containing a fullerene–trinitrofluorenone (C60–TNF) dyad in which the two moieties are linked by a conformationally flexible polyether chain are reported. The PR performance of this compound as a sensitizer in polymer composites based on poly(N-vinylcarbazole) (PVK) as hole charge transporter, piperidindicyanostyrene (PDCST) as non-linear optical chromophore and butyl benzyl phthalate (BBP) as plasticizer, has been compared to those of the corresponding C60 and TNF derivatives. Results show that the linkage of C60 and TNF leads to an improvement in the photoconductivity of the composite leading to an increase of the PR speed. [Copyright &y& Elsevier]

Abstract: The concentration dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene (PS) films containing a variable concentration (between 2.5 and 100wt%) of the luminescent and hole-transporting organic molecule N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) is reported. It is observed that both, the ASE threshold and linewidth above threshold, decrease with concentration up to 20wt% doped films and then keeps a constant value up to concentrations of 100wt% (neat films). Accordingly, photoluminescence (PL) efficiency increases with concentration up to 20wt% and then saturates. Results also show that the position of ASE can be tuned between 413 and 421nm by changing the concentration of TPD. These shifts are also observed in the PL emission and are presumably due to the increase of absorption, as well as polarity of the medium, with concentration. Measurements and modeling of the waveguiding characteristics of the films, show that the observed shifts are not due to cut-off thickness limitations. In addition, although the different confinement of the waveguide modes might be influencing the ASE performance, it seems to be determined mostly by the PL efficiency. Results show that the most efficient films are those doped with 20–30wt% TPD. Finally, a study of the photostability of the samples is also included. [Copyright &y& Elsevier]

High-transparency polymers, called optical polymers (OPs), are used in many thin-film devices, for which the knowledge of film thickness (h) and refractive index (n) is generally required. Spectrophotometry is a cost-effective, simple and fast non-destructive method often used to determine these parameters simultaneously, but its application is limited to films where h > 500 nm. Here, a simple spectrophotometric method is reported to obtain simultaneously the n and h of a sub-micron OP film (down to values of a few tenths of a nm) from its transmission spectrum. The method is valid for any OP where the n dispersion curve follows a two-coefficient Cauchy function and complies with a certain equation involving n at two different wavelengths. Remarkably, such an equation is determined through the analysis of n data for a wide set of commercial OPs, and its general validity is demonstrated. Films of various OPs (pristine or doped with fluorescent compounds), typically used in applications such as thin-film organic lasers, are prepared, and n and h are simultaneously determined with the proposed procedure. The success of the method is confirmed with variable-angle spectroscopic ellipsometry. [ABSTRACT FROM AUTHOR]

• The potential of the Variable Pump Intensity (VPI) method is thoroughly assessed. • Scattering losses of pasive devices and leaky-mode resonances are easily reveiled. • The VPI method does not perform too well in samples with multiple species. • Information about the excited state absorption spectrum can be retrieved. The increasing number of solution-processed laser compounds that can be implemented as low-cost, flexible, and/or integrated devices, makes necessary the development of reliable methods to delineate all their amplifying signatures and thus to open the door to appropriate cross-sample comparisons. Seeking to solve this problem, a new formalism to retrieve the losses and the optical gains from Amplified Spontaneous Emission (ASE) spectra as a function of the excitation density has been recently reported. In this manuscript, we explore the potential of this methodology to unveil relevant information on the photonic properties of the waveguiding devices and on the photophysics of the active materials. We demonstrate that the Variable Pump Intensity method opens the door to understand the relationship between the ASE thresholds and the optical gains and losses, it enables the extraction of the scattering/modal losses of the passive devices, and it can unveil the presence of leaky-modes and excited state absorption. In contrast, it does not perform too well in samples with multiple active species in its current implementation. We have substantiated all these findings using organic semiconductor thin films, several dye-doped polymer thin films and solutions of boron hydride. [ABSTRACT FROM AUTHOR]

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 deposited over high-quality DFB gratings engraved on SiO2 by nanoimprint lithography. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Chemically synthesized zigzag-edged nanographenes (NG) have recently demonstrated great success as the active laser units in solution-processed organic distributed feedback (DFB) lasers. Here, we report the first observation of dual amplified spontaneous emission (ASE) from a large-size NG derivative (with 12 benzenoid rings) dispersed in a polystyrene film. ASE is observed simultaneously at the 685 and 739 nm wavelengths, which correspond to different transitions of the photoluminescence spectrum. Ultrafast pump-probe spectroscopy has been used to ascertain the underlying photophysical processes taking place in the films. DFB lasers, based on these materials and top-layer nanostructured polymeric resonators (i.e., one or two-dimensional surface relief gratings), have been fabricated and characterized. Lasers emitting close to either one of the two possible ASE wavelengths, or simultaneously at both of them, have been prepared by proper selection of the resonator parameters. [ABSTRACT FROM AUTHOR]

• Film thickness in the range 400–40 nm is calculated from spectrophotometric trace. • Simulation and experimental data are compared in the transparent spectral window. • Accuracy of 5 nm is achieved even for films with low refractive index (1.5–1.7). • Refractive index errors hinder the method applicability as thickness decreases. The design and fabrication of thin-film based organic optoelectronic devices require knowledge of the film optical properties. A low-cost and non-destructive method often used for optical characterization of films is the well-established spectrophotometric envelope method. However, this method is typically limited to thickness above 400 nm, a value often higher than that of the films involved in these devices. This paper studies a procedure to obtain the thickness of sub-400 nm active films from their spectrophotometric trace when the refractive index is previously known. The proposed procedure is based on comparing the experimental transmission spectrum in the transparent spectral window with that obtained by simulation. The capabilities of the proposed method are demonstrated here by its application in the fabrication of organic distributed feedback lasers, for which a fine control of film thickness is important to obtain an optimized and reproducible response. Results are verified with other techniques, such as ellipsometry and profilometry. Thus, with the proposed method, film thickness can be easily determined down to 40 nm maintaining an accuracy of about 5 nm even for films with low refractive index (1.5–1.7). Different methods to determine refractive index of these films are also discussed. [ABSTRACT FROM AUTHOR]

We report on the photorefractive (PR) properties at the 633 nm laser wavelength of a polymer composite based on the polymer poly(n-vinyl carbazole) (PVK), doped with the dicyanostyrene derivative 4-piperidinobenzylidene-malonitrile (PDCST) as nonlinear optical chromophore and the liquid plasticizer butyl benzyl phthalate (BBP), without the presence of sensitizer. The PR-effect is observed only when samples are previously subjected to an electric field (i.e., 20 V/μm for 10 min). Photoconductivity and birefringence of the composite become significant when the electric field treatment is performed at temperatures higher than room temperature (24 °C). Gain coefficient and PR speed, determined from two-beam coupling experiments, are compared to those obtained with the PVK/PDSCT/BBP/C60 standard sensitized composite. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Surface-emitting distributed feedback (DFB) lasers with both, resonator and active material based on solution-processable polymers, are attractive light sources for a variety of low-cost applications. Besides, the lasers should have competitive characteristics compared to devices based on high-quality inorganic resonators. Here, we report high performing all-solution-processed organic DFB lasers, consisting of water-processed photoresist layers with surface relief gratings located over the active films, whose emission properties can be finely tuned through resonator design. Their laser threshold and efficiency are simultaneously optimized by proper selection of residual resist thickness and grating depth, d. Lowest thresholds and largest efficiencies are obtained when there is no residual layer, while a trade-off between threshold and efficiency is found in relation to d, because both parameters decrease with decreasing d. This behaviour is successfully explained in terms of an overlap factor r, defined to quantify the interaction strength between the grating and the light emitted by the active film and traveling along it, via the evanescent field. It is found that optimal grating depths are in the range 100–130 nm (r ~ 0.5−0.4). Overall, this study provides comprehensive design rules towards an accurate control of the emission properties of the reported lasers. [ABSTRACT FROM AUTHOR]