Organic Lasers Research Articles

Design and Synthesis of Conjugated Starburst Molecules for Optoelectronic Applications.

In recent years, conjugated starburst molecules, which possess a core unit with radial arms linked to the central axle, have become the research topic owing to their well-defined chemical structures, good solution processability, excellent reproducibility, and superior optoelectronic properties. The increasing interest in starburst systems is evidenced by progressively more frequent investigation on the use of these materials in optoelectronics. The ability to modify chemical structures through control over the core and arms on a molecular level can directly affect the electronic and electroluminescent characteristics of the resulting materials. In this review, we summarize and discuss main progress in our group concerning the rapidly developing field, in which strategies for the design and construction of starbursts are presented at length. Moreover, their application in organic light-emitting diodes (OLEDs) and organic semiconductor lasers (OSLs) are demonstrated as well, exploring the influence of molecular structures on the optoelectronic properties. Challenges and outlooks are also given at last.

Graphitic coated Al nanoparticles manufactured as superior energetic materials via laser ablation synthesis in organic solvents

The large heat release predicted in the early investigations of energetic aluminum nanoparticles (Al NPs) used in solid-state propulsion and pyrotechnics has been offset by hindered diffusion-limited detonation rates due to excess oxide shell formations and surface area loss from aggregations. We address these challenges by manufacturing graphitic shell coated Al NPs (<20 nm sizes) via laser ablation synthesis in solution (LASiS) to preserve high surface areas and interfacial properties of Al NPs. Specifically, we use a high-energy laser to ablate Al pellets confined in either acetone or toluene to coat the laser-ablated Al NPs with graphitic shells generated from the thermal pyrolysis of the organic solvents. Energetic activities of the C/Al NPs were tested via the laser-induced air shock from energetic materials (LASEM) technique. We demonstrate that synthesis parameters such as organic solvents, laser flux and ablation times can be tuned to provide superior control of NP sizes/aggregation with the aid of the C shell formations and, in turn, their energetic behavior. This study unveils the synthesis-structure-property relations in LASiS-based manufacturing of energetic nanocapsules within graphitic shells that are safe to handle and can undergo kinetically controlled spontaneous energy release under desired conditions.

Koloratzaile-laserra, ezinbesteko argi iturria eguneroko bizitzan

Gure eguneroko bizitzan erabiltzen ditugun hainbat gailu elektroniko laser-igorpenean oinarritzen dira. Erradiazio mota honen ezaugarri bereziak, hots, monokromatizitatea, koherentzia eta norabide zuzena, aplikazio arrakastatsu eta eraginkorraren arrazoi nagusia dira. Lan honetan, gure helburu nagusia izan da laserren garrantzia agerian jartzea eta azaltzea zergatik agertzen den erradiazio mota hori. Horretarako, laserren oinarria azaldu da eta argiaren igorpen-mota hori lortzeko beharrezkoak diren eskakizunak aipatu dira. Bukaeran, laser organiko izeneko gailuetan ipini dugu arreta; horietan, koloratzaile fluoreszenteak erabiltzen dira ingurune aktibo gisa. Ildo honetatik, gure ikerkuntza-taldean azken urteetan orokorrean egindako aurrerapausoak iruzkintzen dira. Egindako ikerkuntzaren helburua, laser-seinalearen eraginkortasuna eta ponpaketaren aurrean egonkortasuna areagotzeaz gain, espektro-eremu ultramore ikusgai osoan zehar koloratzaile-laserrak egoera likidoan zein solidoan lortzea izan da.

Glass-forming derivatives of 2-cyano-2-(4H-pyran-4-ylidene) acetate for light-amplification systems

A series of 2-cyano-2-(4H-pyran-4-ylidene) acetate derivatives with triphenyl and 9H-carbazole moieties were synthesized and investigated, mostly for potential applications in organic solid state lasers. Synthesized compounds show remarkable amorphous film formation ability, tunable thermal properties (thermal stability varies from 190 °C to 387 °C and glass transition temperature from 94 °C up to 141 °C) with light absorption from 400 nm to 600 nm and photoluminescence from 600 nm up to 800 nm. Dyes with incorporated mono-styryl- electron donating fragment (KTB, KTBC and KTB3K) showed higher photoluminescence quantum yield (PLQY) (from 16% up to 23%), significantly lower amplified spontaneous emission (ASE) excitation threshold values (24–52 μJ/cm2) in pure thin films comparing to bis-styryl- electron donating fragment containing molecules (Bis-K4C, Bis-K5C and Bis-K4CK) with PLQY of 5–7% and ASE excitation threshold between 165 and 223 μJ/cm2. Some of the investigated KTB-type laser dyes could be perspective materials for organic lasers as well as for laser technology investigation and applications.

Organic Polariton Lasing and the Weak to Strong Coupling Crossover.

Following experimental realizations of room temperature polariton lasing with organic molecules, we present a microscopic model that allows us to explore the crossover from weak to strong matter-light coupling. We consider a nonequilibrium Dicke-Holstein model, including both strong coupling to vibrational modes and strong matter-light coupling, providing the phase diagram of this model in the thermodynamic limit. We discuss the mechanism of polariton lasing, uncovering a process of self-tuning, and identify the relation and distinction between regular dye lasers and organic polariton lasers.

Concurrent Optical Gain Optimization and Electrical Tuning in Novel Oligomer:Polymer Blends with Yellow-Green Laser Emission.

Electrically pumped organic lasing requires the integration of electrodes contact into the laser cavity in an organic light‐emitting diode (OLED) or organic field effect transistor configuration to enable charge injection. Efficient and balanced carrier injection requires in turn alignment of the energy levels of the organic active layers with the Fermi levels of the cathode and anode. This can be achieved through chemical substitution with specific aromatic functional groups, although paying the price for a substantial (and often detrimental) change in the emission and light amplifying properties of the organic gain medium. Here, using host–guest energy transfer mixtures with hosts bearing a systematic and gradual shift in molecular orbitals is proposed, which reduces the amplified spontaneous emission (ASE) threshold of the organic gain medium significantly while leaving the peak emission unaffected. By virtue of the low guest doping required for complete host‐to‐guest energy transfer, the injection levels in the blends are attributed to the host whereas the gain properties solely depend on the guest. It is demonstrated that the ASE peak and thresholds of blends with different hosts do not differ while the current efficiency of OLEDs devices is deeply influenced by molecular orbital tuning of the hosts.

High-performance crystalline organic light-emitting diodes based on multi-layer high-quality crystals

Abstract Crystalline organic light-emitting diodes (OLEDs) are of great potential for the applications in solid-state lighting and electrically pumped organic lasers. Now the performance of crystalline OLEDs is poor because those devices typically use a single organic material with unmatched electron and hole mobilities resulting in imbalance charge injection. Here, we fabricate a crystalline OLED with a multi-layer architecture based on high-quality crystalline films. The device exhibits low driving voltage and high efficiency due to balanced charge injection and exciton confinement. An external quantum efficiency (EQE) of 2.41% is achieved, and the EQE value is the highest efficiency in the field of crystalline OLEDs to date. We expect that our results will be a starting point for further research, providing high-performance crystalline OLEDs for solid-state lighting and electrically pumped organic lasers.

Influence of Blending Ratio and Polymer Matrix on the Lasing Properties of Perylenediimide Dyes

Perylenediimide (PDI) dyes dispersed in polymer films have demonstrated great success as active materials in thin-film organic lasers (TFOLs). The type of matrix used to host the dye and the dye doping rate are both crucial parameters to optimize laser performance. This work reports the study of two soluble PDIs, the comercial derivative perylene orange (PDI-O) emitting at around 580 nm, and a new dye (b-PDI-A) with substituents at the 1,7 bay positions of the PDI core emitting at around 620 nm, dispersed at different doping levels (up to 8 and 50 wt %, for PDI-O and b-PDI-A, respectively) in two widely used polymers for optoelectronics, polystyrene (PS) and poly(methyl methacrylate) (PMMA). The main goal is to determine which of these two polymers, and at which dye concentration, provides the best results for their use in TFOLs. The assessment of the active materials has been carried out through the analysis of their absorption, photoluminescence, and amplified spontaneous emission (ASE) properties. Thei...

Low Amplified Spontaneous Emission Threshold and Efficient Electroluminescence from a Carbazole Derivatized Excited-State Intramolecular Proton Transfer Dye

The search for new classes of efficient electroluminescent organic laser dyes with low amplified spontaneous emission (ASE) threshold (Eth) and low optical losses is crucial toward the realization of organic injection lasers. In this work, we report in-depth studies of an organic semiconducting laser dye, benzo[d]thiazol-2-yl)-5-(9H-carbazol-9-yl)phenol (HBT-Cz), and compare the photophysical study with its parent HBT dye. While HBT is not laser-active in solution, the carbazole derivatized dye (i.e., HBT-Cz) showed a low solution ASE threshold of 21 mJ/cm2, which is comparable to a common high-performing commercial laser dye, Rhodamine 6G (Rh-6G). HBT-Cz also exhibited a low film ASE threshold of 2.4 μJ/cm2 with a low optical loss coefficient of 1.5 cm–1, which is the lowest waveguide loss coefficient reported for solution-processed organic semiconductors. Organic light-emitting diodes based on this material showed a high maximum EQE of 1.9%, which is close to the theoretical EQE limit of the emitter, an...

Suppressing Nonradiative Processes of Organic Dye with Metal–Organic Framework Encapsulation toward Near-Infrared Solid-State Microlasers

Organic materials are an important class of gain media for fabricating miniaturized lasers because they combine fabrication simplicity with wide spectral coverage and tunability. However, progress toward near-infrared (NIR) organic solid-state lasers has been limited because of serious nonradiative processes originating from the severe intermolecular interaction in the condensed state. Here, we develop a strategy to realize room-temperature NIR microscale lasers through encapsulating organic dyes into the cavities of metal-organic frameworks (MOFs). The spatial confinement of the dye molecules within the MOF pores contributes to suppressing the multiple nonradiative processes (i.e., aggregation-caused quenching and exciton-exciton annihilation). This results in a much higher radiative efficiency and thus much easier population inversion and low-threshold NIR lasing. Furthermore, the lasing wavelength can be further expanded based on the tailorable energy levels of the dye molecules. The results will provide useful enlightenment for the development of miniaturized NIR laser sources for new photonic applications.

Organic Semiconductor Laser Platform for the Detection of DNA by AgNP Plasmonic Enhancement.

Organic semiconductor lasers are a sensitive biosensing platform that respond to specific biomolecule binding events. So far, such biosensors have utilized protein-based interactions for surface functionalization but a nucleic acid-based strategy would considerably widen their utility as a general biodiagnostic platform. This manuscript reports two important advances for DNA-based sensing using an organic semiconductor (OS) distributed feedback (DFB) laser. First, the immobilization of alkyne-tagged 12/18-mer oligodeoxyribonucleotide (ODN) probes by Cu-catalyzed azide alkyne cycloaddition (CuAAC) or "click-chemistry" onto an 80 nm thick OS laser film modified with an azide-presenting polyelectrolyte monolayer is presented. Second, sequence-selective binding to these immobilized probes with complementary ODN-functionalized silver nanoparticles, is detected. As binding occurs, the nanoparticles increase the optical losses of the laser mode through plasmonic scattering and absorption, and this causes a rise in the threshold pump energy required for laser action that is proportional to the analyte concentration. By monitoring this threshold, detection of the complementary ODN target down to 11.5 pM is achieved. This complementary binding on the laser surface is independently confirmed through surface-enhanced Raman spectroscopy (SERS).

Enhancing Optical Gain Stability for a Deep-Blue Emitter Enabled by a Low-Loss Transparent Matrix

Efficient organic gain media with high optical gain and good photostability are highly desirable for organic semiconductor lasers. To shed light on the rational molecular design, in this work, a novel deep-blue organic emitter consisting of a carbazole core substituted by a diphenylamine-functionalized triazine unit and two arms of bifluorene with flexible alkyl chains (denoted as C2) was designed and synthesized. Various basic photophysics and amplified spontaneous emission (ASE) characterizations were conducted with a particular focus to unravel the impact of the molecular structure on the photostability. Relatively low ASE threshold of 7.0 μJ/cm2, high net gain coefficient of 79.9 cm–1 (under 40-fold threshold pump fluence), and low optical loss coefficient of 7.0 cm–1 were recorded for C2 in neat films. For comparison, C2 was also dispersed into polystyrene (PS), a low-loss transparent matrix, with various ratios to form PS-C2 blend films. Among them, a 50 wt % PS-C2 blend film exhibited a high gain c...

High‐Speed OLEDs and Area‐Emitting Light‐Emitting Transistors from a Tetracyclic Lactim Semiconducting Polymer

AbstractDevelopment of a new class of luminescent materials that show high‐speed response, high charge carrier mobility, and high brightness is desirable toward realization of next generation of devices, such as electrically pumped organic lasers, visible light communication instruments, and organic light‐emitting transistors. In this paper, high‐speed organic light‐emitting diodes (OLEDs) and high‐performance hybrid light‐emitting transistors from a new type of solution processable luminescent material, poly[thiophene‐2,5‐diyl‐alt‐5,10‐bis((2‐hexyldecyl)oxy)dithieno[3,2‐c:3′,2′‐h][1,5]naphthyridine‐2,7‐diyl] (PTNT), are reported. The OLEDs based on PTNT polymer exhibit a peak brightness of 8 × 105 cd m−2 and 40 MHz modulation frequency under 10 ns pulse operation. This modulation frequency is significantly higher than that of commercially available LEDs, used for visible light communication. Additionally, solution‐processed area‐emitting hybrid light‐emitting transistors with an external quantum yield of 0.25% at brightness of 250 cd m−2 are demonstrated. Finally, the paper provides device physics and optoelectronic properties of PTNT polymer using ultraviolet photon spectroscopy, inversed photoelectron spectroscopy, and photophysical measurements.

Toward Electrically Pumped Polymer Lasing: Light‐Emitting Diodes Based on Microcavity Arrays of Distributed Bragg Gratings

AbstractElectrically pumped organic lasers have been a challenge for years. The most crucial problem is that the density of charge carriers is much lower than the threshold required for lasing as compared with optical pumping. Apparently, increasing the density of charge carriers and reducing the threshold are two practical routes for solving the problem. Reducing the pump threshold depends strongly on the structural design and the operation‐mode control in the device. Here, a large‐area array of microcavities is employed to construct the light‐emitting diodes (LEDs) using distributed Bragg gratings (DBGs) as high‐efficiency reflectors and the homogeneous poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐alt‐co‐(1,4‐benzo‐(2,1′,3)‐thiadiazole)] films in‐between as the active cavity body. Simultaneous lasing of multiple longitudinal modes at optical pumping verifies the optical feedback and oscillation mechanisms in such DBG‐based microcavities. Further successful construction of light‐emitting diodes using these microcavity arrays and the observation of narrowing of the electroluminescence spectrum due to the selectivity by the microcavities and the increase of the excitation electrical field suggest that the LED design here defines a promising step toward electrically pumped polymer lasers.

High quality factor microcavity OLED employing metal-free electrically active Bragg mirrors

Abstract A Fabry-Perot microcavity with a high quality Q-factor is an excellent tool to finely tune and narrow the emission spectrum of embedded organic dyes, leading also to a modification of the radiative emission rate (Purcell effect). For a real application of these properties in light emitting diodes and lasers, high Q-factors should be also provided in electrically-driven organic microcavities, that is still a challenge when organic materials are employed. Metallic contacts can be safely deposited onto organic layers, although they result in strong absorption losses. In this work, we successfully integrated an Organic LED architecture within specifically tailored metal-free electrically active Bragg mirrors, finely optimized to achieve simultaneously high reflectivity and good electrical properties. The resulting electroluminescent microcavity showed a Q-factor of more than 200 (emission linewidth of 2.7 nm at a peak wavelength of 555 nm) and a clear proof of the occurrence of Purcell effect leading to a decrease of exciton radiative lifetime by a factor above 6. Finally, we analysed the parameters that still limit the Q-factor of our architecture, paving the way for future improvements. The proposed approach can be exploited for the fabrication of novel monochromatic organic light sources for telecommunications or biological sensing and it represents an important step towards the realization of electrically driven organic lasers.

Organic Semiconductor Single Crystals for Electronics and Photonics.

Organic semiconducting single crystals (OSSCs) are ideal candidates for the construction of high-performance optoelectronic devices/circuits and a great platform for fundamental research due to their long-range order, absence of grain boundaries, and extremely low defect density. Impressive improvements have recently been made in organic optoelectronics: the charge-carrier mobility is now over 10 cm2 V-1 s-1 and the fluorescence efficiency reaches 90% for many OSSCs. Moreover, high mobility and strong emission can be integrated into a single OSSC, for example, showing a mobility of up to 34 cm2 V-1 s-1 and a photoluminescence yield of 41.2%. These achievements are attributed to the rational design and synthesis of organic semiconductors as well as improvements in preparation technology for crystals, which accelerate the application of OSSCs in devices and circuits, such as organic field-effect transistors, organic photodetectors, organic photovoltaics, organic light-emitting diodes, organic light-emitting transistors, and even electrically pumped organic lasers. In this context, an overview of these fantastic advancements in terms of the fundamental insights into developing high-performance organic semiconductors, efficient strategies for yielding desirable high-quality OSSCs, and their applications in optoelectronic devices and circuits is presented. Finally, an overview of the development of OSSCs along with current challenges and future research directions is provided.

Hole mobility and conductance of iridium complex doped NPB emission layers for organic light-emitting diodes

Iridium complexes are widely used in dye phosphorescent guest-host systems which usually as emission layer of efficient organic light-emitting diodes. Phosphorescent organic light-emitting diodes (OLEDs) with tris(2-phenylpyridine) iridium [Ir(ppy)3] as emissive dopant exhibited high internal quantum efficiency. N, N-diphenyl-N, N-bis(1-naphthylphenyl)-1,1-biphenyl-4,4-diamine (NPB) is a good candidate of host for guest–host system in OLEDs. It is significant that understanding and engineering of charge transport in Ir(ppy)3 doped NPB films to design and optimize OLEDs, even organic lasers. In this paper, hole mobility and conductance of Ir(ppy)3 doped NPB films were investigated by admittance spectroscopy. The results show that for a given applied voltage, with the doping concentration increasing, the conductance first increase reaching a maximum then decrease, which indicate that light doping of NPB with Ir(ppy)3 weakly improves, while heavy doping deteriorates the conductivity. We found that light doping of NPB with Ir(ppy)3 almost do not alter the hole mobility, which coincides with the previous theory. It is concluded that for films thinner than 300 nm, the electric field dependence of hole mobility is negative, that is, the mobility decrease, rather than increase, near exponentially with the square root of the electric field.

Low Amplified Spontaneous Emission Threshold from Organic Dyes Based on Bis‐stilbene

AbstractAdvanced organic laser dyes exhibiting high solubility and bipolar behavior are developed based on a structure combining bis‐stilbene with carbazole (BSBCz). The materials show high photoluminescence quantum yields and large radiative rate constants in solutions, crystals, and blend and neat films. The introduction of alkyl groups significantly improves the solubility of BSBCz, and solution‐processed films of the alkyl‐substituted derivatives exhibit amplified spontaneous emission thresholds as low as 0.59 µJ cm−2, which is comparable to those of vacuum‐deposited BSBCz films. On the other hand, cyano‐substitution on BSBCz (BSBCz‐CN) increases electron‐accepting properties, resulting in a bathochromic shift of the emission wavelength and improved bipolar behavior. In a BSBCz‐CN‐doped film, a low ASE threshold of 0.63 µJ cm−2 is achieved, which is one of the lowest values for organic laser dyes with green emission. In addition, organic light‐emitting diodes based on BSBCz‐CN neat films exhibit external quantum efficiencies of 1.8% and could withstand injection of high current densities of up to 500 A cm−2 under pulse operation. These properties along with low excited‐state absorption cross sections make these materials an outstanding addition to the existing library of organic laser dyes, especially for consideration in electrically pumped lasers.

Design principle of high-performance organic single-crystal light-emitting devices

Organic semiconductor crystals exhibit unique optoelectronic characteristics such as high carrier mobility and laser oscillation. Nonetheless, it remains difficult thus far to achieve such potential activities on an optical device configuration mainly because of lack of knowledge of optical parameters such as refractive indices and their dispersion. Here, we demonstrate a simple but powerful design principle of high-performance organic single-crystal light-emitting devices. The method is based upon observation of emission-angle dependent spectra produced from an organic slab single crystal equipped with a one-dimensional diffraction grating. The emission spectra are characterized by sharply resolved lines whose locations are blueshifted or redshifted as a function of the emission angles. Detailed analysis of these emission lines has enabled us to solve equations of electromagnetic wave motion within and outside the slab crystal and to get solutions under appropriate boundary conditions. The design principle also allows us to relate crystal thicknesses and orders of both longitudinal and transverse modes of emission. Thus, we have been able to optimize the geometry of a slab organic crystal in an organic light-emitting device configuration. The relevant knowledge can directly be used for designing an organic laser either optically excited or electrically excited.

Properties of defect modes in two-dimensional organic octagonal quasicrystals at low dielectric contrast

We study theoretically the localization and coupling properties of defect modes in octagonal quasi-crystals made of conjugated polymers for the light modulation of organic light-emitting devices (OLED) and organic lasers. Due to the rotational symmetry of quasicrystals and the low refractive-index of organic semiconductors, it is observed that localized modes only occur in the complex defect-cavity structures. Unlike conventional photonic crystals with high-index, some unusual characteristics of defect modes coupling in quasicrystals under the low-index contrast are revealed and analyzed, such as the loss of defect modes, composite resonant cavity, and phase-opposition of modal tails. These results will allow us to better utilize the light control in organic photonic crystal microcavity lasers, especially in the localization region, and further to apply in OLED and VCSEL (Vertical Cavity Surface Emitting Laser) fields.