Spontaneous Emission Threshold Research Articles

Benzyl Alcohol-Treated CH3NH3PbBr3 Nanocrystals Exhibiting High Luminescence, Stability, and Ultralow Amplified Spontaneous Emission Thresholds.

We report the high yield synthesis of about 11 nm sized CH3NH3PbBr3 nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH3NH3PbBr3 nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 μJ cm-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.

Decrease of amplified spontaneous emission threshold achieved by core–shell Ag nanocube@SiO2 with ultrasmall shell thicknesses

The amplified spontaneous emission (ASE) threshold reduction of R6G/PMMA film with a quasi-waveguide structure was observed by doping core–shell Ag nanocube@SiO2. When the SiO2 shell thickness was only 3 nm, the ASE threshold declined from 0.8 to 0.2 mJ cm−2, presenting a large decreased ratio of 75%. With the enhancement of the emission intensity, the pulse width of the ASE spectra also narrowed from 8 to 6.8 nm after doping. An efficient coupling between the R6G media and Ag nanocube through the proper ultrathin spacer was concluded as the main reason for such phenomena.

Triple cation mixed-halide perovskites for tunable lasers

Metal halide perovskites are recently attracting strong attention due to their potential in solar cells, LEDs, and lasers. Here, we demonstrate the broad spectral tuning of the optical gain characteristics of triple cation (containing methylammonium, formamidinium and cesium) mixed-halide perovskite thin films. We explicitly study the interrelation between amplified spontaneous emission (ASE) thresholds, the operational stability of the gain, and the material composition. The incorporation of cesium and a deficiency of lead in the precursor solutions is found to be crucial for low ASE thresholds and the improved stability of mixed-halide perovskites. We tune the photoluminescence in mixed-halide perovskites between peak wavelengths of 510 and 790 nm by exchanging the halide from iodide to bromide and chloride in small steps of 10%, while preserving the narrowband emission below a linewidth of 130 meV for all mixtures. The optical gain under ns-excitation can be tuned over a significant portion of this spectral window; we observe ASE emission in regions between 545 to 555 nm and 680 to 810 nm. This is a significant step towards perovskite lasers operating through a broad portion of the visible to near infrared spectrum.

High-Efficiency Optical Gain in Type-II Semiconductor Nanocrystals of Alloyed Colloidal Quantum Wells.

Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm2, long optical gain lifetimes (τgain ≈ 400 ps), and high modal gain coefficients (gmodal ≈ 930 cm-1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.

Towards zero-threshold optical gain using charged semiconductor quantum dots.

Colloidal semiconductor quantum dots are attractive materials for the realization of solution-processable lasers. However, their applications as optical-gain media are complicated by a non-unity degeneracy of band-edge states, because of which multiexcitons are required to achieve the lasing regime. This increases the lasing thresholds and leads to very short optical gain lifetimes limited by nonradiative Auger recombination. Here, we show that these problems can be at least partially resolved by employing not neutral but negatively charged quantum dots. By applying photodoping to specially engineered quantum dots with impeded Auger decay, we demonstrate a considerable reduction of the optical gain threshold due to suppression of ground-state absorption by pre-existing carriers. Moreover, by injecting approximately one electron per dot on average, we achieve a more than twofold reduction in the amplified spontaneous emission threshold, bringing it to the sub-single-exciton level. These measurements indicate the feasibility of 'zero-threshold' gain achievable by completely blocking the band-edge state with two electrons.

Assembling Ordered Nanorod Superstructures and Their Application as Microcavity Lasers

Herein we report the formation of multi-layered arrays of vertically aligned and close packed semiconductor nanorods in perfect registry at a substrate using electric field assisted assembly. The collective properties of these CdSexS1-x nanorod emitters are harnessed by demonstrating a relatively low amplified spontaneous emission (ASE) threshold and a high net optical gain at medium pump intensity. The importance of order in the system is highlighted where a lower ASE threshold is observed compared to disordered samples.

A High Performance Deep Blue Organic Laser Gain Material

High performance organic lasers with both good optical lasing property and desirable carrier mobility still remains a big challenge for realizing electrically pumped organic lasing. In this work, we have developed a carbazole‐end‐capped ladder‐type oligo‐(p‐phenylene) based hybrid oligomer as solution‐processable deep blue laser gain media to reach a good balance between optical property and carrier transporting ability, by introducing moderate steric hindrance into the pi‐conjugated planar oligo‐(p‐phenylene) backbone. This material demonstrates excellent laser performance and moderate hole mobility of 1 × 10–3 cm2 V−1 s−1. The 1‐D distributed feedback (DFB) laser exhibits very low lasing threshold of 0.6 nJ pulse−1 and high slope efficiency of 5.9 ± 0.3% (50% fill factor). More importantly, the prominent ASE performance maintained at very thin film samples. The amplified spontaneous emission (ASE) threshold remains very low (16 nJ pulse−1) in the 45 nm‐thickness film. These results might indicate possibilities for electrically pumped gain materials.

Reduced quenching effects of organic gain media with metallic electrodes via introducing a conjugated macroelectrolyte interlayer

We have demonstrated amplified spontaneous emission (ASE) restoration and threshold reduction by introducing a novel water/alcohol soluble conjugated macroelectrolyte, tris(4-(7-(9,9-di(hexyl-1-N,N-diethanolamino)-9H-fluoren-2-yl)-9,9-di(hexyl-1-N,N-diethanolamino)-9H-fluoren-2-yl)phenyl)amine (TPAOH), serving as an interfacial layer between the gain media layer, dioctyl substituted polyfluorene (PFO), and the Ag electrode layer. By optimizing the film thickness of TPAOH, enhanced ASE performance has been achieved with the lowest threshold of 21 μJ/cm2, demonstrating 3.5-fold reduction from 74 μJ/cm2. Atomic force microscopy results showed good compatibility between the TPAOH film and the PFO layer. The results suggest a facile and low-cost solution-processing interfacial technique to construct efficient organic semiconductor lasers in the presence of metallic electrodes.

Extremely low amplified spontaneous emission threshold and blue electroluminescence from a spin-coated octafluorene neat film

We report on the photophysical, amplified spontaneous emission (ASE), and electroluminescence properties of a blue-emitting octafluorene derivative in spin-coated films. The neat film shows an extremely low ASE threshold of 90 nJ/cm2, which is related to its high photoluminescence quantum yield of 87% and its large radiative decay rate of 1.7 × 109 s−1. Low-threshold organic distributed feedback semiconductor lasers and fluorescent organic light-emitting diodes with a maximum external quantum efficiency as high as 4.4% are then demonstrated, providing evidence that this octafluorene derivative is a promising candidate for organic laser applications.

Boron difluoride hemicurcuminoid as an efficient far red to near-infrared emitter: toward OLEDs and laser dyes.

A hemicurcuminoid boron difluoride complex is used as an emitter in organic light-emitting diodes, showing far red/near-infrared electroluminescence with an external quantum efficiency as high as 2.1%. This dye blended in CBP thin films shows amplified spontaneous emission with a threshold of 22 μJ cm-2 at 750 nm, making this compound attractive for organic semiconductor lasers operating in the near-infrared region.

Bifluorene Single Crystals with Extremely Low‐Threshold Amplified Spontaneous Emission

Organic single crystals offering numerous advantages due to their long‐range molecular order are considered a promising gain medium for realization of electrically‐pumped organic lasers provided their amplified spontaneous emission (ASE) threshold is sufficiently low. Unfortunately, ASE thresholds of such crystals are typically more than one order of magnitude higher (in the range of tens of kW cm−2) as compared to those of amorphous neat/doped films. Here, this issue is addressed by rationally designing bifluorene‐based compounds to express weak intermolecular coupling in the crystalline phase. The twisted molecular backbone as well as out‐of‐plane twisted dimethyl moieties attached at the fluorene end‐groups additionally benefit with enhanced electron–vibronic coupling resulting in a large Stokes shift (0.5 eV) and reduced reabsorption of emission. Endowed with such features the bifluorenes exhibit similarly high radiative decay rates (≈1.5 × 109 s−1) when doped in polystyrene matrix at low concentrations and in sublimation‐grown single crystals. The high radiative rates accompanied by excellent waveguiding properties, favorable orientation of transition dipole moments as well as non‐overlapping excited‐state absorption and gain regions enable achieving extremely low ASE thresholds (≈700 W cm−2) in the bifluorene single crystals. The achieved low threshold encourages employment of rationally designed molecules in organic crystals for lasing applications.

Strongly emissive perovskite nanocrystal inks for high-voltage solar cells

Lead halide perovskite semiconductors have recently gained wide interest following their successful embodiment in solid-state photovoltaic devices with impressive power-conversion efficiencies, while offering a relatively simple and low-cost processability. Although the primary optoelectronic properties of these materials have already met the requirement for high-efficiency optoelectronic technologies, industrial scale-up requires more robust processing methods, as well as solvents that are less toxic than the ones that have been commonly used so successfully on the lab-scale. Here we report a fast, room-temperature synthesis of inks based on CsPbBr3 perovskite nanocrystals using short, low-boiling-point ligands and environmentally friendly solvents. Requiring no lengthy post-synthesis treatments, the inks are directly used to fabricate films of high optoelectronic quality, exhibiting photoluminescence quantum yields higher than 30% and an amplified spontaneous emission threshold as low as 1.5 μJ cm−2. Finally, we demonstrate the fabrication of perovskite nanocrystal-based solar cells, with open-circuit voltages as high as 1.5 V. Despite their impressive performance, more efforts are required to develop industrially scalable perovskite solar cells from less toxic solvents. Towards that aim, this study presents the use of colloidal nanoparticle inks for room-temperature fabrication of CsPbBr3 solar cells.

Ultralow-Threshold Single-Mode Lasing from Phase-Pure CdSe/CdS Core/Shell Quantum Dots.

The development of colloidal quantum dot (QD) lasers is blocked by Auger recombination (AR). Here, phase-pure wurtzite CdSe/CdS core/shell QDs with controlled shell thickness are reported, which possess nearly defect-free core/shell interfaces. Benefiting from increased volume, electron-hole partial spatial separation, and nearly defect-free alloyed interface, this series of QDs exhibit a greater than 3 orders of magnitude decrease in AR rates with increasing shell thickness. Consequently, the amplified spontaneous emission threshold of the QDs with an 11 monolayer CdS shell is found to reach a minimum of 16 μJ cm-2. A record long lifetime (>1000 ps) and extraordinarily large bandwidth (>170 nm) of optical gain are observed by employing ultrafast transient absorption spectroscopy. We leverage the low-threshold gain of the QDs to fabricate microlasers that display single-mode operation and an ultralow threshold of ∼2 μJ cm-2. Our results represent a valuable step toward practical QD lasers.

Novel Fluorene-Based Copolymers Containing Branched 2-Methyl-butyl-Substituted Fluorene-co-benzothiadiazole Units for Remarkable Optical Gain Enhancement in Green-Yellow Emission Range

We develop novel fluorene-based green-yellow emission copolymers (namely, nF1/4F8BT) composed of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and branched 2-methyl-butyl substituted fluorene-co-benzothiadiazole units (F1/4BT) in different ratios. Upon blending nF1/4F8BT copolymers with poly(9,9-dioctylfluorene) (PFO), a systematic increase in photoluminescence quantum efficiency is observed as the ratio of the branched monomers increase. Likewise, nF1/4F8BT and PFO:nF1/4F8BT blends exhibit superior optical gain properties with respect to F8BT, manifested as a 30% reduction of amplified spontaneous emission threshold and 50% increase in optical gain with respect to F8BT blends at the same emission wavelength. The optical-gain-related properties were studied to understand the influence of the branched side chain unit on stimulated emission properties. Femtosecond transient absorption studies confirm exciton-annihilation hindrance in the new copolymers likely caused by interference of branched substi...

Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9′-spirobifluorene)

We demonstrate that ter(9,9′-spirobifluorene) (TSBF) doped in a host matrix layer of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) shows a low amplified spontaneous emission (ASE) threshold (Eth = 1.0 μJ cm−2) and suppressed electroluminescence efficiency roll-off at high current densities (no roll-off up to 100 mA cm−2). One origin of the low ASE threshold is that the TSBF-doped CBP layer possesses a very large radiative decay constant (kr = 1.1 × 109 s−1). Singlet–triplet annihilation is almost suppressed in the TSBF-doped CBP layer, which can be ascribed to the small overlap between the emission and triplet absorption of TSBF. Also, the small energy level difference between TSBF and CBP minimizes carrier trapping in TSBF, leading to the suppression of singlet–polaron annihilation. TSBF showed one of the lowest Eth and the most suppressed efficiency roll-off among organic laser dyes investigated in this study and, therefore, is believed to be a promising candidate to realize electrically pumped organic semiconductor laser diodes in the future.

Optimized Waveguide Geometry for Low Amplified Spontaneous Emission Operation on ITO-Coated Glass

We demonstrated a significant reduction of amplified spontaneous emission (ASE) threshold for an organic optical gain medium operation on an ITO-coated glass by introducing a solution-processed cellulose acetate (CA) thin film as a space layer between the ITO electrode and the optical gain active layer. Using PFO as an optical gain medium, we observed a 4.4-fold threshold reduction from 140 μJ/cm2 for glass/ITO/PFO device to 32 μJ/cm2 for glass/ITO/CA/PFO by controlling the thickness of CA film at 38 nm, and 9.9-fold reduction to 15 μJ/cm2 by further increasing the thickness of CA film to around 152 nm. Furthermore, we investigated symmetrical waveguide of glass/ITO/CA/PFO/CA configuration. It exhibited 3.3-fold lower ASE threshold than that of asymmetrical waveguide of glass/ITO/CA/PFO (6.0 versus 20.0 μJ/cm2), which was even lower than that of quartz/PFO (8.4 μJ/cm2). The CA film was also doped with Ag nanoparticles (AgNPs) in various ratios to improve the conductivity. ASE threshold of glass/ITO/CA(AgNPs)/PFO exhibited no detectable increase with the doping ratio of Ag nanoparticles up to 15 wt%.

Deep Blue Laser Gain Medium Based on Triphenylamine Substituted Arylfluorene With Improved Photo-Stability

We report a novel blue emission triphenylamine substituted arylfluorene (SPhTPA) as organic optical gain medium with low amplified spontaneous emission (ASE) threshold. The optical gain related properties of SPhTPA and its blends with inert polymer, polystyrene (PS), were investigated in detail. This study shows the gain property and photo-stability of SPhTPA have been improved significantly by doping it into PS. The lowest ASE threshold was obtained for the SPhTPA/PS film doped with 20 wt.% SPhTPA. The film of the same doping ratio exhibited high net gain coefficient (27.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and low loss coefficient (7.2 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ). Moreover, the blend film demonstrated a significantly improved photo-stability. The ASE photo-stability half-life of the blend film was 15 fold higher than the pristine SPhTPA film when pumped in open ambient atmosphere. This paper suggests that this triphenylamine substituted arylfluorene is a highly prospective gain medium and doping it into PS is an efficient approach for developing air-stable SPhTPA-based organic lasers.

Singlet-Triplet Exciton Annihilation Nearly Suppressed in Organic Semiconductor Laser Materials Using Oxygen as a Triplet Quencher

We report on the use of oxygen as triplet quencher to reduce singlet-triplet annihilation in light-emitting organic solid films and solvent-free molecular semiconducting liquids. For this purpose, a fluorescent heptafluorene derivative is dispersed either in a wide bandgap 4,4'-Bis (N-carbazolyl)-1,1'-biphenyl (CBP) host or in a solvent-free liquid matrix based on 9-(2-ethylhexyl)carbazole (EHCz). To introduce oxygen in the samples, a modified cold isostatic pressure technique is used in the case of spin-coated CBP blends while oxygen is bubbled in the liquids. The influence of the oxygenation on their photophysical and amplified spontaneous emission properties is examined. Both solid blend films and liquids showed before and after the introduction of oxygen a photoluminescence quantum yield higher than 80% and an amplified spontaneous emission threshold lower than 0.4 μJ/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . While oxygen does not quench significantly the singlet excitons in these systems, singlet-triplet annihilation is strongly reduced in solid thin films and nearly suppressed in the liquid layers. This study demonstrates that liquid organic semiconductors are promising candidates for optically pumped continuous wave lasers and provides important insights for a more effective triplet management in organic semiconductor lasers.

Microsecond-sustained lasing from colloidal quantum dot solids

Colloidal quantum dots have grown in interest as materials for light amplification and lasing in view of their bright photoluminescence, convenient solution processing and size-controlled spectral tunability. To date, lasing in colloidal quantum dot solids has been limited to the nanosecond temporal regime, curtailing their application in systems that require more sustained emission. Here we find that the chief cause of nanosecond-only operation has been thermal runaway: the combination of rapid heat injection from the pump source, poor heat removal and a highly temperature-dependent threshold. We show microsecond-sustained lasing, achieved by placing ultra-compact colloidal quantum dot films on a thermally conductive substrate, the combination of which minimizes heat accumulation. Specifically, we employ inorganic-halide-capped quantum dots that exhibit high modal gain (1,200 cm−1) and an ultralow amplified spontaneous emission threshold (average peak power of ∼50 kW cm−2) and rely on an optical structure that dissipates heat while offering minimal modal loss.

Solution processable 2-(trityloxy)ethyl and tert-butyl group containing amorphous molecular glasses of pyranylidene derivatives with light-emitting and amplified spontaneous emission properties

Small organic molecules with incorporated 4H-pyran-4-ylidene (pyranylidene) fragment as the π-conjugation system which bonds the electron acceptor fragment (A) with electron donor part (D) in the molecule – also well known as derivatives of 4-(dicyano-methylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM) laser dye-have attracted considerable attention of scientists as potential new generation materials for organic photonics and molecular electronics due to their low-cost fabrication possibility, flexibility and low-weight.Six glassy derivatives of 4H-pyran-4-ylidene (pyranylidene) with attached bulky 2-(trityloxy)ethyl and tert-butyl groups are described in this report. Almost all of the synthesized compounds form good optical quality transparent amorphous films from volatile organic solvents and could be obtained in good yields up to 75%. Their light emission in solution and thin solid films is in the range of 600–700nm, they are thermally stable and show glass transition in the range of 108–158°C. The amplified spontaneous emission threshold values of the neat films of the glassy pyranylidene derivatives vary from 155 to 450μJ/cm2 and their HOMO and LUMO energy levels are between of those of tris(8-hydroxy quinolinato) aluminum (Alq3). The photoluminescence quantum yields of the glassy compounds are in the range from 1% to about 7.7% and their electroluminescence properties have been investigated. Therefore, glassy pyranylidene derivatives could be a very potential low-cost solution processable materials for Alq3 hosted light-amplification and light-emitting application studies.