Amplified Spontaneous Emission Properties Research Articles

Solution-based green amplified spontaneous emission from colloidal perovskite nanocrystals exhibiting high stability

Solution-based optical amplification affords a host of benefits ranging from flexibility in the choice of cavity size and shape to high photostability afforded by the constant replenishment of gain media. Works reporting solution-based optical amplification in colloidal semiconductor nanocrystals, however, remain sparse due to the difficulty in achieving high particle number densities required for sustained optical gain. In this work, we demonstrate highly stable amplified spontaneous emission (ASE) from a solution of green-emitting CsPbBr3 perovskite nanocrystals dispersed in a nonpolar solvent after a facile postsynthesis processing step. This processing step not only allows for the purification of nanocrystals from their growth solution, but also allows for long-term colloidal stability at high particle concentrations. Although it is widely reported that perovskite nanocrystals suffer from poor chemical stability, our nanocrystal solutions retain their ASE properties despite long-term storage in excess of five months under ambient conditions. Photostability tests show steady ASE intensities in excess of three hours under constant photoexcitation from a femtosecond pulsed laser beam (>107 shots), far exceeding those of thin films by an order of magnitude. This work opens the possibility of harnessing colloidal CsPbBr3 nanocrystals as highly robust, solution-based optical gain media.

Two-Photon Absorption and Two-Photon-Induced Gain in Perovskite Quantum Dots.

Perovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 106 GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm2. We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process.

Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States.

Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for the realization of electrically driven organic lasers. However, the amplified spontaneous emission (ASE) threshold (1-10 kW/cm2) of SCs is still significantly higher compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging group-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of nonradiative decay, which, along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns), results in high radiative decay rates (∼0.5 × 109 s-1) of the SCs, making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm2 in acetylene-bridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled one to disclose pronounced differences in the excited state dynamics of the studied SCs, pointing out the essential role of radiative traps in achieving a record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically pumped organic laser.

Stimulated emission and optical properties of pyranyliden fragment containing compounds in PVK matrix

Organic solid state lasers are thoughtfully investigated due to their potential applications in communication, sensors, biomedicine, etc. Low amplified spontaneous emission (ASE) excitation threshold value is essential for further use of the material in devices. Intramolecular interaction limits high molecule density load in the matrix. It is the case of the well-known red light emitting laser dye - 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). The lowest ASE threshold value of the mentioned laser dye could be obtained within the concentration range between 2 and 4wt%. At higher concentration threshold energy drastically increases. In this work optical and ASE properties of three original DCM derivatives in poly(N-vinylcarbazole) (PVK) at various concentrations will be discussed. One of the derivatives is modified DCM dye in which the methyl substituents in the electron donor part have been replaced with bulky trityloxyethyl groups (DWK-1). These sterically significant functional groups do not influence electron transitions in the dye but prevent aggregation of the molecules. The chemical structure of the second investigated compound is similar to DWK-1 where the methyl group is replaced with the tert-butyl substituent (DWK-1TB). The third derivative (DWK-2) consists of two N,N-di(trityloxyethyl)amino electron donor groups. All results were compared with DCM:PVK system. Photoluminescence quantum yield (PLQY) is up to ten times larger for DWK-1TB with respect to DCM systems. Bulky trityloxyethyl groups prevent aggregation of the molecules thus decreasing interaction between dyes and amount of non-radiative decays. The red shift of the photoluminescence and amplified spontaneous emission at higher concentrations were observed due to the solid state solvation effect. The increase of the investigated dye density in the matrix with a smaller reduction in PLQY resulted in low ASE threshold energy. The lowest threshold value was obtained around 21μJ/cm2 (2.1kW/cm2) in DWK-1TB:PVK films.

Gamma-Irradiation Effects on the Spectral and Amplified Spontaneous Emission (ASE) Properties of Conjugated Polymers in Solution.

In this paper, we investigate the effects of gamma (γ) radiation on the spectral and mplified spontaneous emission (ASE) properties of two conjugated polymers (CPs) viz., poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH–PPV) (CPM) and poly{[2-[2′,5′-bis(2″-ethylhexyloxy)phenyl]-1,4-phenylenevinylene]-co-[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]} (BEHP-co-MEH–PPV) (BMP) in tetrahydrofuran (THF). Gamma irradiation strongly affected the photophysical properties of these CPs. To explore these changes, gamma radiation, in the range of 2–50 kGy, was used to maintain the temperature at 5 °C constant for all doses at a dose rate of 12.67 kGy/h, using a 60Co gamma ray. The ASE profiles of the CPs in THF were obtained under the high power excitation of a Nd:YAG laser (355 nm), pre- and post-radiation. The result revealed a dramatic blue shift of the fluorescence and the ASE spectra after gamma irradiation. This shift in the luminescence and ASE spectra could be a response to the conformational disorders such as gamma irradiation-induced polymer crosslinking, which was verified using Raman spectra, FTIR, and swelling experiments. This could be the first report on the effect of gamma radiation on the ASE properties of conjugated polymers.

Amplified spontaneous emission from DCJTB encapsulated in mesostructured composite silica SBA-15.

Amplified spontaneous emission (ASE) characteristics of a red dye 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7- etramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) encapsulated in a highly ordered mesoporous SBA-15 were studied. The mesoporous composite silica film loaded with organic dye has been successfully synthesized by a solgel reaction process and a simple postgrafting method at room temperature. The spectrum narrowing phenomena has been observed when the composite film is pumped at λp=532 nm by a Nd:YAG ed laser. There is a substantial reduction in the full width at half-maximum of the emitting light, which is one of the signatures of the presence of ASE. The ASE threshold and net gain, respectively, reached 0.03 mJ pulse^-1 and 34.7 cm^-1 for the DCJTB encapsulated in mesoporous SBA-15 film. The optimized ASE properties owe much to the effects of the better spatial confinement of the molecules in the ordered mesoporous structure of the host SBA-15.

On the correlation between morphology and Amplified Spontaneous Emission properties of a polymer: Polymer blend

We investigate the Amplified Spontaneous Emission (ASE) properties of a prototypical host-guest polymer polymer blend, namely poly(9,9-dioctylfluorene) (PF8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) blend, with different concentration ratio. We show that the initial F8BT content increase causes an increase of the F8BT ASE threshold, even leading to ASE suppression for F8BT contents between 25% and 75%. ASE is then recovered upon further increase of the F8BT relative content. We demonstrate that the ASE properties of the PF8:F8BT are dominated by morphology effects, like submicrometric phase segregation, determining the net gain of the active waveguides.

Amplified spontaneous emission spectra of poly(9,9-dioctylfluorenyl-2,7-diyl) under pulsed laser excitation

Photophysical parameters such as absorption, fluorescence spectra, stokes’ shift, fluorescence quantum yield and amplified spontaneous emission (ASE) of a soluble conducting polymer poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) were measured in different solvents. The absorption spectra of PFO in tetrahydrofuran (THF) have shown two bands at 390 and 437nm. The results showed that the optical density of the band 437nm increased by increasing the concentration. This is a strong indication that PFO in solution aggregates at higher concentrations. On the other hand, at lower concentrations, the fluorescence spectra of PFO in THF have shown three bands; 422 (dominant), 445 (shoulder) and 470nm (hump). For the higher concentrations, the band at 422nm totally vanished while the band 470nm became a dominant. Therefore, the band at 470nm could be attributed to the dimer state. The ASE action was tested in a transverse pumping configuration where the conducting polymer was pumped by laser pulses from the third harmonic of an Nd:YAG laser (355nm). It could be seen that there were three ASE peaks at 425, 450 and 475nm. The spectral and ASE properties of PFO were compared with the conjugated polymer poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PDHF) under the same operating conditions. The most important features are: (1) the ASE intensity and the photochemical stability of PDHF were remarkably high compared with PFO, (2) PFO has an ASE action even under low concentrations (3) and the ASE emission from PFO is tunable in the wavelength region between 425, 450 and 475nm.

Fluorene‐based rib waveguides with optimized geometry for long‐term amplified spontaneous emission stability

ABSTRACTAmongst the different optoelectronic applications of conjugated polymers, waveguide amplifiers and optically pumped lasers are those requiring larger photochemical stability, owing to the large irradiation conditions under operation. In this context, suitable waveguide optimization enabling the reduction of amplified spontaneous emission (ASE) threshold values appears as important as synthetic chemistry protocols to promote polymer robustness against photo‐oxidation. In this work, we develop rib waveguides with different geometries based on four different fluorene‐based compounds and assess the influence of rib confinement on ASE properties. We observe ASE threshold values as low as 8.9 × 10−4 mJ cm−2, being among the lowest threshold values reported so far on blue emitting polymer/oligomer waveguides. We demonstrate that the enhanced ASE efficiency on some of these rib waveguides leads to a fivefold increase in operation lifetime respect to spin‐coated slab waveguides, thus confirming the impact of waveguide geometry on ASE operation stability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1040–1045

Amplified Spontaneous Emission in Pentathienoacene Dioxides by Direct Optical Pump and by Energy Transfer: Correlation with Photophysical Parameters

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.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Operational lifetime improvement of poly(9,9-dioctylfluorene) active waveguides by thermal lamination

We investigate the encapsulation performances of polyethylene terephthalate-poly (ethylene vinyl acetate) laminating pouches, thermally laminated on flexible poly(9,9-dioctylfluorene)(PF8) active waveguides showing amplified spontaneous emission (ASE). We show that the lamination process does not cause significant degradation of the PF8 emission properties and preserves the ASE properties of the waveguide, with an ASE threshold increase of only 15%. A clear increase of the ASE operational lifetime up to 2.3 times is observed after lamination, together with stable ASE threshold for more than 6 months after the device realization, thus proposing thermal lamination as a straightforward technique to encapsulate organic active waveguides, preserving the possibility to realize flexible devices.

Low threshold amplified spontaneous emission from dye-doped DNA biopolymer

In this study, we investigate the amplified spontaneous emission (ASE) properties and conduct a comparative study for two kinds of dye-doped deoxyribonucleic acid (DNA) biopolymers. The system consists of optical films made of DNA modified by two types of surfactants and doped with a common laser dye rhodamine 6G (Rh6G). The ASE properties of the optical films were characterized by a pulsed Nd:YAG laser. The results show that low threshold of DNA biopolymer can be achieved by the employment of suitable surfactant in the system, resulting from an efficient energy transfer process. Coupled with the fluorescence enhancement exhibited in DNA, this effect can further advance biopolymers toward highly efficient media for lasing applications and organic solid-state lasers.

Cross-Linkable Polyspirobifluorenes: A Material Class Featuring Good OLED Performance and Low Amplified Spontaneous Emission Thresholds

We report on an in-depth study of cross-linkable polyspirobifluorene (PSF) copolymers for organic lasing applications. We investigate the performance of the materials in organic light-emitting diodes (OLED) and the optically pumped amplified spontaneous emission (ASE) properties of the new PSF-based devices. The influence of chemical composition and cross-linking of the copolymers is investigated. The cross-linkable polymers presented here exhibit low ASE thresholds and simultaneously good OLED efficiencies. This is exceptional, as we demonstrate that usually only one of these properties is optimized in semiconducting polymers. The ASE threshold in the green-emitting materials is as low as 4.4 μJ/cm2 under nanosecond excitation. The chemical cross-linking of the polymers into an insoluble network has little effect on the ASE properties. With the material exhibiting the best figure-of-merit we fabricated a slab-waveguide OLED. This OLED exhibits a low, almost preserved ASE threshold compared to a single la...

Effect of ring fusion on the amplified spontaneous emission properties of oligothiophenes

We report the observation of amplified spontaneous emission (ASE) in oligothienoacenes, oligothiophenes composed solely of fused thiophene rings, doped into polystyrene films. The effect of ring fusion on the ASE properties has been investigated by comparing the performance of a pentathienoacene derivative, 2,6-bis-triisopropylsilanyl-α-pentathienoacene (BT-PTA), with that of its corresponding non-fused oligothiophene, 5,5″-bis-triisopropylsilanyl-α-terthiophene (BT-α3T), whose synthesis is also reported here. The spectral properties of both systems have been studied by UV-vis optical absorption and photoluminescence in solution at both room and low temperature and interpreted by means of density functional theory (DFT) and time dependent DFT quantum chemical calculations. The dependence of ASE in terms of threshold, linewidth and emission wavelength on the concentration of active material in the films has been investigated in detail, allowing for optimization of the ASE performance. The best results were obtained for oligomer concentrations between 10 and 30 wt%. BT-PTA-doped films show ASE thresholds as low as 10 kW·cm−2, one order of magnitude lower than those of films doped with BT-α3T. These results indicate that the materials are good candidates as active laser materials and that their performance can be improved by adjusting the concentration of material in the film.

Amplified spontaneous emission in α-phase and β-phase polyfluorene waveguides

Amplified spontaneous emission (ASE) of α- and β-phase poly(9,9-dioctylfluorene) (F8) based waveguides has been studied and ASE wavelengths have been determined as 451 nm and 468 nm, respectively. β-phase waveguides are found to be with higher net gain and lower light propagation loss as compared to α-phase waveguides. Such important improvement of ASE properties, making β-phase waveguides superior over α-phase counterparts is shown to be achieved at the expense of higher refractive index and photoluminescence quantum efficiency of β-phase F8.