Stimulated Emission Band Research Articles

Low threshold amplified spontaneous emission and lasing from single crystals and polymer microspheres based on a blue aggregation-enhanced emission molecule

Organic semiconductors as a gain medium have attracted much attention in organic lasers due to their unique properties, i.e., large stimulated emission cross section, widely tunable spectrum region, and mechanical flexibility. Aggregation-caused quenching broadly exists in amorphous solid films and single crystals, which result in high threshold. Here, we studied amplified spontaneous emission (ASE) and lasing properties of an aggregation-enhanced emission (AEE) molecule, namely, 2PB-AC, who owns high photoluminescence quantum efficiency in a solid film and a single crystal. The grown long rod-like single crystals exhibited very good ASE characteristics with a threshold of 48 μJ cm−2 and a full width at half-maximum (FWHM) of 8.4 nm. Furthermore, when forming polymer microspheres by a surface tension-induced self-assembly method, a low lasing threshold of 0.32 mJ cm−2 and a FWHM of 0.41 nm were also obtained well, and their lasing modes could be modulated by changing the diameter of the microspheres. The investigations on transient photoluminescence dynamics and femtosecond transient absorption analysis demonstrated that the high radiative decay rate and the well separation between a stimulated emission band and an excited state absorption band are the main reasons of good lasing performance, indicating that AEE molecules are promising lasing materials.

Asymmetric AZA-BODIPY with Optical Gain in the Near-Infrared Region.

In recent years, there has been a lot of interest in the development of organic compounds emitting in the near-infrared (NIR) region due to their stimulating applications, such as biosensing and light detection and ranging (LiDAR). Moreover, a lot of effort has been devoted to finding organic emitters with optical gain in the NIR region for lasing applications. In this paper, we present the ultrafast spectroscopy of an asymmetric AZA-BODIPY molecule that shows relevant photophysical changes moving from a diluted solution to a concentrated solution and to a spin-coated film. The diluted solution and the spin-coated film show a bleaching band and a stimulated emission band in the visible region, while the very concentrated solution displays a broad (150 nm) and long-living (more than 400 ps) optical gain band in the NIR region, centered at 900 nm. Our results pave the way for a new organic laser system in a near-infrared spectral region.

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Vinyl Groups Containing Tetraphenylethylene Derivatives as Fluorescent Probes Specific for Palladium and the Quenching Mechanism†

Main observation and conclusionWe previously reported that a series of tetraphenylethylene (TPE)‐containing all‐hydrocarbon conjugated polymers exhibited a specific fluorescence quenching by palladium ions (Pd2+). To understand the quenching mechanism, herein we investigate the quenching behaviors of three TPE derivatives in the presence of Pd2+. Each TPE derivative, consisting of TPE unit and terminal vinyl groups, shows an aggregation‐induced emission (AIE) and its fluorescence turns off specifically to Pd2+. The sensitivity is enhanced with increasing numbers of vinyl groups in the molecules. By time‐resolved fluorescence measurement, a dynamic quenching is observed where the fluorescence lifetime is reduced with Pd2+. Specifically, the quenching occurs via the electron transfer in the excited states, as suggested by the disappearance of the stimulated emission band in transient absorption spectra. A theoretical calculation on the excited states identifies intermolecular electron transfer from TPE derivatives to Pd, especially for electron–rich TPE derivative. The mechanism can be general for the design of novel AIE active chemosensors.

Spontaneous and Stimulated Emission in Thin Films of Cu(In1 –xGax)(SySe1 –y)2 Solid Solutions in the Сomposition of Solar Cells

The emission spectra of thin nanocrystalline films of Cu(In1 – xGax)(SySe1 – y)2 (CIGSSe) direct-gap solid solutions in the structure of solar cells, recorded upon continuous-wave laser excitation (~0.5 W/cm2) and nanosecond pulsed laser excitation with a power density in the range of 0.1–53 kW/cm2 at temperatures ranging from 10 to 300 K, are analyzed. It is found that stimulated emission (SE) occurs in thin CIGSSe films at temperatures from 10 to 90 K in the spectral range hν = 1.062–1.081 eV, with a minimum threshold-pump level of ~1 kW/cm2. It is shown that the spontaneous emission band is blue-shifted with an increase in the excitation intensity. It is also established that, with an increase in temperature, the photoluminescence (PL) band (upon weak excitation) and the stimulated-emission band are blue-shifted, whereas the PL band is red-shifted upon strong excitation. The possible reasons and mechanisms for the influence of temperature and excitation intensity on the spectral positions of the spontaneous and stimulated emission bands for the solid-solution films are discussed.

Multiexponential dynamics of Mn2+(3d5) excitation in manganese doped ZnCdS quantum dots: Stimulated emission band in femtosecond transient spectra reveals ultrafast nonradiative energy transfer to Mn2+(3d5)

Stimulated emission (SE) 4T1 → 6A1 bands of Mn2+(3d5) in ZnxCd1−xS or ZnxCd1−xS/ZnS quantum dots were detected in the femtosecond transient absorption spectra at a time delay of 120 fs. Multiexponential rise of SE(Mn2+) amplitude due to nonradiative energy transfer to Mn2+(3d5) occurs both in the femtosecond and picosecond time domain. Transient kinetics of SE(Mn2+) band reveals the dynamics of energy transfer to Mn2+(3d5) with time constants τ1 ~ 400–600 fs, τ2 ~ 4–6 ps and τ3 ~ 87–112 ps. A possible mechanism of ultrafast energy transfer to the Mn2+(3d5) center accompanied by spin-flip is discussed.

Спонтанное и стимулированное излучение в тонких пленках твердых растворов Cu(In-=SUB=-1-x-=/SUB=-Ga-=SUB=-x-=/SUB=-)(S-=SUB=-y-=/SUB=-Se-=SUB=-1-y-=/SUB=-)-=SUB=-2-=/SUB=- в составе солнечных элементов

The results of a study of the emission spectra of thin nanocrystalline films of Cu(In1-xGax)(SySe1-y)2 (CIGSSe) direct-band-gap solid solutions in the structure of solar cells at ~ 0.5 W/cm2 continuous wave and nanosecond pulsed laser excitation in the range of excitation power density 0.1 - 53 kW/cm2 and temperatures of 10-300 K are presented. It was found that stimulated emission (SE) occurs in thin CIGSSe films in the temperature range from 10 K to 90 K in the spectral region h = 1.062 - 1.081 eV with a minimum threshold pump level of about 1 kW/cm2. It was shown, that, with increasing intensity of the exciting emission, the spontaneous emission bands shift toward higher energies. It was found that the photoluminescence bands at low excitation levels and the SE bands shift with increasing temperature toward higher energies, and the PL bands at high excitation levels shift toward low energies. Possible causes and mechanisms of the influence of temperature and excitation intensity on the spectral positions of spontaneous and SE of the films of solid solutions are discussed.

Photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in ionic liquids: Effect of cation alkyl-chain length

Transient absorption spectroscopy has been applied to study the photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in twelve imidazolium/phosphonium cation-based ionic liquids (ILs) with alkyl chains of different lengths. The transient absorption spectra showed an initial bleach signal around the absorption center (400 nm) and a stimulated emission band around 500 nm. The bleach signal was recovered mostly within a few picoseconds, and a hot band absorption appeared around 450 nm. The time profiles of the band integrals for the bleach and absorption spectral regions were simulated by a sum of four exponential decays. The internal conversion rate corresponding to the back electron transfer (b-ET) rate from the charge-separated excited state, was determined by the time profile of the band integral of the bleach region. The vibrational energy relaxation (VER) rate in the ground state was determined from the time profile of the hot band absorption decay band integral. The b-ET rate decreased with increasing number of alkyl carbons in the cation (#C). The solvent dependence was discussed in terms of the free energy change at the Marcus inverted region, which was indicated by the blue-shift of the ground state absorption band. The change of the free energy is ascribed to the change of the charge concentration of ILs. The VER rate decreased slightly with increasing #C. The reason was discussed in terms of the interaction of DMpNA with the polar region of ILs. The intersystem crossing yield also increased with increasing #C, reflecting an increase in the non-polar region of the ILs.

Tailoring optical properties and stimulated emission in nanostructured polythiophene

Polythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices.

Optical absorption properties of laser-driven matter

Characterizing and controlling matter driven far from equilibrium represents a major challenge for science and technology. Here, we develop a theory for the optical absorption of electronic materials driven far from equilibrium by resonant and nonresonant lasers. In it, the interaction between matter and the driving light is treated exactly through a Floquet analysis, while the effects of the probing light are captured to first order in perturbation theory. The resulting equations are reminiscent to those for equilibrium absorption but with the Floquet modes playing the role of the pristine eigenstates. The formalism is employed to characterize the optical properties of a model nanoscale semiconductor dressed by nonresonant light of intermediate intensity (nonperturbative, but nonionizing). As shown, nonresonant light can reversibly turn this transparent semiconductor into a broadband absorber and open strong absorption and stimulated emission bands at very low frequencies ($\ensuremath{\sim}\mathrm{meV}$). Further, the absorption spectra of the driven material exhibit periodic features energetically spaced by the photon energy of the driving light that reflect the periodic structure of the Floquet bands. These developments offer a platform to understand and predict the emergent optical properties of materials dressed by the electric field of light, and catalyze the design of laser-driven materials with desired optical properties.

Photoinduced Charge Shifts and Electron Transfer in Viologen-Tetraphenylborate Complexes: Push-Pull Character of the Exciplex.

Viologen-tetraarylborate ion-pair complexes were prepared and investigated by steady-state and time-resolved spectroscopic techniques such as fluorescence and femtosecond transient absorption. The results highlight a charge transfer transition that leads to changes in the viologen structure in the excited singlet state. Femtosecond transient absorption reveals the formation of excited-state absorption and stimulated emission bands assigned to the planar (kobs < 1012 s-1) and twisted (kobs ∼ 1010 s-1) structures between two pyridinium groups in the viologen ion. An efficient photoinduced electron transfer from the tetraphenylborate anionic moiety to the viologen dication was observed less than 1 μs after excitation. This is a consequence of the push-pull character of the electron donor twisted viologen structure, which helps formation of the borate triplet state. The borate triplet state is deactivated further via a second electron transfer process, generating viologen cation radical (V•+).

Ultraviolet Light Makes dGMP Floppy: Femtosecond Stimulated Raman Spectroscopy of 2'-Deoxyguanosine 5'-Monophosphate.

The ultrafast dynamics of 2'-deoxyguanosine 5'-monophosphate after excitation with ultraviolet light has been studied with femtosecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). TA kinetics and transient anisotropy spectra reveal a rapid relaxation from the Franck-Condon region, producing an extremely red-shifted stimulated emission band at ∼440 nm that is formed after 200 fs and subsequent relaxation for 0.8-1.5 ps, consistent with prior studies. Viscosity dependence shows that the initial relaxation, before 0.5 ps, is the same in water or viscous glycerol/water mixtures, but after 0.5 ps the dynamics significantly slow down in a viscous solution. This indicates that large amplitude structural changes occur after 0.5 ps following photoexcitation. FSRS obtained with both 480 and 600 nm Raman pump pulses observe very broad Raman peaks at 509 and 1530 cm-1, as well as a narrower peak at 1179 cm-1. All of the Raman peaks decay with 0.7-1.3 ps time constants. The 1530 cm-1 peak also shows an increasing inhomogeneous linewidth over the first 0.3 ps. Our TA and FSRS data are consistent with a structurally inhomogeneous population in the S1 (La) state and, in particular, with previous theoretical models in which out-of-plane distortion at C2 and the amine move the molecule toward a conical intersection with the ground state. These FSRS data are the first to directly observe the structural inhomogeneity imparted upon the excited-state population by the broad, flat potential energy surface of the S1 (La) state.

Stimulated emission from aluminium anode oxide films doped with rhodamine 6G

The spectral and luminescent properties of the rhodamine 6G dye in a porous matrix of aluminium anode oxide are studied. The films with a highly-ordered porous structure are produced using the method of two-stage anodic oxidation. By means of raster electron microscopy it is found that the diameter of the pores amounts to nearly and the separation between the adjacent channels is almost . The thickness of the films is equal to , and the specific surface area measured using the method of nitrogen capillary condensation is . Fluorescence and absorption spectra of rhodamine 6G molecules injected into the pores of the aluminium anode oxide are measured. It is found that under the excitation of samples with the surface dye concentration by the second harmonic of the Nd : YAG laser in the longitudinal scheme with the pumping intensity , a narrow band of stimulated emission with the intensity maximum at the wavelength appears against the background of the laser-induced fluorescence spectrum. A further increase in the pumping radiation intensity leads to the narrowing of the stimulated emission band and an increase in its intensity. The obtained results demonstrate the potential possibility of using the porous films of aluminium anode oxide, doped with laser dyes, in developing active elements for quantum electronics.

Distributed feedback and random lasing in DCNP aggregates dispersed in a polymeric layer.

Here, we report on the realization of random lasing (RL) and distributed feedback (DFB) lasing in a layer of luminescent 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) organic nonlinear optical dye that has been dispersed in a poly(methyl methacrylate) (PMMA) matrix. The RL phenomenon appears due to the presence of spontaneously formed micro- and nano-crystals of DCNP in the bulk of the PMMA during the sample preparation. DFB can be realized in an optical system by using degenerated two-wave mixing in the pumping beams. The period of the interference pattern can be easily changed by changing the intersection angle of the pumping beams, resulting in a real time, fully reversible method of DFB lasing emission tuning. Because of the two neighboring stimulated emission bands of DCNP, it is possible to tune the lasing wavelength over a long range of about 65nm.

Laser action and photoexcitations dynamics in PbI_2 films

We have grown high quality PbI2 films by thermal evaporation. We obtained in these films two amplified spontaneous emission (ASE) lines around ~2.23 eV at relatively low pump excitation threshold using 10 ns pulsed laser, along with a broad photoluminescence (PL) band that results from trapped electron and hole (e-h) pairs in shallow traps. We discuss the two ASE lines as due to the anisotropic PbI2 crystal structure. In addition, we also studied the ultrafast transient response of photoexcitations in the PbI2 films using a broadband pump-probe correlation spectroscopy in the broad spectral range of 0.3-2.4 eV with 150 fs time resolution. The transient photomodulation spectrum is dominated by two photoinduced absorption bands and a stimulated emission band that agrees with the obtained ASE band. We ascribed the low threshold laser action in PbI2 films as due to the e-h confinement in ‘quantum wells’ like structures formed between the main 2H-PbI2 crystalline and other polytypes with lower optical gap.

Stimulated emission in cryogenic samples doped with free-base tetraazaporphine.

Thin cryogenic samples of inert gas solids doped with free-base tetraazaporphine (H2TAP) were irradiated with a tunable pulsed laser. Under resonant electronic excitation of the guest, specific vibronic transitions of the fluorescence spectra were found to be strongly enhanced with only a moderate increase of the laser power. This enhancement is due to stimulated emission (SE). The characteristics of SE bands are described in the three hosts (Ar, N2, and Ne) explored, as well as their excitation spectra. SE is observed in transitions involving different vibrational modes of the guest, depending on the host and the electronic excitation. The results are discussed in comparison with previous works on other tetrapyrrolic molecules trapped in inert gas matrices. From this comparison the key features required to observe SE are deduced to be: (1) SE can be obtained with various tetrapyrrolic molecules; (2) free-base molecules are preferable to their metallo-counterparts; (3) the results highlight a specific molecular vibrational mode involved in the process; and (4) cryogenic crystal structures are also of importance in the detection of SE.

Direct Evidence of Torsional Motion in an Aggregation-Induced Emissive Chromophore

The aggregation induced emission (AIE) behavior shown by organic chromophores is very interesting for the development of efficient solid state devices. The restriction of intramolecular rotation is by far the most frequently assumed mechanism to explain this behavior; by blocking or reducing this rotation, upon rigidification of the environment, molecular luminescence is restored. By means of ultrafast pump–probe spectroscopy combined with density functional theory (DFT) and time-dependent DFT calculations, we show direct evidence of intramolecular rotation in a simple push–pull organic chromophore,4-diethylamino-2 benzylidene malonic acid dimethyl ester, possessing AIE properties. The spectral evolution of the stimulated emission band of the chromophore in the first 45 ps after photoexcitation is fully consistent with the presence of a torsional relaxation toward the equilibrium geometry of the excited state, taking place on time scales that depend on the solvent viscosity. The structural features of the excited state fully account for the different photoluminescence efficiencies observed in solvents with different viscosities.

Investigation of Fluorescence Emission from CdSe Nanorods in PMMA and P3HT/PMMA Films

Complementary fluorescence microscopy and ultrafast transient absorption spectroscopy measurements spanning a range of time scales from seconds to femtoseconds probe the interfacial dynamics of charge carriers in CdSe nanorod/polymer blends. Together, these very different techniques provide new information about the origin and dynamics of below-band-edge emission from CdSe nanorods in CdSe/PMMA and CdSe/P3HT/PMMA films [PMMA = poly(methyl methacrylate); P3HT = poly(3-hexylthiophene)]. Emission below the band edge of the CdSe nanorods is associated with surface defects (traps) at the nanoparticle/polymer interface, where conduction band electrons radiatively relax to the intraband defect sites. The fluorescence microscopy experiments simultaneously monitor both the trap emission and the band edge emission from single nanoparticles, and reveal that the two emission channels are distinct. Transitions between the two emissive states occur on time scales longer than ∼20 ms, and always involve an intermediate dark state in which no emission is observed. The presence of P3HT increases the relative band edge emission intensity and reduces the fluorescence intermittency (blinking) of both emissive states. The ultrafast transient absorption experiments monitor the evolution of a stimulated emission band below the CdSe band edge following excitation of P3HT. The measurements reveal ultrafast electron transfer from photoexcited P3HT to the CdSe nanorods within the instrument response time of approximately 140 fs, and confirm that there is strong coupling between the nanorods and P3HT in these dilute blends. Analysis of separate CdSe nanorod etching experiments suggests that the trap states are formed by the removal of atoms from the ends of the nanorods in the presence of chloroform. Mechanisms for charge trapping at the nanoparticle/polymer interface are discussed.

Second-chance forward isomerization dynamics of the red/green cyanobacteriochrome NpR6012g4 from Nostoc punctiforme.

The primary ultrafast Z-to-E isomerization photodynamics of the phytochrome-related cyanobacteriochrome NpR6012g4 from Nostoc punctiforme was studied by transient absorption pump-dump-probe spectroscopy. A 2 ps dump pulse resonant with the stimulated emission band depleted 21% of the excited-state population, while the initial photoproduct Lumi-R was depleted by only 11%. We observed a red-shifted ground-state intermediate (GSI) that we assign to a metastable state that failed to isomerize fully. Multicomponent global analysis implicates the generation of additional Lumi-R from the GSI via crossing over the ground-state thermal barrier for full isomerization, explaining the discrepancy between excited-state and Lumi-R depletion by the dump pulse. This second-chance ground-state dynamics provides a plausible explanation for the unusually high quantum yield of 40% for the primary isomerization step in the forward reaction of NpR6012g4.

Local nanotailoring of polymeric photophysics by Au nanoparticles implantation

AbstractWe studied the excitation dynamics and the photoluminescence spectra in polymeric matrix implanted with metallic nanoparticles. A supersonic beam of gold nanoparticles (sizes 2‐7 nm) was generated by using a Pulsed Microplasma Cluster Source (PMCS). The nanoparticles were implanted in a thin polyfluorene film (thickness around 100 nm) spin coated on a PMMA (poly methyl methacrilate) substrate. We performed photoluminescence and pump‐probe measurements using a Ti:Sapphire amplified laser system, the excitation pump pulse was centered at 400 nm with a 150 fs time duration. The experimental results show a suppression of the amplified spontaneous emission from the polyfluorene (PFO) film due to the presence of the implanted nanoparticles. The transient transmission spectra revealed the presence of a photoinduced absorption band due to the nanoparticles which overlaps with the stimulated emission band from the PFO, hindering the possibility of having amplified spontaneous emission. These results demonstrate the possibility of a local nanotailoring of the polymeric photophysics. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)