Low‐Threshold Amplified Spontaneous Emission in Quasi‐2D Perovskite Films Enabled by 3‐Phosphonopropanoic Acid Additives

By careful engineering of the core and shell dimensions in CdSe/CdS colloidal hetero-nanocrystals, amplified spontaneous emission can be triggered from either the core, shell, or both states simultaneously. The ASE threshold is almost constant over a temperature interval of 5–325 K. This feature is unique to quantum dots and highlights their potential as a gain material, suitable for lasing at elevated temperatures. Among the different photonic applications envisioned using semiconductor quantum dots (Qdots), Qdot lasers are one of the most investigated. In contrast to bulk materials or quantum wells (Qwells), the delta-like density of electronic states predicts a low, temperature-independent lasing threshold,1 providing enhanced device performance compared to other gain media, especially at elevated temperatures. Amplified spontaneous emission (ASE) and lasing have already been observed both in epitaxial2, 3 and colloidal4 Qdots. The molecular beam epitaxy (MBE)-grown Qdots, mostly based on III-V materials, have recently reached a level mature enough to move toward commercialization. Nevertheless, colloidal Qdots offer an interesting low cost alternative, being synthesized by wet chemistry approaches at low temperatures (25–350°C) and standard pressure. However, here progress toward efficient laser devices has been hampered by material quality and enhanced Auger recombination due to the small Qdot size (2–5 nm). Only recently, the latter bottleneck was overcome by the use of quasi-type II CdSe/CdS giant-shell Qdots5 or anisotropic seeded quantum rods (Qdot-in-rods).6 Indeed, here the Auger recombination time could be extended from 10–50 ps to over 10 ns, resulting for instance in biexciton photoluminescence (PL) with a near-unity quantum efficiency (QE).7 Additionally, due to the reduced contribution of Auger recombination, a low ASE threshold5 in combination with a prolonged gain lifetime and a broad gain spectrum has been observed.6 However, it should be noted that these quasi-type II heterostructures may also intrinsically be of better quality compared to previously investigated colloidal Qdot-based gain media: as the lattice mismatch between the CdS shell and the CdSe core is reduced to 4% (much smaller than for instance the 12% mismatch in CdSe/ZnS Qdots or 8.5% in InP/ZnS Qdots), the formation of interface defects, which may reduce the QE and prevent ASE, will be suppressed. In this respect, we recently reported that the CdSe/CdS Qdot-in-rod PL is nearly mono-expontential from 80 K up to room temperature,8 which confirms that our samples do not suffer from significant carrier trapping once the exciton has reached the band-edge state. CdSe/CdS Qdots offer additional advantages unique to this materials system. Preferential localization of the hole in the CdSe core and electron delocalization over both core and shell introduces a strong Coulomb repulsion in the biexciton, blue shifting its absorption with respect to the single exciton.9 Consequently, gain may occur near the single-exciton regime, further suppressing Auger recombination.10 In this letter, we demonstrate an important prerequisite toward the realization of solution processed efficient laser devices: a nearly temperature-independent ASE in a close-packed thin film of colloidal Qdots. So far, only Kazes et al.11 have reported on such behavior in colloidal nanocrystals. Using CdSe/ZnS Qdot-in-rods, they observed that a temperature-independent threshold could only be maintained below 100 K, as the nanosecond pulses employed lead to strong, non-equilibrium Auger heating and concomitant suppression of the gain. Here, we observed efficient ASE even at room temperature, with thresholds Ith = 0.15–1.5 mJ·cm−2 and 0.4–2 mJ·cm−2 for core and shell emission, respectively, up to an order of magnitude lower than previously studied CdSe colloidal Qdots.12, 13 ASE threshold measurements as a function of pump wavelength highlight the importance of employing long CdS Qrod arms, as they enhance the absorption cross section and significantly reduce the gain threshold. Increasing the CdS Qrod length triggers ASE from shell states, which allows probing the carrier dynamics in view of the ASE core and shell recombination time. Finally, power- and temperature-dependent measurements yield the characteristic temperature coefficient for the ASE threshold. By determining a T0 of at least 350 K, we demonstrate that colloidal CdSe/CdS Qdot-in-rod gain materials could enable temperature-insensitive lasing, pivotal for miniaturized and integrated laser sources. This peculiar feature, not present in Qwells and bulk material, triggered wide-spread research on MBE-grown Qdots and has now been extended to colloidal systems. Anisotropic colloidal CdSe/CdS hetero-nanocrystals offer size-tunable ASE over a wide range of wavelengths. The growth of the CdS shell provides surface passivation of the core and reduces the amount of surface defects, resulting in quasi type-II Qdots with high photoluminescence quantum efficiency.14 Additionally, it allows further extending the ASE spectral range as ASE in these systems can arise either from the core or the shell.15 Indeed, Table 1 and Figure 1(b) show that, by careful selection of the core and shell dimensions, the ASE can be tuned from a wavelength λ of 475 nm to 610 nm. Furthermore, by precise engineering of all dimensions, some samples display ASE from both core and shell states simultaneously, offering prospects for dual-emission applications.2, 16 In line with the PL data reported by Sitt et al.,9 the ASE from the CdSe core is blue shifted with respect to the PL peak, and this shift increases with decreasing size (Table 1). The blue-shift is expected to originate from Coulomb repulsion in the multi-exciton regime, which suggests that the ASE still occurs above the single exciton gain regime. (a) Typical TEM image of the Qdot-in-rods (sample S07). (b) Absorbance spectra (dashed line) and emission spectra above the ASE threshold (full line) for samples showing shell-only (bottom, S04), dual (middle, S02) and core-only (top, S06) ASE. In all samples, the shell clearly dominates the absorption spectrum over the barely visible core absorption. (c) Typical output intensity vs. input power (sample S02). The PL (dots) saturates at high I0. On the other hand, we observe a strong rise of the ASE for core (squares) and shell (diamonds) at this input power. (d) The CdSe core ASE threshold strongly decreases when exciting the CdS Qrod shell, due to its enhanced absorption cross-section (sample S03). This yields the ASE threshold Ith, which typically varies from 0.15 to 1.5 mJ·cm−2 for the core ASE and from 0.4 to 2 mJ·cm−2 for the shell ASE. Values are comparable to organic17 and other efficient Qdot gain materials13, 15, 18, 19 (typically achieving 0.01–1 mJ·cm−2), demonstrating their potential for highly efficient, low-cost light-emitting devices with an enhanced photo-stability compared to organic materials, yet no need for high-vacuum fabrication techniques as used in the case of MBE-grown Qdot devices. The key to obtain the low threshold here lies with the growth of a long CdS Qrod, which strongly enhances the short wavelength absorption cross section.19 Figure 1(d) shows the dependence of Ith as a function of the pump wavelength (sample S03). A strong reduction of Ith for off-resonant pumping is observed. In particular, we achieved an 8-fold decrease of Ith in sample S03, and a 3-fold decrease in sample S06 when pumping above the CdS absorption onset. The reduction, however, is not proportional to the increase in absorption. Comparing it to the ratio of the band-edge shell to band-edge core absorption (AR), we observe that the reduction is less pronounced for sample S03 (CdS Qrod length 52 nm, AR 65:1) than for sample S06 (CdS Qrod length 23 nm, AR 17:1). We can conclude that, by pumping well above the CdS band-edge, hot carrier trapping in the shell (at a rate τtr) may still limit the number of excitons reaching the lowest energy state in the CdSe core. Further support for this notion stems from PL excitation spectroscopy in CdSe/CdS Qdot-in-rods, showing a reduced PL QE when pumping the material above the CdS absorption edge,20 and from transient absorption measurements on CdSe Qdots, in which fast (ps-timescale) hot exciton trapping has also been observed.21 For such bare nanocrystals it lead to strongly suppressed gain when pumping them at 400 nm. Surface passivation using a ZnS shell,21 or increasing the nanocrystal volume by moving to single composition Qrods13 lead to reduced ASE thresholds. Here, using Qdot-in-rod heteronanocrystals we are able to further improve, decoupling the enhanced absorption cross section above the CdS band-gap from the emission tuning by the CdSe core size, thus adding a distinct advantage over single-composition Qrods. Further optimization of the Qdot-in-rods toward enhanced shell passivation may then lead to reduced shell trapping rates. Table 1 also reveals a second limit for the Qrod length. Increasing it first leads to dual emission and eventually to shell-only ASE. This transition from core to shell ASE cannot merely be due to the increased carrier trapping rate in longer Qrods, as shell transitions should have an increased Ith as well. However, it can be rationalized by considering that, next to trapping, the shell-to-core exciton transfer time plays an important role in the ASE decay dynamics. In the regime of ASE, balancing the exciton relaxation rates is key to tuning the ASE between different emitting states,16 in contrast to spontaneous dual-color emission, where emission from excited states requires phase-space filling.22 To gain further insight, we extracted the PL (τPLc, τPLsh) and ASE (τASEc, τASEsh) lifetimes of our Qdot-in-rods using spectrally-resolved streak camera measurements (∼2 ps time resolution), and spectrally integrated fluorescence decay traces (time-correlated single photon counting). Figure 2 shows typical streak camera images (sample S03, dual emission), measured at pump intensities I0 just below and above the ASE threshold. In Figure 2(a) a weak PL signal from the shell can be discerned, with a τPLsh = 6 ps. The typical lifetime for radiative recombination in pure CdS Qrods is about 20 ns,23 hence the picosecond lifetime observed here already indicates that the decay is dominated by non-radiative relaxation into the CdSe core. When increasing I0 (Figure 2(b)), a sharp red-shifted ASE peak appears, with a detector response-time limited τASEsh = 2 ps. For the core emission, the single exponential PL decay at low intensities (with τPLc = 20 ns) becomes bi-exponential with the addition of a 72 ps component at higher I0 (Figure 2(c)). This component is blue shifted and has a linewidth similar to the main PL peak, hence we can exclude ASE and assign it to emission from a biexciton (or possibly multi-exciton) state. Being still below the ASE threshold, the image confirms our previous assumption that ASE occurs in the multi-exciton regime in CdSe/CdS Qdot-in-rods. Further increasing I0 allows to observe a sharp ASE peak (Figure 2(d)), here with τASEc = 3 ps. Streak camera images collected on sample S06 (core-only emission, τPLc = 12 ns) at low and high I0 revealed a similar behavior, where we determined τASEc = 4 ps. The relevant carrier dynamics are summarized in Figure 2(e). Our time-resolved PL data reveal that the lifetime of core and shell ASE are comparable to the picosecond shell-to-core exciton transfer time τtf determined using transient absorption (TA) measurements.24 Taking into account that TA data have already shown that longer CdS arms lead to a reduced transfer rate to the core,25 our measurements now correlate this with a suppression of the core ASE in combination with the enhancement of the shell ASE. Streak camera images of sample S03. (a) Below the ASE threshold, the shell emission consists of a broad, rapidly decaying PL. (b) Above threshold, a sharp red-shifted ASE peak emerges. (c) For the core emission below the ASE threshold, a blue-shifted biexciton emission with fast decay is superimposed on the long-lived PL (observed as a flat vertical band due to its 20 ns lifetime). (d) Above threshold, a sharp ASE peak again appears. The curvature/chirp in (a), (b) and (d) is an artifact from the streak system at this high time resolution. (e) Schematic illustration of the relevant carrier dynamics in the CdSe/CdS heterostructure. The wavelength-dependent and time-resolved measurements substantiate how careful engineering of the rod length allows to tailor enhanced absorption and increasing transfer time in order to optimize the ASE properties, for instance toward low threshold, core-emitting samples. Such a sample (S06), together with sample S03 which shows dual emission, was used to determine Ith as a function of temperature. Figure 3(a) shows Iout as a function of I0 for S06, demonstrating that efficient ASE is observed up to 325 K. Ith is again determined via a fit to Equation (1). From the resulting Figure 3(b) it is evident that, despite the large temperature range going from 5 K to 325 K, the threshold remains largely constant for both samples. (a) Output intensity vs. input power at various temperatures (sample S06). (b) ASE threshold (log-scale) vs. temperature for the core ASE of sample S06 (squares), and core (dots) and shell (circles) ASE of sample S03. T0 is the characteristic temperature; a high T0 implies a nearly-temperature independent ASE threshold. Bulk or Qwell-based lasers have a typical T0 around 100 K,1 its value mostly being limited by the increased population of higher energy states at elevated temperature, with a concurring depopulation of the band-edge state. In contrast, fitting Equation (2) to our Qdot-in-rod data (Figure 3(b)) results in a T0 equal to 350 ± 75 K and 950 ± 565 K for samples S06 and S03, respectively. The larger error in the latter sample is due to its dual-emitting nature, which results to some extent in competition between core and shell ASE and consequently larger fluctuations in the ASE threshold. Interestingly, a nearly temperature-independent threshold is even obtained for the shell ASE of sample S03, which indicates that quasi-0D Qrods, with strong quantum confinement in the lateral dimensions and weak confinement along the 52 nm long Qrod axis, may also lead to enhanced temperature stability. The temperature-independent ASE threshold is explained by considering the sparse density of electronic states in small colloidal Qdots. Indeed, the absorption spectra in Figure 1(b) reveal a typical energy separation of 190–250 meV and 210 meV between the first and second excited state of core and shell transitions, respectively. Hence, as these values are an order of magnitude larger than the thermal energy at room temperature, no significant thermal depopulation of the lowest emitting state will occur even up to 325 K. Support for this conclusion comes from the increased T0 for sample S03 (core diameter 2.2 nm) in comparison to S06 (core diameter 3.0 nm), as the reduced core dimensions in the former case result in a stronger excited-state splitting and thus a higher T0. In summary, we have shown that CdSe/CdS Qdot-in-rods are excellent candidates for Qdot-based lasers, combining the advantages of wide spectral tuning of the ASE by varying the CdSe core size and a reduced gain threshold due to an enhanced absorption by the long CdS Qrod shell. Furthermore, they exhibit a high characteristic temperature T0 for the ASE threshold, enabled by the Qdot-in-rod's delta-like density of electronic states. Although future work on colloidal nanocrystals still needs to resolve the issue of efficient electrical injection (already available in epitaxial Qdot lasers), the first promising steps toward this goal have already appeared. Indeed, the fabrication of colloidal quantum dots with short inorganic ligands26 should allow overcoming the insulating barrier formed by the 1–2 nm thick organic ligand shell, paving the way toward a low cost, solution processable quantum dot laser. CdSe/CdS Qdot-in-rods were synthesized according to an established procedure.14 The optical and structural properties of the samples investigated are summarized in Table 1. The core size (diameter) was varied from 2.2 nm to 3.3 nm. Together with the CdS shell, with a typical thickness from 1.2 ML to 4.0 ML (1 ML = 0.34 nm), this resulted in Qrod diameters of 3.5 nm to 5.7 nm, with accompanying Qrod lengths between 21 nm and 52 nm (see Figure 1(a) for a typical transmission electron microscope image, sample S07). The ASE investigations were performed on close-packed Qdot-in-rod layers, drop-cast from a 10-20 μM solution of Qrods in toluene onto a fused silica substrate. To measure the ASE, the Qrods were pumped at a wavelength of 400 nm (unless specified otherwise) using an ultrafast laser with about 100 fs pulse width and a 1 kHz repetition rate. The pump light is the output from an optical parametric amplifier (Light Conversion TOPAS-C) which is pumped by a regenerative amplifier (Coherent Legend Elite) which in turn is seeded by a Ti:Sapphire fs-laser (Spectra Physics Tsunami). The pump beam was focused onto the sample with a cylindrical lens, forming a 50 μm by 2 mm stripe. Time-integrated PL and ASE spectra were collected from the sample edge and measured by a spectrophotometer (Ocean Optics USB2000+VIS-NIR). Time- and spectrally-resolved PL/ASE data were collected by means of a streak camera (Hamamatsu C5680 synchronized to the Ti:Sapphire seed laser with 80 MHz repetition rate) coupled to a monochromator (Acton Spectra Pro 306). For the low temperature measurements the samples were mounted inside a He-exchange gas flow cryostat (Cryovac). Variation of the pump intensity, pump wavelength and sample temperature allowed to carefully disentangle the role of the core and shell states in the ASE process and to determine T0. I.M. and G.R. contributed equally to this work. This project is partly funded by the EU Seventh Framework Program (EU-FP7 ITN Herodot). J. S. Kamal is acknowledged for performing the TEM measurements.

Inorganic lead halide perovskite quantum dot (QD)-embedded glasses with exceptional optical properties and stability are promising optical gain media for laser applications, but their amplified spontaneous emission (ASE) typically occurs at high pumping thresholds. Here, we report a glass network modulation strategy for low-threshold ASE from CsPbBr3@glass. By adding ZrO2 to enhance the glass network polymerization, high-quality and compact growth of QDs inside glass is promoted rather than uncontrolled self-crystallization. Transient absorption measurements reveal that this method reduces carrier trapping, inhibits biexciton Auger recombination, and accelerates hot exciton cooling, enabling efficient population inversion. Consequently, the CsPbBr3@glass exhibits a record-low ASE threshold of 54.5 μ J cm-2 and a high net modal gain coefficient of 394.4 cm-1 under femtosecond pulse excitation, together with quasi-continuous ASE realized using nanosecond laser pumping. Notably, our CsPbBr3@glass is orders of magnitude more stable than their colloidal counterparts for ASE under identical excitation conditions. This study underscores the potential for developing high-performance lasers using glass-protected perovskite QDs.

Solution-processed organo-lead halide perovskites have emerged as promising optical gain media for tunable coherent light sources. The lasing performance is generally determined by the as-synthesized crystal quality. Noble metal nanostructures have been widely utilized to enhance optical responses due to their unique property of localized surface plasmon resonance. Herein, we report a simple method to enhance the near-infrared amplified spontaneous emission (ASE) performance of MAPbI3 polycrystalline films by solution-processing a PMMA spacer layer and an Au NR-doped PMMA top layer on perovskite thin films. As a result, the ASE threshold of the triple-layer perovskite film was significantly reduced by around 36% and the ASE intensity increased by 13.9-fold, compared to the pristine film. The underlying mechanism was attributed to the combined effects of surface passivation by PMMA and plasmon resonance enhancement of Au NRs. The passivation effect results in suppressing the nonradiative recombination and prolonging excited state decay, which have been investigated by transient absorption and pump-probe measurements. The plasmon effect is systematically studied through distance-dependent and spectra-dependent plasmon enhanced emission. The perovskite films with PMMA and Au NR coating showed great stability for 180 min under intense pulse laser continuous irradiation. The improved ASE performance still remained after leaving the film under the atmosphere for more than one month. We have successfully demonstrated a highly stable and sustained ASE output from MAPbI3 films under pulse laser excitation. This study provides a general approach for exploring plasmonic nanostructures in combination with polymers in the development and application of low-cost solution-processed semiconductor lasers.

In this article we report the characterization of the energy transfer process in the reconstituted isoforms of the plant light-harvesting complex II. Homotrimers of recombinant Lhcb1 and Lhcb2 and monomers of Lhcb3 were compared to native trimeric complexes. We used low-intensity femtosecond transient absorption (TA) and time-resolved fluorescence measurements at 77 K and at room temperature, respectively, to excite the complexes selectively in the chlorophyll b absorption band at 650 nm with 80 fs pulses and on the high-energy side of the chlorophyll a absorption band at 662 nm with 180 fs pulses. The subsequent kinetics was probed at 30-35 different wavelengths in the region from 635 to 700 nm. The rate constants for energy transfer were very similar, indicating that structurally the three isoforms are highly homologous and that probably none of them play a more significant role in light-harvesting and energy transfer. No signature has been found in the transient absorption measurements at 77 K for Lhcb3 which might suggest that this protein acts as a relative energy sink of the excitations in heterotrimers of Lhcb1/Lhcb2/Lhcb3. Minor differences in the amplitudes of some of the rate constants and in the absorption and fluorescence properties of some pigments were observed, which are ascribed to slight variations in the environment surrounding some of the chromophores depending on the isoform. The decay of the fluorescence was also similar for the three isoforms and multi-exponential, characterized by two major components in the ns regime and a minor one in the ps regime. In agreement with previous transient absorption measurements on native LHC II complexes, Chl b --> Chl a energy transfer exhibited very fast channels but at the same time a slow component (ps). The Chls absorbing at around 660 nm exhibited both fast energy transfer which we ascribe to transfer from 'red' Chl b towards 'red' Chl a and slow transfer from 'blue' Chl a towards 'red' Chl a. The results are discussed in the context of the new available atomic models for LHC II.

A series of platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl phenylacetylide complexes (5-8) with 4'-naphthyl, 4'-phenanthryl, 4'-anthryl, and 4'-pyrenyl substituents have been synthesized and characterized. The emission properties of these complexes and their corresponding platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl chloride complexes (1-4) at room temperature and 77 K have been systematically investigated. Except for the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex that emits from an admixing state consisting of metal-to-ligand charge-transfer (3MLCT), intraligand charge-transfer (3ILCT), and 3pi,pi characters, emissions of 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl phenylacetylide complexes all originate from a 3MLCT-dominant state. The emission lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex (8) is longer than 2 mus at room temperature, and more than 300 mus at 77 K, while the other three complexes possess an emission lifetime of 200-400 ns at room temperature and tens of microseconds at 77 K. Replacing the chloride ligand in the 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl chloride complexes by a phenylacetylide ligand significantly increases the emission efficiency by an order of magnitude, and the emission lifetimes become longer. In contrast, such an alternation has no pronounced effect on the emission efficiency and lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl complexes. In the transient difference absorption (TA) spectra of 5 and 6, a moderately intense absorption band from 470 to 830 nm and a bleaching band between 400 and 470 nm were observed. For 7, the TA spectrum features a narrow, weak bleaching band at approximately 380 nm and a strong, narrow band at approximately 420 nm, as well as a broad, structureless band from 470 to 750 nm. In addition, a fourth, positive band appears above 800 nm. Complex 8 exhibits a strong, narrow bleaching band at approximately 340 nm and a broad, positive band extending from 370 to 830 nm, with the band maximum appearing at approximately 520 nm. The lifetimes obtained from the kinetic transient absorption measurement coincide with those from the kinetic emission measurement, indicating that the transient absorption originates from the same excited state that emits or, alternatively, from a state that is in equilibrium with the emitting state. All complexes exhibit optical limiting for 4.1 ns laser pulses at 532 nm, with 8 giving rise to the strongest optical limiting, presumably because of the much longer triplet excited-state lifetime and the stronger transient absorption at 532 nm.

Solvated electrons in water have long been of interest to chemists. While readily produced using intense multiphoton excitation of water and/or irradiation with high-energy particles, the possible role of solvated electrons in electrochemical and photoelectrochemical reactions at electrodes has been controversial. Recent studies showed that excitation of electrons to the conduction band of diamond leads to barrier-free emission of electrons into water. While these electrons can be inferred from the reactions they induce, direct detection by transient absorption measurements provides more direct evidence. Here, we present studies demonstrating direct detection of solvated electrons produced at diamond electrode surfaces and the influence of electrochemical potential and solution-phase electron scavengers. We further present a more detailed analysis of experimental conditions needed to detect solvated electrons emitted from diamond and other solid materials through transient optical absorption measurements.

Photoinduced absorption spectra and their temporal profiles of poly(para-phenylenevinylene) thin film exhibit a strong correlation with an appearance of spectrally narrowed emission bands. The dependence of transient absorption signals on excitation and probe power denotes much lower saturation intensity in photoinduced absorption than that in stimulated emission arising from the identical photoexcitations, singlet excitons. We interpreted the peculiar emission bands in terms of superradiance or superfluorescence with a time constant of around 1 ps rather than amplification of spontaneous emission from uncorrelated emitters.

Nonlinear optical properties of MXene and applications in broadband ultrafast photonics

Hybrid metal-halide perovskites are attracting huge research interest for possible applications in optoelectronic and photonic devices. In particular, the demonstration of optical gain and amplified spontaneous emission (ASE) at room temperature stimulates their development as active materials in light amplifiers and lasers. However, understanding of the basic photophysics of the processes affecting the ASE properties to date is still limited. In this work, we report a systematic investigation of the temperature dependence of the ASE and the photoluminescence (PL) of an MAPbBr3 thin film in the 20-300 K range. We confirm that the ASE threshold is strongly temperature dependent, due to the thermal activation of non-radiative processes. In addition, the ASE temperature dependence shows clear discontinuities at around 90 K and 190 K, related to the orthorhombic-tetragonal and tetragonal-cubic phase transitions, respectively. The film spontaneous emission under nanosecond and continuous wave pumping shows the interplay of emission of Free Excitons (FEs), Bound Excitons (BEs), and trap states, with relative contributions depending on the temperature and the excitation regime. Our findings result in a detailed description of the energy states generating the ASE and the ASE properties of the different crystalline phases.

The performance of Cr:forsterite amplifiers is reported for the first time. Small signal gain, temporal response, amplified spontaneous emission (ASE) and polarisation anisotropy are analysed with a 1300nm CW signal. Peak gain cross sections and spontaneous emission and effective lifetimes are determined. Chromium ion concentration and passive internal loss are found to play an important role. Results indicate that increased Cr4+ dopant level leads to lower gain and lower ASE noise.

Monitoring the viscosity of polymers in real-time remains a challenge, especially in confined environments where traditional rheological measurements are hard to apply. In this study, we have utilized the luminescent complex [Cu(diptmp)2]+ (diptmp = 2,9-diisopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline) as an optical probe for real-time sensing of viscosity in various adhesives during the curing process (viscosity increases). The emission lifetime of the triplet metal-to-ligand charge transfer (3MLCT) state of [Cu(diptmp)2]+ in epoxy adhesive increased exponentially during curing, similar to viscosity values obtained from oscillatory rheology. The longer lifetime in higher viscosity materials was attributed to changes in the excited-state deactivation processes from a known Jahn-Teller distortion in the Cu(I) geometry from tetrahedral in the ground state to square planar in the excited state. The real-time viscosity was also monitored reversibly by emission lifetime during polymer swelling (viscosity and lifetime decrease) and unswelling (viscosity and lifetime increase). Monitoring emission lifetime, unlike measuring the excited-state lifetime via transient absorption measurements in our previous study, allowed us to measure viscosity in opaque samples which scatter light. The optical probe [Cu(diptmp)2]+ in Gorilla Glue adhesive showed a clear correlation of the emission intensity or lifetime to viscosity during the curing process. We have also compared these lifetime changes using [Ru(bpy)3]2+ (bpy = bipyridine) as a control. [Cu(diptmp)2]+ showed not only a higher emission lifetime but also more ubiquity as a real-time viscosity sensor.

The amplified spontaneous emission (ASE) that develops on pulsed excitation of concentrated solutions of 2,5-diphenyloxazole (PPO) and related molecules is a nonlinear optical response that is considered anomalous. It consists of dual laser spikes whose relative intensity depends on laser excitation intensity: At the highest laser powers, the dominant spike corresponds to the less intense vibronic band in the normal fluorescence spectrum. This behaviour was attributed to electromers, a class of excited state conformers distinguished by having either the 2- or the 5-phenyl moiety coplanar with the oxazole ring and the other phenyl moiety twisted out-of-plane. Here, it is reported that under the same excitation conditions (266 nm laser excitation) the same ASE response is exhibited when nonplanarity at the 5 position is sterically favoured by 4-alkyl substitution (methyl or tert-butyl, MePPO and t-BuPPO, respectively). Although the X-ray structure of t-BuPPO shows the 5-phenyl group twisted 72° relative to the oxazole plane, t-BuPPO develops two ASE spikes and, on raising laser excitation intensity, the spike corresponding to the weaker vibronic band gains in intensity relative to the spike corresponding to the Franck–Condon (F–C) maximum, mirroring the behaviour of PPO. Accordingly, the electromer model for anomalous ASE spikes must be abandoned. Interestingly, imposing coplanarity of the 5-phenyl group in the rigid phenylindenooxazole (PIO) analogue leads to normal ASE behaviour. A single ASE spike develops that corresponds to the F–C maximum in the fluorescence spectrum of PIO. The different responses of these molecules may reflect different vibronic relaxation rates in the ground state manifold that control the ability of the molecules to maintain inverted populations with respect to ASE active transitions. Another possibility is that changes in relative spike intensity are due to weak differential transient absorption of the two ASE spikes. In the course of this work, a new method for the synthesis of 2,4,5-trisubstituted oxazoles was developed which may be of general use.

Recent studies of nanofibers on organic field‐effect transistor (OFET) photonic memory devices have gained significant attention due to their versatility and adaptability for achieving high performance. This research explores the integration of amplified spontaneous emission (ASE) from the organic laser dye pyrromethene 597 (PM597) with cellulose acetate (CA) polymer through electrospinning, serving as the floating‐gate layer in OFET photonic memory devices. Four fabrication approaches are employed to examine how different morphologies and optical properties affect device performance: mixed composite film, bilayer film, mixed composite fibers, and two‐step fibers. The findings indicate that the optimized memory device achieves a uniform morphology, enhanced scattering effects, and a lower ASE threshold of 4.68 ± 0.46 µJ utilizing two‐step fibers comprising PM597 and CA. As a result, the device exhibits superior memory characteristics, including a high memory ratio of ≈105, retention exceeding 10 000 s, endurance over 20 cycles, and relatively low illumination energy requirements. By refining fabrication methods to optimize ASE properties and memory performance, this study highlights a positive correlation between ASE and enhanced charge storage efficiency, further improving the device's optical performance and offering valuable insights into the intersection of laser physics and nonvolatile memory, an area with significant yet underexplored potential.

The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr3 nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.

The relaxation dynamics of cresyl violet H-aggregate dimers adsorbed onto SnO2 or SiO2 colloidal particles has been examined with ca. 200 fs time resolution. These experiments were performed by monitoring both the ground state recovery of the excited dye molecules and the transient absorption signal in the region of the dye radical cation. For cresyl violet−SiO2, the ground state recovery is a single exponential with a 2.9 ± 0.2 ps time constant. Transient absorption measurements that monitored the excited electronic state of the dye show a similar 2.5 ± 0.4 ps decay. The observed dynamics for cresyl violet−SiO2 is assigned to internal conversion followed by vibrational relaxation of the adsorbed cresyl violet dimers. The similarity of the transient absorption and bleach recovery time constants shows that vibrational relaxation is extremely rapid, i.e., internal conversion is the rate-limiting step. For cresyl violet−SnO2, the ground state recovery is biexponential with time constants of 2.4 ± 0.4 ps (∼80% of the amplitude) and 11.3 ± 0.5 ps (∼20%). Transient absorption measurements that monitor both the electronically excited dye aggregates and the dye radical cation also show a biexponential decay with time constants of 2.3 ± 0.3 and 12.3 ± 0.5 ps. The 2.4 ps process is attributed to internal conversion/vibrational relaxation of the excited dye aggregates, analogous to the results for the cresyl violet-SiO2 system. The 12 ps process is assigned to the decay of the cresyl violet dimer radical cation that is produced by electron transfer to the SnO2 semiconductor particles. The radical cation only contributes to the signal for the cresyl violet−SnO2 system because electron transfer to SiO2 is not energetically allowed. The decay mechanism for the radical cation is back electron transfer from SnO2.