Distributed Feedback Lasing in Amorphous Polymers with

We present the synthesis and characterization of a new type of organic materials for light amplification purposes. These materials consist in branched...
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Distributed Feedback Lasing in Amorphous Polymers with Covalently Bonded Fluorescent Dyes: The Influence of Photoisomerization Process Kacper Parafiniuk,*,† Cyrille Monnereau,‡ Lech Sznitko,† Bastien Mettra,‡ Monika Zelechowska,† Chantal Andraud,‡ Andrzej Miniewicz,† and Jaroslaw Mysliwiec† †

Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ‡ ENS de Lyon, CNRS UMR 5182, Université Lyon 1, Laboratoire de Chimie, Univ Lyon, F69342 Lyon, France S Supporting Information *

ABSTRACT: We present the synthesis and characterization of a new type of organic materials for light amplification purposes. These materials consist in branched polymers based on 9,10bis(4-(diethylamino)phenylethynyl)anthracene or 2-(3-(4(diethylamino)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile chromophore cores with covalently attached poly(methyl methacrylate) chains, named Ant-PMMA and Lem-PMMA, respectively. In both cases, polymers with controlled molecular weight of about 30 kDa were synthesized by atom transfer radical polymerization (ATRP), using functionalized chromophores as initiators. Thin layers of the two polymers were fabricated by a simple drop-casting technique. We describe the spectroscopic properties of these materials and their ability for light amplification through the measurements of amplified spontaneous emission, random lasing process, and distributed feedback (DFB) lasing achieved via holographic-type excitation. Considering the different chemical structures of the chromophores, and related distinct interaction pathways with light, we postulate two slightly different DFB lasing mechanisms in investigated organic solid-state gain media. The Lem-PMMA ability to undergo photoisomerization, providing material refractive index modulation upon holographic-type pumping, is supposed to be responsible for superior DFB lasing performance as compared to Ant-PMMA, for which similar type of excitation results in lasing coupling solely dominated by gain modulation.

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INTRODUCTION Nowadays a global trend in photonic research focuses on the replacement of inorganic materials by organic ones. This evolution is well-documented in such fields as sensing, switching, photovoltaics, light-emitting diode technologies, or gain media for lasing purposes.1−11 Within that framework, works aiming at the incorporation of organic gain materials (i.e., those showing stimulated photon emission) in integrated photonic devices have most often relied on π-conjugated macromolecules or dye-doped polymers.11,12 In addition to some spectroscopic features of the designed materials, such as fluorescence spectral range or quantum efficiency, further practical requirements are also important. Those include in particular the ease and robustness of the synthetic procedure as well as of the material processing. In that context, the first type of materials often relies on tedious synthesis and purification procedures, and is also prone to optical damage through photooxidation reactions.11 Therefore, its use is mostly limited to low-energy output lasing systems, and the aforementioned advantage over inorganic materials in terms of fabrication cost is merely lost. Conversely, dye-doped polymers undergo chromophore concentration depending effects, such as © XXXX American Chemical Society

luminescence quenching (caused by nonemissive H-aggregates formation, reabsorption, or self-energy transfer processes) or luminescent J-aggregates formation emitting in the often undesired, red-shifted spectral region.13−16 In the present paper we propose a new type of polymeric materials for lasing applications that combine, in an unique object, a single 9,10-bis(4-(diethylamino)phenylethynyl)anthracene or 2-(3-(4-(diethylamino)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile chromophore covalently bonded to multiple poly(methyl methacrylate) (PMMA) chains. The structures of these compounds, named AntPMMA (as anthracene derivative) and Lem-PMMA (as Lemke’s chromophore derivative), are shown in Scheme 1. The polymers were synthesized using atom transfer radical polymerization (ATRP), starting from chromophores “macroinitiators”. The nature of the reaction provides us with a precise control over the product molecular weight.17 Since it yields compounds containing only one luminescent center per Received: April 28, 2017 Revised: July 24, 2017

A

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Macromolecules Scheme 1. Synthetic Route for Ethynylanthracene and Lemke’s Chromophore Derivative-Attached PMMA

functionality. This particular polymer, containing a specific type of photoisomerizable chromophore, simultaneously provides optical gain and temporary resonator formation as well as plays the role of self-sufficient waveguiding medium (for laser light propagation).

macromolecule, it results in well-separated chromophore sites in the bulk system.18,19 This precludes the above-mentioned aggregation or phase-segregation processes often encountered in that class of materials.20 Furthermore, immobilization of the chromophores within the gain medium strongly limits their diffusion upon laser light irradiation. Finally, such kind of luminescent polymers may simultaneously play the role of matrix itself. The layers of luminescent polymers, as obtained by the drop-casting technique on glass substrate, were studied for their fundamental light enhancement abilities through the measurements of amplified spontaneous emission phenomenon (ASE), random lasing process (RL), and lasing with distributed feedback (DFB), utilizing a holographic method for formation of a quasi-temporary resonator. The lack of permanent DFB resonator structure, characteristic for that kind of lasers,21 allows to simply test and compare gain abilities of the studied organic media, in contrast to time-consuming relief grating prefabrication providing fixed output wavelength.22,23 Our choice of the chromophores was driven by their presumably distinct interaction with light. It was expected that the polymer containing the highly dipolar Lemke’s chromophore derivative can undergo cis−trans photoisomerization process, unlike the one containing a rigid quadrupolar ethynylanthracene derivative. Consequently, these two polymers should exhibit considerably different behavior (in particular regarding light-induced orientation processes and changes of their refractive index) upon spatially modulated intensity of nanosecond pulsed laser excitation and thus exhibit DFB laser action dominated by refractive index coupling, in the former case, or gain coefficient coupling, in the latter case. Our hypothesis was proven by consistent results concerning light amplification studies and confirmed by light-induced birefringence measurements performed in optical Kerr effect (OKE) experimental configuration. So far, a few works presenting an attempt to realize tunable DFB laser with noticeable index coupling have been reported. Such devices were based on double-layer polymeric systems doped with two different dyes (first responsible for diffraction grating formation, second providing gain),24 photorefractive matrices doped with laser dyes,25,26 or passive polymers doped with luminescent dyes able to produce refractive index modulation.27 Herein, the tunable DFB lasing in Lem-PMMA material is probably the first demonstration of monolayer organic gain medium composed of single compound (a branched polymer) performing triple

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EXPERIMENTAL SECTION

Synthesis and Materials Preparation. Synthesis of ethynylanthracene and Lemke’s chromophore derivative based initiators, Ant-In and Lem-In, respectively (Scheme 1), was based on earlier reported procedures.28,29 In both cases, polymerization of PMMA was achieved by classical ATRP of methyl methacrylate (MMA) at 60 °C, using a tris(2-(dimethylamino)ethyl)amine/copper(I) bromide catalyst (Me6TREN/CuBr) and anhydrous tetrahydrofuran (THF) as a solvent, until complete conversion of the monomer. The polymer was recovered and purified by multiple cycles of solubilization in THF and precipitation in cold methanol. Details of the synthesis are enclosed in Table S1 of the Supporting Information. Final products, Ant-PMMA and Lem-PMMA, were characterized by proton nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC) (cf. Supporting Information Figures S1 and S2; experimental details for GPC are enclosed in the caption for Figure S2). Integration of the NMR signals confirmed the addition of ca. 220 and 210 monomer units per object for Ant-PMMA and Lem-PMMA, respectively, which is in good agreement with the precalculated value, taking full monomer conversion as an hypothesis (222 equiv of monomer in both cases; cf. Table S1). The polydispersity indices (PDI), measured by GPC for both polymers, are higher than those usually measured for linear polymers, i.e., PDI = 2.0 and PDI = 1.5 for Ant-PMMA and Lem-PMMA, respectively, although in good agreement with those found in previous works.28,29 Furthermore, the average molecular weight values ca. Mn = 31 kDa for Ant-PMMA and Mn = 27 kDa for Lem-PMMA found by GPC, using the universal calibration methodology, were found close to the theoretical target values of about 23 kDa. The synthesized polymers were dissolved in dichloromethane (DCM) to obtain 2% w/w mixtures. After complete mixing, 0.4 mL of each solution was drop-casted onto previously cleaned 2.5 × 2.5 cm2 glass substrates, covering the entire surface. The layers were dried slowly under an atmosphere of DCM solvent at room temperature overnight and then additionally dried in atmospheric conditions for 1 day. The thickness of resulting layers, forming asymmetric planar waveguides for light, was determined by a Dektak profilometer and, measured in the places utilized later for achieving light amplification and lasing, was found to be 28 μm for Ant-PMMA and 8 μm for LemPMMA. Methods. All fluorescence measurements were performed at room temperature. Quantum yields of photoluminescence for studied B

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producing two beams of equal intensities. Those two beams were then intersected in the investigated polymeric layer by the use of two mirrors. As a result of beam interference, the sample was irradiated by periodically modulated light intensity pattern, with a period Λ dependent on intersection angle θ, pumping light wavelength λpump = 532 nm and the corresponding material effective refractive index n(532 nm) according to the equation13

polymers were determined using a Horiba-Jobin-Yvon Fluorolog-3 spectrofluorometer, with a three-slit double-grating excitation and emission monochromator with dispersions of 2.1 nm/mm (1200 grooves/mm). A R928 detector was used in the visible range (400− 845 nm). The sample cavity was equipped with a GMP G8 integration sphere. Samples (DCM solution or solid) were placed in an open capillary quartz tube, inserted into the sphere. In both cases, quantum yields were obtained using the methodology originally described in full by de Mello et al.30 In order to minimize differences between the peak intensities of the lamp and emission profiles, and ensure that the collected signal remained in the linear range of the detector, density filters (0.5%) were used to attenuate the intensity of the lamp profile. Excitation and emission spectra of investigated polymers were recorded using a Hitachi FL-4500 spectrofluorometer. Excitation spectrum measurement of Ant-PMMA polymer was performed at an emission wavelength λem = 570 nm, while fluorescence emission spectrum was collected using excitation at λex = 480 nm. In the case of Lem-PMMA polymer the excitation spectrum was obtained at λem = 640 nm, while the emission spectrum was achieved by the use of λex = 490 nm excitation. The luminescence lifetimes were estimated from measurements of luminescence decays with Becker & Hickl time-correlated single photon counting (TCSPC) setup consisting of data acquisition module (SPC-130-EM) and hybrid PMT detector (HPM-100-06) mounted to the Princeton Instruments spectrograph (Acton SpectraPro-2300i). Excitation was provided by a picosecond λpump(PS) = 516 nm laser diode (BDL-516-SMC). The values of the fluorescence lifetimes for Ant-PMMA (measured at λem = 580 nm) and LemPMMA (measured at λem = 600 nm) were extrapolated from a singleexponential decay fit after deconvolution of measured instrument response function (IRF), with the use of dedicated Becker & Hickl SPCImage software. The characterization of the casted polymer layers, in regard to their ability to provide light amplification under an optical pulsed laser excitation (i.e., gain), was achieved in a few steps. In each case, a frequency-doubled neodymium-doped yttrium aluminum garnet nanosecond pulsed laser was used as a source of photoexcitation (Nd:YAG, Surelite II by Continuum). The laser was working at repetition rate equal to 10 Hz, delivering circular-shaped beam (3 mm in diameter in cross section behind a diaphragm) with wavelength of λpump = 532 nm and pulse duration equal to 5 ns. The pumping beam energy density reaching the sample was controlled by rotation of a half-wave plate azimuth with respect to a Glan laser linear polarizer. In the first step, we focused our attention on ASE measurements in order to get information about spectral position and shape of the gain profile as well as about energy density threshold for achieving energy states population inversion in the absence of any resonator.31 In this case, the pumping beam remained unchanged, and the emission from the sample edge was collected by an Ocean Optics USB2000+ optical fiber spectrometer. In the second step, we determined the ability of the investigated materials to provide feedback for RL based on the samples inhomogeneities (giving a contribution to light scattering and formation of local resonators).32 In order to achieve the degree of light confinement required for RL, the shape of excitation beam was changed into a narrow stripe (4 mm × 0.5 mm in the cross section). For that purpose, we used two biconvex spherical lenses expanding the circular-shaped beam and one plano-convex cylindrical lens focusing it into stripe. As a consequence of such an approach, a quasi-onedimensional planar waveguide is created in the layer, where the light localization alongside the excitation stripe presents a higher probability than any other directions.33 Because observation of RL requires high spectral resolution, we used an Andor Solis Shamrock fiber spectrometer (0.1 nm resolution) coupled with pumping laser, allowing collection of the data during each excitation pulse. Emission was acquired in series of 25 spectra in order to get better insight into an evolution of the phenomenon in time. In order to perform DFB laser action measurements, we prepared a so-called degenerate two-wave mixing (DTWM) optical setup.34,35 The circular-shaped pumping beam was directed onto a beam splitter

Λ=

λpump 2n(532 nm) sin(θ /2)

(1)

The interaction of the excitation light interference pattern with a fluorescent polymer layer causes refractive index and/or gain coefficient modulation and in fact a diffraction grating. If the period Λ of this grating satisfies the Bragg condition for light propagating in the layer

m

λlas = n(las)Λ 2

(2) 36

the gain or index coupled DFB lasing may occur. In eq 2, m stands for the Bragg diffraction order, λlas is the wavelength of DFB laser emission, and n(las) is the effective refractive index of the gain medium at DFB lasing wavelength. Therefore, by choosing proper beams intersection angle θ, it is possible to get laser emission at wavelength selected from the spectral range of a material gain profile. The order of Bragg diffraction in our studies was equal to m = 2, which was the lowest possible for λpump = 532 nm pumping wavelength. The DFB laser emission was monitored with the use of both above-mentioned spectrometers. The values of pumping energy density in the case of DFB laser action experiments (e.g., values of the lasing threshold) were calculated from the output energy density of Nd:YAG irradiation incident on the beam splitter. It means that locally, in the maxima of bright fringes, energy density is twice higher (if we assume the highest possible contrast). The final part of our studies was designed to get insight into the mechanism of DFB lasing in studied samples by testing their susceptibilities of undergoing photoinduced refractive index changes. In order to observe photoinduced birefringence two CW lasers were used: a diode pumped solid state laser (DPSS, Compass 315M-100 by Coherent operating at the wavelength of λpump(CW) = 532 nm) serving as a pump source and a helium−neon laser (He−Ne, SP-127 by Spectra-Physics operating at the wavelength of λprobe(CW) = 633 nm) serving as a probe.37 The linearly polarized red light coming from the probing laser was directed with a normal incidence onto the measured sample positioned between two 90° crossed polarizers. When the pumping beam, with a linear polarization set at 45° with respect to probing beam polarization, is inducing the birefringence, the probing beam becomes elliptically polarized after passing through the sample. Thus, a photodiode (PDA 55 by Thorlabs) set behind the analyzer can detect a part of probing light intensity. The dynamics of photoinduced birefringence increase was monitored by coupling the detector signal to a Tektronix TD oscilloscope.

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RESULTS AND DISCUSSION Both synthesized polymers show substantial differences in their basic luminescent properties. The fluorescence quantum yield of Ant-PMMA is very high, equal to ΦDCM = 0.87 in DCM solution and Φsolid = 0.67 in the solid state. Conversely, LemPMMA shows a considerably lower fluorescence quantum yield, being ΦDCM = 0.03 in DCM solution and Φsolid = 0.09 in the solid state. This observation is likely connected with the strong push−pull character of the Lemke’s chromophore derivative which in weak polar environment, such as DCM, could effectively dissipate excitation energy through the twist of dicyanoethenyl group.29,38 Such process is significantly restricted in more viscous environment, explaining the notable quantum yield enhancement in solid state for Lem-PMMA, what is a general trend for this compound family.39 C

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Figure 1. Normalized excitation, emission, and ASE spectra of Ant-PMMA (a) and Lem-PMMA (b) layers. Presented ASE spectra were measured for excitation with the pumping laser energy densities equal to ρ = 5.9 mJ/cm2 and ρ = 1.3 mJ/cm2 for Ant-PMMA and Lem-PMMA samples, respectively. The plots below are showing estimation of ASE thresholds ρth for Ant-PMMA (c) and Lem-PMMA (d).

of ASE band for Ant-PMMA is positioned at λASE = 563 nm with FWHM = 15 nm, while that of Lem-PMMA appears at λASE = 643 nm with FWHM = 16 nm. We observed a marked difference in the threshold energy densities at which the ASE processes occur, as it can be seen in Figure 1c,d. The threshold of ASE for Ant-PMMA was estimated to be ρth = 3.7 mJ/cm2, whereas for Lem-PMMA it was equal to ρth = 0.9 mJ/cm2. The change of the excitation area shape into a stripe results in randomly emerging multiple narrow spikes from the emission spectra. This phenomenon can be ascribed to random feedback, generated on the layer irregularities. When the emitted light is confined into quasi-one-dimensional space of stripe-shape geometry, only the photons propagating along the excited region of the sample undergo amplification. Since the dimension of gain medium is reduced, the probability that photons, due to multiple scattering, can return to the place where they were generated is nonzero. This can give rise to an interference pattern generation, which results in weakly localized light.43 Representative examples of spectra showing RL are presented in Figure 2. In the case of circularly shaped excitation area, this effect has lower probability, first because of weak scattering (the obtained layers show good optical quality, with no chromophore−polymer segregation, and thus weak light diffusion) and second due to the higher number of possible trajectories for photons amplification (i.e., reduced probability for photon closed loop paths formation). That in turn constitutes the typical behavior for ASE phenomenon (cf. Figure 1). Observed RL spectra are generally different depending on the excited region of the material and on the excitation energy density, and may furthermore vary from a pulse to another one

Excitation and emission spectra of the investigated layered materials are shown in Figure 1. Ant-PMMA shows broad excitation bands, with maximum at λex = 520 nm. The spontaneous emission spectrum for this compound is quite narrow, with full width at half-maximum FWHM = 50 nm, and positioned at λem = 547 nm. In the case of Lem-PMMA the excitation spectrum is shifted to the red, having its maximum at λex = 566 nm. The linear fluorescence emission spectrum of that compound is relatively broad, with FWHM = 85 nm, and centered at λem = 603 nm. Fluorescence lifetime measurements of the fabricated layers show near monoexponential decays. The results confirm that, as expected, the shielding effect of the PMMA chains provides efficient separation of the chromophores within the material.13,40 By fitting the emission decays, fluorescence lifetime values of τ = 2.6 ns and τ = 1.2 ns (at room temperature) were found for Ant-PMMA and Lem-PMMA, respectively, which is consistent with what had been determined in previous studies.29,41,42 The fluorescence decay curves together with their fitting and IRF signal are enclosed in Figure S3. Preliminary results concerning basic spectroscopic properties of the materials are in accordance with predictions based on chemical structure analysis, i.e., the more rigid Ant-PMMA chromophore possesses lower number of possible nonradiative relaxation pathways than Lem-PMMA and therefore exhibits longer lifetime of the excited state, narrower emission band, and higher quantum yield. Despite the differences in excitation spectra of the compounds, both can be effectively excited by λpump = 532 nm pulsed laser light to reach ASE level. Resulting representative spectra are shown in Figure 1, where maximum D

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Figure 2. Examples of RL spectra observed for Ant-PMMA (a) and Lem-PMMA (b) measured at excitation energy density equal to ρ = 1.9 mJ/cm2 for both samples. The graphs below are showing estimation of RL thresholds ρth for Ant-PMMA (c) and Lem-PMMA (d) samples. The intensity error bars were estimated on the basis of 25 individual RL spectra from a series.

of the pumping laser. Therefore, the spectra in Figure 2a,b are representative of the RL emission recorded during a given pumping pulse. Energy density required for the formation of random resonators was estimated by averaging the integrated intensities of each spectrum in series and plotting them as a function of the excitation energy density (Figure 2c,d). One can notice that the thresholds of the RL process given by the inflection points, as compared to the ASE thresholds, decrease noticeably to ρth = 1.0 mJ/cm2 in the case of Ant-PMMA and to ρth = 0.7 mJ/cm2 in the case of Lem-PMMA, which may result from the presence of proper feedback conditions for occurrence of RL process. However, the threshold values, given by the curves inflection point, might be overestimated because some RL characteristic spectra appeared even at lower pumping fluence as a result of the high instability of random laser performance at the threshold conditions.44 Such unstable behavior results in increase of the output emission intensity error margins, even if the pumping power fluctuations seem to be negligible (as it is witnessed by the conversely small amplitude of the energy density error bars in Figure 2c,d). Therefore, the points at which error bars are becoming significant (indicated by red circles in Figure 2c,d) may be more adequate for threshold assignment in the case of random lasers. RL spectra series corresponding to these points are shown in Figure S4. Generally, higher pumping fluence gives rise to more pronounced RL spectra, which in turn is associated with higher standard deviation of the integrated average intensities, as a consequence of higher number of possible trajectories for photons amplification.

In order to provide positive and well-controlled feedback in the excitation region of the studied gain materials, we employed the holographic technique to form DFB resonator. Spatially modulated intensity of pumping beam resulting in formation of temporary population diffraction grating enables DFB lasing with an option of tunable laser output in real time.35 As expected, the two investigated compounds showed drastically different responses in the DTWM experiment. DFB laser action spectra achieved for different pumping energy densities and corresponding plots showing estimation of lasing thresholds are shown in Figure 3. In both cases the intersection angle θ between pumping beams was adjusted to get laser action in the central region of gain profiles, i.e., at λlas = 564 nm for AntPMMA (θ = 93°, Λ = 366 nm, n(las) = 1.542) and at λlas = 642 nm for Lem-PMMA (θ = 78°, Λ = 422 nm, n(las) = 1.522). A very low threshold for laser action equal to ρth = 0.6 mJ/cm2 was found for Lem-PMMA. As expected, it was lower than the threshold observed for ASE, but still in the same order of magnitude. In the case of Ant-PMMA layer, DFB lasing line emerges from a fairly intense ASE band. Relatively large contribution of ASE in overall stimulated emission is reflected by the dependence of emission intensity on pumping energy density, where an increase of the slope above DFB lasing threshold is insignificant. The threshold, as estimated from the plot, amounts to ρth = 6.9 mJ/cm2, which is far above ASE threshold for the same material, in distinct contrast to LemPMMA based layer. The occurrence of considerably lower lasing threshold for Lem-PMMA based material could be explained on the basis that the interfering pump beams create not only a population E

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Figure 3. DFB lasing spectra for different pumping energy densities for Ant-PMMA (a) and Lem-PMMA (b) layers. The plots below the spectra show estimation of DFB laser action thresholds ρth for Ant-PMMA (c) and Lem-PMMA (d) gain media.

may be possible only far above the threshold of ASE, what is observed in our experiment. First the ASE arises because the average gain exceeds losses, and after that when spatial modulation of gain coefficient is strong enough, mode selection can be achieved and thus DFB laser emission occurs. On the basis of our observations, it can be concluded that DFB lasing for these two compounds involves different coupling mechanisms, i.e., refractive index or gain coupling.36 Even a single laser shot in DTWM configuration is able to leave in the Lem-PMMA sample a trace of photoinduced molecular ordering, and subsequent ones may lead to buildup of much stronger grating, thus forming conditions for true DFB laser. There is no similar possibility in rigid ethynylanthracene-based chromophores, although this does not prevent DFB lasing in the case of Ant-PMMA. Population grating is sufficient to form DFB conditions, but the process requires considerably higher pumping level (cf. Figure 3). To prove this hypothesis, we performed the measurements of optically induced birefringence using a so-called OKE experimental setup, described in details in the Experimental Section. Results of the measurements performed with continuous illumination by linearly polarized laser light of wavelength λpump(CW) = 532 nm are shown in Figure 4, which clearly reveals the qualitative difference between Ant-PMMA and Lem-PMMA samples. In the case of Lem-PMMA sample, irradiation results in a strong signal, which indicates the changes in the probe λprobe(CW) = 633 nm beam polarization state after passing throughout the layer, which is a clear evidence for the occurrence of photoinduced birefringence. Immediately after switching-off the pumping beam, a decrease of photoinduced birefringence is observed,

grating but also a diffraction grating based on periodic refractive index changes. These changes could be rationalized considering light induced cis−trans photoisomerization processes in Lemke’s chromophore derivative, favored by double bond bridge in the π-conjugated system with highly delocalized electron cloud.29,45 It is well documented that photoisomerization processes facilitate the molecular reorientation process perpendicular to polarization plane orientation,46,47 which can in turn induce after each pulse a local (i.e., at bright fringes of the pumping interference pattern) birefringence and thus refractive index grating formation in the material bulk.46,48,49 Therefore, when the pumping light intensity increases, the refractive index grating is being formed, resulting in the constitution of 1-D photonic band gap before the threshold conditions for ASE are reached. The appearance of the effective feedback loop in Lem-PMMA results in generation of laser line slightly below the ASE threshold for that material. In the case of Ant-PMMA, the chromophore chemical structure exhibits a high degree of symmetry and rigidity with no possibility for photoisomerization, thus preventing (or strongly limiting) reorientation of the chromophores under laser irradiation. We can state that for Ant-PMMA layer the mechanism of DFB lasing is rather based on the constitution of gain coupled feedback which arises when gain coefficient becomes periodically modulated in space. For this type of coupling, no photonic band gap is formed. Thus, in order to achieve wavelength selection, some certain amplitude of gain modulation has to be reached. If the feedback strength is not sufficient, for instance when the contrast of interference fringes is weak, the mode selection according to the Bragg conditions F

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The second explanation for the appearance of two lasing modes may be associated with the 1-D photonic crystal structure formed due to periodic refractive index spatial modulation throughout the Lem-PMMA gain medium, for which we proved contribution of chromophore photoisomerization process in photoinduced birefringence experiment. Such behavior was not encountered for Ant-PMMA material, further demonstrating that refractive index changes within the excitation area are extremely low. The observation of two lasing lines, which positions are determined by the highest density of states at the edges of 1-D photonic band gap structure in the case of Lem-PMMA, could serve for estimation of refractive index modulation amplitude Δn, following the model equation36,50 Figure 4. Photoinduced birefringence in Ant-PMMA and Lem-PMMA layers. Power density of λpump(CW) = 532 nm wavelength pumping beam was equal to P = 0.42 W/cm2.

Δn =

Δλ n(gap) λgap

(3)

which led us to calculate Δn = 0.0022. In eq 3, λgap = 632.15 nm represents the center of the photonic band gap and Δλ = 0.9 nm stands for its spectral width, while n(gap) = 1.527 is the refractive index calculated for the foregoing wavelength. Since in the DFB lasing experiment we have not observed progress of modes separation with increase of excitation pulses number, we deduced that the saturation of the chromophores alignment upon nanosecond pulsed pumping was quickly achieved. Conversely, in the experiment with CW laser excitation, birefringence saturation is not reached after 1 min (cf. Figure 4), meaning that the highest degree of chromophores ordering upon such pumping was not achieved. Anyhow, it has to be mentioned that the values of Δn achievable in both experiments are not comparable. In the case of nanosecond pulsed pumping we would expect a lower degree of chromophores ordering, first because of the much shorter time slot for their reorientation (pulse duration 5 ns) and second because stimulated emission process competes with the excited states depopulation through the photoisomerization. Therefore, the photoinduced birefringence experiment proved qualitatively the possibility of chromophores ordering by polarized light excitation, while the value of DFB lasing modes separation in DTWM experiment allowed to calculate the actual amplitude of refractive index modulation. The highly unequal intensities of lasing peaks can be explained by simple reasoning based on the specific method of material pumping utilized in our studies. The holographic excitation results in the otherwise uniform gain medium irradiated by spatially modulated light intensity becoming a periodic one. Because the pumping beams in DTWM experiment have the same intensities, the highest possible contrast mc ∼ 1 of the interference pattern was achieved. An obvious consequence of that kind of excitation is spatial modulation of gain but in the case of Lem-PMMA of refractive index as well. As described above, such modulation results in 1D photonic band gap and thus two lasing modes appearance for Lem-PMMA. However, due to their nature, the mode of frequency ω1 is in phase with the population grating, while the mode of frequency ω2 is in antiphase to the population grating.51 Then, the exponential gain coefficients for the two modes, averaged over the grating period, differ significantly. This results in considerably different modes amplifications for the same photon propagation distance, which is in fact observed. The mode of frequency ω1 is propagating with higher effective refractive index, while the mode of ω2 is with

indicating that the material is slowly returning to its initial isotropic state. In the case of Ant-PMMA, under identical excitation conditions, no photoinduced birefringence was observed. The observation of DFB emission with an Andor Solis high resolution spectrometer provided us with closer insight into the lasing process performance after each pumping light pulse. Exemplary spectra series obtained for a few sets of pumping beams intersection angle, with excitation energy density far above the estimated threshold value, confirming easy wavelength tuning possibility in the DTWM optical system, are shown in Figures S5 and S6 (see also Table S2 containing the corresponding data). The first obvious difference is that in the case of Lem-PMMA DFB lasing occurs during every excitation pulse and contribution of ASE background in overall emission can be neglected, while for Ant-PMMA, the process is unstable and lasing spikes are emerging occasionally from the gain profile. The second noteworthy observation is that the spectral shape of the laser line (also for reciprocal wavelength units plot) is more symmetric in the case of Ant-PMMA than for Lem-PMMA. It can relate to the fact that DFB emission from the latter material consists of two closely lying spikes, meaning two-mode laser action. Indeed, an unambiguous example of two laser lines appearance for Lem-PMMA material is shown in the inset to Figure S6a (the longer wavelength line having about 7 times higher intensity than the shorter wavelength one). There are at least two possible explanations for such an observation. First, the origin of each peak may be attributed to two perpendicularly polarized lasing modes propagating within planar waveguide layer, i.e., red-shifted transverse electric (TE) and blue-shifted transverse magnetic (TM) ones. Guessing from the position of laser lines within the gain profile, as well as from the layer thickness, being much thicker than the lasing wavelength, these two peaks should have similar intensities. However, considering that the plane formed by the pumping beams is perpendicular to the layer, we can tentatively explain unequal intensities by mean of excitation light linear polarization state. Namely, mostly the chromophores oriented in the layer plane undergo excitation, which in turn gives higher contribution for the TE mode. In order to verify this hypothesis, we checked the polarization of output laser light by a polarizer. It turned out that the two peaks of DFB laser light have the same TE polarization. Therefore, the concept of two orthogonally polarized lasing modes has to be ruled out. G

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Figure 5. Explanation of unequal intensities of two lasing modes. (a) Scheme of two branches of waves propagating within 1-D photonic crystal. The phase difference of π between two waves makes difference both in their frequencies and in received gain. (b) Overlap integral of phase-shifted waves with gain modulation in function of propagation distance L. (c) The position and relative intensities of two lasing modes in Lem-PMMA excited with periodically modulated light intensity.

lower one. Thus, we can finally infer that in bright places of the interference pattern the refractive index of the gain medium is higher. The above-mentioned explanation is schematically illustrated in Figure 5. The lasing modes separation during holographic-type excitation was observed by Tsutsumi and Fujihara27 in dye-doped polymeric layer. However, presumably due to the lower interference pattern contrast supported by the used Lloyd mirror (the highest possible interference pattern contrast is achievable only for the intersection angle value of 90°), the peaks intensities were not as different as what we observed. The lines of DFB lasing presented in Figures S5 and S6 were estimated to have FWHM = 0.31−0.45 nm for Lem-PMMA, while a slightly broader with FWHM = 0.44−0.50 nm was obtained for Ant-PMMA. This small spectral broadening of laser line for Ant-PMMA, despite the few times higher energy density used for excitation and the shorter wavelength of emission, may be connected with the presence of two closely located modes, which are indistinguishable with the spectral resolution used in this study. The above-mentioned observations performed with higher resolution spectrometer are therefore consistent with the earlier stated hypothesis.

molecular weight (here of about 30 kDa). The resulting material can be processed with utilization of common and straightforward solution based techniques. In the present case, we employed a drop casting technique to form layers of good optical quality and no sign of chromophore aggregation or phase segregation. Both compounds showed efficient ASE and RL in light confinement conditions. However, they exhibited significant differences in DFB lasing threshold values upon holographic-type excitation. In both cases fully reversible and tunable DFB lasing is possible, but different coupling mechanisms are responsible for feedback constitution. In the case of Lem-PMMA system, the DFB mechanism was identified to be dominated by coupling with refractive index grating, while for Ant-PMMA system only gain modulation coupling was possible. Lem-PMMA compound provided additional modulation of refractive index via orientation of the chromophores, which explained the more efficient feedback and lasing performance compared to that of Ant-PMMA. We would like to specifically draw the reader’s attention to the fact that LemPMMA single compound supports optical gain, introduces index-coupled DFB resonator, but also plays the role of matrix for covalently attached chromophores. These features make the Lemke’s chromophore derivative-based luminescent polymer outstanding among existing laser dye systems and constitutes, to the best of our knowledge, an unprecedented upgrade and an interesting alternative to existing strategies. Moreover, the simple fabrication and processing methods used for the presented type of compounds predisposes them to be utilized as thin layer optically pumped compact lasers, for instance in fully integrated systems. Finally, the proposed approach for the

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CONCLUSIONS To conclude, we have shown the synthesis route toward new polymeric laser materials based on ethynylanthracene and Lemke’s chromophore derivative luminescent groups covalently attached to PMMA chains, forming an insulating shell around the chromophore backbone. Macromolecules were synthesized via ATRP allowing to get the compounds with controlled H

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synthesis opens new possibilities in functionalized polymers design for photonic purposes. More detailed characterization of the physics underlying the peculiar response of that class of materials to excitation by spatially modulated laser light were beyond the scope of this article; however, we plan to continue such investigations in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00878. Synthesis details table, 1H NMR spectra, GPC chromatograms, fluorescence decay graphs, RL spectra, DFB lasing spectra, DFB lasing parameters table (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kacper.parafi[email protected] (K.P.). ORCID

Kacper Parafiniuk: 0000-0002-8545-6415 Cyrille Monnereau: 0000-0002-8928-2416 Andrzej Miniewicz: 0000-0003-2470-6246 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Centre, Poland (2016/21/B/ST8/00468), statutory fund of Faculty of Chemistry at Wroclaw University of Science and Technology, École Normale Supérieure de Lyon, Centre National de la Recherche Scientifique, and Université Lyon 1. Authors thank Dr. Dominika Wawrzyńczyk for the luminescence lifetime TCSPC measurements as well as Dr. Julien Massin and Dr. Yann Bretonniére for their inspiring initial investigations on Lem derivatives.

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REFERENCES

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