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Reversible Laser Threshold Modulation in Dithienylethene Conjugated Polymer Blends - A Concept for q-switching in Organic DFB Lasers Ann-Kathrin Steppert, Annabel Mikosch, Tamás Haraszti, Robert Göstl, and Alexander J.C. Kuehne ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01641 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019
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ACS Photonics
Reversible Laser Threshold Modulation in Dithienylethene Conjugated Polymer Blends - A Concept for q-switching in Organic DFB Lasers Ann-Kathrin Steppert,† Annabel Mikosch,† Tamás Haraszti,† Robert Göstl,† and Alexander J. C. Kuehne*,†,‡ †DWI
– Leibniz Institute for Interactive Materials, Forckenbeckstraße 50, 52074 Aachen, Germany. of Organic and Macromolecular Chemistry, Ulm University, Albert-Einstein-Allee 11 , 89081 Ulm, Germany.
‡Institute
Keywords: 1D and 2D diffractive feedback gratings, organic lasers, nanolithography, photochromism. ABSTRACT: Tuning the resonator quality is a long established technique for inorganic lasers giving access to extremely short or temporally precise laser pulses. However, this so-called q-switching, where the resonator can be “on” and sustain lasing or be switched “off”, and inhibit lasing is widely unknown for conjugated polymer lasers. Here, we admix thermally stable photo-chromic dithienylethenes with conjugated polymer laser gain materials on 1D and 2D diffractive feedback gratings, which are generated by two-photon laser writing. By irradiation with differently colored light, the thermally bistable dithienylethenes reversibly modulate the refractive index of, and exhibit competing absorption with the gain medium, effectively suppressing laser emission in the one state and allowing low threshold (2.15 mJ/cm2) laser emission in the other state.
Optically pumped conjugated polymer lasers are compelling devices with many advantages compared to their inorganic counterparts: Conjugated polymer laser gain materials possess excellent biocompatibility, their synthesis does not require high temperatures or vacuum techniques, and their emission is tunable across the entire visible range simply by varying the composition of the conjugated polymer.1–4 Diffractive feedback (DFB) gratings are often applied as resonator structures offering an open thin film resonator structure and facile fabrication by spin-coating the conjugated polymer onto the nanofabricated optical grating.5 This geometry allows for single mode, low threshold lasing at resonance conditions described by the Bragg equation (1): 2𝛬
𝑚𝜆laser = 𝑛
eff
inorganic lasers, this effect can be used to produce extremely short, high intensity, or precisely delayed pulses. By contrast, q-switching in organic and polymer lasers is currently being explored for tuning of the laser wavelength or switching the laser on and off.11,12 Tuning of the resonance condition can be achieved by mechanically stretching a silicone elastomer DFB resonator, which has been coated with a conjugated polymer gain medium14 or by changing the refractive index for example by voltage actuating a liquid crystal resonator material.15,16 By contrast, light as a physicochemical stimulus delivers superior spatial, temporal, and energetic resolution rendering photo-addressable molecular switches ideal candidates for controlling the material properties.17,18 This considerable advantage has been exploited with the incorporation of a spiropyran19,20 into an organic gain medium composed of a dielectric poly(methyl methacrylate) matrix doped with a rhodamine laser dye.21 Switching the spiropyran to its merocyanine form by irradiation with light alters the refractive index of the gain medium and therefore the resonance condition. With this method, the lasing wavelength can be shifted by as much as 14 nm, a result of the change in neff as well as competing absorption of the merocyanine form with the emission spectrum of the rhodamine laser dye. However, with every switching cycle the threshold increases, which has been attributed to fatigue of the photochromic moiety. Furthermore, spiropyran is a thermally reversible (Ttype) switch, limiting precise control over the device performance. A photoswitchable gain material where both isomers are thermally stable (P-type) would overcome these problems and enable a new type of organic laser device. In such a device the laser threshold, emission wavelength and refractive
(1)
where m is the diffraction order of the DFB grating, λlaser is the laser wavelength, Λ is the period of the grating, and neff is the effective refractive index. Therefore, we have two parameters (Λ and neff) to control the resonance condition, while lasing is only observed when λlaser lies within the gain spectrum of the laser medium. If λlaser is shifted out of the gain spectrum, light amplification cannot occur and no laser emission is observed. Another means to modulate the resonator and emission properties of such lasers is by controlling the absorption (i.e. transmission losses) around λlaser to change the quality of the resonator. In inorganic lasers, a common approach is to modulate the refractive index distribution within the cavity using ultrasound irradiation (acousto-optical effect)6–8 or applied voltages (electro-optical-, e.g. Pockels cells),9,10 effectively modulating the laser threshold. The principle of switching the quality of a resonator is termed q-switching. In
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absorption of the conjugated polymer and the ring-open DTE should not overlap as this could impair the switching efficiency. Functionalization of the DTE with electron withdrawing groups moderately shifts the absorption spectra of the ring-open and closed isomers bathochromically and additionally renders it more fatigue resistant.30 We therefore synthesize diphenyl-DTE (Ph-DTE) with maximum absorption λmax,o = 285 nm in the ring-open form and λmax,c = 585 nm for the ring-closed from, and di(nitrophenyl)-DTE (NO2-DTE) with λmax,o = 350 nm and λmax,c = 620 nm, respectively (see black lines in Figure 2).
index could be altered with temporal precision overcoming the problem of thermal back-switching. n-C8H17
n-C8H17
n-C8H17 n-C8H17 n
n PFO F
F8DVB
F F
F
F F
F F F
285 nm S
S
585 nm
S Ph-DTEo
F
S Ph-DTEc
F F
F O 2N
F
F
F
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F
F F 350 nm
S S NO2-DTEo
O 2N
F
F F F
S S
620 nm NO2
F
NO2-DTEc NO2
Figure 1. Chemical structure of conjugated polymers PFO and F8DVB, which are explored as gain media. Structure of ring-open (o) and ring-closed (c) forms of Ph-DTE and NO2-DTE with corresponding wavelengths for isomerization.
Dithienylethenes (DTEs) prominently belong to the class of P-type switchable molecules and can undergo a reversible 6πelectrocyclization induced only by illumination with light of different wavelengths, but not by heat (see Figure 1).22 Precise functionalization of the DTE π-system allows tuning of the absorption spectrum of their ring-closed form. This feature has been used to regulate the photoluminescence of nanoparticles by Förster resonance energy transfer (FRET).23–29 Furthermore, DTEs are generally recognized as more fatigue resistant than other molecular switches,30 potentially granting reproducible switching performance and stable laser thresholds for organic laser devices. However, such DTE-based conjugated polymer laser devices remain unexplored to date. Here, we report two different DTE photoswitches admixed to high gain conjugated polymers on 1-dimensional and 2-dimensional DFB grating, which are specifically adjusted to the refractive index and lasing wavelengths of the composites. For the best combination of DTE and conjugated polymer gain medium we find modulation of the refractive index upon photoswitching of the DTE and control of the laser threshold with good stability over several switching cycles. We find laser thresholds as low as 2.15 mJ/cm2 on 2D gratings, which represents the lowest threshold in photoswitchable polymer laser devices to date.
Figure 2. Spectral properties of DTE/conjugated polymer combinations.
the
respective
Normalized (to maximum) absorption spectra of PFO/Ph-DTE b) F8DVB/Ph-DTE c) PFO/NO2-DTE d) F8DVB/NO2-DTE. Absorption spectra of the ring-opened DTEs are represented as black solid lines and ringclosed as black dashed lines. Absorption spectra of the conjugated polymers are presented as blue solid lines and the photoluminescence as blue dashed lines. The DTEs are measured in acetonitrile solution and the conjugated polymers as thin films.
PFO exhibits a relatively small stokes-shift and the absorption and emission spectra overlap only little with the PhDTE, while F8DVB exhibits some overlap with ring-opened Ph-DTE (see blue lines in Figure 2). The ring-open NO2-DTE shows substantial overlap with the absorption of both conjugated polymers and less overlap with their emission profiles in the ring-closed form than Ph-DTE (see Figure 2). Both DTEs are synthesized and received as 100% ring-open isomers (see experimental section). Ph-DTE is literature-known and reaches a composition at the photostationary state (PSS) of 97% ring-closed form when irradiated at 312 nm.31 The PSS of NO2-DTE has not yet been reported. We determine the PSS for this new compound and find 89% ring-closed form when irradiated with 365 nm using 1H-NMR analysis. This value is confirmed exactly by ultra-high performance liquid chromatography (UPLC, see experimental section). To further investigate the switching performance of these tailored chromophore/conjugated polymer combinations, we blend 20 wt% of the as-prepared ring-open DTEs31 with respect to conjugated polymer (overall 5 wt%) in toluene solution and spin-coat thin films on silicon substrates yielding a layer thickness of ~230 nm for PFO/DTE and ~190 nm for F8DVB/DTE. Assuming exciton diffusion lengths in the
RESULTS AND DISCUSSION DTEs absorb light in the near UV spectrum for the ring-open form and in the yellow part of the visible spectrum for the ringclosed form. To avoid spectral overlap between the DTE-switch and the laser gain medium, we opt for high gain conjugated polymers (PFO and F8DVB) with emission in the blue part of the visible spectrum (see Figure 1 and Figure 2). With the conjugated polymer emission placed between the absorption maxima of the DTE isomers, self-switching of the system by the conjugated polymer emission can be avoided. Also, the
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ACS Photonics conjugated polymers of about 5-10 nm32 and that the DTE molecules are well distributed in the polymer matrix, 20 wt% of DTE should allow for efficient electronic interaction. Subsequently, we perform variable angle spectral ellipsometry (VASE) on these films to determine their refractive index n before (ring-open) and after irradiation with UV-light of 312 nm (Ph-DTE) or 365 nm (NO2-DTE). In the blend films we achieve between 85 and 97% ring-closed DTE at the PSS. The VASE results are dominated by n profiles characteristic for the respective conjugated polymer.33,34 For all produced films we observe small deviations in n before and after irradiation, which can be attributed to the different isomers of the admixed DTEs. As we know from UV-vis analysis, the ring-closed isomers absorb in the yellow part of the spectrum and due to the complex correlation between the refractive index and absorption, n deviates in the same spectral ranges (see Figure 3).
F8DVB exhibits a much lower PLQY (also as pure material Φ = 3.8%). This underperformance could originate from the low molecular weight of F8DVB (with an average degree of polymerization of 10), which is the result of around 5% contamination with ethylvinylbenzene, which acts as a polymer end-capper and cannot be separated from the DVB monomer. This reduced molecular weight leads to increased crystallization and therefore quenching in thin solid films.
Figure 4. Emission spectra of DTE/conjugated polymer thin films before irradiation (black), after irradiation with UV-light (blue) and after irradiation with a red LED source (red). Below the reproducibility of the switching process is monitored by recording the absorption band at 585 nm for Ph-DTE and 620 nm for NO2DTE over 10 cycles. Uneven numbers correspond to UVirradiation, even numbers to red-light irradiation for films of: a) PFO/Ph-DTE b) F8DVB/Ph-DTE c) PFO/NO2-DTE d) F8DVB/NO2-DTE.
Figure 3. Spectral dependency of the refractive index n before (black solid) and after irradiation (black dashed) with UV-light (312 nm for Ph-DTE; 365 nm for NO2-DTE) for the combinations of DTE and conjugated polymer: a) PFO/Ph-DTE b) F8DVB/PhDTE c) PFO/NO2-DTE d) F8DVB/NO2-DTE. Even these seemingly small changes in refractive index and absorption will have a great effect on the supported Bragg wavelength in DFB gratings and the resonator quality (cf. equation 1). We move on to investigate the emission behavior of our conjugated polymers with 20 wt% of incorporated DTE before and after irradiation with UV light. To observe whether the switching process is reversible and whether the fluorescence can be modulated effectively and repeatedly inside the conjugated polymer matrix, we irradiate with films with UV to ring-close the DTEs and then expose the films to a red LED light source (λmax = 600 nm) to isomerize the DTEs back into their ring-open forms. For ease of fabrication and to avoid virgin cycle behavior we admix the DTEs (20 wt%) in their ring-open form to the conjugated polymer. Blend films with ring-open DTEs show fluorescence characteristic of the applied conjugated polymer (see Figure 4) with a photoluminescence quantum yield (PLQY) of Φ = 14.3% for PFO and Φ = 3.4% for F8DVB. This is a reduction by about half compared to pristine PFO, with Φ = 26.8%. This reduction could be the result of the dilution of PFO with the non-fluorescent DTE. By contrast,
The PFO-based films exhibit fluorescence typical for the βphase with well resolved vibrational transitions (Figure 4a and c).35 The β-phase morphology is induced by processing from toluene and is desirable as it exhibits higher gain than the amorphous PFO modification.36,37 F8DVB is not known for other modifications.34,38 Ph-DTE allows almost complete fluorescence quenching in conjugated polymer films (Φ ≈ 0% for both), when switched to the ring-closed form (see Figure 4a and b). When inspecting the absorption spectrum of ring-closed Ph-DTE, a small overlap with the emission profiles of the conjugated polymers is apparent (see Figure 2a and b). This is sufficient to almost fully quench fluorescence in Ph-DTE/PFO blends and fully quench in Ph-DTE/F8DVB blends via an energy transfer mechanism (see Figure 4a and b). By contrast, the NO2-DTE is not able to quench the conjugated polymer fluorescence fully after switching into the closed form (neither in PFO nor in F8DVB). We attribute this behavior to the fact, that the absorption wavelength of the ring-closed NO2-DTE is shifted bathochromically compared to Ph-DTE, leading to less
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Irradiation with yellow light inducing back-switching to the ring-open Ph-DTE, which allows to recover low threshold lasing.
spectral overlap with the conjugated polymers (see Figure 2c and d). Furthermore, the PSS of NO2-DTE is smaller than for Ph-DTE for switching into the ring-closed state, which could also add to less effective quenching in the NO2-DTE/conjugated polymer blends. While in Ph-DTE switching is largely independent of the conjugated polymer fluorescence, in NO2DTE switching is in competition with the excitation of fluorescence and potential energy transfer phenomena from DTE to the conjugated polymer impeding efficient switching. Also overlap of the conjugated polymer fluorescence with the ring-closed form might lead to back-switching to the ring-open form via radiative or non-radiative energy transfer. This backreaction would also reduce the switching efficiency of NO2DTE (in the forward direction from open to closed). Therefore longer irradiation periods are required to reach the PSS for NO2DTE loaded films, which might also lead to aging of the films by photo-degradation. For conjugated polymer blends with NO2-DTE we indeed observe decreasing emission intensity with every completed switching cycle (especially in Figure 4d). The fluorescence intensity can be switched reproducibly with both DTEs but only fully on and off for the Ph-DTEs, as we demonstrate over 10 switching cycles (see Figure 4a to d). Due to the superior performance of Ph-DTE over NO2-DTE, we continue with the former to produce laser devices. With the determined refractive indices of blend films with Ph-DTE in the open- and closed form and knowledge about the optimum gain and ideal laser wavelengths of the conjugated polymers38,39 we can now determine ideal Λ for 1D and 2D DFB gratings to apply our blend systems as photo-switchable q-modulated lasers. To produce these DFB gratings, we use 3D nanolithography based on two-photon polymerization with a commercially available photoresist (IP-L from nanoscribe). We write gratings (1D: parallel lines, 2D: two line gratings orthogonal in plane) with Λ = 250 nm for PFO and Λ = 314 nm for F8DVB at heights of ~430 nm (see Figure 5a). The resulting grating lines are triangular in their cross-section and allow for spin-coating of the DTE/conjugated polymer blends on top to produce the laser geometries (see Figure 5b). We spin-coat toluene solutions with 20 wt% of Ph-DTE with respect to the conjugated polymers. This produces smooth layers of DTE loaded conjugated polymers with a thickness of ca. 470 nm (Figure 5b). We optically pump the DTE/conjugated polymer blends on DFB gratings by irradiating into the conjugated polymer absorption (PFO: λex= 410 nm; F8DVB: λex= 420 nm) with 10 ns pulses at 20 Hz. Laser emission from 2D gratings can be achieved for both polymers blended with Ph-DTE in the open form. While the Ph-DTE/PFO system displays negligible background fluorescence and a sharp laser line dominates the spectrum, in the Ph-DTE/F8DVB laser spectrum we observe some residual fluorescence. We account this background fluorescence to less efficient lasing conditions due to lower PLQY of F8DVB. In the open Ph-DTE form the laser thresholds are at 9.4 mJ/cm2 for PFO and 2.15 mJ/cm2 for F8DVB (see Figure 5e and f). The low threshold for laser emission in F8DVB is surprising considering the lower PLQY; however, the emission intensity is much lower than in PFO. When switching the Ph-DTE to the ring-closed form, the laser threshold increases to 14.4 mJ/cm2 for the PFO-based material, while the threshold is beyond the upper detection limit in PhDTE/F8DVB blends (see also Figure 5g and h).
Figure 5. Switchable laser devices: a) Atomic force microscopy image of 2D gratings. b) Scanning electron microscopy image of the cross-section of a lasing device with the polymer film on top of the 1D grating (scale bars represent 1 µm). The triangular shape of the grating is indicated by the faded (red-to-gray) overlay. c,d) Emission spectra of the switchable laser systems on 2D DFB gratings with (c) Ph-DTE/PFO and (d) Ph-DTE/F8DVB. Determination of the laser threshold after repeated open (“on”) and closed (“off”) switching of the corresponding systems in (c and d). ). e,f) Thresholds of the laser devices in the open (open circles) and closed form (full circles). (e) shows the thresholds for a PhDTE/PFO film on a 2D grating; (f) shows the threshold behavior for a Ph-DTE/F8DVB film on a 2D feedback grating. g,h) Optical laser switching behavior of the films: Ph-DTE open – laser on, PhDTE closed – laser off.. g) shows reversible optical switching of the laser threshold of a Ph-DTE/PFO film on a 1D grating. The threshold of the open DTE form increases over the 15 switching events, probably due to fatigue or photooxidation. h) Optical laser switching of a Ph-DTE/F8DVB film on a 1 D grating. The thresholds of the closed form DTE are too high to be determined with our laser setup. (For full color datapoints in (g) a full threshold dataset was recorded. For the faint colored datapoints, the threshold was approached from the low energies until lasing occurred and this pulse power was taken as the threshold. This was done to reduce exposure to the pump laser as all experiments have been conducted in air.)
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ACS Photonics We thankfully acknowledge funding from the DFG (Grant No. KU 2738/3-1). This work was performed in part at the Center for Chemical Polymer Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02).
We perform these switching cycles five times for the PhDTE/F8DVB and 15 times for the Ph-DTE/PFO blend to demonstrate the repeatability and reproducibility of the switching process and the fatigue resistance of Ph-DTE conjugated polymer blends (see Figure 5g and h). During these 15 cycles we observe an increase of the laser threshold for the open DTE form, while the threshold of the ring-closed composite remains constant. We account this slight increase of 2.5 µJ/pulse over 15 cycles to photo-oxidation of PFO as the experiments are conducted at ambient temperatures and atmosphere (photo-oxidation study in the Supplementary Information). This leads to a window of operation between 7 and 10 µJ/pulse for the Ph-DTE/PFO system, which can be driven at around 8 µJ/pulse to be able to stay below the threshold for the closed DTE state but above the threshold of the open state to allow for photoswitching of the laser. The Ph-DTE system is more controllable with lower lasing thresholds for the “on”state and thresholds for the “off”-state that were so high that we were unable to determine them. This concept can be reproduced in 1D gratings; however, laser thresholds are much higher in Ph-DTE/PFO blends (27.5 mJ/cm2for the ring-open and 38.1 mJ/cm2 for the ring-closed forms), while no laser emission could be observed in Ph-DTE/F8DVB blends. This can be explained by improved confinement and therefore more efficient resonance in 2D gratings and has previously been observed and characterized in pure conjugated polymer systems.40,41
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CONCLUSION In conclusion, we have demonstrated that blending photoswitchable DTEs into suitable conjugated polymer matrices affords low threshold DFB lasers whose resonator quality can be switched reliably and repeatedly by irradiation with differently colored light. We demonstrate quality switching using Ph-DTE in two fluorene-based gain media – PFO and F8DVB. This new strategy opens up a pathway for stable refractive index q-switching and the possibility to explore the platform of DTE molecules with further spectrally guiding electron-donating or withdrawing moieties in combination with other laser emitting gain media and laser resonator geometries.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section and photo-oxidation study on the pure conjugated polymers (PDF)
AUTHOR INFORMATION Corresponding Author * Email:
[email protected] ACKNOWLEDGMENTS We thank Nadine Jansen, Michaela Schicho, Jil-Lorean Gieser, Lara Riemann, and William Atkinson for support with the synthesis, characterization and preparative work with DTE-units.
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ACS Photonics
Insert Table of Contents artwork here Reversible Laser Threshold Modulation in Dithienylethene Conjugated Polymer Blends - A Concept for qswitching in Organic DFB Lasers Ann-Kathrin Steppert,† Annabel Mikosch,† Tamás Haraszti,† Robert Göstl,† and Alexander J. C. Kuehne*,†,‡ By blending a light-switchable, thermally stable Dithienylethene (DTE) into a conjugated polymer film, it is possible to reversibly change the quality of a distributed feedback laser (q-switching) and turn the system from “lasing” to “non-lasing”.
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