nanosoftpolymers/MPEGHZ/251720001g/1g

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. AbstractOrganic semiconductors enable the fabrication of a range of lightweight and mechanically flexible optoelectronic devices. Most organic semiconductor lasers, however, have remained rigid until now, predominantly due to the need for a support substrate. Here, we use a simple fabrication process to make membrane-based, substrate-less and extremely thin ( 500鈥塶m) organic distributed feedback lasers that offer ultralow-weight (m/A 0.5鈥塯m鈭?) and excellent mechanical flexibility. We show operation of the lasers as free-standing membranes and transfer聽them onto other substrates, e.g. a banknote, where the unique lasing spectrum is readily read out and used as security feature. The pump thresholds and emission intensity of our membrane lasers are well within the permissible exposures for ocular safety and we demonstrate integration on contact lenses as wearable security tags. IntroductionOptically pumped organic solid-state lasers have gained widespread attention as coherent light sources that are easy to fabricate, have emission tunable across the whole visible range, and are potentially disposable and biocompatible1,2,3,4,5,6,7,8. These lasers hold great promise for a number of applications, e.g. for on-chip spectroscopy9,10, data-communication11, biosensing12, and chemosensing for detecting explosives13,14. However, while organic LEDs, solar cells, and field-effect transistors are now routinely made in bendable or even stretchable formats and with extremely low specific weights15,16,17,18,19, most organic lasers have remained rigid and relatively bulky, largely due to a need for macroscopic and solid support substrates (typical substrate thickness, 100鈥壜祄). Many organic lasers use distributed feedback (DFB) resonators, which provide strong in-plane optical feedback20. There have also been examples of flexible DFB designs21,22,23,24,25. However, these use substrates or matrices of macroscopic thickness, or require metal oxide intermediate layers and femtosecond pumping schemes. These requirements have limited applications of flexible DFB lasers so far.Here, we introduce a different organic laser that maximizes mechanical flexibility and reduces the thickness of the laser to its ultimate limit, by using an architecture that comprises only the organic semiconductor and a DFB resonator and that is fabricated by a wholly solution-based process. The resulting 200-nm thick membrane lasers were operated freestanding in air or readily transferred onto a new substrate, on which direct fabrication of a laser may otherwise be impossible or impractical. As an example, we show how membrane lasers that were designed to produce a well-defined and unique lasing spectrum can be used as counterfeit-resilient, barcode-type security labels on bank notes. In another example, a laser beam was emitted from a bovine eye onto which a contact lens with a membrane laser had been mounted. Due to the low threshold of our membrane laser, a similar configuration is expected to be safe to use in the human eye, e.g. to complement biometric iris recognition.ResultsMembrane laser design and fabricationTo produce transferable and thin membrane lasers, we developed a water-based lift-off technique that releases the final device from a carrier substrate at the end of the fabrication process (Fig.聽1a). The laser was produced via solution-based deposition and UV nanoimprint lithography. The stack initially consisted of a thick and rigid carrier glass substrate, a ~50-nm thick water soluble sacrificial layer of (poly(3,4-ethylenedioxythiophene)-polystyrene-sulfonate (PEDOT:PSS), a UV curable imprint resist defining the DFB resonator and a (180鈥壜扁€?0)-nm thick layer of an organic semiconducting polymer (e.g. F80.9BT0.1, Methods) as gain material (Fig.聽1b). When immersing this stack in water, a hydrophobic membrane detached from the substrate and spread out on the water surface (Fig.聽1c, d). The membrane was readily picked up, and then either suspended in air or transferred onto another substrate (Fig.聽1e, f). The lift-off procedure had no detrimental effect on the photoluminescence quantum yield (PLQY) of the organic polymer (Supplementary Table聽1). We applied the above technique to fabricate membrane lasers with different periodicities and gain materials based on one-dimensional second-order and mixed-order DFB gratings26 and obtained groove depths of (106鈥壜扁€?)鈥塶m and (86鈥壜扁€?)鈥塶m, respectively (Supplementary Fig.聽1).Fig. 1Fabrication and physical properties of the membrane laser. a Schematic of the laser stack immersed in water. b Composition of the laser stack before lift-off (not to scale) consisting of a glass substrate, the sacrificial layer (PEDOT), the polymer grating (UVCur), and the polymer gain material (F8BT). c Schematic of a floating membrane post lift-off. d Image of a floating membrane post lift-off. Black arrows indicating the position of three second-order distributed feedback (DFB) gratings. Scale bar, 3鈥塵m. e Schematic of the vertical laser emission from a second-order DFB laser membrane (pump spot not shown). f Image of a free-standing membrane laser suspended over a hole in a glass substrate. Scale bar, 5鈥塵m. g Mode profile of the TE0 mode intensity ((left| {E_y} right|^2)) in a free-standing membrane laser (black solid line) and a conventional laser on a glass substrate (blue dashed line). Profilometer measurement of the membrane before lift-off (green). The yellow area indicates the gain layer, the light gray area the residual grating layer and the patterned dark gray area the sacrificial layer. 螕 quantifies the overlap of the TE0 mode with the gain materialFull size imageRemoving the substrate not only rendered the laser flexible and lightweight; replacing the glass substrate (refractive index, n鈥?鈥?.52) by air also improved the confinement of the lasing mode to the gain material (n鈥?鈥?.7) and reduced spontaneous emission losses by eliminating leaky substrate modes. The mode overlap with the gain material improved from 螕鈥?鈥?.36 for a typical laser stack on a glass substrate to 螕鈥?鈥?.59 for our membrane design (Fig.聽1g).Membrane laser emissionDevices based on mixed-order gratings and F80.9BT0.1 as the gain material showed laser emission under pulsed excitation (excitation wavelength, 位鈥?鈥?50鈥塶m; pulse duration, 5鈥塶s) with a threshold pump fluence of 3.3鈥塳W鈥塩m鈭? (Fig.聽2a and Supplementary Fig.聽2), in line with state-of-the-art organic DFB lasers27,28. Figure聽2b shows emission spectra of the same membrane laser and Fig.聽2c summarizes the evolution of spectral line width with pump fluence. At low pump fluences the spectrum was dominated by the fluorescence background (螖位鈥夆増鈥?0聽nm) with a broad Bragg mode (螖位鈥夆増鈥?鈥?鈥塶m) at a wavelength of 位鈥夆増鈥?40鈥塶m. Above threshold, a single lasing mode (螖位鈥夆増鈥?.2鈥?.5鈥塶m) appeared and gained superlinearly in relative intensity, eventually completely dominating the emission spectrum. Well above threshold, our second- and mixed-order membrane lasers showed emission linewidths of (133鈥壜扁€?4) pm and (498鈥壜扁€?0) pm, respectively (full width at half maximum, Supplementary Fig.聽3). We also investigated the near and far field emission of mixed- and second-order DFB grating membrane lasers (Fig.聽2d). The near field emission data for the mixed-order grating indicate that most laser light was coupled out from the narrow second-order region in the center of the grating structure26. Due to the narrow width of this region, the emitted laser beam was rather divergent (spread of far field emission at half maximum, 卤(2.60鈥壜扁€?.04)掳). However, if necessary, the divergence can be reduced by adjusting the number of light-extracting second-order periods inserted between the first-order period feedback structure29. For the pure second-order DFB membrane gratings, the light extraction was enhanced and spread over a larger area as can be seen in the near field emission pattern. This led to higher lasing thresholds (typically, 60鈥塳W鈥塩m鈭?), but to reduced beam divergence (spread, 卤(0.34鈥壜扁€?.04)掳) and a sharp far field emission pattern with a fine double-lobe structure30. Well-defined far field emission and low divergence are clear indications of spatial coherence and further evidence for laser action in our membranes2.Fig. 2Characterization of membrane lasers. a Input鈭抩utput characteristics for mixed-order membrane lasers with different gain materials. The lasing thresholds for F80.9BT0.1 (red), F8BT (blue) and Super Yellow (yellow)-based devices are 3.3鈥塳W鈥塩m鈭?, 13.8鈥塳W鈥塩m鈭? and 75.6鈥塳W鈥塩m鈭?, respectively. b Emission spectra of the F80.9BT0.1-based membrane laser for pump fluences below (2.3鈥塳W鈥塩m鈭?), around (5.0鈥塳W鈥塩m鈭?) and well above (33鈥塳W鈥塩m鈭?) the lasing threshold. c Spectral linewidth (full width at half maximum, FWHM) vs. input-power density for the devices based on F80.9BT0.1 (red), F8BT (blue) and Super Yellow (yellow). d Near and far field emission from a mixed- and a second-order distributed feedback (DFB) membrane laser. The location of regions with first- and second-order grating period is indicated on the right-hand side of the near field emissionFull size imageThe process for laser membrane fabrication is compatible with a range of conjugated polymers of different chemical structure and molecular weight. So far, we tested F80.9BT0.1, F8BT and Super Yellow (SY) and found F80.9BT0.1 to provide the best laser performance (i.e., lowest threshold) among these, in line with earlier findings on rigid substrates and with the PLQY of these materials31,32 (Supplementary Table聽1).Membrane lasers as security features on banknotesTwo important features of our membrane lasers are their transferability and mechanical flexibility. After lift-off, the membranes can be transferred onto a wide range of substrates, independent of the substrate composition and surface topology. After the water used for the lift-off has evaporated, the membranes stick tightly to the new surface. This enables the use of membrane lasers as novel barcode-like security labels for objects requiring authenticity control (e.g. banknotes, ID documents, etc.). The emission spectrum of membrane lasers can be tuned by the grating period, the choice of gain material and the waveguide design. The distinct single-mode emission can either be used as an identification feature on its own or be further enhanced by combining a number of different gratings on a single membrane. This creates a well-defined and discrete lasing spectrum that resembles a binary barcode, which is unique to each membrane laser and which can be read out rapidly (ns pumping) and without physical contact (Fig.聽3a). If the laser lines generated by different gratings are spaced by 1鈥塶m (which appears feasible, see long-term measurement below) and cover the 50鈥塶m wide gain spectrum of the organic polymer, at least 50 independent spectral channels can be encoded. This translates to about 250鈥夆増鈥?015 unique labels.Fig. 3Membrane lasers as security features on banknotes. a Barcode-like narrow-band lasing emission from a combination of different second-order gratings with periods ranging from 340 to 360鈥塶m. b Photograph of a 拢5 banknote with a series of membrane lasers transferred on the transparent window of the banknote. Two second-order distributed feedback (DFB) gratings are indicated by black arrows. Scale bar, 5鈥塵m. c Photograph of same banknote when bent parallel to the grating grooves. d Input鈭抩utput characteristics of a mixed-order DFB grating membrane laser on a banknote. e Lasing spectra of a banknote with a second-order membrane laser after repeated bending (radius of curvature, ~ 8鈥塵m). Black dashes mark the measured peak wavelength after each bending cycle. f Lasing spectra acquired from a banknote containing a membrane with three different gratings over a time span of 200 days. For clarity, the spectra are offset verticallyFull size imageIn the following, we illustrate the application of this concept as a security feature on banknotes. Figure聽3b, c shows a polymer banknote with a membrane laser transferred onto the transparent window of the banknote. One of the reasons an increasing number of countries exchange cotton banknotes for polymer notes is their improved counterfeit resilience33. Individual DFB lasers can be identified by their reflection from a white light source and the flexible nature of the membrane laser allowed repeated flexing and bending without delamination or damage. Upon pulsed excitation of the section of the banknote containing the membrane, laser action was readily observed (Fig.聽3d). The lasing threshold was 38.2鈥塳W鈥塩m鈭?, larger than for the free-standing membrane. We attribute this increased threshold to a change in waveguiding properties, with the polymer banknote acting as a substrate that reduced mode confinement and introduced leaky substrate modes.The flexible nature of our membrane lasers also allowed reversible tuning of the emission wavelength by gradual bending parallel to the grating grooves (Supplementary Fig.聽4). When the membrane was straightened out again, the original laser wavelength was accurately restored (standard deviation of laser wavelength over 20 bending cycles with ~8鈥塵m radius of curvature, 蟽(位max)鈥?鈥?0.7鈥塸m; Fig.聽3e).To study the stability of our devices further, we repeatedly recorded the lasing spectrum of a banknote containing a membrane with three different gratings over the course of several months (Fig.聽3f, Supplementary Fig.聽5). During this test, the banknote was stored under ambient conditions and no encapsulation was employed. Compared to the free-standing membrane lasers (Fig.聽2c, Supplementary Fig.聽3), the maximum spectral linewidth of the laser emission from the banknote increased to 螖位max鈥?鈥?.2鈥塶m (FWHM, Supplementary Fig.聽5a). However, due to the large signal-to-background ratio of the lasing spectra, the spectral position of the lasing peak can be localized to a much higher precision using peak fitting. Using Gaussian fits to the lasing spectra, we find that the standard deviation of the lasing wavelengths varies by 蟽(位max)鈥?lt;鈥?5鈥塸m over the entire test period (Supplementary Fig.聽5b). Hence, we conclude that the desired 1鈥塶m precision needed to obtain 1015 unique labels is readily achievable.Membrane lasers as wearable security tagThe high optical transparency of the membrane lasers, combined with their low thresholds and ultrathin design, also inspired us to explore their use as a wearable security tag on contact lenses where they may complement a biometric authentication via an iris scan. Post lift-off, we transferred membrane lasers onto commercially available contact lenses (Fig.聽4a) and mounted these on an explanted bovine eyeball (Fig.聽4b, c). The bovine eye is an excellent and widely used model for the human eye due to its similar structure, slightly larger size, and general availability34. Upon optical excitation with pulsed blue light, we observed a well-defined green laser beam emerging from the eye (Fig.聽4d). The modest divergence of the beam in the far field pattern is consistent with expectations for the second-order DFB laser used for this experiment. Mounting the membrane laser on the eye did not impede narrow linewidth, single-mode operation (peak wavelength, 位鈥?鈥?43.4鈥塶m; Fig.聽4e). Lasing action was again also confirmed by the superlinear relation between pump fluence and output power (Fig.聽4f). The threshold pump fluence to achieve lasing was 56.8鈥塳W鈥塩m鈭?, i.e. higher than the free-standing membrane laser, which we again attribute to the contact lens acting as a substrate that weakens mode confinement. Importantly, however, the power density required to operate the membrane laser is well within the maximum permissible exposure for intentional and repeated ocular exposure (ANSI 2000)35. For a divergent pump beam with a full visual angle of 伪鈥?鈥?0掳, a wavelength of 位鈥?鈥?50鈥塶m, a pulse duration of 5鈥塶s and a repetition rate of 5鈥塇z, the maximum permissible corneal irradiance (thermal limit) is 505.1鈥塳W鈥塩m鈭? (red area in Fig.聽4f), i.e. almost one order of magnitude higher than the pump power density required to operate our laser. According to the ANSI 2000 standard, a membrane laser on a contact lens could thus鈥攗nder appropriate pumping conditions鈥攂e safely operated while being worn in the eye.Fig. 4Membrane lasers as wearable security tags. a Membrane laser transferred onto a contact lens. b Contact lens laser being placed on a bovine eye. c Reflection of a white light source from a second-order membrane laser on bovine eye (white dashed line: outline of the contact lens; white arrow: position of grating). d Photograph of laser beam emitted by same bovine eye with contact lens laser, viewed on a screen placed ~50鈥塩m away. The laser is optically pumped with blue light from the right. e Emission spectrum recorded from a bovine eye with contact lens containing a laser membrane. f Input鈭抩utput characteristics of a mixed-order distributed feedback (DFB) laser on a bovine eye ball. The red area marks pump power densities that exceed the safety limit for intentional and repeated use on the human eye. g Membrane laser transferred onto a finger nail. h Optical excitation of a membrane laser on a finger nail. i Emission spectrum recorded from the same laserFull size imageTo further illustrate the transferability and possibility for bio-integration, we also placed a membrane laser onto a finger nail (Fig.聽4g) where the spectrum can again be readily read out (Fig.聽4h, i), thus providing a possible augmentation for biometric finger print scans.DiscussionIn summary, we have demonstrated the fabrication and operation of ultra-thin, substrate-less organic lasers with extreme mechanical flexibility and lightweight. These physical properties combined with the low lasing threshold and the ability to generate unique output spectra allows the application of membrane lasers as security label that can be applied to a wide range of substrates including banknotes, contact lenses, and finger nails. In the future, the effective gain spectrum of the membrane laser can be broadened further by combining several organic polymers, which would enable a further exponential increase in the number of unique output spectra that can be generated, to (1015)n for n different organic polymers. Further optimization of the DFB grating will likely allow lower lasing thresholds and facilitate LED pumping of membrane lasers. By combining recently developed roll-to-roll nanoimprint and organic ink jet printing technology36, membrane lasers could be mass-produced with high reproducibility and at low cost.MethodsMembrane fabricationA ~50鈥塶m-thick sacrificial layer of PEDOT:PSS (Clevios P VP AI 4083, Heraeus) was spin-coated onto an oxygen plasma-treated glass substrate (25鈥塵m鈥壝椻€?5鈥塵m) at 4000鈥塺pm for 60鈥塻 and baked for 10鈥塵in at 100鈥壜癈. Subsequently, a thin ( 10鈥塶m) adhesion promoter (mr-APS1, Micro Resist Technology) and the photo-curable nanoimprint lithography resist (mr-UVCur21-200nm, Micro Resist Technology) were spin-coated and baked according to the manufacturer鈥檚 guidelines (in brief, mr-APS1 was spin-coated at 4000鈥塺pm for 60鈥塻 and baked for 60鈥塻 at 150鈥壜癈 and UVCur21-200nm was spin-coated at 3000鈥塺pm for 60鈥塻 and baked for 20鈥塻 at 100鈥壜癈). A transparent perfluoropolyether soft stamp鈥攃omprising a negative of the final grating structure (mixed- and second-order with nominal grating periods of 340鈥塶m, 350鈥塶m and 360鈥塶m, in second-order)鈥攚as molded into the UV curable polymer layer using a UV imprint alignment system (EVG620, EV Group; 位鈥?鈥?65鈥塶m; dose 56鈥塵W鈥塩m鈭?; exposure time 220鈥塻). After removing the soft stamp, the grating surface (average thickness, 50鈥塶m) was treated with an oxygen plasma to remove any remaining organic residues and reduce hydrophobicity. Three different gain materials were tested in this study: poly(9,9-dioctylfluorene-co-benzothiadiazole) with a 9:1 ratio of F8 to BT units (F80.9BT0.1; Mw鈥?鈥?2,000鈥塯鈥塵ol鈭?; ADS233YE, American Dye Source Inc.), poly(9,9-dioctylfluorene-co-benzothiadiazole) with a 1:1 ratio of F8 to BT units (F8BT; Mw鈥?鈥?1,000鈥塯鈥塵ol鈭?; ADS133YE, American Dye Source Inc.) and a poly(para-phenylene-vinylene) copolymer (Super Yellow; Mw鈥?鈥?.7脳106鈥塯鈥塵ol鈭?; PDY-132, Merck). These were dissolved in toluene at 25鈥塵g鈥塵l鈭? (F80.9BT0.1 and F8BT) and 10鈥塵g鈥塵l鈭? (Super Yellow), respectively, and the resulting solutions spin-coated at 2000鈥塺pm for 60鈥塻, yielding a gain layer of (180鈥壜扁€?0)鈥塶m (for F80.9BT0.1 and F8BT; see Supplementary Fig.聽6 for concentration-dependent layer thickness) and (250鈥壜扁€?0)鈥塶m (Super Yellow). To release the membrane from the rigid glass substrate, the sample was immersed in deionized water heated to 55鈥壜癈 for 1鈥塰.The thickness of the different layers was measured using a profilometer (Dektak 150 Surface Profilometer; Veeco). The DFB grating properties were checked using an AFM. Each DFB grating covered an area of 1鈥塵m鈥壝椻€?鈥塵m.Optical characterizationMembrane lasers were investigated on a custom-built inverted fluorescence microscope. The pulses produced by an optical parametric oscillator (OPO; Opolette 355, Opotek Inc.) tuned to 450鈥塶m (repetition rate, 5鈥塇z; pulse duration, 5鈥塶s) were passed through a dichroic beam splitter (cut-on wavelength, 500鈥塶m) and focused onto the membrane lasers with a 40脳 or 10脳 objective. The emission from the membrane was collected with the same objective and passed into the collection path via the dichroic beam splitter. Bright field images were recorded with a CCD camera. To record the spectrally resolved laser emission, the light was focused onto the entrance slit of a spectrograph (Shamrock 500i, Andor) and recorded with an attached EM-CCD camera (Newton 971, Andor). The minimum peak width that can be resolved with this system is 50鈥塸m. The pump spot size varied from 35 to 400鈥壩糾 diameter.For far field emission measurements, the back focal plane of the objective was imaged onto the entrance slit of the spectrometer by inserting an additional lens in the emission path one focal length away from the back focal plane of the objective and one focal length away from the projection lens in front of the spectrograph. To resolve the far-field pattern along both axes, the spectrograph was operated in zero-order reflection, i.e. without any wavelength dispersion.To control the pump power density of single pulses and record the input鈭抩utput characteristics of the membrane lasers, the OPO emission was passed through a pair of angle-adjustable birefringent polarizers with the angle between the polarization of the OPO emission and the first polarizer adjusted by a computer-controlled stepper motor. 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Mater. 20, 2044鈥?049 (2008).CAS聽 Article聽 Google Scholar聽 Download referencesAcknowledgementsThe authors thank G. Scarcelli (University of Maryland) and F. Delori and R. Webb (both Harvard Medical School) for fruitful discussion of maximum permissible ocular laser exposure and for providing the Ocular Light Calculator software. They also thank M. Jacquet and N. Panova for their help with taking photographs. The authors acknowledge financial support from the European Research Council (ERC StG ABLASE, 640012), the Scottish Funding Council (via SUPA) and EPSRC (EP/P030017/1). M.K. and J.M.E.G. acknowledge funding from the EPSRC DTG (EP/M506631/1 and EP/L505079/1). M.S. acknowledges funding from the European Commission for a Marie Sklodowska-Curie Individual Fellowship (659213). I.D.W.S. acknowledges funding from a Royal Society Wolfson research merit award.Author informationAffiliationsOrganic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UKMarkus Karl,聽James M. E. Glackin,聽Marcel Schubert,聽Nils M. Kronenberg,聽Graham A. Turnbull,聽Ifor D. W. Samuel聽 聽Malte C. GatherAuthorsMarkus KarlView author publicationsYou can also search for this author in PubMed聽Google ScholarJames M. E. GlackinView author publicationsYou can also search for this author in PubMed聽Google ScholarMarcel SchubertView author publicationsYou can also search for this author in PubMed聽Google ScholarNils M. KronenbergView author publicationsYou can also search for this author in PubMed聽Google ScholarGraham A. TurnbullView author publicationsYou can also search for this author in PubMed聽Google ScholarIfor D. W. SamuelView author publicationsYou can also search for this author in PubMed聽Google ScholarMalte C. GatherView author publicationsYou can also search for this author in PubMed聽Google ScholarContributionsM.K. designed and conducted the fabrication and experiments, analyzed the data, performed the simulations and prepared the manuscript. J.M.E.G. fabricated membranes, and performed PLQY and optical measurements. M.S. interpreted data and performed optical measurements. N.M.K. characterized the gratings. G.A.T, I.D.W.S and M.C.G conceived and supervised the project and prepared the manuscript. All authors discussed the data and contributed to the paper.Corresponding authorCorrespondence to Malte C. Gather.Ethics declarations Competing interests The University of St Andrews has filed a patent application related to the work described here. 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If material is not included in the article鈥檚 Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Reprints and PermissionsAbout this articleCite this articleKarl, M., Glackin, J.M.E., Schubert, M. et al. Flexible and ultra-lightweight polymer membrane lasers. Nat Commun 9, 1525 (2018). https://doi.org/10.1038/s41467-018-03874-wDownload citationReceived: 06 December 2017Accepted: 19 March 2018Published: 01 May 2018DOI: https://doi.org/10.1038/s41467-018-03874-w Young-Shin Park, Jeongkyun Roh, Benjamin T. Diroll, Richard D. Schaller Victor I. 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