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Random-Cavity Lasing from Electrospun Polymer Fiber Networks Sarah Krämmer, Christoph Vannahme, Cameron L. C. Smith, Tobias Grossmann, Michael Jenne, Stefan Schierle, Lars Jørgensen, Ioannis S. Chronakis, Anders Kristensen,* and Heinz Kalt* Electrospun polymer fibers and fiber networks feature extraordinarily high surface-to-volume ratios that are of keen interest to sensing,[1–4] tissue-growth,[5,6] biocidal,[7,8] filtering[9], and protective clothing[10,11] applications. Electrospinning is an established technique across many scientific disciplines, since it provides low-cost, high-throughput fabrication of the polymer fiber networks as well as the means to control the fibers’ material properties.[12] Parameters such as stiffness, strength, fiber diameter, network density, and biocompatibility may all be chosen from a large variety.[6,11–15] In the field of photonics, the electrospinning procedure is particularly promising since it offers a flexible approach to select the optical refractive index, gain and absorption of the fibers. The inclusion of active dopants such as nanoparticles or fluorescent dyes into the electrospun polymer represents a straightforward method to realize broadband light sources.[16–19] This feature opens up exciting possibilities for developing new types of integrated photonic devices that further enhance the utility enabled by the porous nature of the fiber network.[4,11,14] In this regard, laser light sources are especially attractive since they are spectrally narrow and coherent and therefore ideal to incorporate as sensor elements where a change in the laser wavelength serves as the sensor signal. Recent demonstrations of polymer lasers produced by electrospinning have been reported including waveguides acting as Fabry–Pérot cavities[20] or bottlelike microcavities.[21] However, these singular and microscale devices are difficult to handle and their functionality is limited to the subset of systems for which they may be integrated. Here, we report on deterministic laser emission from randomly distributed cavities within polymer fiber networks. The resonators form coincidentally during the fabrication process
S. Krämmer, Dr. T. Grossmann, M. Jenne, S. Schierle, Prof. H. Kalt Institute of Applied Physics (APH) Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Str. 1, 76131, Karlsruhe, Germany E-mail: [email protected] Dr. C. Vannahme, Dr. C. L. C. Smith, Prof. A. Kristensen Department of Micro- and Nanotechnology Technical University of Denmark (DTU) DTU Nanotech, Ørsteds Plads Building 345E, DK-2800 Kgs., Lyngby, Denmark E-mail: [email protected] L. Jørgensen, Prof. I. S. Chronakis DTU-Food, Technical University of Denmark (DTU) Søltofts Plads, Building 227, DK-2800 Kgs., Lyngby, Denmark
DOI: 10.1002/adma.201402995
Adv. Mater. 2014, DOI: 10.1002/adma.201402995
and the network geometry differs from other intentionally designed polymer-fiber structures showing lasing emission. Our experimental results are also in contrast to the phenomenon of random lasing[22]—where light spreads through the medium over statistically varying paths—since here the light paths are fixed to cavity locations and the lasing occurs reproducibly. The Rhodamine 6G (R6G) dye-doped polymer fiber networks are fabricated via electrospinning. By spatially resolved micro-photoluminescence (µ-PL) measurements and spectral analysis, it is verified that randomly distributed individual ring resonators with comb-like emission spectra are responsible for deterministic lasing. These random-cavity lasers provide a novel platform for integrated photonic devices that combine the advantages of both laser emission and electrospun fiber networks. We form the R6G-doped polymethyl methacrylate (PMMA) fiber network via electrospinning,[23] (see Figure 1a, for details see the Experimental Section). Figure 1b,c shows scanning electron microscope (SEM) images of the resulting fiber network. The random distribution of the fibers is clearly visible and under higher magnification in Figure 1c it is evident that the fibers are often in contact where they cross. The density of the fiber network is varied on the wafer substrate during the electrospinning process, which allows for an investigation into its effect. Figure 2a depicts the µ-PL setup used for optical characterization.[24,25] The setup features a nanosecond (i.e., quasi-stationary), spatially homogeneous laser excitation; further details are provided in the Experimental Section. A recording of both microscopic images and luminescence spectra is taken from the same sample area with a lateral image resolution of ≈1 µm, determined by the numerical aperture of the objective. The four examples in Figure 2b–e show different locations with varying fiber density. The spectrum in Figure 2b displays the bare fluorescence of R6G, whereas the spectra in Figure 2c–e feature narrow peaks in addition. The latter is identified to be laser modes from their threshold-like onset. Distinct spectral signatures can be observed at sample regions with either low or high fiber density: in Figure 2d, peaks appear irregularly whereas in Figure 2e, the peak spacing is periodic, exhibiting a free spectral range (FSR) of 0.36 nm. A comparison of Figure 2b,d shows that similar fiber network densities can exhibit considerably different emission features. In regions with medium or high fiber densities as shown in Figure 2b,d,e, lasing emission is observed at many locations on the sample and the spectra mostly exhibit aperiodic peak spacing. For low fiber densities as shown in Figure 2c, laser emission is more uncommon
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Figure 1. Electrospun polymer fiber networks. a) Schematic of the electrospinning process. b,c) SEM images of the electrospun dye-doped polymer fibers showing the disordered arrangement of the resulting network. Scale bar is (b) 200 µm and (c) 10 µm.
but still frequent enough to find routinely and typically comblike spectra with constant FSR appear. The lasing emission originates from resonators formed within the network and the differences in the spectra are attributed to variations in the density of resonators not just the density of the fiber mesh. The resulting emission displays bare fluorescence when no resonator is present, periodic features when there is a single resonator and aperiodic features when there are either multiple, overlapping resonators or potentially a single resonator with additional higher order modes. In order to prove the laser characteristics of the emission, we acquire a power series of spectra and obtain a lasing threshold.[26] In Figure 3a, an input–output curve at one location of a sample is shown, where pump energy refers to the incident energy density. The output intensity is determined
by integrating over the spectral power distribution of a single lasing peak. This is done for two separate peaks appearing at different pump energies, finding their thresholds to be 31 and 51 µJ mm−2, respectively. Figure 3b depicts spectra for excitation energies in the range of these threshold values showing the onset of lasing. Besides the values listed above, lasing thresholds as low as 13 µJ mm−2 have been measured. To investigate the origin of the laser emission, we perform spatially resolved µ-PL measurements. In Figure 4a, an example of a recorded spectra-image pair is shown. By investigating sample locations with a low fiber density, we identify single polymer fibers as emitters of fluorescence and laser light appearing as combs with regular peak spacing. Tracking fibers featuring lasing emission by moving the sample verifies that the polymer fibers serve as waveguides. This is in accordance with previous results.[27] Central to the results of our experiments is that fiber ring-resonator arrangements can be identified as the sources of the laser emission. Figure 4b depicts a typical example of such a ring resonator where the corresponding emission spectra taken at different locations of the ring and the connected fiber parts exhibit the same comb of lasing peaks. The microscope image shows that the ring resonator is formed by the fiber being in contact with itself for 73 µm after completion of a loop. There is no evidence that laser emission couples between nonparallel fiber crossings, in agreement with the expected behavior of light guiding in fibers.[28] On the basis of these results, we exclude the amplification of stimulated emission experienced over a random light-path, referred
Figure 2. Laser emission from the fiber network. a) Scheme of the µ-PL setup. b–e) Microscope images of the fiber network under laser excitation and corresponding µ-PL spectra. The data were taken at different locations with varying fiber network densities. Regions with both low and high fiber densities show lasing emission. A region exhibiting only fluorescence emission is shown in (b). The peaks appear irregularly in (c,d), a periodic mode comb on a fluorescence background can be identified in (e). Scale bar is 30 µm.
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To further verify that the ring resonators are responsible for lasing, we present a numerical analysis of the fiber modes, FSR, and fiber-to-fiber coupling. We calculate the waveguide modes of the fibers via finite element method (FEM) in COMSOL. Within the spectral range of the laser emission, the material dispersion of PMMA and the influence of the R6G dopant on the refractive index can be neglected and we use a constant PMMA refractive index of 1.49. Figure 5a shows the effective refractive Figure 3. Lasing threshold. a) Input–Output curve for two different lasing modes yielding index, neff, of the fundamental single fiber lasing thresholds of 31 and 51 µJ mm− 2. b) µ-PL spectra for different pump energies showing modes as a function of the fiber diameter, the onset of lasing. The spectrum below the threshold of modes 1 and 2 is shown in black, with the two extremity electric field distributhe spectra above the threshold of mode 1 in blue and the spectra over both thresholds in red. tions included as insets. The FSR and the effective refractive index are used to determine the length of the resonator ring, LR, to as random lasing,[22] since random lasing would require λ1 λ2 a multiple scattering of the laser light between neighboring , where λ1 and λ2 are the wavelengths given by LR = polymer fiber sections, which is not observed. neff Δλ of two adjacent peaks. Using an FSR of Δλ = 0.56 nm obtained from Figure 4b, LR is found to be 418–401 µm for fiber diameters ranging between 0.8 and 2 µm. These values correspond well with the measured resonator length of 392 µm obtained from a microscope image, especially when considering that the microscope measurement is a projection of the 3D ring structure into 2D. A ring resonator exists when the polymer fiber forms a loop where the fiber is in contact with itself for a significant coupling length, Lc.[29,30] In order to simulate the power coupling between the overlapping parts of the fiber, we calculate the symmetric and antisymmetric super modes for both TE and TM polarization, assuming an air cladding around the fibers.[31] The modes experience a phase difference of 2πΔneff L , when λ propagating through the fibers over a length L, where Δneff is the difference between the two effective refractive indices of the respective modes. Figure 5b shows the evolution of Δneff as a function of the fiber diameter for both TE and TM polarization. The power coupling coefficient k, which corresponds to the power coupled from one fiber to another, can be expressed as Figure 4. Example of a ring resonator and its spectra found with spectrally resolved µ-PL. a) Allocation of an image of the measured area and its corresponding spatially resolved luminescence spectra where the white lines mark the entrance slit of the spectrometer. It is clearly visible that the fluorescence and the lasing originate from the polymer fibers. The laser emission of a single fiber shows regularly spaced peaks. b) The sample position and the corresponding µ-PL spectrum are marked with the same color and number. The peaks corresponding to laser modes originate from a fiber forming a closed-loop ring resonator by being in contact with itself for approximately 73 µm (region between white bars). The same lasing modes can be observed along this ring and the connected fiber parts whereas other fibers in this area only show fluorescence. Scale bar is 20 µm.
Adv. Mater. 2014, DOI: 10.1002/adma.201402995
⎛ πΔneff LC ⎞ k = sin 2 ⎜ ⎟⎠ ⎝ λ
(1)
From exponentially decaying fits to the data of Figure 5b, the coupling coefficient is calculated and plotted in Figure 5c,d. k directly affects the quality factor (Q factor) of a cavity as it induces losses to the resonator when k is less than 1. Thereby, the lasing
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Figure 5. Simulation results. a) Effective refractive index of the fundamental fiber mode as a function of the fiber diameter. Inset: Mode profiles of fibers with diameter of 0.8 and 2.0 µm, scale bar is 0.5 µm. b) Simulation of the directional coupler effective refractive index difference between the symmetric and antisymmetric super modes for both TE and TM polarization, as well as the fitted exponential curves. Calculated coupling coefficient for λ = 570 nm as function of c) fiber diameter and d) coupling length for TE and TM polarization.
threshold depends on k, since it is inversely proportional to the resonator’s passive Q factor.[32] SEM measurements taken at various sample positions indicate fiber diameters of ≈1.0 µm, corresponding to a coupling coefficient of 0.6 for TE modes. Coupling coefficients less than 1 are in agreement with the observation that lasing peaks appear in fiber sections closely connected to the resonator (compare Figure 4b). Coupling can also occur for higher modes, which have lower effective refractive indices and, therefore, different coupling constants. For the occurrence of lasing modes, however, a high overlap of the mode with the gain medium is essential. Since we observe only one dominant mode in the spectra of Figure 4b, we assume this to be the fundamental mode for which the optical gain is highest. The presented results confirm that light can couple efficiently between two parallel polymer fibers in contact, thus facilitating the formation of ring resonator loops, which serve as cavities for the laser emission. These fiber loops can be considered as isolated ring resonators randomly distributed throughout the fiber system, each experiencing minimal interaction with the rest of the greater network. In this light, we can explain spectra from locations of high density of fibers (e.g., Figure 2d) to be a superposition of multiple ring resonators each contributing with at least one lasing peak to the spectrum. In summary, we have demonstrated that electrospun R6Gdoped PMMA fibers feature deterministic laser emission. By using spatially resolved spectroscopy, we have identified randomly distributed ring resonators to be responsible for
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comb-like laser spectra. We quantitatively confirm this result by determining the resonator length via analysis of the FSR, found to be in close agreement with ring sizes determined from microscope images. Furthermore, we show by simulations that directional coupling between parts of the fibers is responsible for the formation of ring resonators. The electrospinning procedure to fabricate the random-cavity lasers is a facile, versatile, and exceptionally low-cost technique that requires only small material volumes. These lasers represent clear opportunities in the context of novel sensor devices since the waveguided modes in the fibers can be expected to yield high sensitivities due to the large surface-to-volume ratios. Additionally, the lasers are innately integrated in the system and provide a readily measurable transduction mechanism. Accordingly, the electrospun polymer fiber platform represents a flexible and conveniently integrated approach for developing novel photonic devices that simultaneously exploit the advantages offered by the unique fiber network structure.
Experimental Section Preparation of Electrospinning Solutions: PMMA (Mw: 350 kDa from Sigma–Aldrich) is dissolved in DMF (dimetylformamid) at ambient temperature with polymer concentration of 12%–14% w/v. The concentration of dye (Rhodamine 6G from Sigma–Aldrich) in polymeric spinning solutions is 0.3 mg mL−1. Magnetic stirring is performed for at least 3 h at room temperature in order to obtain homogenously dissolved solutions (the vials are covered with aluminum foil to protect from light).
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Acknowledgements This work has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe, by a grant from the Ministry of Science, Research, and the Arts of Baden-Wuerttemberg (Az:7713.14-300). T.G. acknowledges financial support of the Deutsche Telekom Stiftung. S.K. and T.G. thank the Karlsruhe School of Optics and Photonics (KSOP) for continuous support. C.V. and C.L.C.S. acknowledge financial support from the Danish Council for Independent Research (FTP Grant Nos. 12-126676 and 12-126601). L.J. and I.S.C. acknowledge financial support from the Danish Strategic Research Council (DSF project FENAMI, Contract No. 10-93456). Received: July 7, 2014 Revised: September 22, 2014 Published online:
[1] X. Wang, C. Drew, S.-H. Lee, K. J. Senecal, J. Kumar, L. A. Samuelson, Nano Lett. 2002, 2, 1273. [2] C.-C. Kuo, C.-T. Wang, W.-C. Chen, Macromol. Mater. Eng. 2008, 293, 999.
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Electrospinning Processing: A high voltage power supply (ES50P-10W, Gamma High Voltage Research, Inc., USA) is used to provide high voltages in the range of 0–50 kV. To avoid air bubbles, spinning solutions are carefully loaded in a 5-mL syringe to which a stainless steel capillary metal-hub needle is attached. The inside diameter of the metal needle is ≈0.9 mm. The positive electrode of the high-voltage power supply is connected to the needle tip. The grounded electrode is connected to a metal collector covered with a 4-inch silicon wafer with a silicon oxide top layer of 200 nm thickness. The silicon oxide ensures that the surface of the substrate is optically transparent while the silicon serves for conduction. The electrospinning process is carried out at room temperature. A fixed electrical potential of 15 kV is applied across a distance of 20 cm between the tip and the collector. The feed rate of solutions is controlled at 0.05 mL min−1 by means of a single syringe pump (New Era Pump Systems, Inc., USA). µ-PL Setup: For optical excitation a frequency-doubled Nd:YVO4 laser with 532 nm emission wavelength and pulse length of 10 ns is used (see Figure 2a). The laser power is regulated by a Pockels cell and a polarizer. The light is focused onto the sample yielding pump spot sizes between 200 µm and 1 mm in diameter for spatially homogeneous optical pumping. The emitted light is collected with a microscope objective (NA = 0.42, 50×) and guided to a beam splitter where 50% of the light is sent to a camera to obtain an image of the sample. The remaining 50% of the emission light is directed to a spectrometer (50 cm focal length, grating with 1200 lines mm−1) combined with a CCD camera. To obtain spatially resolved spectra of the sample, each pixel of the CCD camera is read out separately and the position of the entrance slit on the sample image is determined. A translation stage is used to move the sample in the x- and y-directions, so that different spots on the sample can be investigated.
[3] Y. Zhang, Y. Guo, Y. Xianyu, W. Chen, Y. Zhao, X. Jiang, Adv. Mater. 2013, 25, 3802. [4] D. Yang, X. Niu, Y. Liu, Y. Wang, X. Gu, L. Song, R. Zhao, L. Ma, Y. Shao, X. Jiang, Adv. Mater. 2008, 20, 4770. [5] Y. Jun, E. Kang, S. Chae, S.-H. Lee, Lab Chip 2014, 14, 2145. [6] S. Agarwal, J. H. Wendorff, A. Greiner, Adv. Mater. 2009, 21, 3343. [7] C. Kriegel, K. M. Kit, D. J. McClements, J. Weiss, Polymer (Guildf ). 2009, 50, 189. [8] X. Ren, H. B. Kocer, S. D. Worley, R. M. Broughton, T. S. Huang, J. Appl. Polym. Sci. 2013, 127, 3192. [9] R. Gopal, S. Kaur, Z. Ma, C. Chan, S. Ramakrishna, T. Matsuura, J. Memb. Sci. 2006, 281, 581. [10] S. Lee, S. K. Obendorf, Text. Res. J. 2007, 77, 696. [11] P. Gibson, H. Schreuder-Gibson, D. Rivin, Colloids Surf. A: Physicochem. Eng. Asp. 2001, 187–188, 469. [12] D. Li, Y. Xia, Adv. Mater. 2004, 16, 1151. [13] R. Dersch, M. Steinhart, U. Boudriot, A. Greiner, J. H. Wendorff, Polym. Adv. Technol. 2005, 16, 276. [14] R. V. N. Krishnappa, K. Desai, C. Sung, E. C. Sungumledu, J. Mater. Sci. 2003, 8, 2357. [15] A. Frenot, I. S. Chronakis, Curr. Opin. Colloid Interface Sci. 2003, 8, 64. [16] H. Yu, H. Song, G. Pan, L. Fan, S. Li, X. Bai, S. Lu, H. Zhao, J. Nanosci. Nanotechnol. 2008, 8, 6017. [17] N. Tomczak, N. F. van Hulst, G. J. Vancso, Macromolecules 2005, 38, 7863. [18] D. Di Camillo, V. Fasano, F. Ruggieri, S. Santucci, L. Lozzi, A. Camposeo, D. Pisignano, Nanoscale 2013, 5, 11637. [19] J. M. Moran-Mirabal, J. D. Slinker, J. A. DeFranco, S. S. Verbridge, R. Ilic, S. Flores-Torres, H. Abruña, G. G. Malliaras, H. G. Craighead, Nano Lett. 2007, 7, 458. [20] A. Camposeo, F. Di Benedetto, R. Stabile, A. R. Neves, R. Cingolani, D. Pisignano, Small 2009, 5, 562. [21] A. J. Das, C. Lafargue, M. Lebental, J. Zyss, K. S. Narayan, Appl. Phys. Lett. 2011, 99, 263303. [22] D. S. Wiersma, Nat. Phys. 2008, 4, 359. [23] J. Doshi, D. H. Reneker, 1995, 35, 151. [24] J. Fallert, R. J. B. Dietz, J. Sartor, D. Schneider, C. Klingshirn, H. Kalt, Nat. Photonics 2009, 3, 279. [25] T. Grossmann, T. Wienhold, U. Bog, T. Beck, C. Friedmann, H. Kalt, T. Mappes, Light Sci. Appl. 2013, 2, e82. [26] I. D. W. Samuel, E. B. Namdas, G. A. Turnbull, Nat. Photonics 2009, 3, 546. [27] H. Liu, J. B. Edel, L. M. Bellan, H. G. Craighead, Small 2006, 2, 495. [28] L. Tong, J. Lou, R. R. Gattass, S. He, X. Chen, L. Liu, E. Mazur, Nano Lett. 2005, 5, 259. [29] H. Li, J. Li, L. Qiang, Y. Zhang, S. Hao, Nanoscale 2013, 5, 6297. [30] L. F. Stokes, M. Chodorow, H. J. Shaw, Opt. Lett. 1982, 7, 288. [31] M. J. F. Digonnet, H. J. Shaw, IEEE Trans. Microw. Theory Tech. 1982, 30, 592. [32] F. P. Schäfer, (Ed.), Dye Lasers, Springer, Berlin, 1990.
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Random-Cavity Lasing from Electrospun Polymer Fiber Networks Sarah Krämmer, Christoph Vannahme, Cameron L. C. Smith, Tobias Grossmann, Michael Jenne, Stefan Schierle, Lars Jørgensen, Ioannis S. Chronakis, Anders Kristensen,* and Heinz Kalt* Electrospun polymer fibers and fiber networks feature extraordinarily high surface-to-volume ratios that are of keen interest to sensing,[1–4] tissue-growth,[5,6] biocidal,[7,8] filtering[9], and protective clothing[10,11] applications. Electrospinning is an established technique across many scientific disciplines, since it provides low-cost, high-throughput fabrication of the polymer fiber networks as well as the means to control the fibers’ material properties.[12] Parameters such as stiffness, strength, fiber diameter, network density, and biocompatibility may all be chosen from a large variety.[6,11–15] In the field of photonics, the electrospinning procedure is particularly promising since it offers a flexible approach to select the optical refractive index, gain and absorption of the fibers. The inclusion of active dopants such as nanoparticles or fluorescent dyes into the electrospun polymer represents a straightforward method to realize broadband light sources.[16–19] This feature opens up exciting possibilities for developing new types of integrated photonic devices that further enhance the utility enabled by the porous nature of the fiber network.[4,11,14] In this regard, laser light sources are especially attractive since they are spectrally narrow and coherent and therefore ideal to incorporate as sensor elements where a change in the laser wavelength serves as the sensor signal. Recent demonstrations of polymer lasers produced by electrospinning have been reported including waveguides acting as Fabry–Pérot cavities[20] or bottlelike microcavities.[21] However, these singular and microscale devices are difficult to handle and their functionality is limited to the subset of systems for which they may be integrated. Here, we report on deterministic laser emission from randomly distributed cavities within polymer fiber networks. The resonators form coincidentally during the fabrication process
S. Krämmer, Dr. T. Grossmann, M. Jenne, S. Schierle, Prof. H. Kalt Institute of Applied Physics (APH) Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Str. 1, 76131, Karlsruhe, Germany E-mail: [email protected] Dr. C. Vannahme, Dr. C. L. C. Smith, Prof. A. Kristensen Department of Micro- and Nanotechnology Technical University of Denmark (DTU) DTU Nanotech, Ørsteds Plads Building 345E, DK-2800 Kgs., Lyngby, Denmark E-mail: [email protected] L. Jørgensen, Prof. I. S. Chronakis DTU-Food, Technical University of Denmark (DTU) Søltofts Plads, Building 227, DK-2800 Kgs., Lyngby, Denmark
DOI: 10.1002/adma.201402995
Adv. Mater. 2014, DOI: 10.1002/adma.201402995
and the network geometry differs from other intentionally designed polymer-fiber structures showing lasing emission. Our experimental results are also in contrast to the phenomenon of random lasing[22]—where light spreads through the medium over statistically varying paths—since here the light paths are fixed to cavity locations and the lasing occurs reproducibly. The Rhodamine 6G (R6G) dye-doped polymer fiber networks are fabricated via electrospinning. By spatially resolved micro-photoluminescence (µ-PL) measurements and spectral analysis, it is verified that randomly distributed individual ring resonators with comb-like emission spectra are responsible for deterministic lasing. These random-cavity lasers provide a novel platform for integrated photonic devices that combine the advantages of both laser emission and electrospun fiber networks. We form the R6G-doped polymethyl methacrylate (PMMA) fiber network via electrospinning,[23] (see Figure 1a, for details see the Experimental Section). Figure 1b,c shows scanning electron microscope (SEM) images of the resulting fiber network. The random distribution of the fibers is clearly visible and under higher magnification in Figure 1c it is evident that the fibers are often in contact where they cross. The density of the fiber network is varied on the wafer substrate during the electrospinning process, which allows for an investigation into its effect. Figure 2a depicts the µ-PL setup used for optical characterization.[24,25] The setup features a nanosecond (i.e., quasi-stationary), spatially homogeneous laser excitation; further details are provided in the Experimental Section. A recording of both microscopic images and luminescence spectra is taken from the same sample area with a lateral image resolution of ≈1 µm, determined by the numerical aperture of the objective. The four examples in Figure 2b–e show different locations with varying fiber density. The spectrum in Figure 2b displays the bare fluorescence of R6G, whereas the spectra in Figure 2c–e feature narrow peaks in addition. The latter is identified to be laser modes from their threshold-like onset. Distinct spectral signatures can be observed at sample regions with either low or high fiber density: in Figure 2d, peaks appear irregularly whereas in Figure 2e, the peak spacing is periodic, exhibiting a free spectral range (FSR) of 0.36 nm. A comparison of Figure 2b,d shows that similar fiber network densities can exhibit considerably different emission features. In regions with medium or high fiber densities as shown in Figure 2b,d,e, lasing emission is observed at many locations on the sample and the spectra mostly exhibit aperiodic peak spacing. For low fiber densities as shown in Figure 2c, laser emission is more uncommon
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Figure 1. Electrospun polymer fiber networks. a) Schematic of the electrospinning process. b,c) SEM images of the electrospun dye-doped polymer fibers showing the disordered arrangement of the resulting network. Scale bar is (b) 200 µm and (c) 10 µm.
but still frequent enough to find routinely and typically comblike spectra with constant FSR appear. The lasing emission originates from resonators formed within the network and the differences in the spectra are attributed to variations in the density of resonators not just the density of the fiber mesh. The resulting emission displays bare fluorescence when no resonator is present, periodic features when there is a single resonator and aperiodic features when there are either multiple, overlapping resonators or potentially a single resonator with additional higher order modes. In order to prove the laser characteristics of the emission, we acquire a power series of spectra and obtain a lasing threshold.[26] In Figure 3a, an input–output curve at one location of a sample is shown, where pump energy refers to the incident energy density. The output intensity is determined
by integrating over the spectral power distribution of a single lasing peak. This is done for two separate peaks appearing at different pump energies, finding their thresholds to be 31 and 51 µJ mm−2, respectively. Figure 3b depicts spectra for excitation energies in the range of these threshold values showing the onset of lasing. Besides the values listed above, lasing thresholds as low as 13 µJ mm−2 have been measured. To investigate the origin of the laser emission, we perform spatially resolved µ-PL measurements. In Figure 4a, an example of a recorded spectra-image pair is shown. By investigating sample locations with a low fiber density, we identify single polymer fibers as emitters of fluorescence and laser light appearing as combs with regular peak spacing. Tracking fibers featuring lasing emission by moving the sample verifies that the polymer fibers serve as waveguides. This is in accordance with previous results.[27] Central to the results of our experiments is that fiber ring-resonator arrangements can be identified as the sources of the laser emission. Figure 4b depicts a typical example of such a ring resonator where the corresponding emission spectra taken at different locations of the ring and the connected fiber parts exhibit the same comb of lasing peaks. The microscope image shows that the ring resonator is formed by the fiber being in contact with itself for 73 µm after completion of a loop. There is no evidence that laser emission couples between nonparallel fiber crossings, in agreement with the expected behavior of light guiding in fibers.[28] On the basis of these results, we exclude the amplification of stimulated emission experienced over a random light-path, referred
Figure 2. Laser emission from the fiber network. a) Scheme of the µ-PL setup. b–e) Microscope images of the fiber network under laser excitation and corresponding µ-PL spectra. The data were taken at different locations with varying fiber network densities. Regions with both low and high fiber densities show lasing emission. A region exhibiting only fluorescence emission is shown in (b). The peaks appear irregularly in (c,d), a periodic mode comb on a fluorescence background can be identified in (e). Scale bar is 30 µm.
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To further verify that the ring resonators are responsible for lasing, we present a numerical analysis of the fiber modes, FSR, and fiber-to-fiber coupling. We calculate the waveguide modes of the fibers via finite element method (FEM) in COMSOL. Within the spectral range of the laser emission, the material dispersion of PMMA and the influence of the R6G dopant on the refractive index can be neglected and we use a constant PMMA refractive index of 1.49. Figure 5a shows the effective refractive Figure 3. Lasing threshold. a) Input–Output curve for two different lasing modes yielding index, neff, of the fundamental single fiber lasing thresholds of 31 and 51 µJ mm− 2. b) µ-PL spectra for different pump energies showing modes as a function of the fiber diameter, the onset of lasing. The spectrum below the threshold of modes 1 and 2 is shown in black, with the two extremity electric field distributhe spectra above the threshold of mode 1 in blue and the spectra over both thresholds in red. tions included as insets. The FSR and the effective refractive index are used to determine the length of the resonator ring, LR, to as random lasing,[22] since random lasing would require λ1 λ2 a multiple scattering of the laser light between neighboring , where λ1 and λ2 are the wavelengths given by LR = polymer fiber sections, which is not observed. neff Δλ of two adjacent peaks. Using an FSR of Δλ = 0.56 nm obtained from Figure 4b, LR is found to be 418–401 µm for fiber diameters ranging between 0.8 and 2 µm. These values correspond well with the measured resonator length of 392 µm obtained from a microscope image, especially when considering that the microscope measurement is a projection of the 3D ring structure into 2D. A ring resonator exists when the polymer fiber forms a loop where the fiber is in contact with itself for a significant coupling length, Lc.[29,30] In order to simulate the power coupling between the overlapping parts of the fiber, we calculate the symmetric and antisymmetric super modes for both TE and TM polarization, assuming an air cladding around the fibers.[31] The modes experience a phase difference of 2πΔneff L , when λ propagating through the fibers over a length L, where Δneff is the difference between the two effective refractive indices of the respective modes. Figure 5b shows the evolution of Δneff as a function of the fiber diameter for both TE and TM polarization. The power coupling coefficient k, which corresponds to the power coupled from one fiber to another, can be expressed as Figure 4. Example of a ring resonator and its spectra found with spectrally resolved µ-PL. a) Allocation of an image of the measured area and its corresponding spatially resolved luminescence spectra where the white lines mark the entrance slit of the spectrometer. It is clearly visible that the fluorescence and the lasing originate from the polymer fibers. The laser emission of a single fiber shows regularly spaced peaks. b) The sample position and the corresponding µ-PL spectrum are marked with the same color and number. The peaks corresponding to laser modes originate from a fiber forming a closed-loop ring resonator by being in contact with itself for approximately 73 µm (region between white bars). The same lasing modes can be observed along this ring and the connected fiber parts whereas other fibers in this area only show fluorescence. Scale bar is 20 µm.
Adv. Mater. 2014, DOI: 10.1002/adma.201402995
⎛ πΔneff LC ⎞ k = sin 2 ⎜ ⎟⎠ ⎝ λ
(1)
From exponentially decaying fits to the data of Figure 5b, the coupling coefficient is calculated and plotted in Figure 5c,d. k directly affects the quality factor (Q factor) of a cavity as it induces losses to the resonator when k is less than 1. Thereby, the lasing
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Figure 5. Simulation results. a) Effective refractive index of the fundamental fiber mode as a function of the fiber diameter. Inset: Mode profiles of fibers with diameter of 0.8 and 2.0 µm, scale bar is 0.5 µm. b) Simulation of the directional coupler effective refractive index difference between the symmetric and antisymmetric super modes for both TE and TM polarization, as well as the fitted exponential curves. Calculated coupling coefficient for λ = 570 nm as function of c) fiber diameter and d) coupling length for TE and TM polarization.
threshold depends on k, since it is inversely proportional to the resonator’s passive Q factor.[32] SEM measurements taken at various sample positions indicate fiber diameters of ≈1.0 µm, corresponding to a coupling coefficient of 0.6 for TE modes. Coupling coefficients less than 1 are in agreement with the observation that lasing peaks appear in fiber sections closely connected to the resonator (compare Figure 4b). Coupling can also occur for higher modes, which have lower effective refractive indices and, therefore, different coupling constants. For the occurrence of lasing modes, however, a high overlap of the mode with the gain medium is essential. Since we observe only one dominant mode in the spectra of Figure 4b, we assume this to be the fundamental mode for which the optical gain is highest. The presented results confirm that light can couple efficiently between two parallel polymer fibers in contact, thus facilitating the formation of ring resonator loops, which serve as cavities for the laser emission. These fiber loops can be considered as isolated ring resonators randomly distributed throughout the fiber system, each experiencing minimal interaction with the rest of the greater network. In this light, we can explain spectra from locations of high density of fibers (e.g., Figure 2d) to be a superposition of multiple ring resonators each contributing with at least one lasing peak to the spectrum. In summary, we have demonstrated that electrospun R6Gdoped PMMA fibers feature deterministic laser emission. By using spatially resolved spectroscopy, we have identified randomly distributed ring resonators to be responsible for
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comb-like laser spectra. We quantitatively confirm this result by determining the resonator length via analysis of the FSR, found to be in close agreement with ring sizes determined from microscope images. Furthermore, we show by simulations that directional coupling between parts of the fibers is responsible for the formation of ring resonators. The electrospinning procedure to fabricate the random-cavity lasers is a facile, versatile, and exceptionally low-cost technique that requires only small material volumes. These lasers represent clear opportunities in the context of novel sensor devices since the waveguided modes in the fibers can be expected to yield high sensitivities due to the large surface-to-volume ratios. Additionally, the lasers are innately integrated in the system and provide a readily measurable transduction mechanism. Accordingly, the electrospun polymer fiber platform represents a flexible and conveniently integrated approach for developing novel photonic devices that simultaneously exploit the advantages offered by the unique fiber network structure.
Experimental Section Preparation of Electrospinning Solutions: PMMA (Mw: 350 kDa from Sigma–Aldrich) is dissolved in DMF (dimetylformamid) at ambient temperature with polymer concentration of 12%–14% w/v. The concentration of dye (Rhodamine 6G from Sigma–Aldrich) in polymeric spinning solutions is 0.3 mg mL−1. Magnetic stirring is performed for at least 3 h at room temperature in order to obtain homogenously dissolved solutions (the vials are covered with aluminum foil to protect from light).
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Acknowledgements This work has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe, by a grant from the Ministry of Science, Research, and the Arts of Baden-Wuerttemberg (Az:7713.14-300). T.G. acknowledges financial support of the Deutsche Telekom Stiftung. S.K. and T.G. thank the Karlsruhe School of Optics and Photonics (KSOP) for continuous support. C.V. and C.L.C.S. acknowledge financial support from the Danish Council for Independent Research (FTP Grant Nos. 12-126676 and 12-126601). L.J. and I.S.C. acknowledge financial support from the Danish Strategic Research Council (DSF project FENAMI, Contract No. 10-93456). Received: July 7, 2014 Revised: September 22, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201402995
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Electrospinning Processing: A high voltage power supply (ES50P-10W, Gamma High Voltage Research, Inc., USA) is used to provide high voltages in the range of 0–50 kV. To avoid air bubbles, spinning solutions are carefully loaded in a 5-mL syringe to which a stainless steel capillary metal-hub needle is attached. The inside diameter of the metal needle is ≈0.9 mm. The positive electrode of the high-voltage power supply is connected to the needle tip. The grounded electrode is connected to a metal collector covered with a 4-inch silicon wafer with a silicon oxide top layer of 200 nm thickness. The silicon oxide ensures that the surface of the substrate is optically transparent while the silicon serves for conduction. The electrospinning process is carried out at room temperature. A fixed electrical potential of 15 kV is applied across a distance of 20 cm between the tip and the collector. The feed rate of solutions is controlled at 0.05 mL min−1 by means of a single syringe pump (New Era Pump Systems, Inc., USA). µ-PL Setup: For optical excitation a frequency-doubled Nd:YVO4 laser with 532 nm emission wavelength and pulse length of 10 ns is used (see Figure 2a). The laser power is regulated by a Pockels cell and a polarizer. The light is focused onto the sample yielding pump spot sizes between 200 µm and 1 mm in diameter for spatially homogeneous optical pumping. The emitted light is collected with a microscope objective (NA = 0.42, 50×) and guided to a beam splitter where 50% of the light is sent to a camera to obtain an image of the sample. The remaining 50% of the emission light is directed to a spectrometer (50 cm focal length, grating with 1200 lines mm−1) combined with a CCD camera. To obtain spatially resolved spectra of the sample, each pixel of the CCD camera is read out separately and the position of the entrance slit on the sample image is determined. A translation stage is used to move the sample in the x- and y-directions, so that different spots on the sample can be investigated.
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