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Distributed Feedback Imprinted Electrospun Fiber Lasers Luana Persano,* Andrea Camposeo,* Pompilio Del Carro, Vito Fasano, Maria Moffa, Rita Manco, Stefania D’Agostino,* and Dario Pisignano* Laser architectures based on coupling photonic crystals or plasmonics with light-emitting nanostructures have attracted an increasing interest as building blocks of new photonic devices and circuits.[1–5] Possible applications include the embedment of these components in chips for microfluidic, biodiagnostics, as well as quantum communication.[6,7] However, nanofabrication technologies allowing the surface of single micro- or nanoparticles to be patterned are still very difficult, expensive, and complex due to the need of high resolution and three-dimensional texturing capability. Indeed, single particle lasers realized so far include opticallypumped ZnO,[8,9] CdS,[10] and GaAs-AlGaAs core-shell nanowires,[11] organic nanowires[12,13] and nanofibers,[14] electricallypumped CdS[15] and ZnO nanowires,[16] and plasmonic lasers based on coupling nanowires with metal films,[4,5] but none of these nanomaterials have been patterned to embed photonic crystals in them. Recently, milling experiments by focused ion beam have led to the demonstration of single cleavedcoupled cavities in GaN nanowires.[17] These processes involve micromanipulation of particles, metal sputtering to protect individual nanowires, milling, and removal of the protection layer and of other excess pieces. Another interesting explored route has been focused on embedding nanowires into periodic poly(methylmethacrylate) (PMMA)/air claddings, which resulted in significant spectral modulation of the emission.[1] Given the great importance of distributed feedback photonic
Dr. L. Persano,[+] Dr. A. Camposeo,[+] Dr. P. Del Carro, Dr. M. Moffa, R. Manco, Prof. D. Pisignano National Nanotechnology Laboratory of Istituto Nanoscienze-CNR via Arnesano, I-73100, Lecce, Italy E-mail: [email protected]; [email protected]; [email protected] Dr. L. Persano, Dr. A. Camposeo, V. Fasano, Dr. S. D’Agostino, Prof. D. Pisignano Center for Biomolecular Nanotechnologies @UNILE Istituto Italiano di Tecnologia via Barsanti, I-73010, Arnesano, LE, Italy E-mail: [email protected] Dr. S. D’Agostino Dipartimento di Fisica Università di Pavia Via Bassi 6, I-27100, Pavia, Italy V. Fasano, Prof. D. Pisignano Dipartimento di Matematica e Fisica “Ennio De Giorgi” Università del Salento via Arnesano, I-73100, Lecce, Italy [+] These authors contributed equally to this work.
DOI: 10.1002/adma.201401945
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architectures for tailoring localization and suppression of optical modes and ultimately reducing lasing thresholds, integrating these structures on single nanowires or nanofibers may lead to new, highly tailorable and miniaturized active elements and devices for nanophotonics. In this work we introduce a new concept which consists in distributed feedback lasers being integrated directly on single nanofibers (scheme in Figure 1a). Our active material is given by individual electrospun filaments of PMMA and 2-[6-(ethylamino)-3-(ethylimino)-2,7-dimethyl-3H-xanthen-9-yl]benzoic acid, ethyl ester, chloride (Rho-6G). Advantages of electrospinning for producing light-emitting nanofibers stand in its good throughput, straightforward operation, versatility in terms of usable active molecular compounds and polymer matrices, improved quantum yield of the emission of the resulting nanofibers.[18–20] Enabling the realization of large amounts of uniform and smooth fibers, electrospinning is an especially interesting method for obtaining lasing nanomaterials. Fiber diameters down to 50 nm can be easily achieved by controlling the process parameters. Electrospun filaments are demonstrated to be highly flexible (Figure 1b) and to achieve sub-wavelength light confinement capability. Silicon master structures are then used for top-down transferring gratings on individual fibers as shown in Figure 1c and 1d. As a result of pattern transfer, each single filament constitutes a highly portable and compact plastic laser source. The room-temperature emission spectra under ns-pulsed excitation at a wavelength of 532 nm, plotted as a function of the absorbed excitation fluence, are displayed in Figure 2a for distributed feedback filament lasers with period, Λ = 600 nm. The device design workflow is summarized in the Supporting Information. At low excitation fluence (below about 700 µJ/cm2) the emission spectra are moderately intense and featureless, whereas for higher pump energies a narrow peak (full width at half maximum < 0.3 nm) arises at wavelength, λL = 562 nm. This peak dominates over a series of residual, much less intense modes spaced by 0.6–0.7 nm. The Bragg expression, mλL = 2neffΛ, is generally employed to describe waveguide multilayer structures with periodically modulated refractive index or optical gain[21] and to effectively predict the emission wavelength of organic distributed feedback lasers, including nanoimprinted devices.[22–24] Here, neff is the effective refractive index of the filament and m indicates the diffraction order in the nanostructure. Each distributed feedback fiber lases at the third order of diffraction. Along the patterned fiber, both the refractive index and the optical gain are modulated with Λ-periodicity, and the measured threshold excitation fluence is 750 µJ/cm2 as displayed in the light-light (L-L) plot in Figure 2b. We compare the lasing characteristics with those of typical distributed feedback lasers based on organic films, made
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Figure 1. Distributed feedback fiber laser morphology. a, Schematics of device. The vertical, downward arrows and the diagonal, upward arrow indicate the excitation pump in form of stripe, and lasing from the excited fiber, respectively. b, Scanning electron micrograph of nanostructured polymeric filaments. The inset shows a zoom of a bent patterned polyacrylonitrile fiber. c, d, Surface of the nanostructured lasing filaments imaged by confocal laser scanning microscopy showing a top-view (c) and a three-dimensional view (d), respectively. Period, Λ = 600 nm. The reference grid highlighted by the white-edged box in (d) has length (parallel to the filament longitudinal axis), width (parallel to the grating features), and height of about 48 µm, 7 µm and 4 µm, respectively. Here, fibers are intentionally produced with diameters in the µm-scale in order to favour optical imaging.
Figure 2. Lasing spectra and input-output characteristics. a, Lasing spectra from an individual distributed feedback nanofiber with Λ = 600 nm. From bottom to top, excitation fluence = 400 µJ cm−2, 600 µJ cm−2, 700 µJ cm−2, 800 µJ cm−2, 900 µJ cm−2, 1.00 mJ cm−2, 1.10 mJ cm−2, 1.30 mJ cm−2 and 1.40 mJ cm−2. b, Corresponding L-L plot. Each single patterned filament exhibits a threshold pumping fluence of about 750 µJ cm−2. c, Lasing spectra from a conventional, thin-film distributed feedback laser with the same active material. Film thickness, h = 450 nm. From bottom to top, excitation fluence = 800 µJ cm−2, 1.20 mJ cm−2, 2.00 mJ cm−2, 2.40 mJ cm−2, 2.70 mJ cm−2, and 3.40 mJ cm−2. d, Corresponding L-L plot, showing a threshold pumping fluence of about 1.50 mJ cm−2. All the lasing spectra are collected by using 10 ns excitation pulses at λexc = 532 nm. e, Map showing the angle-dependent cw photoluminescence emission from gratings with period, Λ = 600 nm, patterned on our active material. The lightest regions in the map highlight the angular dispersion of the emission coming from Bragg-outcoupled modes (full spectra in Figure S1 in Supporting Information). f, Schematics of the experimental configuration for angle-resolved photoluminescence measurements. The collection angle, ϑ, is measured in the plane defined by the normal direction to the sample and by the emission spot-optical fiber line, and it is perpendicular to the grating features (features not to scale).
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of the same active medium and realized by spin-casting and nanoimprinting. A thickness from 450 nm (equalling the fiber value) to 600 nm is obtained for films by varying the solution concentration. Interestingly, the distributed feedback filaments allow threshold to be reduced by 40–50% with respect to conventional, nanopatterned thin-film lasers. Indeed, by the same material and pattern period as in filaments, the threshold excitation fluence in films is of 1.20–1.50 mJ cm−2, slightly varying with sample thickness. Exemplary emission spectra and the L-L plot for films are shown in Figure 2c and 2d, respectively (further data are in Figure S3 in the Supporting Information). The output lasing wavelength is at 586–587 nm, related to possibly different local molecular environment in deposited films or to overall different effective refractive index conditions. As expected, the superimposed photonic crystal structure affects the angular distribution of the emission from our active material as shown in the two-dimensional map in Figure 2e. The wavelength-scale periodic pattern clearly determines an enhancement of the forward output from the active compound (complete spectra are reported in Supporting Information). For instance, a bright emission peak is visible at wavelength ranging from 650 nm (at 24°) to 550 nm (at 35°), as found by measuring the collection angles according to the scheme in Figure 2f. The enhancement of the forwardly out-coupled light is rationalized by the conservation of the in-plane component of the free-space photon wave vector (2π/λ) and of the guided mode (2neff/λ), through the reciprocal lattice vector of the photonic crystal (2π/Λ). In this way, the light trapped in the organic material can be effectively scattered out at certain forward angles. However, Bragg scattering while reducing light self-absorption in the active medium can hardly account for the different thresholds found by distributed feedback films and fibers. As potential mechanism, we firstly study the possibly increased optical gain of the active medium due to the different processing and manufacturing methods used to produce nanofibers and films. To this aim, we provide a quantitative assessment of the gain
properties of electrospun and spin-cast materials by varying the length of the excitation stripe used to pump the active media. In this way, the net optical gain spectrum, g(λ), of the active medium is obtained through the relation between the intensity, I, of the light emitted by the samples, and the length of an excitation stripe length, L:
{
}
I ( λ ) = I 0 ( λ ) exp ⎡⎣ g ( λ ) L ⎤⎦ − 1
(1)
where I0 is a pre-factor corresponding to the intensity signal measured at very short L. The emission spectra collected with different excitation stripes are reported in Figure 3a and b for nanofibers and films, respectively. The different samples are also imaged in the confocal micrographs in the insets. Linenarrowing in untextured fibers and films is related to amplified spontaneous emission (ASE), which is enhanced by longer pump lengths (Figures 3c and d). Both nanofibers and films exhibit a gain peak, centred at about 600 nm with a spectral width of about 20 nm and a maximum of about 8 cm−1. Therefore we cannot find significantly different net optical gain in differently processed materials. Furthermore, by the value of the maximum optical gain, we can extract the ASE threshold length, LTH = ln2/g. We obtain LTH = (900 ± 100) µm for our nanomaterial, together with a net gain cross section (σ = g/N, where N is the excitation density) of the order of 10−20 cm2. To our knowledge, these are the first measurements of these properties for light-emitting polymer nanofibers. We point out that no lasing emission is found in the untextured polymer nanofibers. For observing lasing from untextured Rho-6G/PMMA nanofibers, short fiber segments should be prepared to favour the formation of longitudinal, Fabry-Pérot microcavities.[14] The reduced threshold in distributed feedback nanofibers has to be related to differently effective microcavity effects, namely in different spatial distribution of optical fields. We investigate these effects by Finite-Difference Time-Domain (FDTD)[25] simulations of distributed feedback fibers and films,
Figure 3. ASE and optical gain of active media. a,b, Emission spectra from Rho-6G/PMMA nanofibers dense mats (a) and Rho-6G/PMMA films (b) under excitation stripes of different lengths. From bottom to top, stripe length is varied from 0.5 mm to the maximum value, by steps of 0.5 mm. The insets show confocal fluorescence micrographs of samples. Scale bars: 50 µm. c,d, Dependence of the ASE intensity (left scales) on the excitation stripe length, for nanofibers (c) and films (d), respectively. e,f, Corresponding measured optical gain spectra for nanofibers (e) and for films (f), respectively. The lines are guides for the eye.
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Figure 4. Light propagation and far-field emission in the distributed feedback fibers. FDTD results for the fundamental TE mode sustained by the distributed feedback fiber at 562 nm (a-c) and for a distributed feedback film at 587 nm (d-f). For both the structures the grating period is Λ = 600 nm. (a,d): Maps of the normalized real part of the z-component of the Poynting vector (m−2), calculated on a cross-section (x, y) below the first grating feature impacted by light. Note the significantly different x scales, evidencing the much larger spreading of fields along the film plane. (b,e): Corresponding longitudinal (z,y) sections for x = 0, highlighting the field enhancement through the grating, which is significantly higher in the fiber. (c,f): Corresponding angular distributions of far-field scattered light.
carefully modelling the actual experimental geometries. For light travelling along the longitudinal axis of fibers or along the film (z-axis) at our lasing wavelengths, we study both fundamental transverse electric (TE) and transverse magnetic (TM) modes, focusing on the real part of the z-component of the Poynting vector which is related to the propagating power. Results of simulations for the fundamental TE mode are shown in Figure 4 (the TM mode is available in Figure S4 in the Supporting Information). Here we show the real part, Re(Sz), of the the z-component of the Poynting vector (W/m2), normalized to the source power (W). Such source power is the integral of Re(Sz) for the analyzed mode, calculated over the input crosssection. The normalized Re(Sz) values displayed in the maps so indicate the ratio of the local intensity to the source power, in each point (x, y, z) of the device, thus providing a quantitative insight on the localization of e.m. field. This allows us to highlight the strongly localized hot-spot in fibers, where normalized Re(Sz) reaches values as high as 3.7 × 10−3 m−2 (Figure 4a,b). On the contrary, the films present a much more delocalized field, spreading along the in-plane (x) direction (Figure 4d,e). Therefore, the different confinement of the field, leading to correspondingly higher photonic density i.e. higher peak intensity values experienced by emitters inside the fibers, has to be at the origin of the lowering of the laser threshold. In fact, the threshold reduction following the integration of the photonic crystal structure in single nanofibers depends on the reduced active volume, similarly to the behaviour observed in heterojunction lasers as a function of the thickness of active layers.[26] Here, the fiber waveguide structure channels spontaneous emission to through the gain region with better efficiency and directionality than a thin film. While in a linear system this would just lower the measured intensity, such effect is crucial for the onset of non-linear phenomena such as lasing. Waveguiding in the light-emitting filaments so reduces the sideways emission which in films does not contribute to optical gain and ultimately to distributed feedback lasing. In addition, the intensity of light diffracted-up by the fiber is higher than that of the film, which exhibits a larger transmission along the entire z length (Figure 4b,e). This evidence for the fundamental TE modes, as well as the larger number
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of propagating modes which can most probably be sustained in the planar films, explains the better performance of distributed feedback fibers also in terms of radiation out-coupling and collection. In fact, in imprinted films the stimulated emission depleting excited molecular states is allowed to travel effectively across the sample, i.e. in the plane of the organic layer, whereas in patterned fibers a larger fraction of the same emission is preferentially out-coupled through the grating. The maximum polar angle for external collection of far-field scattered light is calculated to be at ϑ = 32°, in excellent agreement with experiments and mainly related to the grating period. The azimuthal angular behaviour of the far-field (Figure 4c,f) highlights that the emission from the patterned fibers is clearly more spatiallydispersed, with a ∼60° aperture around the z-axis (ϕ from 60° to 120°, Figure 4c) due to the more relevant diffraction. Instead, untextured fibers do not exhibit either localized hot-spots (Figure S7) or significant diffracted-up light. The difference in lasing performances for nanowires or nanofibers and corresponding films reported so far are uneven. For instance, lasing in ZnO and CdS nanowires shows thresholds significantly lower than thin-films.[8,15] Instead, para-sexiphenyl polycrystalline films do not show gain, likely due to inter-domain optical losses, whereas lasing is observed in crystalline nanowires.[27] The threshold reported for poly(9,9-dioctylfluorene) nanowires[12] is quite higher than typical values of film-based distributed feedback lasers,[28] and so on. Here, the comparison of fiber and film-based imprinted lasers is made easier by using the same active materials and characterization set-up. Finally, we point out that a further reduction of the lasing threshold of individual distributed feedback nanofibers can be achieved by controlling and optimizing the photonic crystal geometry. For instance, thresholds further lowered by 15% are measured by using nanofibers with grating period of 400 nm, which corresponds to exploiting the second Bragg order. The versatility of the used nanofabrication methods, producible geometries and utilizable materials, is especially promising in this respect. Next developments will likely include the application of direct imprinting to semiconductor[29] and metal[30] particles, to extend distributed feedback lasers to inorganic nanowires and plasmonics.
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Nanofiber Fabrication: The solution used for electrospinning active fibers is composed of 600 mg of PMMA and 3 mg of Rho-6G dissolved in 1.6 mL of chloroform. The solution is sonicated for 2 hours in order to allow complete polymer dissolution in the solvent. The polymer solution is then loaded in a plastic syringe with a 27 gauge stainless steel needle and injected through the needle at constant flow rate (1 mL/h) by a syringe pump (Harvard Apparatus, Holliston, MA). A 10 kV voltage is applied at the needle using a high-voltage power supply (EL60R0.6–22, Glassman High Voltage, High Bridge, NJ). Fibers are collected on square (1 × 1 cm2) quartz substrates, mounted on a grounded rotating disk collector (3000 rpm) at a distance of 18 cm from the needle. Typical ambient relative humidity and temperature conditions are around 45% and 23°C, respectively. Reference films (thickness of the active layer = 450–600 nm) are obtained by solution spin casting. Nanofiber Nanopatterning: Wavelength-scale 1D Bragg gratings are directly patterned onto PMMA-based or polyacrylonitrile nanofibers by a single-step imprinting. To this aim, master templates are firstly produced by patterning Silicon by electron-beam lithography at 20 kV (Raith150 system), defining parallel grooves with period (Λ) of 600 and 400 nm, and with depth of about 200 nm, over areas of about 0.5 × 4 mm2. Reactive ion etching of Silicon is carried out by a CF4/Ar mixture. Finally, samples are placed on the lower plate of a precision manual press (PW100 P/O/Weber) and in contact with the patterned side of the master for imprinting, which is then performed in air with an applied pressure of 60–80 MPa for times ≤ 15 minutes. Prior to imprinting the masters are aligned with their grooves roughly perpendicular to the longitudinal axis of nanofibers. Due to imprinting, the fibers may undergo partial flattening, resulting in a height reduction up to 40%. Morphological Characterization: Scanning electron microscopy is carried out with an acceleration voltage of 5 kV and an aperture size of 20 µm. Typical features transferred onto active filaments have height of about 100 nm. Optical Characterization: All the optical experiments are performed on fibers of comparable diameter. Confocal fluorescence images are acquired by an A1R MP confocal system (Nikon), coupled to an inverted microscope (Eclipse Ti, Nikon). The fibers are excited by an Ar+ ion laser (λexc = 488 nm) through an oil-immersion objective with numerical aperture of 1.4. The emission is measured by a spectral detection unit equipped with a multi-anode photomultiplier (Nikon). The photoluminescence absolute quantum efficiency (η, that is the number of PL photons emitted per absorbed photon), of films and fibers is measured by placing samples in an integrating sphere (LabSphere), exciting by a cw laser system (Coherent Verdi V10, λexc = 532 nm) and detecting emission by a fiber-coupled spectrometer (Ocean Optics USB 4000). All the measured spectra are corrected by the spectral response of the used experimental setup (integrating sphere, optical fiber and spectrometer), determined by means of a calibrated lamp. η is measured to be 53% for nanofibers, which is higher than the corresponding value in thin films by more than 20%. Angle-resolved PL experiments are carried out by an optical fiber mounted on a rotation stage, allowing the emitted light in a small solid angle (105 dB/m), in order to excite samples over a length of many mm this scheme is more convenient than end-pump configurations used for fiber lasers. End-pump methods based on evanescent coupling have been instead employed with organic nanofibers for exciting spontaneous emission.[31] The stimulated emission and lasing signal from single
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Experimental Section
excited fibers are here collected by the objective and through a dichroic mirror and an external lens coupled into an optical fiber, and then spectrally analyzed by a monochromator (Yobin Yvon, iHR320, grating with 1200 grooves/mm) using a Peltier-cooled charge-coupled device (CCD, Yobin Yvon, Symphony). The spectral resolution of the optical system is approximately 0.1 nm. For gain and reference film lasing characterization, samples are pumped by a rectangular excitation stripe of 2 mm × 300 µm, under vacuum at ∼10−4 mbar. FDTD Calculations: FDTD simulations are performed with the Lumerical[32] software. The time-dependent Maxwell's equations are solved by using a computational grid with absorbing perfectly matched layers boundary conditions and an auto non-uniform mesh type. To model the actual experimental geometries, a thickness (h) of 450 nm and of 450–600 nm is assumed for deposited fibers and for films, respectively, together with a period, Λ = 600 nm, a pattern duty cycle of 50% and a depth of the imprinted features of 100 nm for both the structures. All the simulated structures are centred in the point (0, 0, 0) of our coordinates system. Corners are not assumed to be sharp, but instead present a finite curvature radius (rc = 0.2 h), in order to simulate the partial sample flattening observed upon imprinting. The size of the polymeric region along the z-axis (fiber longitudinal axis) is 15 µm whereas the size along the x-axis (fiber width) is of 1 µm and 15 µm, for the fiber and the film, respectively. All the materials are approximated as lossless with a real refractive index of 1.54, and lye on a quartz substrate (n = 1.46). Frequency domain power monitors are used to record the mode profile over the simulation region under the first feature of the pattern. The maps in Figure 4 and in Figures S4-S8 show the real part, Re(Sz), of the the z-component of the Poynting vector (W/m2), normalized to the source power (W).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We gratefully thank Dr. Lee Carroll and Prof. Lucio C. Andreani for the high-level assistance provided on the use of the code and for the precious scientific contribution given to the work. Dr. Salvatore Girardo is also acknowledged for the realization of some nanofiber samples. The authors acknowledge the financial support from the Italian Minister of University and Research through the FIRB project RBFR08DJZI “Futuro in Ricerca”, and from the Apulia Regional Projects ‘Networks of Public Research Laboratories’, Wafitech (9) and M. I. T. T. (13). Received: April 30, 2014 Revised: June 7, 2014 Published online:
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[19] J. H. Wendorff, S. Agarwal, A. Greiner, Electrospinning. Materials, Processing and Applications, Wiley-VCH, Weihneim, 2012. [20] G. Morello, A. Polini, S. Girardo, A. Camposeo, D. Pisignano, Appl. Phys. Lett. 2013, 102, 211911. [21] H. Kogelnik, C. V. Shank, Appl. Phys. Lett. 1971, 18, 152. [22] M. Gaal, C. Gadermaier, H. Plank, E. Moderegger, A. Pogantsch, G. Leising, E. J. W. List, Adv. Mater. 2003, 15, 1165. [23] P. Del Carro, A. Camposeo, R. Stabile, E. Mele, L. Persano, R. Cingolani, D. Pisignano, Appl. Phys. Lett. 2006, 89, 201105. [24] E. B. Namdas, M. Tong, P. Ledochowitsch, S. R. Mednick, J. D. Yuen, D. Moses, A. J. Heeger, Adv. Mater. 2009, 21, 799. [25] H. Ichikawa, J. Opt. Soc. Am. A 1998, 15, 152. [26] A. Yariv, P. Yeh, Photonics: optical electronics in modern communications (6th Ed.), Oxford University Press, Oxford, 2007. [27] F. Quochi, F. Cordella, R. Orrù, J. E. Communal, P. Verzeroli, A. Mura, G. Bongiovanni, A. Andreev, H. Sitter, N. S. Sariciftci, Appl. Phys. Lett. 2004, 84, 4454. [28] G. Heliotis, R. Xia, D. D. C. Bradley, G. A. Turnbull, I. D. W. Samuel, P. Andrew, W. L. Barnes, Appl. Phys. Lett. 2003, 83, 2118. [29] S. Y. Chou, C. Keimel, J. Gu, Nature 2002, 417, 835 [30] G. Kumar, H. X. Tang, J. Schroers, Nature 2009, 457, 868. [31] L. P. Wang, Y. Wang, L. Tong, Light: Science & Applications 2013, 2, e102. [32] https://www.lumerical.com/tcad-products/fdtd/
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Distributed Feedback Imprinted Electrospun Fiber Lasers Luana Persano,* Andrea Camposeo,* Pompilio Del Carro, Vito Fasano, Maria Moffa, Rita Manco, Stefania D’Agostino,* and Dario Pisignano* Laser architectures based on coupling photonic crystals or plasmonics with light-emitting nanostructures have attracted an increasing interest as building blocks of new photonic devices and circuits.[1–5] Possible applications include the embedment of these components in chips for microfluidic, biodiagnostics, as well as quantum communication.[6,7] However, nanofabrication technologies allowing the surface of single micro- or nanoparticles to be patterned are still very difficult, expensive, and complex due to the need of high resolution and three-dimensional texturing capability. Indeed, single particle lasers realized so far include opticallypumped ZnO,[8,9] CdS,[10] and GaAs-AlGaAs core-shell nanowires,[11] organic nanowires[12,13] and nanofibers,[14] electricallypumped CdS[15] and ZnO nanowires,[16] and plasmonic lasers based on coupling nanowires with metal films,[4,5] but none of these nanomaterials have been patterned to embed photonic crystals in them. Recently, milling experiments by focused ion beam have led to the demonstration of single cleavedcoupled cavities in GaN nanowires.[17] These processes involve micromanipulation of particles, metal sputtering to protect individual nanowires, milling, and removal of the protection layer and of other excess pieces. Another interesting explored route has been focused on embedding nanowires into periodic poly(methylmethacrylate) (PMMA)/air claddings, which resulted in significant spectral modulation of the emission.[1] Given the great importance of distributed feedback photonic
Dr. L. Persano,[+] Dr. A. Camposeo,[+] Dr. P. Del Carro, Dr. M. Moffa, R. Manco, Prof. D. Pisignano National Nanotechnology Laboratory of Istituto Nanoscienze-CNR via Arnesano, I-73100, Lecce, Italy E-mail: [email protected]; [email protected]; [email protected] Dr. L. Persano, Dr. A. Camposeo, V. Fasano, Dr. S. D’Agostino, Prof. D. Pisignano Center for Biomolecular Nanotechnologies @UNILE Istituto Italiano di Tecnologia via Barsanti, I-73010, Arnesano, LE, Italy E-mail: [email protected] Dr. S. D’Agostino Dipartimento di Fisica Università di Pavia Via Bassi 6, I-27100, Pavia, Italy V. Fasano, Prof. D. Pisignano Dipartimento di Matematica e Fisica “Ennio De Giorgi” Università del Salento via Arnesano, I-73100, Lecce, Italy [+] These authors contributed equally to this work.
DOI: 10.1002/adma.201401945
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architectures for tailoring localization and suppression of optical modes and ultimately reducing lasing thresholds, integrating these structures on single nanowires or nanofibers may lead to new, highly tailorable and miniaturized active elements and devices for nanophotonics. In this work we introduce a new concept which consists in distributed feedback lasers being integrated directly on single nanofibers (scheme in Figure 1a). Our active material is given by individual electrospun filaments of PMMA and 2-[6-(ethylamino)-3-(ethylimino)-2,7-dimethyl-3H-xanthen-9-yl]benzoic acid, ethyl ester, chloride (Rho-6G). Advantages of electrospinning for producing light-emitting nanofibers stand in its good throughput, straightforward operation, versatility in terms of usable active molecular compounds and polymer matrices, improved quantum yield of the emission of the resulting nanofibers.[18–20] Enabling the realization of large amounts of uniform and smooth fibers, electrospinning is an especially interesting method for obtaining lasing nanomaterials. Fiber diameters down to 50 nm can be easily achieved by controlling the process parameters. Electrospun filaments are demonstrated to be highly flexible (Figure 1b) and to achieve sub-wavelength light confinement capability. Silicon master structures are then used for top-down transferring gratings on individual fibers as shown in Figure 1c and 1d. As a result of pattern transfer, each single filament constitutes a highly portable and compact plastic laser source. The room-temperature emission spectra under ns-pulsed excitation at a wavelength of 532 nm, plotted as a function of the absorbed excitation fluence, are displayed in Figure 2a for distributed feedback filament lasers with period, Λ = 600 nm. The device design workflow is summarized in the Supporting Information. At low excitation fluence (below about 700 µJ/cm2) the emission spectra are moderately intense and featureless, whereas for higher pump energies a narrow peak (full width at half maximum < 0.3 nm) arises at wavelength, λL = 562 nm. This peak dominates over a series of residual, much less intense modes spaced by 0.6–0.7 nm. The Bragg expression, mλL = 2neffΛ, is generally employed to describe waveguide multilayer structures with periodically modulated refractive index or optical gain[21] and to effectively predict the emission wavelength of organic distributed feedback lasers, including nanoimprinted devices.[22–24] Here, neff is the effective refractive index of the filament and m indicates the diffraction order in the nanostructure. Each distributed feedback fiber lases at the third order of diffraction. Along the patterned fiber, both the refractive index and the optical gain are modulated with Λ-periodicity, and the measured threshold excitation fluence is 750 µJ/cm2 as displayed in the light-light (L-L) plot in Figure 2b. We compare the lasing characteristics with those of typical distributed feedback lasers based on organic films, made
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Figure 1. Distributed feedback fiber laser morphology. a, Schematics of device. The vertical, downward arrows and the diagonal, upward arrow indicate the excitation pump in form of stripe, and lasing from the excited fiber, respectively. b, Scanning electron micrograph of nanostructured polymeric filaments. The inset shows a zoom of a bent patterned polyacrylonitrile fiber. c, d, Surface of the nanostructured lasing filaments imaged by confocal laser scanning microscopy showing a top-view (c) and a three-dimensional view (d), respectively. Period, Λ = 600 nm. The reference grid highlighted by the white-edged box in (d) has length (parallel to the filament longitudinal axis), width (parallel to the grating features), and height of about 48 µm, 7 µm and 4 µm, respectively. Here, fibers are intentionally produced with diameters in the µm-scale in order to favour optical imaging.
Figure 2. Lasing spectra and input-output characteristics. a, Lasing spectra from an individual distributed feedback nanofiber with Λ = 600 nm. From bottom to top, excitation fluence = 400 µJ cm−2, 600 µJ cm−2, 700 µJ cm−2, 800 µJ cm−2, 900 µJ cm−2, 1.00 mJ cm−2, 1.10 mJ cm−2, 1.30 mJ cm−2 and 1.40 mJ cm−2. b, Corresponding L-L plot. Each single patterned filament exhibits a threshold pumping fluence of about 750 µJ cm−2. c, Lasing spectra from a conventional, thin-film distributed feedback laser with the same active material. Film thickness, h = 450 nm. From bottom to top, excitation fluence = 800 µJ cm−2, 1.20 mJ cm−2, 2.00 mJ cm−2, 2.40 mJ cm−2, 2.70 mJ cm−2, and 3.40 mJ cm−2. d, Corresponding L-L plot, showing a threshold pumping fluence of about 1.50 mJ cm−2. All the lasing spectra are collected by using 10 ns excitation pulses at λexc = 532 nm. e, Map showing the angle-dependent cw photoluminescence emission from gratings with period, Λ = 600 nm, patterned on our active material. The lightest regions in the map highlight the angular dispersion of the emission coming from Bragg-outcoupled modes (full spectra in Figure S1 in Supporting Information). f, Schematics of the experimental configuration for angle-resolved photoluminescence measurements. The collection angle, ϑ, is measured in the plane defined by the normal direction to the sample and by the emission spot-optical fiber line, and it is perpendicular to the grating features (features not to scale).
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of the same active medium and realized by spin-casting and nanoimprinting. A thickness from 450 nm (equalling the fiber value) to 600 nm is obtained for films by varying the solution concentration. Interestingly, the distributed feedback filaments allow threshold to be reduced by 40–50% with respect to conventional, nanopatterned thin-film lasers. Indeed, by the same material and pattern period as in filaments, the threshold excitation fluence in films is of 1.20–1.50 mJ cm−2, slightly varying with sample thickness. Exemplary emission spectra and the L-L plot for films are shown in Figure 2c and 2d, respectively (further data are in Figure S3 in the Supporting Information). The output lasing wavelength is at 586–587 nm, related to possibly different local molecular environment in deposited films or to overall different effective refractive index conditions. As expected, the superimposed photonic crystal structure affects the angular distribution of the emission from our active material as shown in the two-dimensional map in Figure 2e. The wavelength-scale periodic pattern clearly determines an enhancement of the forward output from the active compound (complete spectra are reported in Supporting Information). For instance, a bright emission peak is visible at wavelength ranging from 650 nm (at 24°) to 550 nm (at 35°), as found by measuring the collection angles according to the scheme in Figure 2f. The enhancement of the forwardly out-coupled light is rationalized by the conservation of the in-plane component of the free-space photon wave vector (2π/λ) and of the guided mode (2neff/λ), through the reciprocal lattice vector of the photonic crystal (2π/Λ). In this way, the light trapped in the organic material can be effectively scattered out at certain forward angles. However, Bragg scattering while reducing light self-absorption in the active medium can hardly account for the different thresholds found by distributed feedback films and fibers. As potential mechanism, we firstly study the possibly increased optical gain of the active medium due to the different processing and manufacturing methods used to produce nanofibers and films. To this aim, we provide a quantitative assessment of the gain
properties of electrospun and spin-cast materials by varying the length of the excitation stripe used to pump the active media. In this way, the net optical gain spectrum, g(λ), of the active medium is obtained through the relation between the intensity, I, of the light emitted by the samples, and the length of an excitation stripe length, L:
{
}
I ( λ ) = I 0 ( λ ) exp ⎡⎣ g ( λ ) L ⎤⎦ − 1
(1)
where I0 is a pre-factor corresponding to the intensity signal measured at very short L. The emission spectra collected with different excitation stripes are reported in Figure 3a and b for nanofibers and films, respectively. The different samples are also imaged in the confocal micrographs in the insets. Linenarrowing in untextured fibers and films is related to amplified spontaneous emission (ASE), which is enhanced by longer pump lengths (Figures 3c and d). Both nanofibers and films exhibit a gain peak, centred at about 600 nm with a spectral width of about 20 nm and a maximum of about 8 cm−1. Therefore we cannot find significantly different net optical gain in differently processed materials. Furthermore, by the value of the maximum optical gain, we can extract the ASE threshold length, LTH = ln2/g. We obtain LTH = (900 ± 100) µm for our nanomaterial, together with a net gain cross section (σ = g/N, where N is the excitation density) of the order of 10−20 cm2. To our knowledge, these are the first measurements of these properties for light-emitting polymer nanofibers. We point out that no lasing emission is found in the untextured polymer nanofibers. For observing lasing from untextured Rho-6G/PMMA nanofibers, short fiber segments should be prepared to favour the formation of longitudinal, Fabry-Pérot microcavities.[14] The reduced threshold in distributed feedback nanofibers has to be related to differently effective microcavity effects, namely in different spatial distribution of optical fields. We investigate these effects by Finite-Difference Time-Domain (FDTD)[25] simulations of distributed feedback fibers and films,
Figure 3. ASE and optical gain of active media. a,b, Emission spectra from Rho-6G/PMMA nanofibers dense mats (a) and Rho-6G/PMMA films (b) under excitation stripes of different lengths. From bottom to top, stripe length is varied from 0.5 mm to the maximum value, by steps of 0.5 mm. The insets show confocal fluorescence micrographs of samples. Scale bars: 50 µm. c,d, Dependence of the ASE intensity (left scales) on the excitation stripe length, for nanofibers (c) and films (d), respectively. e,f, Corresponding measured optical gain spectra for nanofibers (e) and for films (f), respectively. The lines are guides for the eye.
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Figure 4. Light propagation and far-field emission in the distributed feedback fibers. FDTD results for the fundamental TE mode sustained by the distributed feedback fiber at 562 nm (a-c) and for a distributed feedback film at 587 nm (d-f). For both the structures the grating period is Λ = 600 nm. (a,d): Maps of the normalized real part of the z-component of the Poynting vector (m−2), calculated on a cross-section (x, y) below the first grating feature impacted by light. Note the significantly different x scales, evidencing the much larger spreading of fields along the film plane. (b,e): Corresponding longitudinal (z,y) sections for x = 0, highlighting the field enhancement through the grating, which is significantly higher in the fiber. (c,f): Corresponding angular distributions of far-field scattered light.
carefully modelling the actual experimental geometries. For light travelling along the longitudinal axis of fibers or along the film (z-axis) at our lasing wavelengths, we study both fundamental transverse electric (TE) and transverse magnetic (TM) modes, focusing on the real part of the z-component of the Poynting vector which is related to the propagating power. Results of simulations for the fundamental TE mode are shown in Figure 4 (the TM mode is available in Figure S4 in the Supporting Information). Here we show the real part, Re(Sz), of the the z-component of the Poynting vector (W/m2), normalized to the source power (W). Such source power is the integral of Re(Sz) for the analyzed mode, calculated over the input crosssection. The normalized Re(Sz) values displayed in the maps so indicate the ratio of the local intensity to the source power, in each point (x, y, z) of the device, thus providing a quantitative insight on the localization of e.m. field. This allows us to highlight the strongly localized hot-spot in fibers, where normalized Re(Sz) reaches values as high as 3.7 × 10−3 m−2 (Figure 4a,b). On the contrary, the films present a much more delocalized field, spreading along the in-plane (x) direction (Figure 4d,e). Therefore, the different confinement of the field, leading to correspondingly higher photonic density i.e. higher peak intensity values experienced by emitters inside the fibers, has to be at the origin of the lowering of the laser threshold. In fact, the threshold reduction following the integration of the photonic crystal structure in single nanofibers depends on the reduced active volume, similarly to the behaviour observed in heterojunction lasers as a function of the thickness of active layers.[26] Here, the fiber waveguide structure channels spontaneous emission to through the gain region with better efficiency and directionality than a thin film. While in a linear system this would just lower the measured intensity, such effect is crucial for the onset of non-linear phenomena such as lasing. Waveguiding in the light-emitting filaments so reduces the sideways emission which in films does not contribute to optical gain and ultimately to distributed feedback lasing. In addition, the intensity of light diffracted-up by the fiber is higher than that of the film, which exhibits a larger transmission along the entire z length (Figure 4b,e). This evidence for the fundamental TE modes, as well as the larger number
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of propagating modes which can most probably be sustained in the planar films, explains the better performance of distributed feedback fibers also in terms of radiation out-coupling and collection. In fact, in imprinted films the stimulated emission depleting excited molecular states is allowed to travel effectively across the sample, i.e. in the plane of the organic layer, whereas in patterned fibers a larger fraction of the same emission is preferentially out-coupled through the grating. The maximum polar angle for external collection of far-field scattered light is calculated to be at ϑ = 32°, in excellent agreement with experiments and mainly related to the grating period. The azimuthal angular behaviour of the far-field (Figure 4c,f) highlights that the emission from the patterned fibers is clearly more spatiallydispersed, with a ∼60° aperture around the z-axis (ϕ from 60° to 120°, Figure 4c) due to the more relevant diffraction. Instead, untextured fibers do not exhibit either localized hot-spots (Figure S7) or significant diffracted-up light. The difference in lasing performances for nanowires or nanofibers and corresponding films reported so far are uneven. For instance, lasing in ZnO and CdS nanowires shows thresholds significantly lower than thin-films.[8,15] Instead, para-sexiphenyl polycrystalline films do not show gain, likely due to inter-domain optical losses, whereas lasing is observed in crystalline nanowires.[27] The threshold reported for poly(9,9-dioctylfluorene) nanowires[12] is quite higher than typical values of film-based distributed feedback lasers,[28] and so on. Here, the comparison of fiber and film-based imprinted lasers is made easier by using the same active materials and characterization set-up. Finally, we point out that a further reduction of the lasing threshold of individual distributed feedback nanofibers can be achieved by controlling and optimizing the photonic crystal geometry. For instance, thresholds further lowered by 15% are measured by using nanofibers with grating period of 400 nm, which corresponds to exploiting the second Bragg order. The versatility of the used nanofabrication methods, producible geometries and utilizable materials, is especially promising in this respect. Next developments will likely include the application of direct imprinting to semiconductor[29] and metal[30] particles, to extend distributed feedback lasers to inorganic nanowires and plasmonics.
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Nanofiber Fabrication: The solution used for electrospinning active fibers is composed of 600 mg of PMMA and 3 mg of Rho-6G dissolved in 1.6 mL of chloroform. The solution is sonicated for 2 hours in order to allow complete polymer dissolution in the solvent. The polymer solution is then loaded in a plastic syringe with a 27 gauge stainless steel needle and injected through the needle at constant flow rate (1 mL/h) by a syringe pump (Harvard Apparatus, Holliston, MA). A 10 kV voltage is applied at the needle using a high-voltage power supply (EL60R0.6–22, Glassman High Voltage, High Bridge, NJ). Fibers are collected on square (1 × 1 cm2) quartz substrates, mounted on a grounded rotating disk collector (3000 rpm) at a distance of 18 cm from the needle. Typical ambient relative humidity and temperature conditions are around 45% and 23°C, respectively. Reference films (thickness of the active layer = 450–600 nm) are obtained by solution spin casting. Nanofiber Nanopatterning: Wavelength-scale 1D Bragg gratings are directly patterned onto PMMA-based or polyacrylonitrile nanofibers by a single-step imprinting. To this aim, master templates are firstly produced by patterning Silicon by electron-beam lithography at 20 kV (Raith150 system), defining parallel grooves with period (Λ) of 600 and 400 nm, and with depth of about 200 nm, over areas of about 0.5 × 4 mm2. Reactive ion etching of Silicon is carried out by a CF4/Ar mixture. Finally, samples are placed on the lower plate of a precision manual press (PW100 P/O/Weber) and in contact with the patterned side of the master for imprinting, which is then performed in air with an applied pressure of 60–80 MPa for times ≤ 15 minutes. Prior to imprinting the masters are aligned with their grooves roughly perpendicular to the longitudinal axis of nanofibers. Due to imprinting, the fibers may undergo partial flattening, resulting in a height reduction up to 40%. Morphological Characterization: Scanning electron microscopy is carried out with an acceleration voltage of 5 kV and an aperture size of 20 µm. Typical features transferred onto active filaments have height of about 100 nm. Optical Characterization: All the optical experiments are performed on fibers of comparable diameter. Confocal fluorescence images are acquired by an A1R MP confocal system (Nikon), coupled to an inverted microscope (Eclipse Ti, Nikon). The fibers are excited by an Ar+ ion laser (λexc = 488 nm) through an oil-immersion objective with numerical aperture of 1.4. The emission is measured by a spectral detection unit equipped with a multi-anode photomultiplier (Nikon). The photoluminescence absolute quantum efficiency (η, that is the number of PL photons emitted per absorbed photon), of films and fibers is measured by placing samples in an integrating sphere (LabSphere), exciting by a cw laser system (Coherent Verdi V10, λexc = 532 nm) and detecting emission by a fiber-coupled spectrometer (Ocean Optics USB 4000). All the measured spectra are corrected by the spectral response of the used experimental setup (integrating sphere, optical fiber and spectrometer), determined by means of a calibrated lamp. η is measured to be 53% for nanofibers, which is higher than the corresponding value in thin films by more than 20%. Angle-resolved PL experiments are carried out by an optical fiber mounted on a rotation stage, allowing the emitted light in a small solid angle (105 dB/m), in order to excite samples over a length of many mm this scheme is more convenient than end-pump configurations used for fiber lasers. End-pump methods based on evanescent coupling have been instead employed with organic nanofibers for exciting spontaneous emission.[31] The stimulated emission and lasing signal from single
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Experimental Section
excited fibers are here collected by the objective and through a dichroic mirror and an external lens coupled into an optical fiber, and then spectrally analyzed by a monochromator (Yobin Yvon, iHR320, grating with 1200 grooves/mm) using a Peltier-cooled charge-coupled device (CCD, Yobin Yvon, Symphony). The spectral resolution of the optical system is approximately 0.1 nm. For gain and reference film lasing characterization, samples are pumped by a rectangular excitation stripe of 2 mm × 300 µm, under vacuum at ∼10−4 mbar. FDTD Calculations: FDTD simulations are performed with the Lumerical[32] software. The time-dependent Maxwell's equations are solved by using a computational grid with absorbing perfectly matched layers boundary conditions and an auto non-uniform mesh type. To model the actual experimental geometries, a thickness (h) of 450 nm and of 450–600 nm is assumed for deposited fibers and for films, respectively, together with a period, Λ = 600 nm, a pattern duty cycle of 50% and a depth of the imprinted features of 100 nm for both the structures. All the simulated structures are centred in the point (0, 0, 0) of our coordinates system. Corners are not assumed to be sharp, but instead present a finite curvature radius (rc = 0.2 h), in order to simulate the partial sample flattening observed upon imprinting. The size of the polymeric region along the z-axis (fiber longitudinal axis) is 15 µm whereas the size along the x-axis (fiber width) is of 1 µm and 15 µm, for the fiber and the film, respectively. All the materials are approximated as lossless with a real refractive index of 1.54, and lye on a quartz substrate (n = 1.46). Frequency domain power monitors are used to record the mode profile over the simulation region under the first feature of the pattern. The maps in Figure 4 and in Figures S4-S8 show the real part, Re(Sz), of the the z-component of the Poynting vector (W/m2), normalized to the source power (W).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements We gratefully thank Dr. Lee Carroll and Prof. Lucio C. Andreani for the high-level assistance provided on the use of the code and for the precious scientific contribution given to the work. Dr. Salvatore Girardo is also acknowledged for the realization of some nanofiber samples. The authors acknowledge the financial support from the Italian Minister of University and Research through the FIRB project RBFR08DJZI “Futuro in Ricerca”, and from the Apulia Regional Projects ‘Networks of Public Research Laboratories’, Wafitech (9) and M. I. T. T. (13). Received: April 30, 2014 Revised: June 7, 2014 Published online:
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