APPLIED PHYSICS LETTERS 91, 031109 共2007兲
Amplified spontaneous emission from dye-doped polymer film sandwiched by two opal photonic crystals Feng Jin and Yang Song Laboratory of Organic NanoPhotonics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China and Graduate School of Chinese Academy of Sciences, Beijing 100080, China
Xian-Zi Dong, Wei-Qiang Chen, and Xuan-Ming Duana兲 Laboratory of Organic NanoPhotonics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China
共Received 31 December 2006; accepted 4 July 2007; published online 20 July 2007兲 The authors observed amplified spontaneous emission 共ASE兲 from dye-doped polymer film sandwiched by two opal photonic crystals 共PhCs兲. The ASE effect occurred at 599 nm with a full width at half maximum of about 5.1 nm, which corresponds to the L-point gap edge of the opal PhCs. Photoluminescence lifetimes of both dye-doped polymer films with and without opal PhCs were measured and corroborated that the ASE of the dye-doped polymer film emission is due to the presence of the photonic stop band. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2766652兴 Photonic crystals 共PhCs兲 are materials with periodic dielectric structure which hold a forbidden region for electromagnetic waves1 and have drawn much interest for their many potential applications2–7 since their inception. The possibility for modulating spontaneous emission of light emitters by using the photonic band gap 共PBG兲 of PhCs is of particular interest. At the photonic band edges, light propagates at reduced group velocities owing to resonant Bragg scattering, which can enhance optical gain leading to stimulated emission.8 Optical gain enhancement is of significant importance in the development of solid-state lasers, since it could open the possibility to fabricate low threshold, even thresholdless lasers.3,9,10 To date, numerous reports have confirmed optical gain enhancement by using one-,11 two-,12 and three-dimensional13 共3D兲 PhCs. Among 3D-PhCs, artificial opals constructed by silica nanospheres packed in a face-centered cubic 共fcc兲 lattice are attractive due to their ease of fabrication and the tunability of their PBG in a wide region from ultraviolet 共UV兲 to near infrared, which is achieved via controlling the size of the silica nanospheres which comprise the PhC. Although only a pseudo-PBG can be obtained in opal structures, there is a distinct L-point gap in opal PhCs forbidding light propagation in the 共111兲 direction, which can be utilized to manipulate the spontaneous emission of light emitters. Recently, opal PhCs have been infiltrated with fluorescent dyes14 or semiconductor nanocrystals13,15 and exhibited optical gain enhancement effects. However, there has been little investigation on the optical gain enhancement effect of a gain medium sandwiched by two opal PhCs, although it is expected to show better effect for inhibited spontaneous emission than that with infiltration. In a previous study,16 we demonstrated the lasing emission of a cavity constructed from an allyl-fluorescein doped FAX: ⫹86-10-82543597; electronic mail:
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polymer film and two opal PhCs, and observed lasing output at an extraordinary angle which was attributed to the superprism effect of the opal PhCs. In this letter, we demonstrate the amplified spontaneous emission 共ASE兲 of rhodamine B 共RhB兲-doped polymer film sandwiched by two opal PhCs. To prepare the gain medium, RhB and polymethyl methacrylate 共PMMA兲 were dissolved in chloroform and spin coated onto a glass substrate to form a thin film. The ratio of RhB to PMMA is 3.0 wt %. The absorption and photoluminescence 共PL兲 spectra of RhB-doped PMMA film with a thickness of about 5 m are shown in Fig. 1. The RhBdoped polymer film shows maximum absorption at 552 nm. The PL emission band of the RhB-doped PMMA film exhibits a peak at 580 nm with a full width at half maximum 共FWHM兲 of 35 nm under the excitation wavelength of 532 nm. It is clear that there is an overlap between the absorption band and the PL band in the spectral range of 530– 590 nm. The optical gain is low in this spectral range due to the significant self-absorption, which comes from a high absorption coefficient combined with a high concentra-
FIG. 1. UV-visible absorption 共dashed兲 and photoluminescence 共solid兲 spectra of the RhB-doped PMMA film under excitation wavelength of 532 nm. The inset shows the chemical structure of RhB.
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FIG. 2. 共Color online兲 共a兲 Transmission spectrum of the opal PhC, relative to the normal of 共111兲 planes of opal PhCs. The inset shows the SEM image and digital camera photo of the opal PhC, and the scale bar is 2 m. 共b兲 Schematic configuration of the “sandwich” structure. Incident angle and detection angle are 50° and 0°, relative to the normal of 共111兲 planes of the opal PhCs.
tion of RhB. Consequently, ASE at wavelengths above 590 nm can be expected, where absorption losses are small and the gain is relatively high. Opal PhCs were prepared by the same method as reported in our previous study.16 The opal PhCs were formed by monodisperse silica nanospheres with a diameter of about 250 nm. A scanning electron microscopy 共SEM兲 image showing the cross section of the opal PhCs is presented in the inset of Fig. 2共a兲. It is clear that the silica nanospheres have formed an fcc close-packed lattice structure oriented in the 共111兲 direction on the top surface of the substrate. The opal PhCs show a brilliant green color under white light illumination owing to Bragg diffraction from the 共111兲 crystal planes, as shown in the inset of Fig. 2共a兲. The transmission spectrum of opal PhCs characterizes the optical stop band structure, as shown in Fig. 2共a兲. At normal incidence, the opal PhCs exhibit a minimum transmissivity of 7% at the wavelength of 553 nm. The pronounced attenuation and emergent stop band correspond to the L-point band gap of the opal PhCs and coincides with the emission band of the gain medium. An overlap between the stop band of the opal PhCs and the emission band of the gain medium is necessary for the excitation of stimulated emission, which leads to gain enhancement due to the enhanced density of states at the band gap edges.17 A RhB-doped PMMA film with a thickness of 5 m was sandwiched by a pair of opal PhCs of about 2 m thickness
Appl. Phys. Lett. 91, 031109 共2007兲
FIG. 3. 共a兲 Emission spectra of the RhB-doped PMMA film without opal PhCs pumped by 122.7 J/pulse at 532 nm 共A兲 and RhB-doped PMMA film with opal PhCs pumped by 48 and 111.5 J/pulse 共B and C兲 as well as the transmission spectrum of the opal PhC. 共b兲 Emission spectra of RhB-doped PMMA film with opal PhCs at various pumping energies. The inset shows output emission intensities 共squares兲 of RhB-doped PMMA film and the FWHM 共circles兲 of the ASE spectra under various pumping intensities.
to form the “sandwich” structure, as shown in Fig. 2共b兲. The L-point gap of the opal PhCs was utilized to modify the spontaneous emission of the RhB-doped PMMA film in the sandwiched structure. Because of the overlap between the stop band of the opal PhCs and the emission band of the gain medium, emitted light at the wavelength range corresponding to the L-point gap is reflected back into the gain medium. The self-absorption of spontaneous emission causes reemission at longer wavelengths corresponding to a lower emission band of RhB. Simultaneously, at the edge of the L-point gap, light propagates at reduced group velocities owing to resonant Bragg scattering, which can enhance optical gain, facilitating the development of ASE.15 Consequently, ASE can be expected at the PBG edge instead of within the PBG, although the sandwiched structure is likely to provide multiple feedback for emitted light within the PBG. For the ASE measurement, we used an excitation beam produced by a Q-switched frequency-doubled neodymium doped yttrium aluminum garnet pulsed laser operating at 532 nm 共8 ns, 10 Hz兲. The excitation beam was focused to a spot diameter of 100 m at oblique incidence 共inc = 50° 兲 to the 共111兲 planes of the opal PhCs. The optical emission was measured from the opposite side of the excitation beam by using a fiber-coupled grating spectrometer 共USB 2000, Ocean Optics兲. Opal PhCs show a significant effect on the emission spectra of RhB-doped PMMA film when comparing to bare RhB-doped PMMA film. Figure 3共a兲 shows the laser pumped emission spectra of RhB-doped PMMA films with and with-
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FIG. 4. 共Color online兲 PL decay lifetime curves for RhB-doped PMMA film without 共red兲 and with 共blue兲 opal PhCs. The data were recorded at 599 nm with excitation wavelength at 532 nm.
out opal PhCs. At low excitation intensity, the RhB-doped PMMA film sandwiched by opal PhCs showed a broad emission band with a peak at about 610 nm 关spectrum B in Fig. 3共a兲兴, which is about 30 nm redshifted compared to that of bare RhB-doped PMMA film 关spectrum A in Fig. 3共a兲兴. With increasing excitation intensity, a remarkably narrow emission peak emerges at 599 nm 关spectrum C in Fig. 3共a兲兴. Figure 3共b兲 further shows pump energy dependence of the RhBdoped polymer film emission spectra sandwiched by opal PhCs. Above the threshold pump intensity of 55.5 J/pulse, the FWHM of the emission peak decreased abruptly from 59 to 5.1 nm, and a simultaneous change in slope of the input-output behavior was observed. This spectral-line narrowing and superlinear growth of the emission intensity indicate the development of ASE. The redshift in the emission spectrum and the ASE of the gain medium originate from the L-point stop band in opal PhCs. Spontaneous emission within the L-point band gap is suppressed and therefore missing from the PL spectrum due to inhibition by the PBG.18,19 Self-absorption of the inhibited emission by dye molecules results in reemission near 610 nm, leading to a redshift of the emission spectrum with a wider emission band. Simultaneously, the rate of decay of emitted light is expected to increase when its frequency matches the photonic band gap edge,20 causing the enhancement of spontaneous emission. ASE of the RhB-doped PMMA film with opal PhCs, which emerges at the L-point band edge of the opal PhC, agrees well with prediction. As a result, the spectrally narrow spontaneous emission from the active medium sandwiched by two opals occurred at a longer wavelength compared to that of the PBG frequency range of opals. To further investigate the effect of the PBG on dye emission, PL lifetime experiments were carried out using single photon count fluorescence spectrometry 共F900, Edinburgh Instrument Co. UK兲. Figure 4 shows PL lifetime data of dye-doped PMMA films with and without opal PhCs. Evaluation of PL decay traces reveals nonexponential decay behavior of the dye ensemble, which can be well described by estimating the decay components using a multiexponential least-squares fitting procedure.21 The biexponential nature of the emission decay is attributed to different subensembles of
dye molecules,22 rather than photonic effects of opal PhCs, because it was observed for samples both with and without opal PhCs. Similar to the phenomena reported by Barth et al.,22 we suppose that only the longer decay component, originating from RhB molecules, will be influenced by the presence of the opal PhCs, while the shorter component 共lifetime of about 1 ns兲 is attributed to molecules undergoing some rapid nonradiative process, e.g., due to a strong dyedye interaction. The longer PL lifetime at 599 nm for a gain medium with opal PhCs is 4.1 ns, which is much shorter than that of RhB-doped PMMA without opal PhCs, at 7.6 ns. The decay rate is accelerated due to the increase of the local density of states at the PBG edge, which is in agreement with the result reported by Lodahl et al.20 This measurement of PL lifetime further corroborates that the enhancement of the dye emission is due to the presence of the photonic stop band. In conclusion, we observed a strong effect of the L-point gap on the spontaneous emission of RhB-doped PMMA film sandwiched by two opal PhCs. ASE of the gain medium was observed at 599 nm with a FWHM of 5.1 nm. PL lifetime experiments corroborated that the ASE of the dye emission is due to the presence of the photonic stop band. This work leads toward solid-state laser emission from small structures, which can benefit from the easy manipulation of both dye emission properties and PhC band positions. E. Yablonovitch, Phys. Rev. Lett. 58, 2059 共1987兲. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O’Brien, P. D. Dapkus, and I. Kim, Science 284, 1819 共1999兲. 3 J. Yoon, W. Leea, J. M. Caruge, M. Bawendi, E. L. Thomasb, S. Kooi, and P. N. Prasad, Appl. Phys. Lett. 88, 091102 共2006兲. 4 Y. Y. Li, F. Cunin, J. R. Link, T. Gao, R. E. Betts, S. H. Reiver, V. Chin, S. N. Bhatia, and M. J. Sailor, Science 299, 2045 共2003兲. 5 J. H. Holtz and S. A. Asher, Nature 共London兲 389, 829 共1997兲. 6 E. Chow, S. Y. Lin, J. R. Wendt, S. G. Johnson, and J. D. Joannopoulos, Opt. Lett. 26, 286 共2001兲. 7 J. Dong and Q. W. Ping, J. Phys. Chem. B 110, 16823 共2006兲. 8 K. Yoshino, S. B. Lee, S. Tatsuhara, Y. Kawagishi, M. Ozaki, and A. A. Zakhidov, Appl. Phys. Lett. 73, 3506 共1998兲. 9 J. D. Joannopoulos, R. Meade, and J. Winn, Photonic Crystals, Molding the Flow of Light 共Princeton University Press, Princeton, NJ, 1995兲. 10 H. Hirayama, T. Hamano, and Y. Aoyagi, Appl. Phys. Lett. 69, 791 共1996兲. 11 V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, Opt. Lett. 23, 1707 共1998兲. 12 M. Meier, A. Mekis, A. Dodabalapur, A. Timko, R. E. Shusher, J. D. Joannopoulos, and O. Nalamasu, Appl. Phys. Lett. 74, 7 共1999兲. 13 Y. A. Vlasov, K. Luterova, I. Pelant, B. Honerlage, and V. N. Astratov, Appl. Phys. Lett. 71, 1616 共1997兲. 14 M. N. Shkunov, Z. V. Vardeny, M. C. DeLong, R. C. Polson, A. A. Zakhidov, and R. H. Baughman, Adv. Funct. Mater. 12, 21 共2002兲. 15 G. R. Maskaly, M. A. Petruska, J. Nanda, I. V. Bezel, R. D. Schaller, H. Htoon, J. M. Pietryga, and V. I. Klimov, Adv. Mater. 共Weinheim, Ger.兲 18, 343 共2006兲. 16 F. Jin, C.-F. Li, X.-Z. Dong, W.-Q. Chen, and X.-M. Duan, Appl. Phys. Lett. 89, 241101 共2006兲. 17 K. Sakoda, Opt. Express 4, 167 共1999兲. 18 F. Fleischhaker, A. C. Arsenault, J. Schmidtke, R. Zentel, and G. A. Ozin, Chem. Mater. 18, 5640 共2006兲. 19 E. Palacios-Lidón, J. F. Galisteo-López, B. H. Juárez, and C. López, Adv. Mater. 共Weinheim, Ger.兲 16, 341 共2004兲. 20 P. Lodahl, A. F. V. Driel, I. S. Nikolaev, A. Irman, K. Overgaag, D. Vanmaekelbergh, and W. L. Vos, Nature 共London兲 430, 654 共2004兲. 21 J. Enderlein and R. Erdmann, Opt. Commun. 134, 371 共1997兲. 22 M. Barth, A. Gruber, and F. Cichos, Phys. Rev. B 72, 085129 共2005兲. 1 2
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