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Random lasing from dye doped polymer within biological source scatters: The pomponia imperatorial cicada wing random nanostructures
Organic Electronics 13 (2012) 2342–2345
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Random lasing from dye doped polymer within biological source scatters: The pomponia imperatorial cicada wing random nanostructures Dingke Zhang a,⇑, Gorgi Kostovski a, Christian Karnutsch b, Arnan Mitchell a a b
Microplatforms Research Group, School of Electrical and Computer Engineering, RMIT University, G.P.O. Box 2476, Melbourne, Victoria 3001, Australia Department of Electrical Engineering and Information Technology, University of Applied Sciences, Karlsruhe Moltkestrasse 30, D-76133 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Received 11 May 2012 Accepted 9 June 2012 Available online 6 July 2012 Keywords: Random lasing action Cicada Nanopillars Random scattering media
a b s t r a c t Photonic structures found in biological organisms are often startling in their complexity and surprising in their optical function. In this paper we explore whether biologically derived nanostructures can be utilized to form the resonator structures of organic dye doped polymer lasers. Surprisingly, we find that the random nanostructures on the wing of the pomponia imperatoria cicada can support coherent random lasing when covered with a layer of dye doped polymer film. Due to the scattering role of cicada wing nanostructures, the device emits a resonant multimode peak centered at a wavelength of 605 nm with a mode linewidth of <0.55 nm and exhibits a threshold excitation intensity as low as 70.4 mW/cm2. Our results indicate that abundant, naturally occurring biological nanostructures can provide effective platforms for the study of random lasing, and that the laser properties may provide insight into the degree of disorder exhibited by these natural structures. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Unlike typical lasing action which occurs within carefully configured resonant cavities, random lasers are the simplest sources of stimulated emission, occurring without a cavity, with feedback provided by a random scattering media [1]. To achieve resonance in a field of random scatterers, a sequence of multiple light scattering events must occur to return a photon to its original position and to achieve constructive interference, which leads to resonant localization of light. Furthermore, if the system has optical gain, this resonance can convert to oscillation and lasing action may build up within each loop path [2]. Since the concept of random lasers was originally proposed by Letokhov and was demonstrated in organic systems by Lawandy et al. in 1994 [3], random lasers have received much attention. Various applications including material labeling and tumor detection have been proposed in recent ⇑ Corresponding author. E-mail address: [email protected] (D. Zhang). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.029
years [4,5]. Recent research in self organizing semiconductor nanostructures has allowed observation of random lasing in semi-ordered low-dimensional semiconductor systems, such as ZnMgO nanoneedles [6], ZnO nanosheets [7], GaN nanocolumns [8], and SnO2 nanowires [9]. However, millenia before humans were artificially synthesizing nanostructures, biological systems were using nanometerscale architectures to produce striking optical effects. Recently, the complex optical properties of various biological structures have been systematically studied and modeled [10,11] and potential applications, such as surface-enhanced Raman scattering [12,13], solar cells and light emitting diodes [14], have been explored. However, to our knowledge, these abundant natural nanostructures have yet to be utilized for random lasers. In this work, we show that biological nanostructures embedded within an organic dye doped polymer can serve as a convenient platform for the exploration of random lasing action. Here, the nano-structure on a cicada wing (Pomponia Imperatoria, as shown in Fig. 1a) was selected as the random scattering media. With a wingspan in excess
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D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
Fig. 1. (a) Photograph of the pomponia imperatoria cicada, (b and c) SEM micrograph of the nanopillars present on the wings of this insect in different magnification. The inset is a schematic diagram showing the formation of a closed-loop path for light through multiple optical scattering by cicada wing nanopillars.
of 12 cm, these cicadas provide ample nanostructured surface area. This nanostructure consists of a two dimensional array of nanopillars which have a centre–centre separation of approximately 200 nm, and a height in excess of 300 nm, as shown in the scanning electron microscope images (SEM) in Fig. 1(b–c). The distribution of nanofeatures is roughly hexagonal within small domains, but is disordered across large areas, suggesting that they may be appropriate as a two dimensional field of scatterers for the observation of random lasing action.
SU8 film was measured as 1.6. The refractive indexes of the cicada wings and glass substrate were about 1.5 [15] and 1.52, respectively. The experimental setup to investigate the lasing action followed Ref. [16]. The pump source was a pulsed neodymium-doped yttrium aluminum garnet (Nd-YAG) laser and an optical parametric oscillator (OPO) from Spectra Physics, delivering 2 ns pulses at 532 nm with a 10 Hz repetition rate. The output pulse energy of the pump laser was controlled using a polarizer and a beam splitter for coarse attenuation and fine tuning was achieved with neutral density filters. An adjustable slit and a cylindrical lens were used to shape the beam into a narrow stripe with a continuously variable length on the sample film. The films were pumped at normal incidence with the long axis of the pump beam perpendicular to the edge of the sample. The output signals were detected by fiber-coupled charge-coupled device (CCD) spectrometer (Ocean Optics HR4000). The pumped energies from the laser were measured using a laser power and energy meter from Newport. For comparison, the photoluminescence (PL) was also measured using a continuous wave (CW) 532 nm laser.
3. Results and discussion Fig. 2 shows the emission spectra of the untextured DCM:SU8 film on the glass substrate produced by pumping with a CW laser and pumping with the 2 ns pulsed laser. The emission spectrum pumped by the CW laser exhibits a broad peak at 600 nm which originates from PL emission of the DCM molecules, whereas the spectrum produced by 2 ns pulsed pumping is dramatically narrowed to a 26 nm half-width peak at 613 nm. The laser pulses create a significant population inversion within the DCM and thus the narrow emission spectrum can be attributed to amplified spontaneous emission (ASE) [17].
In our experiments, the spin-coating method was used to apply the dye doped polymer film directly onto a segment of cicada wing. The gain material 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) was first added to SU8 thinner and fully dissolved by the assistance of ultrasonic agitation. Then polymer SU8-2005 was added into the dissolved DCM solution. The ratio of DCM to SU8 solids was 1.5% by weight. The DCM doped SU8 solution was then spin-coated onto the cicada wing segment with a thickness of 2.5 lm (500 rpm for 5 s in spread cycle and 2000 rpm for 30 s in spin cycle). The SU8 was soft-baked on a hotplate at 65 °C for 1 min, 90 °C for 2 min and 65 °C for another 1 min to dispel solvents. It was then cured using a 30 s UV exposure, followed by further baking on a hotplate at 65 °C for 1 min, 90 °C for 2 min and 65 °C for another 1 min. For comparison, a planar DCM doped SU8 film was prepared with the same dye concentration and film thickness by spin-coating onto an untextured glass substrate. Using a prism coupler (Metricon 2010), the refractive index of the DCM doped
Normalized Output Intensity (a.u.)
2. Experimental section 1.0
PL spectrum ASE spectrum lasing spectrum
Δλ =0.962nm
0.8 0.6 595
600
605
610
615
620 625
0.4 0.2 0.0 550
600
650
700
Wavelength (nm) Fig. 2. The emission spectra of the DCM:SU8 waveguide pumped by 532 nm constant laser (triangle) and 532 nm pulsed laser (star) and the emission spectrum of the cicada wing nanopillar scattered DCM:SU8 film pumped by 532 nm pulsed laser (line). The inset shows a portion of the spectrum of the cicada wing nanopillar scattered DCM:SU8 film under higher magnification.
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D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
The emission spectrum from the nanotextured DCM:SU8 embedded cicada wing with 2 ns pumping is also presented in Fig. 2. This exhibits a further narrowed spectrum with discrete peaks emerging. The inset of Fig. 2 presents a magnified view of these discrete peaks. The separated mode linewidth is less than 0.55 nm, which is 47 times smaller than that of the ASE peak of the DCM:SU8 waveguide. The significantly narrowed linewidth and discrete peaks suggests that this emission may be the result of random lasing action. Because the gain media possess the same concentration and thickness for both samples, the obvious difference in emissive actions between DCM:SU8 film on glass and the nanotextured DCM:SU8 embedded cicada wing must be due to the presence of the nanopillar scattering centers on the cicada wing. As described previously, to achieve random lasing, photons will undergo multiple scattering events with some non-zero probability of being returned to their starting point, forming a closed loop. Along different paths, the probability of a photon being scattered back to its starting point is different. There are many such closed-loop paths in the nanotextured DCM:SU8 embedded cicada wing. A potential example of such a closed loop path is illustrated in Fig. 1(d) [5,18]. The path which the photons traverse must be an integer number of wavelengths. To achieve constructive interference and to achieve lasing action, they must accumulate sufficient gain to overcome the losses incurred by scattering. The lasing threshold phenomenon is one the most important properties to confirm lasing action. Similar to regular lasers, random lasers are characterized by a dramatic change of their behavior above the threshold. Fig. 3 shows the output emission intensity integrated over all wavelengths as a function of the pump intensity of the nanotextured DCM:SU8 embedded cicada wing. An abrupt change in the slope, followed by a linear increase in the output signal as the excitation energy is further increased, is clearly observed and this can be interpreted as a lasing
threshold. The threshold pump energy for random lasing can be easily discerned at 107 lW, corresponding to a real energy density of 70.4 mW/cm2 (pump stripe is 19 800 lm). The inset of Fig. 3 shows the evolution of the side emission spectra at different pump energies. At low excitation intensity, the spectrum consists of a single broad spontaneous emission peak. As the pump power increases, the emission peak becomes narrower and stronger due to the preferential amplification at frequencies close to the maximum of the gain spectrum. As the pump power increases further above the threshold, the integrated emission intensity increases much more rapidly with the pump power and more sharp peaks appear. This series of discrete peaks provides clear evidence for the presence of random lasing action on multiple distinct localized closed loops. The occurrence of the threshold further proves the lasing action and provides strong support to the scattering role of the cicada wing random nanostructures. Our samples show different lasing spectra at different viewing angles with respect to the planar emission direction. Fig. 4 displays the angular distribution of the emitted light of our nanotextured DCM:SU8 embedded cicada wing sample. It is noted that the random lasing emission is clearly seen from a range of viewing angles within the plane of the wing. This is reasonable since there are many hexagonal domains with a distribution of pillar height, width and period. Moreover, the cylinders on the cicada wing are not strictly vertical or parallel to each other and the light generated by the DCM undergoes scattering at the cylinder boundaries and forms different closed-loop paths randomly; thus light can potentially be scattered in many directions. In addition, the emitted light from the DCM is spatially confined in some angular range around the edge emission, which is due to the semi-ordered nanostructures from the wings of cicadas, reducing the randomness of the cicada system. A polarizer in the direction perpendicular [transverse electric (TE)] and parallel [transverse magnetic (TM)] to the sample surface was also used to analyze the polarization properties of the lasing light. The inset of Fig. 4 plots output emission intensity as a function of collection angles for both TE and TM polarizations.
12000
Output Intensity(a.u.)
Excitation θ
10000 8000 6000 4000 2000 0 0
Fig. 3. Output emission intensity integrated over all wavelengths as a function of the pumped intensity for the cicada wing nanopillar scattered DCM:SU8 film. The inset is the edge emission spectra at corresponding pump intensities.
10
20
30
40
50
60
O
Viewing angle ( ) Fig. 4. Output emission intensity of the cicada wing nanopillar scattered DCM:SU8 film as a function of viewing angles.
12000 9000 6000 3000 0
1 cm
Output Intensity (a.u.)
D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
0.8 cm
*15
0.3 cm 580
590
600
610
620
630
640
Wavelength (nm) Fig. 5. The emission spectra with different excitation area at constant pump intensity of 10 mW.
Similar with the angle dependence of the random lasing emission, the integrated intensity for both polarizations decreases with the reduction of the viewing angle and the decrease is faster for TE than TM. The increased directionality of the TE polarized emission suggests that the oscillation of this polarization is more coherent, which can be attributed to the predominantly vertical orientation of the nanopillars on the cicada wing providing stronger and more coherent feedback. However, a direct comparison between the two cases may not be appropriate, as the output intensity is also dependent on the excitation volume and the cicada wing planarity can vary over a large area. To further explore the nature of the observed random lasing, the dependence of emission properties on the excitation area was studied. Fig. 5 shows the variation of emission spectra with excitation area (from 5.7 10 4 cm2 to 19 10 4 cm2) at a constant pump power density of 790 mW/cm2. For small areas, no sharp lasing peak is observed. However, when the excitation area increases, sharp lasing peaks appear. The number of sharp lasing peaks increases with further increase of the excitation area. This behavior is characteristic of random lasing and can be explained as follows. In a small excitation area, the light formed by the closed-loop paths is not strong enough and hence the gain experienced by the recirculating photons will be too weak to compensate for the scattering experienced along the closed loop, so laser oscillation is not established. As the pump area increases, it is possible first for a single oscillation loop to exist and then multiple oscillating closed-loop paths can be formed, which results
2345
in the each corresponding to a discrete lasing peak observed in the emission spectrum. These observations of the dependence of lasing action on illumination area provide further evidence of random lasing. 4. Conclusion and future work We have observed random lasing action within a two dimensional field of scatterers provided by the nanostructures on a cicada wing which has been coated by organic dye doped polymer. The observed random lasing action can be attributed to the cicada wing’s semi-ordered nanopillar patterns distributed over a large area. The dependence of random lasing emission on the collecting angle and excitation area agrees well with the random laser theory. Our results indicate that the biologically derived cicada wing is an attractive candidate to realize organic random lasers. Further investigations are currently underway to investigate the effect of local order and long-range disorder on the random lasing behavior and also to compare these biologically derived scatterers with deliberately engineered nano-pillar arrays. References [1] V.S. Letokhov, Sov. Phys. JETP 26 (1968) 835–840. [2] D.S. Wiersma, P. Bartolini, A. Lagendijk, R. Righini, Nature (London) 390 (1997) 671–673. [3] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature (London) 368 (1994) 436–438. [4] D.K. Zhang, Y.P. Wang, D.G. Ma, Appl. Phys. Lett. 91 (2007) 091115– 091117. [5] H. Cao, Waves Random Media 13 (2003) R1–R39. [6] H.Y. Yang, S.P. Lau, S.F. Yu, M. Tanemura, T. Okita, H. Hatano, K.S. Teng, S.P. Wilks, Appl. Phys. Lett. 89 (2006) 081107–081109. [7] D.K. Zhang, S.J. Chen, Z.Q. Deng, Y.P. Wang, Y.C. Liu, D.G. Ma, J. Nanosci. Nanotech. 10 (2010) 6744–6747. [8] M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, K. Kishino, Appl. Phys. Lett. 97 (2010) 151109–151111. [9] H.Y. Yang, S.F. Yu, S.P. Lau, S.H. Tsang, G.Z. Xing, T. Wu, Appl. Phys. Lett. 94 (2009) 241121–241123. [10] L. Plattner, J. R. Soc. Interface 1 (2004) 49–59. [11] J.P. Vigneron, J.F. Colomer, N. Vigneron, V. Lousse, Phys. Rev. E 72 (2005) (1909) 061904–61906. [12] G. Kostovski, D.J. White, A. Mitchell, M.W. Austin, P.R. Stoddart, Biosens. Bioelectron. 24 (2009) 1531–1535. [13] G. Kostovski, C. Udayakumar, S. Jayawardhana, P.R. Stoddart, A. Mitchell, Adv. Mater. 23 (2011) 531–535. [14] S.H. Hong, J.H. Wang, H. Lee, Nanotechnology 20 (2009) 385303– 385307. [15] M. Sun, A. Liang, Y. Zheng, G.S. Watson, J.A. Watson, Bioinsp. Biomim. 6 (2011) 026003–026012. [16] W. Lu, B. Zhong, D.G. Ma, Appl. Opt. 43 (2004) 5074–5078. [17] D.K. Zhang, S.M. Zhang, D.G. Ma, Gulimina, X.T. Li, Appl. Phys. Lett. 89 (2006) 231112–231114. [18] Z.Q. Zhang, Phys. Rev. B 52 (1995) 7960–7964.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Random lasing from dye doped polymer within biological source scatters: The pomponia imperatorial cicada wing random nanostructures Dingke Zhang a,⇑, Gorgi Kostovski a, Christian Karnutsch b, Arnan Mitchell a a b
Microplatforms Research Group, School of Electrical and Computer Engineering, RMIT University, G.P.O. Box 2476, Melbourne, Victoria 3001, Australia Department of Electrical Engineering and Information Technology, University of Applied Sciences, Karlsruhe Moltkestrasse 30, D-76133 Karlsruhe, Germany
a r t i c l e
i n f o
Article history: Received 11 May 2012 Accepted 9 June 2012 Available online 6 July 2012 Keywords: Random lasing action Cicada Nanopillars Random scattering media
a b s t r a c t Photonic structures found in biological organisms are often startling in their complexity and surprising in their optical function. In this paper we explore whether biologically derived nanostructures can be utilized to form the resonator structures of organic dye doped polymer lasers. Surprisingly, we find that the random nanostructures on the wing of the pomponia imperatoria cicada can support coherent random lasing when covered with a layer of dye doped polymer film. Due to the scattering role of cicada wing nanostructures, the device emits a resonant multimode peak centered at a wavelength of 605 nm with a mode linewidth of <0.55 nm and exhibits a threshold excitation intensity as low as 70.4 mW/cm2. Our results indicate that abundant, naturally occurring biological nanostructures can provide effective platforms for the study of random lasing, and that the laser properties may provide insight into the degree of disorder exhibited by these natural structures. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Unlike typical lasing action which occurs within carefully configured resonant cavities, random lasers are the simplest sources of stimulated emission, occurring without a cavity, with feedback provided by a random scattering media [1]. To achieve resonance in a field of random scatterers, a sequence of multiple light scattering events must occur to return a photon to its original position and to achieve constructive interference, which leads to resonant localization of light. Furthermore, if the system has optical gain, this resonance can convert to oscillation and lasing action may build up within each loop path [2]. Since the concept of random lasers was originally proposed by Letokhov and was demonstrated in organic systems by Lawandy et al. in 1994 [3], random lasers have received much attention. Various applications including material labeling and tumor detection have been proposed in recent ⇑ Corresponding author. E-mail address: [email protected] (D. Zhang). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.06.029
years [4,5]. Recent research in self organizing semiconductor nanostructures has allowed observation of random lasing in semi-ordered low-dimensional semiconductor systems, such as ZnMgO nanoneedles [6], ZnO nanosheets [7], GaN nanocolumns [8], and SnO2 nanowires [9]. However, millenia before humans were artificially synthesizing nanostructures, biological systems were using nanometerscale architectures to produce striking optical effects. Recently, the complex optical properties of various biological structures have been systematically studied and modeled [10,11] and potential applications, such as surface-enhanced Raman scattering [12,13], solar cells and light emitting diodes [14], have been explored. However, to our knowledge, these abundant natural nanostructures have yet to be utilized for random lasers. In this work, we show that biological nanostructures embedded within an organic dye doped polymer can serve as a convenient platform for the exploration of random lasing action. Here, the nano-structure on a cicada wing (Pomponia Imperatoria, as shown in Fig. 1a) was selected as the random scattering media. With a wingspan in excess
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D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
Fig. 1. (a) Photograph of the pomponia imperatoria cicada, (b and c) SEM micrograph of the nanopillars present on the wings of this insect in different magnification. The inset is a schematic diagram showing the formation of a closed-loop path for light through multiple optical scattering by cicada wing nanopillars.
of 12 cm, these cicadas provide ample nanostructured surface area. This nanostructure consists of a two dimensional array of nanopillars which have a centre–centre separation of approximately 200 nm, and a height in excess of 300 nm, as shown in the scanning electron microscope images (SEM) in Fig. 1(b–c). The distribution of nanofeatures is roughly hexagonal within small domains, but is disordered across large areas, suggesting that they may be appropriate as a two dimensional field of scatterers for the observation of random lasing action.
SU8 film was measured as 1.6. The refractive indexes of the cicada wings and glass substrate were about 1.5 [15] and 1.52, respectively. The experimental setup to investigate the lasing action followed Ref. [16]. The pump source was a pulsed neodymium-doped yttrium aluminum garnet (Nd-YAG) laser and an optical parametric oscillator (OPO) from Spectra Physics, delivering 2 ns pulses at 532 nm with a 10 Hz repetition rate. The output pulse energy of the pump laser was controlled using a polarizer and a beam splitter for coarse attenuation and fine tuning was achieved with neutral density filters. An adjustable slit and a cylindrical lens were used to shape the beam into a narrow stripe with a continuously variable length on the sample film. The films were pumped at normal incidence with the long axis of the pump beam perpendicular to the edge of the sample. The output signals were detected by fiber-coupled charge-coupled device (CCD) spectrometer (Ocean Optics HR4000). The pumped energies from the laser were measured using a laser power and energy meter from Newport. For comparison, the photoluminescence (PL) was also measured using a continuous wave (CW) 532 nm laser.
3. Results and discussion Fig. 2 shows the emission spectra of the untextured DCM:SU8 film on the glass substrate produced by pumping with a CW laser and pumping with the 2 ns pulsed laser. The emission spectrum pumped by the CW laser exhibits a broad peak at 600 nm which originates from PL emission of the DCM molecules, whereas the spectrum produced by 2 ns pulsed pumping is dramatically narrowed to a 26 nm half-width peak at 613 nm. The laser pulses create a significant population inversion within the DCM and thus the narrow emission spectrum can be attributed to amplified spontaneous emission (ASE) [17].
In our experiments, the spin-coating method was used to apply the dye doped polymer film directly onto a segment of cicada wing. The gain material 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) was first added to SU8 thinner and fully dissolved by the assistance of ultrasonic agitation. Then polymer SU8-2005 was added into the dissolved DCM solution. The ratio of DCM to SU8 solids was 1.5% by weight. The DCM doped SU8 solution was then spin-coated onto the cicada wing segment with a thickness of 2.5 lm (500 rpm for 5 s in spread cycle and 2000 rpm for 30 s in spin cycle). The SU8 was soft-baked on a hotplate at 65 °C for 1 min, 90 °C for 2 min and 65 °C for another 1 min to dispel solvents. It was then cured using a 30 s UV exposure, followed by further baking on a hotplate at 65 °C for 1 min, 90 °C for 2 min and 65 °C for another 1 min. For comparison, a planar DCM doped SU8 film was prepared with the same dye concentration and film thickness by spin-coating onto an untextured glass substrate. Using a prism coupler (Metricon 2010), the refractive index of the DCM doped
Normalized Output Intensity (a.u.)
2. Experimental section 1.0
PL spectrum ASE spectrum lasing spectrum
Δλ =0.962nm
0.8 0.6 595
600
605
610
615
620 625
0.4 0.2 0.0 550
600
650
700
Wavelength (nm) Fig. 2. The emission spectra of the DCM:SU8 waveguide pumped by 532 nm constant laser (triangle) and 532 nm pulsed laser (star) and the emission spectrum of the cicada wing nanopillar scattered DCM:SU8 film pumped by 532 nm pulsed laser (line). The inset shows a portion of the spectrum of the cicada wing nanopillar scattered DCM:SU8 film under higher magnification.
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D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
The emission spectrum from the nanotextured DCM:SU8 embedded cicada wing with 2 ns pumping is also presented in Fig. 2. This exhibits a further narrowed spectrum with discrete peaks emerging. The inset of Fig. 2 presents a magnified view of these discrete peaks. The separated mode linewidth is less than 0.55 nm, which is 47 times smaller than that of the ASE peak of the DCM:SU8 waveguide. The significantly narrowed linewidth and discrete peaks suggests that this emission may be the result of random lasing action. Because the gain media possess the same concentration and thickness for both samples, the obvious difference in emissive actions between DCM:SU8 film on glass and the nanotextured DCM:SU8 embedded cicada wing must be due to the presence of the nanopillar scattering centers on the cicada wing. As described previously, to achieve random lasing, photons will undergo multiple scattering events with some non-zero probability of being returned to their starting point, forming a closed loop. Along different paths, the probability of a photon being scattered back to its starting point is different. There are many such closed-loop paths in the nanotextured DCM:SU8 embedded cicada wing. A potential example of such a closed loop path is illustrated in Fig. 1(d) [5,18]. The path which the photons traverse must be an integer number of wavelengths. To achieve constructive interference and to achieve lasing action, they must accumulate sufficient gain to overcome the losses incurred by scattering. The lasing threshold phenomenon is one the most important properties to confirm lasing action. Similar to regular lasers, random lasers are characterized by a dramatic change of their behavior above the threshold. Fig. 3 shows the output emission intensity integrated over all wavelengths as a function of the pump intensity of the nanotextured DCM:SU8 embedded cicada wing. An abrupt change in the slope, followed by a linear increase in the output signal as the excitation energy is further increased, is clearly observed and this can be interpreted as a lasing
threshold. The threshold pump energy for random lasing can be easily discerned at 107 lW, corresponding to a real energy density of 70.4 mW/cm2 (pump stripe is 19 800 lm). The inset of Fig. 3 shows the evolution of the side emission spectra at different pump energies. At low excitation intensity, the spectrum consists of a single broad spontaneous emission peak. As the pump power increases, the emission peak becomes narrower and stronger due to the preferential amplification at frequencies close to the maximum of the gain spectrum. As the pump power increases further above the threshold, the integrated emission intensity increases much more rapidly with the pump power and more sharp peaks appear. This series of discrete peaks provides clear evidence for the presence of random lasing action on multiple distinct localized closed loops. The occurrence of the threshold further proves the lasing action and provides strong support to the scattering role of the cicada wing random nanostructures. Our samples show different lasing spectra at different viewing angles with respect to the planar emission direction. Fig. 4 displays the angular distribution of the emitted light of our nanotextured DCM:SU8 embedded cicada wing sample. It is noted that the random lasing emission is clearly seen from a range of viewing angles within the plane of the wing. This is reasonable since there are many hexagonal domains with a distribution of pillar height, width and period. Moreover, the cylinders on the cicada wing are not strictly vertical or parallel to each other and the light generated by the DCM undergoes scattering at the cylinder boundaries and forms different closed-loop paths randomly; thus light can potentially be scattered in many directions. In addition, the emitted light from the DCM is spatially confined in some angular range around the edge emission, which is due to the semi-ordered nanostructures from the wings of cicadas, reducing the randomness of the cicada system. A polarizer in the direction perpendicular [transverse electric (TE)] and parallel [transverse magnetic (TM)] to the sample surface was also used to analyze the polarization properties of the lasing light. The inset of Fig. 4 plots output emission intensity as a function of collection angles for both TE and TM polarizations.
12000
Output Intensity(a.u.)
Excitation θ
10000 8000 6000 4000 2000 0 0
Fig. 3. Output emission intensity integrated over all wavelengths as a function of the pumped intensity for the cicada wing nanopillar scattered DCM:SU8 film. The inset is the edge emission spectra at corresponding pump intensities.
10
20
30
40
50
60
O
Viewing angle ( ) Fig. 4. Output emission intensity of the cicada wing nanopillar scattered DCM:SU8 film as a function of viewing angles.
12000 9000 6000 3000 0
1 cm
Output Intensity (a.u.)
D. Zhang et al. / Organic Electronics 13 (2012) 2342–2345
0.8 cm
*15
0.3 cm 580
590
600
610
620
630
640
Wavelength (nm) Fig. 5. The emission spectra with different excitation area at constant pump intensity of 10 mW.
Similar with the angle dependence of the random lasing emission, the integrated intensity for both polarizations decreases with the reduction of the viewing angle and the decrease is faster for TE than TM. The increased directionality of the TE polarized emission suggests that the oscillation of this polarization is more coherent, which can be attributed to the predominantly vertical orientation of the nanopillars on the cicada wing providing stronger and more coherent feedback. However, a direct comparison between the two cases may not be appropriate, as the output intensity is also dependent on the excitation volume and the cicada wing planarity can vary over a large area. To further explore the nature of the observed random lasing, the dependence of emission properties on the excitation area was studied. Fig. 5 shows the variation of emission spectra with excitation area (from 5.7 10 4 cm2 to 19 10 4 cm2) at a constant pump power density of 790 mW/cm2. For small areas, no sharp lasing peak is observed. However, when the excitation area increases, sharp lasing peaks appear. The number of sharp lasing peaks increases with further increase of the excitation area. This behavior is characteristic of random lasing and can be explained as follows. In a small excitation area, the light formed by the closed-loop paths is not strong enough and hence the gain experienced by the recirculating photons will be too weak to compensate for the scattering experienced along the closed loop, so laser oscillation is not established. As the pump area increases, it is possible first for a single oscillation loop to exist and then multiple oscillating closed-loop paths can be formed, which results
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in the each corresponding to a discrete lasing peak observed in the emission spectrum. These observations of the dependence of lasing action on illumination area provide further evidence of random lasing. 4. Conclusion and future work We have observed random lasing action within a two dimensional field of scatterers provided by the nanostructures on a cicada wing which has been coated by organic dye doped polymer. The observed random lasing action can be attributed to the cicada wing’s semi-ordered nanopillar patterns distributed over a large area. The dependence of random lasing emission on the collecting angle and excitation area agrees well with the random laser theory. Our results indicate that the biologically derived cicada wing is an attractive candidate to realize organic random lasers. Further investigations are currently underway to investigate the effect of local order and long-range disorder on the random lasing behavior and also to compare these biologically derived scatterers with deliberately engineered nano-pillar arrays. References [1] V.S. Letokhov, Sov. Phys. JETP 26 (1968) 835–840. [2] D.S. Wiersma, P. Bartolini, A. Lagendijk, R. Righini, Nature (London) 390 (1997) 671–673. [3] N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature (London) 368 (1994) 436–438. [4] D.K. Zhang, Y.P. Wang, D.G. Ma, Appl. Phys. Lett. 91 (2007) 091115– 091117. [5] H. Cao, Waves Random Media 13 (2003) R1–R39. [6] H.Y. Yang, S.P. Lau, S.F. Yu, M. Tanemura, T. Okita, H. Hatano, K.S. Teng, S.P. Wilks, Appl. Phys. Lett. 89 (2006) 081107–081109. [7] D.K. Zhang, S.J. Chen, Z.Q. Deng, Y.P. Wang, Y.C. Liu, D.G. Ma, J. Nanosci. Nanotech. 10 (2010) 6744–6747. [8] M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, K. Kishino, Appl. Phys. Lett. 97 (2010) 151109–151111. [9] H.Y. Yang, S.F. Yu, S.P. Lau, S.H. Tsang, G.Z. Xing, T. Wu, Appl. Phys. Lett. 94 (2009) 241121–241123. [10] L. Plattner, J. R. Soc. Interface 1 (2004) 49–59. [11] J.P. Vigneron, J.F. Colomer, N. Vigneron, V. Lousse, Phys. Rev. E 72 (2005) (1909) 061904–61906. [12] G. Kostovski, D.J. White, A. Mitchell, M.W. Austin, P.R. Stoddart, Biosens. Bioelectron. 24 (2009) 1531–1535. [13] G. Kostovski, C. Udayakumar, S. Jayawardhana, P.R. Stoddart, A. Mitchell, Adv. Mater. 23 (2011) 531–535. [14] S.H. Hong, J.H. Wang, H. Lee, Nanotechnology 20 (2009) 385303– 385307. [15] M. Sun, A. Liang, Y. Zheng, G.S. Watson, J.A. Watson, Bioinsp. Biomim. 6 (2011) 026003–026012. [16] W. Lu, B. Zhong, D.G. Ma, Appl. Opt. 43 (2004) 5074–5078. [17] D.K. Zhang, S.M. Zhang, D.G. Ma, Gulimina, X.T. Li, Appl. Phys. Lett. 89 (2006) 231112–231114. [18] Z.Q. Zhang, Phys. Rev. B 52 (1995) 7960–7964.