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Random lasing from dye-Ag nanoparticles in polymer films: Improved lasing performance by localized surface plasmon resonance
Optics & Laser Technology 91 (2017) 193–196
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Random lasing from dye-Ag nanoparticles in polymer films: Improved lasing performance by localized surface plasmon resonance
MARK
⁎
Daisi He, Weiying Bao, Li Long, Peng Zhang, Maohua Jiang, Dingke Zhang
School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Random lasers Ag nanoparticles Localized surface-plasmon resonance Threshold
Random multimode lasers are achieved in 4(dicyanomethy-lene)−2- tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9enyl)4H-pyran (DCJTB) doped Polystrene (PS) thin films by introducing silver nanoparticles (Ag NPs) as scatterers. Ag NPs were prepared by polymer protection method. By optimizing the concentration of reactant of AgNO3 and the ratio of Ag NPs to DCJTB, the devices emit a resonance multimode peak at a center wavelength of 625 nm and the threshold excitation intensity is as low as 0.0031 mJ/pulse. It can be seen that the microscopic random resonance cavities can be formed by multiple scattering of Ag NPs which supply the localized surface-plasmon resonance (LSPR) coupling with the lasing emission to enhance the lasing efficiency. Our results demonstrate that Ag NPs are promising candidate as alternative sources of coherent light emission to realize low-threshold organic random lasers.
1. Introduction Surface plasmons (SPs) are surface charge density oscillations that exist at a metal/dielectric interface. These oscillations, excited by the interaction between light and metal surfaces [1], are known to increase the density of states and the spontaneous emission rate in the semiconductor [2–4] and lead to the enhancement of light emission. They have been extensively used in various applications, such as surface enhanced Raman [5], organic light-emitting diodes (OLEDs) [6], and biosensors. In recent years, the coupling between a SPs and an optically active medium became a hot topic. For example, Noginov et al. [7] provide a major step forward for plasmon-based nanolasers based on a gold core centered in a dye-doped silica shell, where the optical gain is supplied by organic dye molecules embedded in the silica shells and the surface plasmons are provided by the gold cores. In addition, the metallic NPs with periodic arrangement can act as a grating and lead to distributed feedback lasers [8,9]; gold or silver NPs with random distribution were also found to lead to the random lasing by the effects of enhanced localized EM fields and scattering [10–13]. Recently, particular interest has been drawn to the effect of noble metal nanoparticles on the random lasing characteristics [14,15]. In previous reports, the lasing threshold was reduced and its emission was greatly enhanced by the introduction of Ag or Au NPs [16,17]. Recent analysis has shown that a large fraction of light generated in a typical random laser is coupled into SP modes, especially in devices
⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2016.12.036 Received 31 August 2016; Accepted 27 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
incorporating small molecules such as organic dye. Heydari et al. reported the emission enhancement in the plasmon assisted random laser by coupling between the dyes and the localized surfaceplasmon resonance (LSPR) of Au NPs [17]. Dice et al. demonstrated incoherent random lasing from a suspension of Ag NPs in a methanol solution of Rh6G [18]. In these works, noble metal nanoparticles may strongly scatter and absorb visible light due to the LSPR. The electric field can be confined in the vicinity of the surface of metal NPs due to LSPR; as a consequence, such confinement can be very effective for the excitation of active centers to provide high optical gain for laser oscillation [13]. Recent studies show that the scattering properties of metal NPs are very sensitive to particle size and morphology [19]. In order to evaluate the effect of metallic particle size on random lasing of dyes, in this work, the optical pumped random lasing properties were usually investigated in presence of dye-Ag nanoparticles in polymer films. Ag NPs in different sizes were synthesized by the chemical reduction method and introduced into the gain media of the 4(dicyanomethy-lene) −2-tert–butyl −6(1,1,7,7–tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) doped polystrene (PS). The objective of this paper is to study the influence of the excited SPs from the Ag NPs with different sizes on the lasing properties. By optimizing the concentration of the reactant of AgNO3, we found that surfaceplasmon-enhanced lasing emission with reduced thresholds had been obtained.
Optics & Laser Technology 91 (2017) 193–196
D. He et al.
2. Experimental section 2.1. Synthesis of Ag nanoparticles In our experiments, Ag NPs were prepared by polymer protection method. Here, polyvinylpyrrolidone (PVP), as protective agent, were added into the reactant silver nitrate (AgNO3) solution (the molar ratio of PVP to AgNO3 is 1:2). At the temperature of 60 °C, the mixed solution was fully stirred, and then 0.5 mol/L hydrazine hydrate (N2H4·H2O) solution as reducing agent was slowly added. The obtained Ag NPs were washed by alcohol, acetone, ultrapure water in turn, then dried in the room temperature. To determine the effect of the size of Ag NPs on lasing action, we prepared Ag NPs in different sizes by changing the molar concentration of AgNO3 in N2H4·H2O. The concentrations of AgNO3 in N2H4·H2O were changed from 0.01 to 0.6 M, respectively.
2.2. Fabrication of Ag nanoparticles/organic dye random laser Fig. 2. Emission spectra of the neat DCJTB:PS film pumped by 532 nm constant laser (black curve) and 532 nm pulsed laser (red curve) and the emission spectrum of the Ag NPs @ DCJTB:PS film pumped by 532 nm pulsed laser (blue curve). The image shows a portion of the spectrum of the Ag NPs @ DCJTB:PS film under higher magnification. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
DCJTB, as the gain media, was first dissolved in PS by using chloroform as solvent. The optimized weight ratio of DCJTB to PS was kept constant to 4% by weight. Then, Ag NPs, as scattering centers, were added into the DCJTB:PS blend. To avoid the cluster of the Ag NPs, the blend was ultrasounded 10 min and then spincoated onto the quarz substrate to form a Ag NPs dispersed DCJTB:PS film (Ag NPs @ DCJTB: PS). For comparison, a DCJTB:PS film without Ag NPs was also fabricated. Furthermore, to optimize the concentration of Ag NPs, four samples with different ratios of Ag NPs to DCJTB (1:0.5, 1:1, 1:2, and 1:4) were prepared.
3. Results and Discussions Fig. 1a) shows the scanning electron microscopy (SEM) images of the Ag NPs, when the molar concentration of AgNO3 is 0.01 M. It can be seen that a large number of Ag NPs have been successfully synthesized, and mostly in the shape of sphere and the average diameter is 30 nm, when the concentration of AgNO3 is 0.01 M. With the concentration of AgNO3 increasing, the particle size of Ag powder is gradually increased. When the molar concentration of AgNO3 is 0.03 M, the transmission electron microscopy (TEM) image of Ag NPs were measured and shown in Fig. 1b). The average diameter Ag NPs is increased to 100 nm. The increase of AgNO3 concentration leads to the acceleration of reduction reaction. The formation of the crystal nucleus increases at the beginning, and the trends of agglomeration are greater than the diffusion during the formation of the grains. Thus the size of Ag NPs gradually increased. In order to reveal the role of Ag NPs in the DCJTB:PS film, we also prepared the neat DCJTB:PS film for comparison. The emission spectra from the neat DCJTB:PS film and the Ag NPs scattered DCJTB:PS film (the concentration of AgNO3 is 0.03 M and the ratio of Ag NPs to DCJTB is 1:1 by weight) are shown in Fig. 2. The photoluminescence (PL) spectrum exhibits a broad peak at 600 nm, whereas the spectrum excited by 2 ns pulsed pumping is dramatically narrowed to a 19 nm half-width peak at 628 nm. The laser pulses create a significant
2.3. Measurement of random lasing properties The experimental set-up to investigate the laser action followed Ref. [20]. The pump source was a frequency tripled Nd-YAG laser (SpectraPhysics) delivering 2 ns pulses at 532 nm with a 10 Hz repetition rate. The output pulse energy of the pump laser was controlled using neutral density filters. An adjustable slit and a cylindrical lens were used before the beam splitter in order to shape the beam into a narrow stripe with a continuously varied 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 fibercoupled CCD spectrometer (JYSPEX CCD3000). The pumped energies from the laser were measured using a calibrated laser power and energy meter (Gentec). For comparison, the photoluminescence (PL) was also measured using a continuous wave (CW) 532 nm laser. The morphologies of the as-synthesized Ag NPs were characterized by a field emission scanning electron microscope (FE-SEM, TESCAN MARI3) and a transmission electron microscopy (TEM, Libra-200FE).
Fig. 1. a) The scanning electron microscopy (SEM) image of Ag NPs prepared by polymer protection method with the AgNO3 concentration of 0.01 M; b) Transmission electron microscopy (TEM) image of the Ag NPs with the AgNO3 concentration of 0.03 M.
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0.8 0.6 0.4 0.2
620
630
640
Wavelength (nm)
650
tio
Ag
610
NO 3 C on c
en
tra
0.6 0.05 0.03 0.01
n
(M
0.0
)
1.0
Output Intensity (a.u.)
D. He et al.
Fig. 3. Emission spectra of Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M at pumped intensity of 0.07 mJ/pulse.
population inversion within the DCJTB molecules and thus the narrow emission spectrum can be attributed to amplified spontaneous emission (ASE) [21]. The emission spectrum from the Ag NPs @ DCJTB:PS film with 2 ns pulsed laser 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 0.27 nm, which is 70 times smaller than that of the ASE peak of the neat DCJTB:PS film. The significantly narrowed linewidth and discrete peaks suggests that this emission is the result of random lasing action. It is found that the occurrence of random lasing action depends on the ratios of Ag NPs to DCJTB and the size of Ag NPs which depends on the AgNO3 solution concentrations. Fig. 3 shows the emission spectra of Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively. The optically pumping energy is 0.07 mJ/pulse. For the case of Ag NPs synthesized by low AgNO3 molar concentrations (0.01 M or 0.03 M), very narrow multimode peaks emerge in the emission spectrum. However, as the AgNO3 molar concentration increasing, the multimode sharp peaks decreases relatively. When the AgNO3 reaches a high molar concentration (0.6 M), a similar ASE spectrum with some short peaks is observed. The sharp multimode emission should indicate the random lasing action, and obviously depends on the size of Ag NPs. In the random lasing medium, the gain strongly depends on the medium scatters to form closed-loop paths for random lasers [22,23]. Actually, there are many such closed-loop paths in Ag NPs @ DCJTB:PS film, and these loops could serve as ring cavities, thus photons are multiplied by scattering. Because the lasing action in random lasers is dependent on the scattering mean free length, it is concluded that the Ag NPs grown by different reactant concentrations possess different scattering mean free length, thus showing different lasing actions. Fig. 4 gives the comparison of the output emission intensity integrated over all wavelengths as a function of the Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively. The laser thresholds are all clearly observed. There exists an abrupt change in the slope of the output energy versus input energy curves, followed by a linear increase of the output energy as the excitation energy. It can be seen that the threshold pump energies reach ∼0.026 mJ/pulse, ∼0.003 mJ/pulse, ∼0.03 mJ/pulse, and ∼0.6 mJ/pulse, corresponding to a real energy density of 0.325 mJ pulse−1 cm−2, 0.0375 mJ pulse−1 cm−2, 0.375 mJ pulse−1 cm−2 and 7.5 mJ pulse−1 cm−2, respectively. This observation should provide strong support to the formation of random laser resonators in Ag NPs
Fig. 4. Output emission intensity integrated over all wavelengths as a function of the Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively.
Output Intensity (a.u.)
1.0 0.8 0.6 0.4 0.2 0.0
580
600
620
640
660
A
560
g
N
Ps
ra
tio
1:0.5 1:1 1:2 1:4
Wavelength (nm) Fig. 5. Emission spectra of the Ag NPs @ DCJTB:PS film (concentration of AgNO3 solution is 0.03 M) with the different ratios of Ag NPs to DCJTB (1:0.5, 1:1, 1:2, 1:4) at pumped intensity of 0.07 mJ/pulse. The inset is the threshold for the four samples.
@ DCJTB:PS film and the optimum AgNO3 concentration is 0.03 M for the Ag NPs synthesization. Selecting this optimum concentration, the ratio of Ag NPs to DCJTB is also optimized. Fig. 5 shows the spectra of the Ag NPs @ DCJTB:PS film (concentration of AgNO3 solution is 0.03 M) with the different ratios of Ag NPs to DCJTB(1:0.5, 1:1, 1:2, 1:4) at pumped intensity of 0.07 mJ/pulse. It can be seen that the emission spectra of Ag NPs @ DCJTB:PS film are affected by the ratios of Ag NPs. When the ratio of Ag NPs to DCJTB is 1:1, much more narrow sharp peaks are observed. The thresholds of the Ag NPs @ DCJTB:PS film with the different ratios of Ag NPs to DCJTB is also illustrated in the inset of Fig. 5. A low concentration of Ag NPs shows a weak feedback of backscattered light, thus a higher threshold pump energy is needed, whereas at higher concentration of Ag NPs, the reduction of the transport mean free path length also leads to an increase in the lasing threshold [24]. Therefore, it is necessary to optimize the concentration of the Ag NPs, thus obtaining the best laser threshold property. For this case, the lower threshold of 0.0031 mJ/pulse is obtained when the ratio of Ag NPs to DCJTB is 1:1. In random lasing medium, the gain strongly depends on the medium scattering strength, which is inversely proportional to the 195
Optics & Laser Technology 91 (2017) 193–196
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Department of Chongqing Municipality (KJ1400501) and the Foundation for the Creative Research Groups of Higher Education of Chongqing (No. CXTDX201601016).
Table 1 The threshold values and mean free path length of the Ag NPs dispersed DCJTB:PS films for different concentration ratios of Ag NPs to DCJTB. Ag NPs to DCJTB:
1:0.5
1:1
1:2
1:4
Threshod (mJ/pulse) Δλ (nm) mean free path length ( μm )
0.009 0.31 420
0.0031 0.22 590
0.0052 0.30 434
0.0162 0.57 228
References [1] S.C. Kitson, W.L. Barnes, J.R. Sambles, Full Photonic Band Gap for Surface Modes in the Visible, Phys. Rev. Lett. 77 (1996) 2670–2673. [2] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, Origin of laser-induced nearsubwavelength ripples: interference between surface plasmons and incident laser, ACS Nano 3 (2009) 4062–4070. [3] F. Valmorra, M. Broll, S. Schwaiger, N. Welzel, D. Heitmann, S. Mendacha, Strong coupling between surface plasmon polariton and laser dye rhodamine 800, Appl. Phys. Lett. 99 (2011) 051110–051112. [4] F.F. Ren, K.W. Ang, J. Song, Q. Fang, M. Yu, G.Q. Lo, D.L. Kwong, Surface plasmon enhanced responsivity in a waveguided germanium metal-semiconductor-metal photodetector, Appl. Phys. Lett. 97 (2010) (091102-091102-3). [5] S. Nie, S.R. Emory, Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering, Science 275 (1997) 1102–1106. [6] D.K. Gifford, D.G. Hall, Emission through one of two metal electrodes of an organic light-emitting diode via surface-plasmon cross coupling, Appl. Phys. Lett. 81 (2002) 4315–4317. [7] M.A. Noginov, G. Zhu, A.M. Belgrave, R. Bakker, V.M. Shalaev, E.E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner, Demonstration of a spaser-based nanolaser, Nature 460 (2009) 1110–1112. [8] M. Fukushima, H. Yanagi, Y. Taniguchi, Distributed feedback lasing from dyedoped glass films using photopatterned gold nanoparticles, J. Appl. Phys. 97 (2005) (106104 - 106104-3). [9] J. Stehr, J. Crewett, F. Schindler, R. Sperling, G. von Plessen, U. Lemmer, J.M. Lupton, T.A. Klar, J. Feldmann, A.W. Holleitner, M. Forster, U. Scherf, A low threshold polymer laser based on metallic nanoparticle gratings, Adv. Mater. 15 (2003) 1726–1729. [10] G.D. Dice, S. Mujumdar, A.Y. Elezzab, Plasmonically enhanced diffusive and subdiffusive metal nanoparticle-dye random laser, Appl. Phys. Lett. 86 (2005) (131105-131105-3). [11] O. Popov, A. Zilbershtein, D. Davidov, Random lasing from dye-gold nanoparticles in polymer films: Enhanced at the surface-plasmon-resonance wavelength, Appl. Phys. Lett. 89 (2006) (191116-191116-3). [12] X. Meng, K. Fujita, Y. Zong, S. Murai, K. Tanaka, Random lasers with coherent feedback from highly transparent polymer films embedded with silver nanoparticles, Appl. Phys. Lett. 92 (2008) (201112-201112-3). [13] X. Meng, K. Fujita, S. Murai, K. Tanaka, Coherent random lasers in weakly scattering polymer films containing silver nanoparticles, Phys. Rev. A 79 (2009) (053817-053817-7). [14] K. Aslan, I. Gryczynski, J. Malicka, E. Matveeva, Metal- enhanced fluorescence: an emerging tool in biotechnology, Curr. Opin. Biotechnol. 16 (2005) 55–62. [15] J.R. Lakowicz, Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission, Anal. Biochem. 337 (2005) 171–194. [16] T. Zhai, X. Zhang, Random laser based on waveguided plasmonic gain channels, Nano Lett. 11 (2011) 4295–4298. [17] E. Heydari, R. Flehr, J. Stumpe, Influence of spacer layer on enhancement of nanoplasmon-assisted random lasing, Appl. Phys. Lett. 102 (2013) (133110133110-133113). [18] G.D. Dice, S. Mujumdar, A.Y. Elezzab, Plasmonically enhanced diffusive and subdiffusive metal nanoparticle-dye random laser, Appl. Phys. Lett. 86 (2005) 131105–131107. [19] S.Y. Ning, Z.X. Wu, H. Dong, F. Yuan, J. Xi, L. Ma, B. Jiao, X. Hou, Enhanced lasing assisted by the Ag-encapsulated Au plasmonic nanorods, Opt. Lett. 40 (2015) 990–993. [20] W. Lu, B. Zhong, D.G. Ma, Amplified spontaneous emission and gain from optically pumped films of dye-doped polymers, Appl. Opt. 43 (2004) 5074–5078. [21] N. Tessler, Lasers based on semiconducting organic materials, Adv. Mater. 11 (1999) 363–370. [22] Z.Q. Zhang, Light amplification and localization in randomly layered media with gain, Phys. Rev. B 52 (1995) 7960–7964. [23] D.K. Zhang, Y.P. Wang, D.G. Ma, Random lasing emission from a red fluorescent dye doped polystyrene film containing dispersed polystyrene nanoparticles, Appl. Phys. Lett. 91 (2007) (091115-091115-3). [24] H. Cao, Y.G. Zhao, H.C. Ong, S.T. Ho, J.Y. Dai, J.Y. Wu, R.P.H. Chang, Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films, Appl. Phys. Lett. 73 (1998) 3656–3658. [25] P.P. Pal, N. Jiang, M.D. Sonntag, N. Chiang, E.T. Foley, M.C. Hersam, R.P.V. Duyne, T. Seideman, plasmon-mediated electron transport in tip-enhanced raman spectroscopic junctions, J. Phys. Chem. Lett. 6 (2015) 4210–4218. [26] D. Kurouski, R.P.V. Duyne, In situ detection and identification of hair dyes using surface-enhanced raman spectroscopy (SERS), Anal. Chem. 87 (2015) 2901–2906.
transport mean free path length, L . In the case of the Ag NPs dispersed DCJTB:PS film, the emitted light may be returned by scatterers from which it was scattered before, thereby forming a closed-loop path [24]. Actually, there are many such closed-loop paths in Ag NPs scattered DCJTB:PS film, and these loops could serve as ring cavities, thus photons are multiplied by scattering. The transport mean-free path length can be obtained by the following equation [24]:
L=
λ2 2nΔλ
(1)
Where, L is the transport mean free path length and n is the effective polymer refraction index, which in our case is close to 1.5. Table 1 summarizes the threshold values and the mean free path length of the Ag NPs dispersed DCJTB:PS films at different ratios of Ag NPs to DCJTB. It can be seen that the appropriate concentration of the Ag NPs forms the transport mean free path length and finally leads to the lower threshold of random lasing emission. In this random laser system, Ag NPs are not only as scatters, but also supply LSPR coupling with the lasing emission. It is well known that the metallic NPs have been used to enhance the lasing efficiency [13,17,18], and one mechanisms is accepted as: enhancement of localized EM field in the vicinity of metal NPs. The effect of the enhanced localized EM field can increase the density of pump light available for the gain media, and consequently increases the probability that more dye molecules are excited simultaneously to the higher energy levels; then the excitation rate can be enhanced [19]. Also, some other effects, such as light-induced plasmon-induced electron transport [25] and surface-enhanced Raman effect enhanced by metal nanoparticles [26], will be possible to influence such random lasing performance. 4. Conclusions In conclusion, we presented a detailed study of random lasing performance of DCJTB:PS film with silver nanoparticles as scatters. It was found that the concentration of reactant AgNO3 and the ratio of Ag nanoparticles to DCJTB influenced the lasing performance greatly. When the concentration of AgNO3 solution was optimized to 0.03 M and the ratio of Ag NPs to DCJTB was 1:1, the lower threshold of 0.0031 mJ/pulse was obtained. The low threshold should be attributed to the localized surface-plasmon resonance from the Ag nanoparticles which coupled with the lasing emission to enhance the lasing efficiency. Our work demonstrates the possibility of the metal nanoparticles as alternative sources of coherent light emission to realize random organic lasers. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 11304406 and 61307035), Science and technology research Foundation of the Education
196
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Random lasing from dye-Ag nanoparticles in polymer films: Improved lasing performance by localized surface plasmon resonance
MARK
⁎
Daisi He, Weiying Bao, Li Long, Peng Zhang, Maohua Jiang, Dingke Zhang
School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Random lasers Ag nanoparticles Localized surface-plasmon resonance Threshold
Random multimode lasers are achieved in 4(dicyanomethy-lene)−2- tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9enyl)4H-pyran (DCJTB) doped Polystrene (PS) thin films by introducing silver nanoparticles (Ag NPs) as scatterers. Ag NPs were prepared by polymer protection method. By optimizing the concentration of reactant of AgNO3 and the ratio of Ag NPs to DCJTB, the devices emit a resonance multimode peak at a center wavelength of 625 nm and the threshold excitation intensity is as low as 0.0031 mJ/pulse. It can be seen that the microscopic random resonance cavities can be formed by multiple scattering of Ag NPs which supply the localized surface-plasmon resonance (LSPR) coupling with the lasing emission to enhance the lasing efficiency. Our results demonstrate that Ag NPs are promising candidate as alternative sources of coherent light emission to realize low-threshold organic random lasers.
1. Introduction Surface plasmons (SPs) are surface charge density oscillations that exist at a metal/dielectric interface. These oscillations, excited by the interaction between light and metal surfaces [1], are known to increase the density of states and the spontaneous emission rate in the semiconductor [2–4] and lead to the enhancement of light emission. They have been extensively used in various applications, such as surface enhanced Raman [5], organic light-emitting diodes (OLEDs) [6], and biosensors. In recent years, the coupling between a SPs and an optically active medium became a hot topic. For example, Noginov et al. [7] provide a major step forward for plasmon-based nanolasers based on a gold core centered in a dye-doped silica shell, where the optical gain is supplied by organic dye molecules embedded in the silica shells and the surface plasmons are provided by the gold cores. In addition, the metallic NPs with periodic arrangement can act as a grating and lead to distributed feedback lasers [8,9]; gold or silver NPs with random distribution were also found to lead to the random lasing by the effects of enhanced localized EM fields and scattering [10–13]. Recently, particular interest has been drawn to the effect of noble metal nanoparticles on the random lasing characteristics [14,15]. In previous reports, the lasing threshold was reduced and its emission was greatly enhanced by the introduction of Ag or Au NPs [16,17]. Recent analysis has shown that a large fraction of light generated in a typical random laser is coupled into SP modes, especially in devices
⁎
Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2016.12.036 Received 31 August 2016; Accepted 27 December 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
incorporating small molecules such as organic dye. Heydari et al. reported the emission enhancement in the plasmon assisted random laser by coupling between the dyes and the localized surfaceplasmon resonance (LSPR) of Au NPs [17]. Dice et al. demonstrated incoherent random lasing from a suspension of Ag NPs in a methanol solution of Rh6G [18]. In these works, noble metal nanoparticles may strongly scatter and absorb visible light due to the LSPR. The electric field can be confined in the vicinity of the surface of metal NPs due to LSPR; as a consequence, such confinement can be very effective for the excitation of active centers to provide high optical gain for laser oscillation [13]. Recent studies show that the scattering properties of metal NPs are very sensitive to particle size and morphology [19]. In order to evaluate the effect of metallic particle size on random lasing of dyes, in this work, the optical pumped random lasing properties were usually investigated in presence of dye-Ag nanoparticles in polymer films. Ag NPs in different sizes were synthesized by the chemical reduction method and introduced into the gain media of the 4(dicyanomethy-lene) −2-tert–butyl −6(1,1,7,7–tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) doped polystrene (PS). The objective of this paper is to study the influence of the excited SPs from the Ag NPs with different sizes on the lasing properties. By optimizing the concentration of the reactant of AgNO3, we found that surfaceplasmon-enhanced lasing emission with reduced thresholds had been obtained.
Optics & Laser Technology 91 (2017) 193–196
D. He et al.
2. Experimental section 2.1. Synthesis of Ag nanoparticles In our experiments, Ag NPs were prepared by polymer protection method. Here, polyvinylpyrrolidone (PVP), as protective agent, were added into the reactant silver nitrate (AgNO3) solution (the molar ratio of PVP to AgNO3 is 1:2). At the temperature of 60 °C, the mixed solution was fully stirred, and then 0.5 mol/L hydrazine hydrate (N2H4·H2O) solution as reducing agent was slowly added. The obtained Ag NPs were washed by alcohol, acetone, ultrapure water in turn, then dried in the room temperature. To determine the effect of the size of Ag NPs on lasing action, we prepared Ag NPs in different sizes by changing the molar concentration of AgNO3 in N2H4·H2O. The concentrations of AgNO3 in N2H4·H2O were changed from 0.01 to 0.6 M, respectively.
2.2. Fabrication of Ag nanoparticles/organic dye random laser Fig. 2. Emission spectra of the neat DCJTB:PS film pumped by 532 nm constant laser (black curve) and 532 nm pulsed laser (red curve) and the emission spectrum of the Ag NPs @ DCJTB:PS film pumped by 532 nm pulsed laser (blue curve). The image shows a portion of the spectrum of the Ag NPs @ DCJTB:PS film under higher magnification. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
DCJTB, as the gain media, was first dissolved in PS by using chloroform as solvent. The optimized weight ratio of DCJTB to PS was kept constant to 4% by weight. Then, Ag NPs, as scattering centers, were added into the DCJTB:PS blend. To avoid the cluster of the Ag NPs, the blend was ultrasounded 10 min and then spincoated onto the quarz substrate to form a Ag NPs dispersed DCJTB:PS film (Ag NPs @ DCJTB: PS). For comparison, a DCJTB:PS film without Ag NPs was also fabricated. Furthermore, to optimize the concentration of Ag NPs, four samples with different ratios of Ag NPs to DCJTB (1:0.5, 1:1, 1:2, and 1:4) were prepared.
3. Results and Discussions Fig. 1a) shows the scanning electron microscopy (SEM) images of the Ag NPs, when the molar concentration of AgNO3 is 0.01 M. It can be seen that a large number of Ag NPs have been successfully synthesized, and mostly in the shape of sphere and the average diameter is 30 nm, when the concentration of AgNO3 is 0.01 M. With the concentration of AgNO3 increasing, the particle size of Ag powder is gradually increased. When the molar concentration of AgNO3 is 0.03 M, the transmission electron microscopy (TEM) image of Ag NPs were measured and shown in Fig. 1b). The average diameter Ag NPs is increased to 100 nm. The increase of AgNO3 concentration leads to the acceleration of reduction reaction. The formation of the crystal nucleus increases at the beginning, and the trends of agglomeration are greater than the diffusion during the formation of the grains. Thus the size of Ag NPs gradually increased. In order to reveal the role of Ag NPs in the DCJTB:PS film, we also prepared the neat DCJTB:PS film for comparison. The emission spectra from the neat DCJTB:PS film and the Ag NPs scattered DCJTB:PS film (the concentration of AgNO3 is 0.03 M and the ratio of Ag NPs to DCJTB is 1:1 by weight) are shown in Fig. 2. The photoluminescence (PL) spectrum exhibits a broad peak at 600 nm, whereas the spectrum excited by 2 ns pulsed pumping is dramatically narrowed to a 19 nm half-width peak at 628 nm. The laser pulses create a significant
2.3. Measurement of random lasing properties The experimental set-up to investigate the laser action followed Ref. [20]. The pump source was a frequency tripled Nd-YAG laser (SpectraPhysics) delivering 2 ns pulses at 532 nm with a 10 Hz repetition rate. The output pulse energy of the pump laser was controlled using neutral density filters. An adjustable slit and a cylindrical lens were used before the beam splitter in order to shape the beam into a narrow stripe with a continuously varied 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 fibercoupled CCD spectrometer (JYSPEX CCD3000). The pumped energies from the laser were measured using a calibrated laser power and energy meter (Gentec). For comparison, the photoluminescence (PL) was also measured using a continuous wave (CW) 532 nm laser. The morphologies of the as-synthesized Ag NPs were characterized by a field emission scanning electron microscope (FE-SEM, TESCAN MARI3) and a transmission electron microscopy (TEM, Libra-200FE).
Fig. 1. a) The scanning electron microscopy (SEM) image of Ag NPs prepared by polymer protection method with the AgNO3 concentration of 0.01 M; b) Transmission electron microscopy (TEM) image of the Ag NPs with the AgNO3 concentration of 0.03 M.
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Fig. 3. Emission spectra of Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M at pumped intensity of 0.07 mJ/pulse.
population inversion within the DCJTB molecules and thus the narrow emission spectrum can be attributed to amplified spontaneous emission (ASE) [21]. The emission spectrum from the Ag NPs @ DCJTB:PS film with 2 ns pulsed laser 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 0.27 nm, which is 70 times smaller than that of the ASE peak of the neat DCJTB:PS film. The significantly narrowed linewidth and discrete peaks suggests that this emission is the result of random lasing action. It is found that the occurrence of random lasing action depends on the ratios of Ag NPs to DCJTB and the size of Ag NPs which depends on the AgNO3 solution concentrations. Fig. 3 shows the emission spectra of Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively. The optically pumping energy is 0.07 mJ/pulse. For the case of Ag NPs synthesized by low AgNO3 molar concentrations (0.01 M or 0.03 M), very narrow multimode peaks emerge in the emission spectrum. However, as the AgNO3 molar concentration increasing, the multimode sharp peaks decreases relatively. When the AgNO3 reaches a high molar concentration (0.6 M), a similar ASE spectrum with some short peaks is observed. The sharp multimode emission should indicate the random lasing action, and obviously depends on the size of Ag NPs. In the random lasing medium, the gain strongly depends on the medium scatters to form closed-loop paths for random lasers [22,23]. Actually, there are many such closed-loop paths in Ag NPs @ DCJTB:PS film, and these loops could serve as ring cavities, thus photons are multiplied by scattering. Because the lasing action in random lasers is dependent on the scattering mean free length, it is concluded that the Ag NPs grown by different reactant concentrations possess different scattering mean free length, thus showing different lasing actions. Fig. 4 gives the comparison of the output emission intensity integrated over all wavelengths as a function of the Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively. The laser thresholds are all clearly observed. There exists an abrupt change in the slope of the output energy versus input energy curves, followed by a linear increase of the output energy as the excitation energy. It can be seen that the threshold pump energies reach ∼0.026 mJ/pulse, ∼0.003 mJ/pulse, ∼0.03 mJ/pulse, and ∼0.6 mJ/pulse, corresponding to a real energy density of 0.325 mJ pulse−1 cm−2, 0.0375 mJ pulse−1 cm−2, 0.375 mJ pulse−1 cm−2 and 7.5 mJ pulse−1 cm−2, respectively. This observation should provide strong support to the formation of random laser resonators in Ag NPs
Fig. 4. Output emission intensity integrated over all wavelengths as a function of the Ag NPs @ DCJTB:PS film with Ag NPs synthesized by AgNO3 solution in four different molar concentrations of 0.01 M, 0.03 M, 0.05 M and 0.6 M, respectively.
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Wavelength (nm) Fig. 5. Emission spectra of the Ag NPs @ DCJTB:PS film (concentration of AgNO3 solution is 0.03 M) with the different ratios of Ag NPs to DCJTB (1:0.5, 1:1, 1:2, 1:4) at pumped intensity of 0.07 mJ/pulse. The inset is the threshold for the four samples.
@ DCJTB:PS film and the optimum AgNO3 concentration is 0.03 M for the Ag NPs synthesization. Selecting this optimum concentration, the ratio of Ag NPs to DCJTB is also optimized. Fig. 5 shows the spectra of the Ag NPs @ DCJTB:PS film (concentration of AgNO3 solution is 0.03 M) with the different ratios of Ag NPs to DCJTB(1:0.5, 1:1, 1:2, 1:4) at pumped intensity of 0.07 mJ/pulse. It can be seen that the emission spectra of Ag NPs @ DCJTB:PS film are affected by the ratios of Ag NPs. When the ratio of Ag NPs to DCJTB is 1:1, much more narrow sharp peaks are observed. The thresholds of the Ag NPs @ DCJTB:PS film with the different ratios of Ag NPs to DCJTB is also illustrated in the inset of Fig. 5. A low concentration of Ag NPs shows a weak feedback of backscattered light, thus a higher threshold pump energy is needed, whereas at higher concentration of Ag NPs, the reduction of the transport mean free path length also leads to an increase in the lasing threshold [24]. Therefore, it is necessary to optimize the concentration of the Ag NPs, thus obtaining the best laser threshold property. For this case, the lower threshold of 0.0031 mJ/pulse is obtained when the ratio of Ag NPs to DCJTB is 1:1. In random lasing medium, the gain strongly depends on the medium scattering strength, which is inversely proportional to the 195
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Department of Chongqing Municipality (KJ1400501) and the Foundation for the Creative Research Groups of Higher Education of Chongqing (No. CXTDX201601016).
Table 1 The threshold values and mean free path length of the Ag NPs dispersed DCJTB:PS films for different concentration ratios of Ag NPs to DCJTB. Ag NPs to DCJTB:
1:0.5
1:1
1:2
1:4
Threshod (mJ/pulse) Δλ (nm) mean free path length ( μm )
0.009 0.31 420
0.0031 0.22 590
0.0052 0.30 434
0.0162 0.57 228
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transport mean free path length, L . In the case of the Ag NPs dispersed DCJTB:PS film, the emitted light may be returned by scatterers from which it was scattered before, thereby forming a closed-loop path [24]. Actually, there are many such closed-loop paths in Ag NPs scattered DCJTB:PS film, and these loops could serve as ring cavities, thus photons are multiplied by scattering. The transport mean-free path length can be obtained by the following equation [24]:
L=
λ2 2nΔλ
(1)
Where, L is the transport mean free path length and n is the effective polymer refraction index, which in our case is close to 1.5. Table 1 summarizes the threshold values and the mean free path length of the Ag NPs dispersed DCJTB:PS films at different ratios of Ag NPs to DCJTB. It can be seen that the appropriate concentration of the Ag NPs forms the transport mean free path length and finally leads to the lower threshold of random lasing emission. In this random laser system, Ag NPs are not only as scatters, but also supply LSPR coupling with the lasing emission. It is well known that the metallic NPs have been used to enhance the lasing efficiency [13,17,18], and one mechanisms is accepted as: enhancement of localized EM field in the vicinity of metal NPs. The effect of the enhanced localized EM field can increase the density of pump light available for the gain media, and consequently increases the probability that more dye molecules are excited simultaneously to the higher energy levels; then the excitation rate can be enhanced [19]. Also, some other effects, such as light-induced plasmon-induced electron transport [25] and surface-enhanced Raman effect enhanced by metal nanoparticles [26], will be possible to influence such random lasing performance. 4. Conclusions In conclusion, we presented a detailed study of random lasing performance of DCJTB:PS film with silver nanoparticles as scatters. It was found that the concentration of reactant AgNO3 and the ratio of Ag nanoparticles to DCJTB influenced the lasing performance greatly. When the concentration of AgNO3 solution was optimized to 0.03 M and the ratio of Ag NPs to DCJTB was 1:1, the lower threshold of 0.0031 mJ/pulse was obtained. The low threshold should be attributed to the localized surface-plasmon resonance from the Ag nanoparticles which coupled with the lasing emission to enhance the lasing efficiency. Our work demonstrates the possibility of the metal nanoparticles as alternative sources of coherent light emission to realize random organic lasers. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 11304406 and 61307035), Science and technology research Foundation of the Education
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