Tunable random lasing behavior in plasmonic nanostructures

Abstract

Random lasing is desired in plasmonics nanostructures through surface plasmon amplification. In this study, tunable random lasing behavior was observed in dye molecules attached with Au nanorods (NRs), Au nanoparticles (NPs) and Au@Ag nanorods (NRs) respectively. Our experimental investigations showed that all nanostructures i.e., Au@AgNRs, AuNRs & AuNPs have intensive tunable spectral effects. The random lasing has been observed at excitation wavelength 532 nm and varying pump powers. The best random lasing properties were noticed in Au@AgNRs structure, which exhibits broad absorption spectrum, sufficiently overlapping with that of dye Rhodamine B (RhB). Au@AgNRs significantly enhance the tunable spectral behavior through localized electromagnetic field and scattering. The random lasing in Au@AgNRs provides an efficient coherent feedback for random lasers.

Yadav et al. Nano Convergence (2017) 4:1 DOI 10.1186/s40580-016-0095-5

Tunable random lasing behavior inplasmonic nanostructures

Ashish Yadav1, Liubiao Zhong1, Jun Sun1, Lin Jiang1*, Gary J. Cheng2,3* and Lifeng Chi1*

1 Background

The research on plasmonics has led to extensive applications in the eld of optoelectronics such as light emitting diodes, waveguides, and nano-lasers, due to the unique property known as localized surface plasmon resonance (LSPR) exhibited by metallic nanostructures [14]. In studies based on spontaneous emission, both uorescence enhancement and quenching, have been observed for uorophores in the vicinity of metallic nanostructures [57]. The uorescence enhancement and radiativenonradiative transitions of uorophores are both found to be strongly dependent on the separation between uorophores and metallic nanostructures [5]. In random lasers, gain medium is strongly dependent on the scattering strength [8, 9] and light interact with disordered amplifying media in such systems [10, 11]. The scattering is mainly caused by dielectric or metallic scattered light. This mechanism gives to resonating structures with a high quality factor (Q factor). The phenomena of random

lasing in some other systems such as nanoparticles, [12] conjugated polymer lms, [13] organic dye-doped gel lms, [14] suspensions containing laser dyes, silver nano-particles, [15, 16] and coherent feedback, dielectric materials with high refractive index TiO2, and ZnO have also been studied [1719].

Metal nanoparticles (MNPs) play an important role in spectral narrowing. MNPs have much larger scattering cross section than that of dielectric NPs with the same dimensions. MNPs are enriched with their unique property of surface plasmon resonance (SPR) that may spatially conne light wave near particle surface to give high gain in lasing [20]. SPR position strongly depends on material, shape, size and environment of the NPs. These parameters give spectral tuning of the plasmon resonance to overlap the emission spectrum of the desired active medium. Plasmonic resonances change the local density of optical states to close the NPs and strongly enhance the yield. These NPs can modify the non-radiative and radiative transition rates of nearby dye molecules [2123]. Generally, lasing dyes have large Stoke shifts between their absorptions and emissions, which could reduce the self-absorption and achieve the lower lasing threshold [24]. The emission intensity may be enhanced in the plasmon assisted random laser by coupling between the dye and localized LSPR of AuNPs

*Correspondence: [email protected]; [email protected]; [email protected]

1 Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, Jiangsu, Peoples Republic of China

2 School of Industrial Engineering, Purdue University, 315 N. Grant St, West Lafayette, IN 47907, USAFull list of author information is available at the end of the article

The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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[25] provided that there is sufficient overlap between the LSPR spectrum of AuNPs and emission spectra of the dyes. Meng etal. found enhanced emission of coherent random lasing in polymer lms embedded with Ag NPs [12]. Ning etal. reported enhancement in the lasing eect of Ag encapsulated with Au NRs [26]. In these reports, the enhanced localized electromagnetic (EM) eld was considered to be the dominant mechanism for the occurrence of random lasing, especially for small sizes of Ag NPs. The random lasing could be induced by the eects of both scattering and the enhanced localized EM eld of metallic nanostructure.

In this paper, we presented the tunable random lasing properties of AuNRs, AuNPs and Ag encapsulated with Au nanorods (Au@AgNRs). Au@AgNRs showed intensive tunable random lasing behavior, due to their broad spectrum and multiple peaks, covering the emission spectrum of RhB and nearly overlap. Our experiments indicate the phenomena of lasing and variation with different excitation powers. The plasmonic eect was optically excited by the second harmonic of the 1064nm line of a Nd:YAG laser. The samples were pumped by the second harmonic wavelength at 532 nm, 10 Hz repetition rate, and 6ns pulse duration. A 532nm notch lter was used to suppress the detection of the scattered excitation light.

2 Methods

2.1

Synthesis ofAu@AgNRs, AuNRs andAuNPs

Au@AgNRs were prepared by following the procedure as reported in [27, 28]. The AuAg bimetallic nanostructures were prepared using following methods: for AuNRs samples, three aliquots (1 mL) of the AuNRs solution were centrifuged at 800rpm for 15min and re-dispersed into CTAC solutions (0.08M) at the same volume 0.12, 0.24 and 0.48 of AgNO3 (0.01M) were subsequently into the three aliquots of the AuNRs solution, followed by the addition of ascorbic acid (AA, sigma Aldrich) solutions (0.1M), respectively. The volume of the AA solution was half of that of the AgNO3 solution for each aliquot. The resultant solutions were kept in an isothermal oven, present at 65C for 3h. The AuNPs (13nm) were synthesized by using chemical reduction method, which was carried out as follows: 5mL of 10mM chlorauric acid (HAuCl4.3H2O) solution was heated to boiling 100mL beaker with 90mL di-ionized water, stirring at 400rpm. Then, 5mL of 3.8102M trisodium citrate dihydrate (Na3C6H5O72H2O) was added. The solution color changed within several minutes red wine as continue stirring for 15min [29]. We used 13nm AuNPs as a seeds for 40nm particles. 0.2mL 13nm seeds+5mL 0.5mM HAuCl4 was stirred at 400rpm then 30L then 363 mM NH3OHHCl were added. Size of the AuNPs

40nm is conrmed by SEM. The substrate was carefully cleaned by piranha solution to improve the wet ability of the surface [30, 31]. The rhodamine B aqueous solution mixed with AuNRs was stirred over night and the lm is prepared by casting drop method. We spread the solution by pipette (300 L) and dried at room temperature. We made the device using following steps, polyvinylpyrrolidone (PVP, Mw ~55,000 sigma Aldrich) were used without further treatment. The PVP was dissolved in de-ionized water (DI water) and was stirred for 4h. RhB dye molecule and NPs solution were mixed together and placed for few hours for stirring. Then prepared the lm for laser characteristic.

2.2 Film preparation

The substrate was carefully cleaned in order to improve the wetability of the surface. The silicon substrate (2 2 cm2) were washed in a mixture containing concentrated sulfuric acid (95-98%) and hydrogen peroxide (40%) (H2SO4:H2O2 = 3:1, volume ratio) for 10 min and were treated in an ultrasonic bath containing ammonium hydroxide solution (40%), hydrogen peroxide and de-ionized water with a volume ratio of NH4OH:H2O2:H2O=1:0.6:0.8 for 5min. Thereafter, the substrates were washed with copious de-ionized water and dried in nitrogen gas ow before use [30]. Small amounts of the solution (300 L) were put on the substrate and were carefully spread to fully cover on the substrate. The solution is allowed to slowly dry at room temperature.

3 Results anddiscussion

A typical sketch of the fabrication of lm and experiment are shown in Fig.1.

Scanning electron microscope (SEM) of Au@AgNRs, AuNRs & AuNPs has been shown in Fig. 2. All nano structures are quite uniform with size and shape. It is clear from Fig. 2a that the average length of AuNRs is found to be 86 6 nm and thickness of Ag around

AuNRs is 426nm. The length of AuNRs is observed to be ~60nm and width is 12nm (Fig.2b). The sizes of

AuNPs are found around 40nm. Figure 2d, e and f are shown SEM after lm fabrication. AuNPs are very easy to get the aggregation. To avoid the aggregation we continued stirring then made the lm. The absorption and photoluminescence (PL) behavior of RhB are clearly shown in Fig.3a. Figure3b shows absorption spectrum of Au@AgNRs. The LSPR peak of Au@Ag NRs has been observed to have four peaks (341, 387, 435, and 597nm). RhB dye used an acceptor which can be explained by Frster elegant theory (Frster resonance energy transfer, FRET) [3235]. We have shown comparative study of absorption spectrum AuNRs, Au@AgNRs and AuNPs

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and emission of RhB dye molecules. Absorption spectra of the Au@AgNRs exhibited the multi-LSPR peaks and broad spectra which have a sufficient overlap with the emission spectra of RhB.

Figure4 shows the luminescence behavior of dierent samples excited by 532 nm wavelength. PVP RhB lm

has no spectral behavior at high excitation power. We recorded the spectrum at dierent powers. We collected the emission spectrum of PVA RhB lm as a reference (Fig. 4a). We observed the random lasing in dierent shapes of gold nanostructures, prepared by casting lm deposition method on silicon substrate. Edge-emission

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spectra of the gain medium with MNPs are shown in Fig. 4c. As described in previous reports [36], the net gain medium lm exhibits an obvious lasing behavior. We performed measurements by varying the excitation power from low to high values. MNPs are capped with dye molecules and covered with PVP lm are found to excite at high power, which can be explained by FRET [32] and surface energy transfer (SET) [35]. Fluorophores involved in resonance energy transfer aect the spectral properties of the donor and acceptor. MNPs can aect the radiative rate of uorophores [33, 34]. Modication of the uorophores radiative rate by the metallic structures can lead to enhanced uorescence intensity [7], and the change in the uorophores radiative rate can be explained in terms of the coupling of the molecular and NPs dipoles. Constructive interference of the dipoles has led to the increased radiative rate and resulted possible enhancement of uorescence intensity. Radiative and non-radiative decay rates are dependent on the uorophores dipole relative to the particle surface [37]. Dye molecules could be excited simultaneously to higher energy bands and de-excited with an emission. However, the average power absorbed by each dye molecule is very small as compared to spot illumination (with the same pump power). Each molecule will de-excite at a longer wavelength. When the pump power is increased, the radiative transition probability is enhanced at the shorter/longer wavelength end of the spectrum creating a shift in the spectrum towards blue/red wavelengths. Power-dependent spectral peak shifts with dierent sizes of particles have been reported previously by some other groups [38].

Au@AgNRs exhibited broad spontaneous emission spectra with full width half maximum (FWHM) about

38 nm at low pump power (288.5 J). Once the excitation energy becomes large enough, the emission spectrum became much narrow with FWHM 14 nm at 3.62mJ laser power (Fig.4c). Intensity vs. FWHM variation of emission spectrum with dierent laser powers has been shown in Fig. 4d. Red shift observed due to locally enhanced eld of gold nano structures, when dye molecules are accumulating on the outer surface of the gold nano structures. [39]. Emission intensity is found to linearly increase with the laser pump powers. On the other hand AuNRs has shown blue shift in our experiment. AuNRs showed the spectral narrowing at 587nm at 850 J in Fig. 4e. Figure 4f, the pump power behavior corresponds to FWHM and intensity. In the case of AuNPs, we got the spectral narrowing @ 591 nm at very high laser power as compared to Au@AgNRs and AuNRs shown in Fig.4g. We observed random lasing at high pump power. Enhanced localized electromagnetic eld (EM) in the vicinity of metal nanostructures may enhance the density of pump light available for the gain media, and consequently may increase the probability of the dye molecules that are to be excited simultaneously to the higher energy levels. AuNPs can aect the radiative rate of a uorophores [21, 40]. Some research groups reported lasing efficiency enhanced by metallic NPs [4143]. There are two kinds of mechanism: (1) Enhancement of localized EM eld in the vicinity of metal NPs and (2) Enhancement of scattering strength [41]. Increasing the quantum yield of the gain media will depend on the degree of the overlap between the LSPR spectra and the emission of the RhB. When metallic NPs are excited resonantly, they scatter the energy of emitters with the greater scattering cross sections, and then easily lead to the occurrence of spectral narrowing random lasing. We

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observed that Au@AgNRs absorption have the most sufcient overlap with emission of RhB. Enhancement in the local eld is rather moderate for gold nanospheres because the losses dominate over a possible gain due to feedback from multiple scattering events. Nanorods efficiently scatter the photons emitted by the RhB and more over the high local eld provide an additional excitation enhancement of the molecules. This can be reason to signicant increase in the eective emission rate [44]. In this work we found the best random lasing behavior on Au@AgNRs structure.

PL measurements performed by uoromax-4 spectrouorometer instrument. PVP RhB PL intensity is very less as compare with gold nano-structures clearly seen in Fig.5.

We used time correlated single photon counting (TCSPC) for this experiment. Time-resolved measurements were performed by Nano LEDs (455 nm; FWHM<750ps) with repetition rates between 10kHz and 1 MHz were used to excite the sample. We were used IBH Data Station Hub photon counting module and data analysis. The PVP RhB lifetime was found 2.28ns, PVP-RhB-AuNPs 2.10ns, PVP-RhB-AuNRs 2.08ns and PVP-RhB-Au@AgNRs 1.59ns observed in Fig.5 (right). Fluorescence life time is an intrinsic molecular property.

Further, we conrmed the surface plasmonic eect on metallic nanostructures (Au@AgNRs, AuNRs and AuNPs) (Fig.6a, b, c). Theoretical optical properties were calculated by the nite dierence time-domain (FDTD; Lumerical Solutions, Inc.). The electric proles of Au@ AgNRs strongly aect the local surface electromagnetic eld. Au@AgNRs has unique plasmonic characteristic

and broad spectra. EM eld focalized at the corners or the edges of metal nano structures. Then, we observed very large enhancement factors of the electric eld. The FDTD simulation of the electric-eld distribution of the Au@AgNRs, AuNRs, and AuNPs with emission light wavelengths at 611nm is shown in Fig.6. The color scale indicates the electric eld enhancement factors, normalized to the incident wave. It is found that the electrical eld of the Au@AgNRs obviously enhanced compared with that of the Au NRs and Au NPs. At the same time, we nd that the electrical eld of the Au NRs is stronger than that of AuNPs. The simulation conrmed the unique local eld enhancement of the Au@Ag NRs, which plays an important role in plasmonic enhanced lasing.

4 Conclusion

In this work, we have investigated a tunable random lasing behavior of plasmonic nanostructures (AuNRs, AuNPs and Au@AgNRs) capped with RhB dye. We observed that the plasmonic eect of Au@AgNRs have signicantly improved the lasing behavior of the gain medium and showed the best property in comparison of AuNPs and AuNRs. The broader absorption and multiple peaks of LSPR of Au@Ag NRs overlap with both absorption and emission spectrum of the donoracceptor of the gain medium. These result in the enhanced spectral behavior by the eect of both localized electromagnetic eld and scattering. The random lasing in Au@AgNRs provides an efficient coherent feedback for random lasers. This study provides a new approach to achieve the random lasing by tuning the LSPR spectrum of the metallic nanostructures.

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Authors contributions

AY carried out the random lasing behavior in plasmonic nanostructures and drafted the manuscript. LZ synthesized metal nanoparticles. JS performed the simulation part of this manuscript. GJC, LJ, and LC discussed and helped draft the manuscript. All authors read and approved the nal manuscript.

Author details

1 Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, Jiangsu, Peoples Republic of China. 2 School of Industrial Engineering, Purdue University, 315 N. Grant St, West Lafayette, IN 47907, USA. 3 Birck Nanotechnology Center, Purdue University, 1205 W State St, West Lafayette, IN 47907, USA.

Acknowledgements

L.C. and L.J. acknowledge the nancial support from National Natural Science Foundation of China (Project Codes: 91227201, 21527805 and 21373144), Natural Science Foundation of Jiangsu Province (Project Code: BK20150007 and BK20130287), Collaborative Innovation Center of Suzhou Nano Science& Technology and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). G.J.C. acknowledges the nancial support from National Science Fundation of USA, Division of Civil, Mechanical and Manufacturing Innovation, and National Research Council fellowship program. It is a pleasure to acknowledge the help of Prof. Zhi-Gang Zheng from East China University of Science and Technology, Shanghai, China, who provided the laser facility in his lab.

Competing interests

The authors declare that they have no competing interests.

Received: 16 November 2016 Accepted: 13 December 2016

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