Gold-nanoparticle-assisted random lasing from powdered GaN

Gold-nanoparticle-assisted randomlasing from powdered GaN

Toshihiro Nakamura,* Tomohiro Hosaka, and Sadao AdachiGraduate School of Engineering, Gunma University, Kiryu-shi, Gunma 376-8515, Japan

*[email protected]

Abstract: We demonstrate Au-nanoparticle-assisted random lasing from apowdered GaN sample. In the presence of Au nanoparticles on GaN powdersurfaces, several lasing lines are observed in photoexcited luminescencespectra near the center of the GaN band-edge emission peak. The randomlasing is considered to arise from a decrease in the lasing threshold dueto the suppression of crystal defect loss by surface plasmon excitation onAu. From spatially resolved lasing emission spectra and their FT analysisresults, the formation of random lasing cavities at different spatial positionsis confirmed. The size of the random lasing spot is determined to be largerthan that of the scattered light speckle of the pumping source on a thinpowdered GaN sample.

© 2011 Optical Society of America

OCIS codes: (140.5960) Semiconductor lasers; (160.6000) Semiconductor materials;(240.6680) Surface plasmons.

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in active random media,” Phys. Rev. Lett. 82, 2278–2281 (1999).3. H. Y. Yang, S. F. Yu, S. P. Lau, S. H. Tsang, G. Z. Xing, and T. Wu, “Ultraviolet coherent random lasing in

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GaN nanocolumns,” Appl. Phys. Lett. 97, 151109 (2010).5. M. A. Noginov, G. Zhu, I. Fowlkes, and M. Bahoura, “GaAs random laser,” Laser Phys. Lett. 1, 291–293 (2004).6. T. Takahashi, T. Nakamura, and S. Adachi, “Blue-light-emitting ZnSe random laser,” Opt. Lett. 34, 3923–3925

(2009).7. H. Y. Lin, C. L. Cheng, Y. Y. Chou, L. L. Huang, and Y. F. Chen, “Enhancement of band gap emission stimulated

by defect loss,” Opt. Express 14, 2372–2379 (2006).8. T.-H. Lin, T.-T. Chen, C.-L. Cheng, H.-Y. Lin, and Y.-F. Chen, “Selectively enhanced band gap emission in

ZnO/Ag2O nanocomposites,” Opt. Express 17, 4342–2379 (2009).9. C. W. Cheng, E. J. Sie, B. Liu, C. H. A. Huan, T. C. Sum, H. D. Sun, and H. J. Fan, “Surface plasmon enhanced

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#138910 - $15.00 USD Received 30 Nov 2010; revised 18 Dec 2010; accepted 21 Dec 2010; published 3 Jan 2011(C) 2011 OSA 17 January 2011 / Vol. 19, No. 2 / OPTICS EXPRESS 467

16. T. Karakouz, A. B. Tesler, T. A. Bendikov, A. Vaskevich, and I. Rubinstein, “Highly stable localized plasmontransducers obtained by thermal embedding of gold island films on glass,” Adv. Mater. 20, 3893–3899 (2008).

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(1986).

1. Introduction

Random lasers can be used as a new light source without any precise external laser cavities.Such random lasers have been extensively studied by a number of groups because of theirunique characteristics, such as spatial emission and a relatively high spontaneous emission fac-tor. Random lasing action is caused by multiple scattering of lights in a gain medium consistingof highly concentrated and randomly shaped particles or column scatterers [1]. Since the coher-ent random lasing action in ZnO was reported by Cao et al. [2], such a study has been performedon various semiconductor materials, such as SnO2 [3], GaN [4], GaAs [5], and ZnSe [6].

For improvement of the random lasing properties of semiconductor materials, it is neces-sary to suppress radiative recombination loss. One origin of this loss is the presence of surfaceor internal defects in semiconductors. In particular, the surface defects are easily introducedduring grinding bulk semiconductor materials for preparing powdered samples to achieve ran-dom lasing. These defects lead to a decrease in quantum efficiency of the emitters. Recently,it has been reported that the emission efficiencies from nanorods are markedly improved bydepositing metal nanoparticles on these nanorods [7–9]. This improvement is considered to bedue to the effect of surface plasmons on the metal nanoparticles. It is natural to consider thatthe same strategy can be applicable to powdered semiconductors and may lead to the realiza-tion of a random laser. It is interesting to develop a random laser using a coupled system ofsemiconductor gain materials and metal nanoparticles. Note that the surface plasmon accom-panies strong localized fields and that the light scattering cross section in a coupled system ofmetal nanoparticles and gain materials is strongly enhanced [10, 11]. Concerning the surface-plasmon-enhanced random laser, most studies use dye molecules as a gain material (see, e.g.,Refs. [10] and [11]), but only one study uses ZnO as a gain material [12].

In this work, we demonstrate a random laser using powdered GaN combined with Aunanoparticles. Several lasing lines are observed from a powdered GaN/Au nanoparticle sys-tem near the center of the GaN band-edge emission peak by excitation above the threshold (≥2MW/cm2). Note that the spatially resolved lasing emission profile yields useful informationon random lasing modes [13–15]. Therefore, we measure such a profile in the present randomlaser system. The obtained result suggests that random lasing cavities are located at differentspatial positions and that the size of the random lasing spot is larger than that of the scatteredlight speckle of the excitation source.

2. Experimental procedure

Commercially available GaN powder (Furuuchi Chemical Co., Ltd) with a mean diameter of300 nm was dispersed in methanol. To obtain a very thin powdered GaN film, this methanolsolution was dropped on a silicon substrate and then rotated at 5000 rpm on a spin coater. Fig-ure 1(a) shows the scanning electron microscope image of the sample surface. As shown in


Fig. 1(a), such a thin GaN film consists of aggregates of GaN particles. These aggregates arewell separated with each other and inter-distances between them are found to be in the range of10–100 μm. In Fig. 1(b), we show the scanning electron microscopy image of a GaN powderaggregate. The size of the aggregate is several tens of micrometers. A Au layer with a massthickness of several nanometers was deposited on the powdered GaN film by vacuum evapora-tion. The Au layer may cover only one side of the GaN aggregates. Thus, the deposited Au layeris regarded as island-type nanoparticles and may act as a light absorber in the red wavelengthrange, owing to the excitation of localized surface plasmons [16]. The Au nanoparticle size wascontrolled by the deposited film thickness monitored using quartz microbalance system.

Photoluminescence measurements were performed by excitation with a continuous light of325 nm from a He-Cd laser (Kimmon IK3302R-E). For lasing experiments, a frequency tripledlight pulse of 355 nm from a Nd:YAG laser with 5 ns pulse duration was used. The excitationarea was about 300 μm in diameter. The excited light was incident at an angle of 45◦ to thesample surface. Emission spectra were measured using a single monochromator equipped witha charge-coupled device (CCD, Princeton Instruments, PIXIS:100B). Spatially resolved lasingemission spectra were measured using the same CCD in the imaging mode, where a spatialresolution of about 6 μm was obtained from the vertical dimension (2 mm) of the CCD chip.Emission from the sample was collected using two double convex lenses, 74 and 220 mm infocal length and 50 mm in diameter.

3. Results and discussion

Figure 1(c) shows the photoluminescence spectra of powdered GaN with and without the de-position of Au nanoparticles measured by excitation with a continuous He-Cd laser. The broademission band with a peak at 650 nm observed for the sample without Au deposition cor-responds to the defect-related emission band previously reported [17]. A small peak is alsoobserved at around 390 nm (∼3.2 eV). This peak corresponds to the band-edge emission inGaN. The deposition of a Au layer with a mass thickness of 2 nm decreases the broad emissionband intensity and simultaneously greatly enhances the band-edge emission at 390 nm.

The same enhancement as that shown in Fig. 1(c) is observed in ZnO nanowires and nanorodsdeposited with Au or Ag2O nanoparticles [7–9]. The two independent mechanisms of this typeof photoluminescence enhancement are proposed [7, 9]. One mechanism is the resonance ab-sorption of defect emission by the excitation of surface plasmons on metal nanoparticles andsubsequent charge back-transfer process from metal to ZnO [7]. The other mechanism is thelocal electric-field-induced enhancement of the band-edge emission due to the surface plasmonexcitation on metal nanoparticles [9]. The resonance energy of the surface plasmons should bethe defect emission peak energy in the former case and the band-edge emission energy in thelatter case. In the present study, the embedded Au nanoparticles are found to show an absorp-tion peak at around 660 nm. This wavelength exactly coincides with the defect emission peakobserved in the powdered GaN. Therefore, at least for our sample, the former mechanism isconcluded to be dominant. Indeed, such an enhancement was not observed when the depositedAu layer thicknesses were larger or smaller than 2 nm shown in Fig. 1(c) (dashed lines).

Figure 2 shows the evolution of emission spectra from powdered GaN with Au nanoparticlesexcited by a pulsed Nd:YAG laser. The spectra are plotted after integration over 50 excitationpulses. At low excitation powers, only a weak emission band is observed at 390 nm (3.2 eV).At excitation powers above the threshold (≥ 2 MW/cm2), sharp emission peaks appear nearthe center of the broad spontaneous emission band. With a further increase in excitation power,the emission intensity increases and becomes slightly broader. In the inset of Fig. 2, we showthe lasing output intensity as a function of excitation power density. We observed no lasingspectrum at lower excitation power densities. At above lasing threshold powder density (≥


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Fig. 1. Scanning electron microscope images of (a) random-laser sample surface and (b)a GaN aggregate on it. (c) Photoluminescence spectra of powdered GaN with and withoutAu nanoparticles.

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Fig. 2. Evolution of photoexcited emission at powdered GaN with Au nanoparticles excitedat various powers from 1 to 18 MW/cm2. The spectra are plotted after integration over 50excitation pulses. The inset shows output intensity as a function of excitation power density.


∼2 MW/cm2), the output intensity steeply increased with increasing excitation power density.Because the GaN aggregates are well separated with each other [see Fig. 1(a)], single aggregatemay act as a random-mode cavity, which is formed due to refractive index inhomogeneity [13].Thus, our observed sharp peak in Fig. 2 may arise from random lasing due to the resonantfeedback in each cavity.

It should be noted that such a lasing emission never occurs in powdered GaN without Au evenat the highest excitation power of our pulsed laser. In the case of GaN without Au nanoparticles,almost all the excited electrons may be trapped at the defect sites and recombine with theemitted light in the defect emission peak region (∼660 nm). This leads to a decrease in band-edge emission intensity. Such loss of the band-edge emission inhibits random lasing. On theother hand, in the case of GaN with Au the loss can be compensated by the surface plasmonabsorption of the defect-emitted light and the subsequent charge back-transfer process fromAu nanoparticles to powdered GaN. Therefore, the lasing emission is attained via the surface-plasmon-related processes above a threshold excitation power.

The lasing emission lines observed in the present study are broader than those in other semi-conductor random lasers [2, 4, 6]. The typical full-width at half maximum (FWHM) value ofZnSe random laser is 0.4 nm [6], while that observed in the present study is 2.5 nm. The largebroadening is caused by the fact that each spectrum in Fig. 2 was obtained after integrationover 50 Nd:YAG laser excitation pulses. In Fig. 3(a), we show the pulse-to-pulse spectra ex-cited sufficiently above the threshold power (∼20 MW/cm2) at a fixed excitation spot. Severalnarrow spikes with an FWHM value of ∼0.3 nm are observed on each spectrum. This FWHMvalue is nearly the same as that observed in a ZnSe random laser [6]. As shown in Fig. 3(a), theobserved lasing spike pattern fluctuates from pulse to pulse. This fluctuation is a typical featureof random laser and has been observed in other random laser systems [15,18,19]. Note that nolasing spike appears in the spectrum P1, although the lasing probability is very high because ofsufficient lasing excitation power.

We performed the Fourier transformation (FT) of the emission spectra to estimate the cavitylength of the random laser [13]. The cavity length is obtained from the relation

d = mDn/2, (1)

where d is the Fourier component, m is an integer denoting the FT harmonic, D is the circularcavity diameter, and n represents the refraction index of the lasing medium.

Figure 3(b) shows the FT spectra of the three pulse-to-pulse spectra, P1, P2, and P3, inFig. 3(a). The spectral resolution of the FT spectra is about 0.3 μm, which is estimated from thespectral resolution of lasing spectra in Fig. 3(a), ∼0.1 nm. No clear FT component is observedin the FT spectrum of P1. This is because no clear lasing signal is detected in its emissionspectrum. The main peak at d = 7 μm and its higher-order peaks are clearly observed in the FTspectrum of P2. In P3, peaks at d = 11 and 17 μm as well as the FT component at d = 7 μmare observed. By introducing the refraction index value of n = 2.617 at 3.1 eV (387.5 nm) [20],the cavity diameters are determined to be D = 5 μm for P2 and 5, 8, and 13 μm for P3. Thesevalues are comparable to that obtained in a ZnSe random laser (∼6 μm) [21]. Note that thesecavity diameters are much smaller than the spot size of a Nd:YAG excitation source (∼300μm).

The FT analysis indicates the formation of lasing cavities with different diameters by pulse-to-pulse excitation. These cavities are considered to be located at spatially different positionsin the lasing medium. To investigate the spatial-dependent random lasing properties in detail,spatially resolved lasing emission spectra were measured. Figure 4(a) shows spatially (verticalaxis) and spectrally (horizontal axis) resolved lasing emission profiles. The color scale barrepresents the emission intensity. Each spectrum is obtained after integration over 50 excitationpulses.


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Fig. 3. (a) Pulse-to-pulse spectra obtained at fixed excitation spot. The excitation powerdensity is 18 MW/cm2. (b) FT spectra of lasing spectra in (a).

Several lasing spots are observed in Fig. 4(a). In the rectangular area A1, the large lasing spotis observed at the 400 to 450 μm CCD position. This lasing spot exhibits the highest intensity.A small lasing spot is also observed at around 350 μm in the rectangular area A2. This spotemits a relatively weak lasing light. Note that the lasing intensity fluctuates in space due topulse-to-pulse fluctuation, and thus the reproducibility of the spatial mode map is low in short-range order. This spatial intensity fluctuation was also observed in the literature [15]. However,the overall (long-range order) feature, i.e., the positions of large lasing spot A1 and small lasingspot A2, is not changed from pulse to pulse.

In the inset of Fig. 4(b), we show spatially resolved lasing emission spectra in the rectangularareas A1 and A2. Because these spectra are obtained after integration over 50 excitation pulses,a broad peak together with spikelike structures is observed in both spectra. Figure 4(b) showsthe FT spectra obtained from these emission spectra. The broad FT component in the spectrumof A2 peaks is observed at around d = 14 μm. This component is the second-order peak ofthe main component observed at d = 7 μm in the pulse-to-pulse FT spectra [Fig. 3(b)]. In theFT spectrum of A1, the weak FT component at 22 μm as well as the component at 14 μm areobserved. The 22 μm component may be the second-order peak of the main FT component at11 μm observed in Fig. 3(b).

The spatially resolved FT analysis revealed that the A1 area has several cavities, at least twocavities with D = 5 and 8 μm. This result can be easily understood from the fact that the A1


area has widely extended and strongly lasing spots [see Fig. 4(a)]. It should be noted that thesize of the smaller cavity in the A1 area is the same as that in the A2 area, D = 5 μm. This mayindicate that there is a threshold cavity size for lasing, which is determined, for example, fromthe gain and loss values, the mean free path of scattered light, and the lasing wavelength.

Finally, we determine the difference between the lasing spatial profile and that of the scat-tered excitation light at the random medium (i.e., GaN aggregates). Figure 5(a) shows the spatialintensity profile for the scattered excitation light (355 nm) in the same area and using the sameCCD system as that in Fig. 4(a). The intensity profile in Fig. 5(a) shows a highly irregularlypattern. The irregularity is due to the inhomogeneous scattering event of coherent light at therandom medium. The observed irregular pattern, which is termed the “speckle pattern,” con-tains much information on the random medium [22]. The speckles in Fig. 5(a) appear in theCCD position range from 200 to 500 μm. This pattern is clearly different from the intensityprofile of the lasing emission in Fig. 4(a).

To clarify the difference between the lasing emission profile [Fig. 4(a)] and the speckle pat-tern [Fig. 5(a)], we calculate the intensity-intensity autocorrelation function C(Δr) [15]

C(Δr) =∑r[I(r)−< I(r)>][I(r′)−< I(r′)>]

∑r[I(r)−< I(r)>]2, (2)

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Fig. 4. (a) Spatially (vertical axis) and spectrally (horizontal axis) resolved lasing emissionprofiles. (b) FT spectra of integrated emission spectra (inset) in two six-pixel regions (A1and A2) shown in (a).


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Fig. 5. (a) Spatially (vertical axis) and spectrally (horizontal axis) resolved intensity profilesfor scattered light from excitation source. (b) Intensity-intensity autocorrelation functionsat random lasing spots (390 nm) and scattered light area (355 nm).

where I(r) represents the measured intensity at the CCD position r, Δr is the correlation length,Δr2 = (r′−r)2, and triangular brackets represent ensemble averaging. Because C(Δr) representsthe correlation intensities between the intensities of the emissions at r and r′, the decay lengthof C(Δr) gives the size of the lasing spot or speckle (a bright or dark spot) [22]. Figure 5(b)shows the calculated correlation intensities C(Δr) as a function of the correlation length Δr forthe lasing emission (390 nm) and scattered light (355 nm). The calculated data in the randomlaser was normalized to the value at Δr = 6 μm because the value at Δr = 0 μm has largeexperimental noise [15].

As shown in Fig. 5(b), the lasing correlation length plot decays more slowly than that ofthe scattered light. The decay lengths were estimated by fitting the correlation intensity plotswith a single exponential function. The obtained decay lengths are 33 and 11 μm for the las-ing emission and scattered light, respectively. The longer decay length for the lasing emissionindicates that the size of the lasing spots is larger than that of the speckles. The same tendencywas reported in Ref. [15]. The size of the lasing spots is also confirmed to be smaller than thatof the excitation area (∼300 μm). Note that the size of the speckles (∼ 11 μm) is much largerthan that predicted on the basis of a theory (∼0.5 μm) [22]. This may be due to the low spatialresolution of our measurement setup (∼ 6 μm).


4. Conclusion

We observed random lasing emission from powdered GaN with Au nanoparticles and deter-mined the spatial extent of lasing spots by analyzing the spatially and spectrally resolved lasingemission spectra. In the presence of Au nanoparticles on GaN aggregates, several lasing lineswere observed near the center of the GaN band-edge emission above the excitation threshold(≥2 MW/cm2), while GaN aggregates without Au exhibited only spontaneous emission. Thelasing was due to loss compensation by surface plasmon excitation on Au and to the resonantfeedback in each random-mode cavity, i.e., single GaN aggregates. From the spatially resolvedlasing emission spectra and their FT analysis results, the formation of random lasing cavities atdifferent spatial positions was confirmed. The size of the random lasing spots for the GaN/Auaggregates was found to be larger than that for the scattered light of the excitation source usingthe intensity autocorrelation analysis.

Acknowledgments

This work was partly supported by a Grant-in-Aid for Young Scientists (B) (22760050) fromthe Ministry of Education, Culture, Sports, Science and Technology, Japan.


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Gold-nanoparticle-assisted random lasing from powdered GaN