LETTERS Demonstration of a spaser-based nanolaser M. A. Noginov1, G. Zhu1, A. M. Belgrave1, R. Bakker2, V. M. Shalaev2, E. E. Narimanov2, S. Stout1,3, E. Herz3, T. Suteewong3 & U. Wiesner3 One of the most rapidly growing areas of physics and nanotech- nology focuses on plasmonic effects on the nanometre scale, with possible applications ranging from sensing and biomedicine to imaging and information technology1,2. However, the full develop- ment of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed3 that in the same way as a laser generates stimulated emission of coherent photons, a ���spaser��� could generate stimulated emission of surface plasmons (oscillations of free electrons in metallic nano- structures) in resonating metallic nanostructures adjacent to a gain medium. But attempts to realize a spaser face the challenge of absorption loss in metal, which is particularly strong at optical frequencies. The suggestion4���6 to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently7���10 and even allowed the amplification of propagating surface plasmons in open paths11. Still, these experi- ments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles12���14 lack the feedback mechanism present in a spaser. Here we show that 44-nm-diameter nanoparticles with a gold core and dye-doped silica shell allow us to completely overcome the loss of localized surface plasmons by gain and realize a spaser. And in accord with the notion that only surface plasmon resonances are capable of squeezing optical frequency oscillations into a nanoscopic cavity to enable a true nanolaser15���18, we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531 nm makes our system the smallest nanolaser reported to date���and to our knowledge the first operating at visible wavelengths. We anticipate that now it has been realized experimentally, the spaser will advance our fundamental understanding of nanoplasmonics and the development of practical applications. Aspasershouldhaveamediumwithopticalgaininclosevicinityto a metallic nanostructure that supports surface plasmon oscillations3. To realize such a structure, we employed a modified synthesis technique for high-brightness luminescent core���shell silica nanoparticles19,20 known as Cornell dots. As illustrated in Fig. 1a, the produced nano- particles are composed of a gold core, providing for plasmon modes, surrounded by a silica shell containing the organic dye Oregon Green 488 (OG-488), providing for gain. Transmission and scanning electron microscopy measurements give the diameter of the Au core and the thickness of the silica shell as ,14 nm and ,15 nm, respectively (Fig. 1b, c). The number of dye molecules per nanoparticle was estimated to be 2.7 3 103, and the nanoparticle concentration in a water suspension was equal to 3 3 1011 cm23 (Methods). A calculation of the spaser mode of this system (Fig. 1d) yields a stimulated emission wavelength of 525 nm and a quality (Q)-factor of 14.8 (Methods). We note that in gold nanoparticles as small as the ones used here, the Q-factor is domi- nated by absorption. But as we show below, the gain in our system is high enough to compensate the loss. The extinction spectrum of a suspension of nanoparticles shown in Fig. 2 is dominated by the surface plasmon resonance band at ,520 nm wavelength and the broad short-wavelength band corres- ponding to interstate transitions between d states and hybridized s���p states of Au. The Q-factor of the surface plasmon resonance is esti- mated from the width of its spectral band as 13.2, in good agreement with the calculations. The spectra in Fig. 2 also illustrate that the surface plasmon band overlaps with both the emission and excitation bands of the dye molecules incorporated in the nanoparticles. As illustrated in Fig. 3, the decay kinetics of the emission at 480 nm were non-exponential. Fitting the data with the sum of two expo- nentials resulted in two characteristic decay times, 1.6 ns and 4.1 ns. The absorption and emission spectra of OG-488 (Fig. 2) are nearly symmetrical to each other, as expected of dyes, and this allows us to assume that the peak emission cross-section, sem, is equal to the peak absorption cross-section, sabs 5 2.55 3 10216 cm2, determined from the absorption spectrum of OG-488 in water at known dye concen- tration. With this value and using the known formula relating the strength and the width of the emission band with the radiative life- time t (see ref. 21 and Methods), we obtain an estimated radiative 1Center for Materials Research, Norfolk State University, Norfolk, Virginia 23504, USA. 2School of Electrical & Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA. 3Materials Science and Engineering Department, Cornell University, Ithaca, New York 14850, USA. 1 0 b a c d Gold core Sodium silicate shell OG-488 dye doped silica shell Figure 1 | Spaser design. a, Diagram of the hybrid nanoparticle architecture (not to scale), indicating dye molecules throughout the silica shell. b, Transmission electron microscope image of Au core. c, Scanning electron microscope image of Au/silica/dye core���shell nanoparticles. d, Spaser mode (in false colour), with l 5 525 nm and Q5 14.8 the inner and the outer circles represent the 14-nm core and the 44-nm shell, respectively. The field strength colour scheme is shown on the right. Vol 460|27 August 2009|doi:10.1038/nature08318 1110 Macmillan Publishers Limited. All rights reserved ��2009

evidence that the stimulated emission occurs in individual hybrid Au/silica/dye nanoparticles, rather than in the macroscopic volume of the cuvette. The diameter of the nanoparticle (hybrid Cornell dot) is 44 nm��� too small to support visible stimulated emission in a purely photonic mode. But modelling of the system predicts that stimulated emission can occur in a much smaller surface plasmon mode if the number of excited dye molecules per nanoparticle exceeds 2.0 3 103 (Methods) this number is smaller than the number of OG-488 molecules available per nanoparticle in the experimental sample, which is ,2.73 103. The pumping photon flux in our measurements (,1025 cm22 s21) exceeds the saturation level for OG-488 dye mole- cules (,1024 cm22 s21), so almost all the dye molecules were excited. The gain in the system was thus sufficiently large to overcome the overall loss, enabling the first experimental demonstration of a spaser, which we report here and regard as the central finding of the present work. But another important result is that the outcoupling of surface plasmon oscillations to photonic modes (facilitated by the radiative damping of the localized surface plasmon mode) constitutes a nanolaser that is realized by each individual nanoparticle, making it the smallest reported in the literature and the only one to date operating in the visible range. Thedemonstrated phenomenon, involving resonantenergy transfer from excited molecules to surface plasmon oscillations and stimulated emission of surface plasmons in a luminous mode, is consistent with the original theoretical proposal of a spaser3 and the more recent concept of a ���lasing spaser���25, which share many common features despite their differences in detail. We note that this phenomenon is very different from that exploited in quantum cascade lasers26, in which the surface plasmon mode (almost indistinguishable at the mid-infrared wavelength and the geometry of the experiment from the photonic transverse electromagnetic mode) is used as a guiding mode in an otherwise normal laser cavity. In contrast, in the reported spaser, the oscillating surface plasmon mode provides for feedback needed for stimulated emission of localized surface plasmons. The ability of the spaser to actively generate coherent surface plasmons could lead to new opportunities for the fabrication of photonic meta- materials, and have an impact on technological developments seeking to exploit optical and plasmonic effects on the nanometre scale. METHODS SUMMARY The Methods section presents a detailed discussion of the following experimental and theoretical studies: (1) synthesis and cleaning of hybrid Au/silica/dye nano- particles, (2) theoretical modelling of the spaser effect in hybrid core���shell nanoparticles, (3) emission kinetics measurements, and (4) calculation of the radiative decay lifetime from the emission spectra. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. Received 15 September 2008 accepted 24 July 2009. Published online 16 August 2009. 1. Maier, S. A. Plasmonics: Fundamentals and Applications (Springer, 2007). 2. Brongersma, M. L. & Kik, P. G. (eds), Surface Plasmon Nanophotonics (Springer Series in Optical Sciences, Vol. 131, Springer, 2007). 3. Bergman, D. J. & Stockman, M. I. Surface plasmon amplification by stimulated emission of radiation: quantum generation of coherent surface plasmons in nanosystems. Phys. Rev. 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I., Prosvirnin, S. L., Papasimakis, N. & Fedotov, V. A. Lasing spaser. Nature Photon. 2, 351���354 (2008). 26. Sirtori, C. et al. Long-wavelength (l 8���11.5 mm) semiconductor lasers with waveguides based on surface plasmons. Opt. Lett. 23, 1366���1368 (1998). Acknowledgements The work was supported by NSF PREM grant DMR 0611430, NSF NCN (EEC-0228390), NASA URC (NCC3-1035), an ARO-MURI award (50342-PH-MUR) and a United States Army award (W911NF-06-C-0124). We thank M. I. Stockman for discussions, and J. Chen and J. Irudayaraj for the assistance with the kinetics measurements. S.S. was a member of the Summer Research Program at the Center for Materials Research, Norfolk State University. Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to M.A.N. (mnoginov@nsu.edu). LETTERS NATURE|Vol 460|27 August 2009 1112 Macmillan Publishers Limited. All rights reserved ��2009