We make a step towards quantum nanoplasmonics: surface plasmon fields of a nanosystem are quantized and their stimulated emission is considered. We introduce a quantum generator for surface plasmon quanta and consider the phenomenon of surface plasmon amplification by stimulated emission of radiation (spaser). Spaser generates temporally coherent high-intensity fields of selected surface plasmon modes that can be strongly localized on the nanoscale, including dark modes that do not couple to far-zone electromagnetic fields. Applications and related phenomena are discussed.

[1] We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NO y , NH x ) and sulfate (SO x ) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60-70% of the model-calculated wet deposition rates agree to within ±50% with quality-controlled measurements. Models systematically overestimate NH x deposition in South Asia, and underestimate NO y deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NO y , NH x , and SO x , leading to ±1 s variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36-51% of all NO y , NH x , and SO x is deposited over the ocean, and 50-80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the ''critical load'' threshold of 1000 mg(N) m À2 yr À1 . The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia

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As an efficient nanolens, we propose a self-similar linear chain of several metal nanospheres with progressively decreasing sizes and separations. To describe such systems, we develop the multipole spectral expansion method. Optically excited, such a nanolens develops the nanofocus ("hottest spot") in the gap between the smallest nanospheres, where the local fields are enhanced by orders of magnitude due to the multiplicative, cascade effect of its geometry and high Q factor of the surface plasmon resonance. The spectral maximum of the enhancement is in the near-ultraviolet region, shifting toward the red region as the separation between the spheres decreases. The proposed system can be used for nanooptical detection, Raman characterization, nonlinear spectroscopy, nanomanipulation of single molecules or nanoparticles, and other applications.

Li, Stockman, and Bergman Reply: In their Comment [1], Li, Yang, and Xu (LYX) contend that, within their electrodynamic computation, the maximum enhancement is about 2 times smaller than in our original Letter [2], and with the finite-size effects taken into account it is less by an additional factor of 2.There were two main results reported in our original Letter [2]. The first is based on a qualitative idea of transfer of the excitation and field down the scale of sizes in selfsimilar systems, which leads to multiplicative enhancement of the local field. The maximum enhancement occurs in a small nanofocus (''the hottest spot'') in the gap between the smallest nanospheres. This is the main part of our Letter, and it is not contested in the Comment, which actually confirms the existence of this nanofocus and large field enhancement in it. The second part of Ref.[2] provides an illustration: some computations for three-, five-, and six-sphere nanolenses, which show that the fields are enhanced at the nanofocus, depending on the geometry, by a factor from 500 to 2200. These numbers are stated by LYX to be by a factor of 2 -4 larger than their results.In our original Letter, we clearly stated that we solve the problem in the quasistatic approximation, neglecting spatial dispersion and Landau damping. The parameters of our systems were deliberately chosen to be at the limits of applicability of that approximation. The Comment actually shows that these parameters are reasonable: An error by factor 2, when the total enhancement is 1200, is a reasonably good accuracy given that there are many other physical effects which were ignored in our model, and also in the Comment (see below). We do maintain that our model was solved accurately. A careful perusal of Fig. 1(a) shows that the field at the nanofocus, seen as the very narrow high peak just outside the smallest sphere in the 1.5 nm intersphere gap, changes by more than an order of magnitude in the tangential direction and in the radial direction as soon as that sphere is entered. This abrupt change occurs over a distance of less than 0.5 nm-the grid size used by LYX is too coarse.The great enhancement of field strength arises due to the fact that the frequency is close to that of one or more localized scattering resonances of the system [2]. The generalized Mie theory used by LYX is a multiple scattering theory [3,4]. Such a procedure cannot be expected to be accurate at a frequency close to a scattering resonance: Precisely at the resonance, it will diverge.For our part, we do not understand why the two different approaches, which were used by LYX, yield results that are so close to each other -perhaps this is fortuitous? Finally, LYX discuss numerical consequences for the surfaceenhanced Raman scattering enhancement factor G RS , ignoring the fact that G RS Þ jEj 4 , where jEj is the enhancement factor of the local electric field. Figure 1(b) shows our original Green's function computations [5] of G RS along with jEj 4 . Evidently, G RS differs from jEj 4 by orders...