纳米激光器Demonstration of a spaser-based nanolaser

LETTERS

Demonstration of a spaser-based nanolaser

M.A.Noginov 1,G.Zhu 1,A.M.Belgrave 1,R.Bakker 2,V.M.Shalaev 2,E.E.Narimanov 2,S.Stout 1,3,E.Herz 3,T.Suteewong 3&U.Wiesner 3

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 technology 1,2.However,the full develop-ment of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields.It has been proposed 3that 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 suggestion 4–6to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently 7–10and even allowed the amplification of propagating surface plasmons in open paths 11.Still,these experi-ments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles 12–14lack 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 nanolaser 15–18,we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531nm 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.

A spaser should have a medium with optical gain in close vicinity to a metallic nanostructure that supports surface plasmon oscillations 3.To realize such a structure,we employed a modified synthesis technique for high-brightness luminescent core–shell silica nanoparticles 19,20

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 ,14nm and ,15nm,respectively (Fig.1b,c).The number of dye molecules per nanoparticle was estimated to be 2.73103,and the nanoparticle concentration in a water suspension was equal to 331011cm 23(Methods).A calculation of the spaser mode of this system (Fig.1d)yields a stimulated emission wavelength of 525nm 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.2is dominated by the surface plasmon resonance band at ,520nm 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.2also 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 480nm were non-exponential.Fitting the data with the sum of two expo-nentials resulted in two characteristic decay times,1.6ns and 4.1ns.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,s em ,is equal to the peak absorption cross-section,s abs 52.55310216cm 2,determined from the absorption spectrum of OG-488in 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.21and Methods),we obtain an estimated radiative

1

Center 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

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 5525nm and Q 514.8;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|27August 2009|doi:10.1038/nature08318

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