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1、NanoSuwtwedMaterials,Vol.12.pp.135-138,1999ElsevierScienceLtdPergamonQ1999ActaMetallureicaInc.PrintedintheUSA.Allrights~eserved0965.9773/99/$-seefrontmatterPIISO9659773(99)00082-3OPTICALPROPERTIESOFQUASIFRACTALMETALNANOPARTICLEAGGREGATESJ.A.Sotelo*andG.A.Niklasson#*
2、Dpto.deFisicayMatematicas,UniversidadPeruanaCayetanoHeredia,Apto.5045,Lima,Peru#DepartmentofMaterialsScience,TheAngstrijmLaboratory,UppsalaUniversity,P.O.Box534,S-75121Uppsala,SwedenAbstract-Wecalculatetheopticalextinctionefficiencyoftwo-dimensionalsmallquasifractal
3、aggregatesconsistingofmetalnanoparticles.Computationswerecarriedoutintheoctupolarapproximationforaggregatesof2.5particles.Calculationsforgoldparticlesmuchsmallerthanthewavelengthoflightshowedtwomajorpeaksintheextinctioninthevisiblewavelengthrange.Theseareduetotheloc
4、alizedplasmaresonanceoftheconductionelectronsinasingleparticleandinthewholeaggregate,respectively.Goingtolargeraggregatesizes,orincludinghighermultipolesshiftsthelowfrequencypeaktowardsstilllowerfrequencies.Forparticleswitharadiusof30Atheoctupolarsolutioniswellappro
5、ximatedbyincludinginteractionsuptothethirdnearestneighborparticlesonly.Ontheotherhandasimpleiterativemethodearlierproposedbyusdoesnotgivegoodresultsatlowfrequencies.01999ActaMetallurgicaInc.INTRODUCTIONTheopticalpropertiesoffractalaggregatesconsistingofsmallparticle
6、shaverecentlybeenstudiedinconsiderabledetail.Aggregatesofdielectricparticleshavebeentreatedbythemeanfieldtheory(1,2),withintheanomalousdiffractionapproximation(3)andbythecoupleddipolemethod(4).TheoreticalworkonaggregatesofmetallicparticleshasbeenreviewedbyShalaev(5)
7、.Thecoupleddipolemethodhasbeenappliedalsotothiscase(6);otherapproachesencompasscomputationsofthesurfacemodespectrum(7)andmodelingtheaggregateasaLCR-network(8).Computationsofopticalpropertiesoftouchingmetallicparticlesareespeciallydemanding,sincealargenumberofmultipo
8、lesmaybeneededforhighaccuracy.ForsmallnanoparticlesintheRayleighlimitmorethan10multipolesmaybenecessary,becauseofthehighlynonuniformfields
