David Citrin, professor of Optics and Photonics in the School of Electrical and Computer Engineering, gave a lecture on September 27, 2007 at 12 noon in room 102 of the MiRC buildingRuntime: 53:43 minute

Citrin, David S.

Scholarly Materials And Research @ Georgia Tech

Nanoplasmonic Waveguides:Optical Interconnects, Lasers, and Near-Field ProbesDavid S. CitrinSchool of Electrical and Computer EngineeringGeorgia Institute of TechnologyAtlanta, Georgia 30332-0250 USAandUnité Mixte Internationale 2958 Georgia-Tech-CNRSGeorgia Tech Lorraine, 2-3, rue Marconi, 57070 Metz Francecitrin@ece.gatech.eduOutline• Research Overview• Metal nanoparticle arrays– Nanoplasmonics• ConclusionsNanostructures Nonlinear OpticsUltrafast Phenomena/Terahertz TechnologyResearch OverviewPhotoconductive THzGenerationVSI-GaAssub-ps optical pulseTHz pulseS. E. Ralph, D. Denison-Georgia TechUC Santa Barbara Free-Electron LaserM. S. Sherwin-UCSBTHz Sourcesand various other approachesLarge & $$$$Nonlinear OpticsTHz/Optical Nonlinearities in SemiconductorsOptical beam THzOutline• Research Overview• Metal nanoparticle arrays– Nanoplasmonics• ConclusionsThe Lycurgus Cup (glass; British Museum; 4th century A. D.)When illuminated fromoutside, it appearsgreen.  However, whenIlluminated from withinthe cup, it glows red.Red color is due to verysmall amounts of goldpowder (about 40 partsper million)Metal Nanoparticles2-200 nmAu, Ag,… Other shapes (cups, rods, wires,core-shell structures,…)Metal Nanoparticle Surface PlasmonsDisplacementof the mobileelectrons-+RestoringForceSurface Plasmons: Periodic (but damped) charge oscillationsQ~10Surface Plasmon at Dielectric-Metal Interfaceplasmonsurface plasmonDispersion Relation for Surface PlasmonBoundary Conditions Result in Discrete SP ModesOptically important n=1 modeElectric-dipole moden=1 n=2 n=3 n=4Can be obtained from Mie theoryMetal Nanoparticle ChainsBend LossesDefectsCrosstalkMetal Nanoparticle Chains• Potential: Fabricate optical waveguides withlateral dimensions << l --Nano-opticalInterconnects, near-field optical probes• Challenges: Control attenuation!Metal Nanoparticle Chains• 40-100 nm radius Au/Ag nanoparticles onglass/semiconductor substrate; chains with 75-300nm period• Single nanoparticles: broad plasmon resonance• Optical propagation along chains measured(indirectly)Electron micrograph   Optical Extinction2D FDTD Calculation of Electromagnetic Propagation inMetal Nanoparticle ChainsMetal Nanoparticle ChainsNanoparticle surface plasmons modeled as static point dipoleswith nearest-neighbor coupling, PhD Thesis, S. A. MaierEffects of Higher Multipoles• Distortion of dispersion relation, compared toretaining only dipole-dipole interaction--effectmost pronounced when particles approachcontactBut Hard to Incorporate!What we do: only keep electric dipole moment;correctly account for retardationExcellent account of optical properties providedspacing is not too small (G. Schatz)Plasmon-Polariton DispersionRelationw(q)- wp-S[w(q),q]=0q=2p/lSelf-Energy:Real part: mode frequencyImaginary part: radiative lossComplex!Self-Energy:Retarded Dipole-Dipole EnergyS(w, r12)=E (r12,tret).d21 2d2,r12Retarded dipole-dipole coupling:• Polarization perpendicular to r12:  r12-1, r12-2, r12-3• Polarization parallel to r12 : r12-2, r12-3Long-range coupling only forperpendicular polarization•Fourier transform r12 --> q•Express excitation wavelength andoptical wavelength in dimensionless unitsk=qd, c=kdk=we1/2/c•Write S in units of single nanoparticle radiative widthDispersion and Radiative Width in Infinite ChainsExcitation wavevectorDispersionRe S(k,c)Radiative Width-Im S(k,c)T-modes L-modesk=qd, c=kdD. S. Citrin,Nano Lett. 4,1561 (2004).Nano Lett.CloseSpacingZone-folded light lineDispersion in Infinite ChainsExcitation wavevectorDispersionRe S(k,c)T-modes L-modesDispersionPlasmon-poleapproximationRadiative Width in Infinite ChainsExcitation wavevectorRadiative Width-Im S(k,c)T-modes L-modesRadiativeWidthSurface-Plasmon LaserGain MediumMetalHeavily-doped n-typeMIR and FIR QCLA. Tredicucci et al., Appl. Phys. Lett. 76, 2164 (2000).J. Kalkman et al., Appl. Phys. Lett. 83, 30 (2003).Attenuation Management via a Gain MediumGainAttenuation Management via a Gain MediumGainAttenuation Management via a Gain MediumGainSurface plasmon polaritons benefit from gain; radiativePlasmon polaritons do not!Radiative decay rate goes negative for surface plasmon polaritons!D. S. Citrin, Opt. Lett., in press.Perfect dielectricLossGainIn-Coupling to and Out-Coupling from Semi-Infinite Nanoparticle Chains:Far-Field CouplingIn-Coupling to Semi-Infinite Nanoparticle Chains:Far-Field CouplingkInduced dipole moment    ReImnL TL TConclusion:  Dipole distribution only modified within ~10 nanoparticles nearest chain termination.In-Coupling to Semi-Infinite Nanoparticle Chains:Far-Field Radiation PatternsConclusion: Kirchhoff approximation is lousy.LT TDashed lines are the radiation distributions in theKirchhoff approximationIn plane of polarization In plane perpendicular to polarizationOut-Coupling to Semi-Infinite Nanoparticle Chains:Far-Field CouplingkInduced dipole moment    ReImnL TL TConclusion:  Dipole distribution only modified within ~10 nanoparticles nearest chain termination.In-Coupling to Semi-Infinite Nanoparticle Chains:Far-Field Radiation PatternsConclusion: Kirchhoff approximation is lousy.LT TDashed lines are the radiation distributions in theKirchhoff approximationIn plane of polarization In plane perpendicular to polarizationNear-Field Coupling to Semi-Infinite Nanoparticle ChainsNear-Field Coupling to Semi-Infinite Nanoparticle ChainsRecall: Dispersion of T-modes on nanoparticle chainexhibits negative group velocityInduced dipole moment    ReImnL TL TNear-Field Coupling to Semi-Infinite Nanoparticle ChainsLTLocal spatial phase is positive:Phase velocity away from termination/excitationLocal spatial phase is (mostly) negative:Phase velocity toward termination/excitationNear-Field Coupling to Semi-Infinite Nanoparticle ChainsDefects in Nanoparticle ChainsSubstitutional ImpurityVacancyDefects in Nanoparticle ChainsOff-diagonaldisorderDiagonaldisorderExact inversion of D+-1nmTwo-step approach• Diagonal term using standard approach• Off-diagonal termExact T-matrix Tqq’: Describes the vacancy-induced scattering of plasmon polaritonsL-modesT-modesOther Topics Investigated• Finite length chains (D. S. Citrin, Nano Lett. 5,985 (2005).Nanoparticle RingNano-optical resonatorNano-optical corralNano-antennaNano-optical trap…FrequencyWavevectorRe S-Im SWavevectorRadiativeWidthDispersionT LBright modeD. S. Citrin, J. Opt. Soc. Am. B 22, 1763 (2005).What are the bright modes?Wavevector:         n=1     n=0NanoantennaMolecularTrapConclusions• Optical propagation on nanoparticle chains involvesinterplay of long/short-range coupling, polarizationeffects, radiative decay,…• Attenuation is severe, but there might be strategiesfor its management.• A nascent understanding of in- and out-coupling tonanoparticle chains is emerging.• Nanoparticle rings might form subwavelengthoptical resonators.

Nanoplasmic Waveguides: Optical Interconnects, Lasers, and Near-Field Probes

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Nanoplasmic Waveguides: Optical Interconnects, Lasers, and Near-Field Probes

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