Nanoscale Systems for Opto-Electronics

Transcription

1 Nanoscale Systems for Opto-Electronics 675 PL intensity [arb. units] 700 Wavelength [nm] µm Energy [ev]

2 Nanoscale Systems for Opto-Electronics Lecture 5 Interaction of Light with Nanoscale Systems - general introdcution and motivation - nano-metals (Au, Ag, Cu, Al...) introduction to optical properties mie scattering mie scattering in the near-field mie scattering with nano rods resonant optical antennas - artificial quantum structures (semiconductor quantum dots,...) - quantum dot lasers Optical Interactions between Nanoscale Systems - Förster energy transfer (dipole-dipole interaction) - super-emitter concept - SERS (surface enhanced Raman spectroscopy: bio-sensors) Beating the diffraction limit with Nanoscale Systems - surface plasmon polariton (SPP) - light confinement at nanoscale - plasmonic chips - plasmonic nanolithography 2

3 Last Time: Basic Architecture of Dipole Antenna transmission line ~U feed gap antenna arms of length L perfect metal wires emitting / receiving antenna 3

4 Last Time: RLC Series Curcuit representing Dipole Antenna V0cosωt ω= 1 LC At resonance V Z= 0 j0 Z = R + ix R = Rrad + ROhm ω= 1 LC Z=R C( z) = ε 0 z L( z ) = µ0 z Xc = XL phase ϕ = 0 Q= L CR 2 4

5 Last Time: Linear Center-fed Dipole Antenna power P transmitted as e.m. wave: 1 V02 1 V02 P= Z = ( R + ix ) 2 Z2 2 Z2 R = Rrad + ROhm Summary: antenna resonance exists once the feed gap current density is high and in phase the the driving excitation field 5

6 Last Time: Antenna Feed Gap Confinement and Enhancement Half-wave antenna Full-wave antenna I(z) I(z) ρ(z) ρ(z) I(z) λ/4 λ/4 E0 L = λ/2 z λ/2 λ/2 L=λ Strength of field enhancement depends on current density j(z) Maximum field enhancement for half-wave antenna (L ~ λ/2) 6 z

7 Introduction: Resonant Optical Antennae antennas as receivers cover art Science 308 (2005)

8 Scaling Radio wave antenna: f ~ 1 MHz λ ~ 400 m ~ 200 m Optical antenna: f ~ 1000 THz λ ~ 400 nm Nanofabrication X 10-9 ~ 200 nm SEM image 8

9 Scaling - what is different for antennas at optical frequencies 1. Skin depth 2. Localized surface plasmons Influence on the current distribution 9

10 Skin Depth (δ) E0 E(x) Metal x 10

11 Skin Depth (δ) E0 E(x) Metal x 1 λ δ= = α 4πk absorption coefficent 11

12 Skin Depth Radio Wave Antenna E0 e. m. fields can be assumed to be restricted to the outside of an antenna Optical Antenna E0 e. m. fields are no longer restricted to the outside of an antenna 12

13 FDTD Simulation case: Al dipole antenna λex = 830 nm Finite-Difference Time-Domain Estimate effective wavelength: λ eff/2 ~ λ/2(nglass + nair) = 330 nm 13

14 Particle Plasmon Resonance Quasi-static limit: Particles smaller than the skin depth of metal E Field enhancement is determined only by the material properties, not the size of the particle - but smaller particles give more confinement Field enhancement ~ 20 on the surface (for gold) 14

15 Dielectric function of gold Re[ε ]=-2 15

16 Localized Surface Plasmons quasi-static approximation for elliptical particles: a, b, c << λ z c a b Minimize : ε1 x y εm Polarizability (a.u.) L Plasmon resonance are expected for aluminium gold rods in the NIR; α Dielectric ( L,ω ) function of gold c = b =20 nm; λ = 830 nm and aluminum gold Length L (nm) 16

17 Summarizing: Scaling Effects Large skin depth - change of boundary conditions Localized surface plasmons-resonance of the current due to a resonance in the polarizability Influence on antenna performance at optical frequencies efficiency and resonance length? 17

18 Experimental Approach Prove antenna resonance: vary only one parameter overall length L, fixed wavelength Prove field enhancement in the feed gap: use non-linear processes e.g. two photon-excited Photoluminescence M. R. Beversluis, A. Bouhelier, L. Novotny, Phys. Rev. B 68, (2003). P. J. Schuck, D. P. Fromm, A. Compare rods with antennas Sundaramurthy, G. S. Kino, W. E. Moerner, 94, (2005). no feedgap Phys. Rev. Lett. feedgap 18

19 Fabrication of Optical Antennas I. Electron beam lithography, metallization and lift off electron beam gold (40 nm) resist ITO (10nm) glass cover slip 19

20 Fabrication of Optical Antennas Result of e-beam lithograph, metallization and lift off 50 µ m SEM Patch dimensions: 40 nm x 400 nm x 800 nm 200 nm AFM Surface roughness: ~1 nm rms 20

21 Fabrication of Optical Antennas II. Focused ion-beam milling antennas & rods Analysis by SEM and AFM 200 nm SEM AFM 21

22 Sample Antennas & rods in various length rods 19 antennas x 2.5 µ m2 22

23 Experimental Setup photodiode bandpass (600/300) notch (830) polarizer neutral density bandpass (830/10) optical fiber & fiber polarizer 23

24 Measurements: Confocal experiment Log. Scale! 20 µ m 24

25 Polarization excitation polarization emission polarization (excitation under 45 ) 25

26 Results Only optical antennas in a limited length range show strong white-light continuum emission upon illumination with ps pulses White light generation only for an incident polarization along the antenna long axes 26

27 Spectrum 830 nm, FWHM 8 ps Intensity (a.u.) 2 # µw 80 µw 40 µw M. R. Beversluis, A. Bouhelier, L. Novotny, Phys. Rev. B 68, (2003). x Wavelength (nm) 27

28 Sl op e 2 Sl op e 2 Slo pe Slo pe 4 4 Power Dependence 28

29 White-Light Supercontinuum Precondition: High localization of e. m. fields in space and time strong non-linear optical interaction Characteristic: Intensity threshold ~ 1 GW/cm2 (glass, water) e.g. water ~ 10 GW/cm2 (air) Abrupt spectral broadening Kandidov et al., Appl. Phys. B 77, 149 (2003) 29

30 Slo pe Slo pe 4 4 Power Dependence 2 Estimated field enhancement factor: > 100 Sl op e Sl op e 2 Laser focus peak power at threshold: ~ 0.01 GW/cm2 30

31 Near-field Simulations z H = 50 nm y Antenna dimensions : 40 nm x 40 nm x L x L/2 200 L/2 E0 Near-field intensity distribution X 200 nm Integration 0 R(L) Simulations performed by Prof. O. J. F. Martin (EPFL) using Green s tensor technique [M. Paulus, O. J. F. Martin, J. Opt. Soc. Am. A 18, 854 (2001)]. 31

32 Resonant Optical Antennas Near-field int. enhancement nm 0 4 μw μw μw R(L) 2 R(L). 32

33 Comparison: Au and Al Dipole Antenna 33

34 Results Only optical antennas in a limited length range show strong white-light continuum emission upon illumination with ps pulses White light generation only for an incident polarization along the antenna axes White-light supercontinuum generation at very low threshold intensity (~ 0.01 GW/cm2); estimated field enhancement factor > 100 Experiment and computer simulation show both similar antenna resonance length L ( ~ 250 nm) considerably shorter than λ/2 Calculated field enhancement factor in antenna feed gap >

35 Summary/Conclusion Fabrication of nanometer-scale gold dipole antennas Strong field enhancement in the antenna feed gap white-light supercontinuum generation. Resonance length of a gold antenna is considerably shorter than one-half of the wavelength of the incident light. The high field enhancement and strong resonance shift of a gold dipole antenna compared to that of aluminum antenna may be explained by surface plasmon resonance. 35