Nano-optics. Topics: How do we image things on the nanoscale? How do we use nanofabrication for new optical devices? COSMOS 2006 Lecture 1

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Elfreda Annabella Bruce
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1 Nano-optics Topics: How do we image things on the nanoscale? How do we use nanofabrication for new optical devices?

2 Wave Optics 1. Electromagnetic wave x Ex λ Direction of Propagation y z z plane wave r 2π E( x, y, z, t) = Acos( z ωt) λ light spectrum: E

3 Wave Optics 2. Interference cos(x) cos(x) + = + = cos(x) -cos(x) 2cos(x) 0 phase matters! (constant phase surfaces) k=k z z

4 Wave Optics Huygens principle Every unobstructed point of a wavefront serves as a source of spherical secondary waves. aperture The total field at a point P is the superposition of all secondary wavelets. P interference between all secondary waves

$waves remain confined to center little diffraction effect$

5 Wave Optics λ<<a λ a max. path difference a>λ all phases appear across aperture waves remain confined to center little diffraction effect λ>a max. path difference a<λ waves interfere constructively extend in all directions and angles

6 r i R Wave Optics 3. Fraunhofer Diffraction light distribution at z=d given aperture at z=0 one-dimension: single slit y P θ a Fourier transform y -a/2 a/2 aperture diffraction pattern ka/2 sinθ

7 Wave Optics rectangular aperture a b circular aperture δ Airy rings Bessel function Rλ first minimum δ =1. 22 a

8 Wave Optics 4. Resolution smallest separation between two features to appear separate in far-field Can we see this? 10nm 10nm quantum dots

$diffraction limited resolution δ = 0.$

9 Wave Optics y S 1 A 2 S 1 θ s θ S 2 A 1 S 2 L (after Kasap) I Screen Airy disks need to be separated diffraction limited resolution δ = λ NA Example: λ=0.5µm, NA=1 (after Kasap) δ = 0.3µ m = 300nm Now what??

10 Wave particle duality 5. Electron as wave double-slit experiment with electrons DeBroglie wavelength λ = h p wavelength 1/momentum Example: tennis ball, m=50g, v=20m/s λ=h/(mv)=6.6x10-34 m electron, m=m e =9x10-31 kg, V=100V W=100eV=m e v 2 /2 λ=h/(mv)=0.12 nm

11 Wave-particle duality diffraction: θ d path length difference: L = 2d sinθ constructive interference: L = mλ (Bragg condition) key: d λ for diffraction

12 Lattice types simple cubic (e.g. NaCl) face-centered cubic (fcc) (e.g. Au, Ag) a a a body-centered cubic (bcc) (e.g. Na) diamond (fcc with basis) (e.g. Si) C zinc blende (fcc with basis) (e.g. GaAs, InP) a S a a Zn a a a a

13 Nanostructure imaging

14 SEM 1. Scanning Electron Microscope (SEM) most useful imaging technique focus beam on surface generates secondary electrons (SE) collect SE

15 SEM focused spot E~5-25keV best resolution: nm Depth of focus SEM advantage: large depth of focus

16 SEM Versatility: many things can be imaged semiconductor devices (dead) animals individual nanoparticles biological material processed cheese human teeth

17 SEM gallery spider hair honey bee aphid head louse neuron pollen mitochondria (TEM) termite??

18 Scanning optical microscopy NSOM NSOM = SNOM = Near-field Scanning Optical Microscope scanning probe microscope derivative sub-λ aperture within sub-λ distance to surface feedback for monitoring aperture-sample distance

19 Scanning optical microscopy Imaging modes: collection illumination collection/ illumination oblique collection oblique illumination dark field (after Paesler)

20 Near-field probes: Fiber tips: Scanning optical microscopy sharp fiber tip, etched or pulled usually metal-coated (Al) on sides low coupling: ~10-6 -> problem for collection/illumination mode polarization purity/conservation pulse broadening limited reproducibility

21 Scanning optical microscopy Hollow tips: Si cantilever 100nm Al tip (Witec GmbH) Al tip Si cantilevers with hollow Al tips batch (!!!) microfabrication -> better reproducibility

22 Scanning optical microscopy rat neuron cells human blood cell VCSEL emission chromosome DRAM test structure Polysterene particles

23 5. Infinite potential well confine electron wave in small space One-dimensional potentials V(x) I II III? 0 L x particle in a box What happens to the electron???

24 Infinite quantum well BC at x=l: ψ ΙΙ (L) = A sin(kl)=0 k n π = n L L k n = nπ ; n =1,2,3,... wave vector quantized π h 2mL E n n 2 = particle energy quantized nπ ψ II, n( x) = An sin( x) L 0 L

25 Infinite quantum well V(x) E 3 E 2 E 1 0 L x ψ symm./antisymm. w.r.t. center of well number of nodes: n-1

26 Nano-optics Confinement of electrons on the nanoscale bulk material D(E) ~ E 1/2 quantum well D(E) staircase quantum wire D(E) ~ E -1/2 quantum dot/box D(E) ~ δ(e-e i ) atom-like

27 Optical processes 1. Absorption hυ E n=1 n=1 E 1 L 1 L 2 absorbed energy depends on size (L) different energy corresponds to different wavelength/color

28 Optical processes 2. Emission hυ E 2 E 3 hυ E2 E 1 (b) Spontaneous emission E 1 (d) Fluorescence (molecules) photoluminescence (semiconductors) energy released = light emitted at discrete wavelengths single particle detection -> suitable for nanoparticles can design emission wavelength with size ( E~1/L 2 in infinite well) E n = 2 π h 2mL 2 2 n 2 fluorescence from CdSe/ZnS quantum dots (Bawendi et al)

29 Optical processes 2.1 Organic molecules molecules can vibrate vibration energies are quantized -> levels vibronic transitions in infrared band-like structure above electronic states

30 Optical processes Application: Fluorescence labeling in biology fluorescent molecule = fluorophore selectively tag cell parts with fluorophore locate and identify

constants differ (stress) layer breaks -> islands (dots) form

31 Optical processes Epitaxial nanoparticles (quantum dots) a) Top-down z mask for 70nm GaAs/AlGaAs dots b) Bottom-up wetting layer lattice constants differ (stress) layer breaks -> islands (dots) form self-organization pyramids rather than spheres overgrowth possible -> devices (after Grundmann)

Optical processes Chemically synthesized nanoparticles

relatively narrow size distribution size tunable

32 Optical processes Chemically synthesized nanoparticles (dots) based on group II (Zn,Cd) and group VI (S,Se,Te) elements synthesized from solution core-shell (ZnSe/ZnS) or organic cap layer spherical, relatively narrow size distribution size tunable emission wavelength dots mark tumor location (Seydel, Science, 300, 80, (2003))

Lecture 9: Introduction to Diffraction of Light

$Lecture 9: Introduction to Diffraction of Light$ Lecture 9: Introduction to Diffraction of Light Lecture aims to explain: 1. Diffraction of waves in everyday life and applications 2. Interference of two one dimensional electromagnetic waves 3. Typical