Lecture 12 – Nanophotonics in Microscopy

EECS 598-002 Winter 2006 Nanophotonics and Nano-scale Fabrication P.C.Ku Schedule for the rest of the semester

„ Introduction to light-matter interaction (1/26):

„ How to determine ε(r)?

„ The relationship to basic excitations. „ Basic excitations and measurement of ε(r). (1/31) „ Structure dependence of ε(r) overview (2/2) „ Surface effects (2/7):

„ Surface EM wave

„ Surface polaritons

„ Size dependence „ Case studies (2/9 – 2/16):

„ Quantum wells, wires, and dots

„ Nanophotonics in microscopy

„ Nanophotonics in plasmonics „ Dispersion engineering (2/21 – 3/7):

„ Material dispersion

„ Waveguide dispersion (photonic crystals)

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 2 Principles of optical microscopy

„ The information contained in the object/image is carried by the light wave to the detector (e.g. eyes).

„ Information = F(x,y) Æ F(,kkxy )

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 3 Optical vs electron microscopy

„ Advantages of optical microscopes

„ Photon energy is low (~ eV). Electron energy is high (~10-100 keV) -27 „ Photon momentum is low (~ 10 kg m/s). Electron momentum is high (~ 10-23 -10-22 kg m/s)

„ Especially good to study optical processes (including nonlinear optical response) in the sample

„ Ultra-short optical pulse is available to study ultra-fast processes

„ Can make the fluorescence work Æ useful for biological imaging

„ Disadvantages of optical microscopes

„ Photon does not have charge. Cannot study the Coulomb interaction in the sample.

„ Photon does not reveal information regarding the chemical composition of the sample (unless we go to the x-ray wavelength).

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 4 Resolution limits of optical microscopes

„ Small features correspond to large (kx, ky) components.

„ In traditional optical microscopes, the detector sees the light in the far fieldk region.

2 2 222 k==++ωµε0 kkkxyz

22 ⇒+<⇒kkxyω nck/2/&,max =πλ n

17.5 15 Resolving power 12.5

k-space real-space 10 = λλ/2( n)≡ eff /2 7.5 5 = diffraction limit 2.5

−2/π n λ 2/π n λ k& -2 -1 1 2 λ / n

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 5 Finite-size lens

„ In a real system, the cutoff spatial frequency is often limited by the size of the lens which is quantitatively described by a numerical aperture (NA).

NA≡ n sinθ θ

k&,max 2π ⇒=⇒=sink&,max NA k λ

θ Resolving power Æ

λλ/2NA( )≡ eff /2

where λeff = λ /NA

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 6 Resolution enhancement

„ Shortening of wavelength:

„ Decrease the wavelength of light source

„ Increase the refractive index (e.g. immersion microscope)

„ Using nonlinear optical effect (e.g. frequency doubling)

„ Increase the detectable spatial frequency

„ Moire grating

„ Near field optics to detect the evanescent waves

Ref: S. Kawata, chapter 2, figure 1.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 7 Scanning Near-field Optical Microscope

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 8 Different modes of SNOM

Ref: Prasad, chapter 3, figure 7.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 9 Detection of near field intensity modulation

„ The modulation of the near-field intensity can be detected at the far-field region:

e.g. Thin slab photon tunneling d = wavelength/4 1 Transmission 0.9 Reflection 0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0 20 40 60 80 100 Incident Angle

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 10 Configuration of SNOM

Ref: Prasad, chapter 3, figure 8.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 11 Near-field probes

(aperture in an opaque screen)

(e.g. PSTM)

Green: evanescent wave

Ref: S. Kawata, chapter 2, figure 6.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 12 Advantages of apertureless probes

„ No cutoff frequency as existing in a metal-coated fiber optics probe

„ Field enhancement due to surface plasmon polaritons

„ The probe size can be made much smaller

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 13 Controlling the probe-sample distance

feedback (1) laser excitation (2) laser

beam splitter segment ed photodiod e

The tip-sample distance changes the damping sampl e cant i l ever of the resonance oscillator.

Ref: Prasad, chapter 3, figure 9.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 14 Laser trapping for probe

Ref: S. Kawata, chapter 8.

e.g. 2.5W Nd-YLF traps a gold particle (~ 10-100 nm) with a 100X NA=1.35 objective (oil immersion)

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 15 Examples of SNOM images 1

„ Single molecule fluorescence detection

location

Laser detuning

Ref: W. E. Moerner, Science 265 (1994) 46.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 16 Examples of SNOM images 2

„ Single quantum dot

Ref: T. Saiki and Y. Narita, Jpn. J. Appl. Phys., 37 (1998) 1638.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 17 Examples of SNOM images 3

„ Dynamics using pump-probe technique t = 30 ps

t = 0 ps

Ref: Y. Shen et al., J. Phys. Chem. B 107 (2003) 13551.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 18 Examples of SNOM images 4

Topography

0 14500 0 28000

830,25nm

21000 829,53nm 0 21000 0 14000 SNOM 0

6000

0 Background

0 80000 0 1300 0

0 61000 0 18000

VCSEL 15 microns scan range Taken from presentation materials of WITEC. 0 8000 0 120 00 EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 19 Examples of SNOM images 5

MFM - Magnetic Force Microscopy

80 microns scan range 20 microns scan range 5 microns scan range

PC hard drive

Taken from presentation materials of WITEC.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 20 Examples of SNOM images 5

„ Semiconductor Surface Study

Topography SNOM

20% In (a) (b) (c)

24% In (d) (e) (f)

InGaN films

Jeongyong Kim et al., University of Illinois, see also APL Vol. 80, 6, 989 (2002). 27% In (g) (h) (i)

10 microns scan range

Taken from presentation materials of WITEC.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 21 Examples of SNOM images 7

Topography Adhesion 5 microns scan range

Bodyguard adhesive tape Taken from presentation materials of WITEC.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 22 Reading

„ S. Kawata, chapter 2.

EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku 23