Scanning Near-Field Optical Microscopy for Measuring Materials Properties at the Nanoscale

Scanning Near-Field Optical Microscopy for Measuring Materials Properties at the Nanoscale

Outline Background Research Design Detection of Near-Field Signal Submonolayer Chemical Sensitivity Conclusions (Part I) Introduction to Very Small Aperture Lasers C-shape design Surface Plasmons on VSAL Different Orientations of Apertures Conclusions (Part II)

Contemporary Approaches to Scanning Near-Field Microscopy Aperture Probe Laser Light Apertureless Probe Laser Light Transparent Fiber Coated with Aluminum Scattering Probe Scattered radiation Scanning Modulated Scattered Radiation Scanning Spatial resolution Aperture mid-ir λ/10 Apertureless λ/300

Experimental Setup IR Radiation p-wave Oscillating Probe Sample Tunable CO Laser Multimode AFM IR Objective ~10 μm radiation Parabolic Mirror Partial Reflector Lock-in Amplifier IR Detector

Detection of the Near-Field Signal 30 Amplitude, nm 0 10 IR signal, a. u. 4 f f, x10 0 0 0 40 60 80 100 10 140 160 180 00 Z Distance, nm f signal is more surface-specific

Homodyne Detection of the Near- Field Signal detector back-scattered signal partial reflector reference moved by the piezo driver to maximize the signal The weak nearfield signal is amplified by a strong reference beam [ E ( f ) + E ] = E ( f ) + E ( f ) E E I + ~ E = sc r sc sc r r weak amplified near-field signal no modulation

Tuning Homodyne Amplification 3.5 signal reference partial reflector f signal, a.u. 1.5 1 detector 0.5 0 4 6 8 10 1 14 Partial reflector displacement, microns The automated phase feedback adjusts the position of nearfield signal to the maximum

Distance Dependence of the Near-Field Signal Homodyne detection amplifies the signal 100 3 Amplitude, nm 50 AC signal 1 with reference no reference 0 0 100 00 Z position, nm 0 0 100 00 Z position, nm No tuning to optimize the f IR signal was performed

f-signal collected without the reference is self-homodyned by DC component of back-scattered radiation L. Stebounova, B. B. Akhremitchev, G. C. Walker, Rev. Sci. Inst.74, 3670 (003). Distance Dependence of the Near- Field Signal: Model Probe-sample distance: () t = z + A cos( t) z mean πν Effective polarizability for coupled dipoles: α eff 8πa = a 1 4 3 3 ( a + z) 3 Normalized near-field signal, a.u. Fit error 1 0.8 0.6 0.4 0. 0.05 0-0.05 0 100 00 300 400 500 Z distance, nm a 1 a z

Sub-Monolayer Detection of DNA The gold surface was patterned with alternating stripes of hexadecanethiol and 4bp singlestranded DNA. The pattern was prepared by using the soft lithography technique. 0 Topography DNA no DNA nm 3 1 Near-field signal DNA no DNA a.u. 1.1 1.05 10 0 1-1 0.95-0 0 5 10 15 0-3 μm 0 5 10 15 0 0.9 The DNA pattern is clearer in the near-field image than in the topography image B. B. Akhremitchev, Y. Sun, L. Stebounova, G. C. Walker, Langmuir 18, 535 (00).

Spectrally Resolved Near-Field Signal 0.7 Spectrum of DNA grafted on gold (near-field data were collected using homodyne detection) 7 Far-field attenuation, % 0.6 0.5 0.4 0.3 0. 0.1 6 5 4 3 1 Near-field signal decrease, % 0 1100 1050 1000 950 900 Wavenumber, cm -1 Near-field signal closely follows the far-field absorption spectrum Sub-monolayer chemical sensitivity 0

Near-Field IR Spectroscopy: Conclusions (Part I) An apertureless near-field microscope was used as a tool for chemical imaging of heterogeneous surfaces Imaging of DNA molecules grafted on a gold surfaces reveals sub-monolayer chemical sensitivity of the near-field microscope Homodyne detection was used to improve the sensitivity of the near-field apparatus

Very Small Aperture Lasers Motivations VSAL demonstrates very high output power compact NSOM can be designed Application in storage devices high density rewritable recording and readback marks can be obtained

What is a Very Small Aperture Laser?

Experimental Design E x Z Apertureless probe Active layer Heat sink Cantilever Pre-amp X Lock-in Y Laser Diode XYZ Piezo stage Photodetectors on the laser AFM controller

C-Shape Aperture X X X E 50nm 140nm 80nm 80nm 50nm Y 50 nm Y 50 nm Y F. Chen, A. Itagi, J. A. Bain, D. D. Stancil, T. E. Schlesinger, L. Stebounova, G. C. Walker, B. B. Akhremitchev, Appl. Phys. Lett. 83, 345 (003).

F. Chen, A. Itagi, J. A. Bain, D. D. Stancil, T. E. Schlesinger, L. Stebounova, G. C. Walker, B. B. Akhremitchev, Appl. Phys. Lett. 83, 345 (003). Optical Field Confinement in the Ridge Waveguide 53 nm 69 nm μm μm μm μm

Surface Plasmons Observed on VSAL a) b) c) x x y y y x Interference between the transmitted light and SPs? E x = A exp d exp ( ik x) B x sp + E z = C exp x d exp ( ik x) sp Fitting parameters: A=0.1, B=0.0001, C=3.5, λ sp =350 nm, d=380 nm To be submitted in Optics Letters

Aperture Orientation on VSAL Intensity ratio inside of the apertures: I 3 /I 1 ~.5 Aperture L x H, nm Field decay factor, exp(-αd),* d=50 nm Power decay factor, exp(-αd),* d=50 nm Far-field power, (μw) 00 x 100 100 x 00 * ) α = π H 0.4 0.051 16.5 0.537 0.88 4.5 π λ F. Chen, D. D. Stansil, and T. E. Schlesinger, J. Appl. Phys. 93, 5871 (003)

Conclusions (Part II) VSALs: An optical confinement by the ridge waveguide on VSAL has been observed using near-field microscopy Different shapes and orientations of the apertures have been investigated Possible interference of surface plasmons with directly transmitted through the aperture light has been considered

Acknowledgments NSF ONR ARO NIH