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Offset-apertured near-field scanning optical microscope probes

M. C. Quong and A. Y. Elezzabi

Open Access Open Access

Abstract

Near-field scanning optical microscope (NSOM) probe designs consisting of a subwavelength aperture offset of either a metallic or metal-coated dielectric cantilevered tip are investigated using finite-difference time-domain calculations. The offset aperture and metal-coated dielectric tip couple surface plasmons that illuminate the tip apex, which results in a single-lobed probing optical spot having a full-width half maximum (FWHM) similar to the apex diameter. Since the surface plasmons converge at the apex, an offset-apertured probe promises significantly higher throughput light intensities than an apertured NSOM having a comparable spot FWHM.

©2007 Optical Society of America

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Supplementary Material (1)

Media 1: MOV (1387 KB)     

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Figures (8)
Fig. 1.
Fig. 1. Cross-sectional depiction of the geometries (not to scale) of the (a) OAMA, (b) OAMDA, and (c) a typical apertured NSOM tips. Results obtained from the typical apertured NSOM having parameters w=155 nm, h=80 nm, D=60 nm, and φ=45° is used as the baseline for all comparisons.

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Fig. 2.
Fig. 2. (a) Calculated field intensities illustrate the steady-state intensity distribution of an OAMDA probe. The polarization direction of the incident electric field is indicated by the arrow. Surface plasmons are coupled onto the outer and inner surfaces of the tip and propagate toward the apex. An enhanced electric field intensity is clearly observed at the sharp 40-nm apex. (b) (1.4 MB) Movie illustrating the side view of an OAMDA probe showing the dynamics of the intensity distribution. Note that the surface plasmon wave propagates both on the inner and the outer surfaces of the NSOM probe towards the apex.

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Fig. 3.
Fig. 3. Illumination spots obtained 20-nm from the apex of: (a) an OAMA probe having a 40-nm wide apex and (b) typical apertured NSOM probe with a 60 nm-wide opening. Both NSOM probes have similar FWHM of ~70 nm; however, the spot intensity distribution from the apertured NSOM probe is double-lobed. The scale bars represent 200 nm, and the linear color scales are in arbitrary units. [Media 1]

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Fig. 4.
Fig. 4. (a) Total intensity for an OAMA probe optical spot as a function of surface wave propagation length, L, calculated 20 nm from the probe apex. The dotted lines (black) represent Lorentzian fits to the peaks, and the solid (red) line represents the sum of the three Lorentzian lineshapes. The intensities are relative to those of the apertured NSOM of Fig. 1(c). (b) Spot full-width half-maximum as a function of the surface propagation length, L. In both 4(a) and 4(b), the probe has a fixed apex diameter of 40 nm, θ=11.3°, t=100 nm, and d=120 nm.

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Fig. 5.
Fig. 5. Total intensity calculated 20 nm from the apex of an OAMA probe as a function of (a) cantilever silver layer thickness for d=120 nm and (b) aperture diameter for t=80 nm. For both (a) and (b), other parameters include an apex of 40 nm diameter, L=4 µm, and θ=11.3°. Intensity values are relative to a conventional apertured probe of Fig. 1(c).

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Fig. 6.
Fig. 6. (a, c) The electric field lines in the offset aperture as a result of incident electric field polarization (parallel to the double-arrowed lines). The large circle represents the base of the tip while the smaller circle represents the aperture. (b, d) Intensity distributions in a calculated planar cut (at 1.2 µm from the apex) in the solid silver tip due to the polarizations shown respectively to their left. Note that the color scales are in arbitary units. An OAMA probe having a 40-nm apex, L=4 µm, θ=11.3°, t=100 nm, and d=320 nm is used for both polarizations.

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Fig. 7.
Fig. 7. Total intensity for an OAMDA probe as a function of the silver layer thickness on the silicon dioxide core. Intensities are calculated 20 nm from the 40-nm wide tip apex. Other parameters used for the calculations were L=4 µm, θ=11.3°, t=80 nm, and d=320 nm. Intensity values are relative to those of the apertured probe of Fig. 1(c).

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Fig. 8.
Fig. 8. (a) Total intensity for OAMDA probes calculated 20 nm from the tip apex as a function of the surface wave propagation length, L. The dotted lines (black) represent Lorentzian fits to the peaks and the solid (red) line represents the sum of the three Lorentzian lineshapes. The intensities are measured relative to those of the apertured NSOM of Fig. 1(c). (b) Spot full-width half-maximum as a function of the surface propagation length, L. In both 8(a) and 8(b), the probe has a tip apex diameter of 40 nm, θ=11.3°, t=80 nm, T=60 nm, and d=120 nm.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

ε ( ω ) = ε 0 ε ∞ − ε 0 ω p 2 ω 2 + i ω ν
ν d D → d t + d 2 D → d t 2 = ω p 2 ε 0 E → + ν ε ∞ ε 0 d E → d t + ε ∞ ε 0 d 2 E → d t 2
L max ( n ) = λ SP 4 ( 1 + 2 n )
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