The NSOM Probe

The Optics Laboratory

Group of Hans Hallen, North Carolina State University Physics Department

The Probe

• Optical Properties

• Fabrication -- Heat and Pull

• Fabrication -- Etch

• Fabrication -- Metal Coating

• Throughput

• Thermal Loading

• New Ideas

Optical Properties, Theoretical

• Early NSOM Efforts

• L. Novotny, D.W. Pohl, P. Regli, Ultramicroscopy 57, 180-8 (1995).

• Douglas A. Christensen, Ultramicroscopy 57, 189-95 (1995).

• A perfectly conducting plane with a hole.

• H.A. Bethe, Physical Review, 66, 163-182 (1944).

• C.J. Bouwkamp, Phillips Res. Rep. 5, 401 (1950).

- Does a remarkable good job as observed in single molecule and our Raman studies.

• A recent review

• D. Barchiesi et al. Phys. Rev. E54 (4) pt B, 4285-92 (1996).

• An interesting point:

• L. Novotny, D.W. Pohl, B. Hecht, Ultramicr. 61 (1-4) 1-9 (1995).

• L. Novotny, D.W. Pohl, B. Hecht, Opt. Lett. 20 (9) 970-2 (1995).

- Polarized light through an aperture has two spots of maximal electric field intensity.

- Polarized light through a (thinly) covered point has just one maxima, under the point.

Optical Properties, Experimental

Why do we coat the probe tip with metal?

• To contain the light.

• The modes do not remain localized to the dielectric core when it gets small.

• M.A. Paesler and P.J. Moyer, "Near-Field Optics: Theory, Instrumentation and Applications," (Wiley, New York, 1996).

How well does the metal confine the light?

• It depends upon the metal.

• The light intensity falls exponentially into the metal, scaled by the penetration depth.

• Aluminum is best at visible frequencies.

• The minimum confined size for green light is ~10 nm.

• E. Betzig, et al, Science, 251, 1468-1470 (1991).

How do we inspect the probes?

• A point source Abbe pattern should be observed under a good optical microscope for both coated and uncoated fibers, otherwise throw the tip out.

• Scanning electron microscopy - field emission e-gun for resolution - no conductive coating needed for tips - view aperture/shape at different angles

Fabrication of the Tapered Fiber Probe (Heat and Pull)

• B.I. Yakobson, P.J. Moyer, M.A. Paesler, "Kinetic limits for sensing tip morphology in near-field scanning optical microscopes," J. Appl. Phys 73 (11) 7984-6 (1993).

• G.A. Valaskovic, M. Holton, G.H. Morrison, "Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes," Appl. Opt. 34 (7) 1215 (1995).

• R.L. Williamson, M.J. Miles, J. Appl. Phys. 80 (a) 4804-12 (1996).

• Mufei Xiao et al, "Fabrication of Probe Tips for Reflection SNOM: Chemical Etch and Heating Pulling Methods," J. Vac. Sci. Tech. B15 (4) 1516 (1997).

• Heat with a CO2 laser until the fiber begins to soften.

• Then pull hard (solenoid).

• Heating power, timing, and pulling force are adjusted to give the desired tip shape.

• The tip is coated with Aluminum while rotating.

• Inspection verifies lack of pinholes on shank

and presence of hole at tip (optical microscope).

• Tip taper angle studied with optical or electron (SEM) microscopy.

• In practice, a commercial tip puller is used.

An uncoated pulled (large to see details) tip:

• Note the flat cleaved end which will define the aperture (up to diffusion of the evaporant metal).

Etching the fibers:

• HF-based etches are used for silica fibers.

• Basic (ammonium flouride) solutions give smooth surfaces.

• Organic layers floating on the etchant use surface tension to provide controlled angle of the taper (30° for isooctane).

• P. Hoffmann, B. Dutoit, and R.-P. Salathé, Ultramicroscopy 61, 165 (1995).

• Etching based on properties of the core: S. Monobe and M Ohtsu, J. Lightwave Technol, 14 (10) 2231-5 (1996).

Advantages

• High Throughput

• Reproducible Shape

Disadvantages

• Hard to get a well-defined aperture (no flat cleave as in heat&pull method)

Coating the fibers:

• Standard technology.

• Probe must be rotated -- usually a home-made holder.

• Material

- Aluminum is best in the visible.

• Thickness

- greater than a penetration depth.

- maximum set by film stability and by size of the probe.

- not critical.

- different than what the crystal monitor says due to rotation (~p lower) and angle (cos q).

Coating the Probe with Aluminum

A coated probe (again large so the cleave/ aperture

are readily visible):

Coating Problem: Aluminum Diffusion

• Polarization is effected by the lumps.

Solution -- Cool the Probe (Hallen Lab)

Radiation + Conduction

The Throughput Problem (Pulled fiber tips):

How important is spatial resolution to you?

• It is very expensive in signal intensity.

• Recall that one cannot arbitrarily increase the input power.

• Often one does not need the highest spatial resolution to take advantage of NSOM.

Other types of 'resolution'

• Spectroscopic (cm-1)

• Polarization (°)

• These need high signal to noise so compete with spatial resolution.

Throughput Considerations:

• Modeling for linear sections: idea

B.I. Yakobson and M.A. Paesler, "Tip optics for illumination NSOM: Extended zone approach," Ultramicroscopy 57, 204 (1995).

R = fiber radius, f = taper angle, q = critical angle, r = tip aperture

• The optical intensity decreases exponentially beyond (closer to tip than) cut-off, with length scale dependent upon geometry and material.

Damage of the probe:

• Spatial character A. LaRosa, B. I. Yakobson, and H.D. Hallen, "Origins and effects of thermal processes in near-field optical probes," APL 67, (18), 2597-2599 (1995).

Probe shape(a)

Before Damage(b)

After Damage(c)

A Model(d)

• Theory comes from ray tracing (count bounces/length):

P.O. Boykin, M.A. Paesler, B.I. Yakobson, "Energy Dissipation in NSOM Probe

Fiber Tapers: Ray Tracing Assessment," SPIE Proceedings 2677, 148-153 (1996).

Tip Heating:

• Due to imperfect reflections from the metal coating.

• Be careful if you are not using Aluminum.

• Results in metal diffusion to lumps and scattering loss of light (destroyed probe).

• Measures of thermal time constant, models of probe temperature and profile, and thermal expansion of probe: A. LaRosa, B. I. Yakobson, and H.D. Hallen, APL 67, (18), 2597-2599 (1995).

• Temperature profile from external Al reflectance D.I. Karaldjiev, R. Toledo-Crow and M. Vaez-Iravani, APL 67 (19) 2771-3 (1995).

• Temperature profile from 'STM Thermocouple' M. Stahelin et al, APL 68 (19) 2603-5 (1996).

The experimental method: A. LaRosa, B. I. Yakobson, and H.D. Hallen, "Origins and effects of thermal processes in near-field optical probes," APL 67, (18), 2597-2599 (1995).

• Vary temperature (heating) with a visible laser.

• Detect with a CW IR laser.

Measurements of the probe time constants:

• slender probe with a 180 nm thick Al coating

• 2 mW red light, 1 mW of CW IR input

• triangles (after) and circles (before damage)

• 0.23 and 0.026 nW pk-pk Df

• 10.3 and 10.1 msec time constants

• Independent of details of probe shape.

• Model suggests that several hundred microns

of the tip are heated above ambient

• Peak is tens of microns from the tip.

New Ideas, two ways to increase the throughput by ~1000X:

• Use Etched Tips to Avoid Premature Core Leakage • Dieter Zeisel, Stefan Nettesheim, Bertrand Dutoit, and Renato Zenobi, "Pulsed laser-induced desorption and optical imaging on a nanometer scale with scanning near-field optical microscopy using chemically etched tips," Appl. Phys. Lett. 68 (18) 2491-2 (1996). • S.J. Bukofsky and R.D. Grober, "Video rate near-field scanning optical microscopy," Appl. Phys. Lett. 71 (19) 2749-51 (1997). • M.N. Islam et al, APL 71 (20) 2886 (1997).

• Let Total Internal Reflection Work When it Can • M.A. Paesler, H.D. Hallen, B.I. Yakobson, C.J. Jahncke, P.O. Boykin, and A. Meixner, "Near-field optical spectroscopy: enhancing the light budget," Microscopy and Microanalysis 3, 815 (1997).

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