The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University
Fabrication -- Heat and Pull
Fabrication -- Etch
Fabrication -- Metal Coating
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.
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.
Note the flat cleaved end which will define the aperture (up to diffusion of the evaporant metal).
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).
Hard to get a well-defined aperture (no flat cleave as in heat&pull method)
Probe must be rotated -- usually a home-made holder.
- Aluminum is best in the visible.
- 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 (~π lower) and angle (cos θ).
A coated probe (again large so the cleave/ aperture
are readily visible):
Polarization is effected by the lumps.
Solution -- Cool the Probe (Hallen Lab)
Radiation + Conduction
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'
These need high signal to noise so compete with spatial resolution.
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, φ = taper angle, θ = 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.
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).
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).
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.
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 Δφ
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).
More info is in the papers.
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