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Nov7-12, 06:14 AM
P: 661
Hello everybody!

There is interest in making point-like light sources, especially to measure the quality of optics, and possibly to produce optics. Here are some ways I imagine.


Semiconductor technology can make a hole of 20nm diameter for you. One single 20nm hole in an opaque screen. Through standard patterning techniques, or maybe by punching the screen with an electron beam.

One hole in a screen brings you the same optics interference pattern and intensity as one black point over a luminous background, except that
- You avoid the luminous background, so the optics' interference pattern is much easier to observe;
- You can concentrate your light source on the hole, so the optics' interference pattern is brighter.

Alternately, you could let produce a single gold dot by an atomic force microscope, for instance at IBM's research centre in Zürich. Then, less than 20nm would be better.

Perhaps you might consider a very thin metal layer on glass (like 20nm by evaporation) and punch a hole in it with a (small!) sparkle from a needle. Use weak currents and a micropositioner; corona effect, or even tunnel, tell you the distance. Early tunnel microscopes used a needle of broken tungsten to get a single atom tip.

And what about a space blanket? Their aluminium film is very thin and uses to have already holes. Just choose the hole you prefer, and infer its size from the amount of transmitted light. Their stack is: thicker clear polyester - aluminium - coloured varnish. You may add thicker varnish to protect the best hole.


One more bizarre idea...

Take two thin tungsten wires, like 20µm diameter. Hold them taut by their ends, cross them without pulling them much together. Verify that their contact area is 20nm wide by measuring the cold resistance from one wire to the other : the contact resistance must be equivalent to 5mm length of 20µm wire, which 2+4 resistance measures tell independently of where the wires touch. A small current may be first needed to obtain a stable contact.

Put that in pure argon, inject current to let the tiny contact glow, you get a light source of 20nm diameter, emitting white light to the sides. Drawback : it's only 2nW at 3300K. And it may need 1/2W electricity, which brings the wires to ~1100K away from the contact, shining a bit as well. Thicker wires would improve that.

The contact point doesn't get a constant voltage because of the series resistance, and this is bad for its life expectancy; again thicker wires would improve. Make a new contact at a further location if one becomes too wide.


Back to the holed thin metal film. Several metals like Au, pure Al, Ta, Nb... resist oxidation even in 20nm thickness, and can be coated with SiO2, Si3N4... Au and Al have an interesting opacity (and the film can be thicker); uniform films that thin are obtained by semiconductor processes.

The substrate holding the film can be silica, sapphire... If it must be <1µm thin, it can be a small membrane of SiO2, Si3N4... supported at its periphery by a thicker silicon chip.

To open the hole(s):
  • Maybe the micromanipulator with the sharp tungsten tip can punch the hole by mechanical pressure.
  • The electron beam of course.
  • Anodizing (20nm)3 of Al, Ta, Nb to transparent oxides takes ridiculous amounts of electricity, like 80fC, or 4V through a 20fF stray capacitance. Hard to insulate and control. But one might implant oxygen locally in the metal by a focussed ion beam.
  • Vaporizing aluminium between the tungsten tip and an alumina substrate. It may take some 15mW of electrical power, where many radius-dependent effects cooperate to define a precise radius. A constant voltage (or a constant voltage limit) may improve further.
To illuminate the hole:
  • Erbium-doped fibre lasers for datacom repeaters can inject up to 10kW at 1300nm in a monomode fibre. Even a bearable power density leaves a lot on D=20nm!
  • Laser diodes provide several mW at 780nm (for CD burners), 635nm (DVD burners), 405nm (blu-ray burners).
  • The holed metal film could be deposited on the laser diode for ruggedness, or at the focus for power: a blu-ray burner concentrates several mW to D=290nm.
A holed metal film on a laser diode, possibly with a lens, looks compact and convenient, so it can become a catalogue component if enough customers want it.

Marc Schaefer, aka Enthalpy
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