# Point light source

1. Nov 7, 2012

### Enthalpy

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.

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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.

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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.

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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

2. Nov 7, 2012

### Enthalpy

Gold nanoparticles can frustrate locally a total internal reflection, acting as point-like sources over a dark background, see the sketch.This is far better than shadowing uniform light.

I feel it less good than a notch in a metal screen, because in the transparent material, the displacement current d(D)/dt concentrates before entering the nanoparticle, stretching the radiating zone, and dilutes at the sides, reducing the radiated field and sharpening its pattern.

As the conduction current is nearly compensated by nearby displacement current dilution, the particle's protruding position must be the main contributor to radiation.

Surface smoothness elsewhere must be far better than 20nm. Semiconductor processes have worse requirements.

A small notch in the reflecting surface has nearly the same effect as a gold nanoparticle. I'd try the already suggested methods: mechanical action of the tungsten tip; local electric current from the tip, which may bombard the surface with H and F ions alternating...

Because total reflection puts limits on the angle of incoming light, a lens can't concentrate light as strongly as on a punched screen.

A monomode fibre carries concentrated light, so a small notch there radiates more. Bigger notches are made for fibre gratings. The fibre should be cladding-less (¡Ole!) or maybe have a soft polymer cladding over a hard ceramic core. Alternately, an optical chip made by semiconductor processes can have a light guide with a notch (20nm being a present-day size), and allows a protective cover. Nice compact source, especially if the laser diode is integrated. But I prefer a small hole in a metal screen covering a laser diode.

Marc Schaefer, aka Enthalpy

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3. Nov 7, 2012

### Enthalpy

Got data for opacity there, hope it's correct, telling that:
http://refractiveindex.info/?group=METALS&material=Aluminium

- Aluminium is better than gold in the visible spectrum
- 20nm of Al leave 7% at 800nm, 5% at 400nm and 6% at 171nm. Hole definition is efficient, background light is too strong, you're right.

BUT we can have a thicker layer everywhere and leave it thinner locally to keep the contrast. For instance, if the hole is made by pressing the tip through the soft metal against the hard ceramic, then a conical hole can be narrow at the bottom (let's keep D=20nm) and wider at the top. At 90° full angle, only D=60nm have <20nm thickness, and D=100nm have already 40nm thickness, and so on. This looks simple and acceptably efficient.

If the hole is punched by pressing the tip through very pure aluminium (which is ultra-soft), the harder ceramic stops the movement, and this defines a narrow hole. The other previously suggested processes must work as well.

Immediately around the hole, aluminium isn't very opaque, but enough to define a diffraction pattern; farther away, it's thick enough to avoid background light.

Marc Schaefer, aka Enthalpy

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4. Nov 7, 2012

### Enthalpy

I begin to realize that, because the dots' luminosity or opacity is faint as compared with stray light, opticians will probably use many dots at a time.

That is, they would have an eyepiece and CCD that give many pixels within the objective's diffraction limit, collect the diffraction pattern around many dots in the field, and have a software compute the image's autocorrelation to obtain a clear objective's diffraction pattern from many noisy patterns around each dot.

Or more precisely than "autocorrelation", something like an autoadaptive filter should run on the image, for instance a Kalman filter, to obtain first the positions of the nanoparticles. Then, the correlation between the raw image and the filtered one gives the diffraction pattern, accumulated over many particles. Or maybe the proper programmation of the autoadaptive filter gives the diffraction pattern directly.

For that purpose, they would prefer the dots on an irregular pattern, and I'd say, the distance between the dots should better be greater than one diffraction pattern diameter, which must be seriously bigger than 20nm.

Is a mist of hydrofluoric acid, with droplets 50nm wide, any conceivable? They might etch tiny holes in glass.

I hope a single tiny hole illuminated by the very concentrated light of a DVD burner laser diode and its lens does not need this software data processing.

Marc Schaefer, aka Enthalpy

5. Nov 7, 2012

### Enthalpy

There was still room at the bottom. You guessed: I want individual atoms as smaller light sources.

Take an atom that de-excites in, say 50ns. Pump it enough that it emits every 500ns as an average. The optics under test shall catch 1/4 of the photons and the CCD convert 1/2 to carriers. It gives 5000 electrons in 20ms, but we want to observe the diffraction pattern. Imagine the sidelobe carries 1/10 of the power and spreads over 150 pixels, then each pixels gives 3 electrons. The pixel can leak 500 electrons at room temperature with a fluctuation of 23 electrons. But here are solutions.

If the CCD integrates over 60s instead of 20ms, the signals becomes 9000 electrons and the noise 1200 electrons: 7 sigma are usable. Software that integrates 20ms frames over 60s can compensate vibrations.

Also, a field of view of 1mm2 has 10µm2 for each of 100,000 emitting atoms. Use the previously suggested software autoadaptive filter to add the signal over all atoms, get 42 sigma in 20ms.

Or cool the CCD. Or combine several solutions.

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1mm2 emits here some 20nW. This is as much as a pinhole of d=0.03mm over an old green LED. We use a CCD to its noise limit at room temperature: good hobby astronomers achieve it, so it's not easy. Minimize stray light at the CCD and at the source.

Diffusion and stray luminescence must be minimal at the matrix that holds the emitting atoms.

See the attached idealized band diagrams of luminescence:
materials like SiC, GaN, GaP which use to have many radiating deep levels in the forbidden band unlikely fit. This must preclude the injection of minority carriers by a junction to pump the emitting atoms. Instead, the matrix is more likely a very transparent material without fluorescence, like silica, quartz... and the emitting atoms pumped optically.

If the transition is between two deep levels of the emitting atom, pumping can be more selective (and the pumping light possibly created elsewhere by the same species), and the bands can be full, improving transparency and hindering the transitions from parasitic deep levels. Note that at such tiny concentrations, the emitting atom must absorb itself.

Fluorescence time is documented for lasing species, but we need short times. One example - to be improved - is Cerium doping Yttrium Aluminium Garnet (Ce:YAG) which lasts 20ns to 70ns. Several luminescent species of varied colours in a single matrix can help the user.

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Because a flat refraction is astigmatic, and the optic under test has supposedly little field depth, all emitting atoms must be few 10nm under the suface - common for semiconductors. Implanting with 1µA spread over 1dm2 during 160µs achieves the unusual dopant density. Undesired impurities that fluoresce must be even scarcer.

The pumping light can reach the emitting atoms if undergoing an internal reflection, but confining it in a surperficial layer looks better: the power is where needed and pumps fewer undesired fluorescent impurities, which should then be kept well under 1011 cm-3.

The strong pumping light shall not leak from the matrix. Guided propagation allows to absorb it around the emission field over several mm, by means that avoid hard transitions and associated leaks. For instance, some dopant can create non-radiative absorption or lossy conduction there; it can be in the light-carrying layer or within the fading wave in the substrate below, which can itself be a stack. This keeps essentially the same materials and shapes, so the absorption zone's entrance scatters no light.

The pumping light can make several passes at the emitting atoms, for instance by internal reflection.

Marc Schaefer, aka Enthalpy

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6. Nov 7, 2012

### Enthalpy

The d=20nm punched spot illuminated by a Dvd laser diode shouldn't overheat.

Back to (thicker) gold, deposited on thick silica - for instance because this silica makes the lens. It should withstand 300K heating. The silica (1.3 W.m-1.K-1) hemisphere conducts 370µW from a light spot of D=300nm.

Gold reflects >99% (...at least in air! Gold has an index) so the incoming light power can be 37mW. Let's take 20mW as I believe it's a common power for DVD burners; if yours is stronger, reduce the current and gain lifespan; if needed, use a duty cycle at some MHz.

The d=20nm hole at the D=300nm spot lets <90µW pass through, which is half the light of an old green LED. No adaptive filter, no multiple sources, no integration time - my preferred version. This power leaves adjustment margin if, for instance, light reflection is weaker than expected.

Marc Schaefer, aka Enthalpy