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Enthalpy
#5
Nov7-12, 06:30 AM
P: 661
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 10m2 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 1A spread over 1dm2 during 160s 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
Attached Thumbnails
BandDiagram.png   AtomFluo.png