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Now how the heck do you generate ~148.38nm light with a narrow linewidth? Their approach using four-wave mixing inherently results in short pulses. .. and giv

by nullc 2y ago

Now how the heck do you generate ~148.38nm light with a narrow linewidth? Their approach using four-wave mixing inherently results in short pulses. .. and given that it decays through gamma emission, does this mean we could now build an optically pumped gamma ray laser?

deleted 2y ago
[deleted]
M95D 2y ago
The gamma emission would have to re-excite other atoms in a cascade to create a laser. Since the exciting energy is UV, not gamma => no cascade amplification. A "wavelength converter" might be possible. PS: Are you sure it's gamma emission? That takes more energy than the exciting UV photon.
- karma_pharmer 2y ago
  > PS: Are you sure it's gamma emission? That takes more energy than the exciting UV photon. Apparently it is neither: Decay of the 229Th isomeric state of the neutral thorium atom occurs predominantly by internal conversion (IC) with emission of an electron https://www.nature.com/articles/nature17669 https://www.nature.com/articles/nature17669 https://en.wikipedia.org/wiki/Internal_conversion https://en.wikipedia.org/wiki/Internal_conversion This is pretty weird. You shine UV light (with exactly the right wavelength) on 229Th, and it spits out electrons. But not like the photoelectric effect, where the electrons stop as soon as you turn off the light. No no. The Thorium keeps spitting out an exponentially-decaying stream of electrons for hours after you stop illuminating it. Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar").
  - nullc 2y ago
    Apparently in some ionized states it can't produce the electron and will instead produce the gamma, I'm unclear where the extra energy comes from. > Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar"). If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.
    - karma_pharmer 2y ago
      I'm unclear where the extra energy comes from. When the atom ejects an electron, the hole left behind gets filled by an electron from a neighboring atom. Then the same thing happens to the neighbor -- and so on. This is electrical current flowing. Eventually the loop closes and some hole somewhere in the universe gets filled by the original ejected electron. The hole in one atom can get filled by an electron from a higher orbital in a neighboring atom. In that case the energy gained will be greater than the energy lost by the original electron ejection. This is the situation where you get a photon (x-ray) with a higher energy (= shorter wavelength) than the original incident photon (ultraviolet). Of course there's no free lunch. The way this happens is that N thorium atoms eject electrons from some orbital with energy X, the electrons shuffle around, and those N holes get filled by donors from orbitals whose total energy is N*X even though some of the donors are at higher levels and some are at lower levels. If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density. Yeah I've been thinking that if we had really tiny VLSI-integrated UV lasers (which we absolutely don't, not even close) that a bunch of these 229Th atoms embedded in a silicon chip would be a device with totally fascinating properties. We can build waveguides in silicon wafer processes pretty easily but I'm not sure we can do that at UV wavelengths. You could imagine a single, big, off-chip laser whose beam can be steered by waveguides to illuminate any of a few billion 229Th deposits. These could act like the configuration memory bits of an FPGA. They would be "almost nonvolatile" -- you'd have to refresh them every hour or so, instead of several thousand times per second (dram) or never (sram). At such a low refresh rate the steering doesn't need to be particularly fast, and having to share one laser across all the deposits would not be a problem. Unfortunately 229Th is mildly radioactive, but so are household smoke detectors so hopefully people wouldn't freak out about this.
    - drakebake 2y ago
      It doesn't produce an electron, it just donates its energy to one of the electrons bound to it. This energy is then used to free the electron from its bounds and any extra energy is used to give the electron some speed. Effectively the same energy is consumed when emitting the photon or releasing the electron. If the thorium is sufficiently ionized (the more electrons you strip from an atom, the harder it gets to strip the next one), the binding energy of the electrons is so high that the energy from the nuclear excited state is not enough to free the electron and the process is completely blocked from happening.
karma_pharmer 2y ago
The last sentence of the paper seems to imply that this result will give people a reason to want to develop those: The development of dedicated VUV lasers with narrow linewidth will make it possible to access a new regime of resolution and accuracy in laser M¨ossbauer spectroscopy and to perform coherent control of a nuclear excitation" Previously, if you wanted to manipulate nuclear states, you needed a synchrotron. Now, you need an infinitely less expensive instrument. I suppose the idea is that that will generate a lot of interest in improving the less-expensive instrument.
bawana 2y ago
What i was wondering…exactly… how do you make this kind of a laser? And imagine an xray laser… you could fry the guidance system of drone very precisely
- was_a_dev 2y ago
  Here, it appears they have used Four Wave Mixing in Xeon gas https://en.wikipedia.org/wiki/Four-wave_mixing https://en.wikipedia.org/wiki/Four-wave_mixing
- drakebake 2y ago
  One has to first imagine that a photon isn't as immutable as one sometimes believes. Effectively by putting lots of photons together in a medium (the medium makes sure momentum and energy is conserved) one can create all kinds of colors of light. We can already make "lasers" up to x-rays in synchrotrons by using the right means. The definition of laser becomes a bit shady though as the light is coherent and has a small divergence angle, but is not produced through stimulated emission.
drakebake 2y ago
Good question, nobody is sure yet how to do it. Indeed any pulsed laser will result in a broad spectrum (spectrum = inverse of laser pulse time e.g. 1 ns pulse is 1 GHz spectral width). Using frequency combs is a way around this, which are pulsed but the comb teeth are still narrow (approximately 100 Hz). To really go sub Hz one needs to build a continuous wave laser and mostly people are looking towards nonlinear crystals (poled materials, KBBF etc.) to create these using second harmonic generation

by nullc 2y ago

deleted 2y ago

[deleted]

M95D 2y ago

The gamma emission would have to re-excite other atoms in a cascade to create a laser. Since the exciting energy is UV, not gamma => no cascade amplification. A "wavelength converter" might be possible. PS: Are you sure it's gamma emission? That takes more energy than the exciting UV photon.

karma_pharmer 2y ago
> PS: Are you sure it's gamma emission? That takes more energy than the exciting UV photon. Apparently it is neither: Decay of the 229Th isomeric state of the neutral thorium atom occurs predominantly by internal conversion (IC) with emission of an electron https://www.nature.com/articles/nature17669 https://www.nature.com/articles/nature17669 https://en.wikipedia.org/wiki/Internal_conversion https://en.wikipedia.org/wiki/Internal_conversion This is pretty weird. You shine UV light (with exactly the right wavelength) on 229Th, and it spits out electrons. But not like the photoelectric effect, where the electrons stop as soon as you turn off the light. No no. The Thorium keeps spitting out an exponentially-decaying stream of electrons for hours after you stop illuminating it. Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar").
- nullc 2y ago
  Apparently in some ionized states it can't produce the electron and will instead produce the gamma, I'm unclear where the extra energy comes from. > Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar"). If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.
  - karma_pharmer 2y ago
    I'm unclear where the extra energy comes from. When the atom ejects an electron, the hole left behind gets filled by an electron from a neighboring atom. Then the same thing happens to the neighbor -- and so on. This is electrical current flowing. Eventually the loop closes and some hole somewhere in the universe gets filled by the original ejected electron. The hole in one atom can get filled by an electron from a higher orbital in a neighboring atom. In that case the energy gained will be greater than the energy lost by the original electron ejection. This is the situation where you get a photon (x-ray) with a higher energy (= shorter wavelength) than the original incident photon (ultraviolet). Of course there's no free lunch. The way this happens is that N thorium atoms eject electrons from some orbital with energy X, the electrons shuffle around, and those N holes get filled by donors from orbitals whose total energy is N*X even though some of the donors are at higher levels and some are at lower levels. If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density. Yeah I've been thinking that if we had really tiny VLSI-integrated UV lasers (which we absolutely don't, not even close) that a bunch of these 229Th atoms embedded in a silicon chip would be a device with totally fascinating properties. We can build waveguides in silicon wafer processes pretty easily but I'm not sure we can do that at UV wavelengths. You could imagine a single, big, off-chip laser whose beam can be steered by waveguides to illuminate any of a few billion 229Th deposits. These could act like the configuration memory bits of an FPGA. They would be "almost nonvolatile" -- you'd have to refresh them every hour or so, instead of several thousand times per second (dram) or never (sram). At such a low refresh rate the steering doesn't need to be particularly fast, and having to share one laser across all the deposits would not be a problem. Unfortunately 229Th is mildly radioactive, but so are household smoke detectors so hopefully people wouldn't freak out about this.
  - drakebake 2y ago
    It doesn't produce an electron, it just donates its energy to one of the electrons bound to it. This energy is then used to free the electron from its bounds and any extra energy is used to give the electron some speed. Effectively the same energy is consumed when emitting the photon or releasing the electron. If the thorium is sufficiently ionized (the more electrons you strip from an atom, the harder it gets to strip the next one), the binding energy of the electrons is so high that the energy from the nuclear excited state is not enough to free the electron and the process is completely blocked from happening.

karma_pharmer 2y ago

The last sentence of the paper seems to imply that this result will give people a reason to want to develop those: The development of dedicated VUV lasers with narrow linewidth will make it possible to access a new regime of resolution and accuracy in laser M¨ossbauer spectroscopy and to perform coherent control of a nuclear excitation" Previously, if you wanted to manipulate nuclear states, you needed a synchrotron. Now, you need an infinitely less expensive instrument. I suppose the idea is that that will generate a lot of interest in improving the less-expensive instrument.

bawana 2y ago

What i was wondering…exactly… how do you make this kind of a laser? And imagine an xray laser… you could fry the guidance system of drone very precisely

was_a_dev 2y ago
Here, it appears they have used Four Wave Mixing in Xeon gas https://en.wikipedia.org/wiki/Four-wave_mixing https://en.wikipedia.org/wiki/Four-wave_mixing
drakebake 2y ago
One has to first imagine that a photon isn't as immutable as one sometimes believes. Effectively by putting lots of photons together in a medium (the medium makes sure momentum and energy is conserved) one can create all kinds of colors of light. We can already make "lasers" up to x-rays in synchrotrons by using the right means. The definition of laser becomes a bit shady though as the light is coherent and has a small divergence angle, but is not produced through stimulated emission.

drakebake 2y ago

Good question, nobody is sure yet how to do it. Indeed any pulsed laser will result in a broad spectrum (spectrum = inverse of laser pulse time e.g. 1 ns pulse is 1 GHz spectral width). Using frequency combs is a way around this, which are pulsed but the comb teeth are still narrow (approximately 100 Hz). To really go sub Hz one needs to build a continuous wave laser and mostly people are looking towards nonlinear crystals (poled materials, KBBF etc.) to create these using second harmonic generation