Normally, you think of things casting a shadow as being opaque. However, new research shows that under certain conditions, a laser beam can cast a shadow. This may sound like nothing more than a novelty, but it may have applications in using one laser beam to control another. If you want more details, you can read the actual paper online.
Typically, light passes through light without having an effect. But using a ruby crystal and specific laser wavelengths. In particular, a green laser has a non-linear response in the crystal that causes a shadow in a blue laser passing through the same crystal.
The green laser increases the crystal’s ability to absorb the blue laser beam. which creates a matching region in the blue beam that appears as a shadow.
If you read the article, there’s more to measuring shadows than you might think. We aren’t sure what we would do with this information, but if you figure it out, let us know.
Ruby has a long history with lasers, of course. That green laser pointer you have? It might not be all green, after all.
So it is a “NOT” gate… which, IMHO, is very interesting for photonic circuits.
Next step/question, do two green laser beams add together in this effect?
If so it’s a NAND gate. One of the universal gates that all others can be made from.
Easier than that it should already be a NOR gate which can also be used as a universal gate.
Only other one that I know that works as a base is XOR but that doesn’t seem like it’d apply here.
Hmm, I suppose if it works with two lasers at slightly differing angles, that would be a NOR already.
I was thinking more along the lines of two lasers on differing sides that added together.
As both NOR and NAND are universal gates, either would be a big deal.
Why are so many people claiming it’s either NAND, NOR, or both? Afaics, this effect is XOR (both off = no light, both on = shadow), which is not a universal gate.
Depending on how you look at it, it’s an AND-NOT gate, since the shadow area is illuminated when blue AND NOT green. This is a universal gate, but as of yet unmentioned.
@Zom-B Your other reply hints at the reason, NAND and NOR are “universal” and with just one of those you can build all other logic gates out of them.
As to why it’s mentioned, it’s more wishful thinking/hoping. A universal gate means you have all logic gates which means computation purely within the new device.
If this turns out to be possible (yes, big “if”) then this device will be to light what the transistor did for electricity.
So, in what way does this show that “light can cast a shadow”? As far as I can tell this shows that a laser alters a material, affecting its opacity. I don’t think this is any different in concept to transition lenses (aka photochromic lenses) which have been around since the 60s.
I’m sure there is a lot of actual innovative and useful research here, but the reporting (in phys.org and elsewhere) just seems… sensationalist
AFAIK, the photochromic lenses are based on a chemical effect, thus are much slower than this, which, if I get it right, is based on a quantum effect.
“Strictly speaking, it is not massless light that is creating the shadow, but it is the material counterpart of the polariton, which has mass, that is casting the shadow.” -Article. The author argues that a light wave in medium is fundamentally composed of both photons and excitations in the medium, which is why they are saying that the light creates the shadow. It is also a new effect unlike other light-induced changes in opacity.
The ruby is indeed what is blocking photons and casting a shadow, so it’s a bit of a stretched headline. But still cool
It can work as a NOT gate and with multiple lasers potentially as a NOR gate. Too bad the lasers have different wavelengths so you can’t use the output as an input. It would be cool if we could make optical CPUs.
Maybe using quantum dots it would be possible to turn the blue laser into green right in the output. That would allow to feed the output into another crystal as input.
I think quantum dots could convert the blue light to green but I’m not sure if they’d still form a laser which I think would be desirable if not necessary.
The “gate” input power is 15 watts for 500 milliseconds (then stopped to avoid overheating). Literally not cool.
The output power is less than a half watt.
So it has a fanout of 0.03, a clock rate of less than one hertz, and a power consumption of 15 watts per gate.
Not a practical optical CPU, methinks.
In the test setup they used here but it might be better in a setup meant to exploit the effect now that it’s been observed.
At the incident power density of 30 megawatts per square meter they use (close to that of the surface of the sun) it’s hard (read: impossible ) to believe the effect is not simply thermal lensing: the heating changes the local refractive index of the ruby medium.
In other words, it’s a mirage.
They mention strong heating in the paper, and gate the beam to avoid overheating the ruby block in general, but they DON’T mention thermal lensing or local refractive index changes. That’s a pretty glaring omission. It’s hard to see how this passed peer review to get published as-is.
As far as I understand the paper it’s neither thermal lensing nor changing the refractive index. It’s just playing with energy levels/states.
See my other comment.
BTW: 30 MW/m2 is not a very high intensity. With fs pulses (100 fs or below) you can easily reach intensities beyond 10^12 W/m2 using a small table top system. Having worked with Ti:Sa lasers (pumped with 5-10W of green CW laser light) I don’t wonder that cooling is necessary. It’s just to avoid that heating up of the crystal. For example, the Kerr-lens (non-linear effect) which supports “mode-locking” in the cavity of such a Ti:Sa fs laser is created by the pulses of the “generated” laser not by the pump laser.
True, 30 MW/m^2 is not a high power density for optical effects, but it’s enormous for thermal effects: It’s much higher than the surface of even the hottest possible tungsten filament lamp. It can cause heating rates of thousands of degrees per second, easily causing even modest absorbers (like green light on red ruby) to reach incandescent temperatures very quickly.
There’s no question heating is happening, and there’s ample reason to expect an enormous temperature gradient within the ruby block. It’s crazy NOT to consider thermal lensing as a plausible effect, yet there’s no mention of it in the paper.
Well, cool down! ;-)
According to wikipedia the absorbance of ruby is about 60% @ 550 nm for a 1 cm thick ruby crystal. Their crystal is 1.2 cm thick and they are using 532 nm. So, the crystal absorbs perhaps less than 65% of the power over the whole length of the crystal. In total this is about 10W. And this is absorbed along the whole length of the crystal. Yes, it will warm up. Yes, it will change optical properties. They mention it. But it’s definitely not the reason for the shadow.
I would say that they did not really care about cooling and just set their duty cycle that the crystal did not heat up. I would call their cooling setup rather an alibi than a real cooling setup.
Another interesting effect is that ruby (i.e. sapphire) increases its emissivity with temperature. By reciprocity its optical absorption also increases with temperature. It’s hard to say whether this effect is significant in this case though, despite the very high power density and heating rate.
I don’t think it’s thermal lensing; the setup essentially looks like a basic Electromagnetically Induced Transparency experiment with the pump-vs-probe chosen to effectively get Electromagnetically Induced Opacity… but I don’t see why that’d be novel enough to publish when EIT has been known for ages now.
Thermal lensing would be easy to detect from the ramp up and cool down periods and their effect on the shadow line, just shut off the laser and see if the line goes away in a way that matches the predicted cooling rate. The test can be done in vacuum to avoid thermal refraction in the air. You could also just heat the crystal up without the laser and see if that created any shadow effect.
Not adding that to the paper isn’t surprising. Every experiment has dozens of ways that they could be affected by outside factors. If every possible factor was covered the paper would be unreadable.
If they considered thermal issues to be a major effect, you would likely find that in an appendix or attached paper that goes deeper into the design of the experiment.
The key point of the paper is to communicate the core ideas of the experiment and the results, often adding an explanation of the effects being measured. This is an interesting experiment that doesn’t look difficult or expensive to set up. There will be dozens of outside teams testing these results. If there are experimental flaws they will be identified quickly.
The scientific method can be very elegant at times.
I don’t think this is a very surprising effect. I reminds me of spectroscopy on molecules where there are some higher states which cannot be directly excited optically. And there have been publications on very exotic states and behaviours as I remember from my time at university some 30 years ago where some folks in our group worked on such topics.
As it is written in the publication: “The effect will only take place if the absorption cross-section of the second transition (2E to 2T1) is larger than the one of the first transition (4A2 to 4T2), which is the special case exhibited by ruby.”
You “just need to find a material” (which is certainly all but easy) where there is such a “hidden state” which can only be accessed indirectly. As they do it. They “pump” the material to the 4T2 state which is directly depopulated by phonons (thermal vibrations to call it very simplified) into the 2E state. (Have a look into publications about wavepacket dynamics in (larger) molecules where often the image of the optically excited wavepacket gliding down a slope in a multi-dimensional energy level landscape is used. Or https://en.wikipedia.org/wiki/Population_inversion#Three-level_lasers ) If the 2E state is a long-living state – similar to a triplet state in molecules – it will become highly populated. And if there is a suitable higher energy state (2T1) which is optically accessible from there and if you then select light with the right wavelength to optically excite an electronic transition from that (2E) state to a higher state (2T1) this will cause absorption of the second beam. If the absorption is high enough, it can be observed the the transmitted light. Obviously, the absorption can take place only there (spatially) where the first beam has interfered with the material. Thus, there is a “shadow”.
All true and fine, but Occam would say there’s other things in play, with all the thermal effects, and the paper is not really addressing those, other than by limiting the duty cycle so it doesn’t get too hot.
The funny thing is that it’s also been known from the very first use of ruby lasers in the 60s that the crystal changes color and gets darker (more absorbing over a broad wavelength range) when hit by high intensity light.
If it’s really a spectral (absorption band) effect, then adjusting the wavelengths of the two lasers will shed light [sic] on the issue. Too bad high power tunable CW lasers are harder to come by.
Oh, this can be easily shown. Just take a yellowish or so laser for the probe/illumination laser which does not manage to make the 2E -> 2T1 transition. You don’t need much power for that one. Then, there will be no shadow.
What if you take two of those object laser beams and inject them from the side and top of the cube (or maybe make it a plane instead of a cube) . But instead of ‘line’ lasers, you use thin laser beams and scan them from one side to another like an electron beam. And what if you can modulate both beams to be in phase or out of phase of each other, making use of the standing wave principle. Wouldn’t that mean that you would be able to create single-point shadows in the 2D plane?
Ruby sound like a mixer
ONLY the laser shadow knows.
What evil lurks in the hearts of… lasers?
Only the Ruby knows