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A masing grace
August 15, 2012 5:33 PM   Subscribe

Using spare chemicals, a laser bought on eBay and angst from a late-night argument, physicists have got the world's first room-temperature microwave laser working.

"Mark Oxborrow, a physicist at the UK National Physical Laboratory in Teddington [...] came across a decade-old publication by Japanese researchers suggesting that when the electrons in pentacene are excited by a laser, they configure such that the molecule could work as a maser, possibly even at room temperature."

"...the MASER has had little technological impact compared to the LASER because getting it to work has always required extreme conditions that are difficult to produce; either extremely low pressures, supplied by special vacuum chambers and pumps, or freezing conditions at temperatures close to absolute zero (-273.15°C), supplied by special refrigerators. To make matters worse, the application of strong magnetic fields has often also been necessary, requiring large magnets."


The full journal article, for those with subscriptions.
posted by ancillary (49 comments total) 20 users marked this as a favorite

 
Science works!
posted by nancyhua at 5:39 PM on August 15, 2012 [1 favorite]


Fucking awesome post title btw.
posted by mannequito at 5:46 PM on August 15, 2012 [14 favorites]


I was reading the wiki article on masers the other day, coincidentally. I shall read about room temperature fusion next. Chop chop!

More seriously, despite reading about them, I haven't been able to determine what's so special about a maser - it's effectively a laser operating at RF frequencies, right? So rather than having to have parabolic dishes to transmit microwave signals, you could conceivably use point-to-point masers like we (occasionally) use LOS lasers for telecomm?
posted by Kyol at 5:49 PM on August 15, 2012


We can heat frozen burritos at a great distance now, right? But what else?
posted by humanfont at 5:50 PM on August 15, 2012 [1 favorite]


We can heat frozen burritos at a great distance now, right? But what else?

Marshmallow Peeps!
posted by TwelveTwo at 5:53 PM on August 15, 2012 [1 favorite]


*puts on goggles*

They cut my funding! They laughed at me at the University!

Alright you bastards- WHO WANTS PIZZA BAGELS?
posted by TheWhiteSkull at 5:55 PM on August 15, 2012 [8 favorites]


WHO WANTS PIZZA BAGELS?

When you put pizza on a bagel, you can eat pizza.. any time!
posted by curious nu at 6:00 PM on August 15, 2012 [9 favorites]


I was kind of crossing my fingers that at some point in this story he would have turned the auditorium into a swimming pool and invited the hairdressers from the next campus over.
posted by Diablevert at 6:01 PM on August 15, 2012 [11 favorites]


You can buy lasers on eBay? Interesting...
posted by fshgrl at 6:01 PM on August 15, 2012


1.45 GHz, so λ ~= 20 cm. The properties of waves at that wavelength are completely different from those of light; for reference Wifi uses frequencies in the 2.4 GHz ISM band, while older cordless phones used frequencies in the 900 MHz ISM band.
posted by Monday, stony Monday at 6:01 PM on August 15, 2012


Visible light: ~ 0.39 - 0.75 µm -- after that infra-red, to 1 mm.
posted by Monday, stony Monday at 6:06 PM on August 15, 2012


See, this is only a problem if someone invents a big spinning mirror to go with it...
posted by delfin at 6:09 PM on August 15, 2012 [2 favorites]


So, I wonder what would happen if you maybe filled a house with unpopped popcorn and then aimed one of these things at it from a distance...
posted by Joakim Ziegler at 6:11 PM on August 15, 2012 [12 favorites]


Joakim Ziegler, it'd be like lasing a stick of dynamite!
posted by kyrademon at 6:15 PM on August 15, 2012 [5 favorites]


You can buy lasers on eBay?

You can buy lasers at Staples.
posted by benito.strauss at 6:16 PM on August 15, 2012 [4 favorites]


I beleive the correct phrase would be "mazing a stick of dynamite"
posted by humanfont at 6:19 PM on August 15, 2012


But not the fun ones; those cost a little bit more. Check out a picture of one of my lasers right here. And for 'my,' you can read 'my boss' or maybe the US taxpayer, but one tends to get possessive about the lasers you work with.

All the radiation on the lefthand side (the stretcher) is infrared, so you can't really see it with the naked eye. On the right hand side is the regenerative amplifier; the green light comes from a 30W diode pumped Nd:YLF (Neodynium: Ytterbium I'd like to Fluoresce*) crystal.You can see a little bit of glowy red light coming out from the gold coloured box that houses the Titanium-sapphire crystal.

Laser science really isn't that tricky to understand at a hobbyist level. Building your own lasers from discarded junk isn't that hard; finding a good crystal without a bunch of burn marks is usually the trickiest part. But I'd highly recommend learning to build your own laser. It's fun!

*Nope, it's Neodymium-doped yttrium lithium fluoride. But.
posted by samofidelis at 6:25 PM on August 15, 2012 [7 favorites]


The picture of Oxborrow accompanying the article is great:

yeah looks like i invented room temperature masers last night guys
turns out we could have done it years ago, you just point the thing at the thing
try not to be too mad guys it was totally an accident
posted by chaff at 6:26 PM on August 15, 2012 [9 favorites]


You can buy lasers that fit on a keychain at the Job Lot for $3.00, and it comes with etched crystal lenses to project images, such as a heart, a smiley face and an upturned middle finger at great distances.

I was just gonna sneer there, but then I slowly repeated back to myself - Lasers. Projecting images at great distance. Small enough to fit in the pocket, cheap enough to buy on a whim...

...and it's old fashioned.

Yup. Future's here. Still no flying cars.
posted by Slap*Happy at 6:26 PM on August 15, 2012 [6 favorites]


/me emerges from closet, nods hello nervously, exits room
posted by DU at 6:26 PM on August 15, 2012 [4 favorites]


Check out a picture of one of my lasers right here.

Oooooooohhhhhhh no you don't. Your technical description of that marvelous rig was too much of a tease. Interested laymen are about to go all full-moon-were-nerd, you just can't leave us hanging...
posted by Slap*Happy at 6:31 PM on August 15, 2012


Yay, invisible coherent energy rayguns with no Rayleigh scattering. Just what humanity needed!
posted by seanmpuckett at 6:37 PM on August 15, 2012 [1 favorite]


What do you want to know? It's actually an amplifier in a more complicated laser system. A Ti-Sapph oscillator produces a pulse with about 30nm of bandwidth, centered around 800 nm. That's cut down to 10nm (we could have used a different oscillator, which generates a narrower pulse, but the current one is very power stable), then sent to the regen. The laser produces 80 million pulses per second; each one lasts about 30-40 femtoseconds. So, in a sense, it's 'off' most of the time, but even given that, the time-averaged power is about half a watt.

Inside the regen, the pulse is first stretched temporally so that during the amplification process, the instantaneous optical power never exceeds the damage threshold of the optics. The stretching process is performed by splitting the pulse into its different spectral components using a grating, sending the different constituent wavelengths along slightly different optical paths, then recombining them into a single beam -- but one where the pulse is 'chirped,' which is how we refer to a laser pulse where the different frequency components arrive spread out at different times.

The pulse is then sent over to the other side of the regen, where it acts as the seed in an amplifier. There's another Ti-Sapph crystal that is being pumped by the green laser. The green light comes from another pulsed laser system; Aluminum-Gallium-Arsenide diodes pump near-IR light into another kind of crystal, Nd:YLF. That causes emission at two different wavelengths, both in the invisible near IR (1047 nm and 1053 nm). One of those is selected for using some polarization optics (if I recall; that laser is like a black box most of the time). If the laser were just the crystal and two end mirrors to cause light buildup, that would be it, you'd get a moderate amount of continuous power out of it. Instead, it's a Q-switched device; that is to say, the quality factor of the cavity is spoiled most of the time by an acousto-optic modulator -- basically, a crystal or glassy material with sound waves passing through it. The sound waves are pressure waves, and they create a local change in the index of refraction in the crystal or glass medium; those local changes are periodic with the same periodicity as the sound waves, and act as a diffractive optic that scatters light out of the cavity. In this way, the acousto-optic modulator spoils the performance of the laser -- it lowers the Q-factor. A greater excited state population can build up in the laser than would be feasible if it were operating continuously. Turning off the AOM results in a moderately short pulse (think nanosecond timescale) of high power laser emission.

That high power green pulse hits the Ti-Sapph in the regenerative amplifier; it causes a population of excited states inside that crystal. The seed light from the stretcher is switched into the cavity by a high speed device called a Pockel cell; subsequent pulses are rejected by same. The pulse circulates in the regen, building up power by eating the excited state population until it has saturated, at which point it is switched out by another Pockel cell. We get about 4 watts out, with a repetition rate of 5kHz -- so even more 'off' time from the amplifier, but with pulses with much greater power.

At this point, the pulses are still chirped -- they're still stretched out in time. To undo the effect of the stretcher, we basically run it in reverse -- it's the same principle, but a compressor splits the beam into different colours, runs the components along different beam paths, and reassembles them so that they are temporally compressed. Thus, we get a faster pulse out.

We're not too worried about the pulse duration in the experiments I'm currently running, because we then send the beam into another device called a Non-collinear Optical Parametric Amplifier, which is a device that uses nonlinear mixing to produce coherent light at different wavelengths. From that, we go into another compressor, then a more sophisticated pulse-shaping apparatus. That's a lot of work, but the whole process gives us a tunable pulse with broad bandwidth to run our actual experiments with.

I mean, assuming I can get the thing to work. It's been a long month.

Anyway. Home laser systems for the hobbyist. It's a lot of fun.
posted by samofidelis at 6:51 PM on August 15, 2012 [21 favorites]


I am just glad that for once a drunken pub argument followed by heading to a North London warehouse to buy weird things off ebay worked out well because as much as the media publicizes these success stories you have probably not heard of all the times where for instance the guy can't even get a ride to the warehouse anymore once he's had a few because of that one time.
posted by passerby at 6:57 PM on August 15, 2012 [5 favorites]


Can someone explain the difference between a maser and a laser like I'm five?
posted by Eyebrows McGee at 7:29 PM on August 15, 2012


laser = light
maser = microwaves
posted by Freen at 7:35 PM on August 15, 2012


What do you want to know? [awesomeness goes here] Anyway. Home laser systems for the hobbyist. It's a lot of fun.

Sooooo... where do we begin?

NOOOOOOOOO! SHUSH! This is August. There is a prize for best front page post. A post that tells us where to begin, why we should care, and what we should hope to one day achieve... will absolutely win. (Even if only In My Heart.)

Do it. For science! plz.

Also how do the lenses not burn or melt?

posted by Slap*Happy at 7:37 PM on August 15, 2012


Beams from pink crystals?

My god ... does noone remember what happened at Atlantis any more?
posted by Twang at 7:38 PM on August 15, 2012 [3 favorites]


samofidelis: Wow, all that just to make espresso! But seriously, how about a sentence on why you need such a beam?

Eyebrows: They both make highly focused beams of electromagnetic radiation (EMR). Visible light and microwaves are both varieties of EMR, just with very different scales. So think of herding ants and elephants — involve making animals walk in a group in the same direction, but the scales are so different that you're going to need fairly different methods and tools.
posted by benito.strauss at 7:41 PM on August 15, 2012 [1 favorite]


If the energy conversion efficiency is high, this could push satellite-based solar power generation into the realm of feasibility.

Microwaves are too diffuse for useful long-distance power transmission, but have very good conversion efficiency. Lasers are great for long-distance communication, but current photovoltaics have terrible conversion efficiency. A maser could, potentially, combine the best of both worlds.

Or, more likely, the green energy applications will remain perpetually underdeveloped ("Too expensive!" "Unrealistic!"), while the DOD will spend billions of dollars turning maser technology into a network of active denial system satellites, capable of instantly irradiating any square meter of Earth with horrible pain rays from space.
posted by dephlogisticated at 8:03 PM on August 15, 2012 [6 favorites]


Can someone explain the difference between a maser and a laser like I'm five?

So, you know when you and your buddies get really, really drunk and wander all over the place, and speak in partial sentences such as "like, I'm so...ya know?" and "wherza thing that goes with this pickle?" That is the opposite of a laser, more like an LED. The light rays, and you guys, are incoherent i.e. moving in all different directions at different speeds.

Take the opposite of that. Everyone is moving together at the same speed and in the same direction and saying things that are coherent. That's a laser.

Now, replace you and your buddies with the American economy. Right now it's pretty incoherent, no one's working together or saying the same thing. That's like the microwaves in your microwave oven. If, by some miracle the right conditions can be created and get things moving again then we'll have something like a maser (coherent microwaves), but it takes a lot of work. Except these guys have figured out how to make it work under the current conditions and they're gonna get rich exploiting the loophole. Bastards!

At least that's how I picture it in my head.
posted by highway40 at 8:14 PM on August 15, 2012 [4 favorites]


Here's my proposed use for these new high-intensity masers. I want a device that, when I set my phone, iPad, etc. on a certain table or desk, will detect it visually or by RFID or something and recharge the battery wirelessly as it sits there. I know there are already inductive charging "pads" but I'm imagining this would be more forgiving in terms of direct proximity or positioning. Also maybe better efficiency, once they get the kinks worked out.

I mean, I know it's mundane, but they put lasers on keychains these days!
posted by Joey Buttafoucault at 8:33 PM on August 15, 2012


There's a lot of things that microwave light is useful for that visible light is not. There's a lot of things that coherent light (lasers) are useful for that incoherent light (regular light) is not. What lasers do for visible light, masers can do for microwave light.

We've had masers longer than we've had lasers. But relative to lasers, masers are big and weak. Until now. Lasers were also big and weak. Until recently.

The real breakthrough in lasers for 99.9% of practical applications was the implementation of lasers in semiconductors — that is, with the same processes, scale, and industrial base with which which we make electronic circuits.

So this breakthrough with masers represents, at minimum, the step from the very first lasers which were mostly scientific curiosities to being useful in research and to a limited degree useful in industry — it may quickly lead to a breakthrough similar to that in laser technology which I just described, but not necessarily. The technology described here won't do it; but it very possibly will give many researchers ideas about how to do this differently in ways which are much cheaper. But like dephlogisticated implied, this has immediate applications in certain areas that aren't, um, consumer.
posted by Ivan Fyodorovich at 8:36 PM on August 15, 2012


Ivan, lasers are neat because we don't have other convenient sources of coherent light. What does this maser do for us that a regular RF oscillator does not?
posted by ryanrs at 8:52 PM on August 15, 2012


An RF oscillator can not really emit coherent microwave emission. You may be able to emit a narrow frequency range, but you're still broadcasting it over an antenna. Not emitting a coherent beam. So its more like a very narrow spectrum Red LED in a flashlight say, vs a red laser.

That's why people are jumping to space power transmission - laser lights spreads out very little over large distances compared to non-coherent light. So, you would need a much smaller collecting area on the ground to receive it compared to an RF oscillator + antennae, with much lower losses (and microwave should have a better conversion efficiency than visible even if you must go geothermal plus you don't have much less atmospheric scattering). Or, you would need lower power to heat that Missile for the DOD, and it would work farther away. (I would personally like a keychain maser I can reheat my tea with after I neglect it for too long, kthx).
posted by McSwaggers at 9:10 PM on August 15, 2012 [3 favorites]


"What's all that screaming from the lab next door?"

"Oh, it's those physicists having another one of their macho dick-masering contests again."
posted by ShutterBun at 9:32 PM on August 15, 2012 [1 favorite]


Now, replace you and your buddies with the American economy. Right now it's pretty incoherent, no one's working together or saying the same thing. That's like the microwaves in your microwave oven. If, by some miracle the right conditions can be created and get things moving again then we'll have something like a maser (coherent microwaves), but it takes a lot of work. Except these guys have figured out how to make it work under the current conditions and they're gonna get rich exploiting the loophole. Bastards!

Somewhere in the depths of Internet the opening statement of a dissertation on Microwaving in a Post-Marxist World just paraphrased the shit out of this without giving due credit.
posted by passerby at 9:40 PM on August 15, 2012 [4 favorites]


So rather than having to have parabolic dishes to transmit microwave signals, you could conceivably use point-to-point masers like we (occasionally) use LOS lasers for telecomm?

I don't think so. A maser is a device for generating RF energy, but the directivity of the beam is limited by the aperture area. Lasers produce thin beams because optical wavelengths are incredibly small and therefore a 1mm opening is electrically huge. But a maser with an aperture diameter of a few wavelengths could not produce a very tight beam - it would have about the same directivity as a traditional antenna of the same size.

Bottom line, this is a cool device for making microwaves, but it cannot overcome the fundamental diffraction limit, and would not produce a thin "point-to-point" beam like a laser.
posted by jpdoane at 9:46 PM on August 15, 2012


You can buy lasers at Staples.

A big laser, dude, not a cat toy. I'm kind of surprised they sell them on eBay, but I suppose why not?
posted by fshgrl at 10:00 PM on August 15, 2012


Metafilter: Right now it's pretty incoherent, no one's working together or saying the same thing.
posted by exphysicist345 at 10:11 PM on August 15, 2012


All the "what's it for?" questions are weird. We didn't know what lasers were for until we had them. Then we had brilliant breakthrough after brilliant breakthrough. If you aren't exciting to see the laser have a close cousin pop onto the scene, you don't appreciate how accidentally amazing lasers are.

Besides, what are YOU for?
posted by chairface at 11:29 PM on August 15, 2012 [2 favorites]


But... all RF (OK, not _all_, but the stuff you get off normal transmitters through normal antennas) is coherent, isn't it? Most light is incoherent, because it comes from a whole lot of electrons slamming across energy levels in their own good time and until the laser came along there was no way to synchronise them so each photon was generated in lock-step.

With radio, the photons come from masses of electrons accelerating and decelerating at the same time through what can be considered to be a huge set of very similar energy levels, and the photons are in lock-step because they're... actually, I have no good mental model for how RF photons are produced, and would deeply appreciate one from the MeFizicists. Been bothering me for most of my life, that.

But lasers were a Big Thing because before that, light came from things like light bulbs which produced masses of noise across a huge spectrum. I suppose you can consider the spark gap transmitter to be an incoherent RF source, but then we invented the oscillator. The stuff that comes off ordinary transmitters is effectively coherent and monotonic (modulation/noise apart), so is the output from a maser that different?
posted by Devonian at 2:02 AM on August 16, 2012


But... all RF (OK, not _all_, but the stuff you get off normal transmitters through normal antennas) is coherent, isn't it?

Yes - modulating signals on RF (eg AM, FM, CDMA, etc) depends on this. There seems to be some confusion between phase coherence (all the emitted photons being in phase) and beam collimation (all the photons going in the same direction). The laser creates a collimated beam by generating a coherent wavefront across an aperture. A reflector antenna (or array antenna or any highly directive antenna) does the exact same thing to produce a directional beam. The phase coherence is necessary, but the aperture size is what establishes the directivity (tightness) of the beam. Even optical lasers are not perfectly directive, their beams spread out as they propagate.

As far as how RF photons are generated, we typically don't think about quantum aspects at RF (at these frequencies, they are very much waves. Things only become more particle-like at much higher frequencies). One method is with a magnetron. But at RF, the generation of the signal is usually separate from the radiation of the signal (though not with a maser).
posted by jpdoane at 5:16 AM on August 16, 2012


Can someone explain the difference between a maser and a laser like I'm five?
posted by Eyebrows McGee at 21:29 on August 15 [+] [!]

laser = light
maser = microwaves
posted by Freen at 21:35 on August 15 [+] [!]


Freen's got it; it's just the name. The underlying quantum mechanical process is the same in both cases -- stimulated emission. For an excited state (an atom that has absorbed energy, for example, but lasers (or masers, for that matter) don't necessarily have to operate on an electronic transition; instead, a vibrational energy level in a molecule or other similar excited state may play the role of the upper state on the lasing transition. The word 'maser' is actually an acronym -- Microwaves Amplified by Stimulated Emission of Radiation; similarly for laser but with light instead of microwaves.

To back up a bit, that stimulated emission bit is what's really important. There's a paper Einstein wrote that lays out a lot of the relevant processes for optical physics. Interestingly enough, when he wrote on the topic in 1917, this result predated most of what we think of as the foundational work in quantum mechanics, which was largely unexplored until the 1920's (although Planck actually proposed that radiation must be quantized -- which is simply to say that it comes only in discrete chunks of energy, rather than in a continuously variable amount -- in 1900 or so). Anyway, Einstein made arguments from thermodynamics that emission of radiation could occur either spontaneously, as an excited state lost energy, or via a stimulated process, when the arrival of a photon of a specific energy induces the emission of another photon of the same energy. In other words, if you have a chunk of material with sharply defined energy states -- like a crystal with some impurity atoms tightly held in its lattice -- and a random photon comes wiggling past, it can induce an excited state to drop back down to its ground state by emitting a sort of duplicate photon. This is stimulated emission. The trick to getting a laser or maser to work is to keep those two duplicated photons circulating inside the laser -- so you use a series of mirrors to catch them and send them back through the excited material, and those two photons turn into four photons. The next time, eight photons. You get exponential amplification of those photons -- this leads to an intense beam of electromagnetic radiation, where there is a high degree of coherence among the photons (we will elide for a moment a careful discussion of coherence. I think a useful analogy is that of a swingset, with a number of children swinging. When the children all swing back and forth together, they are in phase; when one swings forward whilst the other swings backwards, they are said to be out of phase. In both cases, there is a well defined phase relationship amongst the swingers, but the in phase swingers have a constructive interference effect. The cumulative effect of electromagnetic oscillations that are in phase is great, while the effect of the anti-correlated, out of phase oscillations is minimized, as their motions 'cancel out' one another. On a large swing set, with many swingers moving without any special synchronicity, there will not be coordinated motion between different swingers. They will tend to lose or gain correlation with time, and thus can be said to be incoherent. Normal light is like those incoherent swingers; laser light is like those swingers in lock-step).

Also how do the lenses not burn or melt?
posted by Slap*Happy at 21:37 on August 15 [+] [!]


It's the process of stretching the pulse that does this. It lowers the peak energy -- the laser pulse is drawn out in time, so that at any given moment it's not too intense. Then, when we want to take advantage of its high power and its ability to probe events that happen on a short time scale, we compress it back onto itself.

In fact, while the laser is propagating through the experiment, the intensity is not usually sufficiently great to damage the optics, which are fairly robust (and expensive). It's in the laser cavity itself, where there is a build-up of power, that the optics are vulnerable. Outside the laser, if you bring the beam to a focus, you can burn yourself. And some of the beefier pump lasers will suffice for that even not at a focus. The oscillator is pumped by a 5W continuous green laser; I've been burned by that many times. It doesn't feel like touching a stove, though -- it's more like getting jabbed with a needle, because the beam is relatively narrow, and can penetrate deeply. The regen that I linked the photo of has a 30W pulsed pump beam. It's a larger diameter beam through most of the laser, but it would hurt like the dickens at a focus. The much greater risk, of course, is to your eyes, and we take great care to work safely with laser light.

A big laser, dude, not a cat toy. I'm kind of surprised they sell them on eBay, but I suppose why not?
posted by fshgrl at 0:00 on August 16 [+] [!]


Lasers are generally pretty unregulated. The restrictions that do apply are usually more of a workplace safety variety. The FDA is one of the larger regulatory bodies tasked with laser safety, go figure.


samofidelis: Wow, all that just to make espresso! But seriously, how about a sentence on why you need such a beam?


I work in a research group that studies photosynthesis and some related problems. It has recently -- in the last five or six years, actually -- been shown that the transport of energy in the early stages of photosynthesis depends on coherent processes. This was shocking, since the general framework for understanding energy transport in these types of systems was to assume an incoherent hopping -- the energy kind of lives in one spot of a photosynthetic apparatus, then randomly jumps to the next spot, then the next. It thus ends up at the reaction center, where that energy can be used to perform the chemistry bits of photosynthesis that are important\boring.

Now, however, it seems that coherent energy transport is a big part of how excitation energy is moved from the initial absorption event in a photosynthetic antenna to the reaction center via a multi-step process that appears to involve both wavelike and hopping motion. The wavelike motion is unexpected, and may in fact represent a kind of 'quantum computing' in a soft sense -- the excitation energy is diffused, and samples a number of different pathways as it is transported to the reaction center. In principal, this could result in the photosynthetic apparatus permitting a sort of optimization algorithm, as the energy effectively finds the most efficient pathway. It's not at all clear if this is necessary, or if evolution has selected for this kind of effect, or if this is just something that happens in proteins. I'm trying to resolve that aspect of the general problem of photosynthetic energy transport.

Anyway, we study these processes using ultrafast laser spectroscopy. To do so, we need light that is coherent, of controllable colour, and arriving in very fast pulses -- something on the order of ten femtoseconds; a femtosecond compared to a second is roughly the same ratio of durations as a second compared to the age of the universe. It's not even the limit of what laser science can do, however -- the leading edge of the laser spectroscopy arms race is at the 100 attosecond (0.1 femtosecond) limit. We just don't need that kind of speed to study the processes we're interested in.

But! To return to my original point, that I made so poorly. Laser science is totally hackable, just like these guys did. I'm sitting in a lab space with $500,000 of environmental control systems designed to keep this environment quiescent; there's about a million bucks worth of laser behind me on the optical table, which itself is something like $40k and floats on legs filled with compressed nitrogen to isolate it from acoustic noise. But you can build your own laser at home from junk. It's a great science fair project for a bright teenager, and you can do it for a couple hundo with parts from ebay, government surplus, and the like. When I lived in Texas, I would occasionally wander out to state surplus, and you could find anything you need to put together a crunky old system that just baaaaarely lases. You won't be able to build a state of the art system without special training and a lot of money, but who cares? You'll love your homebrew a lot more.
posted by samofidelis at 8:56 AM on August 16, 2012 [7 favorites]


But... all RF (OK, not _all_, but the stuff you get off normal transmitters through normal antennas) is coherent, isn't it? Most light is incoherent, because it comes from a whole lot of electrons slamming across energy levels in their own good time and until the laser came along there was no way to synchronise them so each photon was generated in lock-step.


I think the answer is coherentish. There are of course, degrees of coherence, but a large number of RF sources are largely coherent. Otherwise, you wouldn't be able to use the modulation techniques that permit you to do all the radio stuff we do -- and here I mean radio in the pedestrian, turn on the car radio and listen sense, as well as the RF measurement and modulation stuff in telecoms or NMR.


With radio, the photons come from masses of electrons accelerating and decelerating at the same time through what can be considered to be a huge set of very similar energy levels, and the photons are in lock-step because they're... actually, I have no good mental model for how RF photons are produced, and would deeply appreciate one from the MeFizicists. Been bothering me for most of my life, that.


If you want to boggle yourself further, look into a free electron laser, which produces coherent light in much the same manner as the coherent RF photons. The classical way to describe RF emission is that all accelerating charges radiate, and you are applying a potential to an antenna that causes current to flow at a certain frequency, oscillating back and forth along that antenna. A cartoon picture of the emission process is simply to think of the constituent charges being oscillated back and forth, and the whole array of them (arranged in a line, for example, in a simple quarter wave dipole antenna) radiate power. Because they're in an array, and have a well-defined phase relationship, you can get beaming.

(Oh, incidentally, plenty of people used to quibble about whether the free electron laser should even be called a laser, because of its weirdness. There are other sorts of oddities out there -- you can get lasing without population inversion, for example)

Now, a proper description of RF emission at the qm level is tough, because there isn't necessarily a clear and obvious set of quantum levels that are relevant. I mean, they are there, but I would have to think for a minute as to what they might be. The relevant energy manifold is obvious in a maser -- indeed, solving the ammonia maser is one of the first qm problems you do at the senior undergrad or grad level -- but it's not immediately obvious what they are in the simple dipole antenna.

Of course; that's all an approximation anyway. Since we're interested here in the fundamental processes of emission and radiation, we should be treating the electromagnetic field as a quantized field. But quantum field theory is hard. So we shall not.

Oh, and, I apologize if I got a bunch of this wrong. Please feel free to yell at me. It's largely stuff that I haven't thought about in a long time, and if I were to be more careful in my writing here, I'd never have gotten past the first paragraph.
posted by samofidelis at 9:08 AM on August 16, 2012 [4 favorites]


Thanks for the responses, samo. I really enjoyed them.
posted by benito.strauss at 10:08 AM on August 16, 2012


No problem. I got a real thing for lasers, I hope I didn't bore everyone else.
posted by samofidelis at 12:03 PM on August 16, 2012 [1 favorite]


Jon Singer has been doing stuff with surplus lasers and miscellaneous crap around his house/lab for a long time. For example, here he is trying to figure out which fountain pen inks will lase best.
posted by seanmpuckett at 12:17 PM on August 16, 2012


When they were at Stanford, Ted Hänsch and Art Schawlow showed laser action in Jello (admittedly doped with some kind of dye molecule). Lots of stuff will lase if you hit it hard enough. “Laser Action of Dyes in Gelatin,” by T.W. Hänsch, M. Pernier and A.L. Schawlow in the January 1971 IEEE Journal of Quantum Electronics, if I found the right citation.
posted by samofidelis at 2:01 PM on August 16, 2012


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