[math-fun] Time-reversed laser absorbs coherent light
FYI -- If you used a laser to "push"/accelerate such a device in outer space, it would continue accelerating until the doppler frequency matched the absorption frequency, at which point it would stop accelerating & explode ? http://www.nytimes.com/2011/02/22/science/22laser.html February 21, 2011 A Dazzling Show Inside a Laser, but a Vacuum of Light Outside By HENRY FOUNTAIN In an elegant melding of theoretical and experimental physics, scientists at Yale University have taken the basic function of a laser and flipped it around  producing a device that absorbs, rather than emits, a beam of light. The device, which the scientists call a Âcoherent perfect absorber or, more popularly, an anti-laser, may lead to the development of new kinds of switches, filters and other components that could be useful in hybrid optical-electronic computers under development, among other applications. A. Douglas Stone, a theoretical physicist at Yale, developed the concept of a backward-running laser in a paper in Physical Review Letters last spring. The actual device, described in a paper published last week in Science, was created in the laboratory of a laser physicist, Hui Cao. In a laser, energy is pumped into a medium  which can be a solid, liquid or gas  between two mirrors, stimulating the emission of photons that are coherent, or of the same frequency and phase. The photons reflect back and forth between the mirrors, resulting in amplification of the light. ÂYou put energy into it, and some of that energy gets converted into that beautiful coherent light beam, Dr. Stone said. In his theoretical work, Dr. Stone said, he made use of the fact that the equations that describe how a laser works have certain symmetrical properties. ÂIf you can make a laser of a certain type, the equations say you can make a reverse device as well, he said. An anti-laser uses mirrors, too, but the other components are the reverse of a laser. The medium that provides amplification is replaced with one that provides absorption, and the outgoing light beam is replaced with an incoming one. (This light needs to be coherent, so it takes a laser to make an anti-laser.) The incoming beam is split in two, and hits the medium from two sides. The photons bounce around between the mirrors and interfere with one another, eventually wiping themselves out in a flurry of electrons and heat. The experimental device absorbed about 99.4 percent of the light. In theory, an anti-laser should be able to absorb 100 percent. ÂItÂs a one-way trap for light, Dr. Stone said. Dr. Cao said the device they built was relatively simple, using silicon as the absorptive medium and a couple of Âbad mirrors. ÂBut we should be able to get coherent perfect absorption in more complicated systems, she said. Eventually it may even be possible to make an Âanti version of a so-called random laser, in which the medium is highly disordered and there are no mirrors. The experimental device works in the near-infrared, outside of the visible spectrum. But Dr. Stone said that in principle anti-lasers would not be limited in terms of frequency. ÂWe could move it into the visible, or the farther infrared, he said. ÂItÂs definitely possible to engineer this across the whole range. Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy who was not involved in the work, said the anti-laser was an Âimportant achievement that was Âexciting and surprising to the scientific community. He said one important characteristic of the device is that the absorption could be turned on or off. This might make anti-lasers extremely useful as optical switching devices. A device that absorbs light perfectly might be considered ideal for solar energy applications, but Dr. Longhi said this is not the case. Sunlight is not coherent, and an anti-laser will not work with incoherent light, he said. A physicist would describe the device as a Âtime-reversed laser, since the symmetrical properties are related to the concept of time reversal. But Dr. Stone said he thought the term anti-laser was a better description for nonscientists, so that no one would think the device had anything to do with time travel. But even Âanti-laser is problematic, he noted. ÂI donÂt want people to think this is some kind of laser shield, he said. ÂIf R2-D2 had our anti-lasers, it would be melted into a puddle.Â
Please excuse me if I show ignorance in Physics, but.. Is it not possible to filter/redirect/polarise sunlight so it would be coherent enough to be absorbed by say 1000 such anti-lasers each designed to absorb specific frequencies, thus giving a very efficient solar power source ? On 21 Feb 2011, at 21:16, Henry Baker wrote:
FYI -- If you used a laser to "push"/accelerate such a device in outer space, it would continue accelerating until the doppler frequency matched the absorption frequency, at which point it would stop accelerating & explode ?
http://www.nytimes.com/2011/02/22/science/22laser.html
February 21, 2011
A Dazzling Show Inside a Laser, but a Vacuum of Light Outside
By HENRY FOUNTAIN
In an elegant melding of theoretical and experimental physics, scientists at Yale University have taken the basic function of a laser and flipped it around producing a device that absorbs, rather than emits, a beam of light.
The device, which the scientists call a “coherent perfect absorber” or, more popularly, an anti-laser, may lead to the development of new kinds of switches, filters and other components that could be useful in hybrid optical-electronic computers under development, among other applications.
A. Douglas Stone, a theoretical physicist at Yale, developed the concept of a backward-running laser in a paper in Physical Review Letters last spring. The actual device, described in a paper published last week in Science, was created in the laboratory of a laser physicist, Hui Cao.
In a laser, energy is pumped into a medium which can be a solid, liquid or gas between two mirrors, stimulating the emission of photons that are coherent, or of the same frequency and phase. The photons reflect back and forth between the mirrors, resulting in amplification of the light.
“You put energy into it, and some of that energy gets converted into that beautiful coherent light beam,” Dr. Stone said.
In his theoretical work, Dr. Stone said, he made use of the fact that the equations that describe how a laser works have certain symmetrical properties.
“If you can make a laser of a certain type, the equations say you can make a reverse device as well,” he said.
An anti-laser uses mirrors, too, but the other components are the reverse of a laser. The medium that provides amplification is replaced with one that provides absorption, and the outgoing light beam is replaced with an incoming one. (This light needs to be coherent, so it takes a laser to make an anti-laser.)
The incoming beam is split in two, and hits the medium from two sides. The photons bounce around between the mirrors and interfere with one another, eventually wiping themselves out in a flurry of electrons and heat.
The experimental device absorbed about 99.4 percent of the light. In theory, an anti-laser should be able to absorb 100 percent. “It’s a one-way trap for light,” Dr. Stone said.
Dr. Cao said the device they built was relatively simple, using silicon as the absorptive medium and a couple of “bad” mirrors.
“But we should be able to get coherent perfect absorption in more complicated systems,” she said. Eventually it may even be possible to make an “anti” version of a so-called random laser, in which the medium is highly disordered and there are no mirrors.
The experimental device works in the near-infrared, outside of the visible spectrum. But Dr. Stone said that in principle anti-lasers would not be limited in terms of frequency.
“We could move it into the visible, or the farther infrared,” he said. “It’s definitely possible to engineer this across the whole range.”
Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy who was not involved in the work, said the anti-laser was an “important achievement” that was “exciting and surprising to the scientific community.”
He said one important characteristic of the device is that the absorption could be turned on or off. This might make anti-lasers extremely useful as optical switching devices.
A device that absorbs light perfectly might be considered ideal for solar energy applications, but Dr. Longhi said this is not the case. Sunlight is not coherent, and an anti-laser will not work with incoherent light, he said.
A physicist would describe the device as a “time-reversed” laser, since the symmetrical properties are related to the concept of time reversal.
But Dr. Stone said he thought the term anti-laser was a better description for nonscientists, so that no one would think the device had anything to do with time travel.
But even “anti-laser” is problematic, he noted. “I don’t want people to think this is some kind of laser shield,” he said. “If R2-D2 had our anti-lasers, it would be melted into a puddle.”
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My dad put up a couple of them in my bedroom when I was a boy --- we used to call them "blackout curtains". Isn't science marvellous? WFL On 2/21/11, Henry Baker <hbaker1@pipeline.com> wrote:
FYI -- If you used a laser to "push"/accelerate such a device in outer space, it would continue accelerating until the doppler frequency matched the absorption frequency, at which point it would stop accelerating & explode ?
http://www.nytimes.com/2011/02/22/science/22laser.html
February 21, 2011
A Dazzling Show Inside a Laser, but a Vacuum of Light Outside
By HENRY FOUNTAIN
In an elegant melding of theoretical and experimental physics, scientists at Yale University have taken the basic function of a laser and flipped it around producing a device that absorbs, rather than emits, a beam of light.
The device, which the scientists call a “coherent perfect absorber” or, more popularly, an anti-laser, may lead to the development of new kinds of switches, filters and other components that could be useful in hybrid optical-electronic computers under development, among other applications.
A. Douglas Stone, a theoretical physicist at Yale, developed the concept of a backward-running laser in a paper in Physical Review Letters last spring. The actual device, described in a paper published last week in Science, was created in the laboratory of a laser physicist, Hui Cao.
In a laser, energy is pumped into a medium which can be a solid, liquid or gas between two mirrors, stimulating the emission of photons that are coherent, or of the same frequency and phase. The photons reflect back and forth between the mirrors, resulting in amplification of the light.
“You put energy into it, and some of that energy gets converted into that beautiful coherent light beam,” Dr. Stone said.
In his theoretical work, Dr. Stone said, he made use of the fact that the equations that describe how a laser works have certain symmetrical properties.
“If you can make a laser of a certain type, the equations say you can make a reverse device as well,” he said.
An anti-laser uses mirrors, too, but the other components are the reverse of a laser. The medium that provides amplification is replaced with one that provides absorption, and the outgoing light beam is replaced with an incoming one. (This light needs to be coherent, so it takes a laser to make an anti-laser.)
The incoming beam is split in two, and hits the medium from two sides. The photons bounce around between the mirrors and interfere with one another, eventually wiping themselves out in a flurry of electrons and heat.
The experimental device absorbed about 99.4 percent of the light. In theory, an anti-laser should be able to absorb 100 percent. “It’s a one-way trap for light,” Dr. Stone said.
Dr. Cao said the device they built was relatively simple, using silicon as the absorptive medium and a couple of “bad” mirrors.
“But we should be able to get coherent perfect absorption in more complicated systems,” she said. Eventually it may even be possible to make an “anti” version of a so-called random laser, in which the medium is highly disordered and there are no mirrors.
The experimental device works in the near-infrared, outside of the visible spectrum. But Dr. Stone said that in principle anti-lasers would not be limited in terms of frequency.
“We could move it into the visible, or the farther infrared,” he said. “It’s definitely possible to engineer this across the whole range.”
Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy who was not involved in the work, said the anti-laser was an “important achievement” that was “exciting and surprising to the scientific community.”
He said one important characteristic of the device is that the absorption could be turned on or off. This might make anti-lasers extremely useful as optical switching devices.
A device that absorbs light perfectly might be considered ideal for solar energy applications, but Dr. Longhi said this is not the case. Sunlight is not coherent, and an anti-laser will not work with incoherent light, he said.
A physicist would describe the device as a “time-reversed” laser, since the symmetrical properties are related to the concept of time reversal.
But Dr. Stone said he thought the term anti-laser was a better description for nonscientists, so that no one would think the device had anything to do with time travel.
But even “anti-laser” is problematic, he noted. “I don’t want people to think this is some kind of laser shield,” he said. “If R2-D2 had our anti-lasers, it would be melted into a puddle.”
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Perhaps there's more to this that exceeded the understanding of the NYT reporter. From what I read here, this anti-laser is designed more to absorb money than photons. Plain ordinary absorption already is the time reverse of stimulated emission, and the absorbed light can be coherent or not, as you please. So then what claims are made for this anti-laser that might not be possible with an ordinary absorber? High absorptance -- but this is just a matter of reducing reflectance, already a well developed technology. Absorptance modulation, optical switching -- can already be done. May work in the visible range -- already have visible light absorbers. It would be much more useful to have transparent materials in the far ultraviolet, beyond 150 nm, the so-called vacuum ultraviolet, in which the only known transparent medium is the vacuum. -- Gene ________________________________ From: Henry Baker <hbaker1@pipeline.com> To: math-fun@mailman.xmission.com Sent: Mon, February 21, 2011 1:16:33 PM Subject: [math-fun] Time-reversed laser absorbs coherent light FYI -- If you used a laser to "push"/accelerate such a device in outer space, it would continue accelerating until the doppler frequency matched the absorption frequency, at which point it would stop accelerating & explode ? http://www.nytimes.com/2011/02/22/science/22laser.html February 21, 2011 A Dazzling Show Inside a Laser, but a Vacuum of Light Outside By HENRY FOUNTAIN In an elegant melding of theoretical and experimental physics, scientists at Yale University have taken the basic function of a laser and flipped it around producing a device that absorbs, rather than emits, a beam of light. The device, which the scientists call a “coherent perfect absorber” or, more popularly, an anti-laser, may lead to the development of new kinds of switches, filters and other components that could be useful in hybrid optical-electronic computers under development, among other applications. A. Douglas Stone, a theoretical physicist at Yale, developed the concept of a backward-running laser in a paper in Physical Review Letters last spring. The actual device, described in a paper published last week in Science, was created in the laboratory of a laser physicist, Hui Cao. In a laser, energy is pumped into a medium which can be a solid, liquid or gas between two mirrors, stimulating the emission of photons that are coherent, or of the same frequency and phase. The photons reflect back and forth between the mirrors, resulting in amplification of the light. “You put energy into it, and some of that energy gets converted into that beautiful coherent light beam,” Dr. Stone said. In his theoretical work, Dr. Stone said, he made use of the fact that the equations that describe how a laser works have certain symmetrical properties. “If you can make a laser of a certain type, the equations say you can make a reverse device as well,” he said. An anti-laser uses mirrors, too, but the other components are the reverse of a laser. The medium that provides amplification is replaced with one that provides absorption, and the outgoing light beam is replaced with an incoming one. (This light needs to be coherent, so it takes a laser to make an anti-laser.) The incoming beam is split in two, and hits the medium from two sides. The photons bounce around between the mirrors and interfere with one another, eventually wiping themselves out in a flurry of electrons and heat. The experimental device absorbed about 99.4 percent of the light. In theory, an anti-laser should be able to absorb 100 percent. “It’s a one-way trap for light,” Dr. Stone said. Dr. Cao said the device they built was relatively simple, using silicon as the absorptive medium and a couple of “bad” mirrors. “But we should be able to get coherent perfect absorption in more complicated systems,” she said. Eventually it may even be possible to make an “anti” version of a so-called random laser, in which the medium is highly disordered and there are no mirrors. The experimental device works in the near-infrared, outside of the visible spectrum. But Dr. Stone said that in principle anti-lasers would not be limited in terms of frequency. “We could move it into the visible, or the farther infrared,” he said. “It’s definitely possible to engineer this across the whole range.” Stefano Longhi, a physicist at the Polytechnic Institute of Milan in Italy who was not involved in the work, said the anti-laser was an “important achievement” that was “exciting and surprising to the scientific community.” He said one important characteristic of the device is that the absorption could be turned on or off. This might make anti-lasers extremely useful as optical switching devices. A device that absorbs light perfectly might be considered ideal for solar energy applications, but Dr. Longhi said this is not the case. Sunlight is not coherent, and an anti-laser will not work with incoherent light, he said. A physicist would describe the device as a “time-reversed” laser, since the symmetrical properties are related to the concept of time reversal. But Dr. Stone said he thought the term anti-laser was a better description for nonscientists, so that no one would think the device had anything to do with time travel. But even “anti-laser” is problematic, he noted. “I don’t want people to think this is some kind of laser shield,” he said. “If R2-D2 had our anti-lasers, it would be melted into a puddle.” _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
On 21 Feb 2011, at 22:05, Eugene Salamin wrote:
High absorptance -- but this is just a matter of reducing reflectance, already a well developed technology.
That's not the only issue, surely it depends on what form the absorbed energy takes on absorption, or I should say the overall efficiency of going from light to accessible/transferable power ?
From: David Makin <makinmagic@tiscali.co.uk> To: math-fun <math-fun@mailman.xmission.com> Sent: Mon, February 21, 2011 3:03:44 PM Subject: Re: [math-fun] Time-reversed laser absorbs coherent light On 21 Feb 2011, at 22:05, Eugene Salamin wrote:
High absorptance -- but this is just a matter of reducing reflectance, already a
well developed technology.
That's not the only issue, surely it depends on what form the absorbed energy takes on absorption, or I should say the overall efficiency of going from light to accessible/transferable power ? _______________________________________________ The NYT article seems to say that, at least so far, absorption in an anti-laser goes into heat. But a standard absorber can be designed to produce useful work, as for example a solar cell. The front surface of a solar cell can be etched into tiny pyramids, subwavelength laterally, several wavelengths tall, to gradually shift the refractive index and thus reduce reflection. -- Gene
That's not the only issue, surely it depends on what form the absorbed energy takes on absorption, or I should say the overall efficiency of going from light to accessible/transferable power ? _______________________________________________ The NYT article seems to say that, at least so far, absorption in an anti-laser goes into heat. But a standard absorber can be designed to produce useful work, as for example a solar cell. The front surface of a solar cell can be etched into tiny pyramids, subwavelength laterally, several wavelengths tall, to gradually shift the refractive index and thus reduce reflection.
-- Gene
I know, but what's the efficiency rating of state-of-the-art solar cells ? Also, probably more relevant, what's the efficiency rating of light->solar->electric heater->heat compared to light->anti-laser->heat ? I realise it's all hypothetical because I'm making the massive assumptions that somehow sunlight could be used with anti-lasers *and* that the method used would be very efficient ;) I suspect that even if possible then converting sunlight to light coherent enough to work with the anti-lasers would be somewhat inefficient anyway.
On Mon, Feb 21, 2011 at 3:32 PM, David Makin <makinmagic@tiscali.co.uk> wrote:
I know, but what's the efficiency rating of state-of-the-art solar cells ?
About 40%, AFAIK, in converting solar to electricity.
Also, probably more relevant, what's the efficiency rating of light->solar->electric heater->heat compared to light->anti-laser->heat ?
A lot of people think that this is exactly the problem: the right way to use solar is light->solar->heat. The electricity step is hugely inefficient. Just paint a big water tank black. OK, so it's not quite that simple, but it's a lot cheaper than photovoltaic and more efficient too. --Joshua
The technology village not that far from here has example housing that uses light->heat via water tanks, it's pretty impressive. To my mind building new housing without this type of facility (or an alternative) is incredibly short-sighted (especially in places less cloudy than Wales). On 21 Feb 2011, at 23:35, Joshua Zucker wrote:
On Mon, Feb 21, 2011 at 3:32 PM, David Makin <makinmagic@tiscali.co.uk> wrote:
I know, but what's the efficiency rating of state-of-the-art solar cells ?
About 40%, AFAIK, in converting solar to electricity.
Also, probably more relevant, what's the efficiency rating of light->solar->electric heater->heat compared to light->anti-laser->heat ?
A lot of people think that this is exactly the problem: the right way to use solar is light->solar->heat. The electricity step is hugely inefficient. Just paint a big water tank black. OK, so it's not quite that simple, but it's a lot cheaper than photovoltaic and more efficient too.
--Joshua
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On 21 Feb 2011 at 15:35, Joshua Zucker wrote:
A lot of people think that this is exactly the problem: the right way to use solar is light->solar->heat. The electricity step is hugely inefficient. Just paint a big water tank black. OK, so it's not quite that simple, but it's a lot cheaper than photovoltaic and more efficient too.
OTOH, it isn't so easy to cook a meal, keep your refrigerator working, run your computers, etc, with a big tank of hot water. [And note that folks have used solar-water-heaters for a long time now. At least 20 yrs ago in MA a friend of my used solar to heat his outdoor pool] /Bernie\ -- Bernie Cosell Fantasy Farm Fibers mailto:bernie@fantasyfarm.com Pearisburg, VA --> Too many people, too few sheep <--
From: David Makin <makinmagic@tiscali.co.uk> To: math-fun <math-fun@mailman.xmission.com> Sent: Mon, February 21, 2011 3:32:54 PM Subject: Re: [math-fun] Time-reversed laser absorbs coherent light
That's not the only issue, surely it depends on what form the absorbed energy takes on absorption, or I should say the overall efficiency of going from light
to accessible/transferable power ? _______________________________________________ The NYT article seems to say that, at least so far, absorption in an anti-laser
goes into heat. But a standard absorber can be designed to produce useful work,
as for example a solar cell. The front surface of a solar cell can be etched into tiny pyramids, subwavelength laterally, several wavelengths tall, to gradually shift the refractive index and thus reduce reflection.
-- Gene
I know, but what's the efficiency rating of state-of-the-art solar cells ? Also, probably more relevant, what's the efficiency rating of light->solar->electric heater->heat ((Equal to efficiency of solar cell, the remaining power being wasted in heating the solar cell.)) compared to light->anti-laser->heat ? ((Equals 100% if we take into account the power needed to operate the anti-laser.)) I realise it's all hypothetical because I'm making the massive assumptions that somehow sunlight could be used with anti-lasers *and* that the method used would be very efficient ;) I suspect that even if possible then converting sunlight to light coherent enough to work with the anti-lasers would be somewhat inefficient anyway. _______________________________________________ In general the efficiency of any passive heating device is 100% -- all the power going in comes out as heat. However, a heat pump can achieve >100%. Recall that a heat engine can absorb heat Q2 from a high temperature T2, discharge heat Q1 to a low temperature T1, and produce useful work Q2-Q1. If the engine is reversible, Q2/T2 = Q1/T1 (i.e. no entropy production). Then the efficiency is (Q2-Q1)/Q2 = (T2-T1)/T2. Now operate the engine in reverse. Do work Q2-Q1 in order to extract heat Q1 from a cold environment at T1 and provide heat Q2 to a high temperature environment at T2. The efficiency is Q2/(Q2-Q1) = T2/(T2-T1), the equality holding for a reversible heat pump. An air conditioner pumps heat from your cool indoors to the hot outdoors. These efficiencies are the maximum possible consistent with thermodynamics, and they will be lower for real engines and pumps. -- Gene
Supposing an anti-laser could be more more efficient at capturing laser energy than a physical absorber, it could be a component in an energy transfer system. For instance, a solar satellite could convert sunlight to a laser beam and direct it to an anti-laser on Earth where it would be caught and converted to useful energy. This system would require a smaller earthly footprint than solar panels, and would be more or less impervious to weather conditions. ----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1204 / Virus Database: 1435/3458 - Release Date: 02/21/11
<flame> Bankers have recently proved far more efficient at destroying money than scientists could ever dream to be. Perhaps Freddie/Fannie Mae/Mac could convert their charters into building superconducting supercolliders instead of empty houses & empty bank accounts ? </flame> At 02:05 PM 2/21/2011, Eugene Salamin wrote:
Perhaps there's more to this that exceeded the understanding of the NYT reporter. From what I read here, this anti-laser is designed more to absorb money than photons.
participants (7)
-
Bernie Cosell -
David Makin -
David Wilson -
Eugene Salamin -
Fred lunnon -
Henry Baker -
Joshua Zucker