Graphene found to emit infrared light

Apr 26, 2012 by Bob Yirka report
Made of a single sheet of carbon atoms, graphene can be spun at the fastest rate of any known macroscopic object. Image credit: Wikimedia Commons.

(Phys.org) -- Ever since its discovery in 2004, graphene, the honey-comb arranged sheet of one atom thick carbon atoms, has continued to make waves in both the physics and engineering worlds. Now comes news from yet another research team heralding a new found property of the fascinating material. This time, as the group describe in their paper published in Physical Review Letters, it’s been found to have optical gain properties as a result of population inversion of electrons when struck with a short blast from a laser.

Optical gain means that if light is shined onto a material, it reflects back more than was sent in; a very useful property for making optoelectric devices such as lasers, amplifiers, modulators and absorbers.

In this latest research effort, the team used graphene that had been epitaxially grown, which is where a crystalline substance is overlaid onto a substrate, resulting in a sheet of high quality graphene. They then excited the graphene with pump laser (1.55 eV.) pulses of very short duration (35 fs). In measuring the amount of light that was reflected back, they found it was more than was sent in. This they say, was because of graphen’s unique physical properties which causes it’s light conduction to change from positive to negative, which of course means that it reflects more than it absorbs.

More specifically, the material’s property occurs because as the pump laser pulse strikes the graphene, its electrons become excited with more charge carriers winding up in the Dirac cone than in the lower cone (population inversion). Because of the imbalance, a probe photon stimulating the excited states causes the emission of infrared light. And what’s more, the gain is more than that for conventional optical devices.
Additional research by the team showed that the optical gain observed with the graphene sample could occur over a wide range of energy pulses from the laser as well, leading to even more potential applications.

While their research is promising, the team acknowledge that much more work needs to be done before any actual real world devices could be created that take advantage of this new-found property of graphene, but eventually, the hope is that such equipment will be able to carry out their tasks faster than what is currently available, allowing for the development of high-speed telecommunications devices capable of keeping up with the ever growing demand for faster networks.

Explore further: Improving printed electronics process and device characterization

More information: Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene, Phys. Rev. Lett. 108, 167401 (2012) DOI:10.1103/PhysRevLett.108.167401

Abstract
We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.

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User comments : 11

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antialias_physorg
4.6 / 5 (9) Apr 26, 2012
And before anyone comes in with strange ideas:
No, This is not a violation of any conservation laws.
Chmegr
not rated yet Apr 26, 2012
And before anyone comes in with strange ideas:
No, This is not a violation of any conservation laws.


Thank you! I was about to write the same thing.
Smellyhat
not rated yet Apr 26, 2012
And if you are confused by the photo caption: no, it has absolutely nothing to do with the article.
Origin
1 / 5 (1) Apr 26, 2012
Well, if nothing else, it's still graphene and not the elephant or something (dis)similar.
This is not a violation of any conservation laws.
Recently one such a strange attempt has been published just in connection to graphene. http://phys.org/n...wer.html

simonl
5 / 5 (1) Apr 26, 2012
And before anyone comes in with strange ideas:
No, This is not a violation of any conservation laws.

Why not? An explanation would bear more weight than an unexplained claim.

The system only receives energy from the laser, afaiu. So how can more energy leave the system than comes in? Unless the material loses something, in which case the effect would soon fade off?

Just wondering.
antialias_physorg
5 / 5 (2) Apr 26, 2012
Why not?

The provided link to the abstract says it pretty well:
We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer.


So you don't get out more than you pumped in (creating the inversion population). But during a very short interval afterwards incident light will cause higher emission (depleting the population inversion in the process - that's where the 'more photons/energy out' comes from). It's pretty similar to how you make laser light, actually
(or rather: I can't find, at first glance, a difference between this and a laser medium)
packrat
1 / 5 (1) Apr 26, 2012
It doesn't say in the abstract and I can't afford the actual article but I'm wondering if the output response would trigger the same effect in a facing layer of graphene. If so possibly aim the original laser at 89 degrees or so and within a couple of inches of bouncing back and forth between the two layers you could get an amplified pulse out. Would this be possible?
FastEddy
not rated yet Apr 27, 2012
" ... which of course means that it reflects more than it absorbs." ... wondering if the total energy equation is balanced? ... "free energy"?? Would the power of the laser light inclement on the Graphene surface be equal to the power "reflected" ... close to the "perfect" mirror? ... 100% more or less?? ... We note that the frequency seems to be changed, visible laser light to infra-red ...
ryggesogn2
2.3 / 5 (3) Apr 27, 2012
Another inaccurate headline. Everything above 0 K emits in the IR.
DaFranker
1 / 5 (1) May 02, 2012
Another inaccurate headline. Everything above 0 K emits in the IR.

AKA Everything period, because true 0 K is only possible where nothing exists, by definition(s).
DaFranker
1 / 5 (1) May 02, 2012
" ... which of course means that it reflects more than it absorbs." ... wondering if the total energy equation is balanced? ... "free energy"?? Would the power of the laser light inclement on the Graphene surface be equal to the power "reflected" ... close to the "perfect" mirror? ... 100% more or less?? ... We note that the frequency seems to be changed, visible laser light to infra-red ...


No. As other commenters so thoughtfully pre-emptively stated just for this occasion: This does not violate any conservation laws. Nor any other current scientific rules/laws, for that matter.

This article basically says "Heat graphene with lazor, graphene emits heat, but emits more heat than it was heated! Even more than other stuffs! HurrDurr!", for those who don't understand how this doesn't violate conservation laws. The above is sort of a figurative simplification, of course.