The Travels of An Exciton [First images of exciting particle]

Focus Physical Review ^ | 22 October 2003 | Kim Krieger

[Russian Sage]

Phys. Rev. Lett. 91, 177401 (issue of 24 October 2003)	22 October 2003
Title and Authors
The Travels of An Exciton

 

K. Matsuda/Kanagawa Acad. of Science and Tech. Trapped and tracked. The first images of the motion of an exciton--a particle essential to modern electronics--and a biexciton show that the lighter exciton (top) roams farther.

Researchers have tracked their first exciton. A team reports in the 24 October PRL that they imaged the wave-like motion of the particle, which is essential to the operation of lasers in CD players and grocery scanners. They detected the light of a single trapped exciton and distinguished it from that of a double-particle called a biexciton. The technique may be used in the future to view the wave nature of other nanoscale particles.

When an electron in a semiconductor moves away from its native atom, the vacated spot is known as a hole, and it can act as if it were a positively charged particle. When a negatively charged electron meets a positively charged hole, they attract and can orbit around one another for a brief time before combining and emitting light. During that nanosecond-long dance, the particle pair is called an exciton.

Excitons produce the light in light emitting diodes (LEDs) and in laser diodes found in CD players and laser pointers. They may also play a role in future quantum computer technologies. Like all quantum-scale particles, an exciton does not exist at a single point but is described by a wave--or wave function--that gives the probability of finding it in any given location. A trapped particle at low energy has a wave function with a central peak where it is most likely to be found and a width that gives the distance the particle is likely to roam from the center. This "roaming distance" is related to many basic physical properties, but previous attempts to gauge it have relied on indirect methods. Images of the light from excitons have only captured them as single points.

Kazunari Matsuda of the Kanagawa Academy of Science and Technology and the Japan Science and Technology Agency, and his colleagues, measured the wave function of an exciton by mapping with high resolution the light it gave off when the electron and hole combined. They repeatedly zapped a 100-nanometer-diameter raised patch of semiconductor called a quantum dot with laser light delivered through a microscopic glass fiber. Each light pulse created a single exciton and biexciton on the dot, and the team used the same fiber to collect the light released with each death of the particles. The researchers kept their 20-nanometer-diameter probe tip in the same spot for many cycles before moving to the next position and gradually built up a complete picture. They distinguished the exciton from the biexciton by observing at characteristic wavelengths of light for each type of particle.

The images show that an exciton strays farther from the center of a quantum dot than does a biexciton. The particles act like balls rolling inside a valley, and the heavier particle--the biexciton--can't roll as far up the sides as the lighter particle can.

Matsuda and colleagues present "a major step forward in nanospectroscopy," says Christoph Lienau of the Max-Born Institute in Berlin. He adds that because the technique can resolve objects less than 30 nanometers in size, direct optical mapping of other nano-size objects is within reach. Dan Gammon of the Naval Research Laboratory in Washington, DC, agrees but says the technique is still very difficult. Matsuda says his group hopes to further develop imaging techniques at nanoscales and explore how light from a nano-sized source interacts with matter.

--Kim Krieger

[dirtboy]

They detected the light of a single trapped exciton and distinguished it from that of a double-particle called a biexciton.

I hate it when excitions go lesbian.

[PoorMuttly]

Hey...he's a personal friend of mine.

If they wanted a photo, I could have sent them one from my last Official Muttly Gala Pool (birdbath in back yard) Party !

[PoorMuttly]

...before everyone asks this...

...his name is Phil.

(but you should see his wife. Hubba-Hubba !)

[Physicist]

They repeatedly zapped a 100-nanometer-diameter raised patch of semiconductor called a quantum dot with laser light delivered through a microscopic glass fiber. Each light pulse created a single exciton and biexciton on the dot, and the team used the same fiber to collect the light released with each death of the particles. The researchers kept their 20-nanometer-diameter probe tip in the same spot for many cycles before moving to the next position and gradually built up a complete picture.

It reminds me of Doc Edgerton with his strobe light.

[Libertarianize the GOP]

ping

[RANGERAIRBORNE]

One thing that struck me in college physics (and has been borne out since) is that, whenever particle physicists decide that some specific particle MUST exist, and build instruments to go looking for it, it always seems to be there. A striking coincidence.

There is a germ of an idea here, nut it would take someone more philosophically inclined than I am to follow it up...

[RANGERAIRBORNE]

"but", not "nut". Probably a Freudian slip.

[Russian Sage]

...whenever particle physicists decide that some specific particle MUST exist, and build instruments to go looking for it, it always seems to be there. A striking coincidence.

You only hear about the ones that are there! Why don't you try predicting a particle that must be there and see how lucky you get.

This isn't a discovery of a particle thing. It's observing particle behavior to see if the behavior matches predicted behavior.

The fact that you can use the theoretical properties of a particle to fine tune real products like LEDs and laser diodes should give a lot of confidence that the particles are real.

[RANGERAIRBORNE]

I didn't say or imply that these particles aren't "real".

I do think that there is some element of the observer determining reality built into our Universe- but if Einstein and Heisenberg couldn't nail this down, I suspect that I'm not going to be able to do it this afternoon...

[Physicist]

One thing that struck me in college physics (and has been borne out since) is that, whenever particle physicists decide that some specific particle MUST exist, and build instruments to go looking for it, it always seems to be there. A striking coincidence.

Off the top of my head, here's a list of particles that either aren't there, do not play the crucial role envisioned for them, are too deeply hidden to see, or have yet to be discovered:

• pomerons
• pseudoscalar axions
• glueballs
• neutral heavy leptons
• Z-prime (and many other potential artifacts from GUT-scale symmetry breaking)
• 4-generation quarks
• leptoquarks
• technicolor scalars
• technipions
• technirhos
• W1, W2, W3, W4
• the Higgs boson
• the entire supersymmetric particle spectrum
• the top meson and baryon spectra
• Goldstone bosons
• tachyons
• Fadeev-Popov ghosts
• gravitons (including Kaluza-Klein towers and other bulk excitations)
• WIMPS
• strange droplets
• dark matter
• the 17-keV neutrino and other figments of the imagination
• magnetic monopoles
• instantons

I'm sure there are some obvious ones I forgot.

[RANGERAIRBORNE]

Well, I'll yirld on this- but some of those are not "particles", as I understand the term- and others would by their nature not be observable with any conceivable instrumentation (tachyons, for instance- how could you find a particle that never travels slower than light?). Many of these things are theoretical constructs to fill a hole in an equation, but are not being seriously searched for as "real" particles.

If I had 50 or 100 more IQ points, I would be able to present a more compelling case- or perhaps realize that I'm on a "non-falsifiable" track and give it up.

[Bob J]

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[Physicist]

E.C.G Sudarshan (who I believe should have received the Nobel Prize for Physics, along with the late Bob Marshak, for the "V minus A" theory of the weak interaction), has constructed detailed theories of particle physics wherein tachyons play a crucial role. He believes that they are physical entities whose existence will one day be proven.

[Physicist]

Many of these things are theoretical constructs to fill a hole in an equation, but are not being seriously searched for as "real" particles.

I should also add that quarks and gluons fell into this category, up until the time they were discovered. Murray Gell-Mann, in proposing the quark model, called them a "mathematical fiction".

There are other mathematical constructs (Skyrmions, Regge poles) which I didn't include for that reason. The only item on my list that should probably be stricken on those grounds would be Fadeev-Popov ghosts (particles which arise naturally in certain quantum field theories--in fact, are sometimes required to make them work--but which have a negative probability to exist). So OK, forget about them.