Can a single molecule behave as a mirror?

Nov 18, 2008 By Miranda Marquit feature

(PhysOrg.com) -- “We have shown for the first time, theoretically, that a single molecule can behave as a perfect mirror,” Mario Agio tells PhysOrg.com. “Imagine that your mirror at home becomes a single molecule and that you put a strong lens between you and it. Well, you could still see the image of your face reflected…[A]mazing if you think that a molecule is just about a nanometer in size.”

Agio is a scientist at the Nano-Optics Group headed by Vahid Sandoghdar at ETH Zurich, Switzerland. Sandoghdar’s team investigates both theoretically and experimentally how light interacts with nanoscale objects. “Theoretically,” Agio says, “we show that a directional dipole wave can be reflected by one point-like oscillating dipole.” The results of the work done by the Group, including Gert Zumofen and Nassiredin Mojarad, can be seen in Physical Review Letters: “Perfect Reflection of Light by an Oscillating Dipole.”

“There are many possibilities for this kind of work,” Agio points out. He explains that, “Once you can realize efficient coupling between a photon and a molecule or atom in free space, there is a great deal that you can do. For example, advanced solar cells, spectroscopy, quantum communications and quantum computation would all benefit from stronger interaction between light and matter.”

However, there are some technical problems that need to be overcome in the lab. One of the more pressing issues is the fact that molecules behave like strongly damped oscillators at room temperature. “We need to cool the system to very low temperatures,” Agio says. Referring to work published earlier this year in Nature Physics (“Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence”), he continues: “Indeed, our experiments have been able to achieve 12 percent extinction of light in the forward direction by a single molecule.

Agio also explains that there are problems with being able to focus. “The reflection is only 100 percent if you can create a focused beam with lenses that just are not available today. But a terrific 80 percent reflection should be possible even with today’s conventional lenses and laser beams.”

Sandoghdar’s group has been considering future developments for this work, and Agio discusses some of the possibilities. “We are considering the use of optical antennas to overcome the limitations of focusing. We should be able to channel and collect light more efficiently, improving the coupling between a photon and a single molecule, very much in the same way the antenna in your cell phone receives and send calls.” He pauses before adding: “We’d have to make it very small, though, scaling everything down to optical frequencies is challenging.”

Agio is optimistic. He points out that the group has already done some work along the lines of optical antennas, and that it, too, has been reported in Physical Review Letters. “We have already shown experimentally that noble metal nanoparticles are very efficient optical antennas for single molecules,” he says, referencing an article titled, “Enhancement of single molecule fluorescence using a gold nanoparticle as an optical nano-antenna.”

“Now,” he continues, describing the next steps, “we are working on how to combine the focusing with the antenna to make all of this workable.”

More Information:

G. Zumofen, N. M. Mojarad, V. Sandoghdar, and M. Agio. “Perfect Reflection of Light by an Oscillating Dipole,” Physical Review Letters (2008). Available online: link.aps.org/abstract/PRL/v101/e180404.

G. Wrigge, I. Gerhardt, J. Hwang, G. Zumofen & V. Sandoghdar. “Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence,” Nature Physics (2008). Available online: www.nature.com/nphys/journal/v… n1/abs/nphys812.html.

Sergei Kühn, Ulf Håkanson, Lavinia Rogobete, and Vahid Sandoghdar. “Enhancement of Single-Molecule Fluorescence Using a Gold Nanoparticle as an Optical Nanoantenna,” Physical Review Letters (2006). Available online: link.aps.org/abstract/PRL/v97/e017402.

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com

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Mercury_01
1 / 5 (2) Nov 18, 2008
No.
raron
not rated yet Nov 19, 2008
So the molekule reflects all waves at once back at were they came from.

Easy Quantum Peasy
:)
axemaster
not rated yet Nov 19, 2008
"The first statement does not lead to the second one, the image of your face is not a dipole wave its a superposition of zillions of interfering dipole waves (from individual atoms) so theres no way the single molecule can work like a perfect mirror."

I don't see how that really makes any sense - isn't that the case with a normal mirror as well? Your argument doesn't seem to work...

Anyhow, this technology, while not useful (in my opinion) for a bathroom mirror, should prove very useful for making ultra high sensitivity sensors, and possibly perfect focussing lenses for lasers that never burn out.
gragg
not rated yet Nov 19, 2008
The first statement does not lead to the second one, the image of your face is not a dipole wave its a superposition of zillions of interfering dipole waves (from individual atoms) so theres no way the single molecule can work like a perfect mirror.


If you could build an optical system that images the (dipole) wave from each atom of an extended object onto the molecule, each wave should be reflected. Like from a tiny spherical mirror. The problem would be more about matching polarization and hitting the molecule's resonance wavelength.

It can be showed that even 2 dipole waves of equal frequency pose problems: if the reflecting dipole happens to be at the location where the interference cancels out the effective field the dipole wont be able to reflect anything.

So you would not see your face.


I guess that two (focused) dipole waves from two different atoms can only totally cancel out in the focus if they either come from exactly the same position, or from opposite directions.
superhuman
not rated yet Nov 20, 2008

I don't see how that really makes any sense - isn't that the case with a normal mirror as well? Your argument doesn't seem to work...

Normal mirror has zillions of atoms, and each one contributes only a tiny amount to the final picture.


I guess that two (focused) dipole waves from two different atoms can only totally cancel out in the focus if they either come from exactly the same position, or from opposite directions.


Depend on how you understand "totally".
If I get your argument right you mean that even if it cancels for one part of molecule it wont for the other part which will then reflect it.
This is not possible due to the wavelength of visible light being about 1000 times bigger then the diameter of the molecule so while it might not be *perfectly* canceled out the molecule placed in the middle of destructive interference zone wont be able to reflect anything.
gragg
not rated yet Nov 20, 2008

Depend on how you understand "totally".
If I get your argument right you mean that even if it cancels for one part of molecule it wont for the other part which will then reflect it.
This is not possible due to the wavelength of visible light being about 1000 times bigger then the diameter of the molecule so while it might not be *perfectly* canceled out the molecule placed in the middle of destructive interference zone wont be able to reflect anything.


Yes, the molecules is much smaller than the wavelength. With total I meant that the sum of the two waves' amplitudes is zero at the molecule's position for all times. So that it sits in a minimum of a standing wave pattern. But I am not sure. Maybe that can also happen for two focused dipole waves that are not co- or counterpropagating.
superhuman
not rated yet Nov 21, 2008
It can happen in other cases also, the only requirement is that the difference between the paths be equal to half the wavelength, then the waves from both dipoles will have equal magnitude and opposite signs and will cancel each other out.