Monday, August 24, 2009

plasmons instead of cavities

Sorry for the delay in posts. The beginning of the new academic year is a hectic time.

This paper
is a very exciting new result. Unfortunately there does not appear to be a publicly accessible version available. Ordinarily, lasing (that is, light amplification by the stimulated emission of radiation) requires a few things. One needs a "gain medium", some kind of optically active system that has (at least one) radiative transition. In this paper, the medium is a dielectric oxide containing dye molecules known to fluoresce at a wavelength of 520 nm. This medium needs to be pumped somehow, so that there are more optically active systems in the excited state than in the ground state. This is called "population inversion". (It is possible to get lasing without inversion, but that's a very special case....) Finally, one generally needs a cavity - an optical resonator of high enough quality that an emitted photon stays around long enough to stimulate the emission of many more photons. The cavity has to be somewhat leaky, so that the laser light can get out. However, if the cavity is too leaky, the optical gain from stimulated emission in the pumped medium can't outpace the cavity losses. The usual approach is to have a rather high quality cavity, made using either dielectric mirrors, total internal reflection, or some other conventional reflectors.

In this paper, however, the authors take a different tactic. They use the near-field from the plasmon resonance of the gold core (not coincidentally, at around 520 nm wavelength) of Au-core-dielectric-shell nanoparticles. Plasmon resonances are often quite lossy, and this is no exception - the Q of the plasmon resonance is around 14. However, the enhanced near field is so large, and the effective mode volume (confinement) is so small, that gain still outpaces loss. When the dye is optically pumped, it is possible to make these nanoparticles lase. This paper is likely to spawn a great deal of further work! It's cool, and there are many clear directions to pursue now that this has been demonstrated.


Joseph Smidt said...

That is really interesting.

Thruxton said...

I feel that the authors went out on a limb with this one. This is because they claim single-particle lasing/spasing even though all of their measurements are performed on a bulk sample. They are probing ~10^9 particles!

Anyone who does colloidal synthesis knows that there are always ugly particles in even the best monodisperse synthesis. Personally, I would worry about a few hundred microscale particles contributing whispering gallery mode lasing.

Thruxton said...

I have an unrelated question:

To get a negative index of refraction in a nanostructured noble metal you need to excite it with a resonant wavelength that induces both a negative permittivity and permeability.

I understand that the permittivity is a materials/wavelength-dependent variable. And I have heard that a negative permeability is caused by a circular (or circulating) plasmon current following the right-hand rule.

My question: are they implying that there is an actual current of electrons moving around the nanostructure?

If I attach a nano-lightbulb to a negative index metamaterial, will it light up?

Doug Natelson said...

Thruxton - The authors did check a bit for the effect you're worried about. They cut the particle concentration by 1000x and saw no change in threshold behavior. This suggests that aggregates or collective interparticle effects aren't important. Still, I agree that single nanoparticle measurements would have been more impressive. I'm sure people are racing to do this.

Regarding your second question.... Sure, there are real electrical currents flowing in metal structures when they're illuminated, nano or not. When you shine light on a piece of metal, currents flow, leading to reflection of much of the radiation as well as damping of the EM wave going into the material. Of course, the currents in question are generally AC currents oscillating at the frequency of the incident radiation, on the order of 10^15 Hz or so.