New paper - plasmons, excitons, and steering energy

We have a new paper out in Nano Letters (arxiv version here), and I wanted to explain a bit about it and why I think it's a really cool result.   

I've written before about the Purcell Effect.  When we study quantum mechanics, we learn that the rates of processes, like the spontaneous emission of light from an atom, are actually malleable.  The rate of a particular process is usually proportional to the number of ways that process can happen - this is quantified in something called Fermi's Golden Rule.  When we are talking about something like emission of light from an atom, the rate is proportional to the number of possible final states of the photon.  We know how to count those states in a given energy range in free space, and Purcell pointed out that by placing that atom in an optical cavity, we alter the density of final states as a function of frequency, \(\rho(\omega)\) from its empty space value, and hence can change the rate of emission.  Pretty wild that placing a system in a cavity can alter the flow of energy in that system away from what it would otherwise be.

I've also written before about what happens we take two resonators and couple them together - we get "hybridization" or "new normal modes".  If you take a mass on a spring (natural frequency \(\omega_0 = \sqrt{k/m}\)) and couple it mechanically to another identical mass on an identical spring, the coupled system will now have two resonances, one above and one below \(\omega_{0}\).  The chemistry analog of this is, bonding two hydrogen atoms (each with 1s orbitals) together leads to two \(\sigma\) orbitals, one bonding and one antibonding.  

In the new paper, we start with a little metal tunnel junction that hosts plasmonic resonances, like the junctions I wrote about here.  We showed in that paper and subsequent work that it is possible to use an applied voltage and current to get some of the electrons, right near where the electrodes almost touch, to become effectively so hot that they glow (emitting light at energies larger than the applied voltage), while the atomic lattice itself remains cold.  The light emission process here is the radiative recombination of hot electrons and holes in the metal, where an electron drops down in energy to fill in a hole and spit out a photon.  The plasmon resonances of the bare metal act like a sort of cavity, shaping the density of photon states \(\rho(\omega)\), as we also showed here.  The plasmons, set by the metal shape and electronic properties, actually affect the rate at which the electrons and holes in that same metal radiatively combine.

Left: A thin flake of WSe2 is placed on a plasmonic
Au junction.  Right: Overbias light emission from the
device at a particular emitted polarization shows a big
peak splitting right around where the exciton resonance
is of the WSe2 (orange curve).  Adapted from the
SI of this paper.

The wrinkle in the new paper is that we couple that metal plasmonic junction with a thin (few nm) layer of 2D semiconductor by placing the semiconductor on top of the metal.  The semiconductor can host excitons, bound electron-hole pairs, and if the semiconductor is excited with enough energy to create them, the excitons can radiatively annihilate, leading to a comparatively narrow resonance at an energy that overlaps the plasmon resonances of the metal junction.  Thanks to hybridization between the plasmons in the metal and the excitons in the semiconductor, the photon density of states now has a split peak structure ("upper and lower plexciton polariton resonances" if you are an expert).  Light emission in this device is still due to recombination of electrons and holes in the metal, but now the recombination dynamics of those electrons "feels" the strong coupling between the excitons and plasmons.  (The polarization of the emitted light is rather complicated because of the polarization properties of the plasmon resonances).  

There are a lot of interesting possibilities on where to go from here, but it's always amazing to me to see how this physics comes together.  In this case, by changing the optical environment of a metal structure, we can alter the fate of energy stored in the electrons of that metal.  Really neat.