Polaritons

large oscillator strength → much bigger in organic materials (because?)

small excitation linewidth (so small inhomogeneous broading for instance)

molecule assemblies/liquids/? with particle density $N/\nu$, in a cavity have enhanced coupling $\omega_{Rabi}\propto \sqrt{N/\nu}$ (REFERENCE?)

collective wave functions are strongly delocalized, P- emission is spatially coherent: “molecules that are micrometers apart emit in-phase!”

Life times: can vary a lot, especially there are a lot of cases, where the lifetime of the Polariton is much longer than the lifetimes of the constituents (matter, photon)! (some sources can be found in [1])

First experiment with such organic semiconductors:
D. G. Lidzey, D. D. C. Bradley, M. S. Skolnick, T. Virgili, S. Walker, and D. M. Whittaker, Strong Exciton-Photon Coupling in an Organic Semiconductor Microcavity, Nature (London) 395, 53
use tetra-(2,6-t-butyl)phenol-porphyrin zinc (4TBPPZn) as the organic semiconductor, which has high oscillator strength and sufficiently small linewidth

organic materials have much more complicated electronic structure than conventional non-organic semiconductors (few level approximations are bad!)

BOA can break down because polariton introduces intermediate energy level in the large gap between electron and nucleus energies

strong coupling due to collective coupling of many molecules to one mode ($w_{Rabi}$ enhanced by $\sqrt(N)$): only small fraction of the modes are coupled, rest is “dark”, but still affected by the strong coupling [1]

minimal model [1]: 1d valence electron coupled to 1 mode of rest atoms (nuclei plus frozen core electrons), which could be a “stretching” of a phonon mode or of a bond, inter-rest-atom potential modeled by Morse potential (with fitting parameters)

First experiment: Kéna-Cohen, S.; Forrest, S. R. Room-temperature Polariton Lasing in an Organic Single-crystal Microcavity. Nat. Photonics 2010, 4, 371−375

“photon and exciton dispersions anticross under strong coupling, resulting in two new dispersion relations for the lower polariton (LP, lower energy branch) and upper polariton (UP, higher energy branch)”

mass (~10^-4 m_e) and lifetime (dependent on Q-factor of cavity, 10-100 ps possible so far) dominated by photon, interactions instead by excitons (mainly coulomb exchange), but this varies with momentum/dispersion

non-equilibrium due to finite lifetime of excitons (induced by scattering) and photons (by mirror quality)

condensation is highly non-linear, governed by two mechanisms: 1) dissipation via phonons, 2) dissipation via polariton-polariton scattering

measurement easy, as polaritons decay through the mirror as photons that have the same energy and momentum!! (why that?)

ongoing discussion on separation/validity: Bose-Einstein-Condensate - Polariton Laser - Photon Laser, main problem lies in the definition of concept either in or out of equilibrium

has Berezinskii–Kosterlitz– Thouless (BKT) and BCS pahse

first experimental relaization (in orgaanic semiconductor cavity):
Plumhof, J. D.; Stöferle, T.; Mai, L.; Scherf, U.; Mahrt, R. F. Room-temperature Bose−Einstein Condensation of Cavity Exciton−Polaritons in a Polymer. Nat. Mater. 2014, 13, 247−252

Slow down of reaction in e.g. vibrational SC (light coupled to vibration mode) as the splitting changes the vibration frequency and thus the bond strength [2]

change of character of reaction: from an associative to a dissociative transition state [2]

delocalized nature of polaritons give them good transport properties, also dark-states play a role [2]
$\rightarrow$ Förster-type nonradiative transfer rate can be increased by a factor of 7 with an efficiency approaching unity

Theory (not read yet):
Y. Todorov and C. Sirtori, Intersubband polaritons in the electrical dipole gauge, Phys.Rev.B 85, 045304 (2012)

Another USC situation: 2d QW of electrons (2 bands) coupled to a 0d-mode (plasmon polaritons)
Y. Todorov, A. M. Andrews, R. Colombelli, S. De Liberato,C. Ciuti, P. Klang, G. Strasser, and C. Sirtori, Ultrastrong Light-Matter Coupling Regime with Polariton Dots Phys. Rev. Lett. 105, 196402 (2010)

proof strong coupling between a “large biological system” (conglomerate of photoactive molecule BChl c that form a chlorosome in a very complicated way) to a cavity: despite a large scattering, polaritons form

could be used to make photosynthetic process more efficient

They imagine: “since the density of chlorosomes within the green sulfur bacteria is high, it may be possible to strongly couple a living bacteria to a cavity mode resulting in a ‘living polariton’”