Ultrastrongly Coupled Exciton–Polaritons in Metal‐Clad Organic Semiconductor Microcavities

Abstract

The large exciton binding energies and oscillator strengths characteristic of organic semiconductors make this class of materials uniquely suited for the study of the strong excitonphoton coupling regime at room temperature. [ 1 ] In this regime, where the exciton-photon interaction exceeds the photon and exciton damping, new coherent light-matter excitations are formed called microcavity polaritons. In inorganic semiconductors, polaritons have been the source of a wealth of fascinating phenomena such as parametric amplifi cation, Bose-Einstein condensation and superfl uidity. [ 2 ] These stem principally from the exciton character of the resulting excitation. [ 3 ] In organic semiconductors, macroscopic occupations of the lower polariton branch minimum have been demonstrated, [ 4 ] but nonlinear phenomena resulting directly from polariton-polariton interactions remain to be shown. [ 5 ] To allow the buildup of a polariton condensate, all of these demonstrations have sandwiched the active material between two dielectric mirrors, each providing almost 100% refl ectivity and thus reducing the decay of polaritons via their photon component.The use of a metal mirror, however, can be benefi cial, principally for three reasons: it reduces the mode volume below that achievable with Bragg mirrors which results in a larger Rabi splitting, it provides the stop band width then required to accommodate both the upper and lower polariton branches and it provides a simple means for electrical injection as demonstrated in the pioneering work by Tischler et al. [ 6 ] and further investigated in other structures. [ 7 ] The enhancement in Rabi splitting was fi rst explicitly demonstrated by Hobson et al., who observed a Rabi splitting of ∼ 320 meV in an allmetal cavity. [ 8 ] Since then, Rabi splittings of ∼ 430 meV, ∼ 450 meV and ∼ 650 meV were observed in refl ectivity from polysilane containing metal/ dielectric cavities, [ 9 ] from polycrystalline tetracene [ 10 ] and photoisomerized merocyanine [ 11 ] all-metal cavities, respectively. Such values are comparable (>20%) to the bare exciton energy E ex . This regime where ∼ E ex has been termed the ultrastrong coupling regime and possesses a number of interesting properties. [ 12 ] For a correct treatment of the resulting polaritons, anti-resonant terms and the contribution arising from A 2 , the squared magnetic vector potential, must be included within the usual Jaynes-Cummings-type Hamiltonian. In the context of bulk exciton-polaritons, the full light-matter Hamiltonian was considered in the seminal work of Hopfi eld and Agranovich. [ 13,14 ] There, it was shown that polaritons are the lowest lying excitations of the coupled light-matter system. One can also show that if the full Hamiltonian is considered, the ground state of the coupled system is modifi ed by the light-matter interaction. This ground state was later explicitly constructed by Quattropani et al. [ 15 ] One of its most interesting aspects was recently highlighted by Ciuti et al. in the context of cavity ex...

Coupling of exciton-polaritons inlow−Qcoupled microcavities beyond the rotating wave approximation

“…This energy is angle-independent and so is expected to give the resonance energy for the coupled exciton-photon system 12 . Comparing these two contour plots, the polariton dispersion of the TM modes is flatter than that of the TE modes consistent with polarization dependence of the bare cavity photon dispersion 22 To connect better to the underlying physics, we plot the dispersion as a function of the wavevector k, shown in Fig. 5(a) for TM and (b) for TE polarization.…”

“…Organic semiconductor-based single microcavities composed by high-Q or low-Q reflectors, exhibiting large vacuum Rabi splitting, have been particularly interesting as strong and ultrastrong exciton-photon coupling can be readily attained at room temperature [12][13][14][15][16][17][18][19][20][21][22][23][24] . In this framework, due to the specific excitonic property of organic materials, allowing the ultrastrong coupling regime with a large Rabi splitting to be reached, the demonstration of such coupled multiple organic microcavities, each in the ultrastrong regime, suggests the potential for new physics and applications for tunable polariton-based devices operating at room temperature 25 , with new concepts in quantum information being one example 26 .…”

“…The organic material used within the microcavities is a wellstudied molecular glass of an organic dye with large inhomogeneous absorption broadening. The large oscillator strength of the material is a key factor to reach the ultrastrong coupling regime 22 . The use of a neat molecular glass provides for a high number density for a large oscillator strength with high optical quality.…”

Nonlinear interactions in an organic polariton condensate

“…In all-metal microcavities, this material has been used to demonstrate ultrastrong coupling with Rabi splittings of Ω~1 eV . 35 It is highly photostable and possesses a quantum efficiency of 0.43. The calculated cavity quality factor for our structure is Q ~ 300.…”

Strong coupling in a microcavity containing β-carotene

“…The resonant, reversible exchange of energy between the semiconductor exciton and confined cavity-photon yields a splitting of these states into lower and upper polaritons, separated by the Rabi splitting energy (ħΩ). This phenomenon has been widely investigated in both inorganic [3][4][5][6] and organic semiconductors [7][8][9][10], the latter of which have the advantage of strongly bound excitons that are stable at room temperature. Recent advances have seen the development of organic exciton-polariton condensates and room-temperature polariton lasing [11,12], and an everexpanding field of molecules capable of undergoing strong exciton-photon coupling [13].…”