Nonlinear interactions in an organic polariton condensate

Abstract

Under the right conditions, cavity polaritons form a macroscopic condensate in the ground state. The fascinating nonlinear behaviour of this condensate is largely dictated by the strength of polariton-polariton interactions. In inorganic semiconductors, these result principally from the Coulomb interaction between Wannier-Mott excitons. Such interactions are considerably weaker for the tightly bound Frenkel excitons characteristic of organic semiconductors and were notably absent in the first reported demonstration of organic polariton lasing. In this work, we demonstrate the realization of an organic polariton condensate, at room temperature, in a microcavity containing a thin film of 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene. Upon reaching threshold, we observe the spontaneous formation of a linearly polarized condensate, which exhibits a superlinear power dependence, long-range order and a power dependent blue shift: a clear signature of Frenkel polariton interactions. 2The last decade has seen the study of the quantum fluidic behaviour of light flourish. 1 One branch of this field has focused on exploiting the properties of cavity polaritons:hybrid light-matter quasiparticles formed in semiconductor microcavities. 2 The substantial interest in strongly coupled semiconductor microcavities stems principally from the possibility to impart weakly interacting cavity photons with a strongly interacting matter component inherited from the exciton. On one hand, this matter component enhances energetic relaxation towards the polariton ground state by allowing interactions with phonons and other polaritons. 3,4 On the other hand, the nonlinearity inherited from the exciton gives rise to the hydrodynamic behaviour of polaritons. 5,6, 7,8,9,10,11 Polaritons have a finite lifetime, determined principally by their photonic component, beyond which they decay through the cavity mirrors. If relaxation is efficient enough, however, a macroscopic population can be accumulated in a single state-often the ground state-via bosonic final state stimulation. 12, 13,14 The threshold corresponding to this process, termed polariton lasing, can be significantly below that required for conventional photon lasing. The resulting macroscopically occupied state then behaves as a non-equilibrium Bose-Einstein condensate of polaritons. 15,16 Although polariton lasing has been mainly observed at low temperature due to the small binding energy typical of Wannier-Mott excitons, 13 recent developments have led to room temperature demonstrations in III-nitrides and ZnO. 17,18,19,20 Frenkel excitons possess binding energies of ~ 1 eV and are thus highly stable at room temperature. 21 Organic polariton lasing was first demonstrated in microcavities containing anthracene single-crystals. 22,23 3The nonlinear character of polariton condensates leads to a wealth of fascinating phenomena such as superfluidity and the formation of dark solitons and vortices. 7,8,9,10,11 This nonlinearity, which in a microscopic pi...

Macroscopic Two-Dimensional Polariton Condensates

“…As bosonic quasiparticles, polaritons experiment final-state stimulated scattering, which results, above a density threshold, in a laser-like emission without population inversion, a collective phenomenon that is explained in the framework of Bose-Einstein condensation [9][10][11]. A unique feature of polariton condensates is their driven/dissipative nature, in which the steady state is reached through a dynamical balance of pumping and dissipation.Polariton condensate have been experimentally observed in different materials, both inorganic [12][13][14][15] and organic semiconductors [16,17], and thanks to their light mass, condensation is achieved also at room temperature [18]. However, differently from their atomic counterpart, these condensates suffer from dephasing and density fluctuations induced by the interactions with the exciton reservoir, effectively resulting in multimode condensates [19][20][21].…”

“…Polariton condensate have been experimentally observed in different materials, both inorganic [12][13][14][15] and organic semiconductors [16,17], and thanks to their light mass, condensation is achieved also at room temperature [18]. However, differently from their atomic counterpart, these condensates suffer from dephasing and density fluctuations induced by the interactions with the exciton reservoir, effectively resulting in multimode condensates [19][20][21].…”

Bose-Einstein condensation of photons from the thermodynamic limit to small photon numbers

“…[12], p362), approaching thermal equilibrium, even if imperfectly. The excitons associated with the condensed polaritons can be free to move (Wannier excitons, typical of inorganic semiconductors [13]) or bound to individual sites (Frenkel excitons, typical of organic fluorescent solids [14,15]). By contrast, thermalization and BEC of photons as described above is performed in the weakcoupling limit, and with liquid-state matter.…”

Bosonic lasers: The state of the art (Review Article)

“…While a majority of research has been focused on semiconductor-based systems, significant advances have also been made in the development of organic based systems, where polariton BEC was reported [22,23] and the spatial coherence of room temperature polariton lasers demonstrated [24].…”