interaction that 2D materials have demonstrated has made them highly attractive for practical device applications. Graphene, the archetype 2D material was explored extensively for a wide array of photonic applications 1 . However, due to the lack of direct bandgap in graphene, considerable attention has shifted towards 2D materials known as layered transition metal dichalcogenides (TMDs) 2,3 . These TMDs are a group of naturally abundant material with a MX 2 stoichiometry, where M is a transition metal element from group VI (M = Mo, W); and X is a chalcogen (M = S, Se, Te). One of the most intriguing aspect of TMDs is the emergence of fundamentally distinct electronic and optoelectronic properties as the material transitions from bulk to the 2D limit (monolayer) [4][5][6][7][8] . For example, the TMDs evolve from indirect to direct bandgap semiconductors spanning the energy range of 1.1 to 1.9 eV in the 2D limit 4-6 .Among the TMDs, molybdenum disulphide (MoS 2 ) is one of the most widely studied systems used to demonstrate 2D light emitters 9 , transistors 10,11 , valleytronics 12-15 and photodetectors 16,17 . The novel excitonic properties of 2D MoS 2 that make it very interesting for both fundamental studies and applications include: (i) the enhanced direct band gap photoluminescence (PL) quantum yield of the monolayer compared with the bulk counterpart 7,8 , (ii) the small effective exciton Bohr radius (0.93 nm) and associated large exciton binding energy (0.897 eV) 18,19 providing the opportunity for excitonic devices that operate at room temperature (RT) and (iii) the 2D nature of the dipole orientation making the excitonic emission highly anisotropic 20 . 3The interaction of a dipole with light can be modified by altering the surrounding dielectric environment. The most widely studied and technologically relevant phenomenon in this context is the Purcell enhancement wherein the spontaneous emission rate of the dipole is enhanced using an optical cavity by altering the photon density of states. Here the coupling between the dipole and the cavity photon is defined to be in the weak coupling regime since the interaction strength is weaker than the dissipation rates of the dipole and the photon. This regime has been demonstrated in the 2D materials using photonic crystal cavities coupled to 2D layers of MoS 2 21 , and WSe 2 22 . It resulted in an enhancement of the spontaneous emission rate and highly directional photon emission.When the interaction between the dipole and the cavity photons occur at a rate that is faster than the average dissipation rates of the cavity photon and dipole, one enters the strong coupling regime resulting in the formation of new eigenstates that are half-light -half-matter bosonic quasiparticles called cavity polaritons. Since with the pioneering work of Weisbuch et al. 23 there have been numerous demonstrations of cavity polariton formation and associated exotic phenomena in solid state systems using microcavities and quantum wells that support quasi 2D excitons [24][25][26] . H...

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Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient four-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here we review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technological applications, the use of new materials for room-temperature operation, and the possibility of exploiting polaritons for quantum computation and simulation.

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...

Superfluidity-the suppression of scattering in a quantum fluid at velocities below a critical value-is one of the most striking manifestations of the collective behaviour typical of Bose-Einstein condensates. This phenomenon, akin to superconductivity in metals, has until now only been observed at prohibitively low cryogenic temperatures.For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has only been demonstrated at liquid helium temperatures. In this case, the limit is not imposed by the mass, but instead by the small exciton binding energy of Wannier-Mott excitons, which places the upper temperature limit. Here we demonstrate a transition from normal to superfluid flow in an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only to table-top studies of quantum hydrodynamics, but also to room-temperature polariton devices that can be robustly protected from scattering.

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...

High-efficiency white organic light-emitting devices (OLEDs) [1][2][3][4] are of interest due to their potential uses in fullcolor active-matrix displays coupled with color filters, and also as solid-state lighting sources. For full-color display backlights, high brightness is required because of loss of light in the optical films and the small aperture ratio (∼ 40 %) in the backplane of the thin-film transistors. [5] Similarly, high brightness (> 800 cd m -2 ) is required for solid-state lighting sources. To achieve both high brightness and efficiency, the stacked OLED (SOLED) [6][7][8] consisting of multiple electroluminescent elements connected in series has been introduced. More recently, high-efficiency green-light-emitting SOLEDs have been demonstrated. These devices use a transparent chargegenerating interlayer such as indium tin oxide (ITO), [9] V 2 O 5 , [9] or an organic p-n junction, [10] where the hole-transporting layer (HTL) and electron-transporting layer (ETL) are doped with FeCl 3 and Li, respectively. In the SOLED structure, the luminance at a fixed current density increases linearly with the number of stacked and independent OLED elements. This can lead to a significant improvement in lifetime, as well as external efficiency, by reducing the degradation that accompanies the high drive currents required to achieve similarly high brightness in a single-element OLED.Here we demonstrate high-efficiency white-light-emitting SOLEDs based on phosphorescent emitters with Li-doped ETLs. To the best of our knowledge, it is the first report using a vacuum-thermally deposited MoO 3 film interposed between stacked elements. Figure 1 shows the transmittance of a layer of MoO 3 . The film is more transparent in the wavelength range from 400 to 500 nm compared with previously reported V 2 O 5 interlayers, [11,12] which is essential to achieve an efficient white SOLED. Furtheremore, it is easier to handle than the corrosive and optically absorbing FeCl 3 . To our knowledge, this also is the first device that utilizes phosphor with pyrazolyl-based ligands, which provides a blue-green emission, in a white-light-emitting OLED. Figure 2 shows the proposed energy diagram of the white electrophosphorescent SOLED consisting of multiple, nearly identical white-light-emitting OLED elements, or subpixels. The highest occupied molecular orbital (HOMO) energies for each material were measured using UV photoemission spectroscopy, and the position of the lowest unoccupied molecular orbital (LUMO) energies were estimated by adding the energy corresponding to the onset of optical absorption to the HOMO energy. This procedure generally leads to a systematic underestimation of the HOMO-LUMO energy gap by 0.5 to 1.0 eV. In the white SOLED, two or three electrophosphorescent subpixels are stacked (corresponding to a 2-or 3-SOLED, respectively), with the thickness of the 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) HTL varying in each element. To obtain both high efficiency and balanced emission intensity from each phospho...

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