Figure 1: (a) Non-resonant pumping scheme that is impeded by exciton annihilation, preventing polariton build up in the well. (b) Intra-cavity pumping scheme that populates polaritons along the entire dispersion curve. (c) Device structure. (d) Spectra showing the overlap of the emission of the DCM pump with the absorption of the strongly coupled J-aggregate.

The strongly coupled states of light and matter in microcavities known as polaritons offer exciting possibilities for the study of condensation, superfluidity, and other condensed matter phenomena and show promise as a radically new class of optoelectronic devices based on the macroscopic coherence of light and matter ^[1] . In particular, organic materials allow for strong coupling and polariton lasing to be achieved at room temperature and with substantially reduced requirements for cavity quality factor ^{[2] [3]} . In our work, we demonstrate room temperature polariton lasing in a lambda-thick microcavity where a highly absorbing thin film of J-aggregates serves as the strong coupling material (Figure 1c). Typically, polaritons in the cavity are created by excitation with non-resonant laser pulses. Subsequently, the hot excitons couple to the cavity and move along the polariton dispersion towards the bottom of the polariton trap (Figure 1a). However, a loss mechanism known as annihilation  is present in organic materials which prevents a high density of polaritons from being formed in the trap, and hence polariton condensation (lasing) cannot be achieved. We employ a new pumping scheme known as intra-cavity pumping that circumvents annihilation losses inherent to organic materials at high excitation densities (Figure 1b) by pumping the polariton mode from within the cavity using another organic material (the laser dye DCM) (Figure 1c and 1d). Using this flexible cavity architecture, polariton lasing at room temperature has been achieved. Current work focuses on the application of the polariton laser as a low-energy, ultra-fast all-optical switch and on a better understanding of the properties of the polariton condensate.

1. H. Deng and Y. Yamamoto, “Exciton-polariton Bose-Einstein condensation,” Reviews of Modern Physics, vol. 82, pp. 1489-1537, May 2010.
2. S. Kéna-Cohen and S. R. Forrest, “Room-temperature polariton lasing in an organic single-crystal microcavity,” Nature Photonics, vol. 4, pp. 371-375, Apr. 2010.
3. J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, and V. Bulovic, “Solid state cavity QED: Strong coupling in organic thin films,” Organic Electronics, vol. 8,  pp. 94-113, 2007.

Figure 1: (a) Enhancement structure schematic. (b) Absorption and emission spectra of materials in the system. (c) Absorption enhancement of J-aggregates due to the critically coupled resonator. (d) Spectrum of DCM emission for control sample and when coupled to JCCR. (e) Photograph of sample under ambient light. (f-g) Photographs of control DCM sample and DCM on JCCR.

The efficient absorption and emission of light is of fundamental importance to the operation of optoelectronic devices such as solid-state lighting, lasers, and photodetectors. The properties that determine the brightness of molecules and other luminescent species—absorption cross-section and emission quantum yield – are fundamental to the material and cannot be readily changed. It is thus highly desirable to have a device or general strategy for decoupling the absorption and emission properties of a luminescent thin film, so that the brightness of a lumiphore can be increased without changing its spectral emission properties or increasing the intensity of light for photoexcitation. Towards this end, work over the past decade has focused on the enhancement of fluorescence by coupling luminophores to metal nanoparticles, such as gold ^[1] .

Here we propose an alternative, purely exciton-based, approach to the enhancement of luminophore emission by coupling the luminophore to a highly absorbing resonant optical structure. The enhancement structure is based on a thin film of strongly absorbing J-aggregates placed at the anti-node of the electric field at a distance λ/4 away from a mirror, where λ is the wavelength of incident light. This placement results in destructive interference between the light reflected from the J-aggregate film and light that is reflected by the mirror and passed by the film. The structure (Figure 1a), here referred to as a J-aggregate critically coupled resonator (JCCR), can absorb up to 97% of the incident light ^[2] . The optical energy, localized in the form of excitons in the J-aggregate film, can then be coupled to luminophores deposited on the JCCR by Förster resonant energy transfer (FRET). As a result, the JCCR acts as a platform for dramatically enhancing the optical absorption cross-section of the target luminophore, leading to a subsequent enhancement in the emission. Such an architecture forms a highly general platform for fluorescence enhancement. A wide range of luminophores, both organic and inorganic, with emission spanning the visible and near IR, can be enhanced by simply choosing the appropriate J-aggregate material. In this work, we show a dramatic 20-fold enhancement in the emission of the laser dye DCM by utilizing the JCCR enhancement structure.

1. J.-H. Song, T. Atay, S. Shi, H. Urabe, and A. V. Nurmikko, “Large enhancement of fluorescence efficiency from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons,” Nano Letters, vol. 5, pp. 1557-61, Aug. 2005.
2. J. R. Tischler, M. S. Bradley, and V. Bulovic, “Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film,” Optics Letters, vol. 31, pp. 2045-2047, 2006