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METALORGANIC MICROCAVITIES
Keep it coherent
Scientists have shown that embedding strips of metal in an organic optical resonator allows its emission properties to be tuned while maintaining coherence.
Michiel Wouters
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The interaction between light and matter is of fundamental importance. Organic microcavities oer particularly
appealing prospects for studying this interaction because of their large oscillator strengths, exciton binding energies and simple layer fabrication. However, studies into the electrical excitation of processes related to the lightmatter interaction require the use of highly absorbing electrodes, which is generally considered to be detrimental to optical coherence.
Reporting in Nature Photonics,Robert Brckner and co-workers from Technische Universitt Dresden in Germany have now shown that coherent emissioncan be achieved in an organic microcavity containing strips of thin metal lm1
(Fig.1). This nding is important becauseit demonstrates that the presence of an embedded metal is not detrimental to the photonic coherence of the resonator output. Moreover, the possibility of laser operation with a metal inside the microcavity is promising for achieving the long-sought goal of organic lasing under electrical pumping.
When a metal layer is embedded in a microcavity, plasmonpolariton states can form at the boundary between the metaland a dielectric Bragg mirror (Fig.1a). Their energy is within the bandgap of the Bragg mirror, making them the optical analog of the electronic Tamm states that form at the edge of a crystal hence they are referred to as Tamm plasmonpolaritons2.
Patterning the embedded metal layer makes it possible to engineer the optical states inside the microcavity. Brckneretal. studied the case of a periodically patterned structure consisting of 40-nm-thick silver strips with a lattice periodicity of around 10m (Fig.1a). They also fabricated two other microcavities to serve as references: one without an embedded metal and one containing a homogeneous 40-nm-thick layer of silver.
Angle-resolved photoluminescence spectra (Fig.1b) show the typical structure expected for a Bloch wavefunction in such a periodic potential, with replica dispersion branches shied in wavenumber by 2N/a, where N is an integer and a is the lattice
periodicity. The states in the conguration of Brckneretal. reach up to N=30, which is much larger than in previous studies into periodically modulated microcavities3,4.
This high order can be explained by thetight localization of the optical modes...