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Flat organic microlasers investigated in two and three dimensions

Stefan Bittner

à 14h en salle C. Brot

Organic solid state microlasers are ideal testbeds to investigate fundamental problems like the ray-wave correspondence in open wave-dynamical systems or the diffraction at dielectric corners. The microlasers are fabricated from a thin film of a dye-doped polymer via electron beam lithography. While the thickness of the cavities is of the order of one wavelength, their transverse dimension is much larger than the wavelength. Therefore one can consider the cavities as two-dimensional photon billiards and investigate the relations between the lasing modes and the dynamics of the corresponding classical billiard system. The lasing modes of microlasers are often localized on periodic ray trajectories that can be deduced from various experimental observables like their spectra and far-field distributions. A particularly interesting case is that of triangular cavities because the periodic orbits of arbitrary triangular billiards are not known. A number of triangular microlasers with different shapes was investigated, and in the case of triangles exhibiting a symmetry, the resonant states showed a clear localization on simple periodic orbits. In the cases of asymmetric triangles, however, no underlying classical orbit could be identified, but the modes seemed instead to be related to so-called diffractive orbits that impinge on one of the corners. Since the microcavities are only one wavelength thick, diffraction at their edges is inevitable. However, the diffraction at dielectric corners and edges remains an unsolved mathematical problem. A novel setup that enables to measure the far-field distribution in the full solid angle was used to measure the emission from different kinds of cavities in three dimensions. It was found that, contrary to intuition, a large part of the lasing light is emitted in directions out of the plane of the cavities. Furthermore, the far-field emission depends strongly on the shape of the cavity, the type of the lasing mode, and on the substrate of the cavity. A simple analytical model was used to explain the far-field distributions of ribbon-shaped cavities, while those of other resonator types are not yet fully understood