Absorption (dashed line) and steady-state photoluminescence at low excitation density (solid line) of the MeLPPP. The arrow indicates the region of interest, where the maximum optical gain is.
Schematic cross section of a circular grating with period . The scheme illustrates the second-order Bragg diffraction, i.e., normal diffraction to the surface (first order) and in-plane feedback (second order). The active polymer MeLPPP was spin coated on top of the circular gratings.
SEM images of two circular gratings before the polymer film coating. The grating grooves appear as dark circles, resembling the cross section of a leek. (a) Cavity radius and (b) with .
Photoluminescence spectra below lasing threshold: A sharp peak appears inside the band gap, for (a) it is located at and for (b) at . The peak intensity increases linearly with the pumping power.
Simulation results: The graphs show the ratio between the power in the first ring and that in the last one as described in Eq. (12). The solid curve represents the monopole , the dashed curve the dipole , and the dash-dotted curve the quadrupole . We see a clear peak inside the band gap for both inner ring radii at and . The two vertical solid lines delimit the band gap for an equivalent linear grating computed with MPB.
Analysis graph of the peak positions. Experiment: The two solid straight lines represent the band gap of the equivalent experimental linear grating and the triangles the experimental peaks. Simulation: The two dotted straight lines represent the band gap of the circular structure modeled, the solid curve the peaks of the monopole resonance , the dashed one those of the dipole , and the dash-dotted one those of the quadrupole . The error bar corresponds to a shift of in the grating period.
PL spectrum of MeLPPP at low excitation intensities (cf. Fig. 1) together with the laser emission, both normalized with respect to maximum intensity. The inset is an enlargement of the laser line with a Gaussian fit through the measured points. The emitted beam has a FWHM of at .
Two PL spectra of an excited circular grating : one below (dotted line) and one above (solid line) the lasing threshold. The energy values denote the energy per pump pulse. The inset shows emission intensity vs excitation pulse energy. The solid squares represent the threshold behavior of the new structures , the empty squares that of the old ones .
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