Lower limit of the lasing threshold in an organic microcavity

Abstract

The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq 3 :DCM. Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq 3 at a wavelength of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic microcavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thickness dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission wavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond to different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinement is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the mirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thickness of approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.