Abstract

In random lasers (RLs) with spatially distributed feedback (SDF), lasing feedback is provided by highly scattering particles randomly distributed inside the active medium [1]. Alternatively, in RLs with spatially localized feedback (SLF), the gain region is placed between two scattering surfaces [2]. Spatial modes of RLs have been previously characterized in devices with SDF [3, 4] but not yet in RLs with SLF. Here, we analyse the the spatial emission of individual RL modes supported by SLF, observing specific intensity patterns for each considered lasing wavelength. The investigated RLs consist of a dye doped polymer film with random defects at polymer/air interfaces, see [2] for fabrication details. A simplified view of our experimental set-up is given in Fig. 1(a). The film is pumped by a ns-pulsed laser at 532 nm shaped in a stripe line with 50 µm width (x direction) and 3 mm length (z direction). The two scattering surfaces between polymer and air, located at the ends of the striped pump, provide optical feedback and act as output couplers [5]. The diverging beam exiting the RL from one scattering surface is collected by a convex lens (CL) with 5 cm focal length, in order to image emission in the x-y plane orthogonal to the z direction. The beam is scanned along x and y by a fibre tip (core diameter 100 µm) connected to a spectrometer.

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