  The optical gain of blue light-emitting electrospun polystyrene fibers doped
with a linear multi-fragment molecular dye based on the combination of
fluorine-carbazole functional units is investigated, with the aim of
correlating emission properties and the specific material architecture made of
either aligned or disordered fibers. Enhanced performance is found in aligned
fibers, whose gain spectrum can be finely tuned by varying the dye
concentration. Instead, randomly oriented fibers show a manifold spectral line
narrowing, resulting in sharp laser peaks superimposed on top of a broad
emission band, ascribable to random lasing. In these systems, the increase of
the dye content turns out to be effective for both decreasing the lasing
threshold by about a factor 6 and for varying the laser emission wavelength.
These results make these arrays and disordered architectures of fibers valuable
active media for variable-gain, broadband lasing, which is remarkably important
in optical sensing and tunable microlaser devices.
