An ultra-narrow linewidth solution-processed organic laser

摘要

Optically pumped lasers based on solution-processed thin-film gain media have recently emerged as low-cost,broadly tunable,and versatile active photonics components that can fit any substrate and are useful for,e.g.,chemo-or biosensing or visible spectroscopy.Although single-mode operation has been demonstrated in various resonator architectures with a large variety of gain media--including dye-doped polymers,organic semiconductors,and,more recently,hybrid perovskites—the reported linewidths are typically on the order of a fraction of a nanometer or broader,i.e.,the coherence lengths are no longer than a few millimeters,which does not enable high-resolution spectroscopy or coherent sensing.The linewidth is fundamentally constrained by the short photon cavity lifetime in the standard resonator geometries.We demonstrate here a novel structure for an organic thin-film solid-state laser that is based on a vertical external cavity,wherein a holographic volume Bragg grating ensures beth spectral selection and output coupling in an otherwise very compact (～cm3) design.Under short-pulse (0.4 ns) pumping,Fourier-transform-limited laser pulses are obtained,with a full width at half-maximum linewidth of 900 MHz (1.25 pm).Using 20-ns-long pump pulses,the linewidth can be further reduced to 200 MHz (0.26 pm),which is four times above the Fourier limit and corresponds to an unprecedented coherence length of 1 m.The concept is potentially transferrable to any type of thin-film laser and can be ultimately made tunable;it also represents a very compact alternative to bulky grating systems in dye lasers.