Grating-assisted spectrally-narrowed emissions from an organic slab crystal excited with a mercury lamp

We report spectrally-narrowed emissions that take place from an organic semiconductor slab crystal of 2,5-bis(4-biphenylyl)thiophene (BP1T) under a low excitation-intensity regime. These emissions are caused with a mercury lamp that operates on a household power supply with an electric current approximately 1 A. The BP1T slab crystal is equipped with a distributed Bragg reflector. To complete this structure the slab crystal is attached to a diffraction grating that is engraved on a surface of a quartz glass substrate. The diffraction gratings have precisely been formed using a focused ion beam with a nanometer-defined precision. The spectral narrowing accompanied by the emission intensity increment is related to the strong mode-coupling between the forward electromagnetic wave and the backward (i.e., reflected) wave within the grating zone. Using a laser we also carried out the emission measurements on the BP1T crystals under a high excitation-intensity regime. The emissions are characterized as the longitudinal multimode laser oscillation, enabling us to determine the group refractive index of 4.56 for the BP1T slab crystal. Under both the low and high excitation-intensity regimes excitons are dominant species of the emission. Their participation in the spectrally-narrowed emissions is briefly discussed.