Mirrorless lasing from thiophene∕phenylene co-oligomer crystals based on stimulated resonance Raman scattering

Very narrowed light emission was observed from single crystals of various thiophene∕phenylene co-oligomers. Under optical pumping at their resonant absorption wavelengths, all the platelet crystals showed amplified spontaneous emission with a full width at half maximum (FWHM) of $5–7\mathrm{nm}$ . When the pumping wavelengths approached the absorption edges, the crystals emitted further narrowed laserlike lines $(\mathrm{FWHM}<0.5\mathrm{nm})$ . From their Stokes shift frequencies compared with the off-resonant Raman spectra, we assigned these emission lines to the stimulated resonance Raman scattering. These lines appeared in single or double according to the Raman active modes determined by their different molecular symmetry. The dependence of line intensity on polarized pumping suggested that the stimulated resonance Raman scattering was related to polaritonic phenomena of vibrationally dressed excitons in the uniaxially oriented molecular crystals.