The authors observed a narrow emission from a 500-nm -thick codeposited organic thin film, 6 wt % 4, 4′ -bis [(N -carbazole)styryl]-biphenyl (BSB-Cz) doped into a 4, 4′ -bis (N -carbazole)-biphenyl host, under continuous-wave excitation. Although they observed no clear threshold characteristics on the spectral narrowing, they found that the emission centered at a wavelength of λASE =462 nm, corresponding to the zero to one transition of BSB-Cz, has a transverse electric mode with a narrow full width at half maximum of λFWHM =3.4 nm, indicating the occurrence of amplified spontaneous emission. No excited state absorptions of singlet and triplet excited states of BSB-Cz are confirmed around λASE, leading to large net gain for the light amplification.
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