Ultraviolet amplified spontaneous emission from thin films of 4,4′-bis(9-carbazolyl)-2,2′-biphenyl and the derivatives

We demonstrate 394 nm ultraviolet amplified spontaneous emission (ASE) with a low pumping power threshold of $E_{\mathrm{th}}\text{=1.3±0.2\hspace{0.17em}μ}{\mathrm{J/cm}}^2,$ from a thin film of an organic semiconductor ${\text{4,4}}^{\prime }-\mathrm{bis}\text{(9-}\mathrm{carbazolyl}{\text{)-2,2}}^{\prime }-\mathrm{biphenyl}$ (CBP) under the pulse excitation of a ${\mathrm{N}}_2$ gas laser (337 nm). 3-methyl and 3,6-dimethyl substituted CBP derivatives also exhibited pronounced ASE in the deep-blue region of 401 and 406 nm and ASE thresholds of less than $E_{\mathrm{th}}\text{=2\hspace{0.17em}μ}{\mathrm{J/cm}}^2.$ We also examined the ASE characteristics of ${\mathrm{N,N}}^{\prime }-\mathrm{di}\mathrm{(m-}\mathrm{tolyl}{\mathrm{)-N,N}}^{\prime }-\mathrm{diphenylbenzidine}$ (TPD), ${\mathrm{N,N,N}}^{\prime }{\mathrm{,N}}^{\prime }-\mathrm{tetraphenylbenzidine}$ (DPABP) and ${\mathrm{N,N}}^{\prime }-\mathrm{di}\mathrm{(\alpha -}\mathrm{naphtyl}{\mathrm{)-N,N}}^{\prime }-\mathrm{diphenylbenzidine}$ (α-NPD). While TPD and DPABP showed low ASE thresholds, α-NPD did not show any ASE. We show that the large radiative decay rate ${\mathrm{(k}}_f)$ of DPABP and TPD, which is derived from their short fluorescence lifetime ${\mathrm{(\tau }}_f)$ and large quantum efficiency ${\mathrm{(\eta }}_f\mathrm{),}$ leads to a low ASE threshold. On the other hand, the lack of ASE from α-NPD is ascribable to the small $k_f$ of ${\text{0.8±0.1×10}}^8\hspace{0.17em}{\mathrm{s}}^{\text{−1}},$ which is due to the rather long ${\tau }_f$ of 3.5 ns and small ${\eta }_f$ of $\text{0.29±0.02.}$ In addition, we observed that the ASE gain is closely correlated with $k_f.$ A larger $k_f$ resulted in a larger ASE gain.