Active narrowband filtering, line narrowing and gain using ladder electromagnetically induced transparency in an optically thick atomic vapour

Abstract: Electromagnetically induced transparency (EIT) resonances using the $5\rm{S}{1/2}\rightarrow5\rm{P}{3/2}\rightarrow5\rm{D}{5/2}$ ladder-system in optically thick Rb atomic vapour are studied. We observe a strong line narrowing effect and gain at the $5\rm{S}{1/2}\rightarrow5\rm{P}{3/2}$ transition wavelength due to an energy-pooling assisted frequency conversion with characteristics similar to four-wave mixing. As a result it is possible to observe tunable and switchable transparency resonances with amplitude close to $100\%$ and a linewidth of 15 MHz. In addition, the large line narrowing effect allows resolution of $^{85}$Rb $5\rm{D}{5/2}$ hyperfine structure even in the presence of strong power broadening.