  We optically excite $^{85}$Rb atoms in a heated vapor cell to a low-lying
Rydberg state 10D$_{5/2}$ and observe directional terahertz (THz) beams at 3.3
THz and 7.8 THz. These THz fields are generated by amplified spontaneous
emission from the 10D$_{5/2}$ state to the 11P$_{3/2}$ and 8F$_{7/2}$ states,
respectively. In addition, we observe ultraviolet (UV) light produced by
four-wave mixing of optical pump lasers and the 3.3 THz field. We characterize
the generated THz power over the detuning and power of pump lasers, and
identify experimental conditions favoring THz and UV generation, respectively.
Our scheme paves a new pathway towards generating high-power narrow-band THz
radiation.
