Detection of far-infrared radiation using Rydberg atoms

A new method for detecting far-infrared radiation is demonstrated. The far-infrared radiation induces transitions between highly excited (Rydberg) levels of Na. These transitions can be detected by selective field ionization. The method is capable of providing narrow-band detection with essentially continuous tuning throughout the infrared and far-infrared range. We have detected radiation from a laser source at 496 and 118 μ with a 1-MHz bandwidth. A noise equivalent power of 5×10−15 W/Hz1/2 was measured at 496 μ.