Quantum counter for far-infrared radiation

Two types of three-level quantum counters, based on the Bloembergen and the inverted scheme, are discussed with respect to their application for up-conversion of far-infrared (FIR) radiation to visible light. It is reported on a tunable quantum counter of the inverted type: in optically pumped ruby, photons at 29 cm
^-1
are absorbed by the transition
\bar{E}
(
²
E
) →
2\bar{A}
(
²
E
) and lead to an increased R
2
- fluorescence radiation. By Zeeman splitting of the
²
E
levels, tuning of the detected frequency from 29 cm
^-1
to higher and lower frequencies is possible (with 1 cm
^-1
/Tesla). Using the 891-GHz radiation of an HCN laser we determine for the quantum counter a power conversion efficiency of the order of 10
^-6
and a frequency bandwidth of 360 MHz. We find the interesting result that phonon bottleneck due to resonant trapping of 29 cm
^-1
phonons can lead to an increase of the quantum efficiency by orders of magnitude.