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^-1are absorbed by the transition (² ) → (² ) and lead to an increased R2- fluorescence radiation. By Zeeman splitting of the² levels, tuning of the detected frequency from 29 cm^-1to 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^-6and a frequency bandwidth of 360 MHz. We find the interesting result that phonon bottleneck due to resonant trapping of 29 cm^-1phonons can lead to an increase of the quantum efficiency by orders of magnitude.