Sensitization of infrared quantum detectors by energy transfer mechanisms

The detection of infrared radiation using the tripositive rare earth ions of Pr, Nd, Dy, Ho, Er, and Tm has been reported by several authors. The energy levels of these ions are employed to convert infrared quanta to visible quanta by a two step excitation process. To date the conversion efficiency of this process has been of the order of 10^-6for the best systems. We have increased the efficiency of the Holmium quantum counter by two orders of magnitude by making use of energy transfer between the Yb and Ho rare earth ions. The single crystal used is CaF2doubly doped with 20% Yb⁺, and 2% Ho³⁺. The heavily doped Ytterbium absorbs in a band around one micron (into the²F5/2 level) and transfers most of its energy to the⁵I6level of Holmium. The electron is then lifted to higher excited states of the Ho³⁺ion by selected pump frequencies. The electron then radiates to the ground state with radiation that can be detected either by the eye or by commercial photomultipliers. Output signals in the red, green, blue, and UV have been detected.