IRQC parameters for Er3+ occupying sites of C2v and C3v symmetry in CdF2

Abstract

Measurements have been made of the optical parameters relevant to the operation of the Er³⁺ infrared quantum counter (IRQC) proposed recently to detect 1.5‐μm radiation. The CdF₂:Er³⁺ crystals used for this purpose were prepared under specific conditions selected to provide different local site symmetries at the Er³⁺ ions. EPR measurements were employed to determine the site symmetries and the relative concentration of ions at the various sites. In particular, high concentrations of C_2v sites were generated by double doping with NaF prior to crystalline growth [CdF₂: (Er³⁺,Na⁺)], whereas C_3v sites were generated preferentially by oxygen firing sodium‐free crystals subsequent to growth [CdF₂: (Er³⁺,O²⁻)]. Selective generation of these two sites permitted the unambiguous determination of their characteristic optical absorption, emission, and excitation spectra, as well as the lifetimes and radiative efficiencies of relevant excited states. In addition, double excitation using two ir sources simultaneously, was used to determine the cross section for transitions between the ⁴I_13/2 and ⁴S_3/2 excited states and the lifetime of the former state. The optical properties of the Er³⁺ ion were observed to be sensitive to the local environment associated with each site and, in particular, to the magnitude of the noncubic component of the crystalline field. Phase‐lag measurements confirmed the successive‐excitation model proposed to explain the conversion of 1.5‐μm radiation to 540‐ or 660‐nm radiation in the presence of an 840‐nm pump. Cooperative‐excitation processes, on the other hand, were observed to produce 660‐nm emission when 790‐ or 970‐nm pumps were employed. The dependence of the over‐all IRQC quantum efficiency on pump flux and on the optical parameters of Er³⁺ is derived from solutions to the appropriate rate equations. The analysis indicates that the performance of the 1.5‐μm Er:IRQC, utilizing an 840‐nm pump, is nearly optimized for CdF₂: (Er³⁺,O²⁻).