The origin of anomalous red-luminescence in nonlinear optical crystals K$_{2}$Al$_{2}$B$_{2}$O$_{7}$ : Fe

In this work, the spectra of anomalously intense luminescence at 1.675 eV (FWHM = 0.173 eV) in wide-gap (180–3600 nm) nonlinear-optical crystals K$_{2}$Al$_{2}$B$_{2}$O$_{7}$ : Fe were observed experimentally for the first time upon excitation in the spectral region of 4–5 harmonics of the fundamental frequency of the Nd$^{3+}$ radiation. Using low-temperature (7–293 K) luminescence-optical spectroscopy with selective excitation by synchrotron radiation, it was shown that red luminescence occurs at intracenter $^4T_1(^4G)\to^{}6A_1(^6S)$ transitions in impurity Fe$^{3+}$ ions with a concentration less than 2 ppm. The high excitation efficiency in the exciton region is due to the partial overlapping of the fundamental absorption edge of the crystal, where mobile excitons are excited, and the broad absorption band with O–Fe charge transfer with a maximum at 6.5 eV. The spectrum and temperature dependence of the luminescence are due to the superposition of two types of luminescence centers (activation energies 9 and 20 meV) based on Fe$^{3+}$ impurity ions located in regular Al$^{3+}$ positions of two nonequivalent Al$_2$O$_7$ clusters.