Linear and nonlinear optical spectroscopy of gadolinium iron borate GdFe3(BO3)4

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Abstract
The optical spectra and the second-harmonic generation (SHG) are studied in a noncentrosymmetric GdFe3(BO3)4 magnet. In the region of weak absorption (α∼20–400 cm−1) below ∼3 eV, three absorption bands are distinguished, which can be unambiguously assigned to forbidden electronic transitions from the ground 6 A 1 state of the Fe3+ ion to its excited states 4 T 1(∼1.4 eV), 4 T 2(∼2 eV), and 4 A 1, 4 E(∼2.8 eV). Intense absorption begins in the region above 3 eV (α∼2–4×105 cm−1), where two bands at ∼4.0 and 4.8 eV are observed, which are caused by allowed electric dipole charge-transfer transitions. The spectral features of SHG in the 1.2–3.0-eV region are explained by a change in the SHG efficiency caused by a change in the phase mismatch. It is shown that in the weak absorption region, phase matching can be achieved for SHG.