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

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.