Using spectroscopic ellipsometry, the room temperature refractive index and absorption versus wavelength of the ferroelectric antiferromagnet bismuth ferrite, $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ , are reported. The material has a direct band gap at $442\mathrm{nm}$ wavelength $\left(2.81\mathrm{eV}\right)$ . Using optical second harmonic generation, the nonlinear optical coefficients were determined to be $d_{15}∕d_{22}=0.20\pm 0.01$ , $d_{31}∕d_{22}=0.35\pm 0.02$ , $d_{33}∕d_{22}=-11.4\pm 0.20$ , and $\mid d_{22}\mid =298.4\pm 6.1\mathrm{pm}∕\mathrm{V}$ at a fundamental wavelength of $800\mathrm{nm}$ .

A prominent band centered at $\sim 1000–1300{\mathrm{cm}}^{-1}$ and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, $T_N\sim 375°\mathrm{C}$ . This band is composed of three peaks, assigned to $2A_4$ , $2E_8$ , and $2E_9$ Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the $2E_8$ mode, in particular, nearly disappears at $T_N$ on heating, indicating a strong spin-two-phonon coupling in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ .

The interplay between spin waves (magnons) and electronic structure in materials leads to the creation of additional bands associated with electronic energy levels which are called magnon sidebands. The large difference in the energy scales between magnons (meV) and electronic levels (eV) makes this direct interaction weak and hence makes magnon sidebands difficult to probe. Linear light absorption and scattering techniques at low temperatures are traditionally used to probe these sidebands. Here we show that optical second-harmonic generation, as the lowest-order nonlinear process, can successfully probe the magnon sidebands at room temperature and up to 723 K in bismuth ferrite, associated with large wave vector multimagnon excitations which linear absorption studies are able to resolve only under high magnetic fields and low temperatures. Polarized light studies and temperature dependence of these sidebands reveal a spin-charge coupling interaction of the type $P_s{L}^2$ between the spontaneous polarization $\left(P_s\right)$ and antiferromagnetic order parameter, $L$ in bismuth ferrite, that persists with short-range correlation well into the paramagnetic phase up to high temperatures. These observations suggest a broader opportunity to probe the collective spin-charge-lattice interactions in a wide range of material systems at high temperatures and electronic energy scales using nonlinear optics.

The main properties of the ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ ${(}^4{F}_{3/2},{}^2{\overrightarrow{F_{7/2}}}^4{I}_{9/2},{}^2{F}_{5/2})$ energy transfer in yttrium aluminum borate nonlinear laser crystal have been studied. This host has been found especially suitable for efficient ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ energy transfer because of the good matching between phonon energy and the ${}^4{F}_{3/2}\left({\mathrm{Nd}}^{3+}{)-}^2{F}_{7/2}\right({\mathrm{Yb}}^{3+})$ energy gap (≅1000 ${\mathrm{cm}}^{\mathrm{-}1}$). Energy-transfer probabilities in excess of 65% have been obtained for a ${\mathrm{Nd}}^{3+}$ (10 at. %) and ${\mathrm{Yb}}^{3+}$ (5 at. %) codoped sample. The influence of both crystal temperature and ${\mathrm{Nd}}^{3+}$ concentration on the transfer probability has been investigated. In the 15–350 K range the ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ energy-transfer efficiency increases with temperature, whereas for crystal temperatures above 350 K the ${\mathrm{Nd}}^{3+}\leftarrow {\mathrm{Yb}}^{3+}$ back transfer is activated, leading to a decrease in the effective ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ energy-transfer probability. The analysis of the decay curves obtained after pulsed excitation has been used to determine the multipole character of the ${\mathrm{Nd}}^{3+}-{\mathrm{Yb}}^{3+}$ interaction. The donor-acceptor energy-transfer microparameter was determined $\left[C_{\mathrm{DA}}\right({\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+})\cong 18\times {10}^{-39}{\mathrm{cm}}^6/\mathrm{s}],$ and compared to those obtained for other ${\mathrm{Nd}}^{3+}-{\mathrm{Yb}}^{3+}$ co-doped materials.