Interfacial charge transfer excitation with large optical nonlinearity in manganite heterostructure

Interfacial electronic states between ${\mathrm{LaMnO}}_3$ and ${\mathrm{SrMnO}}_3$ have been studied by second-harmonic (SH) spectroscopy. The SH contributions from bulk, surface, and interface were separated by measuring four distinct types of manganite thin films. In the SH spectra, we found a broad peak at $2\mathrm{eV}$ unique to the ${\mathrm{LaMnO}}_3∕{\mathrm{SrMnO}}_3$ interface, which was assigned to interfacial charge transfer excitation based on a Hartree-Fock calculation. No metallic phase showed up at the interface. Experimental and theoretical second-order susceptibilities ${\chi }^{\left(2\right)}$ at the interface were estimated to be $\sim {10}^{-6}\mathrm{esu}$, which is ten times as large as the highest ${\chi }^{\left(2\right)}$ value of ${\mathrm{BaTiO}}_3$.

Figure 1

Angular dependence of the SH intensities of manganite thin films at $2.74\mathrm{eV}$ as SH photon energy. The solid line is obtained from a model calculation based on the Maxwell equations for a heterostructure with an effective nonlinear medium localized at the interface. The angle of incidence is given with respect to the sample normal. The inset shows a schematic of the manganite heterostructure $({\mathrm{SrMnO}}_3∕{\mathrm{LaMnO}}_3∕\mathrm{LSAT})$.

Figure 2

Calibrated SH spectra of the manganite thin films as a function of SH photon energy. The spectra are normalized to the peak SH intensity of ${\mathrm{SrMnO}}_3∕{\mathrm{LaMnO}}_3∕\mathrm{LSAT}$ at about $2\mathrm{eV}$.

Figure 3

The $\mid {\chi }_{zzz}^{\left(2\right)}{\mid }^2$ spectra based on the $p$-$d$ model (see Ref. 24). The spectra are normalized so that the peak $\mid {\chi }^{\mathrm{aS}}+{\chi }^{\mathrm{SL}}\mid$ is unity, which corresponds to $1.184\times {10}^{-6}\mathrm{esu}$.