# Nonlinear Optical Microscopy of Interface Layers of Epitaxial Gamet Films
## 2 Experimental Procedure and Samples under Study  
Experiments were performed for 10 \(\upmu\)m thick \((LuBi)_{3}Fe_{5}O_{12}\) epitaxial films grown by liquid phase epitaxy on a (111) facet of centrosymmetric GGG substrates. Figure 1a shows the transmission and optical density (OD) spectra of the studied samples, which reveal high absorption at the wavelengths below 600 nm that is typical for garnet films [2]. Namely, the absorption coefficient in the blue spectral range is about four orders of magnitude larger as compared to the wavelength in the near-IR spectral range, which is rather important for the nonlinear optical experiments. Magnetic force microscopy (MFM) experiments of the garnet films were performed using the set-up described in detail in [42]. In brief, we used the atomic force microscope SmartSPM (AIST-NT, now produced by HORIBA Scientific) with a special magnetic tip composed of a multilayered \([Co(2)/Pt(5)]^{10}\) film, the thickness of the corresponding layers in Angstrom is denoted in brackets. Such a composition of the sensitive layer of the tip provided, first, the perpendicular magnetic anisotropy of the film and, second, its small magnetic moment. As a result, the magnetic structure of the studied sample was not disturbed during the MFM measurements. MFM images of \((LuBi)_{3}Fe_{5}O_{12}\) films were obtained in a standard two-pass technique (the so called lift mode), when at the first step the topography of the studied sample is recorded followed by the registration of the magnetic response in the constant distance surface-probe mode at the second step. Our measurements have shown that the films are flat, the root mean square average of the profile height deviations from the mean value (RMS) being about 1 nm. It is worth noting that the MFM probe reveals the distribution of the magnetic stray field along the normal to the surface, while the field associated with the in-plane components of magnetization remain much less pronounced in the MFM profile. For the nonlinear optical microscopy studies, we used the radiation of an optical parametric oscillator (OPO) system Topol 1050-C (Avesta) with the laser pulse duration of up to 180 fs, wavelength tunable in the spectral range from 740 nm up to 1800 nm, and the mean power of 100 mW. The pump radiation was focused by a Nikon Plan Apo 60\(\times\) oil objective with the NA = 1.4 in a spot of about 0.7-0.8 \(\upmu\)m in diameter, which provided the waist length of the laser beam of a few micrometers, depending on the wavelength of the laser radiation. Mitutoyo M Plan Apo 100x objective with the NA = 0.7 was used for gathering the transmitted radiation at the second or third harmonic's wavelengths, the nonlinear optical signal being detected by the Hamamatsu R4220 photomultiplier (PMT). In our experiments, the \((LuBi)_{3}Fe_{5}O_{12}\) film was fixed at the Ratis three-axes piezo scanner stage, providing the positioning accuracy of a few nanometers. Polarizations of the input and harmonics' radiation were controlled by the Glan-Taylor calcite polarizers and the necessary set of color filters placed in front of the PMT were used to block the pump beam passing through the garnet layer. Figure 1: (**a**) Optical density (OD) and transmission spectra of \((LuBi)_{3}Fe_{5}O_{12}\) film, high absorption region corresponding to the second and third harmonics wavelengths is in green, high transparency range is in red. (**b**) Optical transmission microscopy image of the film measured for the crossed orientations of the polarizer and the analyzer; and (**c**) MFM image of the \((LuBi)_{3}Fe_{5}O_{12}\) film. In accordance with the transmission spectrum of the \((LuBi)_{3}Fe_{5}O_{12}\) layer shown in Figure 1a, the studied garnet films are transparent for the fundamental laser radiation in the whole tuning range marked by red in Figure 1a, while second and third harmonics' wavelengths typically fall into the absorption band.