Fig. 1. XRD results of the BTO thin film deposited on the (001) STO substrate by PLD. (a) A θ-2θ scan of the BTO thin film. The BTO was epitaxially grown along the (00l) direction. (b) A φ scan of the BTO thin film indicated a cube-on-cube growth on the STO substrate.

Fig. 2. (a) Structure of a tetragonal BTO unit cell with one Ti atom in the cell center, eight Ba atoms in the corners, and six O atoms in the center of the facets. (b) The six possible ferroelectric domain variants, X+, X−, Y+, Y−, Z+, and Z−, in the +X→, +Y→, and +Z→ lab-based coordinate system.

Fig. 3. Four possible domain variants in the y−z plane, Y+, Y−, Z+, and Z−, and the crystal axes (U1, U2, U3) associated with each of the four domain variants. The incident light E→0 is linearly polarized, and the azimuthal angle between the incident light and the y-axis is noted as φ.

Fig. 4. Calculated azimuthal-dependent polarized SHG, Iy2ω(φ) and Iz2ω(φ), of a single Y+ domain BTO thin film. The tensors dij from a bulk BTO crystal were used: d15=17  pm/V, d31=15.7  pm/V, and d33=6.8  pm/V. The thickness of the BTO crystal was 500 nm. Iy2ω(φ) and Iz2ω(φ) showed distinct two-lobed and four-lobed SHG patterns, respectively.

Fig. 5. Iy2ω(φ) and Iz2ω(φ) were calculated when the ferroelectric domain fraction ratio, δAY/δAZ, was set to (a) 10, (b) 1, and (c) 0.1. The dij values were from a bulk BTO crystal. At δAY/δAZ=10, Iy2ω(φ) had a two-lobed profile, and Iz2ω(φ) had a four-lobed profile. As δAY/δAZ decreased to 0.1, Iy2ω(φ) became four-lobed and Iz2ω(φ) became two-lobed. In addition, the axis of the two-lobes rotated as δAY/δAZ changed.

Fig. 6. Schematic of the experimental setup for measuring the azimuthal-dependent polarized SHG from a BTO thin film. The pumping laser was a tunable ns pulsed laser and the first filter removed the light that was not at the pumping wavelength λ. The polarization of the light was rotated by the λ/2 phase plate. The mid-IR light was focused on the BTO thin film using the front objective lens, and the SHG was collected by another objective lens in the back. The polarizer selected the polarization of the SHG signals, and the second filter removed the residual mid-IR pumping light. The SHG in the NIR region was measured by a photodetector or a spectrometer.

Fig. 7. (a) Iy2ω(φ) and (b) Iz2ω(φ) obtained from the BTO thin film deposited by PLD. The dashed lines represent the measured data and the solid green lines represent the modeling results. From the fitting, the values d15=12.5  pm/V, d31=9.0  pm/V, d33=10.0  pm/V, and δAY/δAZ=1 were resolved.

Fig. 8. SHG intensity, Iy2ω, versus the square of the input mid-IR laser power, Ilaser, measured at λ=3.5  μm. The black circles indicate the measured data, and the red line represents the fitted curve. A linear relationship between Iy2ω and (Ilaser)2 was found.

Fig. 9. SHG spectrum of the BTO thin film when the mid-IR pumping wavelength was tuned from λω=3.0 to 3.6 μm. Strong SHG signals between λ2ω=1.5 and 1.8 μm were found, indicating that BTO has a broadband second-order optical nonlinearity.