Quasiphase-matched $ee$-$e$ frequency conversion of light in $LiNbO_3:Y$ crystals with a regular domain structure

A study was made of the characteristics of second harmonic generation (phase matching of the $ee$-$e$ type) in $LiNbO_3:Y$ crystals with a regular domain structure created by growth in an asymmetric temperature field when the ($01\bar12$) face formed at the growth front. When the structure period was 6 $\mu m$ and the phase matching was achieved at the wavelength of 1.1 $\mu m$ for the first spatial harmonic of the structure and at 0.896 $\mu m$ for the second harmonic. The frequency conversion efficiency was 12% for the main phase matching when pumping was provided by a nanosecond optical parametric oscillator made of lithium niobate. The pump power density was 200 MW/cm$^2$, the length of the crystal element was 1 mm, the spectral width of the phase-matching process was 3.0 nm, the temperature width was 70 K, and the angular width was $\pm7^\circ$. The spectral width could be increased by employing a regular domain structure with a variable period.