Thanks to its simple process, high accuracy and 3D flexibility, femtosecond (fs) laser direct writing waveguide technology has been widely used in recent years for the fabrication of various types of micro and nano photonic devices. This technique relies on the nonlinear absorption process induced by a focused fs laser inside a transparent material. Depending on the processing parameters and material properties, the irradiated region produces a local increase or decrease in refractive index, and the waveguide structures constructed on this basis can be classified into the following three categories: type-I, type-II and type-III. Among them, type-II and type-III waveguides utilize the low refractive index region as the waveguide boundary and the high refractive index region generated by stress field as the waveguide core. They have better thermal stability and polarization properties compared to type-I waveguides, and the intrinsic properties of the material are not affected, thus they can be widely used in laser and nonlinear optics.

In addition, fs lasers can also change the nonlinear properties of materials. On the one hand, for some ferroelectric crystals, the local temperature gradient field caused by the focused fs laser may induce a thermoelectric field, which causes ferroelectric domain inversion. This phenomenon is extremely demanding on the parameters of the fs laser and the orientation of writing, which is more difficult to achieve experimentally; on the other hand, the focused fs laser can also directly modulate the nonlinear coefficients by destroying the lattice structure. Depending on the degree of destruction, the reduction or even erasure of the nonlinear coefficients can be achieved. Compared with the traditional electric-field poling technique, fs laser modulation of nonlinearity has the advantages of low cost and the ability to process three-dimensional nonlinear structures, based on which one-to-three-dimensional quasi-phase-matching (QPM) structures are developed, playing an important role in nonlinear frequency conversion, nonlinear beam shaping, and nonlinear holographic imaging.

In this paper, we combine the fs laser modulation of the refractive index and nonlinearity, and for the first time fabricate a QPM type-II waveguide structure on a Z-cut lithium niobate crystal with direct fs laser writing, and demonstrate the second harmonic generation process based on the maximum nonlinear coefficient d33 of lithium niobate. Figure 1 shows the fs laser direct writing platform and design of the device, where the wavelength of the fs laser is 1030 nm, the pulse width is 500 fs, and the repetition frequency is 1 k Hz. The nonlinear modulation period of the prepared waveguide is about 6.9 um, which is used to meet the QPM conditions of the second harmonic at room temperature for a laser with a fundamental frequency of 1064 nm. Figure 2 shows the fabricated waveguide structure and the refractive index distribution of the waveguide cross-section obtained based on the simulation. Figure 3 shows the measured second harmonic power versus the input fundamental frequency power. In the type II waveguide with the QPM structure, we obtained a conversion efficiency of 8.76 %/W*cm, which is a significant improvement over the waveguide without QPM structure. Based on the numerical calculation, the modulation depth is about 0.89. This structure will provide a direct-written QPM scheme for fs laser with easy preparation while ensuring high nonlinear conversion efficiency.

Fig 1. Schematic of the fs laser direct writing platform.

Fig 2.  (a) Fs laser direct writing QPM type-II waveguide and normal type-II waveguide. (b) Cross-section images of the type-II waveguide (top) and the nonlinearity modulated region (bottom). (c)(d) Simulated refractive index distribution.

Fig 3.  (a)Second harmonic power versus input fundamental wave power in the QPM type-II waveguide. (b) Comparison of the QPM type-II waveguide with the normal type-II waveguide.

This research is published in “Tingge Yuan, Bing Zhu, Honghuan Tu, Yuping Chen and Xianfeng Chen, Femtosecond laser direct writing quasi-phase matched type-II waveguide in lithium niobate, Optical Materials Express, 13(1), 1-8 (2023)”.

Link: https://doi.org/10.1364/OME.477618